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Catalogue of the Vascular Plants of Ecuador

Main | Introduction | Geography | Geology | Paleoclimates | Climates
Vegetation | History of Collecting | Format of the Catalogue
Results | Acknowledgements | Search the Catalogue

Figures in this article are in PDF format and can only be
viewed using Adobe Acrobat Reader (free download).


Climates

By David A. Neill and Peter M. Jørgensen

A large variety and range of climatic regimes are found in Ecuador, and this variety has a major effect on the extension of the vegetation types and on the diverse flora of the country. The climatic regimes found in Ecuador are influenced by its geographical position astride the equator, the general circulation of the atmosphere, the position and movements of the ocean currents, and by orographic effects produced by the abrupt topography of the Andes as well as the smaller coastal ranges.

Descriptions of climatic patterns in Ecuador are found in Naranjo (1981), Porrut (1983), and Cañadas (1983); each of these works includes tables and diagrams of precipitation and temperature records for a number of sites in the country, and together provide the basis for the following summary of Ecuador's climates. Another useful source is a discussion of climate in the tropical Andes by Sarmiento (1986). The study of climate in Ecuador is hampered by a shortage of long-term records and, particularly, by complete lack of records from many remote, sparsely populated areas where climatic extremes such as very high rainfall are presumed to occur but have not been measured. Only a few sites in the country have meteorological records of 50 years or more—mostly larger cities such as Quito, Guayaquil, and Cuenca. A network of more than 125 meteorological stations throughout Ecuador was established in the early 1960s.

Due to Ecuador's position on the equator, the day length changes very little throughout the year—every day has about 12 hours of sunlight, varying no more than about 30 minutes at any point in the country. On the equator, the total amount of solar radiation reaches a maximum at the equinoxes; this is only 13% higher than the minimum amount of radiation intercepted at the solstices. A consequence of this relative annual constancy in solar radiation is the low seasonal variation in mean air temperature at equatorial latitudes. From month to month, the mean temperatures at all sites in Ecuador are relatively constant; monthly means do not vary more than 3°C at any site, and at many sites vary less than 1°C. In contrast, the daily fluctuations in temperature over 24-hour periods are much more pronounced; the circadian cycle of temperature change is therefore much more important than the annual change in mean temperature. Daily temperature fluctuation at mid- to upper elevations in the Andes is often 20°C or more. In the lowlands, the daily fluctuation in temperature is generally much less, closer to about 10°C. The daily maxima and minima do have significant annual variation at some sites, for example, at high elevations where freezing temperatures are more prevalent during the dry season with clear skies.

Elevation Temperature in ºC Temperature in Ecuador varies rather predictably with altitude. At sea level in coastal Ecuador, the mean annual temperature is about 25°C. On moist tropical mountains, following the adiabatic lapse rate, temperature decreases at about 0.5°C for each increase of 100 m in altitude. The lapse rate, as determined from climatic records at various elevations, is slightly different for the western slopes versus the eastern slopes of the Andes (Cañadas, 1983), and sites in the inter-Andean valleys are somewhat warmer than sites at equivalent elevations on the outer slopes of the cordilleras (Naranjo, 1981). Nevertheless, the relative constancy of the adiabatic lapse rate allows us to approximate the mean temperature at different altitudes in Ecuador (the values at a particular site may differ by as much as 1.5°C from the approximate mean temperatures in Table 1).
Sea level 25.0
500 m 22.5
1000 m 20.0
1500 m 17.5
2000 m 15.0
2500 m 12.5
3000 m 10.0
3500 m 7.5
4000 m 5.0
Table 1. Approximate mean temperature at different elevations.

Freezing temperatures occur at about 3,000 m elevation in the Andes and are increasingly frequent at greater elevations. Frosts are rare below the 3,000-m line and have never been recorded, for example, in the center of Quito at 2,800 m elevation, the only site in Ecuador with a 100-year meteorological record. The lowest temperature recorded in Quito is 2°C.

In contrast to the constancy of temperature regimes in Ecuador, rainfall regimes vary enormously from place to place, in both the annual amount of precipitation and in the patterns of seasonal distribution of rainfall. Different patterns of rainfall are found in the Coastal, Andean, and Amazonian regions of continental Ecuador, and in the Galápagos Islands; variation also occurs from north to south in each main geographical region, and on a local scale according to topography and other factors. Figure 1 is a generalized climatic map of Ecuador, showing elevation contours (which correspond approximately to the mean temperature values indicated in Table 1), annual precipitation isohyets, and Walter climate diagrams (Walter, 1979) for 23 sites throughout the country, selected to represent the range of climatic patterns in Ecuador.

