The Antarctic Circumpolar Wave: A Beta Effect in Ocean–Atmosphere Coupling over the Southern Ocean

Abstract

The Antarctic circumpolar wave (ACW) is a nominal 4-yr climate signal in the ocean–atmosphere system in the Southern Ocean, propagating eastward at an average speed of 6–8 cm s−1, composed of two waves taking approximately 8 years to circle the globe. The ACW is characterized by a persistent phase relationship between warm (cool) sea surface temperature (SST) anomalies and poleward (equatorward) meridional surface wind (MSW) anomalies. Recently, White and Chen demonstrated that SST anomalies in the Southern Ocean operate to induce anomalous vortex stretching in the lower troposphere that is balanced by the anomalous meridional advection of planetary vorticity, yielding MSW anomalies as observed. In the present study, the authors seek to understand how this atmospheric response to SST anomalies produces a positive feedback to the ocean (i.e., an anomalous SST tendency displaced eastward of SST anomalies) that both maintains the ACW against dissipation and accounts for its eastward propagation. To achieve this, we couple a global equilibrium climate model for the lower troposphere to a global heat budget model for the upper ocean. In the absence of coupling, the model Antarctic Circumpolar Current (ACC) advects SST anomalies from initial conditions to the east at speeds slower than observed, taking 12–14 years to circle the globe with amplitudes that become insignificant after 6–8 years. In the presence of coupling, eastward speeds of the model ACC are matched by those due to coupling, together yielding a model ACW of a nominal 4-yr period composed of two waves that circle the globe in approximately 8 years, as observed. Feedback from atmosphere to ocean works through the anomalous zonal surface wind response to SST anomalies, yielding poleward Ekman flow anomalies in phase with warm SST anomalies. As such, maintenance of the model ACW is achieved through a balance between anomalous meridional Ekman heat advection and anomalous sensible-plus-latent heat loss to the atmosphere. This balance requires the alignment of covarying SST and MSW anomalies to be tilted into the southwest–northeast direction, which accounts for the spiral structure observed in global SST and sea level pressure anomaly patterns around the Southern Ocean. Eastward coupling speeds of the model ACW derive from a beta effect in coupling that displaces a portion of the anomalous meridional Ekman heat advection, and its corresponding anomalous SST tendency, to the east of SST anomalies. Therefore, the ACW is an example of self-organization within the global ocean–atmosphere system, depending upon the spherical shape of the rotating earth for its propagation and the mean meridional SST gradient for its maintenance, and producing a net poleward eddy heat flux in the upper ocean that tends to reduce this mean gradient.