Coastal climate change and variability in the Benguela current system

Doctoral Thesis


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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.