Summary paragraph: Extreme Atlantic and Benguela Niño events significantly impact the tropical Atlantic region with far-reaching consequences on local marine ecosystems, African and South American climates, to the El Niño Southern Oscillation. While accurate forecasts of these events are invaluable, state-of-the-art dynamical seasonal forecasting systems have shown limited predictive capabilities. Thus, the extent to which the variability of the tropical Atlantic is predictable is an open question. Here, exploiting a deep learning-based statistical prediction model, we show that tropical Atlantic events can be predicted up to 3-4 months in advance. Notably, our convolutional neural network model excels in forecasting peak-season events with remarkable accuracy extending lead-time up to 5 months. Detailed analysis reveals our model’s ability to exploit known physical precursors, particularly associated with long-wave ocean dynamics, for accurate predictions of Atlantic/Benguela Niños. This study challenges the perception that the tropical Atlantic is inherently unpredictable and highlights the potential of deep learning to advance our understanding and forecasting of critical climate events.

Abstract: The coastal regions off Angola and Namibia are renowned for their highly productive marine ecosystems in the southeast Atlantic. In recent decades, these regions have undergone significant long-term changes. In this study, we investigate the variability of these long-term changes throughout the annual cycle and explore the underlying mechanisms using a 34-year (1982-2015) regional ocean model simulation. The results reveal a clear seasonal dependence of sea surface temperature (SST) trends along the Angolan and Namibian coasts, with alternating positive and negative trends. The long-term warming trend in the Angolan coastal region is mainly explained by a pronounced warming trend in the austral spring and summer (November-January), while the decadal trend off Namibia results from a counterbalance of an austral winter cooling trend and an austral summer warming trend. A heat budget analysis of the mixed-layer temperature variations shows that these changes are explained by a long-term modulation of the coastal currents. The Angolan warming trend is mainly explained by an intensification of the poleward coastal current, which transports more warm equatorial waters towards the Angolan coast. Off Namibia, the warming trend is attributed to a reduction in the northwestward Benguela Current, which advects cooler water from the south to the Namibian coast. These changes in the coastal current are associated with a modulation of the seasonal coastal trapped waves that are remotely-forced along the equatorial waveguide. These long-term changes may have significant implications for local ecosystems and fisheries.

Abstract: This study explores the oceanic connection between the equatorial dynamics and the coastal variability along the northern coast of the Gulf of Guinea on interannual timescales, based on the experimentation with a high-resolution tropical Atlantic Ocean model over 1958-2015. Equatorial Kelvin waves, forced by wind-stress anomalies in the west-central equatorial basin, significantly control the interannual fluctuations of the coastal sea-level and subsurface temperature near the thermocline (>70%), leaving only a marginal role for the local forcing contribution. The dynamical coastal response exhibits a clear propagative nature, with poleward propagations (0.75-1.2 m.s-1) from Cameroon to Liberia. Because their coast is close to the equatorial waveguide, the coastal variability is influenced by both equatorially-forced coastal trapped waves and reflected equatorial Rossby waves. Furthermore, remote equatorial forcing explains more of the surface temperature variance for the coastal systems associated with clear upwelling characteristics such as Côte d’Ivoire and Ghana, where subsurface/surface coupling is more efficient. The surface thermal amplitude and timing is shaped by the coastal stratification and circulation and exhibits a marked seasonal modulation, so that the timing of the SST anomalies relative to the dynamical signature lacks consistency, making SST a less reliable variable for tracking coastal propagations in the Gulf of Guinea. Our findings open the possibility of predicting interannual changes in coastal conditions off Côte d’Ivoire and Ghana a few months in advance, to anticipate impacts on fish habitats and resources, and to facilitate proactive measures for sustainable management and conservation efforts.

Plain Language Summary: Our study explores the interannual fluctuations of the oceanic conditions along the northern coast of the Gulf of Guinea using numerical experiments conducted with an ocean model of the tropical Atlantic at high-resolution. Modeling results show that the coastal interannual variability of sea level and subsurface temperature is driven to a large extent (>70%) by the propagation of oceanic long waves forced distantly in the western central equatorial basin by a modulation of the trade winds. A clear propagative signal is observed along the coast of the northern Gulf of Guinea, which takes about 1.5 months to affect the coastal variability off Liberia. In contrast, coastal surface winds and atmospheric fluxes have only a marginal influence on the coastal variability on interannual timescales. Furthermore, the impact on the sea surface temperature is only substantial and seasonal along the coasts of Côte d’Ivoire and Ghana, where the upwelling dynamics favors an efficient penetration of the subsurface temperature anomalies into the surface layer. This oceanic connection unveils the potential for predicting fluctuations in coastal conditions and fish habitats in the Ivorian-Ghanaian upwelling system several months in advance, for sustainable resource management.

Abstract: The Congolese upwelling system (CUS), located along the West African coast north of the Congo River, is one of the most productive and least studied systems in the Gulf of Guinea. The Sea Surface Temperature minimum in the CUS occurs in austral winter, when the winds are weak and not particularly favorable to coastal upwelling. Here, for the first time, we use a high-resolution regional ocean model to identify the key atmospheric and oceanic processes that control the seasonal evolution of the mixed-layer temperature in a 1°-wide coastal band from 6°S to 4°S. The model is in good agreement with observations on seasonal timescales, and in particular reproduces the signature of the surface upwelling during the austral winter, the shallow mixed-layer due to salinity stratification, and the signature of coastal wave propagation. The analysis of the mixed-layer heat budget reveals a competition between warming by air-sea fluxes, dominated by the solar flux throughout the year, and cooling by vertical mixing at the base of the mixed-layer, as other tendency terms remain weak. The seasonal cooling is induced by vertical mixing, but is not controlled by the local wind. A subsurface analysis shows that remotely-forced coastal trapped waves raise the thermocline from April to August, which strengthens the vertical temperature gradient at the base of the mixed-layer and leads to the mixing-induced seasonal cooling in the Congolese upwelling system.

Plain Language Summary: The Congolese upwelling system is located along the West African coast north of the Congo River. It is one of the highly productive systems in the Gulf of Guinea and has received the least attention due to the lack of historical data in this coastal region. The low temperatures occur during the austral winter when winds are weak in the area. We use a high-resolution regional ocean model to identify the main atmospheric and oceanic forcing controlling the seasonal changes in sea surface temperature. We find a competition between warming by air-sea fluxes, dominated by the solar flux throughout the year, and cooling by vertical mixing. The seasonal occurrence of low temperature induced by vertical mixing is not controlled by the local wind, but rather by remotely forced coastal trapped waves.

Abstract: Eastern boundary upwelling systems are hotspots of marine life and primary production. The strength and seasonality of upwelling in these systems are usually related to local wind forcing. However, in some tropical upwelling systems, seasonal maxima of productivity occur when upwelling favorable winds are weak. Here, we show that in the tropical Angolan upwelling system (tAUS), the seasonal productivity maximum is due to the combined effect of coastal trapped waves (CTWs) and elevated tidal mixing on the shelf. During austral winter, the passage of an upwelling CTW displaces the nitracline upward by more than 50 m. Thereby, nitrate-rich waters spread onto the shelf, where elevated vertical mixing causes a nitrate flux into the surface mixed layer. Interannual variability of the productivity maximum is strongly correlated to the amplitude of the upwelling CTW as seen in sea level data. Given that CTWs are connected to equatorial forcing, a predictability of the strength of the productivity maximum is suggested.

Abstract: This paper presents a comprehensive analysis of the extreme Atlantic and Benguela Niño events that occurred during the boreal spring-summer of 2021. We conducted sensitivity experiments with a regional ocean-atmosphere coupled model of the tropical Atlantic to investigate the phenology of these interannual events, unravel their triggering mechanisms, and quantify the contributions of local and remote processes. The results revealed that both the 2021 Atlantic and Benguela Niños were triggered by anomalous atmospheric fluxes at the model southern boundary (32°S), leading to a significant and persistent weakening of the South Atlantic Anticyclone. The associated poleward anomalous coastal wind off Africa reduced coastal upwelling and evaporation south of 15°S, initiating the Benguela Niño. Then, the relaxation of the equatorial trade winds forced a downwelling equatorial Kelvin wave, which warmed the eastern equatorial region, marking the onset of the Atlantic Niño. The equatorial event reached full maturity in July 2021 through ENSO-like air-sea interactions in the equatorial basin, enhanced by the atmospheric connection associated with low-level winds converging toward the distant coastal warming. While air-sea interactions in the tropical Atlantic acted as a negative feedback for the coastal warming, the ocean connection with the equatorial variability through the propagation of equatorially-forced downwelling coastal waves intensified the coastal warming, peaking end of May 2021. Overall, this research provides valuable insights into the complex dynamics of Atlantic and Benguela Niños, emphasizing the interconnectedness between these two systems. This has important implications for improving Earth system models which currently struggle to simulate these extreme events.

