Contribution of Kelvin waves to thermal anomalies in coastal waters off Peru during El Niño 2015/16 and the 2017 coastal El Niño

Abstract

This paper compares the effect of remote (Kelvin waves) and local (winds) forcing on the thermal structure of the water column during El Niño 2015/16 and the 2017 coastal El Niño. To do this, we analyzed sea level and temperature anomaly data both in the equatorial Pacific and in the Peruvian coastal strip to estimate the propagation speed of the coastal-trapped wave during both events. In summer, temperature anomalies in the surface layer (<30 m) of up to +7 °C were determined for 2017 CEN mainly associated with the weakening of the southeasterly trade winds and, secondly, with the arrival of downwelling Kelvin
waves. Information on subsurface temperature was obtained from fixed coastal stations (≤100 m) located off Paita (5.06°S), Chicama (7.82°S), and Callao (12.03°S). On the other hand, during the previous event, El Niño 2015/16, temperature anomalies of up to +6 °C in a layer above 100 m occurred in the same stations (summer) but only associated with the arrival of downwelling Kelvin waves generated in the equatorial Pacific. The variation of zonal and meridional flow off the Gulf of Guayaquil and northern Peru were closely related to the anomalous warming in the surface layer. We propose that the changes during EN 2015/16 and La Niña 2016 (the occurrence of downwelling and upwelling Kelvin waves), together with the weakening of the South Pacific Anticyclone, contributed to the formation of 2017 CEN. Our results confirm that fixed coastal stations are strategically located for the study of the oceanographic manifestations of ENSO in the Southeast Tropical Pacific, especially in the area off the Gulf of Guayaquil and northern Peru. Also, they are key to monitor the variability of local winds and identify events similar to the 2017 CEN.