Mechanisms controlling the SST air - sea heat flux feedback 1 and its dependence on spatial scale

The turbulent air-sea heat flux feedback (α, in W m−2 K−1) is a major 7 contributor to setting the damping timescale of sea surface temperature (SST) 8 anomalies. In this study we compare the spatial distribution and magnitude of 9 α in the North Atlantic and the Southern Ocean, as estimated from the ERA10 Interim reanalysis dataset. The comparison is rationalized in terms of an upper 11 bound on the heat flux feedback, associated with “fast” atmospheric export of 12 temperature and moisture anomalies away from the marine boundary layer, and a 13 lower bound associated with “slow” export. It is found that regions of cold surface 14 waters (≤10◦ C) are best described as approaching the slow export limit. This 15 conclusion is not only valid at the synoptic scale resolved by the reanalysis data, 16 U. Hausmann · J. Marshall Department of Earth Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA E-mail: uhaus at mit dot edu A. Czaja Department of Physics, Imperial College London, UK 2 Hausmann et al. but also on basin scales. In particular, it applies to the heat flux feedback acting 17 as circumpolar SST anomaly scales are approached in the Southern Ocean, with 18 feedbacks of ≤10 W m−2 K−1. In contrast, the magnitude of the heat flux feed19 back is close to that expected from the fast export limit over the Gulf Stream and 20 its recirculation with values on the order of ≈ 40 W m−2 K−1. Further analysis 21 suggests that this high value reflects a compensation between a moderate thermo22 dynamic adjustment of the boundary layer, which tends to weaken the heat flux 23 feedback, and an enhancement of the surface winds over warm SST anomalies, 24 which tend to enhance the feedback. 25