Plain Language Summary: In this study, we investigate the extreme Atlantic and Benguela Niño events that occurred during the boreal spring-summer of 2021. The 2021 Atlantic Niño was the warmest equatorial event since the satellite era, while the 2021 Benguela Niño, associated with the weakest boreal summer primary production since 2002, ranked 6th in terms of coastal temperature anomalies. The main objectives were to analyze the phenology of these interannual events, to explore the underlying air-sea interactions, and to examine the oceanic and atmospheric connections between equatorial and coastal warmings. By conducting a series of sensitivity experiments with a regional ocean-atmosphere coupled model, we were able to identify the triggering mechanism and quantify the roles played by local and remote processes in driving the 2021 events.
Our results not only allowed us to explain the sequence of events leading to the extreme intensity of the equatorial and coastal events, but also revealed the high degree of interconnectedness between the two events. Indeed, we showed that the strength of the Benguela Niño warming fueled the Atlantic Niño through an atmospheric teleconnection, and the Atlantic Niño, in turn, controlled to a large extent the coastal warming through an oceanic connection associated with long-wave propagation. Adding the fact that the 2021 Atlantic and Benguela Niños share the same external forcing mechanism, our results strongly suggest that these two interannual modes of variability are part of a unified mode of variability in the tropical Atlantic.
The results of our study provide valuable insights into the complex dynamics governing the amplitude and timing of the Atlantic and Benguela Niño events. We have uncovered the intricate interplay between ocean wave dynamics, local air-sea interactions, and distant atmospheric teleconnections. We believe that our findings not only contribute to a deeper understanding of the climate dynamics in the tropical Atlantic but also have important implications for Earth system modeling and the prediction and management of these extreme events.

Abstract: Surface chlorophyll-a concentration (Chl-a) observed by satellite shows a marked seasonal and interannual variability in the Tropical Atlantic. This study analyzes how the remotely-sensed surface Chl-a responds to the leading boreal summer climate modes affecting the interannual Tropical Atlantic variability over 1998-2018, corresponding to a positive Atlantic Multidecadal Oscillation phase. We show that the Atlantic Zonal Mode (AZM) and the North Tropical Atlantic Mode (NTAM) significantly drive the interannual surface Chl-a variability in the equatorial Atlantic, with different timings and contrasted zonal modulation of the Cold Tongue. The AZM involves remotely-forced wave propagations favoring upwelling in the east and Chl-a modulation in the core of the Cold Tongue. Instead, the impact of the NTAM is mainly in the west, in response to locally-forced pumping that modulates the western extension of the Cold Tongue. Such conditions can affect the marine food web, inducing significant variations for ecosystem functioning and fisheries.

Plain Language Summary: The Tropical Atlantic Ocean is characterized by strong year-to-year surface temperature fluctuations which can be classified into basin-scale climate modes. In this study, we examine to which extent these modes of variability have a signature on the surface chlorophyll-a concentration, a proxy of the biological activity at sea. Using two decades (1998-2018) of ocean-color satellite observations, we show that the interannual surface chlorophyll-a modulation in boreal summer is impacted by these climate modes mainly along the equator. They drive contrasted Chlorophyll-a fluctuations in time and space. The first climate mode, the Atlantic Zonal Mode, involves distant mechanisms that propagate along the equator favoring biological activity in the eastern equatorial basin. Conversely, the second mode, the North Tropical Atlantic Mode, involves local mechanisms in the west of the equatorial band where it favors biological activity. Such conditions can affect the marine food web, inducing significant variations for the fisheries.

Abstract: We investigate the lag between warm interannual Sea Surface Temperature (SST) events in the eastern-equatorial Atlantic, the Atlantic Niños, and the occurrence of Benguela Niños along the southwestern Angolan coast. While it is commonly agreed on that both events are associated with equatorial and subsequent coastal-trapped wave propagations driven remotely by a relaxation of the trade-winds, it is surprising that SST anomalies off Angola tend to precede the ones in the eastern-equatorial sector by ~1 month. To explain this counterintuitive behavior, our methodology is based on the experimentation with a tropical Atlantic Ocean model. Using idealized wind-stress perturbations from a composite analysis, we trigger warm equatorial and coastal events over a stationary and then, seasonally-varying ocean mean-state. In agreement with the linear dynamics, our results show that when the interannual wind-stress forcing is restricted to the western-central equatorial Atlantic, the model yields equatorial events leading the coastal ones. This implies that neither the differences in the ocean stratification between the two regions (thermocline depths or modal wave contributions) nor the seasonal phasing of the events explains the observed temporal sequence. Only if wind-stress anomalies are prescribed in the coastal fringe, the coastal warming precedes the eastern-equatorial SST anomaly peak, emphasizing the role of the local forcing in the phenology of Benguela Niños. A weaker South-Atlantic Anticyclone initiates the coastal warming before the development of eastern-equatorial SST anomalies. Then, equatorward coastal wind anomalies driven by a convergent anomalous circulation located on the warm Atlantic Niño stops the remotely-forced coastal warming prematurely.

Plain Language Summary: We investigate the Sea Surface Temperature (SST) interannual fluctuations in the Tropical Atlantic. We focus on the extreme warm events that occur every few years in the Gulf of Guinea, the Atlantic Niños, and along the coast of Angola-Namibia, the Benguela Niños. It is commonly agreed on that both events are forced by equatorial and subsequent coastal waves triggered in the western-central basin by a relaxation of the trade-winds. Yet, we observe that the coastal warming tends to precede the one in the Gulf of Guinea by ~1 month. We explain this counterintuitive fact, using experimentation with a tropical Atlantic Ocean model. Using idealized wind-stress perturbations, we simulate warm equatorial and coastal events on top of a stationary and then, seasonally-varying ocean mean-state. Results show that when wind-stress perturbations are confined to the western-central equatorial Atlantic, the model yield equatorial events leading the coastal variability, consistent with the propagation path of the waves. This implies that neither the ocean mean-state nor its seasonal variability controls the timing between events. We further show that only when wind-stress anomalies are prescribed within the coastal fringe, warm events off Angola precede the eastern-equatorial SST anomaly. Both warmings originate from a reduction of the strength of the South-Atlantic Anticyclone. Nevertheless, local processes initiate the coastal warming before the remotely-forced equatorial waves impact the eastern-equatorial SST. Benguela Niños then also stop earlier due to the development of coastal wind anomalies associated with a convergent anomalous circulation located on the warm Atlantic Niño event.

Abstract: We investigate the dynamics of the interannual Coastal Trapped Waves (CTW) propagations along the south-western African coast and their role in triggering Benguela Niño and Niña events from 1958 to 2008. Using regional ocean model sensitivity experiments, we track equatorially-forced CTW down to the Southern Benguela Upwelling System (SBUS), where they account for 70% of the coastal sea level anomalies (SLA), temperature, and salinity variability. We then decompose the model coastal variability into individual CTW modal contributions and identify periods of energetic downwelling and upwelling propagations. A composite analysis allows for documenting and quantifying the oceanic response (circulation, temperature, and salinity) on the shelf during the passage of remotely-forced CTW. Results reveal that North of ~19°S, the coastal interannual variability is dominated by the second and third CTW modes. In the BUS, their amplitudes decrease and the interannual fluctuations are largely explained (> 70%) by the faster and weakly-dissipative first CTW mode. This dynamic explains the peculiar propagative pattern associated with SLA propagations, in which equatorially-forced fluctuations in the SBUS peak before the waves imprint the variability at ~19°S. The impact of CTW on the temperature in the SBUS is drastically lower than in the NBUS and Angolan regions. At last, we show that 71% of the extreme Benguela Niño and Niña events, in the surface layer, are associated with remotely-forced CTW propagations. The coherence between our CTW index and these extreme events increases when detecting temperature anomalies in the sub-surface rather than at the sea surface.

