Sea surface temperature variation in the Galápagos Archipelago: a comparison between AVHRR nighttime satellite data and in situ instrumentation (1982–1998)

A multi-year study was undertaken to evaluate the relationship between sea surface temperatures (SSTs) derived from two methodologies: Advanced Very High Resolution Radiometer (AVHRR on NOAA satellites) and in situ instrumentation. Comparisons within sites were made at six widely distributed locations in the Galápagos Archipelago: Academy Bay, Santa Cruz, Bartolomé, Champion, Punta Espinosa, Fernandina, and Wolf. Comparisons were made at monthly (1982–1998) and weekly (1997–1998) resolution. Results of within-site comparisons indicated that the differences in monthly SSTs for the two methods, at four primary sites, ranged from a mean difference of −0.62°C to a high of +1.39°C. Weekly comparisons made during the very strong 1997–1998 ENSO event revealed smaller differences between methodologies (−0.33 to + 0.99) due to the widespread warming resulting from ENSO conditions. While a statistical evaluation of the data revealed few overall differences between sites and methods, two sites, Champion and Urvina Bay, showed consistent differences between the two methodologies. At Urvina Bay in situ data were consistently higher than satellite measurements, and vice versa at Champion. These differences could be explained by heating in the shallow nearshore waters of Urvina Bay, and at Champion by the placement of the in situ instrument below the depth of a shallow thermocline. Lastly, a comparison of AVHRR-derived SSTs from 1982–1998 at six widespread sites was used to evaluate patterns of seasonal SST variability within the archipelago. Although several distinct temperature zones were identified, average differences between zones previously suggested by Harris (1969) were found to be lower than proposed.