COSTA RICA THERMAL DOME
The large-scale ecology of this region is driven by the interplay of two vast, rich ecosystems: the Costa Rica Thermal Dome north of the equator and the Humboldt-galapagos system that reaches from South America and spreads along the equator. These systems are two of only three areas on Earth — the other being in the northwest Indian Ocean — where extensive cold-water upwellings bring an abundance of marine life to the open oceans of the tropics. Tropical seas in general are virtual deserts, but where these cool-water systems approach the surface, plankton erupts, and millions of ocean giants feed on trillions of prey animals over several thousand square miles. As the sun moves north and south with the seasons, these two giant systems — particularly the CRTD to the north — shift, expand, and contract with the ebb and flow of the seasons.
The CRTD is a bulge of deep, cold, nutrient-rich water between 100 and 200 miles across, depending on time of year, pulled up from the depths to just a few yards below the surface. When it rises, it brings an explosion of life. Cocos Island — about 5 miles long by a little over 2 miles wide, 340 miles off Costa Rica — is at the southern edge of the CRTD through the middle of the year.
Unlike other islands of the eastern tropical Pacific, Cocos is lush and green. Hammerhead numbers at Cocos may be the highest in the world. From May through September, hammerheads, whale sharks and mantas reach their highest numbers around the island.