Abstract: The oceans have a profound influence on the weather and climate of South Africa. Not only, most rainfall comes from condensation of water vapor originating from the flux of moisture from ocean to atmosphere but the temperature of the remote and surrounding oceans have an impact of the interannual variability of rainfall. A good example of this remote effect is the impact of the ocean during El Ninowhen the temperature of the Pacific and Indian Ocean is higher than normal. This creates more rainfall above the higher sea surface temperature thus modifying the global Walker and Hadley circulation. Air usually rises in the equatorial regions, especially above the ocean creating rainfall which increases further the ascending motion of air. The air is eventually pushed poleward and then cools and sinks in the subtropical region where Southern Africa sits, creating high pressure and subsidence which is not favorable to rainfall. This is one of the teleconnection mechanisms linking remote oceanic region to South Africa. The aim of the project was to better understand the role of the ocean on weather, climate and rivers of Southern Africa. Little work has been done to connect streamflow to the El Nino Southern Oscillation previous to that project. We also looked at the tropical Atlantic Ocean, which is closer to us although but smaller where a similar phenomenon to El Nino occurs, the Benguela Nino. Closer to South Africa, the Agulhas Current was known to impact the atmosphere above it due to high turbulent flux of moisture from sea to atmosphere. In the past, due to low resolution reanalyzed climate data that did not integrate the Agulhas Current, it was not possible to study the impact of the Agulhas Current on local climate with reanalyzed climate dataset such as ERA or NCEP. This project made some important advance to that matter using new high resolution reanalyzed climate data and a model and offer some sounds evidence on the impact of the Agulhas Current on coastal rainfall. A list of publication originating directly from the project and a list of student and thesis supported by the project is found at the end of the executive summary.

Abstract: A systematic study of Benguela Niño and Benguela Niña events during 1958 to 2015 including those that developed before the satellite era (1982) is carried out using an ocean general circulation model in combination with a linear equatorial model. Altogether, 21 strong warm and cold anomalous coastal events are identified among which 6 undocumented extreme coastal events are reported. Results suggest that most of these extreme coastal events including the newly identified ones are linked to remote equatorial forcing via mode 2 equatorial Kelvin waves. The latter propagates after approaching the African coast poleward as coastally trapped waves leading surface temperature anomalies along the Angola‐Benguela current system by 1 month. 1‐2 months before the peak of Benguela Niños or Niñas usually occurring in March‐April, a large‐scale wind stress forcing is observed with both local (variations of alongshore coastal wind stress) and remote forcing developing simultaneously. Results further suggest that surface temperature anomalies off Southern Angola and in the Angola‐Benguela Front are associated with equatorial dynamics and meridional wind stress fluctuations off the southwestern African coast north of 15°S. Similar mechanisms are observed for Northern Namibia in combination with forcing by local meridional wind stress variations.

Plain Language Summary: The Benguela upwelling system located in the southeastern Atlantic Ocean supports a large marine ecosystem due to upwelling conditions. Every few years, anomalous warm and cold coastal events occur in the southeastern Atlantic and are detrimental for Angola, Namibia, and South Africa, as they affect fisheries and rainfall like El-Niño phenomenon in the Pacific. To study these coastal events from 1958 to 2015, we use the output from a tropical Atlantic simulation in combination with the solution of a simple linear equatorial model. We study the anomalous coastal events including the ones that occurred before the satellite era (before 1982) and examine the role of the local wind forcing and the remote forcing associated with equatorial variability. We describe so far undocumented extreme events occurring from 1958-2015. Results suggest that most of the extreme coastal warm and cold events are associated with the propagation of equatorial Kelvin waves along the equatorial wave-guide which trigger poleward-propagating coastal trapped waves along the southwestern African coast. 1-2months before the peak season (usually March-April) of the anomalous coastal events, a large-scale wind pattern is observed, encompassing both variations of alongshore coastal wind in the southeastern Atlantic and zonal wind along the equatorial Atlantic.

Abstract: The tropical Atlantic is home to multiple coupled climate variations covering a wide range of timescales and impacting societally relevant phenomena such as continental rainfall, Atlantic hurricane activity, oceanic biological productivity, and atmospheric circulation in the equatorial Pacific. The tropical Atlantic also connects the southern and northern branches of the Atlantic meridional overturning circulation and receives freshwater input from some of the world’s largest rivers. To address these diverse, unique, and interconnected research challenges, a rich network of ocean observations has developed, building on the backbone of the Prediction and Research Moored Array in the Tropical Atlantic (PIRATA). This network has evolved naturally over time and out of necessity in order to address the most important outstanding scientific questions and to improve predictions of tropical Atlantic severe weather and global climate variability and change. The tropical Atlantic observing system is motivated by goals to understand and better predict phenomena such as tropical Atlantic interannual to decadal variability and climate change; multidecadal variability and its links to the meridional overturning circulation; air-sea fluxes of CO₂ and their implications for the fate of anthropogenic CO2; the Amazon River plume and its interactions with biogeochemistry, vertical mixing, and hurricanes; the highly productive eastern boundary and equatorial upwelling systems; and oceanic oxygen minimum zones, their impacts on biogeochemical cycles and marine ecosystems, and their feedbacks to climate. Past success of the tropical Atlantic observing system is the result of an international commitment to sustained observations and scientific cooperation, a willingness to evolve with changing research and monitoring needs, and a desire to share data openly with the scientific community and operational centers. The observing system must continue to evolve in order to meet an expanding set of research priorities and operational challenges. This paper discusses the tropical Atlantic observing system, including emerging scientific questions that demand sustained ocean observations, the potential for further integration of the observing system, and the requirements for sustaining and enhancing the tropical Atlantic observing system.

Abstract: The oceanic connection between the coastal variability along the southwestern African coasts and the linear equatorial dynamics at subseasonal time-scales (<120 days) is examined using a variety of model outputs, ranging from linear to general circulation models. We focus on the equatorially-forced weakly-dissipative first-mode coastal trapped waves which are shown to propagate down to the southern tip of Africa. In the eastern equatorial Atlantic, the first-mode equatorial forcing is tangled with the higher-order Kelvin wave modes and is overshadowed by the dominant second baroclinic mode. The latter is slower and peaks 10 days after the concealed first-mode contribution. Within this time frame, the fast remotely-forced first-mode coastal trapped waves impinge on the variability of the Benguela upwelling ecosystem, almost in phase with the subseasonal sea level fluctuations in the Gulf of Guinea. Over 1993-2008, the equatorial forcing undergoes a substantial interannual modulation. Periods of energetic first-mode equatorial Kelvin waves coincide with a strong subseasonal coastal wind activity that breaks the stronger equatorial connection. This suggests the existence of a large-scale atmospheric connection between the equatorial wave forcing and the along-shore winds in the Benguela, modulating the maximum latitude at which the equatorial dynamics impacts the local marine resources.

Plain Language Summary: The ocean variability in the coastal fringe along southwestern Africa is connected to the equatorial dynamics through the southward propagation of coastal trapped waves excited in the Gulf of Guinea by incoming equatorial waves. Recent studies suggested that, at subseasonal timescales (<120 days), these remotely-forced waves do not reach the very productive Benguela Upwelling System. This study challenges this knowledge. We focus on some specific equatorial waves that are fast and weakly-dissipative when transmitted along the African coast. Analyses of ocean models of various complexity reveal that their signature in the Gulf of Guinea is overshadowed by other dominant processes and are therefore hardly detectable from altimetry. These waves indeed propagate down to the southern tip of Africa and can affect local marine-ecosystems balance. We then concentrate on periods during which the equatorial forcing is more energetic, prone to a strong equatorial connection that can outshine the signature of the locally wind-forced variability. Results however suggest the existence of a large-scale atmospheric connection such as a strong wave activity coincide with intense coastal wind variability in the northern Benguela that breaks the equatorial connection and controls the maximum latitude at which the equatorial dynamics can impact the local marine resources.

Abstract: The fate of the Organic Matter (OM) produced by marine life controls the major biogeochemical cycles of the Earth’s system. The OM produced through photosynthesis is either preserved, exported towards sediments or degraded through remineralisation in the water column. The productive Eastern Boundary Upwelling Systems (EBUSs) associated with Oxygen Minimum Zones (OMZs) should foster OM preservation due to low O₂ conditions, but their intense and diverse microbial activity should enhance OM degradation. To investigate this contradiction, sediment traps were deployed near the oxycline and in the OMZ core on an instrumented moored line off Peru, providing high temporal resolution O₂ series characterizing two seasonal steady states at the upper trap: suboxic ([O₂]<25 μmol.kg^-1) and hypoxic/oxic (15<[O₂]<160 μmol.kg^-1) in austral summer and winter/spring, respectively. The OMZ vertical transfer efficiency of Particulate Organic Carbon (POC) between traps (Teff) fits into three main ranges (high, intermediate, low) suggesting that both predominant preservation (high T_eff >50%) and remineralisation (intermediate T_eff 20<50% or low T_eff<6%) configurations can occur. An efficient OMZ vertical transfer (T_eff>50%) has been reported in summer and winter associated with extreme limitation in O₂ concentrations or OM quantity for OM degradation. However, higher levels of O₂ or OM, or less refractory OM, at the oxycline, even in a co-limitation context, can decrease the OMZ transfer efficiency below 50%, especially in summer during intraseasonal wind-driven oxygenation events. In late winter and early spring, high oxygenation conditions together with high fluxes of sinking particles trigger a shutdown of the OMZ transfer (T_eff<6%). Transfer efficiency of chemical elements composing the majority of the flux (nitrogen, phosphorus, silica, calcium carbonate) follows the same trend than for carbon, with the lowest transfer late winter and early spring. In terms of particulate isotopes, vertical transfer of δ¹⁵N suggests a complex behaviour of ¹⁵N impoverishment or enrichment according to Teff modulation. This OM fate sensitivity to O₂ fluctuations and particles concentration calls for further investigations on OM and O₂-driven remineralisation processes and the consideration of intermittent OMZ behaviour on OM in past studies and climate projections.

