El Niño and Global Warming

By Rasmus Benestad & Raymond Pierrehumbert

This is the first part of a planned mini-series of 3 posts on tropical climate, circulation, and oceanic response in conjunction with a global warming. Climate change related to a global warming is more than just temperature and precipitation -massive atmospheric circulations change too, and these changes can have consequences.

The name ‘El Niño’ originally was given to a change in the coastal current (usually flowing from south to north) near the Peruvian coast during anos de abundancia. Sir Gilbert WalkerPaita sailors who used to sail north-south direction along the coast called the counter-current ‘El Niño’, after the Child Jesus because it had a tendency to appear soon after Christmas (the reason for this seasonality is not yet fully understood, and some of the strongest events peaked earlier in the year). The counter-current (reversal to north-to-south flow) usually would appear in concert with rains in otherwise dry regions. El Niño is also associated with warm surface water in the eastern tropical Pacific.

However, the name ‘El Niño’, which originally has its origin from changes in the ocean, is linked to changes in the atmospheric circulation. The understanding of the atmospheric circulation changes, later to be discovered to be connected to the appearance of El Niño events, was originally motivated by reasons other than ocean currents. Sir Gilbert Walker (photo to the left) was motivated by the question why the South Asian Monsoon sometimes failed from one year to another. There was a catastrophic drought and a subsequent famine in 1877, and the question was why do such events take place? Walker discovered that the sea level pressure fluctuations over the Indian Ocean and tropical Pacific tend to vary with opposite phase. He named this dipole of opposite variations ‘the Southern Oscillation’. The atmospheric circulation associated with this dipole pattern is known as the Walker circulation. The Walker circulation refers to the mean (steady) ciculation where air over the warm pool in the western part of the tropical Pacific rises, being fed by the easterly surface trade winds across the Pacific, and subsidence over eastern Pacific. The Southern Oscillation refers to the inter-annual variations in this circulation.

Figure showing the Walker circulation (left) and the Hadley circulation (right).

It is important not to confuse the Walker Circulation with the Hadley Circulation (also known as the ‘Hadley Cell’), which also involves deep convection in the tropics. Whereas the Walker Circulation (or ‘Walker Cell’) refers to an air flow parallel with the equator – all in the tropics – the Hadley Cell involves air rising in the tropics (follows the solar equator and gives rise to the Inter-Tropical Convergence Zone (ITCZ) which then flows polewards before sinking in the subtropics. The Walker Circulation involves an east-west asymmetry, whereas the Hadley Cell in principle does not.

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