When the Alisios Drop: Why September and October Run Drier on the Caribbean Side

In September the Pacific slope of Costa Rica takes its heaviest rain of the year. Tropical cyclones, monsoon wind off the equatorial Pacific, sea breezes. The Instituto Costarricense de Turismo lists all three.

On this side of the mountains the same weeks often run the other way. The rain eases. The wind drops. The sea goes flat for days at a time.

It sounds like a story a lodge would tell. It is in the meteorological record, with dates.

The alisios

The alisios are the trade winds. They come off the Caribbean Sea and run west into Costa Rica. The mountains behind this coast stand across their path. Air that meets a ridge has one direction left. It rises, it cools, and it drops its water on the seaward slope.

That is a large part of why this coast is green in July, the peak of its second rainy stretch. The rain is manufactured by wind and rock together, most of the year.

In boreal autumn the alisios reach their weakest magnitude of the year. A 2024 paper in Revista de Ciencias Ambientales sets this out. Alfaro Martínez, Alvarado Gamboa, Fallas López, Mora Sandí and Hidalgo León wrote it. They read the daily rain gauges along the Central American Caribbean slope, year after year, and looked for the dip.

Weaker wind means weaker forced ascent. Less air pushed up the slope means less cloud made over it. The machine keeps running, at lower speed.

The Instituto Meteorológico Nacional says the same thing in one line. The Caribbean regime is rainy throughout the year, with a decrease in February, March and October.

The Caribbean Low-Level Jet

The alisios do not weaken alone. The same study ties the September and October decrease to a weakening of the Caribbean Low-Level Jet. That is the fast band of wind running across the Caribbean Sea. The study also names wind components from the southwest and the west. Those come with the South Pacific Subtropical High.

Then the storms. The Caribbean Sea near Central America hits its yearly peak of cyclone frequency in exactly these weeks. Those systems pull westerly wind across the isthmus. Westerly wind feeds the Pacific slope and starves this one.

So it is one weather pattern, not two. Rain on the Pacific coast in late September, a calm morning at Cahuita. One fact, seen from two sides of a ridge.

The paper adds two associations on top. Phases 1, 2 and 8 of the Madden-Julian Oscillation, when the trade winds run weak. And periods when the North Atlantic sits warm relative to the equatorial Pacific.

Limón, 1981 and 1998

An average is not a promise. The window opens in early September and closes in mid-October. The dates and the day count sit in the block below.

Some years go much further. The Limón station recorded 54 percent less rain than normal in September 1981. In September 1998 the deficit was 76 percent. Other years it barely shows at all. A mean drawn across three decades hides both kinds of year inside it.

Scale matters here too. A relative minimum on this coast is still a wet place. Drier than the same coast in July. Not dry. What a wet place looks like from the garden is an early hour of water and a clear day behind it. The light shell travels either way.

The reef at Cahuita

Cahuita National Park was created by decree on 7 September 1970. It holds 1,106 hectares of land, a 600-hectare coral zone and 22,400 hectares of sea. SINAC describes the reef as the most developed on Costa Rica's Caribbean coast: 35 coral species, 123 fish species, 140 molluscs, 44 crustaceans, 128 algae.

What weak wind does to water is not complicated. Wind makes waves. Less wind, smaller waves. Flat sea and clear water tend to arrive in the same week, because the same thing produces both. We are not going to print a visibility figure. We have not found an honest one.

On a morning like that the surface at Playa Grande holds the light instead of breaking it. The reef edge shows as a dark line from the sand. Eight hundred metres of path, ten minutes down through the jungle. The sea is already flat when you get there. March and April do this too, in a normal year. September is not the only good water on this coast. It is the stretch the record explains best.

Rain matters to that water for a second reason. The reef takes sediment and agrochemicals from the La Estrella River. The currents carry them down and spread them along the coast. That pressure runs all year and has nothing to do with the season. It is the reef's real problem. No calm week fixes it.

Tortuguero

North up the same coast, the turtles are still working. The Sea Turtle Conservancy has run research at Tortuguero since the 1950s. It calls this the largest nesting population of green turtles in the Atlantic. The nesting season covers these weeks. A female can reach 420 pounds, which is 190 kilos. She hauls that weight up a beach in the dark, digs, lays, and goes back down.

Tourism sites will name a peak month for you. The Conservancy gives the season and no peak, so neither will we. The dates it does give are in the block above.

The rest holds up on its own. The wind that makes this coast wet takes its yearly rest. The rain follows the wind, and so does the sea. Then the alisios pick up again, and the window closes.

What the record holds is smaller than a forecast and steadier than a hunch. On this coast the calm weeks and the wet season are the same season. One engine, running slower once a year, on the far side of a ridge from the storms that drive it. The road, the drive from San José and the rest of the calendar are on the practical page.