The anomalous 2017 coastal El Nino event in Peru

Remarkably heavy and devastating rainfalls affected large parts of Peru during the austral summer 2016-2017. These rainfalls favoured widespread land sliding and extensive flooding and generated one of the most severe disasters of Peru since the 1997-1998 El Nino event. The amount of rainfall recorded between January and March 2017 only compares to the biggest El Nino events of the last 40years (i.e. 1982-1983 and 1997-1998) and exceeded the 90th percentile of available records (1981-2017) over much of the northern and central coasts of Peru, the Andean region and Amazonia. The occurrence of these heavy rainfalls was highly anomalous as it occurred during the first austral summer following the development and decay of a very strong El Nino in 2015-2016. Here, we propose that the likely cause of the anomalous rainfalls is linked to the combination of an especially intense wet spell over the Central Andes related to a deep, long-lasting anticyclone located adjacent to the Chilean coast, and to the unusual development of warm water off the coast of Peru in the nominal El Nino 1+2 region. This warming has been related to an anomalous weakening of the mid-upper level subtropical westerly flow, which in turn led to a weakening of the southeasterly trades off the coast, thus hindering the upwelling near the Peruvian coast and favoring the eastern Pacific warming. This development is counter to the usual evolution of sea surface temperature in the eastern equatorial Pacific following very strong El Nino events, such as those occurred in 1982-1983, 1997-1998, and 2015-2016. This paper explores the unusual nature of this event in the observational record and illustrates its consequences.