Response of the atmosphere–ocean mixed‐layer system to anomalous ocean heat‐flux convergence

We investigate the role of the extratropical oceans in climate variability. Rather than following the traditional approach of studying the atmospheric response to prescribed sea‐surface‐temperature (SST) anomalies, we focus on the role of anomalous ocean heat transport. We employ an atmosphere model coupled to a fixed‐depth (‘slab’) ocean mixed‐layer model, forced by a field of ocean heat‐flux convergence (OHFC). We study the response of this coupled atmosphere–ocean mixed‐layer system to an idealized OHFC anomaly. We find that the OHFC forcing is mainly balanced by anomalous surface latent‐heat flux, i.e. enhanced evaporation. Notably, the anomalous SST is not co‐located with the forcing, nor with the anomalous air–sea flux, nor with the anomalous diabatic heating. Contrary to conventional wisdom, in our experiment the influence of the extratropical ocean on the atmosphere is not manifest primarily in an SST signature. The anomalous diabatic heating is associated with a downstream surface low‐pressure anomaly, in line with the simple theory of the steady response to extratropical heating. Transient‐eddy anomalies play only a minor role. The principal SST anomaly appears to be a passive response to this anomalous atmospheric flow. Our results suggest that the traditional narrow focus on SST as the key variable involved in ocean–atmosphere interaction may offer only a limited, perhaps even misleading, guide to the influence of the extratropical oceans on climate variability.