Environmental Research Division

The tropics and extratropics are closely linked via the atmospheric Hadley-Walker circulation (see schematic below). Trade winds from the North and South Pacific Highs (H) warm as they move toward a low (L) in the western tropical Pacific. Rising warm, moist air feeds tropospheric winds that return to the extratropics via the Hadley-Walker circulation. This air gradually cools and sinks as it moves to the northeast and southeast, supplying mass to the highs (H) that are the sources of the trade winds, important vectors for the transport of momentum, energy, and mass between the tropics and extratropics. During a La Niña (El Niño) this circulation is stronger (weaker).

The above schematic shows that a large region of low SLP near Indonesia is coupled to two areas of high SLP in the extratropics: the North Pacific High (NPH) in the northeast Pacific and the South Pacific High (SPH) in the southeast Pacific. On interannual time scales, SLP anomalies at the NPH and SPH are have the opposite sign of those over the western tropical Pacific-southeast Asia. Differences between Tahiti and Darwin mainly reflect changes in the zonal Walker circulation, the zonal component of the atmospheric flow

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