Climate Monday: Visualizing El Niño and La Niña

February 26th, 2018

Continuing our survey of climate and weather visualizations, this week we have a few ways of visualizing El Niño and La Niña, which are two flavors of the El Niño-Southern Oscillation (or ENSO).  This is a relevant topic for this winter, because the world is currently experiencing a La Niña episode.

The best way to fully grasp the El Niño Southern Oscillation is probably through animations that can give a 3-dimensional perspective, because the whole system depends on interactions between the ocean and atmosphere throughout the entire equatorial Pacific Ocean — which stretches for a little under 1/2 of the entire Equator.  It’s a complicated system, and using just words is inadequate.  Here’s one example from Keith Meldahl, a professor at MiraCosta College:

If you prefer a British accent and a more formal presentation, here’s an animation from the UK Met Office:

To summarize, these animations are showing how ENSO works and how it impacts precipitation in the tropical Pacific. Normally, ocean currents and wind at the surface both bring air and water  from east to west, pulling water away from South America.  This keeps the coast of Peru and Ecuador cooler and drier than you might expect, because cold water from the south and from deep in the ocean moves in to replace the water being pushed to the west.  Meanwhile, Indonesia, Papua New Guinea, and Oceania receive ample rain from the warm currents and warm winds.  This hints at a key concept in hydrology and meteorology: air that starts out cold is unlikely to provide much rain, but air that starts out warm and then rises and cools? That’s a rainmaker. During an El Niño event, the winds and ocean currents are weaker, so there’s less pushing of the warm air to the west, and the area where rain occurs drifts to the east.  During a La Niña event, the winds and currents are stronger, so there’s more pushing to the west, and the area where rain occurs drifts west.

That’s great for visualizing the physics, but to see what’s going on right now, a great place to visit is the National Oceanic and Atmospheric Administration’s ENSO website. The easiest way to measure whether we’re in neutral conditions, El Niño, or La Niña is to measure the temperature of the ocean surface (a.k.a. “sea surface temperature” or SST) using satellites. When El Niño occurs, there’s weaker currents and less upwelling of cold water off the coast of Peru, so the sea surface is warmer than normal.  When La Niña occurs, there’s more upwelling of cold water than normal, and the sea surface is colder than normal.  We’re are in a modest La Niña right now, and it’s starting to weaken. Here’s the data from January 2018:

This map shows how sea surface temperatures along the Equator compared to normal for January of 2018. Blue color shows that the sea surface was colder than normal along the Equator — a La Niña event (from NOAA). Data come from a combination of satellites managed by the USA, Japan, and Europe.

The last question we might consider is: Does this have any impact on the USA?  The answer is: some impact, but it’s indirect.  El Niño and La Niña influence the location of the jet streams, narrow regions of strong winds that direct most of our weather in the USA. The jet streams bring rain. The USA is mostly dominated by the polar jet stream, but during El Niño years, the polar jet stream is pushed to the north, and a secondary jet stream develops in the south — often right through Arizona, Texas, and Florida. So the southern tier of the USA tends to be wetter during El Niño events and drier during La Niña events.  La Niña events are often some of the coldest in the northern Great Plains of the US and Canada, and El Niño some of the warmest.

For Ohio, La Niña events actually end up being a little wetter because the polar jet stream is more often sitting right over us (like it was nearly all of last week!). Note, ENSO has only a weak to moderate influence in much of the USA, but it is part of what shapes our winter weather!

Typical winter weather patterns for North America during La Niña and El Niño events. (from NOAA)

More El Niño:

An overview from the UK Met Office

The 2015-2016 El Niño Event (by ECMWF)

El Niño for Kids (by NASA)

 

 

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