Abstract: Mesoscale sea surface temperature (SST) variability plays an important role in shaping local atmospheric boundary layers through thermodynamic processes. This study focuses on the upscaling effects of mesoscale SST gradients in sensitive areas on the southern Africa regional atmospheric circulation. Using regional atmospheric model sensitivity experiments which differ only in the mesoscale SST forcing characteristics (either the full spectrum of SST variability or only its large-scale components are included), we first quantify the importance of SST gradients on regional atmospheric conditions. Agulhas eddies and meanders influence the vertical air column up to the troposphere, and mesoscale ocean patterns significantly modify incoming landwards moisture fluxes. The austral summer mean state is then modified in terms of air temperature, cloud cover and mean rainfall, with notable differences in tropical rainbands over southwestern Africa. Mesoscale SST variability favours tropical-extra-tropical interactions and cloudband development over the continent. These results stress the importance of high-resolution ocean forcing for accurate atmospheric simulations.

Abstract: The Humboldt and the Benguela upwelling systems are connected to the equatorial variability through the coastal waveguide, so that a large variance of the coastal sea level and current variability can be described as an infinite sum of ortho-normal free Coastal Trapped Wave (CTW) modes. The objective of this study is to infer the CTW mode contributions to the coastal variability in both systems at sub-seasonal timescale (<120 days) from regional ocean circulation model simulations. We develop and validate twin regional model configurations of the southeastern Pacific and Atlantic Oceans. Cross-shore spatial structures of the four first free CTW modes are then derived from model mean stratification and topography along the southwestern African and South American continents. We introduce and validate a new methodology to estimate the gravest CTW mode contributions to model pressure and alongshore current. Our formulation draws on the ortho-normality of the CTW modal structures, and uses a simple projection of the coastal and bottom model pressure onto each CTW structure. Results give confidence in the ability of this modal decomposition methodology to disentangle CTW mode contributions from complex non-linear coastal processes that control the coastal sub-seasonal variability. In both systems, it allows to successfully extract the gravest poleward propagating CTW modes with velocities close to the theoretical values and amplitudes consistent with the solutions of a simple multi-mode linear CTW model. Furthermore, results show that both systems exhibit relatively different CTW dynamics and forcings which are discussed in the companion paper [Illig et al., 2018].

Plain Language Summary: Coastal trapped waves propagate in the ocean along the continental shelves, with the coast on their left in the southern hemisphere. They exert an important influence on the coastal circulation and mixing, with notable implications for productive ecosystems. We introduce a new methodology to estimate the amplitude of these waves and their contribution to the coastal sea level and alongshore current variability. It benefits from a relatively simple implementation and is adapted to ocean model solutions. We validate this method using twin regional ocean model configuration of the southeastern Pacific and Atlantic Oceans. This novel approach allows to successfully extract the contribution of the southward propagating coastal waves in these two different systems, with velocities close to the theoretical values and amplitudes consistent with the dynamics of a simple linear coastal model. This gives confidence in the ability of this new technique to disentangle coastal wave contributions from complex non-linear processes that control the ocean variability in coastal fringes. Furthermore, results show that both systems exhibit drastic differences in the coastal wave dynamics at sub-seasonal timescales (<120 days) which are discussed in the companion paper [Illig et al., 2018].

Abstract: The objective of this study is to compare the characteristics of the oceanic teleconnection with the linear equatorial dynamics of two upwelling systems along the southwestern South American and African continents at sub-seasonal timescales (<120 days). Altimetric data analysis shows that the coastal variability remains coherent with the equatorial signal until 27°S in the Southeastern Pacific (SEP), while in the Southeastern Atlantic (SEA) it fades out south of 12°S. To explain this striking difference, our methodology is based on the experimentation with twin regional model configurations of the SEP and SEA Oceans. The estimation of free Coastal Trapped Waves (CTW) modal structures and associated contribution to coastal variability allows inferring and comparing the characteristics of each CTW mode in the two systems; namely, their forcings, amplitude, dissipation rate and scattering. Results show that the Pacific sub-seasonal equatorial forcing is only 20% larger than in the Atlantic, but important differences in the relative contribution of each baroclinic mode are reported. The first baroclinic mode dominates the eastern equatorial Pacific variability, while in the eastern equatorial Atlantic the second mode is the most energetic. This leads to a drastic increase in the dissipation and scattering of the remotely-forced CTW in the SEA sector, compared to the coastal SEP. Concomitantly, south of 15°S, the sub-seasonal coastal wind stress forcing is significantly more energetic in the SEA and participate in breaking the link between the equatorial forcing and the coastal variability. Our results are consistent with the solutions of a simple multi-mode CTW model.

Plain Language Summary: The Humboldt and the Benguela upwelling systems are connected to the equatorial variability. Part of the incoming eastward equatorial wave energy is transmitted southward along the South American and African coasts as coastal trapped waves, where they imprint on the ecosystem variability. At sub-seasonal timescales (<120 days), altimetry reveals that the coastal variability remains coherent with the equatorial signal until 27°S in the southeastern Pacific, while in the Atlantic counterpart it fades out south of 12°S. To explain this striking difference, we compare the characteristics of coastal waves between the two systems: their forcing at the equator, their dissipation and scattering along their propagation, and the energization by the coastal wind-stress. We use a variety of ocean models of different complexity ranging from regional general circulation models to simple linear coastal models. Results show that the difference between the two systems regarding the connection with the equatorial variability can be attributed to the distinct characteristics of their equatorial forcing. The latter favors fast and weakly-dissipative coastal wave in the Humboldt. Off southwestern Africa, the equatorially-forced coastal waves dissipate at ~13°S, where the sub-seasonal coastal wind-stress forcing is energetic and participates in breaking the link between the equatorial and coastal variabilities.

Abstract: A Benguela Niño developed in November 2010 and lasted for 5 months along the Angolan and Namibian coastlines. Maximum amplitude was reached in January 2011 with an interannual monthly Sea Surface Temperature anomaly larger than 4 °C at the Angola Benguela Front. It was the warmest event since 1995. Consistent with previous Benguela Niños, this event was generated by a relaxation of the trade winds in the western equatorial Atlantic, which triggered a strong equatorial Kelvin wave propagating eastward along the equator and then southward along the southwest African coast. In the equatorial band, the associated ocean sub-surface temperature anomaly clearly shows up in data from the PIRATA mooring array. The dynamical signature is also detected by altimetry derived Sea Surface Height and is well reproduced by an Ocean Linear Model. In contrast to previous Benguela Niños, the initial propagation of sub-surface temperature anomalies along the equator started in October and the associated warming in the Angolan Benguela Front Zone followed on as early as November 2010. The warming was then advected further south in the Northern Benguela upwelling system as far as 25°S by an anomalously strong poleward sub-surface current. Demise of the event was triggered by stronger than normal easterly winds along the Equator in April and May 2011 leading to above normal shoaling of the thermocline along the Equator and the south-west African coastline off Angola and an associated abnormal equatorward current at the Angola Benguela Front in April and May 2011.

