Sea-level pressure patterns in South America and the adjacent oceans in the IPCC AR4 models

A. DI LUCA; CAMILLONI, INÉS; V.BARROS

Foz de Iguazú

Conferencia; 8th International Conference on Southern Hemisphere Meteorology and Oceanography; 2006

American Meteorological Society

The objective of the present work is to evaluate the ability of a set of global climate models available for the preparation of the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC) to represent the predominant patterns of sea level pressure (SLP) over an extense region of the Southern Hemisphere that comprises Southern South America and the South Atlantic and South Pacific oceans. Monthly mean SLP fields derived from the different models, some of them with more than one integration differing only in initial conditions, were compared with the NCEP/NCAR reanalyses for the period 1978/2000. This relatively short period was selected because the reanalyses are more representative of real data over the oceanic areas only after 1978 when satellite data began to be fully used. Monthly correlation coefficients between the output from the models and the NCEP/NCAR reanalyses show that all of them represent adequately the SLP fields during most of the year with the lower correlations during the austral autum (March-April-May). The only exception is one of the realizations of the ECHAM5/MPI-OM that shows poor correlations and was discarded for the following analyses. A T-mode principal component (PC) analysis of the monthly mean SLP obtained from NCEP/NCAR reanalyses shows that there are three predominant patterns that characterize SLP fields over the region. The first one (PC1) that explains 43.2% of the variance, represents the summer surface circulation with the South Atlantic and South Pacific highs in their southernmost positions. The second and third patterns that explain respectively 27.3% and 14.1% of the total variance, represent the winter circulation with the South Atlantic and South Pacific in the northernmost position (PC2) and a surface pattern that characterizes the frontal activity during that season (PC3). Temporal series of the three PCs show a positive linear trend of the summer mode and slightly negative trends for the winter ones. The same PC analysis was performed to the output of the eight global climate models that better represent the monthly mean SLP fields. Results show that most of them represent the same dominant patterns. Linear correlation coefficients between the PCs derived from the NCEP/NCAR reanalyses and the model output show that the models with the best performance are the IPSL-CM4, GFDL CM2.0, UKMO-HadCM3 and ECHAM5/MPI-OM. Nevertheless only few of these models are able to represent the observed linear trends in the temporal series that shows a slight increase of the summer mode at expense of the winter ones.