Over the last few days, NOAA issued an El Niño watch starting late this summer but, more interestingly, put up a 65% chance of an El Niño during the 2018/2019 Winter. I know it is still summer, but this can give us an opportunity to look at potential for the up coming Winter. In tonight's blog, I am going to share with you some of NOAA's notes on the upcoming El Niño but more importantly discuss how an El Niño can affect our Winter in the Ohio Valley. Let's start with an update on the El Niño per NOAA...

June 2018 ENSO Update: El Niño Watch!

Author: 
June 14, 2018
 

Well, well, well… what have we here? Favorable conditions for El Niño to develop? The June ENSO forecast estimates a 50% chance of El Niño developing during the late summer or early autumn, and an approximately 65% chance of El Niño conditions in the winter, so forecasters have instituted an El Niño Watch.

Who’s on first

Before we get into the potential for El Niño, let’s talk about right now. We are in neutral, and forecasters expect that ENSO-neutral conditions will play on through the summer. The surface temperature of the tropical Pacific Ocean is close to the long-term average in most areas, including the Niño3.4 region (our primary monitoring region for ENSO), which was smack-dab on the average in the latest weekly measurement.

May 2018 sea surface temperature departure from the 1981-2010 average. Graphic by climate.gov; data from NOAA’s Environmental Visualization Lab.

Another interesting thing this map shows us is the prominent pattern of warmer-than-average surface temperatures north of the equator, and cooler-than-average waters south of the equator. This illustrates the strongly positive phase of the Pacific Meridional Mode… which I’ll get to in a minute.

The atmosphere is also looking pretty ENSO-neutral. Remember, warmer-than-average waters tend to evaporate more water and warm the air above them, creating more rising motion and clouds than average. Cooler waters are the reverse, resulting in less cloud cover than average. During a La Niña event such as this past winter, we expect fewer clouds over the central Pacific, and more over Indonesia, which demonstrates the atmospheric response to La Niña’s cooler-than-average central Pacific.

Places that were more (purple) or less (orange) cloudy than the 1981-2010 average during May 2018, based on satellite observations of outgoing longwave radiation (heat). Thick clouds block heat from radiating out to space, so less radiation = more clouds, and more radiation = clearer skies. Climate.gov map from CPC data.

Some reduced cloudiness and rain remained over the central Pacific during May, but as we move into June, this pattern is weakening further.

What’s on second

Computer model forecasts made in June are usually more reliable than those made earlier in the spring, thanks to the progression beyond the spring predictability barrier. While many of these models have been hinting at warming east-central Pacific sea surface temperatures for a few months, we can trust their predictions a bit more now that we’re moving past the spring barrier.  

Most of the dynamical models predict sea surface temperatures in the tropical Pacific will be more than 0.5°C warmer than the long-term average by this fall—i.e., above the threshold for El Niño conditions. Several statistical models, which make predictions by applying statistics to historical conditions, are also currently predicting sea surface temperatures in the El Niño realm by late fall. These models are often more conservative than the dynamical models, and the fact that both sets are largely in agreement is lending forecasters some confidence.

Climate model forecasts for the Niño3.4 Index. Dynamical model data (purple line) from the North American Multi-Model Ensemble (NMME): darker purple envelope shows the range of 68% of all model forecasts; lighter purple shows the range of 95% of all model forecasts. Statistical model data (dashed line) from CPC’s Consolidated SST Forecasts. NOAA Climate.gov image from CPC data.

But wait, there’s more! The temperature of the water below the surface of the equatorial Pacific has been elevated since March, as a downwelling Kelvin wave formed and slowly moved from west to east under the surface. Recently, a second downwelling Kelvin wave reinforced the warmer-than-average subsurface conditions. The May 2018 subsurface heat content is about the 6th highest since 1979.

Area-averaged upper-ocean heat content anomaly (°C) in the equatorial Pacific (5°N-5°S, 180º-100ºW) during May. The heat content anomaly is computed as the departure from the 1981-2010 base period mean. Climate.gov figure from CPC data.

We care about the subsurface because it can provide a supply of warm water to the surface. As  a downwelling Kelvin wave moves to the eastern part of the Pacific, the warmer waters will rise to the surface. Thus, elevated subsurface temperatures are often an early indicator that El Niño is on the way.

The positive phase of the Pacific Meridional Mode (visible in the north-south warm-cool pattern from the sea surface temperature map above) can also be an early indicator that conditions are favorable for El Niño to develop. This pattern can encourage the trade winds (the near-surface east-to-west winds along the equator) in the central Pacific to relax. The trade winds ordinarily serve to cool the surface and keep warmer waters “piled up” in the far western Pacific. Often, relaxed trade winds lead to downwelling Kelvin waves, as the warmer western waters are allowed to slosh eastward.

Speaking of relaxing trade winds… as we go to press, the trade winds in the eastern tropical Pacifichave weakened as a “westerly wind burst” is taking place. (The effect of the Pacific Meridional Mode is usually farther to the west, and attributing the cause of any particular wind burst can be a difficult chicken-and-egg problem.) This westerly wind burst could work to enhance warming conditions.

A note of caution’s on third

With all that said… it’s early yet. The winds along the equator are difficult to predict more than a week in advance, and as much as westerly wind bursts can help El Niño develop, so easterly wind bursts (when the trade winds strengthen) can discourage El Niño. Forecasters feel that current conditions are favorable for El Niño, so an El Niño Watch has been raised. But we’ll keep a weather eye on those conditions as summer progresses, and pitch it to you straight, here at the ENSO Blog.

 

I figure some will finish reading the NOAA article saying what does this mean for me? Let's discuss that...

 

How Does An El Niño Affect Winter In The Ohio Valley?

 

This is the part of the blog that is the most interesting. We actually have very distinct impacts in the US when an El Niño is present during Winter. I spent a little time looking at the teleconnections for a Winter episode of the El Niño and how it can impact temperatures / precipitation. An El Niño does vastly affect the jet stream pattern in across the US. During El Niño episodes, we normally see the storm track shift southward with more active storm track along the Gulf Coast. Below you can see the difference between La Niña and El Niño jet stream patterns. 

 

Facebook 2 pic

 

Precipitation During El Niño Events

We can actually look at the precipitation anomalies (drier or wetter) during El Niño events. Looking backward through the data, the picture is quite clear on precipitation. During El Niño winter events, we normally see drier than normal in the Ohio Valley and as you can see it is a distinct signal.

 

ENSO Precip

 

 

Temperatures During El Niño Events

Again, we will do the same exercise for temperatures during winter El Niño episodes. Looking through the data, we can see the temperature anomalies are quite pronounced in the northern US and southern US. Along the Canadian border, El Niño winter events demonstrate a distinct signal for warmer than normal. In contrast, during El Niño winter events, there is a strong signal for colder than normal in the southwest through Texas into the Carolinas. In the Ohio Valley, the signal is a bit more mixed. The data shows a signal toward slightly warmer than normal in southern Indiana, but not much of a signal in Kentucky. 

 

ENSO Temp

 


We are still many months away from Winter, but if the emerging El Niño does take over during winter, the past data strongly shows a drier than normal winter with no major signal for colder or warmer than normal in the Ohio Valley. It would be premature to speak to any potential impact to snowfall, but obviously we will be looking more at this in the coming months.

 

  

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