Simulation capability of tropical and extratropical seasonal climate anomalies over South America

Abstract

An ensemble of 20 extended integrations of the atmospheric model CSIRO Mark 2, forced with the sea-surface temperature observed during the 1986–1998 period, was performed to analyze the simulation capability of seasonal climate anomalies over South America and adjacent oceanic areas. Variations of the simulation skill within the region and during the experimental period were assessed through standard statistical measures and compared to the signal-to-noise ratio distribution. Before the skill assessment, model systematic errors were thoroughly evaluated. The results confirm that the simulation skill is very high in tropical oceanic areas, and decreases rapidly towards middle and high latitudes. Model performance at mid and high atmospheric levels is substantially better than at low levels. Relatively high simulation capability was found over the Pacific Ocean between the equator and the Antarctic coast, which is coherent with the presence of three relative maximums in the signal-to-noise ratio, similar to the increase of the forced variance found by several authors over much of the Pacific–North American pattern region. Rainfall rate and second-order moments associated with the cyclonic activity and the meridional eddy fluxes of heat and humidity are better simulated in a narrow strip parallel to the SPCZ and extending further southeast into mid latitudes of the continent. The simulation skill noticeably improves during the warm and cold ENSO phases, in correspondence with an intensification of the signal-to-noise ratio, and useful rainfall anomaly simulations can be obtained over the Amazonas and Rio de la Plata river basins.