Costa Rica Thermal Dome

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A wind- and current-driven upwelling feature in the eastern tropical Pacific whose mean position is near 9°N 90°W, at the eastern end of a thermocline ridge between the westward North Equatorial Current and the eastward North Equatorial Countercurrent. It can be up to about 1,000 km across and it moves: in some years it sits closer to the coast, in others further offshore. The shallow, seasonally predictable thermocline makes it far more productive than the surrounding tropical ocean, which is why a warm-water destination can also be a feeding destination - blue whales tagged off California in summer have been tracked to it and to the waters west of it in winter.