Читать книгу Exploring the Solar System - Peter Bond - Страница 79

Changes in oceanic circulation can lead to variations in heat transport and very different weather patterns. One of the most important of these periodic variations in the ocean current and atmospheric circulation is the El Niño – Southern Oscillation (ENSO) which occurs over the Pacific Ocean (Figure 3.17).

Under normal conditions, the trade winds blow towards the west across the tropical Pacific. These winds pile up warm water in the western Pacific, so that the sea surface is about 0.5 m higher in Indonesia than off the coast of Peru. As a result, the sea surface temperature is about 8°C higher in the western Pacific. Moist air rising over the warm water results in convectional clouds and heavy rainfall. In contrast, the upwelling of cold water from depth off the western coast of South America causes the overlying air to be stable, so rainfall is low.

This pattern changes every two to five years. During an El Niño⁸, the trade winds weaken and the equatorial countercurrent strengthens, sending warm water eastward towards Ecuador and Peru. The storm pattern also shifts toward the east and the upwelling of cold water in the eastern Pacific is suppressed. Instead of drought conditions, the west coast of the Americas experiences heavy rainfall, but there is decreased rainfall in the western Pacific (e.g. Australia and Indonesia).

El Niño is often replaced by La Niña, which is characterized by unusually cold ocean temperatures in the equatorial Pacific and a tongue of cool water which extends farther westward than usual. The reasons for these major changes are not well understood.

Changes in the ENSO system of the Pacific Ocean also influence wind patterns, rainfall, and temperature in many other parts of the world.