Recorded presentation4B.2
A general theory for the spatial and transient behavior of the South Pacific Convergence Zone
Matthew Widlansky, Georgia Institute of Technology, Atlanta, Georgia; and P. J. Webster
The South Pacific Convergence Zone (SPCZ) is a band of deep convection and dynamic activity that extends in a southeasterly direction from the equatorial warm pool to the central higher latitude South Pacific Ocean. Atmospheric convection associated with the SPCZ significantly impacts large-scale circulation patterns over the Pacific, influencing climate variability on intraseasonal to decadal timescales. The SPCZ and its variability remains poorly represented in climate models. The principal concern is to understand physically why the SPCZ contains a pronounced orientation and why transients of the SPCZ have significantly longer periods at low latitudes than those at higher latitudes. General circulation models (GCMs) provide few clues due to a bias towards simulating a double Intertropical Convergence Zone (ITCZ) in the central and eastern Pacific.
We present an analysis of the dynamics responsible for the temporal and spatial variability of the SPCZ. The basis of the hypothesis is the manner in which variations of the background basic flow formed by the slowly varying sea-surface temperature (SST) form regions of negative stretching deformation (dU/dx) that alter the group speed and the scale of higher frequency transient waves causing modifications in group speed, scale, and energy density. Regions of convection in both the equatorial and subtropical portions of the SPCZ are shown to be associated with dU/dx<0. These associations are confirmed by TRMM precipitation rates and NCEP reanalysis. Band-pass filtering shows that energy of mid-latitude wave trains accumulates in the region of slowly varying negative stretching deformation, leading to enhanced convection. Nearer the equator, larger scale and lower period equatorial modes accumulate energy in a location corresponding to the northwest sector of the SPCZ. Interannual variability of the large-scale zonal wind pattern is also strongly correlated with location of the SPCZ, suggesting an interaction with ENSO, or more likely, the changing background SST associated with the different phases of ENSO. We conclude with an analysis of a simulated SPCZ, using the Weather Research and Forecasting (WRF) model initialized with idealized boundary conditions, in an effort to measure the sensitivity of tropical and extratropical wave accumulation to basin-scale SST patterns. From these analyses comes a more general theory of the SPCZ accounting for its spatial orientation, its temporal variability between low and high latitudes, and its longer term variability.
Session 4B, Basin-Scale Forcing of Synoptic Weather Patterns
Tuesday, 13 January 2009, 8:30 AM-9:45 AM, Room 128B
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