Ekman modulation of the sea-surface temperature on the Eastern South Pacific

Abstract

Abstract Nine years of data (1992–2000) were used to study the evolution of the sea–surface temperature (SST) over the eastern south Pacific. Our analysis shows that the variability in the SST on the interannual scale is attributed mainly to the equatorial El Nino-La Nina events while cooling, associated with the wind-Ekman drift, is an important component of the annual cycle. The anomalies at the interannual scale show up as a front that parallels the coast. At the seasonal scale, anomalous warming and cooling of coastal waters is strongly altered by the local effect of the wind-driven Ekman transport. This latter effect is especially significant along the Peruvian and Northern Chilean coast where wind induced upwelling and cooling is out-of-phase with other important annual forcings (i.e radiation and annual Kelvin waves). These local wind effects show up in the time-latitude plots as a thinning of the warm-water bands along the Peruvian coast (∼10°S y 17°S) and a thickening of the bands further south (∼18°S y 22°S). Three anomalous events that stand out clearly at the interannual scale are El Nino 92–93, La Nina 96 and El Nino 97–98. Their individual evolutions over time and along the coast are very different. Though the least intense of the three, the 92–93 event had the longest duration. The 96 event was the shortest in duration and its strongest anomaly occurred around the Peruvian coast, similar to the warm event of 92–93. The strongest event for the period occurred in 97–98, its warm anomaly showing strongly along the whole latitudinal extent of the study area. Contrary to what occurs at the seasonal scale, the increase on east/west Ekman transport on the interannual scale seems to be in phase with other relevant cooling and warming mechanisms. The interannual Ekman anomalous transports are at their maximum near the Peruvian-northern Chilean coast; however, they do not seem to alter the SST in a significant way. A similar situation has been described for sea-level data in the California Current System, i.e. interannual variability being associated mostly with variability of equatorial origin while seasonal variability is associated to both wind-forced local variability and remotely forced variability (J. Geophys. Res. 94 (1989) 3159)