What Exactly is El Niño and La Niña? Bonus: What does this mean for us?

El Niño and La Niña are weather catch phrases that are thrown around pretty frequently, but how many of us truly know what these features are? They are complex atmospheric-oceanic circulations, and to be honest, it's more than we could even explain in a short blog post, but we will cover some of the basics here.

Let's start with El Niño since we currently have these conditions in place. El Niño is the warm phase of the El Niño Southern Oscillation (ENSO) and refers to warming in the equatorial Pacific sea surface temperatures (SSTs) compared to normal. Here is a quick video explaining this phenomenon:



La Niña is the cool phase of ENSO and refers to cooling in the equatorial Pacific SSTs compared to normal. One more quick video to explain:





The strength of El Niño and La Niña episodes is determined by the difference in SSTs compared to normal and are measured in a region of the east-central Pacific Ocean known as the Niño 3.4 Region:


The Oceanic Niño Index (ONI) is a measure of the departure from the normal sea surface temperature in this region. This is the standard by which the strength of El Niño and La Niña episodes are measured. The average sea surface temperature in the Niño 3.4 Region is calculated each month and then averaged with values from the previous month and following month. This running three-month average is then compared with the average sea surface temperature for the same three months during the 1981-2010 30-year climate normal period giving a value for the ONI. Positive values of 0.5°C or greater reference the warm phase, or El Niño, while negative values of 0.5°C or less reference the cold phase, or La Niña. Curious how these have measured up through the years? All the values can be found here, or you can also check out this graph which shows January 1950 through late spring 2015:
The data for this graph was provided by the Climate Prediction Center (CPC), with the graph produced by Golden Gate Weather Services



Now that we know what these phenomena are, let's talk about what is really important - how does each impact us locally. Quick...is El Niño or La Niña more likely to bring the Sierra and northwest Nevada a wet winter? Do you have your answer? Chances are it isn't what you think. There seems to be the notion that El Niño equates to a wetter than average winter and La Niña the opposite. How many of you remember the incredible winter of 2010-2011? Here's a couple pictures to refresh your memory:



Believe it or not, this was a moderate to strong La Niña year! 

Being in the Sierra and northwest Nevada puts us right in between the greatest El Niño impacts, which brings better chances for rain to southern California and better chances for drier conditions to the Pacific Northwest. We have had both wet and dry El Niño years and La Niña years as well as ENSO neutral years. Unfortunately there is just not much correlation with ENSO for our part of the world. Since the 1930s we have had 6 strong El Niños and even these have ranged from dry to very wet in the Sierra and western Nevada. There have been two winters with particularly strong El Niños (1982-83 and 1997-98) and both of these winters were very wet in the Sierra with instances of significant flooding in both California and Nevada. However, this is only 2 data points in the admittedly short 65-year data set and does not guarantee that we will have a wet winter this year! Anyone who has lived in the region since the mid-1990s will likely remember the destructive January 1997 floods, but this occurred in the previous winter (1996-97) during an ENSO neutral year.



Now, I know what you're thinking...wasn't last year forecast to be an El Niño? Well, that is true, but the difference is last year we were in an ENSO neutral phase and only a weak El Niño was forecast, which took longer to develop than expected. This year we are already observing moderate El Niño conditions in the tropical Pacific Ocean (image below) with the forecast for it to only intensify. There is now high confidence in strong El Niño conditions persisting through the winter of 2015-2016 (strong being ONI values of +2 degrees C or warmer). I'm sure many of you have been hearing the hype about this being a "Godzilla-like" El Niño, and while a strong one is forecast, whether this becomes a record-breaker remains uncertain.

This graphic from the Climate Prediction Center is showing sea surface temperature (SST) anomalies in the Niño 3.4 region


There is one other big question mark...the Blob! The warming in the eastern Pacific Ocean along the west coast is unprecedented and climate experts have no idea how this feature will interact with a strong El Niño. This "blob" has been linked to the ridge of high pressure which has been plaguing the west the past 4 years and could affect how El Niño drives weather patterns into California.

