"Considering that these islands are placed directly under the equator, the climate is far from being excessively hot; this seems chiefly caused by the singularly low temperature of the surrounding water, brought here by the great southern Polar current. Excepting during one short season, very little rain falls, and even then, it is irregular; but the clouds generally hang low. Hence, whilst the lower parts of the islands are very sterile, the upper parts, at a height of a thousand feet and upwards, possess a damp climate and a tolerably luxuriant vegetation. This is especially the case on the windward sides of the islands, which first receive and condense the moisture from the atmosphere".
Voyage of the Beagle, 1835.
The Galapagos climate is essentially a function of its position on the equator, the ocean currents that intersect in this region, and the winds that help push those currents. There are two seasons, the cool or "garua" season, and the hot, or rainy season.
The garua season usually starts around May / June and ends between December and January.
The southeast trade winds are particularly strong during this period, reaching their maximum strength in August or September. The Inter-tropical Convergence Zone, (ITCZ), where the trade winds of the northern and southern hemispheres meet, lies a few degrees to the north of the geographical equator. At this time of year the southeast trade winds push cold water from the south to the north. For this reason the currents tend to be strongest during this season. The wind also pushes cold air over a layer of warm moist tropical air, constant at the Equator, which blows over the cold ocean surface. This creates an inversion layer of warm air, in between, cold air above and cold water below; producing a dense saturated layer of moist air or fog. As this humid air is pushed towards a landmass it loses atmospheric pressure, condenses and precipitates as mist, called “garua”.
The rainy season usually begins in December / January and ends May / June. At this time of year, the ITCZ moves a couple of degrees south and the southeast trade winds slacken, eventually stopping. As a result, the cold water from the south no longer affects the islands to such an extent, and warm surface waters begin flooding back from the north where they have been held up by the constant trade winds. The inversion layer weakens and disappears, to be replaced by convectional air movement, and rapid cloud build up which produces localised heavy rains. Water and air temperatures begin to rise, and the climate reverts to being tropical for 5 – 6 months.
Every few years the warm water from the north and the rains arrive with more strength than in the intervening years, bringing about the global weather phenomena known as "El Niño Southern Oscillation", (ENSO), or simply el Niño. The eastern Pacific waters become super – heated and the islands are subjected to torrential downpours not unlike the monsoon season in India.
Fresh water lakes, streams, rivers, and waterfalls appear overnight, the vegetation becomes lush and dense, and the land flora and fauna thrive. Conversely, however the hot water brings sterility and mortality to the marine environment, the surrounding ocean becomes devoid of life and the Galapagos marine creatures that cannot migrate great distances perish. Major Niño events have most recently been recorded in 1982 – 83 and 1997 – 98. Reconstruction of the paleo-climate of the Galapagos back to the late 15th century, by the use of hermatipic, (reef forming), coral growth ring analysis, has indicated that these events are occurring with increasing frequency and each event tends to surpass the previous one in intensity, (personal communication Dr Jerry Wellington). This would be synonymous with the theory of global warming.
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