Much of the rainy weather has its origin in Africa

Tropical waves are lined up across the Atlantic is the explanatory graphic. Graphic: U.S. National Oceanic and Atmospheric Administration

For about eight months of the year, Costa Rica is under siege from weather patterns that form over Africa. These are the infamous tropical waves that bring clouds and rain. Sometimes they combine with home-grown weather instability to really drench the country.

Although these waves cannot be seen, they are far from abstract. Mexico’s Bay of Campeche is getting rain at this writing thanks to a tropical wave. Meteorologists with the U.S. Hurricane Information Center estimate that there is a 50 percent chance that this disorganized system will become a significant storm or cyclone.

Meanwhile there are two more tropical waves making their way westward across the Atlantic. One is off the coast of Venezuela moving at about 20 knot or about 23 mph, said the center. The low-pressure trough of this system stretches north to the Dominican Republic. This likely will have an effect on Costa Rica’s weather in a few days.

Yet a third wave is several days behind, according to the center.

A lot of expats from the United States and Canada are unfamiliar with tropical waves because only infrequently do these systems touch the U.S. Southwest and create monsoon conditions over the dry landscape.

In Costa Rica they are a major weather factor. Some scientists also think that cyclones in the Pacific may have a relationship with wave that have passed into that ocean. Some waves can actually go around the globe.

There are about 60 generated from instability over Africa each year, according to the U.S. National Oceanic and Atmospheric Administration. While only about 60 percent of the Atlantic tropical storms and minor hurricanes originate from
easterly waves, nearly 85 percent of the intense or major hurricanes have their origins as easterly waves, said the agency.

The waves usually appear to be about three to four days apart or separated by 2,000 to 2,500 kilometers, some 1,200 to 1,500 miles, said the agency.

Scientists have been studying this phenomenon since the 1930s, but they still have a lot of questions. For one thing, according to the National Oceanic and Atmospheric Administration, scientists do not know how the waves change each year in both intensity and location.

In June 2010 the region from Golfito to Jacó took it on the chin as one wave passed through. High winds downed trees and telephone lines and probably did millions of dollars of damage to the tourism industry. That was tropical wave No. 8 for the year. Then No. 9 came through a day later bringing more rain and even some to the Central Valley. No. 11 was not far behind.

This is the storm that closed Parque Nacional Manuel Antonio and caused tourists to leave.

Many of the major storms that have hit Costa Rica were associated with tropical waves, although some waves pass overhead with little notice.

The Instituto Meteorológico Nacional put out an alert Monday afternoon based on high temperatures during the day. The weather service predicted heavy thunderstorms in the evening and early morning. The Central Valley appeared late Monday to have been spared, but the prediction for Tuesday is the same except that the heavy downpours are supposed to be in the afternoon in the Central Valley, the northern zone and the Pacific coast.

A tropical wave is not involved in this forecast, but if it were, weather experts would call it an onda tropical in Spanish.

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