Oceanography

El Nino

Wind driven reversal of the pacific equatorial currents resulting in the movement of warm water toward the coasts of the Americas, so called because it generally develops  just after Christmas.

On the sheltered or lee side of the tropical landmasses, under the trade winds of the Pacific, the upwelling of deep oceanic water is nearly constant process. From time to time, this process falters; the trade winds strengthen, and warm tropical surface water accumulates on the west side of the pacific. This even is followed by the winds losing their driving force; the upwelling lessens, and the mass of warm western Pacific water moves eastward across the ocean to accumulate along the coast of the Americas. This sequence of events is known as El Nino.

A severe El Nino event affects weather systems over large areas of the earth. In 1982-83 the polar jet stream was displaced far southward over the Pacific Ocean, bringing unusually dry conditions to Hawaii and a strong low pressure system to the Gulf of Alaska that resulted in high winds and high precipitation along the west coast of the United States. Heavy rains occurred in Ecuador, Peru, and Polynesia, while droughts came to Australia, the Sahel of Africa, southern India, and Indonesia. At the same time lower surface temperatures in the North Atlantic made the hurricane season the quietest in over fifty years.

The exact cause of El Nino is still in doubt, but certain processes have been identified with its appearance. One process, in which atmospheric pressure increase on one side of the Pacific, decreases on the other, and then reverses, is known as the Southern Ocean Oscillation. The pressure centers associated with this Oscillation lie over Easter Island in the eastern Pacific and Indonesia in the western Pacific. Under normal conditions, there is a high-pressure system over Easter Island and a low-pressure system over Indonesia, the trade winds are strong and constant and upwelling occurs along the coast of Peru. When the atmospheric pressure system reverses, the southeast trade winds break down.

A severe El Nino uses the changes in the Southern Ocean Oscillation and the surge in strength of the trade winds, which appears to precede their decline and the development of the westerlies.

The alternation between these two events has been quite regular for the last hundred years, except for the periods between 1880 and 1900 when La Nina conditions prevailed, and the 1975-88 periods of El Ninos. Look carefully at figure 7.23 and notice that the 0 ⁰ C line represents the long-term average sea-surface temperature over the last forty years. If this line were redrawn between 0 ⁰ C on the left and 0.5 ⁰ C on the right, allowing a half-degree rise in ocean surface temperature, the regular alternation between El Nino and La Nina events would appear as a more regular pattern. Recent analyses of surface water temperatures off the southern California coast during this period (1950-1990) indicate an average water temperature increase of 0.8 ⁰ C. There are at least two possible mechanisms that could lead to such a warming of ocean surface water: (i) the oceans response to global warming or (2) a reduction in upwelling in the Pacific Ocean associated with changes in North Atlantic circulation.