Publication Date

2018-05-07

Availability

Open access

Embargo Period

2018-05-07

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Meteorology and Physical Oceanography (Marine)

Date of Defense

2018-01-31

First Committee Member

Lisa M. Beal

Second Committee Member

Ben P. Kirtman

Third Committee Member

Igor Kamenkovich

Fourth Committee Member

Shenfu Dong

Fifth Committee Member

Arne Biastoch

Abstract

Agulhas leakage has profound impacts on the climate system via importing heat and salt anomalies and affecting the stability of the meridional overturning circulation in the Atlantic. Existing numerical studies on Agulhas leakage are mostly based on ocean general circulation models, and the few coupled studies suffer from coarse resolution. In this dissertation, we investigate Agulhas leakage variability and its impacts by analyzing a suite of ocean eddy resolving (1/10 degree) coupled simulations. We find that the high resolution configured NCAR Community Coupled System Model (CCSM3.5) simulations share some discrepancies such as regular ring path and eastward bias of retroflection position with previous studies using models of similar horizontal resolution. The simulated Agulhas Current is much broader and the mean transport is larger than the latest mooring observations at 34S, possibly due to strong recirculation. Agulhas leakage is quantified with the aid of Lagrangian particle tracking. We devise a strategy to track the particles released daily at the 34S section, after a thorough sensitivity tests to find the optimized configurations. In a 20th century simulation, resolving the Agulhas retroflection constrains the inertially choked Agulhas leakage to 11.2 Sv, well within the existing estimates. We show that monthly mean outputs can be used to produce a reliable time series of Agulhas leakage variability on longer-than-seasonal time scales (r=0.88, p<0.01), by comparing to a parallel simulation that archives daily mean fields every five days. We find that only 47% of Agulhas leakage transport is associated with Agulhas Ring shedding. Under a Lagrangian framework, we decompose the velocity field into eddying and large-scale fields then recreate a number of total velocity fields by modifying the eddying component, to assess the dependence of interannual leakage variability on mesoscale dynamics. We find that leakage time series show strong coherence at periods longer than 1000 days, in spite of the modified eddy fields, and that 50% of the variance is independent of eddies at interannual timescales. Such variability is found to be related to a meridional shift and/or strengthening of the westerlies, similar to the wind stress response of the Southern Annular Mode (SAM). We find that high leakage periods are associated with east-west contrasting patterns of sea surface temperature, surface heat fluxes, and convective rainfall, with positive anomalies over the retroflection region and negative anomalies within the Indian Ocean to the east. High leakage periods are also related to reduced inland rainfall over southeastern Africa in austral summer. We investigate the changes of Agulhas leakage associated with increasing CO2 in a coupled setting. Agulhas leakage is found to be 15% higher in the control simulations with a year 2000 CO2 level, as well as stronger westerlies and a southward expanding and intensifying supergyre. The Agulhas retroflection area is characterized by a warmer sea surface, higher surface heat flux lost and kinetic energy, consistent with recent satellite observations. We find that Agulhas leakage has been increasing at a rate of 0.33 Sv per decade since the mid-1950s. Our results imply that existing hindcast studies likely overestimate the positive leakage trend, which is sensitive to the surface forcing employed to drive the ocean models.

Keywords

Agulhas Leakage; AMOC; climate change; coupled model; climate variability

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