El Niño

[Ban my account ffs!]

I probably could've dredged up another thread, but I'll make this one.

You'll be hearing more and more about El Niño in the coming weeks and months, as it appears the strongest event since the 1997/98 "Super Niño" wreaked havoc on weather patterns and caused major disruptions globally.

 

I had written a few weeks ago that this "Kelvin wave" looked unusually powerful and that it might trigger a strong El Niño event.  Only since that time has NOAA even acknowledged a 50% chance of one developing this summer.

El Niños are triggered usually during boreal spring developing rapidly before slackening somewhat during boreal summer (as winter is in full swing in the southern hemisphere) before strengthening again during boreal fall.

Events that begin as early as this one is developing are almost always very strong and disruptive events.

What is remarkable about this compared to 1997/98.. is that ocean heat content was quite a bit higher in the Pacific basin and building throughout the winter of 1996/97.  But the blob of warmth was very similar in late March 1997 compared to the blob shown above.  That means this one developed and is moving that much faster.  Whether this means a stronger event remains to be seen.  There could be counteracting cold water that keeps it from becoming a huge event.

In any case:  If a strong event is triggered during the coming couple months, it could have major impacts globally over the summer, including:

1)  Failure of the monsoon in India and across SE Asia.  Already unusually dry weather has enveloped much of the region.

2)  Drought in Australia.

3)  Unusually strong summer monsoon across Mexico and the western U.S.

4)  Wetter than normal conditions across much of the U.S.

5)  More eastern Pacific hurricanes and fewer Atlantic hurricanes


Other impacts are less certain.  Drought often coincides with El Niño in the Amazon, south-central and southeast Asia, Australia, and across rice belt of China (while the northern regions along the Yellow River often flood).

Going into autumn and winter:  During strong events, precipitation over the western U.S. tends to be above normal with very wet conditions in the Southwest but near average conditions further north.  Weaker El Niños often mean drought for portions of the west... but in strong events, so much warm water moves in directly south of North America that it gives us a big moisture source.

But the impact of any given event can be quite different from others... so it's all a bit uncertain.  But the likelihood of certain impacts (like wetness in California) are much higher during El Niño than during other years.

[Ban my account ffs!]

You can see the blob of warm water near the equator in the Pacific slowly moving eastward.  The figure in the post above is a cross section of the average anomalies of water below the ocean surface.

[Simfan34]

This seems so... esoteric as to come off as ominously supernatural; but I am ignorant about such things. Do keep us informed. It would seem to presage a nice, wet Keremt in Ethiopia- it would certainly help.

[Ban my account ffs!]

It's really not hard to understand. 

Here's a diagram of the Walker Circulation.  This is the normal circulation of air and ocean water that operates in a mirrored conveyor belt.



During La Niña, the whole process illustrated above is "sped up".  The winds and ocean currents move more quickly and more warm water piles up on the west side of the Pacific.

Eventually the bubble of warm water in the west overwhelms the forces that are keeping it there (the constant winds from the east), and the bubble of warm water that has built up beneath the ocean surface "sloshes" eastward and rises to the ocean surface.

During the strongest such events, it can trigger a reversal of the winds and completely turn the whole conveyor belt around for a while. 



It is important not to look at El Niño and La Niña in the simplistic way the government defines them... which is the periodic warming or cooling of the eastern Pacific ocean from the long term average.

It is actually a process in which La Niña events 'charge' or "grow" the warm subsurface bubble of water in the western Pacific... with El Niño being the mechanism that releases that pent up heat into the atmosphere and out to space.

So during La Niña you have lots of colder than normal water at the surface in the east Pacific while the warmer than normal water is kept below the surface where it can't warm the atmosphere.

During El Niño that warm bubble moves east to South America and surfaces.  In it's place, cold water fills the place where the warm bubble usually resides.  The whole process flips.. you have lots of unusually warm water at the surface across the east Pacific.. while the colder than normal water is below the surface where ti can't cool the atmosphere.

[Simfan34]

Wait... is the Pacific not flat? It would seem to imply that. But why does it "speed up", then?

[Ban my account ffs!]

Wait... is the Pacific not flat? It would seem to imply that. But why does it "speed up", then?

The ocean surface is far from flat.  Winds and ocean currents drive pretty substantial differences in sea level height.  This is why you get a "storm surge" during hurricanes.  The intense winds of the storm driving in towards land cause the water to overflow onto the land.  Obviously once this dies down, the water returns to the ocean.

As for why it speeds up.. that's hard to say.  There is a complicated transition in winter/spring when the sun crosses the equator.  It is nearly impossible to predict what will happen in any given year until we're relatively close to it.

Some years the east to west trade winds weaken substantially during the spring as the sun crosses the equator.  This can trigger El Niño.  Other years they don't weaken as much and this causes cool water to begin upwelling faster near South America which can trigger La Niña.

Most years we don't know what is going to occur with ENSO (the cycle of El Niño/La Niña) until mid summer.  It is actually quite unusual to see an El Niño begin so suddenly and so early in the year.  But precedent shows that El Niños that begin suddenly and early in the year tend to be strong events, and strong events tend to be disruptive globally.

I couldn't tell you why a strong event could be in the cards.. except the last El Niño we had was in 2009/10... to have 4 consecutive winter seasons without El Niño is pretty unusual.  The last time we saw 4 winters between El Niños was in 1959/60 to 1962/63.