Contact Me

Your first name
Your last name
Your E-mail*

Enter your Message* Send E-mail

The intraseasonal Kelvin wave and the dynamics of the Central Pacific El Niño events Kobi Alberto MOSQUERA (2010-2015)

The El Niño phenomenon is the dominant mode of climate variability at interannual timescales in the tropical Pacific. It modifies drastically the regional climate in surrounding countries, including Peru for which the socio-economical impacts can be dramatic. Understanding and predicting El Niño remains a top-priority issue for the scientific community. Large progresses in our understanding of El Niño and in our ability to predict it have been made since the 80’s, thanks to the improvement of the observing system of the tropical Pacific (TOGA program and emergence of the satellite era). At the end of the Twentieth century, whereas new theories were proposed and tested, progresses in numerical modeling and data assimilation led to the idea that El Niño could be predicted with at least 2 or 3 seasons in advance. The observations since the beginning of the 21st century have wiped out such expectation : A new type of El Niño, known as the Central Pacific El Niño (CP El Niño) or Modoki El Niño has put the community in front of a new challenge.

This thesis is a contribution to the current international effort to understand the dynamics of this new type of El Niño in order to propose mechanisms explaining its increased occurrence in recent decades. The thesis focuses on the investigation of the role of the oceanic equatorial waves in the dynamics and thermodynamics along the equatorial Pacific Ocean during CP El Niño events. It first takes a close look at the first CP El Niño of the 21st century, i.e. the 2002/03 El Niño, based on an oceanic general circulation model. Then, it documents the characteristics of the IntraSeasonal Kelvin waves (ISKws) over the period 1990-2011. It is shown that the ISKw experience a sharp dissipation in the eastern Pacific that is interpreted as resulting from the scattering of energy associated with the zonal contrast in stratification (i.e. sloping thermocline from west to east). Partial reflection of the ISKw as Rossby waves near 120°W is also identified, which may explain the confinement of CP El Niño warming in the central Pacific. We suggest that the increased occurrence of CP El Niño in recent years may be associated to the La Niña-like state since the 90’s and changes in the seasonality of the thermocline since the 2000’s.

Coastal physical and biogeochemical variability in the Southeastern Atlantic: Role of local atmospheric forcing versus oceanic teleconnection Marie-Lou BACHELERY (2013-2016)

The objective of this thesis is to study the coastal variability in the southeastern Atlantic Ocean, focusing on the Benguela Upwelling System (BUS). More specifically, we aim at determining the contribution of the equatorial connection (and Coastal Trapped Waves (CTW) propagation) compared with the local atmospheric forcing (wind stress and heat fluxes) to this variability. This Ph.D. was led by a need to understand what are the dominant processes associated with extreme warm and cold events in the BUS (Benguela Niños/Niñas) which could have striking effects on the marine ecosystem and fisheries.
We based our methodology on the development of a southeastern Atlantic configuration using a physical-biogeochemical coupled model (ROMS-BioEBUS) as well as several sensitivity experi- ments to the forcing contribution: (1) remote and (2) local. The analysis of the different simulations highlight the respective roles of the remote equatorial and the local forcing on the dynamical (temperature, currents, density) and biogeochemical (oxygen and nitrate concentrations and Primary Production) variability, along the equator and the south- western African coast. At sub-seasonal time scales (11 days–3 months), the oceanic variability is dominated by the local forcing (wind stress and heat fluxes) ; while the interannual variability (13–18 months) and Benguela Niños/Niñas events are explained by the remote equatorial forcing. At interannual time scales, CTW propagate poleward along the southwest African coast with a maximum signature in sub-surface. During their propagation, CTW trigger strong temperature, density, currents and biogeochemical cycles (oxygen and nitrate concentrations) anomalies that could reach the north- ern part of the BUS (24°S). However, their amplitude and the most poleward latitude at which they can be detected is modulated by (1) the interannual variability prescribed at the southern boundary and associated with the equatorward Benguela current, (2) interannual wind locally forced CTW, and (3) the upwelling dynamics along the southwestern African coast. In order to understand the role of CTW on the thermodynamical and biogeochemical balance, we have studied the mechanisms and processes associated to these waves. Interannual anomalies of temperature and nitrate associated with CTW propagation are explained by physical advection processes. Alongshore and vertical currents variations associated with CTW propagation modify the transport of equatorial water poleward as well as deep water upward along the west coast of Africa, respectively.
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. Finally, the results reveal a north (0°S-22°S) / south (23°S-28°S) contrasting conditions in the CTW biogeochemical signature associated with a change in the sign of interannual anomalies in the BUS. This abrupt change observed in the BUS is explained by the modification of the mixed vertical gradient due to the strong local upwelling dynamics.

Our results point out the importance of the equatorial connection to the coastal physical and bio- geochemical interannual variability in the southeast Atlantic Ocean and in the BUS. This should allow forecasting the occurrences of interannual events (but not their magnitude which can be modulated by local atmospheric variability) that have strong ecological and socio-economic consequences.

