MSCI Thesis Qualifying/Comprehensive Exam - Morgan ParisWednesday, May 31, 2017 - 10:00am
The Role of ENSO on the Agulhas Current System and Benguela Upwelling
The Agulhas Current, a western boundary current, is a limb of the wind driven anti-cyclonic circulation of the south Indian Ocean. The current originates south of Madagascar fed by the Mozambique Channel and the East Madagascar Current. Near the tip of Africa, the current retroflects shedding warm saline waters into the Atlantic Ocean. This phenomenon, referred to Agulhas leakage, interacts with the Benguela current and feeds the upper arm of the Atlantic Meridional Overturning Circulation (MOC). Fluctuations in this highly dynamic system impact the strength of overturning sequentially altering climate patterns. Thus far, studies have failed to determine key factors and systems driving small-scale variations in leakage. The combined objectives of this proposed work is that ENSO plays a role in observed small-scale fluctuations of the Agulhas Current system through interactions with source currents, leakage, and Benguela upwelling.
First, the relationship between ENSO events classified by the Oceanic Niño Index (ONI) and fluctuations in Agulhas leakage will be explored using sea surface temperature (SST) and sea surface salinity (SSS). It is predicted that there will be a connection between SSS and SST in which higher temperatures will be accompanied by higher salinity while cooler waters will be fresher. SSS observations can be coupled with altimetry data to better understand leakage process and identify mesoscale eddies. Preliminary results indicate Agulhas retroflection sheds anomalously warm waters in response to El Niño and anomalously cool waters in response to La Niña. Starting at the peak of an ENSO event the signal is transmitted at 12°S and 25°S from the Pacific Ocean into the Indian Ocean basin by Rossby waves. These waves travel westward until they reach Madagascar where they interact with source currents to complete the transfer of an ENSO signal into the Agulhas leakage region. These currents include the East Madagascar Current (EMC) and the Mozambique Channel. The process is believed to occur during a timescale spanning two years. At the end of two years, the anomalous leakage enters the Benguela current system. Agulhas leakage is a dominant source of the southern portion of this system and directly influences upwelling dynamics. We will establish and explore an upwelling index to understand the connection between Agulhas leakage and Benguela upwelling. Then, this relationship can be explored from the perspective of anomalous leakage events produced by ENSO.
The combined objectives should work together to create a complete picture of all components of the Agulhas current beginning with source current dynamics leading to Agulhas leakage and ending in the Benguela current. Specifically, the transfer of ENSO signal will be identified through each of these systems. The overall objective is to explain and prove interannual small-scale variations of Agulhas leakage are a result of ENSO events. To understand the role of ENSO on Agulhas Current system and Benguela upwelling, we will use satellite observations (SST from multi sensor satellites; Salinity from Aquarius, SMOS, and SMAP; winds from QuikSCAT, ASCAT, RapidSCAT and ScatSAT), model simulations from 1/12° Hybrid Corrdinate Ocean Model (HYCOM), and for interannual to decadal study we will use Simple Ocean Data Assimilation (SODA) re-analysis.