The phenomenon is triggered every year between January and April without missing a single time for decades. However, the rise of cold water in Panama has just been missed, due to too weak trade winds.
Off the Pacific, northern trade winds blow over the Bay of Panama every winter and push surface water offshore. The icy and nutrient-laden depths then fill the void left. This year, the rise of cold water in Panama did not take place, unprecedented since the beginning of the surveys.
Forty years of perfect cycle, broken in 2025
From January to April, the cycle usually follows a metronome regularity. The northeastern trade winds, loaded by the Caribbean Sea, cross the narrow isthm and then sweep the Gulf of Panama. Under their effect, the warm waters move away from the coast, while an icy current rises from the depths. Rich in nitrates and phosphates, it then takes over near the coasts. Researchers at the Smithsonian Tropical Research Institute have been following this phenomenon since 1985 and had never seen it derail, either way.
In 2025, however, the sensors delivered a completely different story. The green hue, a sign of the proliferation of phytoplankton, did not spread in the bay as usual. At the same time, the surface temperature climbed several degrees above seasonal normals, and the salinity, a mark of deep water, did not decline. Aaron O’Dea, marine paleobiologist at the Smithsonian, evokes an event « unpublished in the instrumental archive » of the tropical Pacific, but also a life-size test for the resilience of the entire ecosystem.
Trade winds too short to awaken the depths
In 2025, the winds that cross the isthmus blew fewer days than usual and later in the dry season, blurring the triggering of the vertical pump. When they finally activated, their intensity reached the usual levels, but the window was too short to push the surface water sustainably to the sea.
The shifted position of the intertropical convergence zone, combined with a La Niña episode at the end of 2024, seems to be responsible for this atmospheric shortness of breath.
source : science et vie