Abstract: The link between equatorial Atlantic Ocean variability and the coastal region of Angola-Namibia is investigated at interannual time scales from 1998 to 2012. An index of equatorial Kelvin wave activity is defined based on Prediction and Research Moored Array in the Tropical Atlantic (PIRATA). Along the equator, results show a significant correlation between interannual PIRATA monthly dynamic height anomalies, altimetric monthly Sea Surface Height Anomalies (SSHA) and SSHA calculated with an Ocean Linear Model. This allows us to interpret PIRATA records in terms of equatorial Kelvin waves. Estimated phase speed of eastward propagations from PIRATA equatorial mooring remains in agreement with the linear theory, emphasizing the dominance of the second baroclinic mode. Systematic analysis of all strong interannual equatorial SSHA shows that they precede by 1-2 months extreme interannual Sea Surface Temperature Anomalies along the African coast, which confirms the hypothesis that major warm and cold events in the Angola-Benguela current system are remotely forced by ocean atmosphere interactions in the equatorial Atlantic. Equatorial wave dynamics is at the origin of their developments. Wind anomalies in the Western Equatorial Atlantic force equatorial downwelling and upwelling Kelvin waves that propagate eastward along the equator and then polewards along the African coast triggering extreme warm and cold events respectively. A proxy index based on linear ocean dynamics appears to be significantly more correlated with coastal variability than an index based on wind variability. Results show a seasonal phasing, with significantly higher correlations between our equatorial index and coastal SSTA in October-April season.

Abstract: The near-shore surface mesoscale atmospheric circulation in the upwelling systems off Peru and Chile is influential on the Sea Surface Temperature through Ekman transport and pumping. There has been a debate whether or not the so-called “wind drop-off”, that is a shoreward decrease of the surface wind speed near the coast, can act as an effective forcing of upwelling through Ekman pumping. Although the wind drop-off has been simulated by high-resolution atmospheric models, it has not been well documented due to uncertainties in the scatterometry-derived wind estimates associated with land contamination. Here we use the along-track altimetry-derived surface wind speed data from ENVISAT, Jason-1, Jason-2, and SARAL satellites, to document the spatial variability of the mean wind drop-off near the coast as estimated from the inversion of the radar backscattering coefficient. The data are first calibrated so as to fit with the scatterometer observations of previous and current satellite missions (QuikSCAT, ASCAT). The calibrated data are then analyzed near the coast and a wind drop-off scale is estimated. The results indicate that the wind drop-off takes place all along the coast, though with a significant alongshore variability in its magnitude. Differences between products are shown to be related both to the differences in repeat cycle between the different altimetry missions and to the peculiarities of the coastline shape at the coastal latitudes of the incident tracks. The relative contribution of Ekman pumping and Ekman transport to the total transport is also estimated indicating a comparable contribution off Chile while transport associated to Ekman pumping is on average ~1.4 larger than Ekman transport off Peru. Despite the aliasing effect associated with the weak repetitivity of the satellite orbit and the high frequency variability of the winds in this region, the analysis suggests that the seasonal cycle of the surface winds near the coast could be resolved at least off Peru.

Abstract: The recurrent occurrences of interannual warm and cold events along the coast of Africa have been intensively studied because of their striking effects on climate and fisheries. Using sensitivity experimentation based on a coupled physical/biogeochemical model, we show that the oceanic remote equatorial forcing explains more than 85% of coastal interannual nitrate and oxygen fluctuations along the Angolan and Namibian coasts up to the Benguela Upwelling System (BUS). These events, associated with poleward propagations of upwelling and downwelling Coastal Trapped Waves (CTW), are maximum in subsurface and controlled by physical advection processes. Surprisingly, an abrupt change in the CTW biogeochemical signature is observed in the BUS, associated with mixed vertical gradients due to the strong local upwelling dynamics. Coastal modifications of biogeochemical features result in significant primary production variations that may affect fisheries habitats and coastal biodiversity along the southwestern African coasts and in the BUS.

Abstract: In this study, uncoupled and coupled ocean–atmosphere simulations are carried out for the California Upwelling System to assess the dynamic ocean–atmosphere interactions, namely, the ocean surface current feedback to the atmosphere. The authors show the current feedback, by modulating the energy transfer from the atmosphere to the ocean, controls the oceanic eddy kinetic energy (EKE). For the first time, it is demonstrated that the current feedback has an effect on the surface stress and a counteracting effect on the wind itself. The current feedback acts as an oceanic eddy killer, reducing by half the surface EKE, and by 27% the depth-integrated EKE. On one hand, it reduces the coastal generation of eddies by weakening the surface stress and hence the nearshore supply of positive wind work (i.e., the work done by the wind on the ocean). On the other hand, by inducing a surface stress curl opposite to the current vorticity, it deflects energy from the geostrophic current into the atmosphere and dampens eddies. The wind response counteracts the surface stress response. It partly reenergizes the ocean in the coastal region and decreases the offshore return of energy to the atmosphere. Eddy statistics confirm the current feedback dampens the eddies and reduces their lifetime, improving the realism of the simulation. Finally, the authors propose an additional energy element in the Lorenz diagram of energy conversion: namely, the current-induced transfer of energy from the ocean to the atmosphere at the eddy scale.

Abstract: We investigate the respective roles of equatorial remote (Equatorial Kelvin Waves) and local atmospheric (wind, heat fluxes) forcing on coastal variability in the South-East Atlantic Ocean extending up to the Benguela Upwelling System (BUS) over the 2000–2008 period. We carried out a set of six numerical experiments based on a regional ocean model, that differ only by the prescribed forcing (climatological or total) at surface and lateral boundaries. Results show that at subseasonal timescales (<100 days), the coastal oceanic variability (currents, thermocline, and sea level) is mainly driven by local forcing, while at interan- nual timescales it is dominated by remote equatorial forcing. At interannual timescales (13–20 months), remotely forced Coastal-Trapped Waves (CTW) propagate poleward along the African southwest coast up to the northern part of the BUS at 24°S, with phase speeds ranging from 0.8 to 1.1 m/s. We show that two triggering mechanisms limit the southward propagation of CTW: interannual variability of the equatorward Benguela Current prescribed at the model’s southern boundary (30°S) and variability of local atmospheric forcing that modulates the magnitude of observed coastal interannual events. When local wind stress forc- ing is in (out) of phase, the magnitude of the interannual event increases (decreases). Finally, dynamical processes associated with CTW propagations are further investigated using heat budget for two intense interannual events in 2001 and 2003. Results show that significant temperature anomalies (±2°C), that are mostly found in the subsurface, are primarily driven by alongshore and vertical advection processes.

Abstract: An accurate quantification of the role of the ocean as source/sink of greenhouse gases (GHGs) requires to ac- cess the high-resolution of the GHG air–sea flux at the interface. In this paper we present a novel method to reconstruct maps of surface ocean partial pressure of CO₂ (pCO₂) and air–sea CO₂ fluxes at super resolution (4 km, i.e., 1/32° at these latitudes) using sea surface temperature (SST) and ocean color (OC) data at this resolution, and CarbonTracker CO₂ fluxes data at low resolution (110 km). Inference of super-resolution pCO₂ and air–sea CO₂ fluxes is performed using novel nonlinear signal processing methodologies that prove efficient in the context of oceanography. The theoretical background comes from the microcanonical multi- fractal formalism which unlocks the geometrical determination of cascading properties of physical intensive variables. As a consequence, a multi-resolution analysis performed on the signal of the so-called singularity exponents allows for the correct and near optimal cross-scale inference of GHG fluxes, as the inference suits the geometric realization of the cascade. We apply such a methodology to the study offshore of the Benguela area. The inferred representation of oceanic partial pressure of CO₂ improves and enhances the description provided by CarbonTracker, capturing the small-scale variability. We examine different combinations of ocean color and sea surface temperature products in order to increase the number of valid points and the quality of the inferred pCO₂ field. The methodology is validated using in situ measurements by means of statistical errors. We find that mean absolute and relative errors in the inferred values of pCO₂ with respect to in situ measurements are smaller than for CarbonTracker.

Abstract: In this study, we document and interpret the characteristics of the IntraSeasonal Kelvin wave (ISKw) in the Pacific over the 1989–2011 period, based on observations, a linear model, and the outputs of an Ocean General Circulation Model (OGCM). We focus on the wave activity during the Central Pacific (CP) El Nino events contrasting with the extraordinary El Nino of 1997/1998. We find that the ISKw activity is enhanced in Austral summer (spring) in the central Pacific (west of ~120°W) during CP El Nino events. The linear model experiment indicates that the Austral summer peak is wind-forced, while the Austral spring peak is not and consequently results from nonlinear processes. In addition, a strong dissipation of the ISKws is observed east of 120°W which cannot be accounted for by a linear model using a Rayleigh friction. A vertical and horizontal mode decomposition of the OGCM simulation further confirms the sharp changes in characteristics of the ISKws as well as the reflection of the latter into first-meridional Rossby wave at the longitude where the maximum zonal gradient of the thermocline is found (~120°W). Our analysis suggests that the confinement of CP El Nino warming in the central Pacific may result from the reinforcement of the zonal gradient in stratification associated with the La Nina-like conditions since the late of the 1990s, leading to scattering of the energy of the ISKws in the eastern Pacific.

