Contrary to traditional thinking that the atmosphere drives sea surface temperature and surface heat flux variability in the mid-latitudes, researchers from the US have shown that where surface temperature gradients are strong, the ocean drives this variability. The finding highlights the importance of resolving internal oceanic processes as well as atmosphere-ocean interactions in models.
Stuart Bishop from North Carolina State University and his collaborators at the National Center for Atmospheric Research showed that in regions away from these strong mean sea surface temperature gradients, the traditional idea of atmospheric forcing still holds.
Oceanic weather drives the surface temperature variability of the Western Boundary Currents and the Southern Ocean Antarctic Circumpolar Current, particularly the Agulhas Return Current, Bishop and the team found. By dampening the sea surface temperature anomalies generated by eddy stirring using spatial and temporal smoothing, they showed that the oceanic influence on sea surface temperature variability increases with time scale but decreases with spatial scale. These regions transition from ocean- to atmosphere-driven at spatial scales less than 500 km, but this figure varies widely geographically.
This result is in contrast to previous work, where the mid-latitude sea surface temperature variability was atmosphere-driven and the ocean was passive. Many of these earlier studies, however, used rather coarse resolution observational estimates of sea surface temperature and surface heat flux, or coarse coupled global climate models. So ocean eddies were not resolved – the researchers involved noted this limitation, finding their results only applicable to regions away from strong oceanic currents.
Many scientists feel it’s important to understand ocean-atmosphere energy exchange and to capture it in climate models, to represent both the mean state and variability of the climate system accurately. This study highlights the value in precise and detailed observational estimates as well as the importance of using eddy-resolving models to simulate ocean-atmosphere interaction, especially in regions with strong sea-surface temperature gradients.
The researchers used state-of-the-art surface heat flux and ocean temperature observational estimates. A simple energy balance model of coupled air-sea interaction and a lagged correlation between ocean temperature and surface heat fluxes helped discriminate between atmosphere-driven and ocean-driven variability.
Bishop and colleagues published their work in Journal of Climate.
