Thermodynamic Mechanisms Responsible for the Tropospheric Response to SST Anomalies in the Antarctic Circumpolar Wave

Abstract

Tropospheric temperature and vorticity budgets for the Antarctic Circumpolar Wave (ACW) are diagnosed utilizing the National Centers for Environment Prediction?National Center for Atmospheric Research reanalysis datasets from 1983 to 1992, focusing on the eastern Atlantic, Indian, and western and central Pacific sectors of the Southern Ocean where remote forcing from the Tropics has been observed to be weak. There, warm sea surface temperature (SST) anomalies are found in the ACW propagating eastward together with anomalous upward latent heat flux, positive precipitation, low-level convergence, upper-level divergence, midlevel ascent, and poleward surface wind. Diagnosing the anomalous temperature budget finds SST-induced latent heat flux instigating anomalous mid- to upper-level diabatic heating and low-level diabatic cooling in the absence of significant eddy heat flux divergence. This diabatic heating profile is balanced by a combination of vertical and horizontal heat advection, giving rise to anomalous ascent and poleward wind throughout the column. The thermodynamics of this deep diabatic heating scenario are different from those of Palmer and Sun. An intrinsic feedback from atmosphere to ocean is indicated by reduced sensible-plus-latent heat flux displaced 45° to 90° of phase to the east of warm SST anomalies, yielding an anomalous SST warming tendency that contributes both to eastward phase propagation and amplitude maintenance of the ACW. Diagnosing the anomalous potential vorticity budget finds the vertical gradient of anomalous diabatic heating, negative over most of the column, balanced by the anomalous advection of planetary vorticity, the mean advection of anomalous relative vorticity, and net vortex tube advection, together yielding a poleward equivalently barotropic wind response to warm SST anomalies. This deep diabatic heating scenario is contrasted against the remote forcing scenario in the eastern Pacific and western Atlantic sectors of the Southern Ocean where remote forcing associated with the El Niño?Southern Oscillation (ENSO) in the Tropics can now be seen to drive the SST tendency in the ACW.