El Niņo and global warming

1. Introduction

Normally the West Pacific is moist. Warm air rises over there and decends in the East Pacific. This is the Pacific Walker Cell. Over the equator an Easterly wind blows. Together with Southerly winds along the Chilean and Peruvian coasts this causes upwelling of colder water. Most years around Christmas some warmer water appears along the Ecuadorian and Northern Peruvian coasts. This phenomenon was called El Niņo (the boy-child) by the local people. Every 2 - 7 years the warming of the water in the East Pacific is much more widespread, with dramatic (local, but also larger scale) consequences. This phenomenon is referred to in the literature as ENSO (El Niņo / Southern oscillation), or just the initial name of the local people of Peru: El Niņo.
During El Niņo the moist (and warm water) area around Indonesia moves to the middle of the Pacific. Indonesia, The Philipines, SE Asia and Eastern Australia are (much) dryer than normal, while Ecuador and North Peru are much wetter than normal. During strong El Niņo's the (Pacific) Walker Cell circulation is shifted to quite an extent.

2. Events in the past

In the past some dramatic weather incidents took place in which El Niņo may have played a role in. A notorious example is a devastating drought in India and Australia in the years 1789 - 1792. In these years there was a monsoon failure in South Asia. Millions of people perished in India in the famines which followed. In the records the El Niņo of 1791-92 is bookmarked as "very severe", probably about as strong as the 1982-83 and 1997-98 events.
Although the droughts in Australia and India preceeded the onset of El Niņo it may have aggravated them. Also some correlation between El Niņo and the monsoon in South-East and South Asia has been found from records in the past. This makes it interesting to see whether there is a connection between global warming and El Niņo. The potential impacts can be big around the Pacific, but maybe also in other parts of the world, e.g. South Asia.

Now the subject is addressed from three angles:

1. What are the possible impacts of El Niņo?; Do they vary?
2. Is there a connection between El Niņo and global warming?
3. Can the (distant) impacts of El Niņo be changed by global warming?

3. Looking at El Niņo from three angles

3.1. The impacts of El Niņo

As mentioned in the introduction, El Niņo is accompanied by shifts in the so called Walker cell circulations over the equatoral part of The Earth. Two circulation cells can be distinguished: The Hadley cells and the Walker cells.
The Hadley cell is the meridional exchange of air masses between the inter-tropical convergence zone (mostly near the equator, except in Asia during the Monsoon) and the subtropics. Air rises in the inter-tropical convergence zone, subsides in areas at about 30° latitude and returns as trade winds.
In Walker cells air masses along the equator are exchanged in a longitudinal way, like in the Pacific.

If one compares the 1982-83 and 1997-98 El Niņo's in both cases there were droughts in Indonesia and floods in Peru and Ecuador. But there were also differences. The biggest differences were over Australia and Eastern Africa. In 1982-83 East Africa was dry (due to subsiding air), while it was very wet at the end of 1997 (An eightfold of normal rainfall in Mombasa, a clear signal of convective activity). At the start of 1998 NE Australia was deluged, while in 1982-83 droughts dominated the picture.
This implies that, at farther distances, during each El Niņo a different picture arises. The Walker cells around the world do shift, but differently in each case, giving rise to great contrasts in terms of rainfall from case to case.

It is striking that the Monsoon over India was hardly affected by the 1997-98 El Niņo. I will come back on this at the end.

3.2. El Niņo and global warming

Now an interesting subject for further research arises. Is there a connection between the frequency and intensity of El Niņo and possible global warming? A first thing one can do is looking at the records, of which the qualitative records are quite long - going back to the conquest of the Inca Kingdom by the Spanish in the early 16th century.

Looking at these records reveils episodes with severe El Niņo's and less severe El Niņo's: Very active periods were around 1580, the 1720-ties, around 1790, 1880-ties and 1890-ties, and after 1982-83 the 1990-ties.
Periods with less activity: the middle of the 17th century, the middle of the 18th century, 1930-1960.
It is very difficult to recognize any trend in it. Statistically it cannot be proven that there is an appreciable correlation between El Niņo and the temperature on The Earth.

However: This doesn't prove that there isn't a possibility of "passing a critical treshold" boyond which a sort of switch to an other regime in terms of El Niņo frequency and/or severity might be possible.
A publications was made in Nature (ref. 1) in which model studies were made on the possible impact of Global warming on the occurence of El Niņo. In these studies indications were found that El Niņo warm events will become more frequent and more severe. Also stronger cold events would be possible.

The authors of the article correctly point out that there are still processes not well understood, such as cloud feedback, which might change the found results from the model work.

A recent publication at the KNMI site points in the direction that there is no correlation between the temperature on Earth and the activity of the El Niņo cycle. (ref. 3)

Concluding: Up to now no connection between global warming and El Niņo has been found. The most recent publication points in the same direction: No link with global warming.

3.3. Changing impacts of El Niņo

A last interesting question is the question whether the effects of El Niņo themselves could change as well in an event of global warming. In this case a recent publication in Science (ref 2.) puts some more light on it.
A study of the Indian Monsoon and El Niņo showed some correlation between the two. During El Niņo the Monsoon, in average, was weaker than normal. However, it was found that in the last two decades this correlation has disappeared.

One of the possible explanations: The warming of the Eur-Asian continent provides so much enhancement of the Monsoon, that it offsets possible El Niņo effects. It is known that the snow cover of central Asia (Tibetan plateau) in spring has an influence on the Indian Monsoon, especially its timing.

Concluding: This example shows that indirect effects of El Niņo may change to quite an extend from what was seen in the past in case of a certain degree of global warming.

References:

1. A. Timmermann, J. Ohberhuber, A. Bacher, M. Esch, M. Latif and E. Roeckner
Increased El Niņo frequency in a climate model forced by future greenhouse warming
Nature, 1999, Vol. 398, pp. 694-696

2. K.K. Kumar, B. Rajagopalan, M.A. Cane
On the weakening relationship between the Indian monsoon and ENSO
Science, 1999, Vol. 284, pp. 2156-2159

3. Sjoukje Yvette Philip
Exploring El Niņo Mechanisms in Climate Models
pHD thesis, 2009, University of Utrecht