Abstract: The Sea Surface Temperature (SST) intraseasonal variability (40–90 days) along the coast of Peru is commonly attributed to the efficient oceanic connection with the equatorial variability. Here we investigate the respective roles of local and remote equatorial forcing on the intraseasonal SST variability off central Peru (8°S–16°S) during the 2000–2008 period, based on the experimentation with a regional ocean model. We conduct model experiments with different open lateral boundary conditions and/or sur- face atmospheric forcing (i.e., climatological or not). Despite evidence of clear propagations of coastal trapped waves of equatorial origin and the comparable marked seasonal cycle in intraseasonal Kelvin wave activity and coastal SST variability (i.e., peak in Austral summer), this remote equatorial forcing only accounts for ~20% of the intraseasonal SST regime, which instead is mainly forced by the local winds and heat fluxes. A heat budget analysis further reveals that during the Austral summer, despite the weak along-shore upwelling (downwelling) favorable wind stress anomalies, significant cool (warm) SST anomalies along the coast are to a large extent driven by Ekman-induced advection. This is shown to be due to the shallow mixed layer that increases the efficiency by which wind stress anomalies relates to SST through advection. Diabatic processes also contribute to the SST intraseasonal regime, which tends to shorten the lag between peak SST and wind stress anomalies compared to what is predicted from an advective mixed-layer model.

Abstract: The various facets of El Niño and its effects on the coast of Peru The El Niño phenomenon is the dominant mode of inter-annual variability in the Pacific Ocean, which results from the interaction between the ocean and atmosphere in the tropical Pacific. Recent research shows that there are several facets of this phenomenon, which vary according to the modalities of interaction between the ocean and atmosphere, as well as their locations. There are at least two types of El Niño with different expressions on the sea surface temperature in the tropical Pacific and on the coast of Peru: one that takes place in the Central Pacific (which tends to be associated with colder oceanic conditions who favoring the aridity of the Peruvian coast and the ocean conditions hypoxic), and another that takes place in the Eastern Pacific (which transforms the Peruvian coast in a “typical” tropical zone, with warm and oxygenated coastal waters, and heavy rain). Nowadays, research efforts to understand the mechanisms involved in the different types of El Niño have been strengthened, since in recent decades has increased the frequency of these events in the Central Pacific, suggesting that it might be a result of climate change. The improvement of both regional models coupled ocean - atmosphere and ocean - biogeochemical aims to improve the understanding of the vulnerability of the Peruvian biosphere to climate change, and propose a paradigm that represents the bimodality of the inter-annual variability in the tropical Pacific .

Abstract: Subseasonal variability of Sea Surface Temperature (SST) in the central Benguela upwelling system is investigated using TMI satellite-derived data over the period 1999– 2009. Spatial patterns and time-frequency characteristics of subseasonal variability are documented based on Empirical Orthogonal Functions (EOF) decomposition and wavelet analysis. Despite the land contamination of the TMI satellite data within approximately 100 km off the coast the first EOF of SST anomalies allows characterizing the coastal upwelling variability at the subseasonal scale. Two regimes of variability are evidenced: a submonthly (2–30 days) regime with a dominant 11 days oscillation and a lower frequency intraseasonal (30–90 days) regime with a dominant 61 days oscillation. Both regimes are modulated, to a large extent, by the local surface wind stress and are consistent with Ekman dynamics. The seasonality of the relationship between wind stress and SST for submonthly (intraseasonal) regime is characterized by a marked semiannual (annual) cycle, which is explained in terms of the impact of seasonal change of the upper ocean stratification on the vertical advection process. The wind-driven SST subseasonal variability is shown to be associated with eastward propagating disturbances in the midlatitudes corresponding to a wave number 4. The results also suggest an important role of the Antarctic Oscillation in modulating the intraseasonal wind-driven SST variability. The characteristics of the equatorial intraseasonal Kelvin waves are documented in order to discuss possible impact of remote oceanic forcing on SST variability along the coast in the Benguela upwelling system.

Abstract: Simulating the oceanic circulation in Eastern Boundary Upwelling Systems (EBUS) is a challenging issue due to the paucity of wind stress products of a sufficiently high spatial resolution to simulate the observed upwelling dynamics. In this study, we present the results of regional simulations of the Humboldt current system (Peru and Chile coasts) to assess the value of a statistical downscaling model of surface forcing. Twin experiments that differ only from the momentum flux forcing are carried out over the 1992–2000 period that encompasses the major 1997/98 El Niño/La Niña event. It is shown that the mean biases of the oceanic circulation can be drastically reduced simply substituting the mean wind field of NCEP reanalysis by a higher resolution mean product (QuikSCAT). The statistical downscaling model improves further the simulations allowing more realistic intraseasonal and interannual coastal undercurrent variability, which is notoriously strong off Central Peru and Central Chile. Despite some limitations, our results suggest that the statistical approach may be useful to regional oceanic studies of present and future climates.

Abstract: The recent decades have experienced changes in the characteristics of the El Niño phenomenon, with in particular an increased occurrence of so-called Modoki or Central Pacific El Niños. Here the 2002/2003 El Niño, characterized as a Central Pacific El Niño, is studied from an Ocean General Circulation Model simulation. The focus is on the sequence of equatorial waves and their impact on zonal and vertical advection. The wave amplitude according to the most energetic baroclinic modes are first estimated, which allows inferring the sequence of the intraseasonal equatorial Kelvin (IKW) and Rossby (IRW) waves. It is shown that energetic downwelling IKWs, forced in the western-central Pacific, crossed the equatorial Pacific. Reflections of IKWs into IRWs onto the zonally varying thermocline and eastern boundary are also observed. A simplified heat budget of the surface layer is then carried out to infer the dominant processes at work during the evolution of this event focusing on the wave-induced advection terms. The results indicate that the warming phase (April–November 2002) is mainly controlled by zonal advection of mean temperature (accounted for by IKWs and locally wind-driven current) and by vertical advection in the eastern Pacific. The cooling phase (December 2002 to April 2003) is dominated by a reduction in solar radiation and the IRW-induced zonal advection of mean temperature respectively in the central and eastern equatorial Pacific. The recharge-discharge process is also showed to be at work with the recharge (discharge) process operating mainly through the second (first) baroclinic mode.

Abstract: Changes in the mean circulation of the equatorial Pacific Ocean partly control the strong decadal modulation of El Niño–Southern Oscillation (ENSO). This relationship is considered from the linear stability of a conceptual recharge/discharge model with parameters tuned from the observed mean state. Whereas decadal changes in the mean thermocline depth alone are usually considered in conceptual ENSO models, here focus is given to decadal changes in the mean stratification of the entire upper ocean (e.g., the mean thermocline depth, intensity, and thickness). Those stratification changes modify the projection of wind stress forcing momentum onto the gravest ocean baroclinic modes. Their influence on the simulated frequency and growth rate is comparable in intensity to the one of usual thermodynamic and atmospheric feedbacks, while they have here a secondary effect on the spatial structure and propagation of SST anomalies. This sensitivity is evidenced in particular for the climate shift of the 1970s in the Simple Ocean Data Assimilation (SODA) dataset, as well as in a preindustrial simulation of the Geophysical Fluid Dynamics Laboratory (GFDL) model showing stratification changes similar to the ones after 2000. Despite limitations of the linear stability approach, conclusions on the sensitivity to stratification may be extended to interpret the modulation and diversity of ENSO in observations and in general circulation models.

Abstract: The freshest surface waters in the tropical Pacific are found at its eastern boundary. Using in situ observations, we depict the quasi-permanent presence of a far eastern Pacific fresh pool with sea surface salinity (SSS) lower than 33, which is confined between Panama’s west coast and 85°W in December and extends westward to 95°W in April. Strong SSS fronts are found at the outer edge of this fresh pool. We investigate the seasonal dynamics of the fresh pool using complementary satellite wind, rain, sea level and in situ oceanic current data at the surface, along with hydrographic profiles. The fresh pool appears off Panama due to the strong summer rains associated with the northward migration of the ITCZ over Central America in June. During the second half of the year, the eastward-flowing North Equatorial Counter-Current keeps it trapped to the coast and strengthens the SSS front on its western edge. During winter, as the ITCZ moves southward, the northeasterly Panama gap wind creates a southwestward jet-like current in its path with a dipole of Ekman pumping/eddies on its flanks. As a result, upwelling in the Panama Bight brings to the surface cold and salty waters which erode the fresh pool on its eastern side while both the jet current and the enhanced South Equatorial Current stretch the fresh pool westward until it nearly disappears in May. New SMOS satellite SSS data proves able to capture the main seasonal features of the fresh pool and monitor its spatial extent.

