Evolution of the Southeast Pacific ITCZ in Boreal Spring as viewed from SSM/I and SSM/T-2

Abstract

SSM/I and SSM/T-2 microwave radiances are used to document the evolution of the Southern Intertropical Convergence Zone (SITCZ) during Boreal Spring. Combining the SSM/1's Total Precipitable Water (TPW) product with SST for 1993-1995, a Moisture Convergence Parameter (MCP) is defined which removes the total TPW due to local thermodynamic effects. The residual is used to document the dynamic effects of the large scale circulation on the ITCZ. The MCP describes a dynamically-induced ITCZ which forms along 7'S over the eastern Pacific as a near mirror image of its northern counterpart. The evolution of the 1995 Southern ITCZ is subdivided into the developing, mature and dissipating phases, as defined by MCP. In the developing phase, convection occurs in the disorganized moist wake of large scale easterly wave passage as detected by SSM/T-2 upper tropospheric microwave moisture channels. East of the wave, convection organizes along two lines (NITCZ and SITCZ) coupled with a SSM/T-2 nearline channel signal indicating drying regions poleward of the lines; which is not present before the wave passage. While convective systems along the Northern ITCZ experience no significant increase in monthly frequency in the developing phase, they increase fourfold along the Southern ITCZ. Large scale features such as warming SSTS, a double maximum in meridional SSTS, slackening southeast trades and an anomalous jet at 200 hPa are all present during the developing and mature phases of the Southern ITCZ. Only three weeks after commencing development, the Southern ITCZ moist band in SSM/I radiances collapses and the last convective systems along the Southern ITCZ dissipate. The large scale descent signal in SSM/T-2 brightness temperatures surges northward and the NITCZ once again dominates the eastern Pacific domain. The phenomenon of SITCZ evolution is summarized in a conceptual model.