The cold waters of the Humboldt ocean current are generated off the southern tip of South America and flow up along the coast of the continent, creating the world’s driest desert (the Atacama) in western Chile and Peru. The warm waters of the El Niño ocean current, generated offshore of tropical Central America, create the wettest forests in the world, on the coast of Colombia. Coastal Ecuador is located at the convergence of these two ocean currents and the ecological extremes which they create, accounting for the dramatic climatic variations that characterize the Pacific Equatorial Forest.
The point of convergence between the two currents oscillates between the north and south on an annual basis. Starting in December, the warm El Niño current moves southward and brings with it heavy rains that last through June. Starting in July or August, the cold Humboldt current pushes the El Niño current back north and accordingly generates cool, dry weather.
Throughout coastal Ecuador, rainfall varies sharply, along both the north-to-south gradient and as a function of elevation. Average annual rainfall ranges from over 4,000 mm at the Colombian border to as little as 100 mm at the Peruvian border. Even within the same latitude, there can be xeric (very dry) conditions along the shoreline and then pluvial (very wet) conditions as little as six kilometers inland along the peaks of the coastal mountains. Species characteristic of both of these extreme conditions can often be found cohabitating within the same stream valley.
A layer of fog generated from the ocean shrouds the peaks of the coastal mountains most of the year, creating cloud forest. At this elevation much of the annual precipitation is generated in the form of fog drip (also called horizontal precipitation), in which fog condenses on plant foliage and then drips down onto the ground below. The cloud forest is wet and green every day of the year, even during the dry season. Conversely, the forests along the shores of the ocean are deciduous and lose their leaves during the dry season, in the same way that a temperate forest sheds its leaves in winter. In some parts of the Pacific Equatorial Forest, cloud forest and deciduous forest are separated by as little as four kilometers. Between these two local extremes lies tropical moist evergreen forest.