Seasonal Sea Surface Temperature (SST) anomalies showing the "blob" and El Niño. 
A composite showing the overall pattern during the past 4 winters which has driven the west into a major drought.
So, with all these details in mind, what does this mean for us here in the Sierra and northwest Nevada? The latest Climate Prediction Center outlook for January through March 2016 favors better than average chances for above normal precipitation from about Lake Tahoe south, with below normal chances favored in the Pacific Northwest. With this being said, however, it is only an outlook, not a guarantee, and confidence remains low to medium for our region. The other factor is medium to high confidence in above normal temperatures this winter, which could indicate an increased frequency of warm storms with high snow levels.

The official CPC outlook for January through March 2016 precipitation as of August 20, 2015.

We know the drought is on everyone's mind and it would be incredible if we had a wet winter! Just remember, it took 4 years to create the precipitation deficits we now have, so it will take more than one wet winter to end the drought! If we combine the last 4 years, much of northeast California, the Sierra, and northwest Nevada are more than one year behind on precipitation. Even if we had the incredible winter of 2010-2011 again, we still wouldn't be caught up to normal.

For a great reference on the latest state of the ENSO and predictions for the upcoming season check with the Climate Prediction Center. You can also view our short YouTube video on El Niño which is our office's take on keeping it real with what you can expect for the upcoming winter.

Popular posts from this blog

NWS Wind Forecasts: AKA What's the Deal With Friction?

Image
We have had a number of comments on social media which mention much lower observed wind speeds locally than are in the forecast. Well, you are unlikely to know this (it is not exactly advertised!) but National Weather Service wind forecasts are for 10 meters, or almost 33 ft, above the ground. As we are about to explain, that makes all the difference in the wind between eye-level (~5-6 feet) and ~33 feet.  It is natural to wonder the purpose of a wind height forecast well above where you and I typically reside. The main reason for this is the complication in forecasting wind as one nears the ground due to frictional influence caused by the surface. Here is a diagram showing the concept of wind speeds change as one gets away from the ground.  As you can see above, there is a rapid speed reduction (gray dashed line) near the ground with a lesser change away from the surface. Note that the above diagram is generalized. How much reduction is realized depends on the nature of the s...
Read more

What is the Rain Shadow and What's Wind Gotta do With It?

Image
The dreaded rain shadow - something that those of us who live east of the Sierra crest are more than familiar with and hate to hear. So what is this rain shadow and why does it even exist? Lets start with the topography just to our west... The Sierra Nevada mountain range runs 400 miles from north-to-south and is approximately 70 miles wide. It includes the highest peak in the lower 48, Mt Whitney at 14,505 feet, and rises above 10,000 feet in many locations. Left: The Sierra Nevada Mountains highlighted in blue. Right: A relief map showing elevation (in meters) of the Sierra. The Sierra are a significant barrier for Pacific storms which typically move from west to east. The movement of air masses and associated weather cannot go through the mountains and is forced up and over the mountain range. As Pacific moisture is driven eastward by the wind, the air mass is lifted up and over the Sierra. Rising air will cool and once it reaches the point of saturation, condense into...
Read more

Forecasting "Dry Microburst" Potential From Soundings and Observations

Image
Summer afternoons in the valleys of eastern California and western Nevada usually feature dry low level relative humidity. In certain cases where low levels are very dry (single digit relative humidities) and enough instability for showers and/or thunderstorms exists, there is the heightened possibility for severe (over 60 mph) outflow winds. Now, severe winds are possible from strong thunderstorms in many cases, even without very dry surface relative humidity. However, what makes very dry surface or low level conditions special is that severe outflow winds are possible even with w eak  convection such as light showers...or even just cumulus clouds! When severe winds are produced by convection with little or no rainfall reaching the ground, it is referred to as a "dry microburst". Microbursts can be of significant danger to aviation and, if they are strong enough, life and property. Before we move onto what conditions we look for to determine the threat for dry microbursts,...
Read more