GEOL 388: Field Natural History of the Galápagos Islands

Summer Semester I 2004
Climate and Oceanography

Darwin notes some aspects of the local landforms that have to do with climatological conditions:
"it is a remarkable circumstance that every one of the twenty-eight tuff-craters which were examined, had their southern sides either much lower than the other sides, or quite broken down and removed. As all these craters apparently have been formed when standing in the sea, and as the waves from the trade wind and the swell from the open Pacific here unite their forces on the southern coasts of all the islands, this singular uniformity in the broken state of the craters, composed of the soft and yielding tuff, is easily explained"
Darwin, Voyage of the Beagle, Chap. 17

Galápagos Climate:

Climatology
As everywhere, weather is controlled primarily by oceanography, and shallow oceanography (upper 1 km) is controlled by wind. So let's start with global air circulation.

Lower 10-18 km of atmosphere is the troposphere. It is warmest at the base (where warmed by land surface) and cools further up (due to distance from heat source and to expansion (decreased pressure)).

As land surface heats up due to sunlight, warm shallow air rises. Since nature abhors a vacuum, other air must come in replace it.

Since Earth's surface is curved (sphere), get latitudinal effects:

However, Earth also rotates, so it generates Coriolis effects:

In cases of air masses, combined effect of altitudinal, latitudinal, and Coriolis effects generates Hadley circulation cells.

Global circulation pattern of Hadley cells in turn drives global major wind patterns. Equator represents the convergence of the Trade Winds.

Oceanography
Wind patterns drive shallow (<1 km) oceanic circulation. Because of Coriolis forces, net motion of ocean water is perpendicular to wind direction (90° to right in northern hemisphere, 90° to the left in the southern hemisphere). This sets up large gyres (huge circulation patterns) as well as smaller countercurrents in the seas. The combination of all these produce Global circulation patterns.

Galápagos Archipelago lies at confluence of several major currents:

Major effects:

  • Galápagos is washed by cold water, so air temperature is cool, and few tropical storms are generated
  • Net flow is from South America towards the west

    Because flow is away from a landmass, get upwelling:

    Long and short term climate fluctuations:
    Quaternary glaciationIce Ages. Not simply a prolonged period of cold, but instead a fluctuation between two major states:

    Effects of glacial-interglacial cycles on Galápagos

    Shorter Term climate changes: El Niño Southern Oscillation (ENSO) events
    Periodically, starting around Christmas time, a major disruption of coastal Ecuadorian and Peruvian fisheries is disrupted. Because of timing shortly before Christmas, called "The Child". Seen locally as warming of ocean water, and consequently bad fishing and stronger storms.

    El Niño Events:

    For coastal South American communities, El Niño years are very tough: fishing gets bad, storms cause damage, even farming suffers if bird guano is a major fertilizer (birds fly off to better fisheries, so no new guano).

    An intense version of the opposite system was called "Anti-El Niño", but since this would basically mean "Anti-Christ", it has been re-dubbed " La Niña" ("the Girl"). It causes greater dryness in the region.

    El Niño years in Galápagos:

    La Niña years in Galápagos has opposite effect (great for sea life, tough for land)