Twenty-first Century Changes in the Agulhas Leakage: Separating the Effects of Ozone Recovery and Greenhouse Gases increase

Abstract:

The Agulhas leakage represents the transport of warm and saline Indian Ocean waters into the Atlantic Ocean, in the form of rings, eddies and filaments. Upon entering the Atlantic, the Agulhas leakage feeds into the upper, warm branch of the AMOC. Its variability, on decadal time scales, therefore affects the AMOC. During the last decades of the twentieth century, the Agulhas leakage experienced a significant positive trend, associated with the austral summer strengthening of the mid-latitude westerly winds in the Southern Hemisphere. The main driver of the westerlies changes was the formation of the Antarctic ozone hole. Increasing concentrations of atmospheric greenhouse gases (GHG) have a similar effect on the westerlies, but their contribution in the recent past was of secondary importance. This is likely to change in the second half of the twenty-first century, as the GHG levels will continue to increase while the ozone hole is expected to recover, following the ban of ozone depleting substances (ODS). This raises the question of whether the Agulhas leakage will continue to increase in the future, driven by a GHG-induced strengthening of the westerlies, or whether the impacts of ozone recovery and GHG increase will compensate each other, resulting in no change of the Agulhas leakage.

In this talk, we investigate the future changes in the Agulhas leakage using a new, state-of-the-art, coupled climate mode, FOCI. The stratospheric ozone chemistry in FOCI is calculated interactively, ensuring a realistic representation of the future ozone recovery, which occurs in the Antarctic around 2060. In addition, the ocean resolution around South Africa is enhanced to 1/10° in order to resolve the mesoscale features responsible for the Agulhas leakage. We present results from two ensembles of sensitivity experiments, in which ODS are kept constant to 1995 values, while GHG vary following the Shared Socioeconomic Pathway (SSP) 5-8.5 and vice-versa and from one ensemble in which both ODS and GHG follow SSP5-8.5. We systematically separate the impacts of ozone recovery and of increasing levels of GHG in the Southern Hemisphere throughout the twenty-first century. Our focus extends from the polar night jet in the stratosphere to the westerly winds in the troposphere and the effect of their variability on the Agulhas leakage.