View metadata, citation and similar papers at core.ac.uk brought to you by CORE Quarterly Journal of the Royal Meteorological Society Q. J. R. Meteorol. Soc. (2012) provided by University of East Anglia digital repository A multiscale framework for the origin and variability of the South Pacific Convergence Zone Adrian J. Matthews* School of Environmental Sciences and School of Mathematics, University of East Anglia, Norwich, UK *Correspondence to: A. J. Matthews, School of Environmental Sciences, University of East Anglia, Norwich Research Park, Norwich, NR4 7TJ, UK. E-mail:
[email protected] The diagonal South Pacific Convergence Zone (SPCZ) is the major climatological precipitation feature over the Pacific region during the Northern Hemisphere win- ter. 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 vortic- ity 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.