Ning, L. and Bradley, R. S. (2015), Influence of eastern Pacific and central Pacific El Niño events on winter climate extremes over the eastern and central United States. Int. J. Climatol.. doi: 10.1002/joc.4321
El Niño is back in the news with 2015 predicted to be one of the strongest El Niños on record. This comes just a year after last year’s equally strong prediction of an El Niño that never fully formed.
Introduction – What exactly is El Niño and what makes it so difficult to predict?
El Niño is a climate phenomenon that occurs in the tropical Pacific, but it has far-reaching effects. Under “normal” conditions, the trade winds blow westward along the equator, pushing warm surface water toward the western side of the Pacific where it piles up. This creates an opening for cold, deep water to upwell on the eastern side of the Pacific. The warm water in the west leads to strong evaporation, which means a lot of rainfall in the surrounding areas (Fig 1).
During El Niño, the trade winds lose strength and sometimes even reverse. This causes the piled-up warm water to slosh back toward the eastern side of the Pacific. Now the upwelled cold water is covered by warm surface water and the eastern side of the Pacific becomes the area with increased rainfall, rather than the western side. Here’s a link to a helpful video that animates the process. You can watch it now if you’d like. I’ll wait…
Temperature and precipitation during the El Niño winters since 1950 are shown on the maps in Figure 2 as differences from average. It is clear that not all El Niño winters are the same. This variability stems from a few factors. First, El Niño is not the only large-scale climate pattern that affects the United States. Other important climate modes operate on decadal timescales and affect the background atmospheric condition. And, of course, weather conditions change on daily and seasonal cycles so that when you look out your window on any given day of the winter, you see the effects of all the large-scale climate modes and small scale things like storms passing over. Second, El Niños can be centered in different places (depending on how far that warm water sloshes over to the east) and this center determines how atmospheric patterns will be arranged over the U.S. Finally, it’s also important to remember that the maps we usually see show averages that smear out all the variability and extremes of each day that constitutes an El Niño winter.
In this paper, the authors made a distinction between two different types of El Niño – central Pacific and eastern Pacific – and analyzed the extreme climate events associate with each of them. Eastern Pacific El Niños are the ones we usually think of (and the kind that made the three strongest El Niños on record) where a warm pool of water sits on the eastern side of the Pacific Ocean. During central Pacific El Niños, the center of warm water is more toward the center of the Pacific.
Methods – Extremes are more memorable than averages
The authors chose nine different types of extreme weather events to look at. These included the warmest (top 90%) and coolest (bottom 90%) days and nights, the number of frost days, and the number of days with more than 10mm of precipitation. Extreme events are more memorable than the average conditions of the winter and, in some respects, more important. They represent social and economic concerns like the risk of flooding on the rainiest days or strains on the power grid when everyone turns the heat up on the coldest nights.
Results – Eastern Pacific El Niños are warmer and wetter
Because central Pacific and eastern Pacific El Niños are centered at different places, atmospheric circulation patterns across the U.S. also shift for each case. Small changes in the average climate condition can lead to large changes in the frequency and intensity of climate extremes.
During central Pacific El Niño years, winds on the east coast of the United States tend to come from a more northerly direction than they do during eastern Pacific El Niños (Fig 3). Winds from the north bring cold Arctic air so, on the east coast, central Pacific El Niños winters tend to be the cooler kind. During eastern Pacific El Niños, the winds tend to come more directly from the warm central Atlantic. This leads to more extremely warm days and fewer extremely cold days during eastern Pacific El Niño winters.
Most of the central and eastern United States can expect to have more extreme rain events during eastern Pacific El Niño years except for the Ohio Valley, which will have longer periods of dry days (Fig 4).
The key here is to realize how much better prepared we can be if we know the likelihood that a winter will bring abnormal weather events. These results should lead to more helpful predictions about the kind of winters we can expect to have in the United States during El Niño years. As the global climate continues to warm, strong El Niños are expected to increase in frequency, so it’s a good idea from a social and economic perspective to understand what to expect.
Conclusions – Take the predictions with a grain of salt
The El Niño of 2015 looks to be an eastern El Niño, so previous patterns suggest that the east coast can expect warmer weather and more extreme rain and snow events.
Keep in mind, though, that predictions are always talking about probabilities. Just because it’s more probable that something will happen doesn’t guarantee that it will. So with that cautionary tip, go forth and prepare yourself for an El Niño winter, whatever that may mean where you live.