60-year trend shows tropical trade winds weakening in Pacific too – ENSO inclination towards El Niño state

trade winds El NiñoIt is a question that has puzzled climate scientists for a long time:

Under global climate warming, how will the El Niño/La Niña Southern Oscillation (ENSO) respond?

There had been suggestions before that a warmer world would see more dominant El Niño phases – based in part on research indicating there could have been a ‘permanent El Niño’ during the warmer climate of the Pliocene – something a later study disproved.

Without much theoretical background to support expectations for any long-term development – today researchers led by the Department of Meteorology of the University of Hawaii report in a publication in Nature that their observations do show a multi-decadal trend.

The Walker circulation (or Walker cell) which normally – through easterly trade winds – leads to the accumulation of warm tropical waters in the West Pacific (compensated by higher-altitude winds blowing back from West to East) has been weakening in the past 60 years.

As a result the normal sea surface temperature difference between the tropical West and East Pacific has decreased by 0.3-0.4 degrees Celsius, the researchers say – and that’s an ENSO state determining difference.

A weak Walker circulation is the meteorological driver of El Niño [which is indeed also influenced by East Pacific ocean upwelling]. When the trade winds are weak, more tropical warm waters remain in the eastern equatorial sector. This leads to locally increased convection, which sucks in air across the ocean surface from all directions, including the West – establishing a blockade in the normal air cell circulation pattern, typical of El Niño.

As a result, the East-Pacific tropics would then experience increased rainfall – with relatively high air pressure and drier conditions further to the West.

A (semi-)permanent El Niño would be very bad news for Australian farmers, orangutans in the rainforests of Borneo, and all others in the West Pacific who are dependent on the return of wet years after prolonged droughts…

Same trend for tropical Atlantic

What is noteworthy is that in early 2011 the same research group has also published results showing the Atlantic trade winds have decreased – indeed, over that same 60-year timespan.

This could indicate that the wind pattern changes would be atmospherically driven – through a globally decreasing Walker circulation/general circulation – instead of being driven by sea surface temperature* anomalies:

If the weakening trade winds would have been not the cause but the result of a more dominant East-Pacific El Niño state – one would typically expect Atlantic trade winds to pick up in speed, as El Niño-induced storms suck in the trade winds coming – across the Caribbean, from the East.

[*) That is not what the researchers conclude with regards to the decreasing Pacific trade winds. In the new Nature publication they write: "[...]we perform experiments using an atmospheric model, and find that SST warming patterns are the main cause of the weakened Walker circulation over the past six decades (1950–2009).” Well, let’s say it’s all interconnected anyway…]

We do hope you’re still with us. Perhaps looking at a map can help.

[Here is one that shows current West-Pacific El Niño development by the way. Update: this one is better.]

© Rolf Schuttenhelm | www.bitsofscience.org

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