Trade winds. The east-to-west flow of airs across more than ten thousand miles of Pacific waters. Starting just off the coast of Ecuador, these winds typically blow in the range of 15 to 25 miles-per-hour uninterrupted across the vast Pacific before terminating in the South China Sea. The winds are a normal condition in the Equatorial Pacific. So constant that sailors relied on them as a kind of ocean conveyor during the days when sailing ships still ruled the waves. Year in, year out, the trade winds blow. Usually only subject to minor insults and brief interruptions from the massive and powerful weather phenomena that is El Nino.
But, starting yesterday, something rather odd began to happen. A six thousand mile stretch of the trades simply went dead.
(Pacific Ocean wind pattern as of 1 PM EST, June 4. The brighter the green, the higher the intensity, the deeper the blue, the weaker the winds. Direction of flow indicated by tapering lines. Note the large dead zone in the Pacific equatorial wind belt. Image source: Earth Nullschool. Data source: NOAA, GFS, MMAB, EMC, NCEP, OSCAR, UCAR.)
Draw a line due south of Kauai to the equator and there you will find a cyclone hovering just to its north.
Cyclones here usually have their wind fields dilated by the ongoing pressure of the east-to-west trade winds. As such, typical circular wind flow around a normal cyclone near the Pacific equator is distorted, turning instead into a kind of wind hump where the trades slow at the base and speed up at the top. West winds generally never completely wrap around these small storms.
But our cyclone is a bit unusual. For not only is it featuring a west wind flow of about 10 mph over about a 500 mile stretch of water, it also pushes ahead of it a trade wind killing frontal boundary. A sinking and rolling in the atmosphere that is acting like a kind of wall to the trades — keeping them from further progress.
The storm is the tip of a spear aimed at the heart of the trades and around it they bisect, shifting above the 10 degree North Latitude line in the north and below the 10 degree South Latitude line in the south. This wide gap features only weak and confused airflows. North-to-south they meander with the occasional weak east wind and numerous anomalous west winds filling in this rift. A broad, nearly 1,000 mile wide hole, that continues on west past the Solomons, past New Guinea, and on all the way to the Philippines.
To the East, a second 2,000 mile stretch of west winds running from south of California and on to the South American coast crowds out the trades. Together with the great wind gap to the west, these two patterns combine to cut off the trades from much of the Equator. What is left is only about 3,000 miles of uninterrupted flow. A mere 30% of the pattern’s typical range.
The El Nino Feedback
So why all the drama? What’s so important about trade winds anyway? Well, from the point of view of the developing monster weather event that is El Nino — almost everything.
For El Nino to grow and progress, in essence, for the massive pile of warm water that has accumulated in the Western Pacific to keep flowing east, the trade winds have to fail. They do this either through strong west wind events that open the gates to warm surface water flow eastward. Or they do it through a kind of trade wind collapse.
(Equatorial Pacific Ocean temperatures warmed to near +0.70 this week as global sea surface temperatures remained in an extraordinarily hot range near +1 C above the already hotter than normal 1979 to 2000 average. A rising El Nino combined with global warming pushed April of 2014 to its hottest temperatures on record and likely had the same effect on May. Any further intensification of El Nino is likely to push this dire trend into even more extreme territory. Image source: University of Maine. Data Source: GFS.)
It is this kind of event that climate experts call an El Nino feedback — an atmospheric condition that sets in place the features that allow Pacific Ocean surface warming to intensify along a strengthening El Nino path. As of yesterday, and continuing on through today, that feedback is readily visible in what appears to be a mass trade wind die-off. A great hole punched through the heart of equatorial air flow.
Such a condition, according to past weather observations, should give what is already a strengthening El Nino a boost. So it appears the potential for a monster El Nino today again ramped higher.
UPDATE: The most recent NOAA/CPC official forecast calls for a 70% chance of El Niño by this summer with the overall intensity forecast to be moderate. However, CPC El Niño forecast discussion shows a rough historic potential of 60% for a strong El Niño and a 20% chance for a very strong event due to very rapid Pacific Equatorial warming during May.