A multiscale framework for the origin and variability of the South Pacific Convergence Zone

Abstract

AbstractThe diagonal South Pacific Convergence Zone (SPCZ) is the major climatological precipitation feature over the Pacific region during the Northern Hemisphere winter. However, the basic mechanisms that control its structure and variability are only partly understood. Here, an analysis of the SPCZ is carried out in a multiscale framework. This identifies two modes that dominate: a (westward) shifted SPCZ and an enhanced SPCZ, which occur independently of each other. Within both modes, the primary mechanism for the initiation of precipitation is a transient synoptic wave propagating along the subtropical jet, which is then refracted by the basic state toward the westerly duct over the central equatorial Pacific. Individual vorticity centres in the wave become elongated, with a diagonal (northwest–southeast) tilt. Convection then occurs in a diagonal band in the poleward flow ahead of the cyclonic vorticity anomaly in the wave. However, latent heat release in the convection leads to upper‐tropospheric divergence and anticyclonic vorticity forcing, which dissipates the wave, shutting off the convective forcing and stopping the precipitation. Hence, each individual wave or event only lasts a few days and contributes a discrete pulse of diagonally oriented precipitation to the region. The sum of these events leads to the diagonal climatological SPCZ. Event occurrence is a stochastic process, the probability of which is modified by lower‐frequency variability of the basic state, including the Madden–Julian Oscillation (MJO) and El Niño–Southern Oscillation (ENSO). For example, during periods of enhanced convection over the eastern Indian Ocean to western Pacific (MJO phases 3–6 and La Niña) the westerly duct expands westwards, allowing synoptic waves to refract equatorwards earlier and increasing the probability of westward‐shifted SPCZ events. Hence, both the existence and variability of the SPCZ depend fundamentally on scale interactions between dynamical processes on time‐scales ranging from daily to interannual. Copyright © 2011 Royal Meteorological Society