Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Michael Ventrice , 2:36 PM GMT on June 07, 2014
Today's guest blog post is by Dr. Michael Ventrice, an operational scientist for the Energy team at Weather Services International (WSI). This is a follow-up post to the ones he did on February 21 and April 4 on the progress of El Niño. Today's post is quite technical! - Jeff Masters
The June 5, 2014 El Niño update from NOAA's Climate Prediction Center gives a 70% chance that El Niño will form this summer, and an 80% by fall, but El Niño odds are higher than this. A strong Madden Julian Oscillation (MJO) event is forecast to develop over the central-eastern Pacific later this month in through early July (the Madden Julian Oscillation (MJO) is a pattern of increased thunderstorm activity near the Equator that moves around the globe in 30 - 60 days.) This MJO location favors for another period of westerly wind bursts over the Central Pacific, an atmospheric signature that is likely to be the final kick needed for a blossoming El Niño event.
As I blogged about on February 21, there has been a noticeable warming in the eastern Pacific over the past few months in response to one of the most impressive downwelling oceanic Kelvin waves observed since satellite measurements began in the late 1970s. Recall that we observed a series of strong westerly wind bursts over the western-central Pacific Ocean this past winter. These westerly wind bursts can be tied to the state of the MJO as well as other equatorial waves and tropical cyclones.
Figure 1. A time-longitude plot of the departure from average of the depth of the 20°C isotherm shows the impressive nature of this downwelling oceanic Kelvin wave at the end of May, which had finally completed its trip all the way to the eastern Pacific Ocean. The series of these waves like has been observed in 2013 - 2014 is very typical of what one sees before the onset of an El Niño event. Note that since the downwelling oceanic Kelvin wave has emerged at the surface in recent weeks, we have seen a rise in the standard ENSO 3.4 index to anomalies approaching +0.5°C, which is the threshold for classification of El Niño conditions. Image credit: NOAA/CPC.
Figure 2. In our history, we have observed strong oceanic Kelvin waves to be driven by westerly wind bursts, which are often timed with the state of the MJO. Note in this time-longitude plot made during the onset of the 1997 super-El Niño, the shading is anomalous outgoing long wave radiation (OLR; thunderstorms are represented by blue shading), the convectively active phase of the MJO (coincides with low-level westerly winds) are represented by solid-red contours, and downwelling Oceanic Kelvin waves are indicated by the blue-solid contours. It is evident that consecutive MJO events play a critical role in facilitating a basin wide transition to El Niño. Note here time is going up!! Figure courtesy of Dr. Paul Roundy, SUNY Albany.
Figure 3. Departure of ocean temperature from average along the Equator in the Pacific Ocean on May 28, 2014 (top), shows an area of 5°C (8°F) ocean temperature anomalies at a depth of 50 - 150 meters, the signature of an oceanic Kelvin wave. In addition, warmer than average sea-surface temperatures extend along the Equator from South America all the way to the Date Line. This is a classic “Full-basin” El Niño expression. A time lapse is available here. Image credit: NOAA/CPC.
There is potential for a big MJO event this month
Over the past month, the MJO signature was relatively weak. During the latter half of May, we observed a period of enhanced trade flow (easterlies) over the eastern half of the Pacific Basin, which can often counter, or “stall” the El Niño for a period of time due to favoring a period of upwelling in the eastern part of the Basin. But while the atmosphere favored a weakening of the El Niño expression, the ocean did not a skip a beat. Warm ocean currents continue to rip towards the east, advecting warm waters from the western half of the basin to the east. The Pacific Warm Pool, which was well established in the western half of the basin for the past couple of years, has now shifted past the Date Line. You can watch the remarkable evolution of the eastward shifting Warm Pool on the Climate Prediction Center (CPC)’s link here.
Since the ocean remains in a state that is evolving towards El Niño, all we need is the atmosphere to behave…and it does appear that the atmosphere soon will! For the past five to six European weekly forecasts, the model has been becoming more and more aggressive with a developing MJO signature to push across the central-eastern Pacific later in June through early July. In response, we should expect another period of westerly wind bursts over the western-central part of the Basin. And what do you know, the model is keying on a period of anomalous lower-tropospheric flow that very well may be the final kick needed to facilitate a moderate-to-strong El Niño expression later this Fall. Forecast models continue to show the peak of the El Niño will occur later this Fall, with a magnitude near +1.5°C above average, as defined by the ENSO 3.4 Index. Now this is not a “Super El Niño” by any means, but it is very strong and there is still uncertainty regarding how strong it will get.
Figure 4. Weekly European model forecast of the MJO made over the past month have increasingly shown a strong MJO episode developing in late June and early July.
Regardless of what amplitude the ENSO 3.4 Index achieves, it only matters of the atmosphere responds. There are number of ways to identify the expression of El Niño in atmospheric data fields. One popular way is to look is to look at the Southern Oscillation Index (SOI), the difference in surface pressure between Darwin, Australia and the island of Tahiti. The SOI tends to drop to very low values during the presence of an El Niño atmosphere. This can be illustrated in the time-series below, where 1997 and 1982 marked the lowest points in the index over the past 35 years.
Figure 5. Southern Oscillation Index (SOI) from 1982 - 2014 shows the two strongest El Niño events of the past 35 years, in 1982 and 1997, had strongly negative SOIs.
Figure 6. The Southern Oscillation Index (SOI) for the past two years shows a downward trend in recent months, but has yet to cross into the sustained negative territory that will indicate the atmosphere has responded to warming SSTs in the equatorial Pacific.
My thoughts on this summer: Since the atmosphere has NOT yet locked into an “El Niño state”, we might expect the typical cooler-than-average summer conditions that the eastern 2/3 of the U.S. typically experiences during an El Niño event to be delayed. This means we can expect periods of hot weather across the major natural gas and power markets this summer. We do not believe these heat waves will be prolonged in nature, and it is difficult to pin-point the timing and magnitude of such events. But we can expect them to continue until the atmosphere “feels” the El Niño developing beneath in the ocean. With a possible strong MJO event on the horizon, it does suggest however that the atmosphere could lock into an El Niño state sometime between July and August.
Dr. Michael Ventrice is an operational scientist for the Energy team at Weather Services International (WSI), who provide market-moving weather forecasts and cutting-edge meteorological analysis to hundreds of energy-trading clients worldwide. Follow the WSI Energy Team on Twitter at @WSI_Energy and @WSI_EuroEnergy.
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