Galapagos Islands seasons - seasonal weather changes

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 Niņo, and they are coupled to a reversal in atmospheric circulation known as the Southern Oscillation. Together they are sometimes called ENSO (for El Niņo-Southern Oscillation) events. When an El Niņo 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 Niņo 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 Niņo 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 Niņo 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 Niņo seems not to have devastated marine life quite so badly as did the 1982-83 event. Nevertheless, as the El Niņo 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 Niņa 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 Niņo-La Niņa 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.