South Pacific Convergence Zone Variability and Biases in Models

Ben Lintner is the atmospheric sciences graduate program director at Rutgers as well as an associate editor of Journal of Climate.  His October 27, 2014 presentation at Yale focused on little understood fundamental aspects of the South Pacific Convergent Zone, (SPCZ), an area of intense deep convection and low-level convergence extending southeastward from the western Pacific warm pool into Southern Hemisphere mid-latitudes that is a dominant feature of the tropical Pacific. One theorized control on the position of the SPCZ is the amount of low-level inflow from the relatively dry southeastern Pacific basin. Building on the analysis of observed SPCZ-region synoptic scale variability by Lintner and Neelin (2008), composite analysis is performed here on two reanalysis products as well as output from 17 models in phase five of the Coupled Model Inter-comparison Project (CMIP5). Using low-level zonal wind as a compositing index, it is shown that the CMIP5 ensemble mean, as well as many of the individual models, captures patterns of wind, specific humidity, and precipitation anomalies resembling those obtained for reanalysis fields between strong- and weak-inflow phases. Lead-lag analysis of both the re-analyses and models is used to develop a conceptual model for the formation of each composite phase. This analysis indicates that an equator ward displaced Southern Hemisphere storm track and an eastward displaced equatorial eastern Pacific westerly duct are features of the weak-inflow phase, though as indicated by additional composite analyses based on these features, each appears to account weakly for the details of the low-level inflow composite anomalies. Despite the presence of well-known biases in the CMIP5 simulations of SPCZ region climatologies, the models appear to have some fidelity in simulating synoptic scale relationships among low-level winds, moisture, and precipitation, consistent with observations and simple theoretical understanding of interactions of dry air inflow with deep convection.

January 5, 2015