Abstract: The spatial and temporal variability of nearshore winds in eastern boundary current systems affect the oceanic heat balance that drives sea surface temperature changes. In this study, regional atmospheric and oceanic simulations are used to document such processes during an atmospheric coastal jet event off central Chile. The event is well reproduced by the atmospheric model and is associated with the migration of an anomalous anticyclone in the southeastern Pacific region during October 2000. A robust feature of the simulation is a sharp coastal wind dropoff, which is insensitive to model resolution. As expected, the simulated oceanic response is a significant sea surface cooling. A surface heat budget analysis shows that vertical mixing is a major contributor to the cooling tendency both in the jet core area and in the nearshore zone where the magnitude of this term is comparable to the magnitude of vertical advection. Sensitivity experiments show that the oceanic response in the coastal area is sensitive to wind dropoff representation. This is because total upwelling, i.e., the sum of coastal upwelling and Ekman pumping, depends on the scale of wind dropoff. Because the latter is much larger than the upwelling scale, coastal wind dropoff has only a weak positive effect on vertical velocities driven by Ekman pumping but has a strong negative effect on coastal upwelling. Interestingly though, the weakening of coastal winds in the dropoff zone has a larger effect on vertical mixing than on vertical advection, with both effects contributing to a reduction of cooling.

Abstract: The tropical Pacific variability has experienced changes in its characteristics over the last decades. In particular, there is some evidence of an increased occurrence of El Niño events in the central Pacific (a.k.a. 'Central Pacific El Niño' (CP El Niño) or 'El Niño Modoki'), in contrast with the cold tongue or Eastern Pacific (EP) El Niño which develops in the eastern Pacific. Here we show that the different flavours of El Niño imply a contrasted Equatorial Kelvin Wave (EKW) characteristic and that their rectification on the mean upwelling condition off Peru through oceanic teleconnection is changed when the CP El Niño frequency of occurrence increases. The Simple Ocean Data Assimilation (SODA) reanalysis product is first used to document the seasonal evolution of the EKW during CP and EP El Niño. It is shown that the strong positive asymmetry of ENSO (El Niño Southern Oscillation) is mostly reflected into the EKW activity of the EP El Niño whereas during CP El Niño, the EKW is negatively skewed in the eastern Pacific. Along with slightly cooler conditions off Peru (shallow thermocline) during CP El Niño, this is favourable for the accumulation of cooler SST anomalies along the coast by the remotely forced coastal Kelvin wave. Such a process is observed in a high-resolution regional model of the Humboldt Current system using the SODA outputs as boundary conditions. In particular the model simulates a cooling trend of the SST off Peru although the wind stress forcing has no trend. The model is further used to document the vertical structure along the coast during the two types of El Niño. It is suggested that the increased occurrence of the CP El Niño may also lead to a reduction of mesoscale activity off Peru.

Abstract: The EBUS (Eastern Boundary Upwelling Systems) and OMZs (Oxygen Minimum Zone) contribute very significantly to the gas exchange between the ocean and the atmosphere, notably with respect to the greenhouse gases (hereafter GHG). From in-situ ocean measurements, the uncertainty of the net global ocean-atmosphere CO 2 fluxes is between 20 and 30%, and could be much higher in the EBUS-OMZ. Off Peru, very few in-situ data are available presently, which justifies alternative approaches for assessing these fluxes. In this contribution we introduce

Abstract: The Tropical Rainfall Measuring Mission Microwave Imager sea surface temperature (SST) and QuikSCAT wind stress satellite data are used to investigate the intraseasonal upwelling variability along the coat of Peru over the period 2000–2008. Two regions of peak variance correspond to the central Peru region (Pisco region, 15°S) and the northern Peru region (Piura region, 5°S). A covariance analysis reveals a significant coherency between winds and SST anomalies off Pisco, consistent with Ekman pumping and transport dynamics. The upwelling cell consists in a meridionally extended fringe of colder (warmer) water extending as far as 250 km from the coast at 15°S. In the Piura region, the intraseasonal covariability pattern is represented by two modes, one relevant to the direct Ekman dynamics and the other one associated with the remote forcing of intraseasonal oceanic Kelvin wave. Two regimes of variability are evidenced. A low‐period regime (10–25 days) is the signature of Ekman transport/pumping dynamics and is remotely forced by the migratory atmospheric disturbances across the southeastern Pacific anticyclone. A high‐period regime (35–60 day band) is associated with the combined forcing of oceanic equatorial Kelvin waves and migratory atmospheric disturbances in the midlatitudes. In particular, the modes of covariability exhibit a prominent ∼50 day period energy peak. It is shown that this period arises from the impact of the first two baroclinic modes Kelvin wave, with the second baroclinic mode Kelvin wave being more influential on the Piura region.

Abstract: Intraseasonal equatorial Kelvin wave activity (IEKW) at a low frequency in the Pacific is investigated using the Simple Ocean Data Assimilation (SODA) oceanic reanalyses. A vertical and horizontal mode decomposition of SODA variability allows estimation of the Kelvin wave amplitude according to the most energetic baroclinic modes. A wavenumber–frequency analysis is then performed on the time series to derive indices of modulation of the IEKW at various frequency bands. The results indicate that the IEKW activity undergoes a significant modulation that projects onto baroclinic modes and is not related in a straightforward manner to the low-frequency climate variability in the Pacific. Linear model experiments corroborate that part of the modulation of the IEKW is tightly linked to change in oceanic mean state rather than to the low-frequency change of atmospheric equatorial variability.

Abstract:

Abstract: We investigate the impact of submonthly rain fluctuations on the daily-to-interannual variations of the salinity in the tropical Indian Ocean. A three-year record of daily observed precipitation and wind are used to force an Indian Ocean model in two experiments that differ only by their rain forcing, which is either daily- (Exp_day) or monthly- (Exp_month) averaged. Results show that submonthly precipitations significantly impact the Surface Layer Salinity (SLS) in the northern and eastern Bay of Bengal during the wet season from May to November. In September when the Bay reaches its SLS minimum, the Andaman Sea is saltier by as much as 1.3 psu for Exp_day. The frequency shift between the submonthly rain forcing and the yearly salinity response is due to accumulation in time of nonlinear mechanisms: sudden rain deficits increase the SLS by entraining salty waters from below, whereas rain excesses are inefficient to decrease the salinity of the thick surface layer.

Abstract: Warmer than average sea surface temperatures were observed by the Tropical Rainfall Mission Microwave Imager in the Angola Benguela Current system in late austral summer 2001 and persisted for about three months. These coastal anomalies extended offshore by 1 to 4° longitude and were not due to local ocean atmosphere interaction or relaxation of the upwelling favorable southerly winds. Instead, they were remotely forced by ocean atmosphere interaction in the Tropical Atlantic. Satellite remote sensing and a linear ocean model suggest that relaxation of trade winds along the equator triggered Kelvin waves that crossed the basin within a month in early 2001. Westerly wind anomalies were also observed in December 2000 and January 2001 over most of the Tropical Atlantic contributing to a warm preconditioning due to an enhancement of the oceanic annual cycle. This led to abnormal sea level heights near equatorial Africa that propagated southwards along the coast towards the Angola Benguela Frontal zone. This process increased the seasonal penetration of warm and salty water of tropical origin into the Angola Benguela upwelling system.

Abstract: We investigate the interannual warm event that occurred in the equatorial Atlantic in boreal spring- summer 1996. The role of local coupled air-sea interactions versus Tropical Pacific remote forcing is analysed using observations and ensemble experiments of an intermediate coupled model of the Tropical Atlantic. Results show that the persistent anomalous cold conditions in the Tropical Pacific over 1995 – 96 were favorable to the growth of the local air-sea interactions that led to the 1996 warming in the equatorial Atlantic. Based on the estimation of the changes in the Walker circulation over the Pacific and Atlantic for the meteorological reanalyses and the coupled model, a mechanism of Pacific-Atlantic equatorial connection is proposed to explain this particular warm episode.

