An observational study of the South Pacific Convergence Zone using satellite and model re-analysis data

Abstract

Satellite derived wind, rain rate and sea surface temperature data combined with NCEP analysis data are used to examine the structure of the South Pacific Convergence Zone (SPCZ) during La Nina conditions. Annual means indicate that the axes of maximum surface convergence and rain rates are essentially co-located in the Northern Hemisphere Inter-Tropical Convergence Zone and over the tropical (TR) portions of the SPCZ. However, over subtropical (ST) and middle latitudes (ML), the axis of maximum convergence is eastward and equatorward of the axis of maximum rain rates such that surface divergence predominates over the these portions of the SPCZ rain rate maximum. Analysis of NCEP data also exhibit a similar convergence distribution at 1000 hPa; however, at the 925, 850 and 700 hPa levels convergence replaces divergence and is co-located with the axis of maximum rain rates. Seasonal rain rate patterns indicated that the TR and ST SPCZ are more convectively active during Austral Summer and Fall while the ML SPCZ is more active during the Fall and Winter. Vertical wind shear and the variability of the meridional flow indicate the ML SPCZ is strongly associated with the mid-latitude storm track while the TR SPCZ is associated with low wind shear. The ST SPCZ represents a transition zone between an environment dominated by low shear and one that is more baroclinic. Lag correlation analysis using rain rate, 200 hPa divergence and height anomaly data as well as composite analysis of high rain fall events revealed wave like patterns over the ST and ML SPCZ. The estimated wavelengths range from 4700 to 5500 km, values that correspond to wave numbers 7 and 8; individual anomalies tend to move eastward at speeds of 5 to 7 m/s. The vertical structure of the height anomalies suggest baroclinic dynamics are important to the development of the mean rainfall pattern in the ML SPCZ and to a lesser extent the ST SPCZ. During high rain rate events, only a given portion of the SPCZ, approximately 3000 to 4000 km in length, is convectively active.