A study of Benguela Niños and Niñas from 1958 to 2015. Rodrigue Anicet IMBOL KOUNGUE (2015-2018)

Prediction and Research Moored Array in the Tropical Atlantic (PIRATA) records in combination with outputs from an Ocean Linear Model (OLM) and altimetric data are used to investigate the link between the equatorial Atlantic Ocean dynamics and the variability in the coastal region of Angola-Namibia at interannual timescales over 1998 to 2012. The PIRATA records help to define an index of equatorial Kelvin wave activities in the Equatorial Atlantic. There is a good agreement between PIRATA monthly dynamic height anomalies, altimetric monthly sea surface height anomalies (SSHA), and sea level anomalies calculated with an OLM at interannual time scales. This allows the interpretation of PIRATA records in terms of equatorial Kelvin wave propagations. Extreme warm or cold events in the Angola – Namibia area lag strong anomalous eastward equatorial propagations by 1-2 months. Remote equatorial forcing via equatorial Kelvin waves which propagate poleward along the west African coast as coastal trapped waves is at the origin of their developments. Results show a seasonal phasing, with significantly higher correlations between the equatorial index and coastal sea surface temperature anomalies (SSTA) off Angola-Namibia in October-April season. Then, a systematic study of all the Benguela Niño and Benguela Niña events before 1982 is done using an Ocean general circulation model in combination with the OLM outputs from 1958 to 2015. 26 anomalous strong coastal events (16 warm and 10 cold) are identified. The analysis of their evolution confirms the remote equatorial origin of most of these coastal anomalous strong events. Modelled meridional transport anomalies across the Angola Benguela Front (ABF) contribute to the development of these anomalous coastal warm events. Across the ABF, the results obtain with the net temperature transport are similar to the ones with net mass transport. Most anomalous events peak in October - April season. Lagged composites of surface temperature and wind stress anomalies in the equatorial and southeastern Atlantic reveal that both local and remote forcings develop simultaneously 1-2 months before the peak of Benguela Niño or Niña. At the monthly scale, local atmospheric forcing is more correlated with anomalous coastal events occurring in Southern Angola which is a non-wind-upwelling driven region. The results from this thesis open the possibility to predict Benguela Niño and Benguela Niña events using an index depicting the equatorial interannual variability associated with Interannual Equatorial Kelvin Wave propagation, especially from October to April when the coastal stratification is favourable to the imprint of coastal trapped waves in the surface layer.

Coastal climate change and Variability in the Benguela Current System. Folly Serge TOMETY (2017-2022)

The thesis aims to seek to document the long-term change and decadal variability in the Benguela Upwelling System and study the possible mechanisms behind these changes. The Benguela Upwelling System is one of the four most productive fisheries areas in the world, and it is therefore important to understand the mechanisms leading to changes at different time and space scales before developing scenarios or forecasts for the future of the region. The first part of the thesis (chapter 3) uses four satellite-derived Sea Surface Temperature (SST) datasets combined with various climate reanalysis data to investigate the long-term SST trends in the Benguela Upwelling System over the period 1982-2017. The use of different datasets shows different trends depending on the dataset, which is a concern. However, after a thorough examination, there is some consensus. Results show that the Angola-Benguela Upwelling System has significantly changed during the last three decades. The changes vary in space and depend on season. Cooling trends are observed in the southern part of the Benguela Upwelling System in the austral summer and autumn. The cooling trend is consistent with a positive trend in upwelling-favourable equatorward winds due to the intensification and poleward expansion of the South Atlantic Subtropical high-pressure atmospheric system. A warming trend is observed in Southern Angola and Northern Benguela in late spring and summer. Results also show that the warming or cooling trends in the Benguela Upwelling System are not as linear as the trend in global air temperature. Indeed, when studying trends for the 1982-2017 period, trends tend to slow down and can reverse sign in some regions and recent time, suggesting decadal variability. Most discrepancy between SST datasets occurs from 1982 to 1985, the start of the satellite era. The second part of the thesis (chapter 4) focuses on understanding the mechanisms leading to the warming trends along the Angolan and Northern Benguela coast. To do so, the Ocean General Circulation Model NEMO (OGCM NEMO) is used. The model produces an unrealistic cooling trend in the Northern Benguela due to a positive trend in upwelling-favourable wind model forcing. The modelled warming trend in Southern Angola is properly simulated which allows me to use the model to study the mechanisms leading to the warming trend in Angola. Analysis of the model net heat budget components and their contribution to the overall SST trend suggests that the warming trend observed along the Angolan and Namibian coasts through the austral summer is primarily associated with the intensification of the poleward flow along the coast, bringing more warm water from the tropics to the region and also due to weakening of the vertical flow of cold water to the surface. Locally, the net surface heat flux has decreased and tends to create a negative SST trend but does not offset the warming trend created by the intensification of the flow. The poleward intensification of the Angola Current is attributed to the intensification of the cyclonic circulation around the Angola Dome. Lastly, in chapter 5, the decadal variability in the Benguela upwelling system, identified in Chapter 3, is investigated using a long-term ocean model simulation of 110 years (1900 - 2010) of the global ocean-ice components of the Norwegian Earth System Model (NorESM). The results reveal the presence of three dominant scales of variability: the interannual (2-8 years), quasi-decadal (9-14 years) and interdecadal (19-26 years) variability in the Southern Benguela upwelling system. The Southern Benguela SST correlations with the global SST reveal that at quasi-decadal scale the Southern Benguela SST is linked to the south Atlantic SST and the north-east Pacific SST fluctuations, while at the interdecadal scale the Southern Benguela SST modulation is linked to the equatorial and northern Pacific SST, Indian SST and Atlantic SST fluctuations except the equatorial Atlantic SST.