Distinct impacts of two types of La Nina events on Australian summer rainfall

摘要

This study investigates impacts of two types of La Nina events, eastern Pacific (EP) La Nina and central Pacific (CP) La Nina, on Australian summer rainfall during 1951-2009. Results show that Australian summer rainfall is sensitive to the change in the location of sea surface temperature (SST) anomalies in equatorial Pacific. During CP La Nina, maximum cold SST anomaly is located in the equatorial CP west of 150 degrees W, and significant northeasterly wind anomalies tend to prevail over northeastern Australia during austral summer. This brings more moist and warm flow from the tropics to Australia and leads to significant positive rainfall anomalies over northern and eastern Australia. In contrast, during EP La Nina, maximum cold SST anomaly is confined to equatorial EP east of 150 degrees W and atmospheric circulation anomalies tend to be weak. As a result, rainfall anomalies are generally weak over Australia in EP La Nina. The differences in the Australian summer rainfall anomalies between CP La Nina and EP La Nina are attributed to the differences in atmospheric circulation anomalies. Specifically, the atmospheric circulation anomalies over tropical Pacific tend to be stronger and located more westward in CP La Nina. Higher climatological SST in the equatorial CP than equatorial EP, larger magnitude and westward shift of cold SST anomaly centre in CP La Nina than EP Nina may explain stronger and westward shift of the atmospheric anomalies in CP La Nina. Atmospheric model numerical experiments confirm the contribution of stronger circulation response in CP La Nina to the positive rainfall anomalies in Australia. Results in this study suggest that it is important to classify the La Nina events into different types when predicting Australian summer rainfall.