Water mass seasonal variability in the Galápagos Archipelago

Abstract

Three hydrographic surveys were conducted within the Galápagos Archipelago during 2005–2006. The surveys captured the surface properties (<80 m) near the extremes and midpoint of the annual cycle of the mean sea surface temperature (SST) and winds. A cooler SST occurs in boreal summer and fall as the southeast trades strengthen. Current data at 110°W show that this coincides with the Equatorial Undercurrent (EUC) becoming weaker and deeper below a strengthening westward South Equatorial Current (SEC). Opposite conditions are generally found in the spring. Meanwhile, the sea surface salinity (SSS) freshens in late winter/spring when the archipelago receives large rainfalls as the Intertropical Convergence Zone (ITCZ) shifts southward, or in late fall when receiving large influxes from the North Equatorial Countercurrent (NECC). As a result, Tropical Surface Waters (TSW) with salinity ( S) <34 fill the archipelago from the late fall through early spring. The SSS becomes saltiest in late spring/early summer as the EUC strengthens, resulting in Equatorial Surface Waters (ESW), S>34, throughout the archipelago. Equatorial Surface Waters are present west of Isabela, where the EUC upwells as it interacts with the Galápagos platform. They also are found east of the archipelago in the cold tongue, which extends westward from South America, and therefore may be advected by the SEC into the archipelago. The upwelling west of Isabela creates a consistently shallow 20 °C isotherm (thermocline), which remains elevated across the archipelago. Linear extrapolation of the thermocline depth along the equator from 110 to 95°W gives a good approximation of the thermocline depth within the archipelago from 92 to 89°W.