Thursday, April 23, 2015

An El Niño For California!

Wednesday, April 22, 2015
10:14 am

Last year, climatologists were warning us that we could have one of the biggest El Niños in history, perhaps one even rivaling the legendary El Niño of 1997-1998. Those forecasts never verified, but you wouldn’t know that by our weather this past winter. El Niño winters are generally warmer and drier than normal in the Pacific Northwest, and boy oh boy were we warm. While we had near-normal rainfall, most of this rainfall occurred in short, heavy spurts due to warm, subtropical systems soaking the area. All of this made for, as I’m sure you are well aware, a less-than-optimal ski season.

Precipitation and temperature departures from average. Credit: Climate Prediction Center's El Niño/Southern Oscillation (ENSO) Diagnostic Discussion

But this winter, it looks like a real El Niño may be in store. While I normally hate El Niños (I’m a skiing fanatic), I’m actually hoping a strong one arrives this year. Why, you might ask? 

One word. California.

Folsom Lake. Credit: California Department of Water Resources

The above picture shows pictures of Folsom Lake, a reservoir 25 miles northeast of Sacramento in Northern California. The picture on the left is from July 2011, and the picture on the right is from January 2014. During July 2011, the reservoir was at 97% total capacity and had 130% of its average capacity for that date. For January 2014, the reservoir was at 17% total capacity and had 35% of its average capacity for that date. I can't even tell if the reservoir reaches the dam in the foreground. The lack of water in California is frightening. Jerry Brown, the governor of California, has called for a 25% reduction of water usage in 2015 compared to 2013 levels, and numerous measures are being put in place to ration water. The latest, a "tiered" measure where the more water you use, the more you would pay per gallon, was recently ruled unconstitutional this past Monday. 

This is the kind of stuff that drives me crazy. There seems to be a common theme in American society of being unable to take action on a pressing environmental issue because it will violate some peoples’ rights. When a situation gets really, really serious, taking swift action is more important than finding a perfect solution. What's more important... having people who use large amounts of water pay unfair sums of money for using it (while farmers get off for free and Nestle (Arrowhead) illegally pumps their water from California’s national forests and sells it to drought-free states for a profit), or continuing to watch California's water supply dwindle? In my opinion, when there is a crisis, it is more important to get an imperfect solution in place that can be edited later than take no action at all. The lack of urgency towards mitigating environmental crises in politics confounds me. But that is a topic for a different blog.

So if California won't help themselves, will Mother Nature come to the rescue? I am cautiously optimistic that she will. But before we discuss how the current El Niño will unfold, let's take a look at the effects of past ones for the continental U.S. I hope you like pictures. :)

Typical climate patterns for the U.S. during El Niño winters. Credit: Climate Prediction Center/NCEP/NWS/The COMET Program

During El Niño years, the jet stream that comes off the Pacific and brings us our big storms tends to shift to the south. As a result, we tend to be warmer and drier, while California tends to be cooler and wetter. This pattern is not unique to the West Coast; during El Niño years, the entire northern and southern tiers of the U.S. follow a similar trend.

The maps below show the composite precipitation and temperature anomalies during El Niño years over the states. Of particular note is the sharp gradient in precipitation between the Cascades and the Sierra Nevada. With warmer and drier-than-normal conditions, the Cascades are often starved of snowfall, while the Sierra Nevada get absolutely crushed.

Composite Precipitation Anomalies (inches) during El Nino years. Credit: NOAA

Composite Temperature Anomalies (F) during El Niño years. Credit: NOAA

Now that we’ve got that covered, we’re ready to take a look at El Niño's current status. But before we do that, let’s quickly review our El Niño “regions.”

Credit: NOAA's National Climatic Data Center

El Niño occurs in the tropical Pacific, and there are different Niño "regions" that are used as an indicator of El Niño strength. They are all important, but Niño 3.4 is the one that is generally the most representative of an El Niño event. The graphs and maps below show the temperature anomalies in these regions over time. All of the pictures below were retrieved from the Climate Prediction Center's latest El Niño/Southern Oscillation (ENSO) diagnostic discussion.

SST anomalies from all of the Niño regions

SST anomalies over the tropical Pacific

As you can see, sea-surface-temperatures (SST) were above average for the entire winter, but for the tropical Pacific to be in an El Niño state, the SST must have a 3-month average of at least 0.5 °C above normal. We barely, barely made El Niño criteria this past winter, but as you can see, all of the Niño regions are warming rapidly, and we are now solidly in weak El Niño conditions.

Changes in ocean temperature at depth along the Equator

Warming is also very apparent at depth, as the positive temperature anomalies have increased throughout the water column in the central and eastern Pacific. 

The majority of models are on board for an El Niño event. There are three charts I like to look at: the "Probabilistic ENSO Forecast" from the Climate Prediction Center (CPC) and the International Research Institute for Climate and Society (IRI), an IRI/CPC compilation of Niño 3.4 SST predictions from different dynamical and statistical models around the world, and the Niño 3.4 SST predictions from the CPC’s own CFSv2 (Coupled Forecast System Model, version 2). As the Probabilistic ENSO Forecast chart below shows, there is a 60-70% chance of the El Niño lasting through 2015, and our chances of a La Niña developing (cooler-than-normal waters in the tropical Pacific) are small.

Graph showing the probabilities of an El Niño, Neutral, or La Niña event for the rest of 2015

The IRI/CPC’s dynamical/statistical model compilation shows all but one model forecasting above-average SST for the rest of the year, with the majority predicting El Niño conditions. The thicker lines are the averages of given models, and these also denote weak-to-moderate El Niño conditions for the rest of 2015.

Niño 3.4 SST anomalies as predicted by different dynamical and statistical models

The CFSv2 also shows a moderate El Niño developing. The solid black line is the measured SST in Niño 3.4 thus far, the colored lines are forecasts from different "ensemble" members; i.e. forecasts using slightly different initial conditions, and the dashed line is the average of these ensemble members. 

Niño 3.4 SST Anomalies from the CFSv2 model

For a more visual depiction of what this graph represents, take a look at the sequence of images below. These images are based on the above predictions from the CFSv2 model and show what the SST throughout the tropical Pacific are forecast to look like as we head into 2015.

Visual evolution of the El Niño as predicted by the CFSv2 model

In conclusion, an El Niño is already developing, and models are pretty confident that it will continue to strengthen and persist throughout 2015 into next winter. The effects of El Niño for North America are most pronounced during our winter, so if California gets buckets of rain this summer, don't drop down on your knees and praise El Niño just yet. On the other hand, if California is getting swamped in the middle of the winter, well, you'll know who to worship. 


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