Impact of the Agulhas Current on storm development

O'Connor, Jason

Impact of the Agulhas Current on storm development

Master Thesis

2020

Publisher

University of Cape Town

Department

Department of Oceanography

Faculty

Faculty of Science

Abstract

A high-resolution atmospheric model (WRF) is used to investigate the impact of the Agulhas Current on synoptic storm development. A sensitivity experiment is conducted to analyse the influence of the Agulhas Current's sea surface temperature (SST) on rain producing, synoptic scale weather features. Two model configurations: Control (CTL) and Smooth (SMTH) are analysed to understand the effect of the Agulhas Current's SST and high latent heat fluxes on storms that develop or track over the Current. The two configurations are identical except that the SMTH simulation has the SST signature of the Agulhas reduced by smoothing out the strong SST gradients associated with the Current. This results in the Agulhas Current core having SSTs reduced by roughly 1.5°C in the SMTH configuration. Consequently, lower (100 - 150 W.m¯²) latent heat fluxes are also simulated at the Current core's location in the SMTH run. Using daily South Africa Weather Service synoptic charts from 2001 - 2005, when the model output is available, two hundred (200) synoptic scale storms are found to track over the Current. Using the TRMM 3B42 3-hourly 0.25 x 0.25° precipitation rain rate product, 70 (of the 200) are found to have produced rainfall. Five model variables are used as proxies for the storm intensity of these 70 storms. Ten storms are found to show storm intensification when passing over the Current. In the CTL simulation, of these ten storms, ten show lower 850mb geopotential heights (m), nine show higher surface wind speeds (m.sˉ¹), seven show higher rain rates (mm.hrˉ¹), eight show higher Eddy Kinetic Energy (EKE) (m².sˉ²) and nine show greater upward moisture flux at the surface (g.mˉ².sˉ¹) compared to the SMTH run once each storm has propagated over the Current. Model output analysis shows sustained or dissipating storm intensity of the other 60 storms while passing over the Current. Nonetheless, these results provide a strong case for the influence of the Agulhas Current on the intensification of synoptic scale, rain producing events.

Keywords

Oceanography

Applied Ocean Sciences

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