The Angola-Benguela Upwelling system: interannual and decadal variability (two talks)

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Marie-Lou Bachelery and Folly Serge Tomety
University of Cape Town
Seminar Date: 
26. November 2019 - 13:00 - 14:00
Lecture room, Ground Floor, NERSC

Title 1: Impacts and characteristics of the interannual Coastal Trapped Waves in the Angola-Benguela Upwelling System

Marie-Lou Bachelery, Serena Illig, and Mathieu Rouault

Located along the coast of Angola, Namibia, and South-Africa, the Angola-Benguela Upwelling System (ABUS) hosts one of the most productive ecosystem of the world, detrimental for the food security of the populations of the surrounding countries. However, the regional climate and fish-stock availability are subject to a large variability with the regular occurrences (every ~2 years) of anoxic events as well as extreme warm/cold events called Benguela Niños/Niñas. Here, we investigate the role of the equatorial dynamics and subsequent remotely-forced Coastal Trapped Waves (CTW) on the phenology of the extreme Benguela Niño/Niña events. 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) and temperature 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 biogeochemical properties) on the shelf during the passage of remotely-forced CTW.

Title 2: Coastal variability and change in the Benguela Upwelling System: decadal variability and trend.

Folly Serge Tomety

The dynamics of the Benguela Upwelling System is maintained by the coastal trade winds controlled by the South Atlantic high-pressure system. In a context of climate change and following Bakun's hypothesis, the amplitude of the upwelling is expected to increase due to the intensification of equatorward coastal wind. Consequently, this should lead to cooler coastal SST.
Here, we investigate the decadal variability and trends of the SST in the Benguela Upwelling System using observations, model and reanalysis datasets. Our results show that the trends vary depending on the region and the products. It can reverse sign depending on the available period of the datasets on which trends are estimated, highlighting the shortage of baseline data to unravel decadal variability from long-term changes. Therefore, we use a 100-year forced global ocean simulation, to improve our understanding of the mechanisms that drive the SST decadal variability and trend in the Benguela Upwelling System. Preliminary results suggest a strong decadal signal of periods of 10 to 20 years in the South Benguela, in agreement with the decadal variability of the coastal trade wind.