During the Beagle's visit to the Galapagos Islands in 1835, Charles Darwin noted that the local climate was far less warm than would be expected from the islands' position on the Equator. The air-conditioning effect is due to the cooling influence of the surrounding oceans — part of which, according to C. Eden and A. Timmermann (Geophys. Res. Lett. 31, L15308; 2004), arises from the very presence of the islands.
As this satellite image shows, the Galapagos are isolated in the vastness of the Pacific Ocean, lying about 1,000 km west of South America. This is nonetheless an oceanographically sensitive location, because the islands obstruct two components of a system of wind-driven ocean currents in the equatorial Pacific. The cool Southern Equatorial Current flows westwards as part of the Pacific subtropical gyre, and splits into a northern and a southern branch at the Galapagos. The subsurface Equatorial Undercurrent transports water eastwards between and beneath these two branches, and almost stops dead where it hits the islands.
Using a high-resolution numerical model, Eden and Timmermann have simulated equatorial Pacific currents with and without the Galapagos topography. The differences are significant. The islands produce a wake-like pattern in both currents, with flow anomalies extending up to 2,000 km in an east–west direction. And as a result of stronger upwelling of cooler water from depth, sea surface temperatures just west of the Galapagos are up to 2 °C lower than they would otherwise be — hence the comparatively temperate climate.