The sea off Playas del Coco turns colder every December. A boat ride that felt like bathwater in September can mean a thermocline sharp enough to make you shiver at depth by February. That change runs the local dive calendar, and the same cold water from the Papagayo winds gives the reefs of the Gulf of Papagayo their yearly relief from heat. In late 2026 it arrives behind an El Niño that the Instituto Meteorológico Nacional places above the 1997 and 2015 events.
The Wind That Empties the Bay
The mechanism starts far to the north. Between December and March, cold air masses push south into Central America and hit hardest in January. Researchers Eric Alfaro and Jorge Cortés described this in a 2012 study in Revista de Biología Tropical. The wind funnels through the low ground of southern Nicaragua and northern Costa Rica. It forms jets known locally as "papagayos." Speeds can reach 50 meters per second, and the jets can last for weeks.
Bahía Culebra runs northeast to southwest, which lines it up with that wind. The wind pushes the warm surface water out of the bay, and colder water rises from below to replace it. During five extreme events the study tracked between 1998 and 2010, sea temperatures dropped by about 8 to 9°C. At Güiri-Güiri reef inside the bay, earlier measurements ranged from 9.9 to 31.6°C beneath the surface.
Monthly averages tell a calmer story than those extremes. A survey of the gulf from October 2016 to February 2018, published in the journal Water, found mean surface temperatures of 26.51°C in the dry season and 29.29°C in the rainy season. The same survey found chlorophyll-a almost three times higher in the dry season, 1.19 mg/m³ against 0.43 mg/m³. The study links that dry-season rise to nutrients brought up by the upwelling. The Área de Conservación Guanacaste reports deep water as cold as 16°C during the windy season while the surface stays around 28°C. That gap is why a single dive can feel like two different oceans.
How the Boat Schedule Changes
Divers out of Coco feel that wind most directly. Rocket Frog Divers says crossing the gulf to the Bat Islands gets "extremely hard" from December to March. During those months the shop runs its trips to local sites and the Catalinas. The Bat Islands, or Islas Murciélago, sit at the edge of Santa Rosa National Park, about 50 minutes from Coco by the shop's boat. Rich Coast Diving runs its bull-shark trips there from June to October, weather permitting.
By the time this is posted, the bull-shark window is closing and the cold-water calendar is about to start.
| Rainy season, May to November | Windy season, December to April | |
|---|---|---|
| Bat Islands | Best visibility, 30 to 100 ft, from May to November | Crossing becomes difficult, so trips mostly pause |
| Catalinas | Best visibility from May to mid-November | Water at its coldest in January and February. Giant mantas gather in the upwelling season |
| Local sites near Coco | Warmer, steadier water | Visibility of 10 to 70 ft that can change almost daily |
The local sites are where most winter diving happens. Rocket Frog lists about 30 within a 15-minute ride of Coco, including Tortuga, Argentina Point, Sorpresa, Monkey Head, Turning Point, Shark Shallows, Punta Gorda, the Aquarium and Palmares. The shop notes that water temperature there can change by as much as 5°C in a single day and that the thermoclines are hard to predict.
The cold also brings the season's biggest draw. Misión Tiburón calls the Catalina Islands Costa Rica's most important gathering site for oceanic giant mantas and says they come during the upwelling season. The same water that limits the Bat Islands trips is what brings the mantas to the Catalinas.
What the Cold Does for the Reefs
The cold water matters beyond diving. A 2025 study published in Diversity traced the reefs of the North Pacific through the 2023–24 El Niño. Sea surface temperatures stayed above the 29.9°C bleaching threshold from May to October 2023 and passed it again in May 2024. Heat stress peaked at 10.2 Degree Heating Weeks in July 2023, more than double the 1997–98 peak of 4.1.
The losses were concentrated in a few places. Across the five sites studied, average live coral cover fell from 42% to 13% by July to November 2024. The authors note that the overall decline was not statistically significant. Jícaro and Sombrero lost nearly all of their coral. Fish that feed on invertebrates had higher biomass on healthy reefs than on bleached or badly damaged ones.
The study also describes recovery. In Culebra Bay, water temperatures start to fall in October as the trade winds strengthen and reach their lowest in February and March. The average difference between the upwelling and non-upwelling seasons can be as much as 4°C. In 2023, temperatures fell steadily after October and dropped below 28°C by December. The authors say this pattern "will help corals recover from bleaching" and suggest these reefs may serve as thermal refuges.
The Variable This Winter
This is where the 2026 forecast comes in. The 2012 study notes that the El Niño-Southern Oscillation drives most of the year-to-year variation in the Papagayo upwelling. This year's event is unusually strong. In its climate outlook updated October 16, 2026, IMN reports that Niño 3 reached about +3.4°C in September, above the same month in 1997 and 2015. It expects the event to peak in December.
The same bulletin addresses the cold surges that produce the papagayos:
"Respecto a los empujes fríos, se prevé una temporada con una frecuencia dentro de los valores normales; no obstante, las lluvias y los vientos asociados a estos sistemas podrían presentar una menor incidencia."
In English, IMN expects the usual number of cold surges this season, but their rain and wind may have less impact. For November 2026 through January 2027, IMN forecasts dry-season conditions on the Pacific slope, with average temperatures 1.5 to 2°C above normal. IMN's shorter-range forecast calls for very dry conditions in the North Pacific from October 25 through November 8.
NOAA's view points the same way. Its October 8 ENSO discussion put Niño-3.4 at +2.1°C. It gave a better than 83% chance of a strong to very strong event through January to March 2027.
Neither agency has forecast the strength of the upwelling itself. IMN meteorologist Daniel Poleo told The Tico Times in September that a very strong El Niño does not scale up every impact by the same amount. He added that Costa Rica has no recent event this size to forecast from. A normal number of cold surges with weaker winds would still produce an upwelling season. How much cooling it delivers is unknown.
What to Watch From the Water
Anyone who swims, boats or dives from Coco this winter can track the season through a few concrete markers from the sources above:
- The October-to-December temperature drop. In 2023, Culebra Bay water was below 28°C by December. Water that stays warmer into December would mean the cooling started late.
- The first cold thermoclines at local sites. Swings of up to 5°C in a day at spots like Tortuga and Punta Gorda are the usual sign that deep water is rising.
- Mantas at the Catalinas. Misión Tiburón links their gathering to the upwelling season, so sightings are one indicator of how the season is going.
Under the Área de Conservación Guanacaste's published marine-sector rules, trips into the protected sector need advance authorization that lists the date, the visitors and the vessel, and fishing of any kind is prohibited there. The bull sharks will come back to Islas Murciélago when the wind drops. In early July, Coco's Festival del Tiburón Toro brought researchers, divers and Misión Tiburón to Parque Amor de Temporada, Hotel El Colono and Casa Selvita, as The Tico Times reported. Between now and then, the gulf has its cold season to get through.
For owners in Playas del Coco, the bay's seasons are part of what the property offers. If you'd like to talk about how the water, the winds and the year-round rhythm fit with owning here, ACT Costa Rica welcomes you to Request a Private Consultation.
