information is vital.
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.