A Case Study of Coastal El Niño Event in Early 2017

The Japan Meteorological Agency (JMA) issues information about extreme climatic events in a timely manner when such events occur and significantly affect the socio-economic activity. One recent example was that JMA issued a climate monitoring report on April 4, 2017, regarding the persistent heavy rains from January to April over the northwestern part of South America. It was reported by the governments that floods and/or landslides due to heavy rains in the region caused totally hundreds of fatalities in Columbia and Peru.

This widespread rainfall is supposed to be led by the coastal El Niño, which is a phenomenon that SST (sea surface temperatures) off the northwestern coast of South America becomes far warmer than that of well-known normal El Niño. In 2017, the coastal El Niño event emerged in January and persisted until April, during which the peak of SST anomalies in the area exceeded the value of three times of its standard deviation. Our preliminary study shows that, in part of the eastern equatorial Pacific in the Southern Hemisphere, trade winds were weakened in January and latent heat fluxes from the sea surface were also weakened. Less evaporation from sea surface prevented the SST from cooling and SSTs were warmer than about 27 degrees C since mid-January, which were favorable for active convection. Enhanced convection induced the north wind anomalies, which in turn weakened the trade winds further. Statistical analysis also indicated that precipitation amount in Peru during the period from January to March has associations with the warmer-than-normal SST in the northwestern coast of South America and the enhanced convective activities over the eastern equatorial Pacific.

To understand the prolonged ocean-atmosphere coupled phenomenon like coastal El Niño events is definitely important because it can lead better understanding of the ENSO mechanism itself and also may be followed by improvement of the ENSO monitoring or forecasting technique. It will be shown how such circulation anomalies in the atmosphere and the ocean were formed and maintained during this coastal El Niño event.