Date of Award

2018

Document Type

Open Access Thesis

Department

Marine Science

Sub-Department

College of Arts and Sciences

First Advisor

Subrahmanyam Bulusu

Abstract

The southern end of the African continent marks the division between the Atlantic Ocean and the Indian Ocean where warm, saline waters propagate into the Atlantic Ocean in the form of rings, eddies, and filaments. These warm, saline waters released during the retroflection of the Agulhas Current are referred to as Agulhas leakage. The Agulhas Current is a poleward flowing western boundary current that forms the limb of the winddriven anti-cyclonic circulation of the south Indian Ocean. The current originates south of Madagascar fed by the Mozambique Channel and the East Madagascar Current. Smallscale variations have been identified in Agulhas leakage that further impact the strength of Meridional Overturning Circulation sequentially altering the climate patterns as well as interact with the Benguela current altering Benguela upwelling. Numerous mechanisms have been identified by previous studies as potential factors resulting in Agulhas leakage fluctuations but these studies fail to determine the driving source of changes to Agulhas leakage. This study proposes El Niño Southern Oscillation (ENSO) as a driving force behind the changes in Agulhas leakage dynamics. Altimetry-derived sea surface height (SSH) anomalies, Advanced Very High Resolution Radiometer (AVHRR) sea surface temperature (SST) data, sea surface salinity (SSS) Simple Ocean Data Assimilation (SODA) reanalysis product, and in-situ datasets of ARGO profiles of salinity and temperature are utilized to explore signal response in both the surface and depth to an ENSO event. This thesis determines that an ENSO event originating in the Pacific Ocean alters the salinity and temperature properties of the Indian Ocean basin in both the surface and subsurface. The resulting signal, which is opposite for El Niño compared to La Niña, further propagates via westward propagating Rossby waves into the Agulhas Current system to influence the Agulhas leakage region between 20 to 26 months after the peak of an ENSO event. The salinity and temperature signal of the Agulhas leakage region [370-450S, 100-200E] is observed as anomalously warm and fresh transitioning to warm and saline in response to El Niño. Oppositely, in response to La Niña, the Agulhas leakage region is anomalously cool and saline transitioning to cool and fresh. Furthermore, it is revealed that the subsurface (~500 m depth) temperature and salinity anomalies across the Indian Ocean and within the Agulhas leakage region reflect this surface response at depth and persist for a longer duration (~1 to 2 years) compared to the surface signal

Share

COinS