Long-term warming in the Southern Ocean: Evaluating the role of air-sea exchange and the upper ocean

The Southern Ocean has undergone significant warming in recent decades. In this study we update comparisons of Argo data with historic in situ upper-ocean profiles, extending the time series back in time to 1900. Observations indicate that warming is surface intensified but has penetrated through the entire 2000-m depth range sampled by Argo. The intensified warming in the upper ocean is consistent with numerical model results and implies that air-sea exchanges may be one of the drivers of the warming trend. In order to assess the mechanisms underlying the warming, observed upper-ocean temperature variability is evaluated on seasonal-to-interannual time scales and compared with estimates of air-sea fluxes, with wind observations, and with large-scale modes of climate variability. Eddy processes are also a potential driver of ocean temperature trends, and this is assessed using satellite altimeter data. The low stratification of the Southern Ocean means that eddies detected by altimetry at the ocean surface extend through the top 2 km of the ocean. While air-sea fluxes are expected to have large impacts within the mixed layer, eddy variability shows a different picture: sea-surface-height anomalies are less correlated with variability in the upper 200 m of the ocean than they are with variability at depths between about 600 and 1400 dbars, implying that eddy processes and air-sea fluxes play distinctly different roles in the heat balance of the ocean.