Long-term Variability in ITCZ Characteristics from an Automated Objective Identification Algorithm

Abstract:

The intertropical convergence zone (ITCZ) is one of the most important features of the tropics, supplying large amounts of latent heat to the atmosphere and driving the ascending branch of the Hadley circulation. Although there have been many studies defining the climatological location of the ITCZ and the strength of general tropical precipitation, there has been less work quantifying the extent and intensity of ITCZ convection. These characteristics of the ITCZ are extremely important as recent work suggests a remote influence of subtropical moisture on the ITCZ. Many recent studies of general circulation models (GCMs) exhibit the “rich-get-richer” mechanism, with tropical precipitation increasing and subtropical precipitation decreasing. As a first step in studying feedbacks between the tropics and subtropics using observations, an automated, objective ITCZ identification and characterization algorithm is developed and applied to ECMWF Reanalysis Interim (ERA-Interim) variables, and TRMM Microwave Imager (TMI) and Global Precipitation Climatology Project (GPCP) rain rates (RRs) to create 15- and 36-year climatologies in the tropical Pacific ocean. Characteristics derived from the algorithm include ITCZ latitude, northern and southern ITCZ convection boundaries, ITCZ extents, and precipitation intensity. Preliminary results show a climatological ITCZ location consistent with previous studies (8°N), while long-term trends in the width and ITCZ precipitation intensity show narrowing and intensification, especially in the central Pacific. The intensification is much stronger near the center of the ITCZ, so a census of precipitation features (PFs) across the ITCZ is also planned using the TRMM Precipitation Radar (PR) PF database. PFs will be analyzed over the entire ITCZ, in the center, and along ITCZ edges to determine how changes in the characteristics of convection drive the observed changes in ITCZ precipitation.