The climate of the Galápagos Archipelago is highly sensitive to the coupled oceanic and atmospheric variability of the tropical Pacific Ocean. The seasonal cycle in the Galápagos consists of a warm and wet season from January to May and a cool and dry season from June to December, a result of the migration of the Intertropical Convergence Zone to the north of the islands and associated changes in southeasterly trade wind strength and oceanic upwelling. It is however, the interannual (2-7 year) El Niño-Southern Oscillation (ENSO) that dominates the spectrum of modern Galápagos climate variability. During an El Niño year, the islands can experience an order of magnitude increase in precipitation, along with much warmer ocean and air temperatures. Reduced upwelling undermines the marine food chain, impacting the foraging of birds and iguanas, as well as fish and marine mammals. Prolonged La Niña events, on the other hand, are associated with drought and cooler temperatures. Galápagos climate also varies on longer timescales, from decades to centuries to millennia. Paleoclimate records from Galápagos lakes, peatlands, and fossil corals, along with nearby marine sediments, are a key source of insight into this longer-term climate variability and its drivers. In addition, these paleoclimate records augment the short period of instrumental climate observations from the Galápagos, aiding in the detection of anthropogenic climate change.
The Galápagos islands also exhibit considerable spatial variability in climate across the archipelago. The northern islands (Wolf and Darwin) experience more tropical conditions – warmer and less variable ocean temperatures, for example – whereas the upwelling of the Equatorial Undercurrent in the western boundary brings strong seasonal cooling and high variability. In the vertical dimension, island topography defines wet and dry areas of the larger islands. The lowlands of the Galápagos islands, near the coasts, are notably arid, especially on the leeward side of the highlands. These highlands (≥~200 meters above sea level) penetrate an atmospheric inversion layer and stratocumulus cloud deck and are quite wet, with much higher relative humidity. The layer of stratocumulus clouds, or garúa, is an important characteristic of Galápagos climate, with significant control on ecosystems and freshwater availability (and human influence). Temporal changes in island geography and elevation can thus alter spatial gradients of Galápagos climate. Examples of this include the lower sea level of glacial periods which changed the archipelago’s geometry, and the uplift of the Galápagos platform and individual peaks over longer time scales. More recent intervals may have experienced ocean and atmospheric changes that could alter the spatial patterns of Galápagos climate, e.g. via changes in ocean circulation associated with ENSO or longer-term ocean-atmosphere modes.