Oceanography and climate information is vital.

In the same paragraph, Darwin identifies the principal reason for the Galapagos' dry and moderate climate: "this seems chiefly caused by the singularly low temperature of the surrounding water, brought here by the great southern Polar current." Today, the "the great southern Polar current" is known as the Peru, or Humbolt, Current. Carrying an enormous volume of cold water northward from the Antarctic region, it keeps the western coast of South America temperate and dry. As it passes northern Peru, the Humbolt current bends to join the Equatorial Current flowing westward across the Pacific, bathing the Galapagos in cool water. The Humbolt current has a mirror image in the northern hemisphere, the southward flowing California Current, which is responsible for California's pleasant climate. Both the Humbolt and California currents are parts of large gyres, called geostrophic currents, separately circulating water in the North and South Pacific. Similar current systems operate in the Atlantic and Indian Oceans
There is another reason for the peculiar climate of the Galapagos, of which Darwin was unaware, and that is ocean upwelling. Upwelling refers to the rise of deep water to the surface; this can occur as a result of both current patterns and winds. Though the actual cause is complex, a simple explanation goes as follows. As water of the Humbolt Current turns westward, it spreads out, or diverges. Since the water is spread out over a greater area, extra water must come from below, or upwell, to make up the difference. A more important reason for upwelling, however, has to do with winds and a phenomenon known as Ekman Transport. The trade winds blow from southeast to northwest in the southern hemisphere and from northeast to southwest in the northern hemisphere. Thus both blow towards the equator. However, the winds push water not straight ahead, but at a 45° angle to the wind direction (45° to the left in the southern hemisphere and 45° to the right in the northern hemisphere). This is Ekman Transport, which, like the Coriolis Force, is a result of the Earth's rotation. Thus although the trade winds are blowing toward the equator, they push water away from it! Once again, the divergence in surface water allows deep water to rise to the surface. The oceans are thermally stratified, so that the water rising from depth is colder than the surface water. In some areas, the water temperature can fall below 20° C (68° F), particularly west of Isabela. For most people, this is too cold for comfortable swimming!
There are two seasons in the Galapagos. The dry, or garua, season, which runs from July to December. "Garua" refers to the fog and mist that common hangs on the higher elevations during this season. The hot or wet season lasts from January through June, with March and April generally being the wettest months. The timing of the seasonal change varies somewhat and there is often a several month transition when either type of weather can occur. These seasons are also governed by oceanographic conditions. Around December, several changes occur in atmospheric and oceanic currents. The trade winds slacken and the Intertropical Convergence Zone, the "climatic equator" that is usually located north of the geographic equator, shifts south toward the Galapagos. The slacking trade winds cause the westward flowing current to slow. These reduces the upwelling and allow warmer water to invade the region. The air warms and the inversion layer breaks down. This allows warm air to rise to the point where rain clouds form and daily afternoon showers occur. Even in this season, however, low elevations, particularly those in the rain shadow of highlands, receive only limited rain. Interestingly, the highlands receive more moisture from the garua than they do from the rain
Every few years, this seasonal warming is more intense and prolonged than usual. These are oceanographic events known as El Niño, and they are coupled to a reversal in atmospheric circulation known as the Southern Oscillation. Together they are sometimes called ENSO (for El Niño-Southern Oscillation) events. When an El Niño occurs, the entire equatorial and atmospheric circulation pattern reverses. Currents and winds reverse and now bring warm water and air from the western Pacific to the Galapagos and coastal South America. In association with this, the normal atmospheric high pressure system in the eastern Pacific is replaced with a low pressure one, and the low pressure system in the western Pacific with a high pressure one (see the adjacent diagram). Areas in northern Australia, New Guinea, and Indonesia suffer drought while heavy rains occur in the Galapagos and the west coast of South America. In the Galapagos, the rains moisten even the dry lowlands, allowing vegetation to flourish. With food abundant, the terrestrial animals, such as the iguanas and finches, do well. At the same time, these changes inhibit the upwelling that enriches the Galapagos waters in nutrients. Sea life suffers as a result, sometimes dramatically. A particularly severe El Niño occurred in 1982-1983. Terrestrial life flourished; finches, for example, raised several broods of young. But it was a catastrophy for marine life. Sea birds of all types were unable to raise their young and there was high mortality among marine iguanas and fur seals. El Niño thus sets a rhythm to Galapagos life, but one in which the fortunes of marine and terrestrial life are exactly out of phase.
The 1997-1998 El Niño has been one of the strongest events of the century. Drought and wildfires plagued Indonesia and Australia while western North and South America suffered from floods and heavy snows. It had the expected effect on the Galapagos: heavy rain fell between March and June of 1997, and again in the wet season of 1998; sea and air temperatures were typically 4 to 5° C above normal. This has an adverse impact on marine life, since upwelling, and hence ocean nutrient levels have been reduced. On the whole, however, this El Niño seems not to have devastated marine life quite so badly as did the 1982-83 event. Nevertheless, as the El Niño drew to a close in June of 1998, one could see sea lion carcasses and bones littering Galapagos beaches. There was high mortality among marine iquanas as well, and the survivors looked emaciated. Many sea birds failed to rear young. On the other hand, the abundant rainfall made the normally arid and brown lowlands verdant and terrestrial animals and birds flourished.
A La Niña event, in which the air and water is cooler than normal, is expected for 1998-99. This will be beneficial to marine life, but will take a toll on land-dwellers. The El Niño-La Niña cycle is part of the rhythm of life in the Galapagos and the fauna and flora there are well adapted to it. Though the weak succumb, the strong survive to pass on their genes to a new generation. Life goes on.