Sunday, November 15, 2015

El Niño, part II: Global & local effects / MWN Winter Outlook

In part 1 of this blog series on El Niño, we examined the basics of ENSO (El Niño Southern Oscillation), including what exactly it is and how ocean temperatures affect global weather patterns, a little history as to how it was discovered, and how it is measured. In this final installment, we examine the typical effects of El Niño globally, as well as what we have experienced locally in prior El Niño years, and finally what we might expect this winter. This blog will also serve as the "2015-'16 MWN Winter Outlook" post that is commonly requested this time of year.

Global effects of El Niño

Though effects on the atmosphere over North America, and the rest of the world, vary with each El Niño, there are some typical atmospheric responses. The effects are most visible during the winter months, since that is when the waters of the Pacific tend to be warmest during El Niño years, and include:
  • Increased precipitation across the southern tier of the U.S, including drought-stricken California, due to a more active Pacific jet stream
  • Decreased precipitation across the Pacific Northwest and Ohio Valley
  • Warmer than average conditions across the northern half of the U.S. and southern Canada, as the polar jet stream is displaced further north
  • More coastal storms affecting the eastern U.S., and
  • In summer, suppressed activity in the tropical Atlantic and increased activity in the eastern Pacific.
Below are the typical El Niño patterns during the winter (top) and summer (bottom) months globally. These "typical" effects are often be offset by other, shorter-term, regional or global patterns that are much less predictable beyond a couple of weeks. For instance, just because a region is usually dry or cool doesn't mean the entire winter will fall into that pattern.

Regional effects of El Niño

Mid-South weather can be fairly fickle during El Niño winters, as we are typically positioned between the persistent Pacific jet stream that brings wet and cool conditions to the southern U.S. and a pronounced dry area over the Ohio River Valley, which can extend as far south of the Tennessee River Valley. If either of those areas shifts slightly due to other variables, Mid-South weather could end up either wetter or drier than average, particularly for short durations over the course of the winter.

Mid-South temperatures over the course of an El Niño winter are generally driven by shorter-term regional variations that occur. El Niño itself generally places us in a "near average" temperature regime, between cooler than average weather to the south (nearer the Pacific jet stream) and warmer than normal weather to the north.
Typical climate pattern over North America during El Niño winters. Graphic courtesy NOAA/CPC.

Other factors: Teleconnections

While El Niño will be the primary factor on our winter weather pattern, there are other climate influences (or "teleconnections") that may last only for a week or two at a time that frequently determine local weather conditions. These are much less predictable more than a couple of weeks in advance. These include the North Atlantic Oscillation (NAO), Arctic Oscillation (AO), Pacific Decadal Oscillation (PDO), Pacific-North American Pattern (PNA), and Madden-Julian Oscillation (MJO), among others. Here is a brief description of a few that can affect our weather:

North Atlantic Oscillation (NAO) - Outside of El Niño, one of the primary influencers of winter weather in eastern North America and Europe is the NAO. A positive NAO occurs when atmospheric pressure over the high latitudes of the North Atlantic (i.e., Greenland) is below average and areas in the central North Atlantic have above average pressure. A strong jet stream across the eastern U.S. into the north Atlantic keeps the coldest winter air bottled up to the north, resulting in above average temperatures for the eastern U.S. A negative phase features above average pressure over the high latitudes and usually results in below normal temperatures and a snowy pattern for the eastern U.S. as cold air is allowed to dip into the region due to a weaker jet stream. The NAO can shift from positive to negative multiple times within a season or may vary in strength but remain in the same phase for several months at a time. At least through the early stages of the winter season, a positive NAO is expected.

