Kevin Leaman
Also available in Spanish.
Circulation in the southwestern Caribbean is highly variable and poorly understood. It is driven in large part by seasonal changes in the trade winds and the Intertropical Convergence Zone, or "ITCZ", and possibly also by interactions with the offshore Caribbean Current. The ITCZ appears in satellite pictures as a band of clouds near the equator, toward which the northeast and southeast trade winds blow from the Northern and Southern Hemispheres, respectively.
In winter, the ITCZ shifts slightly south and the weather over the Caribbean region is often dry, but very windy. These winds cause strong upwelling of nutrient-rich water -- and productive fisheries -- along the Colombian and Venezuelan coasts. In summer, the ITCZ moves north and the winds die away; however, ITCZ clouds produce intense rainfall -- up to three meters during the rainy season. Heavy rainfall in the region can modify the salinity and density of the nearby ocean water and affect current flow.
This forcing produces a large gyre -- the Panama-Colombia Gyre or PCG -- in the ocean between those two countries. The PCG exhibits major changes over the seasons; it may even sometimes reverse direction. The PCG may also connect with the Caribbean Current during part of the year, which may allow fish or other larvae, for example, to be carried offshore from estuaries along the Colombian coast and downstream into the Caribbean Sea and Gulf of Mexico.
Most of the drifters being launched in the passages of the Greater and Lesser Antilles will probably be carried through the Caribbean Sea and the Yucatan Channel, never reaching the southwestern Caribbean. To obtain a more uniform coverage of the region a separate set of drifters is being launched -- eight drifters every season (or, every three months) for one year -- in an initial exploratory effort to describe the PCG and how it changes. This can be done inexpensively by taking advantage of "volunteer observing ships" to launch the drifters -- in this case vessels of the Colombian Navy that make regular trips between Cartagena, Colombia and San Andres Island.
See also Project Results.