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 , 3:19 PM GMT on April 04, 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 one he did on February 21 on the progress of El Niño. Today's post is very technical! - Jeff Masters
There have been tremendous changes in the Pacific Ocean over the past two months which continue to favor a moderate to strong El Niño event later this spring and summer. Since my previous post on February 21, NOAA's Climate Prediction Center (CPC) has issued an El Niño watch.
To begin, we are currently observing what looks to be the strongest downwelling oceanic Kelvin wave event since satellite records began in the 1970s. This still needs to be verified in reanalysis, but a large swath of 6°C (11°F) ocean temperature anomalies at a depth of 100 - 200 meters (Figure 1) clearly illustrates the significance of this event. To review, oceanic Kelvin waves travel only from west to east at extremely slow speeds (2-3 m/s). These waves have been alluded to as the facilitators of El Niño. There are two phases of an oceanic Kelvin wave, the “Upwelling” phase and the “Downwelling” phase. The Upwelling phase of an oceanic Kelvin wave pushes colder water from the sub-surface towards the surface, resulting in cooling at the surface. The Downwelling phase is the opposite, where warmer waters at the surface of the West Pacific warm pool are forced to sink, resulting a deepening of the thermocline and net warming in the sub-surface.
Figure 1. Departure of ocean temperature from average along the Equator in the Pacific Ocean on March 29, 2014 (top), shows a large area of 6°C (11°F) ocean temperature anomalies at a depth of 100 - 200 meters. A time lapse is available here. Image credit: NOAA/CPC.
In the West Pacific, the thermocline is rising in response to strong upwelling (cold ocean temperature anomalies near the surface). In the central and eastern Pacific, the thermocline is deepening as the warm pool has begun to rapidly shift towards the Date Line. An enlightening time lapse can be found on the NOAA/CPC webpage. Note the lens of colder than average ocean temperature anomalies at the surface in the far eastern Pacific. This can be attributed to a surge in the Easterly trade winds over the eastern equatorial Pacific, which pushes water away from the coast, resulting in some upwelling off the west coast of South America. The surge in the trade winds is just an expression of atmospheric processes occurring in the tropics at intra-seasonal (weekly) timescales. Nevertheless, it is evident that the entire West Pacific Warm Pool has begun to shift eastward, and there is a large adjustment in the Pacific Ocean currently underway.
That being said, we still need to see some favorable atmospheric forcing this month to continue the forward advancement of a full-basin El Niño. In particular, west-to-east blowing winds along the Equator are needed to keep pushing warm water eastwards towards South America. Keeping this in mind, there are some signs of an upcoming period of westerly wind bursts along the equatorial Central Pacific in the next few weeks.
Figure 2. Rainfalls rates over the Indian Ocean (shaded colors), departure of the winds at 200 mb from average (arrows), and Kelvin filtered velocity potential at 200 mb (VP200, contours.) Image credit: Michael Ventrice.
An exceptionally strong atmospheric convectively coupled Kelvin wave (CCKW) is currently propagating across the equatorial Indian Ocean. IMPORTANT: An atmospheric CCKW is DIFFERENT than an oceanic Kelvin wave since atmospheric CCKWs are stratospheric waves in the *atmosphere* that are confined to just the equatorial band. Thus we cannot experience a CCKW passage in North America. CCKWs often couple with thunderstorm activity within the troposphere in the tropics. In addition, CCKWs in the atmosphere are non-dispersive in theory, so they can make many circuits around the globe before attenuating from external forcing such as friction; oceanic Kelvin waves can only travel the distance of whatever basin they are in (in this case, the Pacific). However, *both* atmospheric CCKWs and oceanic Kelvin waves propagate from west to east only.
Figure 3. A time-longitude plot of unfiltered VP200 anomalies (shaded) with Kelvin filtered VP200 anomalies (contours; dashed contours represent the upper-level divergent phase of the CCKW or its convectively active phase) illustrates the non-dispersive nature of this CCKW, as it makes a complete circuit around the globe. Image credit: Michael Ventrice.
Figure 4. GFS model forecast for April 6, 2014. Six-hour precipitation rates are shaded. Often during and up to a few days after the passage of a strong CCKW, tropical cyclones can develop on either side of the Equator, depending on the season. An example can be seen over the Southern Indian Ocean this week, where the GFS model is forecasting the development of a tropical “gyre” that could become a tropical depression. Note the equatorial westerlies are a component of the anatomy of the CCKW circulation itself.
The forecast calls for this Indian Ocean CCKW to push across the Date Line during mid-April. This would be a time when we might see another period of westerly winds develop across the equatorial Central Pacific--favorable atmospheric conditions for a full-basin El Niño to emerge. The anticipated westerly wind burst in mid-April may be composed of individual tropical cyclones, or extra-tropical waves intruding the tropics.
In addition to the CCKW itself, there are higher than average probabilities of another developing Madden Julian Oscillation (MJO) to emerge over the West Pacific following the passage of this strong CCKW, in mid-to-late April. A great deal of my graduate study work focused on CCKW-MJO interactions and the plot below is from Ventrice et al. (2012), which is of a time-longitude composite plot of unfiltered VP200 anomalies (shaded), Kelvin filtered outgoing long wave radiation (OLR) anomalies (black contours), and MJO filtered OLR anomalies (orange contours). From selecting only dates where a strong CCKW passed the eastern tropical Atlantic, a lagged composite approach from these dates reveal a remarkable picture. Once the CCKW passes across Africa to over the Indian Ocean, we often observe a developing MJO event over the Indian Ocean that then propagates eastward across the Pacific region thereafter. There are increased chances of a similar scenario to play out over the next few weeks.
Why does this matter for El Niño? Well, within and following the passage the convectively active phase of the MJO, we often observe an increased number of West Pacific typhoons and low-level westerly wind flow. This is what is likely needed to continue the eastward advancement of the West Pacific Warm Pool this spring, and provides more evidence for a full-basin El Niño event to emerge later this spring in through summer. Furthermore, it is important to note that the latest climate model forecasts are now more aggressive with the amplitude of the potential emerging El Niño. This can be seen in both the ECMWF and CFSv2 Niño3.4 forecasts. For the purpose of illustration, below is the CFSv2 model forecast from mid-February 2014:
And here is the CFSv2 model forecast from Early April 2014:
Nearly a +0.5°C adjustment has been made in just one month for the June-July-August period and beyond, indicating that the model is even more bullish on the El Niño this spring in through summer.
Bottom Line: The Pacific Ocean continues to show signs of a developing moderate to strong El Niño event. During strong full-basin El Niño’s, we often observe cooler than average temperatures in summer across the eastern two thirds of the U.S., and lower than average Atlantic hurricane activity.
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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