As Charles Darwin remarked during his visit to the archipelago, the unique climate of the Galapagos Islands “seems chiefly caused by the singularly low temperature of the surrounding water, brought here by the great southern Polar current (Humboldt Current).” The climate and organisms in the Galapagos Archipelago are directly influenced by the conflux of a complex system of ocean currents, as well as trade winds and ocean floor upliftings. These processes provide the nutritional basis for the Galapagos ecosystem.
The Galapagos Islands are predominantly influenced by three major currents: the Humboldt Current (also known as the Peru Coastal Current) – which is fed by the South Equatorial Current; the Cromwell Current (or Pacific Equatorial Undercurrent); and the Panama Current.
The Humboldt Current is most prominent in the archipelago from July to November, which coincides with the dry season. The current originates in the waters of Antarctica. As it moves northward, the Southeast Trade Winds and the rotation of the Earth create the largest upwelling system in the world, bringing cold water up from the ocean floor and pushing an enormous amount of nutrients up into the current’s waters. Therefore, in its wake, the current leaves behind the world’s most productive marine ecosystem. As the majority of marine life lives closer to the surface, when it dies it falls towards the ocean floor where it decomposes, attracting large amounts of phytoplankton and algae – the basis of the food chain and Galapagos life. Then, once it reaches the equator, the Earth’s rotation and seasonal winds push the Humboldt Current west, where it flows into the South Equatorial Current and pushes westward throughout the islands.
On the other hand, the Cromwell Current originates in the Western Pacific Ocean, and like the Humboldt, is extremely cold – a mere 13°C at its core. This enormous ocean current (250 miles wide and 3500 miles long) flows eastward 300 feet beneath the ocean surface. Once it reaches the Galapagos Platform, it is pushed upwards, bringing deep, nutrient-rich waters to the shores of the islands. In fact, this current is the reason for much of the unusual marine life that can be seen in the Galapagos, including Mola Mola, seahorses, bull sharks, red-lipped bat fish and king angel fish. However, this current is rarely seen in the eastern part of the archipelago as it dissipates around the central islands once it hits the Galapagos platform.
The cool waters of the Humboldt and Cromwell currents play an essential role in the climate of the archipelago. Generally, air temperature gradually decreases with elevation; however, with the arrival of these cold currents, from July to November this effect is reversed, and the air closer to the ground is cooler than the air at elevations. This effect is known as temperature inversion. However, a number of the volcanoes are tall enough to obstruct the inversion layer, causing the moisture in the air to condense- which is why the garua mist that can be seen from May to December. It is because of this process that the lowlands are unusually dry.
Beginning in December, the Humboldt Current loses much of its strength as the Northeast Trade winds shift, bringing warm waters south from Central America via the Panama Current. As the waters warm to around 27°C, the inversion layer dissipates and the climate becomes much more tropical and more characteristic of equatorial regions. The rains arrive in short, sharp showers and the land quickly dries out thanks to the porous soil. Thus, it is generally characterized by blue skies and sunshine. While this current is not as nutrient-rich as the Humboldt and Crowell Currents, the warm waters attract a variety of fish, sharks and rays. Additionally, because of decreases in plankton and algae, the waters are significantly clearer, making for excellent scuba diving and snorkeling conditions.
Because several of these currents do not extend throughout the entire archipelago, the waters surrounding each island can vary greatly, in turn affecting the climate and ecosystem of each island. Thus the water temperature, salinity and productivity of the waters vary depending on the predominant current that flows around an island at a given time.