Is there a connection? And if so, how does it work? Does a warming world lead to more El Ninos? More La Ninas? Or more intense El Ninos and La Ninas?
Lots of processes affect the climate. Certainly greenhouse gases have an effect and it's clear that rising atmospheric concentrations of greenhouse gases such as carbon dioxide (CO2) are causing a long-term rise in global temperatures. Another process that affects the climate is the El Nino-Southern Oscillation.
The El Nino-Southern Oscillation
To understand what the El Nino-Southern Oscillation or ENSO is about, you should think of the tropical Pacific Ocean as a huge bathtub with water sloshing back and forth. But a bathtub with an important characteristic: in this tub, the surface water at the western end (near Indonesia and Australia) tends to be warm while the water on the eastern end (near South America) tends to be cool.
When the water sloshes toward the east, the surface of the central Pacific Ocean fills up with warm water, triggering more rain over the central Pacific and suppressing rain over Asia, among a wide variety of other types of meteorological effects throughout the globe, including, as illustrated in the graphic below, in the United States. (See here and here.) This condition is referred to as El Nino.
Conversely, when the water sloshes to the west, the central Pacific fills up with cool water, isolating the warm surface water at the western edge of the Pacific and limiting precipitation to that region as well. (See here and here.) This condition is known as La Nina. (See more graphics for help visualizing the different events.)
The Atmosphere-Ocean Dance
The ocean is not alone in this oscillation. While the ocean sloshes back and forth, the atmosphere is doing its own thing over the Pacific. Climate scientists refer to the wind patterns over the tropical Pacific as the Walker circulation [pdf] after meteorologist Sir Gilbert Thomas Walker who worked out the nature of this circulation pattern in the early 1900s. Under normal conditions the surface winds of the Walker circulation over the Pacific run from east to west. During La Nina, the surface winds intensify -- thus moving the cool waters of the eastern Pacific over the central Pacific. During El Ninos the surface winds reverse, blowing the warm waters from the west over the central Pacific.
This "ongoing dialog between the ocean and atmosphere" is called the El Nino-Southern Oscillation.
The ENSO-Climate Connection
All this sloshing back and forth can affect the weather around the globe in complex and in some cases surprising ways. For example, El Ninos tend to bring heavy rainfall to California and wetter conditions to the Southeast. La Ninas tend to bring less rain to the Midwest and the arid Southwest. In recent years the conditions in the equatorial Pacific have been dominated by La Ninas (more on this later) and is probably one of the reasons for the ongoing drought in much of the Midwest.
The sloshing back and forth can also influence global temperatures. During an El Nino, the tropical Pacific fills up with warm water, which enhances the transfer of heat from the ocean to the atmosphere. This tends to raise global temperatures. Conversely, during La Nina, the tropical Pacific fills up with cool water, which tends to depress heat transfer and therefore global temperatures.
Now think about what happens when we superimpose a long-term rise in global temperature from greenhouse gases with the ENSO sloshing. We should find year-to-year variations in the global temperature rise with upward spikes in temperature during El Nino years and flatter or even modest short-term decreases during La Ninas. And this is exactly what we find when we examine the global temperature record (see graphic).
Note, for example, that two (1998 and 2010) of the five years with the highest recorded temperatures (1998, 2002, 2003, 2005, 2010) occurred during El Ninos. The other three were neutral years; i.e., neither El Nino or La Nina.
Also note that since 2000, there has been only one year with an El Nino (2010). Some have argued that this is a major reason why global temperatures in the 2000s have been flat.
In fact, Grant Foster of Tempo Analytics and Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research have shown that when you use statistical techniques to filter out the influence of La Nina and El Nino, one finds that global temperatures have been steadily rising over the last 30 years. (See here and here.) Note, for example, in the graphic that 2006, 2008 and 2012 were the warmest La Nina years on record for this period.
Is There A Climate-ENSO Feedback?
Given the influence of ENSO on global temperatures, we'd like to know if the frequency of La Ninas and El Ninos and their intensity are changing over time and more specifically if climate change itself is driving such a change.
If these things are occurring, the implications could be significant.
Imagine that global warming is causing more La Ninas. That would imply a negative feedback -- warmer temperatures leading to cooler ocean temperatures, less heat transfer from the ocean to the atmosphere, which would slow the rise in global temperature at least for some period of time.
Conversely, a positive feedback would arise from global warming causing more El Ninos.
Some Say Yes and Here Come the El Ninos
For more than a decade scientists have been trying to figure out, with mixed results, if such a climate-ENSO connection exists. Some have concluded on the basis of analyses of data that global warming is increasing the likelihood of El Ninos. For example see this 1996 paper by Kevin Trenberth of the National Center of Atmospheric Research and co-authors and this 2005 paper by A.A. Tsonis of the University of Wisconsin-Milwaukee and co-authors, both published in the journal Geophysical Research Letters.
A number of other investigators have used climate model simulations to explore how global warming would affect ENSO. Most have concluded that global warming will lead to more and more intense El Ninos. (See here, here and here.)
Others: Not So Much
A somewhat more equivocal conclusion was reached by a team headed by Kim Cobb of Georgia Tech. In a January 2013 report published in the journal Science, Cobb and her co-authors reported on the results of an analysis of oxygen isotopes in coral reefs that yielded information on ENSO cycles over the past 7,000 years.
The authors did find a significant increase in the intensity and variability in ENSO in the 20th century, but also found that similar departures occurred in the past, for example in the 17th century when global temperatures were not elevated. It is therefore not at all clear that the 20th century ENSO trend that Cobb et al saw had any relationship to the contemporary trend in global temperatures.
But Maybe La Ninas on the Way
Now a new study by Michelle L'Heureux of the NOAA Climate Prediction Center and co-authors appearing last week in the journal Nature Climate Change adds a new wrinkle.
Analyzing surface pressure data over the last century, the authors were able to reconstruct the variations of the Walker circulation over that period. Their conclusion: since the 1970s the atmospheric circulation patterns over the Pacific have tended to favor La Nina conditions over El Nino ones. And, they write: "The overall trend towards a stronger, La Niña-like Walker circulation is nearly concurrent with the observed increase in global average temperatures."
This whole ENSO-climate change thing is really fascinating and is critical to our ability to understand and predict interannual and interdecadal trends in global temperatures. But, at least for now, it's a work in progress. Regardless, over the long-term, I foresee rising global temperatures.