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Are More Frequent or Intense La Niñas in Our Future?

September 21, 2011
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As discussed in previous blogs (May 10, May 25, Aug 16), there is a clear link between conditions in the tropical Pacific Ocean and rainfall here in the Southwest.  Although this relationship is the most clear during the winter months (we experience dry winters during La Niña and wet winters during El Niño), conditions in the tropical Pacific may also affect the strength or length of our monsoon (as discussed in the May 25 blog).  

This past year has been another year of precipitation deficit, largely due to below average winter rainfall as a result of La Niña conditions (Aug 16 blog).  Additionally, this year’s summer monsoon rains, although seemingly stronger than in recent years, have still been below average and not enough to offset the prolonged drought.  So the question remains: When will the Southwest get a reprieve? 

Unfortunately, things are not looking good for this next year.  La Niña conditions have returned (image 1) and are expected to strengthen gradually through the winter (NOAA’s Climate Prediction Center), likely giving us another dry winter.  Are such ‘double-dip’ events (Aug 16 blog) going to be more common in the future?  More generally, can we expect more frequent and/or stronger El Niño and La Niña events in the future?  And if so, will these events continue to affect precipitation patterns in the Southwest in the same way they have in the past?

Image 1: The average departure of surface ocean temperatures from normal (i.e., anomalies) experienced last month (August, 2011).  Last month, cooler than normal conditions returned to the eastern tropical Pacific, heralding the arrival of another La Niña event.  (Image created from IRI/LDEO Climate Data Library, iridl.ldeo.columbia.edu/)

Unfortunately, these are not easy questions to answer.  Changes to the mean state (the normal conditions) of the tropical Pacific, the effect of the mean state on the strength and frequency of El Niño and La Niña events, the location of maximum warming during individual events, and the influence of individual events on precipitation patterns in the Southwest all play a role.

First, let’s look at what we know about the changes in the mean state of the tropical Pacific.  Sea level pressure observations over the past century suggest that there has been a weakening of the Walker circulation, the normal circulation pattern within the basin in which air rises in the west and sinks in the east (for more information, see the ENSO summary).  This response has been dubbed ‘El Niño-like,’ as this pattern resembles the conditions that develop during El Niño events.  On the other hand, historical observations and climate models disagree regarding the change in surface ocean temperatures.  Therefore, the response of the tropical Pacific to the imposed warming is likely much more complicated than previously thought, that is, it’s likely not a simple ‘El Niño-like’ or ‘La Niña-like’ response. 

So what can we say about the change in El Niño-related variability (the strength and frequency of events) in the future?  Unfortunately, model projections disagree regarding the change in variability in the tropical Pacific between now and 2100.  When compared with their projections for the mean state in the basin, these models also disagree regarding the effect of mean state on El Niño-related variability.  Some show an increase in variability with La Nina-like conditions, while others show a similar response in variability with an El Nino-like mean state. 

Due to these discrepancies among climate models and historical observations regarding the change in mean state and variability within the tropical Pacific, we need to use paleoclimate records to understand how the system has changed in the past so that we may improve our understanding of how it may change in the future.  As summarized in Jessica Conroy’s recent blog (May 10), North American tree-ring reconstructions, which are sensitive to El Niño-related variability, suggest that variability in the tropical Pacific was stronger in the past when the eastern tropical Pacific was warmer.  Fossil coral records from Palmyra Atoll in the central Pacific Ocean (Cobb et al. 2003) and a tree-ring reconstruction from the southwestern U.S. (D’Arrigo et al. 2005) also display higher variability during periods of reduced solar activity in the Little Ice Age, when the eastern tropical Pacific may have been warm relative to the western Pacific.

So, given what we know about how the tropical Pacific may change in the future, what can we expect in the Southwest?  Although the relationship between the tropical Pacific and winter rain in today’s climate is well established, this relationship may change in the future.  The northward shift of the westerly winds (CLIMAS March 26, 2008) may reduce the influence of the tropical Pacific on the winter storm track.  The spatial pattern of individual El Niño events in the tropical Pacific may also change (e.g., Ashok et al. 2007), with peak warming occurring in the central Pacific, rather than in the eastern Pacific as historically observed (image 2).  If these central Pacific El Niño events become more common, future El Niño events may not have the same effect on winter rains in the Southwest.
Image 2: Departure of surface ocean temperatures from normal (i.e., anomalies) during the peak (December) of two El Niño events: (left) the 1997-1998 event, when warming was greatest in the far eastern Pacific Ocean, as typically observed during historical El Niño events, and (right) the 2009-2010 El Niño, in which most of the warming took place in the central Pacific Ocean.  (Image created from IRI/LDEO Climate Data Library, iridl.ldeo.columbia.edu/)

Nonetheless, despite these uncertainties regarding future changes in the tropical Pacific and the effect of these changes on the Southwest, we do know that rising temperatures will lead to increased evaporation and an increased likelihood of drought.  This increased water demand, combined with the northward shift in the westerly winds (CLIMAS March 26, 2008) will continue to stress our water supply.  So at this point, we can only hope that the tropical Pacific will not exacerbate these effects.

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