资源环境科技发展态势分析平台

The U.S.-Affiliated Pacific Islands (USAPIs), located in the tropical western Pacific, are very susceptible to severe drought. Dry season (December-May) rainfall anomalies have different relationships to ENSO for USAPIs north and south of 7 degrees N. South of 7 degrees N, rainfall exhibits a canonical negative correlation with the Oceanic Nino Index (ONI) (i.e., dry conditions during warm periods). To the north, the dry season falls into either canonical or noncanonical (positively correlated with ONI) regimes. Noncanonical years pose an important forecasting challenge as severe droughts have occurred during cool ONI conditions (referred to here as cool dry cases). Composite analysis of the two regimes shows that for noncanonical cool dry years, anticyclonic circulation anomalies over the tropical western North Pacific (TWNP), with a band of anomalous dry conditions extending from the central Pacific toward Micronesia, result in unexpected droughts. In contrast, canonical cool wet events show cyclonic TWNP circulation and increased rainfall over the northern USAPIs. Maximum SST anomalies are located near the date line during noncanonical years, and farther east during canonical years. While both regimes show negative rainfall and TWNP anticyclonic circulation anomalies before the onset of the December-May dry season, during the dry season these anomalies persist during noncanonical events but rapidly reverse sign during canonical events. SST anomalies in the noncanonical regime extend eastward from the central Pacific rather than intensify in place over the eastern Pacific in the canonical regime. Differences in the evolution of circulation, precipitation, and SST anomalies suggest distinct physical mechanisms governing the two ENSO regimes, with possible ramifications for seasonal forecasts.