On the role of the Agulhas Current on weather and climate of South Africa

Nkwinkwa, Njouodo Arielle Stela

On the role of the Agulhas Current on weather and climate of South Africa

Doctoral Thesis

2018

Abstract

The Agulhas Current is the strongest western boundary current of the Southern Hemisphere. The aim of this thesis is to understand the impact of ocean-atmosphere interaction in the Agulhas Current on the atmosphere and to investigate its importance for Southern African rainfall. This warm Current creates a high temperature gradient with the surrounding ocean, leading to a large turbulent flux of moisture from ocean to atmosphere (also called the turbulent latent heat flux). The dynamics of ocean-atmosphere interaction above the Agulhas Current and its impact on the weather and climate of Southern Africa are not well known. This is due to a) climate reanalyses that do not include the Agulhas Current and b) the lack of regional capacity in ocean-atmosphere modeling. I use ocean observations, various climate reanalyses, and several satellite remote sensing data sets to find out if the new reanalyses (cited below) do represent the intense exchange of moisture that occurs above the Agulhas Current and the Agulhas Retroflection region. The largest turbulent latent heat flux is found in the Retroflection region in winter, while the lowest is off Port Elizabeth in summer. The Climate Forecast System Reanalysis (CFSR) and the ModernEra Retrospective analysis for Research and Applications version-2 (MERRA-2) do represent the turbulent latent heat flux well when compared to high-resolution satellite data. ERA-Interim Reanalysis underestimates the turbulent latent heat flux due to reduced wind speeds. The observation-based National Oceanography Centre Southampton (NOCS) is different from the satellites and the reanalysis products because its annual cycle is reversed, and NOCS underestimates the turbulent latent heat flux compared to the former products. The study of the satellite product air-sea turbulent fluxes (SEAFLUX) shows that east of the Agulhas Current, the specific humidity difference is the main driver of the annual cycle variations of turbulent latent heat flux, while the main driver in the Retroflection is the wind speed and both the specific humidity difference and the wind speed in between (around Port Elizabeth). I use high-resolution annual mean observations from satellites, atmospheric reanalysis, and the Weather Research and Forecasting (WRF) model to show that the warm core of the Agulhas Current drives a band of precipitation along the east coast of South Africa when the Current is adjacent to the coast. To do that, I conduct a classic modeling experiment with one configuration representing the sea surface temperature (SST) of the Agulhas Current relatively well. This WRF experiment reproduces the turbulent latent heat flux well. The second simulation is with SST of the Agulhas Current reduced by up to 2°C compared to the first experiment. From a diagnostic of the pressure adjustment mechanism, results show that the warm SST of the Agulhas Current enhances the formation of coastal precipitation along and above the Current. Finally, I look at the seasonality of oceanatmosphere interaction in the Agulhas Current and its impact on Southern African precipitation. In winter, the impact of the Agulhas Current is confined to the atmosphere above it and mechanisms are similar to those described for annual mean. In summer and autumn, SST differences between the two simulations where the Agulhas Current system is more than 25°C, leads to differences in geopotential height above the ocean, extending along the eastern coast and over the land area. The higher temperature of the control simulation leads to cyclonic circulation anomalies and larger moisture flux anomalies from the Agulhas Current to the continent from the south. The analysis of the high-level moisture flux indicates that the Agulhas Current influences the rainfall and humidity flux of Southern Africa. More moisture flux is then brought inland at a higher level. In the northeast of the region, there is an export of moisture anomalies from land to the ocean, and an import of moisture anomalies from above the Agulhas Current to the landmass. This is due to the wind anomalies between the two simulations. However, the overall result leads to more precipitation over the interior of the continent. The study shows that it is important to integrate the fine structure of the ocean temperature of the Agulhas Current in modeling studies and climate reanalyses. Results of this thesis have implications for the prediction of South African weather and climate, and for understanding past and present climate.

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