Effects of the 2017 coastal El Niño on the thermohaline structure and geostrophic fluxes off the northern Peruvian coast

Abstract

We used satellite wind data and hydrographic information collected in transect surveys between December 2016 and June 2017 to describe the effects of the 2017 coastal El Niño (2017 CEN) on the thermohaline structure, water masses, and geostrophic fluxes within 100 nm off Paita and Chicama. Southern wind speed anomalies, between -3.5 to -1.0 m/s, were recorded (January-March 2017) leading to thermal stratification of the upper layer of the water column, as well as the southward advection of equatorial surface waters (ESW) and tropical surface waters (TSW) to Chicama and Paita, respectively. At the latter location, SST peaked at 27.4 °C in February in the coastal zone (CZ; < 30 nm) and 28.9 °C in March in the oceanic zone (OZ; > 30 nm), while in Chicama, SST reached 25.7 °C in the CZ and 27.9 °C in the OZ, both in February. SSTA off Paita peaked in February (+5.4 °C, CZ; +5.0 °C, OZ). In March 2017, off Chicama, SSTA peaked at +4.3 °C (CZ) and +4.5 °C (OZ). When anomalies are integrated over the water column, the thermal anomalies peaked in March at both transects. Between January and February, the geostrophic velocities in the Paita CZ indicated intense southerly fluxes in the upper 40 m layer and the subsurface layer in March. Conversely, in the Chicama CZ, the fluxes were weak (<10 cm/s on average), heading north in the upper 30 m layer and south in the subsurface layer throughout the entire period. Integrated fluxes in the Paita and Chicama OZs were weak in January and February and headed north in March. Intense southerly fluxes were detected at both transects, between April and May 2017, occupying most of the water column (mainly from depths between 50 and 150), likely resulting from the passage of a downwelling equatorial coastal-trapped wave.