Upwelling

Wind-driven rise of cold, nutrient-rich deep water toward the surface, which drops temperature and visibility but fuels spectacular marine life.

Upwelling is the vertical movement of cold, nutrient-rich water from the depths up toward the surface, most often when persistent wind pushes surface water offshore and deep water rises to replace it. A diver notices it as a sudden cold snap at a normally warm site, sometimes a drop of 8 to 10 degrees C over a few days, along with greener, planktonic water.

The nutrients that upwelling delivers fuel the food chain, which is why the great upwelling coasts, California, Peru, the Galapagos, South Africa, host such dense marine life, from kelp forests to sardine baitballs and the predators that follow them. Short-term, the plankton bloom can crush visibility; the payoff is action: mantas, molas, whale sharks, and feeding frenzies concentrate where the cold water rises.

Practically, pack for the cold side of a site's range during upwelling season and check recent temperature reports rather than trusting the brochure. Logging water temperature on every dive makes these seasonal swings obvious in hindsight. If the water suddenly turns cold and green midweek, do not write the trip off, ask the guides where the action has moved, because the event that ruined the visibility usually switched on the feeding.