Abstract Author: Alice M. Grimm and Marcia T. Zilli
Abstract Title: Predictability of South American Summer Monsoon Precipitation from Previous Conditions in Spring
Abstract: The variability of precipitation in Central-East Brazil in summer is shown to have significant inverse relationship with precipitation in spring, which reflects on the relationship between the first variability modes for spring and summer precipitation in South America. In spring the first mode is dipole-like, with opposite loadings over central-east and southeast South America. It is connected with ENSO. The leading mode of summer also features dipole-like oscillations between central-east and southeast South America, but is not strongly connected with ENSO. A significant relationship is disclosed between these first dipole-like modes of spring and summer rainfall and thus between the rainfall in spring and summer over central-east South America, which is part of the monsoon core region. These dipole-like modes are associated with a rotational circulation anomaly over southeast Brazil that either conveys moisture flux into central-east Brazil (if it is cyclonic) or into southeastern South America (if it is anticyclonic). In spring this anomaly seems to be remotely forced, but after strong rainfall anomalies over central-east Brazil in this season, it tends to reverse sign in peak summer, inverting the dipole-like rainfall anomalies. This reversal is hypothesized to be locally forced by surface-atmosphere feedback triggered by the spring anomalies, as weaker teleconnections in summer allow local processes that are stronger in this season to overcome remote forcing. SST and circulation anomalies associated with the first modes in spring and summer and also the relationship between the first summer mode and surface temperature in spring are consistent with this hypothesis.