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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 variability and change in the Benguela Upwelling System: Decadal trend analysis.

Folly Serge TOMETY (2017-2020)

Sea Surface Temperature is a good indicator to monitor upwelling strength in Eastern Boundary Current such as the Benguela Current. Satellite remote sensing allows estimating sea surface temperature for most of the ocean since the 1980’s. Over the last 35 years, the ocean has warmed globally but some regions are showing a cooling. Detecting trend in coastal upwelling is challenging due to proximity of the coast and clouds. A rigorous analysis of sea surface temperature observations of various data sets over 35 years is used to examine variability and seasonal temperature trends in the Angola, Benguela and Agulhas current systems using four different dataset based all using on AVHRR satellite remote sensing (Reynolds SST 1°x1° resolution, Reynolds SST 0.25°x0.25°, Hadley SST 1°x1°, Pathfinder 4x4 km). Significant annual warming trend were found off of the Angola and Namibian coast (> 0.35°C per decade) and in the Agulhas retroflection region (~ 0.5°C per decade) in all dataset and cooling trend is found in the Southern Benguela and South Coast of South Africa where a strong seasonally depend wind driven upwelling occurs. However, cooling or warming trends are seasonally dependent. Warming trend is more pronounced in early austral summer off the Angola and Namibia coasts and in late summer in the Agulhas Current retroflection region in all datasets. However, in the Benguela upwelling system (BUS), notable differences in SST trends among the datasets occur. Some data indicate a significant cooling trend up to 0.5°C in the Southern Benguela upwelling especially in autumn and early winter while other datasets indicate little change. In addition, we found that the SST difference between products changes over times.

PhD Students

Folly Serge TOMETY ORCID iD icon
Coastal variability and change in the Benguela Upwelling System: Decadal trend analysis.
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Supervisors: M. Rouault and S. Illig

Rodrigue Anicet IMBOL KOUNGUE ORCID iD icon
A study of Benguela Niños and Niñas from 1958 to 2015.
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Supervisors: M. Rouault, S. Illig, and J. Deshayes

Coastal physical and biogeochemical variability in the Southeastern Atlantic: Role of local atmospheric forcing versus oceanic teleconnection.
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Supervisors: I. Dadou and S. Illig

The intraseasonal Kelvin wave and the dynamics of the Central Pacific El Niño events.
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Supervisors: B. Dewitte and S. Illig