Decadal trends in the Southern Ocean meridional eddy heat transport

Abstract Meridional heat transport induced by oceanic mesoscale eddies (EHT) plays a significant role in the heat budget of 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 1993–2016. The results reveal that the surface EHT from both satellite and ECCO2 data consistently show decadal poleward increasing trends in the SO, particularly in the latitude band of 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 per decade 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 eddy kinetic energy (EKE) is the dominant mechanism for the increase of EHT in the SO. By performing energy budget analysis, we further find that the decadal increase of 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 Southern Annular Mode between 1993–2016. The decadal trends in EHT identified here may help understand decadal variations of heat storage and sea-ice extent in the SO.