Climate variability

Climate variability is different from “climate” which, within a given geographical area, is the averages of temperature, wind direction and speed, atmospheric pressure, humidity and a number of other meteorological parameters, calculated over a sufficiently long period so as to be called "normal." On the other hand, “climate variability” is dependent on extreme atmospheric conditions that far exceed the normal. The phenomena that produce these extremes are highly organized cold fronts, dry stationary cells, hurricanes, tropical disturbances, and cells with disproportionate humidity. Paradoxically, extreme meteorological events can include both excessive rainfall and extended drought. As in many parts of the world, such phenomena in this region are thought to be associated with ENSO—the El Niño Southern Oscillation.

ENSO is a cyclical phenomenon initiated with changes in temperatures in the tropical Pacific Ocean (Map 4). The two phases of the cycle (El Niño, the warm phase, and La Niña, the cold phase) influence air pressures, precipitation totals and temperatures worldwide, which can be above or below normal depending on where they are being measured. Significantly, El Niño episodes cause more precipitation across the Pacific to about 86º west latitude and less precipitation from there to the east; La Niña episodes do just the opposite (Figure 1). Consequently, since Costa Rica and Nicaragua are at the precise latitude where the effects of El Niño change, ENSO impacts vary depending on where one is in the two countries. Though well-defined effects do occur when the intensity of El Niño is strong—for example, in 1982 and 1997 when rainfall totals in the area were far below normal. The 1997 occurrence was probably the strongest of the twentieth century and the Pacific Slope and Central Valley of Costa Rica experienced a large deficit of precipitation while the Atlantic slope had a 40% increase above normal. Interestingly, however, during 2001 all of Central America suffered conditions of drought without the presence of an ENSO event.

Each phase has an average period of recurrence of about four years although the historical record shows variations of between two and seven years. Generally, each El Niño episode lasts between nine and twelve months beginning June to August. Most peak during December to April and decay during May to July of the next year. However, some episodes may last two years and longer. Recent research also suggests that ENSO can influence the formation of hurricanes where a strong El Niño can inhibit their formation while a strong La Niña episode can provide relatively more favorable conditions for hurricane formation in the Atlantic.

For example, the regional drought of 1997-98 in Central America was an El Niño event while the two most recent important hurricanes to hit Central America (Cesar in 1996 and Mitch in 1998) occurred when La Niña was active. Hurricane Cesar was one of the more destructive in Costa Rican history and Hurricane Mitch is considered the most destructive in all of Central America causing thousands of deaths and injuries, and billions of dollars in damage to housing, infrastructure, agriculture, ranching, and fishing—all of which continue to inhibit socio-economic development throughout the region.

There is no doubt that individuals and communities in the basin are inadequately protected from climate variability—and they suffer accordingly. Poverty, which acts to magnify the negative effects of climate extremes, does not often allow the extra resources required to prevent or adequately mitigate problems brought on by climate variability. However, communities, old and new, remain in the basin and, however insufficient, coping mechanisms do evidently exist and they appear to have some degree of success.

The basin can be divided by defining an eastern tropical rainforest region with rainfall between 4,000 mm and 6,000 mm per year; a western tropical savanna region in the Lake Nicaragua drainage where rainfall is 700 mm to 2,500 mm and a strong dry season; a middle region with rainfall of more than 2,500 mm and no marked dry season; and a Montane region with rainfall again reaching 3,000 mm to 4000 mm annually (Map 5). Table 2 presents average monthly rainfall data from stations within the SJRB with the three highest months at each station given in bold and the three lowest given in italics to indicate the wet and the dry seasons.

The average temperature in this region is between 22 °C and 24 °C, but in the tropical forests monthly averages are between 23°C and 28°C. In the highest regions of the river basin along its southern limit, temperatures drop to 9°C in the rainy season and 11°C in the dry season. Relative humidity is also high; in the tropical rainforest it attains more than 90%, while in the low northern savanna it is 62%. Evaporation rates, of course, vary with temperature and relative humidity. In the Nicaraguan area of the basin, evaporation rates have been recorded from 2,376 mm in Altagracia (62 meters above sea level) to 1,748 mm in Boaco (360 meters above sea level).

Lake Nicaragua's only outlet is the San Juan River. Initially the river is entirely within Nicaragua, but approximately five kilometers downstream from El Castillo; its right (southern) margin becomes the international border between the two countries. The river strikes a course towards the southeast and, 205 km from its start, separates into two branches that empty into the Caribbean approximately 20 km apart.