Annual patterns of rainfall in the Andean region of Ecuador are profoundly influenced by the oscillations of the Intertropical Convergence Zone (ITCZ), the trough of low pressure between the large currents of continental air masses north and south of the equator, that is associated with cloudiness and heavy precipitation. The ITCZ shifts from a position at about 10°N latitude at the June solstice, to about 5°S latitude at the December solstice. Therefore, the ITCZ passes over Ecuador twice during the year on its northward and southward oscillations. The shifts in the ITCZ produce a bimodal distribution of rainfall at Andean localities in Ecuador, with two rainy periods and two drier periods during the year. This bimodal pattern is illustrated by the climate diagrams for Tulcán, Mira, Ibarra, Quito, Tumbaco, Cotopaxi, Riobamba, and Cuenca (Figure 1). In the Ecuadorian Andes, the major dry season is during July–August and, in some cases, extending to September. A less pronounced dry period is discernible at most sites during January. Periods of high rainfall for most Andean sites are during March–April and again in October. Precipitation frequently occurs as violent afternoon thunderstorms, sometimes with hail at sites above 2,500 m. The mean annual precipitation for the sites indicated varies from about 1,250 mm for Quito, to just 400 mm for Riobamba.

In the coastal region of Ecuador, annual rainfall patterns are under the influence of the two principal ocean currents in the Pacific, near the shore of northwestern South America. These include the cold Humboldt Current, which flows northward along the coast of Chile, Peru, and southern Ecuador, and turns eastward at about the equator and flows past the Galápagos Islands. The second is the warm equatorial current that flows southward from the Gulf of Panama, along the Pacific coast of Colombia, and meets the Humboldt Current near the equator along the north-central coast of Ecuador.

The Humboldt Current brings arid conditions to the adjacent coast, as the cool oceanic air passes over the relatively warmer landmass. Another effect of the Humboldt Current is the overcast skies—the low clouds, known locally as garua—that form a layer about 600 m above sea level and cover most of western Ecuador throughout the dry season.

The warm equatorial current that bathes the northwest coast of Ecuador brings with it moist air and rainfall. During most years, the warm equatorial current pushes farther to the south of the equator for a few months, December to April generally, bringing rainfall and warm, moist air to the areas of the central and southern Ecuadorian coast that are under the influence of the dry, cool Humboldt Current the remainder of the year. This phenomenon is known locally as El Niño (the Christ Child) because the annual rains usually begin in mid- to late December, around Christmas.

Due to the annual southward incursion of the warm equatorial current, most of coastal Ecuador, as well as the Galápagos Islands, has a unimodal pattern of precipitation, with one rainy season extending from December to April or May, and a long dry season from May to December. The length and intensity of the dry season vary at different sites in the coastal region. Salinas, for example, at the western tip of the Santa Elena peninsula, is most strongly affected by the Humboldt Current flowing just offshore, and receives only about 125 mm of rain annually, mostly during February and March. Guayaquil, at the mouth of the Guayas River, and farther from the influence of the Humboldt Current, receives nearly 1,000 mm of rain, with a 7-month dry season. Pichilingue, in the inland Guayas valley and farther north, receives over 2,400 mm but experiences a significant 4-month dry season from August to November. San Lorenzo, in the northwest corner of the country, has a climate influenced by the warm equatorial current and has only a short dry period around November. Inland areas on the coastal plain, near the northern border with Colombia, probably receive more than 5,000 mm of rain annually, but meteorological records are lacking.

At irregular intervals, but averaging about every seven years, the El Niño phenomenon is much stronger than normal along the Pacific coast of South America. During El Niño years, the warm equatorial waters push much farther south into coastal Peruvian waters, displacing the cold Humboldt Current, bringing heavy rains to the Peruvian desert as well as coastal Ecuador. The warm water conditions may last for more than a year before the Humboldt Current again brings dry weather to the coast. The heavy rains associated with El Niño cause flooding in coastal Ecuador and destroy roads, bridges, houses, and crops. The last two major El Niño events were during 1982–1983 and 1997–1998.

In Amazonian Ecuador, rainfall is relatively constant throughout the year. The extensive forests of the Amazon basin recirculate moisture through evapotranspiration, and the relative humidity of the atmosphere above the Amazonian forests remains high throughout the year. Convective and orographic effects produce rainfall, sometimes as afternoon and evening thunderstorms, and sometimes as steady drizzles that may last several days. Amazonian sites in Ecuador, exemplified in Figure 1 by the climate diagrams for Tena, Nuevo Rocafuerte, Papallacta (on the eastern slopes of the Andes, with an Amazonian rainfall regime), Sucúa, and Zamora, receive rain throughout the year; no month shows a moisture deficit for these sites. The Amazonian sites do, however, have periods of somewhat lower rainfall, generally during August and again in January, the months that correspond to the dry season in the Andes associated with the movements of the ICTZ.

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