Puerto Rico DroughtWatch
Climate of Puerto Rico
In general, the east-west trending Cordillera Central and Sierra de Cayey mountains form an insular hydrologic divide that separates the island of Puerto Rico into two climatologically distinct regions. The northern two-thirds of the island has a relatively humid climate whereas the southern one-third of the island is semi-arid.
The National Oceanic and Atmospheric Administration (1982) subdivided the island of Puerto Rico into six climate areas and designated a seventh climate area in the outlying islands (fig 1). Local climate variability within each subdivision and in the outlying islands is attributed to local differences in topographic relief and the effect of prevailing trade winds. Along the northern coast of Puerto Rico and in the outlying islands, prevailing trade winds are from the northeast. Along the southern coast of Puerto Rico, the east-west trending mountains affect wind patterns. In this area, the predominant wind direction is from the southeast during daylight hours, whereas prevailing winds are from the northeast during the period from midnight to early morning. Prevailing winds along the western end of Puerto Rico at Mayagüez differ from other areas on the island. Predominant winds in this area are from the west, which is nearly opposite to the prevailing northeast trade winds. Prevailing west winds are infrequent throughout other areas of the island. For example, in the San Juan area, west prevailing winds occur less than 3 percent of the time and generally are associated with the passing of cold fronts across the island.
Figure 1. Climatic subdivisions of Puerto Rico and outlying islands.
Mean monthly air temperatures in Puerto Rico and the outlying islands vary little throughout the year. Air temperatures generally are warmest during the month of August and coolest during the months of January and February. In coastal areas, annual air temperatures range from a mean maximum of 27 degrees Celsius (°C) to a mean minimum of 24 °C. In interior mountainous areas (that is, Eastern and Western Interior climate subdivisions; fig. 1), annual air temperatures range from a mean maximum of 25 °C to a mean minimum of 22 °C.
Air temperatures fluctuate little throughout the year as a result of relatively constant insolation (that is, solar radiation) and seawater temperatures. The rate of delivery of solar radiation is nearly constant because the difference in daylight hours varies little throughout the year. Between the longest day of the year (13 hours, 13 minutes) and the shortest day (11 hours, 2 minutes), the amount of daylight differs by only slightly more than 2 hours. Mean monthly seawater temperatures vary by only about 4 °C; the mean maximum water temperature of 28 °C occurs in October, and the mean minimum water temperature of 24 °C occurs in January.
The spatial distribution of rainfall in Puerto Rico is variable (fig. 2). Rainfall is greatest in the Sierra de Luquillo rainforest in the eastern part of Puerto Rico. The mean annual total rainfall in Sierra de Luquillo is 169.0 inches per year (in/yr) (4,305 millimeters per year (mm/yr). The least amount of rainfall occurs in the vicinity of Guánica at Ensenada in southwestern Puerto Rico. In this area, the mean annual total rainfall is 30.0 in/yr (768 mm/yr).
Figure 2. Distribution of mean annual precipitation in Puerto Rico
Pronounced orographic effects from the Cordillera Central and the Sierra de Cayey mountains in Puerto Rico result in high rainfall amounts occurring on the windward side of the mountains, which is north of the insular hydrologic divide. The opposite occurs south of the divide on the leeward side of the mountains, where coastal areas lie within a rain shadow resulting in lower rainfall amounts. Orographic effects on rainfall are not as pronounced in the outlying islands, with the exception of Vieques.
Rainfall on the island of Vieques differs substantially between the leeward or west side of the island and the windward or east side of the island. In general, rainfall occurs more frequently on the west side of Vieques. Although rainfall data are insufficient to quantify amounts, estimates indicate that the difference in rainfall between the east and west ends of the island could be as much as 13.6 in/yr (345 mm/yr). Rainfall in the eastern part of the island may be similar to that of the island of Culebra where the mean annual total rainfall is 36.4 in/yr (925 mm/yr). The mean annual total rainfall in the western part of Vieques, at Playa Grande and Resolución, is 50.0 in/yr (1,270 mm/yr). The mean annual total rainfall on the island of Mona is 36.0 in/yr (915 mm/yr).
Although rainfall data are limited, certain vegetation adapted for life with a limited water supply (such as xerophytes) serve as strong indicators about rainfall distributions on Vieques. The growth of xerophytes is prevalent on the east side of the island where rainfall is limited as opposed to the west side of the island where rainfall is more abundant.
Major rainfall events producing substantial volumes of rain in Puerto Rico and the outlying islands are caused by one of two climate mechanisms—the passage of an easterly wave or the passage of a cold front. Easterly waves generally occur during May to November with some having sufficient intensity to evolve into tropical storms and (or) hurricanes. Cold fronts generally occur during November to April and may produce sufficient rainfall to cause flooding even during the period from December to March, which is a relatively dry period. The number of easterly waves or cold fronts passing over the region in any given year ultimately determines whether the region experiences relatively dry conditions or wet conditions. As a result, localized droughts occur yearly within many of the geographic areas of Puerto Rico.