Browsing by Author "Illig, Serena"
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- ItemOpen AccessA study of Benguela Niños and Niñas from 1958 to 2015(2018) Imbol, Koungue Rodrigue Anicet; Rouault, Mathieu; Illig, SerenaPrediction 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.
- ItemOpen AccessCoastal climate change and variability in the Benguela current system(2022) Tomety, Folly Serge; Rouault, Mathieu; Illig, SerenaThe 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.
- ItemOpen AccessModulation of Wind Work by Oceanic Current Interaction with the Atmosphere(2016) Renault, Lionel; Molemaker, M Jeroen; McWilliams, James C; Shchepetkin, Alexander F; Lemarié, Florian; Chelton, Dudley; Illig, Serena; Hall, AlexIn this study uncoupled and coupled ocean-atmosphere simulations are carried out for the California Upwelling System to assess the dynamic ocean-atmosphere interactions, viz.,the ocean surface current feedback to the atmosphere. We 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, we demonstrate the current feedback has an effect on the surface stress and an 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 near-shore 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 re-energizes 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, we propose an additional energy element in the Lorenz diagram of energy conversion, viz., the current-induced transfer of energy from the ocean to the atmosphere at the eddy scale.