Sack Lunch Seminar (SLS)

We present an observational estimate of the signature of the ocean's transient mesoscale eddy field in sea surface temperature (SST), a variable that determines the scope not only for transient (mesoscale) ocean-atmosphere coupling in midlatitudes, but also for any systematic eddy fluxes of heat through the surface mixed-layer. The estimate is provided for the North Atlantic and the Southern Ocean by analysing the relationship between satellite microwave SST and multi-altimeter sea surface height (SSH) both at fixed locations and following eddy tracks.

Following eddy tracks reveals systematic SST signals throughout the eddy lifetimes, observed not only in energetic regions of large SSH variability associated to the Gulf Stream and the ACC, but also in quiescent basin interiors. These are shown to be consistent with simple constraints for the strength of the mesoscale air-sea heat flux feedback derived from in-situ observations and climatology. At fixed-locations, to the contrary, MW-SST variability is well described by the classical paradigm of large-scale midlatitude air-sea interactions and a zero-order contribution of the mesoscale is limited to narrow windows around major oceanic frontal zones. This regional, and also a pronounced seasonal modulation, in the nature of MW-SST variability can be understood in terms of a competition between large-scale atmospheric forcing and mixed-layer eddy isotherm stirring.

The track-following analysis moreover reveals a systematic westward phase shift of eddy SST anomalies with respect to rotating eddy pressure cores, in both quiesent and energetic regions. Although eddy shedding from major currents is appealing as a mechanism for heat transport ("drift'' heat transport), we find that the poleward (equatorward) motion of warm anticyclones (cold cyclones) produces a poleward heat transport in the mixed layer that is about an order of magnitude weaker than that resulting from the observed westward phase shift between SST and SSH fluctuations ("swirl'' heat transport). Mixed-layer eddy diffusivities for heat thus largely reflect the eddy swirl heat transport, and eddy SST signatures are found crucial in setting their magnitude and spatial variations. Simple scaling results suggest an implied poleward mixed-layer eddy heat transport across the ACC region on the order of 0.1 PW.