Decadal Trends in the Southern Ocean Meridional Eddy Heat Transport

Abstract

Abstract Meridional heat transport induced by oceanic mesoscale eddies (EHT) plays a significant role in the heat budget of the Southern Ocean (SO) but the decadal trends in EHT and its associated mechanisms are still obscure. Here, this scientific issue is investigated by combining concurrent satellite observations and Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2) reanalysis data over the 24 years between 1993 and 2016. The results reveal that the surface EHTs from both satellite and ECCO2 data consistently show decadal poleward increasing trends in the SO, particularly in the latitude band of the Antarctic Circumpolar Current (ACC). In terms of average in the ACC band, the ECCO2-derived EHT over the upper 1000 m has a linear trend of 1.1 × 10−2 PW decade−1 or 16% per decade compared with its time-mean value of 0.07 PW. Diagnostic analysis based on “mixing length” theory suggests that the decadal strengthening of eddy kinetic energy (EKE) is the dominant mechanism for the increase in EHT in the SO. By performing an energy budget analysis, we further find that the decadal increase in EKE is mainly caused by the strengthened baroclinic instability of large-scale circulation that converts more available potential energy to EKE. For the strengthened baroclinic instability in the SO, it is attributed to the increasing large-scale wind stress work on the large-scale circulation corresponding to the positive phase of the Southern Annular Mode between 1993 and 2016. The decadal trends in EHT identified here may help understand decadal variations of heat storage and sea ice extent in the SO. Significance Statement Oceanic mesoscale-eddy-induced meridional heat transport (EHT) is a key process of heat redistribution in the Southern Ocean (SO), but the decadal variations of EHT and the associated mechanisms remain obscure. Here, by analyzing satellite and reanalysis data between 1993 and 2016, we find that the poleward EHT has significant decadal increasing trends in the SO, particularly in the Antarctic Circumpolar Current latitude band. Further analysis suggests that the increasing EHT is mainly caused by enhanced eddy kinetic energy converted by the strengthened baroclinic instability of large-scale circulation, which is attributed to the strengthening winds modulated by the Southern Annular Mode. The above findings may improve our understanding of the decadal variations of heat storage and sea ice extent in the SO.