This blog addresses two possibly related questions: The ultimate status of El Niño over the upcoming year, and whether the recent evolution towards El Niño might have influenced this year’s wild winter over North America.
Possible Upcoming Major El Niño
The present conditions of the tropical Pacific Ocean suggest that a major El Niño event might happen this year. Beginning March 10, 2011 I posted a couple of educational blogs on what El Niño is and how it works. http://blog.timesunion.com/weather/el-nino-in-a-cake-pan-basics-of-enso/813/
During summer into fall 2012, there was a “dud” El Niño event that appeared to be developing over the summer but did not amplify into the winter. NOAA’s Climate Prediction Center posted an El Niño watch before the event broke down, and I posted blogs at the time consistent with the likelihood that an event might grow.
Although we understand El Niño probably better than any other natural climate phenomenon besides the seasonal cycle, that kind of failed forecast event illustrates the difficulty of predicting El Niño. The present developing event includes far more amplitude in the signals that often lead to El Niño growth than during 2012, but the specter of past forecast busts in predicting weaker events suggests caution in predicting high confidence in a strong outcome for this event. On the other hand, such experience tends to lead forecasters to understate the actual risk when a major event might occur.
Growth or decay of El Niño events is difficult to predict because weather events that are not easy or not possible to predict influence it. For example, weather events, such as bursts of westerly wind along the equator early in the El Niño’s lifetime would grow the El Niño, while surges in the trade winds in the region of warmest water would weaken it. All El Niño events develop with both westerly wind bursts and surges in easterly wind, but the collective actions over time of these wind events, interacting with the warm water volume of the ocean, determine how big an El Niño even becomes and how rapidly it develops. Westerly wind bursts pile up warm water to the east and push it down deeper in the ocean, creating “Kelvin” waves thousands of miles across that then propagate the effects of the wind events eastward.
If these weather events were completely random with respect to El Niño, then they would make it impossible to predict in advance the ultimate amplitude of an El Nino event. In other words, any developing El Niño event in progress would be equally likely to fail as it is to grow. Yet, especially in big El Nino events, the weather events that force El Niño growth are not completely independent from the El Niño itself: The development towards El Niño encourages (but does not guarantee) westerly wind burst events where they are needed the most to amplify the event. Thus when conditions strongly favor a new El Niño event, the same conditions usually continue to favor the event more often than they disfavor it until the El Niño event matures. In other words, once the process begins in such a big way, it is more likely to continue and to grow than it is to decline and disappear. The 2012 event was never sufficiently strong to yield such a level of enhanced confidence.
Over the last 40 years, there have been only two major El Niño events (1982-1983 and 1997-1998).
Buoys deployed in the tropical Pacific Ocean, beginning mostly during the late 1980s and early 1990s, help scientists track how El Niño events are progressing. Since these buoys were mostly not available during 1982, we only have detailed data from one major El Niño event to analyze. Scientists often look to the past to gain ideas about how present and future events might behave.
Each El Niño event begins with a series of Kelvin waves, a small number of which usually become strong. The biggest wave event leading up to the 1997 El Niño occurred in March. Figure 1 shows the March 1997 average ocean temperature structure. The bottom panel shows how much the temperature in the top panel differs from the average for that time of the year. Figure 2 shows the ocean temperature pattern during the 5-days ending March 20, 2014. The black x’s represent the locations of temperature sensors attached to buoy cables. It is best to focus on the regions of the diagram where x’s are prevalent. The amplitude of the temperature departures from normal in the present event are actually stronger than those achieved during the March 1997 event.
However, only one similar event in observational history to the present event is a very small sample size, so that we can’t look to the past to confirm the range of possible El Niño outcomes when a similar event is occurring. In other words, we cannot tell from such a small sample whether, for example, 40%, 60%, 80%, or more than 90% of similar oceanic wave events are ultimately followed by major El Niño events. Our tiny sample says only that 100% of 1 similar event did so. We have never observed a similar sized wave that was not followed by a major El Niño event, but we don’t know what fraction of a large number of similar events would have similar outcomes.
Another way to anticipate possible outcomes following a big Kelvin wave event is to consult computer models that attempt to simulate the physics of the event. However, none of our models seem to simulate well the development and evolution of strong Kelvin wave events. My own assessment of models statistics suggests that these models tend to under predict the amplification rates of strong El Niño events early in their lifetimes when such high amplitude Kelvin wave events occur.
So, although I do not claim to be certain that a major El Niño event will develop this year, I think that the probability of a major event is substantially higher than that suggested by NOAA’s Climate Prediction Center (their most recent forecast suggests a 50% chance of El Niño developing at all, while I suggest that the chances are probably closer to around 80% for a major El Niño). My motivation to assert higher confidence is that the wind patterns presently observed that favor strong El Niño growth are favored to continue to occur: these wind signals are not completely independent from the progress toward El Niño, so the present progress towards El Niño favors their continued occurrence.
Trouble is that in our lifetimes, we are not likely to ever experience a large enough number of major Kelvin wave events similar to the present event and that of March 1997 to be able to objectively assess through the historical record which of us is right, whether or not this potential event becomes major.
Our Wild Winter
Now I would like to address another important matter that I think is related to the progress toward El Niño: Why were temperatures this winter so extreme, with warmth and drought at the west coast and extreme cold over the eastern United States.
Sometimes the weather and climate patterns leading up to an El Niño event have far ranging impacts as much as the El Niño itself. Could some of this winter’s weather patterns be related to the patterns evolving toward El Niño? Several past years had similar distributions of convection (or thunderstorms) in the tropics to this winter. These include 1986, 2004, 2006, and 1997. All of these years evolved into El Niño events. All of these years January into March had above average sea surface temperature and enhanced rainfall in the region along the equator north of New Guinea and the surrounding regions of the western Pacific Ocean. Although those years did not have some of the same weather extremes observed this year, they showed surprisingly similar overall patterns in atmospheric pressure and wind. I have posted a slide show showing the global pattern of ridges and troughs during these past years for comparison with the pattern this winter.
Above average rainfall on the equator near and east of New Guinea was probably associated with the build up warm water volume in the equatorial Pacific Ocean that helps make conditions favorable for an upcoming El Niño, since warmer water can lead to more rainfall in the tropics. The above observations suggest that this region of enhanced rainfall generated circulation patterns in the atmosphere that set the general overall state of the past winter (and the previous broadly similar winters of 1986, 2004, 2006, and 1997), likely leading to many of the extreme temperature and precipitation outcomes we observed.