Agulhas leakage variability and its climate implications in a coupled system

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. Here we investigate Agulhas leakage variability and its impacts by analyzing a suite of ocean eddy resolving (1/10 degree) coupled simulations. In the high-res NCAR Community Coupled System Model, Agulhas leakage is quantified with the aid of Lagrangian particle tracking. In a 20th century simulation, resolving the Agulhas retroflection constrains the inertially choked Agulhas leakage to 11.2 Sv. 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. We find that only 47% of Agulhas leakage transport is associated with Agulhas Ring shedding. Moreover, At interannual timescales, we find that a significant portion of leakage variability might be controlled by large-scale forcing. Such variability may affect regional climate through air-sea coupling. We also 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. 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.

Jun 13, 2018 1:00 PM