The coupled seasonal hindcasts of the South American monsoon

Abstract

A small set of seasonal hindcasts of the South American monsoon (SAM) from the Center for OceanLand-Atmosphere Studies (COLA) climate model is compared with a multi-decadal integration from the same model. It is shown that there are broad similarities in the December-January-February mean precipitation bias over continental South America, which comprise of the dry (wet) bias over central Amazon, northern South America (central Andes and south-east Brazil) in the seasonal hindcasts and in the long-term simulation. Furthermore, both types of model integrations underestimate (overestimate) the strong (weak) precipitation events over the core region of the SAM. In terms of interannual variability, seasonal hindcasts and the multi-decadal simulation show reasonably similar results. However, the spin-down of the moisture field in the seasonal hindcasts is evident in its distribution of the daily precipitation over the SAM core region. It is also found that in the multi-decadal integration owing to model drift, the Sea Surface Temperature (SST) gradient in the Atlantic Ocean north of the equator is so reduced that results in a weaker low-level cross-equatorial flow into the SAM region compared to either the seasonal hindcasts or the National Centers for Environmental Prediction (NCEP) reanalysis.We then repeated the coupled seasonal hindcasts separately with higher horizontal and vertical resolutions of the atmospheric general circulation model (AGCM) to assess their impact on the SAM climate and its variations. It is shown that a higher horizontal resolution does not significantly improve the precipitation climatology and its interannual variability over continental South America. However, there are some marginal improvements in the frequency of occurrence of heavy precipitation events in the core region of the SAM at higher horizontal resolutions. The higher vertical resolution of the AGCM did not show a comparable benefit as the higher horizontal resolution on the SAM precipitation. However, some of the largest precipitation errors, especially over the north-eastern part of continental South America, over the Atlantic inter-tropical convergence zone and over the central Andes remain in all resolution experiments conducted in this study.