Estimates of Air–Sea Feedbacks on Sea Surface Temperature Anomalies in the Southern Ocean

Abstract

Sea surface temperature (SST) air-sea feedback strengths and associated decay time scales in the Southern Ocean (SO) are estimated from observations and reanalysis datasets of SST, air-sea heat fluxes, and ocean mixed layer depths. The spatial, seasonal, and scale dependence of the air-sea heat flux feedbacks is mapped in circumpolar bands and implications for SST persistence times are explored. It is found that the damping effect of turbulent heat fluxes dominates over that due to radiative heat fluxes. The turbulent heat flux feedback acts to damp SSTs in all bands and spatial scales and in all seasons, at rates varying between 5 and 25 W m 22 K 21 , while the radiative heat flux feedback has a more uniform spatial distribution with a magnitude rarely exceeding 5 W m 22 K 21 . In particular, the implied net air-sea feedback (turbulent 1 radiative) on SST south of the polar front, and in the region of seasonal sea ice, is as weak as 5-10 W m 22 K 21 in the summertime on large spatial scales. Air-sea interaction alone thus allows SST signals induced around Antarctica in the summertime to persist for several seasons. The damping effect of mixed layer entrainment on SST anomalies averages to approximately 20 W m 22 K 21 across the ACC bands in the summer-to-winter entraining season and thereby reduces summertime SST persistence to less than half of that predicted by airsea interaction alone (i.e., 3-6 months).

Fast and slow responses of Southern Ocean sea surface temperature to SAM in coupled climate models

“…Thus we infer from the observed climatology that the step response function of the real SO crosses over from negative to positive SST anomalies on a timescale of at least 5 years, possibly several decades, after a hypothetical step-increase in the SAM. Using a more direct approach based on an observationally-constrained model of the upper SO, (Hausmann et al 2016) evaluate the response of SO SST to SAM and also predict a long crossover timescale in agreement with our result.…”

“…(4) and (5). A recent estimate of the air-sea feedback strength in the SO by Hausmann et al (2016) can provide guidance in the further assessment of modeled air-sea feedbacks and the possible impact of inter-model differences on the response to SAM.…”

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

“…We will thereby focus only on the turbulent contribution (by latent and sensible heat fluxes) to α net = α turb + α rad . As established previously for both NH and the SO (Hausmann et al 2016), α turb typically dominates the feedback. We will simply denote it α in the following (dropping the subscript).…”

“…1, several studies have produced estimates of the turbulent heat flux feedback α in the Northern Hemisphere (e.g., Frankignoul and Kestenare 2002;Park et al 2005) and recently an estimate has become available also for the SO (Hausmann et al 2016). Figure 2 displays an estimate of α obtained by applying the method described in this latter study (as outlined also in Appendix 1) to the ERA-I dataset, for both the NA and the SO.…”

“…In a recent study, Hausmann et al (2016) provide a benchmark calculation for the circumpolar SO, thereby complementing the previous observational estimates of α for the midlatitude ocean basins of the Northern Hemisphere and the low-latitude Southern Hemisphere (Frankignoul et al 1998;Frankignoul and Kestenare 2002;Park et al 2005). These studies have highlighted marked variations in α net over the world's major Abstract The turbulent air-sea heat flux feedback (α, in W m −2 K −1 ) is a major contributor to setting the damping timescale of sea surface temperature (SST) anomalies.…”