By Amanda Delaney
The Caribbean Sea typically has two seasons, a dry season that lasts from December through May, then a wet season that starts in June and ends in November. It is not coincidental that the wet season falls during the hurricane season in the Atlantic Ocean. Periodic tropical waves that move offshore western Africa and progress west to the Caribbean Sea usually deliver much of the rain that the islands experience. These tropical waves become more frequent during late summer through early autumn before diminishing later in autumn.
What drives these seasonal shifts and why are more areas in the Caribbean Sea more susceptible to rain than others? What are signs to watch ahead of tropical waves? Finally, how do El Niño and La Niña affect the amount of rainfall during certain times of the year?
The main weather feature that determines the onset of the wet season is the location of the Intertropical Convergence Zone (ITCZ). The ITCZ is typically located to the north of the Equator where the sun produces the maximum amount of heating. Here is where the northeast and southeast trade winds converge and produce lift, and therefore cloud cover/rainfall. On satellite imagery, the ITCZ can be located as a band of clouds that stretches from western Africa to northern South America or the southern Caribbean Sea.
During the winter, the ITCZ is located between 03N and 10N and the heaviest rain remains south of the Caribbean Sea. Any moisture comes from isolated showers and thunderstorms delivered by the trade winds. However, the air is drier this time of year and any moisture that does occur is typically brief. Sea surface temperatures usually reach their lowest at 26°C-27°C in the Caribbean Sea during this time of year.
As the sun angle shifts to the north, the ITCZ moves northward as well, and reaches its northern apex between approximately 08N and 15N during the summer. The monsoon rains shift farther north over Africa, allowing for tropical waves to develop and move west as early as late May through June across the Atlantic Ocean. The tropical waves become more frequent later in the summer and autumn due to increasing sea surface temperatures that further strengthen the showers and thunderstorms to the east and over the Caribbean Sea (averaging between 28°C-29°C). It is no surprise that the peak of tropical cyclone season occurs in mid-September due to the favorable sea surface temperatures. As we progress through autumn, the ITCZ will shift back to the south and drier conditions will resume by early winter.
Terrain plays a large role in how much rain falls over the islands. If there’s a similar elevation across the island, the rainfall is typically even across the region. If the terrain drastically changes (such as large hills or mountains), then as the moist air mass encounters the higher terrain, this air mass is forced to lift above the mountain and moves into cooler temperatures. The moisture condenses in the cooler temperatures, creating clouds and rain. This process is called orographic lift. By the time the air mass makes it over the mountain, usually the moisture has dried up and the west side (or lee side) of the mountain remains dry. Typically, the east side (or windward side) of the island has more lush vegetation because of the abundant rainfall while the lee side of the island is generally dry due to the mountain’s influence, or what is called a rain shadow.
A prime example can be found in Dominica, as shown in the table of rainfall reported at two airports. The first is Canefield Airport on the western side of Dominica, and the other is Douglas-Charles Airport on the eastern side. These airports are separated by a mountain range where elevations range between 300-1,400 m. The table below shows a rainfall difference during the dry season (December through May), the wet season (June through November, and the total annual rainfall. As you can see, the rainfall is highest at Douglas-Charles Airport where it is exposed to the weather to the east. However, some of this rainfall is blocked from moving over the mountains; therefore, Canefield Airport reported lower amounts.
There are signs several days in advance to watch for ahead of tropical waves. Depending on the strength of a tropical wave, a long-period building swell may develop two to three days ahead of the arrival of the tropical wave, as well as wispy high cirrus clouds. Approximately one to two days in advance, the barometric pressure will start to drop, the cloud deck will lower with more frequent showers and the humidity will increase. The wind will also increase and shift more out of the northeast.
When the tropical wave moves through, the winds will usually peak (ranging between 25-35 knots outside of heavier thunderstorms) and shift from northeast to southeast. There will be abundant showers and thunderstorms which will diminish after the tropical wave has moved through the area. After the tropical wave, temperatures will increase briefly for approximately 24 hours. These warmer temperatures are from sinking warm, dry air behind the tropical wave.
Finally, La Niña and El Niño can impact the amount of rainfall over the Caribbean Sea. If La Niña develops during the wet season, this generally allows for more tropical waves and tropical cyclones to develop, as well as overall greater rainfall over much of the eastern and northern Caribbean Sea. If La Niña occurs during the dry season, rainfall will increase over the western Caribbean Sea, particularly Costa Rica, Panama and Colombia. This occurs because the subtropical jet shifts farther south, drawing Pacific moisture over these areas.
During El Niño, the opposite regions experience higher rainfall than La Niña. During the normal dry season, higher rainfall can be found over the eastern Caribbean Sea while drought conditions typically develop over the western Caribbean Sea and Central America. During the wet season, a drier-than-normal summer/autumn usually occurs over the eastern Caribbean Sea due to a below- normal number of tropical waves and tropical cyclones that develop and move west across the tropical Atlantic Ocean toward the Caribbean Sea.
Although the amount of rainfall that reaches the Caribbean Sea generally dictates the season, terrain can greatly impact the amount of rainfall even on one island. Tropical waves and tropical cyclones usually provide the most rainfall during the rainy season. However, El Niño and La Niña can alter these typical seasons in certain regions of the Caribbean Sea depending on what time of year they occur. Last February, heavy rainfall created floods and landslides across Colombia which was likely caused by La Niña at that time. La Niña weakened back in March, and we are now in ENSO-neutral conditions (neither La Niña nor El Niño). However, a moderate El Niño is forecast to develop late this summer or early autumn. There is about a 25 percent chance this El Niño could become strong toward the end of the year. This has already affected the upcoming hurricane season forecast across the Atlantic Ocean where a below-average season for tropical development is predicted.
Caribbean Compass contributor Amanda Delaney is a senior meteorologist with Weather Routing, Inc. (www.wriwx.com)
