Effects of two different La Nina types on the South American rainfall

摘要

The paper tests if the two La Nina (LN) types can be distinguished from each other using their climate impacts on northeastern Brazil (NEB). To this end, all LN events during the 1901-2010 period followed by a wet or dry rainy season (from February to April) in NEB are classified into two categories: WET-LN and DRY-LN. The global and regional anomalous atmospheric circulation patterns and the rainfall anomaly patterns in South America associated with the two cases are analysed. The WET-LN and DRY-LN events present, respectively, the eastern Pacific LN and central Pacific LN sea surface temperature (SST) anomaly features in the tropical Pacific. On the other hand, the WET-LN features an interhemispheric SST dipole pattern in the tropical Atlantic, and the DRY-LN, colder-than-normal surface waters in the tropical South Atlantic (TSA) and equatorial Atlantic during MAM(+1). The two analysed cases show regional differential circulation patterns in all seasons. The anomalous wetness over northern and northwestern South America occurs for the WET-LN type during JJA(0)-DJF(+1) and for the DRY-LN type, during DJF(+1)-MAM(+1). The anomalous dryness over SESA is more evident for the WET-LN during JJA(0) and MAM(+1), and for the DRY-LN, during SON(0). Anomalous dryness occurs in central and eastern South America noted during JJA(0) and DJF(+1) for both cases analysed. The precipitation anomalies in northern South America during DJF(+1) are stronger and more extensive for the DRY-LN than for the WET-LN events due to the action of both the anomalous (double) Walker and Hadley cells for the DRY-LN, in contrast with the exclusive action of an anomalous Walker cell for the WET-LN case. Also, a double Walker cell drives the dry-wet dipole between northern South America and NEB during MAM(+1) for the DRY-LN. Our results might be useful mainly for climate monitoring purposes.