The seasonally-varying influence of ENSO on rainfall and tropical cyclone activity in the Philippines

An observational study covering the period 1950-2002 examines a seasonal reversal in the ENSO rainfall signal in the north-central Philippines. In boreal Summer of El Nino (La Nina) events, above (below) average rainfall typically occurs in this area. Rainfall anomalies of opposite sign develop across the country in the subsequent fall. This study investigates the seasonal evolution of the anomalous atmospheric circulation over the western North Pacific (WNP) during both El Nino and La Nino and places these features in the context of the large-scale evolution of ENSO events, including an analysis of changes in tropical cyclone activity affecting the Philippines. The results show that during boreal summer of El Nino (La Nina) events, a relatively narrow, zonally elongated band of enhanced (reduced) low-level westerlies develops across the WNP which serves to increase (decrease) the summer monsoon flow and moisture flux over the north-central Philippines and is associated with an increase (decrease) in the strength of the WNP monsoon trough via the anomalous relative vorticity. Tropical cyclone activity is shown to be enhanced (reduced) in the study region during boreal summer of El Nino (La Nina) events, which is related to the increase (decrease) of mid-level atmospheric moisture, as diagnosed using a genesis potential index. The subsequent evolution shows development of an anomalous anticyclone (cyclone) over the WNP in El Nino (La Nina) and the well-known tendency for below (above) average rainfall in the fall. Prolonged ENSO events also exhibit seasonal rainfall sign reversals in the Philippines with a similar evolution in atmospheric circulation.