Walker circulation

Walker circulation In the 1920s and early 1930s Sir Gilbert Walker, an English meteorologist, published a series of papers which suggested that the weather in Djakarta (Indonesia) and Santiago (Chile) was related in such a way that when the pressure was higher than normal at one place it was lower than normal at the other. Since these cities are about 15 000 km apart it was difficult at that time to visualize a mechanism for the connection between these distant locations. It is now understood that what Walker had discovered was part of a teleconnection now known as the Southern Oscillation. Teleconnections are links between one part of the world and another so that variations at one location are linked to variations at the other. The size of the variations is, however, usually very small and may be indistinguishable in the normal day-to-day variations in weather patterns. Modern research shows that the anti-correlation found by Walker extends over the whole of the southern tropical Pacific Ocean and acts like a see-saw pivoting about a line at approximately 170°E. When the region to one side of this line has higher than normal pressure the other side has lower than normal pressure. The see-saw moves extremely slowly and somewhat erratically. The period of oscillation varies between three and seven years.

The large-scale circulation of the atmosphere and oceans must be responsible for these long-term variations. Over the tropical oceans the dominant circulation is the Hadley circulation. Air over the warm oceans near the Equator rises through the troposphere to altitudes up to 16 km and then moves poleward towards both the north and south, gradually descending in the subtropical anticyclone belts around 30° north and south. If water in the west of the tropical Pacific is warmer than that in the eastern Pacific, then there will be a relative strengthening of the rising air in the west and weakening of the rising air in the east. Since this implies that more air is removed upwards in the west and less is removed in the east, a horizontal flow at low levels from east to west is required for the circulation to be sustainable. Also an opposite flow is required at high levels to complete the circulation. The complete three-dimensional circulation is complex but it can be visualized as having three important components: the Hadley circulation in the north–south direction; the effect of the rotation of the Earth, which means that the winds are diverted to the right of the direction implied by the Hadley circulation; and the ‘Walker’ circulation with air rising in the eastern Pacific moving eastward at high levels and sinking over the eastern Pacific to return to the western Pacific near the ocean surface.

The Southern Oscillation is caused by small periodic variations in the Walker circulation. These are not completely understood but are related to the ocean circulation, which periodically produces strong upwelling of cold water in the eastern Pacific of cold water which then travels westward. This linked atmosphere ocean circulation is also referred to as the El Niño circulation.

Charles N. Duncan

Bibliography

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