Sunday, April 17, 2016

Viva La Niña!

Credit: NOAA

Earlier this week, my good friends at NOAA (well, hopefully we'll be best buds someday) issued a La Niña watch for the Tropical Pacific. Some of the models were showing a transition to a La Niña for next winter in the months before, but there was enough spread and uncertainty in the models that the climatologists at NOAA did not put out an official watch. However, as spring rolled on, more and more models hopped on board with the idea of a La Niña for the 2016-2017 winter. Of course, there is still a lot of spread in the models, and some well regarded ones (such as NOAA's own CFSv2 model) actually show yet another El Niño developing next autumn and hanging around for the winter.

Credit: NOAA

At this point though, we look to be headed towards a La Nina not only because the model consensus but because of what has happened during previous strong El Niños. After the record-breaking El Nino of 1997-1998, we saw a massive La Niña the following winter, and it was because of this La Niña that Mt. Baker ski resort got 95 feet snow in a single year, making it the snowiest year ever for anywhere in the world where snow measurements are taken (there are snowier places, including some on Mt. Baker, but observations are not taken there). The models and history are on our side, and that's why the CPC decided to go ahead and issue a La Nina watch.

El Nino refers to a periodic warming of water in the tropical Pacific due to weaker trade winds and less upwelling, while La Nina refers to the opposite. Just like El Nino, La Nina affects weather patterns all over the world, and in many areas, the effects are nearly opposite to what we would expect during an El Nino. While El Ninos shift tropical convection over Indonesia eastward (often resulting in massive fires over southeast Asia), La Ninas shift the convection westward, meaning that Indonesia and many areas in Southeast Asia will likely see enhanced precipitation this year. Hurricanes are often more plentiful in the Atlantic but less plentiful in the Eastern/Central Pacific during El Ninos, so a stormier-than-average Atlantic hurricane season looks on tap for the summer and autumn of 2016.

Credit: NOAA Pacific Marine Environmental Laboratory

In our neck of the woods, La Ninas tend to create a regime nearly opposite to the one that typically shows up during an El Nino year. Instead of having a big low pressure system in the eastern Pacific that allows the jet stream to sag south into California, we have a substantial ridge which nudges the jet stream northward, allowing it to sag into our area from the NW. Indeed, our La Nina years are stormier, cooler, and wetter than average, with mountain snowfall well-above normal. They generally don't have the biggest storms - those tend to occur on neutral years - but they have the most. And that isn't to say we can't have strong storms - one of the strongest storms in Pacific Northwest history, the "Great Coastal Gale" of 2007, pummeled the coast with hurricane force winds and caused massive flooding across much of Western Washington, including submerging a 10-mile stretch of I-5 under 10 feet of water. 2007-2008 was a strong La Niña year and the Cascades got plenty of snow - Alpental was open until Memorial Day (and I was lucky enough to ski the backcountry then!).

Let's take a look at what's happening in the tropical Pacific right now.

Credit: NOAA

As the animated .gif above shows, sea-surface temperatures throughout the tropical Pacific had dropped precipitously over the past month, and they are continuing to drop at a pretty impressive rate. Also, note that there is still a significant amount of warmer-than-average water in the Northeast Pacific. Strong El Niños tend to grow and fizzle out quickly, and this one is no exception. Take a look at the measured SST anomalies in each of the "Niño Regions" across the Pacific. The higher the number, the further west the region.

Temperatures are clearly decreasing everywhere. Niño 1+2 had a little rebound in March, but they'll cool off soon enough. One interesting fact - the El Niño of 1997-1998 that everybody likes to compare to this one had a far bigger influence in the far Eastern Pacific (Niño Region 1+2), while this one was mainly situated over the Central Pacific. The distribution of water plays a significant role in how weather is affected by El Niño, and an El Niño centered over the Central Pacific has different effects than one over the Eastern Pacific. As this year showed, our seasonal forecasts weren't very good at all - so studies into how the distribution of warm water throughout the Pacific affects the effects of an El Niño throughout the world should be further studied.

One of the most telling signs that our El Nino is ending is that even though the temperatures at the sea surface are above normal, those just beneath the surface are actually below normal. In fact, the total heat contained in the upper ocean (upper 300 meters of water) is actually below average now. Ol' El is clearly on his way out.

Credit: NOAA

Credit: NOAA

Adios, El Niño. You and your record-breaking, horribly predicted rains will be missed. See you in a couple years.

In the meantime, Viva La Niña!

Clearing snow at Chinook Pass in June 2011. The 2010-2011 winter was a moderate La Niña winter.

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