Sea-surface temperature forcing on a simulated South Pacific Convergence Zone

Viewed from space, one of the most prominent and robust features is the band of deep convection in the South Pacific extending southeastward, from the tropical warm pool to higher latitudes. This has been called the South Pacific Convergence Zone (SPCZ). Similar diagonal cloud zones, with less intensity, exist in the South Indian and Atlantic oceans where basin-scale surface heating gradients also drive a zonal wind basic state conducive to accumulation of mid-latitude disturbances. Many global climate models fail to simulate diagonal orientation of these convective bands and lack interaction with the mid-latitude storm track. In particular, the simulated SPCZ extends more zonally than observed in almost all coupled General Circulation Models, sometimes approaching the South American coast. Our previous work, investigating physical mechanisms that promote diagonal orientation of the SPCZ, found correlations between slowly varying sea-surface temperatures (SST), wave energy accumulation regions where zonal winds decrease with longitude (i.e., negative stretching deformation), and the observed convective band. Group speeds of eastward propagating synoptic (2-8 day period) Rossby waves decrease in the vicinity of the SPCZ, increasing the local wave energy density. Recent simulations of South Pacific climate variability using a primitive-equation model with simplified parameterizations (nicknamed SPEEDY; Molteni 2003), forced by observed monthly varying SST, shows realistic accumulation of synoptic disturbances near the mid-latitude SPCZ; however, the climatological mean of deep convection remains shifted slightly eastward. We examine, in a series of modeling experiments, sensitivity of the zonal wind basic state and location of convection to perturbations of the basin-scale SST gradient. Results show evidence that the SPCZ's spatial orientation and its longer term variability is influenced by the El Niņo-Southern Oscillation (ENSO), or alternatively, the changing background SST associated with different phases and varieties of ENSO.