Regional Trends in Southern Ocean Eddy Kinetic Energy.

Previous model‐based studies and observation‐based studies suggest that increasing wind energy input into the Southern Ocean will primarily cause increases in eddy kinetic energy (EKE) with no significant change in the mean circulation, a result that has been named the "eddy‐saturation" hypothesis. However, due to the sparsity of the available observations, quantifying and understanding regional EKE changes in the Southern Ocean is challenging. In this study, we examine regional trends in the Southern Ocean EKE using altimetry crossover measurements and an ocean reanalysis product to quantify if undersampling by altimetry will bias EKE trend estimates and to test if previously observed EKE trends are homogenous throughout the Southern Ocean or concentrated in a few regions. To verify that the EKE computed from altimetry crossovers accurately represents yearly averaged EKE over the Southern Ocean, we first conduct a sampling experiment with the HYCOM Global Ocean Forecasting System 3.1 ocean reanalysis. We find that the crossover sampling is sufficient to represent the yearly averaged EKE when averaged over sectors of at least 30° of longitude. We find no coherent increase in EKE over the entire Southern Ocean from altimetry crossover measurements, but instead find significant EKE increase over only one region, primarily south of New Zealand and downstream of the Campbell Plateau. We conclude that the EKE change in the Southern Ocean is not as homogenous as implied by previous studies and more work is needed to understand if this is consistent with the "eddy‐saturation" hypothesis or related more to local dynamics. Plain Language Summary: Eddy kinetic energy (EKE) represents the energy of time‐variable currents. Previous model‐based studies and observational‐based studies have suggested that increasing wind energy input into the Southern Ocean is mainly offset by increases in the EKE without significant change in the mean circulation. However, there have been no studies to quantify if this signal is coherent throughout the Southern Ocean or is regionally biased. Here, we use satellite observations and models to study surface geostrophic EKE in the Antarctic Circumpolar Current region where mean EKE is the highest. We first conduct a sampling experiment with the model to confirm that the more limited sampling of the satellite does not bias EKE when averaged over sectors of at least 30° longitude band. We then use the satellite data to show that EKE has only increased significantly over one region between 1993 and 2019. This suggests that estimates of EKE trends over the Southern Ocean are not certain as previously assumed. Key Points: Eddy kinetic energy (EKE) computed from altimetry crossovers can well represent EKE changes in the Southern OceanNo coherent increase in EKE over all areas of the Southern Ocean was identified since 1993EKE in the Southern Ocean has increased since 1993 in one region that is downstream of the Campbell Plateau [ABSTRACT FROM AUTHOR]