The Costa Rica Thermal Dome (CRTD) phenomenon is permanent in the region. Nevertheless, the upwelling of cold waters towards areas where light penetrates varies seasonally, and even annually depending on intensity of marine currents, Trade Winds and El Niño phenomena. Its horizontal extent fluctuates reaching a maximum of 1.000 km, on its east-west axis, though it can likewise cover less distance.
The CRTD phenomenon acts in a regular annual cycle. It begins in the coastal areas between San Juan del Sur (Nicaragua) and the Papagayo Bay (Costa Rica). At the beginning of the year, the Papagayo Jet Stream (Trade Winds that cross through the Lake of Nicaragua depression) intensifies. During May-June the Dome moves away from the coast, and during July-October it is found in deep waters and migrates north. Even when the Papagayo Jet Stream weakens, the wind effect continues due to a northward movement of the Intertropical Convergence Zone and its associated cyclonic winds. At the end of the year (November- December), the Dome disappears.1,2.
Thus, depending on the stage of the annual cycle and its associated currents and dominating winds, the Dome expands and contracts horizontally and vertically (Figure 3). During seasons when El Niño phenomena dominates, the Dome has very little presence on the top layers of the ocean as it keeps to deep waters.
Figure 3. Dome persistence (isotherm 20°C at 35 meters depth) during the 1980-2009 period. Most of the time the Dome is located in international waters, though during 1 to 5 years of the cycle, it extended over the Mexican and Central American Exclusive Economic Zones.
As a result of annual and inter annual variations, primary productive areas associated with this phenomenon expand and contract, as shown by NASA Sea Star and SeaWiFS images. The animation demonstrates places with the greatest chlorophyll presence and high phytoplankton densities, tracking algae bloom movement.
Figure 4. Animation showing primary productivity fluctuations in the Costa Rican Thermal Dome between 1997 and 2006.4.