A Framework for the Origin of the Diagonally Oriented South Atlantic and South Pacific Convergence Zones

Abstract:

We present a conceptual framework for the diagonal orientation of the South Pacific Convergence Zone (SPCZ) and South Atlantic Convergence Zone (SACZ), based on observations of tropical-extratropical interactions, Rossby wave theory, and vorticity calculations. Wave trains propagate eastward along the Southern Hemisphere subtropical jet, with initially quasi-circular vorticity centres. In the zonally sheared environment on the equatorward flank of the jet, these vorticity centres become elongated and develop a northwest-southeast tilt. Ray tracing diagnostics in a non-divergent, barotropic Rossby wave framework then explain the observed equatorward propagation of these diagonal vorticity structures toward the westerly ducts over the equatorial Pacific and Atlantic. The baroclinic component of these circulations leads to ascent and destabilisation ahead of the cyclonic vorticity anomaly in the wave. Over the high sea surface temperatures in this region, deep convection is triggered. Latent heat release forces ascent and strong upper-tropospheric divergence, with an associated anticyclonic vorticity tendency. In a vorticity budget, this cancels out the advective cyclonic vorticity tendency in the wave train over the SPCZ, and dissipates the wave within a day. The mean SPCZ is then comprised of the sum of these pulses of diagonal bands of precipitation. These mechanisms also operate in the SACZ. However, the vorticity anomalies in the wave trains are stronger, and the precipitation and negative feedback from the divergence and anticyclonic vorticity tendency are weaker, resulting in continued propagation of the wave and a more diffuse diagonal convergence zone.