Abstract: The relative roles played by the remote El Niño–Southern Oscillation (ENSO) forcing and the local air–sea interactions in the tropical Atlantic are investigated using an intermediate coupled model (ICM) of the tropical Atlantic. The oceanic component of the ICM consists of a six-baroclinic mode ocean model and a simple mixed layer model that has been validated from observations. The atmospheric component is a global atmospheric general circulation model developed at the University of California, Los Angeles (UCLA). In a forced context, the ICM realistically simulates both the sea surface temperature anomaly (SSTA) variability in the equatorial band, and the relaxation of the Atlantic northeast trade winds and the intensification of the equatorial westerlies in boreal spring that usually follows an El Niño event. The results of coupled experiments with or without Pacific ENSO forcing and with or without explicit air–sea interactions in the equatorial Atlantic indicate that the background energy in the equatorial Atlantic is provided by ENSO. However, the time scale of the variability and the magnitude of some peculiar events cannot be explained solely by ENSO remote forcing. It is demonstrated that the peak of SSTA variability in the 1–3-yr band as observed in the equatorial Atlantic is due to the local air–sea interactions and is not a linear response to ENSO. Seasonal phase locking in boreal summer is also the result of the local coupling. The analysis of the intrinsic sustainable modes indicates that the Atlantic El Niño is qualitatively a noise- driven stable system. Such a system can produce coherent interdecadal variability that is not forced by the Pacific or extraequatorial variability. It is shown that when a simple slab mixed layer model is embedded into the system to simulate the northern tropical Atlantic (NTA) SST variability, the warming over NTA following El Niño events have characteristics (location and peak phase) that depend on air–sea interaction in the equatorial Atlantic. In the model, the interaction between the equatorial mode and NTA can produce a dipolelike structure of the SSTA variability that evolves at a decadal time scale. The results herein illustrate the complexity of the tropical Atlantic ocean–atmosphere system, whose predictability jointly depends on ENSO and the connections between the Atlantic modes of variability.

Abstract: The skill of a newly designed global atmospheric model of intermediate complexity – QTCM (for quasi-equilibrium tropical circulation model) in simulating the teleconnections is investigated. The role of the ENSO remote forcing over the Pacific surrounding regions is em- phasized from sensitivity experiments to critical parameters of the model. The role of the tropical intraseasonal variability (ITV) on the simulated ENSO teleconnection pattern is esti- mated using the methodology proposed by Lin et al. (2000) allowing to damp the energy of ITV in the model. The re- duction of intraseasonal variability allows emphasizing the forced response of the atmosphere and eases the detection of local coupled atmosphere-ocean patterns. It was shown that the simulated ITV has an impact on the ENSO teleconnection pattern both in the mid-latitudes and in regions of ascending and descending branches of Walker circulation cells in the tropics.

Abstract:
The objectives of this Thesis is to study the coupled interannual variability in the Tropical Atlantic and the teleconection with the Tropical Pacific variability.

We first investigate the low frequency variability of the Tropical Atlantic vertical structure (1981-2000) based on the CLIPPER ocean General Circulation Model (OGCM). We aim at determining to what extent the observed variability can be explained by the low frequency wave dynamics. The linear vertical mode of CLIPPER monthly climatological stratification are es- timated along the equator. Then, the baroclinic mode contributions to surface zonal current and sea level anomalies are calculated and analysed at interannual timescales. The second baroclinic mode is the most energetic. The first (third) mode exhibits a variability peak in the west (east). The summed-up contribution of the high-order baroclinic modes (4-6) is as energetic as the gravest modes and is larger in the east. Wave components are then derived by projection onto the associated meridional structures. The effect of longitudinal boundaries near the equator is taken into consideration. Equatorial Kelvin and Rossby waves propaga- tions, with phases speed close to the theory, are identified for the first three baroclinic modes. The comparison with a 6-baroclinic-mode Ocean Linear Model ( OLM ) corroborates the propagating properties of the CLIPPER waves coefficients. An estimation of the meridional boundary reflection efficiency indicates that wave reflections take place at both boundaries. A 65% reflection efficiency is found at the eastern boundary.
Our study suggest that low-frequency wave dynamics is to a large extent at work in the Tropical Atlantic.

On the basis of what is known on the Pacific El Niño-Southern Oscillation mode ( ENSO ), this may provide a guidance for investigating the ocean-atmosphere mechanisms that can lead to the Atlantic Equatorial Mode. In that way, a coupled Tropical Intermediate Model for the Atlantic Climate Studies (TIMACS) is designed to study the relative part played by remote ENSO forcing and the local air-sea coupling in the Equatorial Atlantic. The oceanic component of TIMACS consists in the OLM coupled to a simple mixed layer model. In a forced context, it is shown to simulate realistic Sea Surface Temperature ( SST ) variability in the equatorial band. The atmospheric component is the global general circulation atmospheric model developed at UCLA (named QTCM. This model is shown to simulate realistic characteristics of ENSO tele-connections on the Tropical Atlantic sector, namely the relaxation of the North East trades winds and the intensification of the equatorial westerlies in the boreal spring usually consecutively to an El Niño event.
The results of coupled experiments with or without Pacific ( ENSO ) forcing and with or without explicit air-sea interaction in the equatorial Atlantic indicate that most of the equatorial Atlantic SST interannual variability is forced by ENSO . For instance without ENSO forcing the model only sustains oscillation in the presence of atmospheric noise. The characteristics of the unstable coupled modes allowed in the equatorial Atlantic system are investigated, which shows that damped modes are favored consistently with the model experiments. They are however sufficiently energetic to modify the spectrum of variability expected from the sole Pacific ENSO forcing towards higher and lower frequencies. In particular, it is demonstrated that the peak in SST variability in the 1 to 3 year −1 frequency band as observed in the equatorial Atlantic is due to the local air-sea interactions that is not a linear response to ENSO. Also, the presence of inter-decadal variability in the model that is not correlated to Pacific indices suggests that part of the decadal variability observed in the tropical Atlantic could be associated to equatorial wave dynamics. Our results point out the complexity of the Equa- torial Atlantic ocean-atmosphere system which predictability depends on the Pacific conditions and/or the high-frequency atmospheric activity.

Abstract: We investigate the tropical Atlantic vertical structure variability (1981–2000) based on the CLIPPER ocean general circulation model (OGCM). We aim at determining to what extent the observed interannual variability can be explained by the low-frequency wave dynamics. The linear vertical modes of the OGCM climatological stratification are estimated along the equator. The baroclinic mode contributions to surface zonal current and sea level anomalies are calculated and analyzed at interannual timescales. The second baroclinic mode is the most energetic. The first (third) mode exhibits a variability peak in the west (east). The summed-up contribution of the high-order baroclinic modes (4–6) is as energetic as the gravest modes and is largest in the east. Wave components are then derived by projection onto the associated meridional structures. The effect of longitudinal boundaries near the equator is taken into consideration. Equatorial Kelvin and Rossby waves propagations, with phases speed close to the theory, are identified for the first three baroclinic modes. The comparison with a multimode linear simulation corroborates the propagating properties of the OGCM waves coefficients. An estimation of the meridional boundary reflection efficiency indicates that wave reflections take place at both boundaries. A 65% reflection efficiency is found at the eastern boundary. Our study suggests that low-frequency wave dynamics is to a large extent at work in the tropical Atlantic. On the basis of what is known on the Pacific El Nin ̃o-Southern Oscillation mode this may provide a guidance for investigating ocean-atmosphere mechanisms that can lead to the Atlantic zonal equatorial mode.

Abstract: An Ocean General Circulation Model (OGCM) of the tropical Pacific in which combined TOPEX/Poseidon and ERS sea level anomalies are assimilated over January 1994 through July 1999, is used to investigate equatorial wave characteristics during the intense 1997–1999 El Niño-La Niña event. Near the equator, the linear vertical modes are estimated at each grid point of the OGCM simulation with and without assimilation. Consistently with an increase of the vertical gradient within the thermocline and a rise of the thermocline depth in the eastern basin, the assimilation results in an increased contribution of the higher- order baroclinic modes in the eastern basin and a decreased contribution of the first baroclinic mode in the western Pacific for the zonal current variability. For pressure, the first baroclinic mode contribution is reduced whereas the higher-order modes contribution is weakly impacted. Kelvin and first-meridional Rossby waves are then derived for the first two more energetic baroclinic modes in the simulation with assimilation. Kelvin waves of both modes constructively contribute to the strong warming observed in 1997, with the first (second) baroclinic mode being more energetic than the second (first) baroclinic mode in the early (mature) stage of the warming. Kelvin waves of both modes reflect as first meridional Rossby waves at the eastern boundary (reflection efficiency of ~95%) and contribute to push back the warm pool westward. The reversal of the warming is apparently initiated by the second baroclinic mode contribution which controls the position of the 28°C isotherm in the surface layer in the far eastern Pacific from January 1998. At the western boundary, reflections of Rossby waves take place for both modes with an estimated total efficiency of ~50% at 165°E. This suggests that, in our model, the delayed oscillator theory is not applicable for explaining the reversal from warm to cold conditions in 1998 while the zonal advective feedback was at work. More generally, the study suggests that it is necessary to take into account the vertical structure of the ocean when interpreting altimetric data, which can be done through an assimilation experiment.