Southern Ocean standing meander and sea level anomalies: over 20 years of trends

Amelie Meyer, Australian Research Council Centre of Excellence for Climate Extremes, Hobart, TAS, Australia, Clothilde Langlais, CSIRO Marine and Atmospheric Research Hobart, Hobart, TAS, Australia, Navid Constantinou, Australian National University, Research School of Earth Sciences, Canberra, ACT, Australia, Benoit Legresy, CSIRO Oceans and Atmosphere, Hobart, TAS, Australia, Andrew M Hogg, Australian National University and ARC Centre of Excellence for Climate Extremes, Research School of Earth Sciences, Canberra, ACT, Australia and Nathaniel L. Bindoff, Univ Tasmania, Hobart, TAS, Australia
Southern Ocean circulation strongly influences the global climate system. Eddy energy in the Southern Ocean is expected to rise with increasing strength of Southern Hemisphere westerly winds. Satellite altimetry data provides a 25-year time series (1993-2018) of sea surface height from which, observed trends in ocean Eddy Kinetic Energy can be derived. These trends currently show strong regional variability, with some estimates suggesting an overall mean increase in Southern Ocean Eddy Kinetic Energy, possibly dominated by regional extremes. Concurrently, both reanalysis products and models show intensification of Southern Hemisphere westerly winds with a poleward shift during the summer (DJF) period and a 20% increase in wind stress forcing on the Southern Ocean during the 1980-2010 period. The impact of such changes on Southern Ocean dynamics, circulation and transformation of water masses is still under investigation.

Here we use existing observations, reanalysis products, and high-resolution ocean model data to estimate changes in Southern Ocean mesoscale circulation over the past 25 years, focusing on the oceanic standing meander and front between South Africa and the Kerguelen Islands. We find that local sea level anomalies have localized positive significant trends both in the satellite observations (~1 cm/yr) and in the ACCESS-OM2 (0.1o) model (~0.5 cm/yr). We also find no meridional shift in the meander location over the observation time period and suggest that the decreasing amplitude of meander is driving local sea level trends. Such a change in the meander’s amplitude has implications for the local carbon and heat budget by impacting regional upwelling and fluxes from deeper waters to the surface.