Mesoscale eddies play an important role in the transport of heat, carbon, and nutrients in the Southern Ocean. Previous studies have documented an increasing intensity of the Southern Ocean eddy field during recent decades; however, it remains unclear whether the mesoscale eddies with different lifetimes have different temporal variations. Using satellite altimeter observations from 1993 to 2020, we found that the increasing trend in the intensity of eddies is dominated by long-lived eddies (with lifetimes ≥ 90 days), whose amplitude has increased at a rate of ~2.8% per decade; the increase is concentrated downstream of topography. In contrast, short-lived eddies (with lifetimes < 90 days) do not appear to have a significant trend in their amplitudes since the early 1990s. An energy conversion analysis indicates that the increased baroclinic instabilities of the mean flows associated with topography are responsible for the amplitude increase of the long-lived eddies. This study highlights the need for a better understanding of the changes in mesoscale eddies owing to their importance in the transport of heat, carbon, and nutrients.

Mesoscale eddies play an important role in both momentum and heat balances in the Southern Ocean. Previous studies have documented an increasing intensity of the Southern Ocean eddy field during recent decades; however, it is still unclear whether the mesoscale eddies with different lifetimes have different temporal variations. Using satellite altimeter observations from 1993 to 2020, we find that the increasing trend in the intensity of eddies is dominated by long-lived eddies (with lifetimes ≥ 90 days), whose amplitude has increased at a rate of ~2.8% per decade; the increase is concentrated downstream of topography. In contrast, short-lived eddies (with lifetimes < 90 days) do not appear to have a significant trend in their amplitudes since the early 1990s. An energy conversion analysis indicates that the increased baroclinic instabilities of the mean flows associated with topography are responsible for the amplitude increase of the long-lived eddies.

Ocean mesoscale eddies are observed to influence mixed layer depth (MLD) with a deepening by anticyclonic eddies (AEs) and a shoaling by cyclonic eddies (CEs). By using Argo profiles, satellite altimeter data and eddy-resolving model data, this study investigates how the AEs and CEs alongside the Kuroshio Extension jet (KEJ) modulate the winter MLD. Results show that although the modulations by the AEs and CEs are roughly symmetric over the whole region on the winter MLD, their impacts are strongly asymmetric to the north and south of the KEJ. The deepening effect by the AEs is much more than the shoaling effect by the CEs to the north of the KEJ, while the shoaling effect by the CEs is much more than the deepening effect by the AEs to the south of the KEJ. Therefore, their overall effect results in a net deepening to the north and a net shoaling to the south of the KEJ. A further analysis finds that this asymmetric feature is closely related to the distribution of eddy amplitudes which implies more energetic AEs to the north but CEs to the south of the KEJ. This study highlights the importance of the distribution of energetic eddies on the modulation of MLD.