Guest Post by Bob Tisdale
The graphics at the NOAA GODAS website were running a few pentads (5-day periods) behind when I published the May 2015 ENSO Update. They’re caught up now, and the Hovmoller diagram of the surface zonal wind stress along the equator, Figure 1, is showing another westerly wind burst during early May 2015 in the western equatorial Pacific.
HOW WESTERLY WIND BURSTS IMPACT THE EQUATORIAL PACIFIC
The “normal” state of the tropical Pacific has trade winds blowing from east to west, which pushes the sun-warmed water to the west. As a result, the surfaces of the western tropical Pacific are naturally warmer than those in the east.
Westerly wind bursts are temporary weather-related reversals of that normal wind flow. That is, the winds blow from west to east along the western equatorial Pacific for a few days and weeks. The westerly wind bursts help to move some of the warm water back to the central and eastern equatorial Pacific, strengthening the eastward flow of the equatorial countercurrent. Westerly wind bursts are also known to initiate downwelling Kelvin waves, which are also associated with additional warm waters being carried eastward below the surface along the equator by the Cromwell Current (a.k.a. the Pacific equatorial undercurrent).
One of the factors that kept the 2014/15 El Niño from strengthening last year was the absence of westerly wind bursts after the initial ones early in the year. See Figure 2. It includes Hovmoller diagrams of surface zonal wind stress along the equator for 2014 on the left, 2015-to-date in the center, and 1997 as a reference for a powerful El Niño on the right. In 1997, there were numerous westerly wind bursts over the course of the year, helping to strengthen it into a “super El Niño”.
Figure 2 (Click to enlarge)
So, if Mother Nature wants the El Niño this year to strengthen, she’ll continue to provide westerly wind bursts in the western equatorial Pacific.
BUT THERE ARE NEGATIVE SUBSURFACE TEMPERATURE ANOMALIES IN THE WESTERN EQUATORIAL PACIFIC
Many readers follow the illustrations of the subsurface temperature anomalies along the equatorial Pacific that the BOM and NOAA provide. NOAA animates them at their Equatorial Pacific Temperature Depth Anomaly Animation webpage. See Animation 1 for the anomalies for the past 2 months, through the pentad centered on May 13th. It’s showing a cross section of the equatorial Pacific to depths of 450 meters (about 1476 feet), with the western equatorial Pacific on the left and eastern to the right. The temperature anomalies are color coded and noted in the scale at the bottom. You’ll note recently there are cooler-than-normal anomalies in the western equatorial Pacific. (You might need to click start the animations.)
Some may be wondering how pushing water from the western Pacific to the east could strengthen an El Niño when that water in the west is cooler than normal. The answer: even though the anomalies in the western equatorial Pacific are negative, in absolute terms, the water in the western equatorial Pacific is actually warmer than it is in the east. See Animation 2, which is the accompanying animation of the subsurface temperatures of the equatorial Pacific, but shown in absolute terms.
Note the depth of the 20 deg C isotherm as time progresses. It grows deeper in the east as warm water migrates eastward during the evolution of this year’s El Niño. Also note how the warm water is still plentiful at depth in the western equatorial Pacific. And at the surface, it’s still warmer in the west than in the east. So there’s still room for the El Niño to grow.
Normally the 20 deg C isotherm in the eastern equatorial Pacific is close to the surface, so that there is a very distinct slope from west to east, with the warmer water noticeably deeper in the west than in the east. But at the peak of the 1997/98 El Niño, so much warm water had migrated to the east that the 20 deg C isotherm was deeper in the east that it was in the west…the slope reversed.
There are of course different hopes for the El Niño from different parts of the world. Californians are hoping a strong El Niño will bring a temporary end to its drought. And Australians are hoping the El Niño fades because El Niños are associated with drought in Australia.
The recent westerly wind burst could very well help to strengthen the El Niño conditions this year. Then again, Mother Nature always has tricks up her sleeves. Time will tell.