El Niño - How it Developed
El Niño, and his counterpart La Niña, are part of “an oscillation of the ocean-atmosphere system in the tropical Pacific having important consequences for weather around the globe” (NOAA). This oscillation system is also referred to as ENSO - El Niño Southern Oscillation. For the conditions to be considered El Niño, the sea surface temperatures (SSTs) must be above average temperatures (and inversely for La Niña - must be lower than average). Additionally, there needs to be evidence of atmospheric involvement, usually characterized in surface wind direction changes over the Central Pacific.
NOAA issued an El Niño advisory in early March, at which point there warmer SST (evidenced in an index that monitors temperatures), increased rainfall over the Central Pacific, westerly wind bursts, and a growing downwelling Kelvin wave. The downwelling Kelvin wave is described as “a warmer-than-average pulse of subsurface waters [that] travel east across the Pacific” by Emily Becker at NOAA. These are all very much telling signs that El Niño conditions are developing.
During March, conditions strengthened, and there was an atmospheric response. Normally, surface winds at the equator are easterly (blowing from the east to the west). When the atmospheric conditions allow, these winds shift to westerly (blowing from west to east). This allows warm water at the surface to move “backwards” into the Central Pacific, or even further to the Eastern Pacific, waters. Another indicator, increased rain over the Central Pacific, was evident.
By May, there was a 90% chance that El Niño conditions would continue through the summer. This is a confident forecast, but the evidence of conditions had strengthened. There were above average SSTs ( >0.5°C warmer), the trade winds had been more westerly than average, and there was higher amounts of rain in the Central Pacific. Most computer models were also forecasting this continuation of El Niño, but at this point no one was sure what the strength of the event would be.
In June, forecasters had confidence in the El Niño event continuing into the fall and the winter, and were expecting a strong event (determined by the 3-month average SST peaking >1.5°C above normal). SSTs continued to be above average, weakening winds, which in turn weakened the Walker circulation. The Walker circulation is the circulation pattern of rising and falling air, and moves horizontally at the equator. During an El Niño event, this cell is offset eastward, which places the rising, turbulent air just off the west coast of North America. The sinking air is placed just east of the coast of Asia. Additionally, a second downwelling Kelvin wave helped move those warm SSTs eastward.
During July, the SSTs increased to 1°C above average in the Central and Eastern Pacific equatorial region and increasing amounts of rain. August brought news of the SSTs continuing to be >1.2°C above average. Atmospheric conditions continue to favor El Niño with increased clouds over Central Pacific and weakened easterlies. Forecasters expected this event to have a 2.0°C above average SST maximum. As of now, in early Sept, there’s a 95% likelihood of El Niño existing through this winter and dying off during the spring.
So, what does this mean for us?
ENSO affects the U.S. most strongly in the winter, because we are more vulnerable to global circulation patterns. The North Pacific Jet stream will be further south, so we can expect to see above average moisture in the storms this winter affecting the southern half of the States. But, the bloggers said it best:
“While we have more confidence in general impacts over the U.S. in an El Niño because of the shifts in the jet stream, they’re not guaranteed, because the ultimate result will also depend on factors (chaos & other climate patterns) that are not predictable months in advance and can also impact the jet stream and large scale atmospheric flow” (Becker 2015).
For those of us in the Southern U.S., it’s time to prepare for an interesting winter!
For further reading on this topic, please visit NOAA’s ENSO blog HERE
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