Each year between January and April, a blob of cold water rises from the depths of the Gulf of Panama to the surface, playing an essential role in supporting marine life in the region. But this year, it never arrived.
"It came as a surprise," said Ralf Schiebel, a paleoceanographer at the Max Planck Institute for Chemistry who studies the region. "We've never seen something like this before."
The blob is as much as 10 degrees Celsius colder than the surface water. It is also rich in nutrients from decomposing matter that falls to the ocean floor, providing food for local fisheries and wildlife.
Dr Schiebel was one of the scientists who recently documented the lack of this yearly upwelling in a paper in Proceedings of the National Academy of Sciences and identified a likely culprit: The lack of strong trade winds, which typically blow across Panama and kick off the dry season in January.
When the trade winds reach the Gulf of Panama they push hot surface water away from the coast, which makes room for cold water to rise from the deep.
Steven Paton, one of the paper's co-authors, runs a large environmental monitoring programme at the Smithsonian Tropical Research Institute. The record he helps maintain shows the upwelling has taken place annually for at least 40 years. With that data and other long term records, "we can very clearly say something very unusual happened that we need to pay attention to," he said.
It's unclear whether a warming planet played a role in the disappearance of the cold blob this year. But the researchers have a few theories about what affected the trade winds.
Trade winds, like the ones that drive the cold upwelling in the Gulf of Panama, typically form when air moves from high pressure to low pressure systems. But this year Panama saw only a quarter of the usual dry season trade winds and when they did emerge, it was only for a short period of time.
Nutrient rich waters are important for Panama's fishing industry, which is concentrated on the Pacific side of the isthmus, rather than in the Caribbean, he said. The upwelling also supports large marine life, like dolphins, rays and migrating whales that pass through the region. The lower temperatures also provide respite for coral reefs, which are made up of living organisms that can bleach white and die when they get too hot.
The Bermuda-Azores High is a high pressure system that moves around the Atlantic Ocean, affecting seasonal weather patterns across Europe, Africa and the Americas. A separate, low pressure system, known as the Intertropical Convergence Zone, wraps around the Equator and moves south of Panama in winter. This southward movement, in combination with the difference in pressure from these two systems, causes the force that drives Panama's dry season trade winds.
New York Times News Service