Seasonal patterns of sea-surface temperature and ocean color around the Galapagos: regional and local influences

Monthly climatologies of satellite-derived sea-surface temperature (SST, from AVHRR) and ocean color (from Ocean Color and Temperature Scanner and Sea-viewing Wide-Field-of-view Sensor) around the Galapagos Archipelago are used to estimate the mean seasonal cycle of these properties and their relationship to the equatorial circulation in this oceanographically complex region. Harmonic analysis of the climatological time series indicates a best fit with annual and semi-annual constituents. The annual amplitude is the dominant signal in SST, corresponding to the basin-wide seasonal cycle of warming and cooling associated with the north-south migration of the intertropical convergence zone. Influx of upwelled water from the Panama Bight into the northeastern part of the study area is also consistent with the annual signal. An empirical orthogonal function decomposition identifies two main spatial patterns with amplitude time series representing out-of-phase annual cycles. The dominant mode corresponds to the strengthening of the Equatorial Front and the South Equatorial Current during the second part of the year. This mode explains 92.2% of the SST variance and 82.9% of the ocean-color variance. The second mode is consistent with the topographically induced upwelling of the Equatorial Undercurrent on the western side of the archipelago, and with influx of upwelled Panama Bight water on the eastern side, both reaching their peak during the first part of the year. This mode accounts for 6% of the SST variance and 7.7% of the ocean-color variance. The seasonal evolution of water-column temperature and nitrate (from World Ocean Atlas 1998 climatologies) is consistent with the satellite-derived patterns. A slight tilt aligned with the east-west axis of the Galapagos Platform (outlined by the 2000-m isobath) is evident in all property fields, suggesting that the presence of the archipelago introduces a small but noticeable perturbation to the large-scale currents and property gradients of the eastern equatorial Pacific. (C) 2004 Elsevier Ltd. All rights reserved.