The North Atlantic Oscillation (NAO) measures pressure anomalies over the North Atlantic Ocean. Higher than normal pressure over the northern latitudes of the north Atlantic typically results in a cold and snowy pattern for the eastern U.S. in winter. Graphic courtesy

Arctic Oscillation (AO) - A positive AO occurs when the ring of winds circulating around the polar region (commonly referred to as the "Polar Vortex") is strong, keeping cold weather confined to the highest latitudes around the North Pole. Higher pressure results in the mid-latitudes, along with less frequent intrusions of Arctic air. In a negative AO phase, this wind circulation weakens, allowing the cold polar air to penetrate south into the middle latitudes and increasing storminess in these areas as the polar jet stream dips south. This is what has been referred to in the media last January as the "Polar Vortex" invading the U.S., when in actuality, it is always there, just not always as far south. The AO is difficult to forecast more than a couple of weeks into the future.

The Arctic Oscillation (AO) measures anomalies in pressure in the Arctic region. A negative phase occurs with higher pressure at the north pole, resulting in intrusions of cold air into the mid latitudes. A positive phase occurs when the cold air stays bottled up at the pole, resulting in higher pressure in the mid latitudes. Graphic courtesy
Pacific Decadal Oscillation (PDO) - As its name implies, this teleconnection generally lasts much longer (years at a time), varying in strength but generally staying in the same phase for long periods. The PDO is defined by ocean temperature anomalies in the northeast and tropical Pacific Ocean, with a positive phase occurring when warmer than average sea surface temperatures are positioned along the Pacific coast and cooler than average temperatures are located in the interior northern Pacific. After about 16 years in a negative phase, the PDO went positive about two years ago and remains that way now. It is a fairly recently-described concept and thus it effects are not yet well understood.

Pacific-North American Oscillation (PNA) - The PNA describes a pattern of mid-level pressure readings in distinct areas across the Pacific and North America. Usually, these pressure readings (or "heights") are similarly anomalous in the Aleutian Islands of Alaska and the southeast U.S. In the positive phase, heights are above average around Hawaii and in western North America and lower in the North Pacific and southeastern U.S. The positive phase tends to result in cooler and drier weather for the eastern portion of the country in the winter and also tends to occur during El Niño conditions, but not always.

Average temperatures during a strongly positive PNA regime in January 1981, courtesy of the State Climate Office of North Carolina. The country was virtually split in half with very cold air in the east and warmer than normal air in the west.
Madden-Julian Oscillation (MJO) - The MJO is an "eastward moving disturbance of clouds, rainfall, winds, and pressure that traverses the planet in the tropics and returns to its initial starting point in 30 to 60 days, on average" ( Because it is an intraseasonal tropical climate variable, it can change over the course of weeks. Its phase (of which there are eight) can have dramatic impacts on mid-latitude weather, including cold air outbreaks over the eastern U.S. in winter, flooding rain, and jet stream changes.

As you can see, the state of each of these teleconnections can alter or even reverse the impacts of a "typical" El Niño season for periods during the winter, which then affects the overall averages. It's important to remember that "climate is what we expect, weather is what we get." Thus there can be a difference between long-term, or seasonal, averages and day-to-day weather that makes up that average.

Winter outlooks from other sources

As I state every year, I am not an expert climatologist. At MWN, the focus is, nearly exclusively, on the short to mid-term forecasts - those out to a week to 10 days. So, as usual, in putting together our Mid-South winter outlook, I consulted multiple sources and researched data from winters past that I felt were comparable to what we might experience this year from a general pattern perspective (called analogs, listed at the end of the post).

Significant weight was given to previous winters that had strong El Niño conditions but consideration was also given to winters with weaker El Niños which had similar sea surface temperatures outside the Pacific ENSO region, namely the eastern north Pacific and western north Atlantic, since they would be most likely to have an effect on U.S. weather. In particular, I feel the warm waters near the Pacific coast and those in the north Atlantic will play more than a passing role in weather patterns over the U.S. and influencing the Mid-South.

The winter outlooks that I gave the most credence to were those from NOAA and WeatherBell Analytics, as their reasoning is sound and I generally agree with their premises of their respective outlooks, even though they differ slightly. You'll find those outlooks shown below.

NOAA Winter Outlook

NOAA's temperature outlook for this winter leans heavily on climatological expectations associated with El Niño. Percentages represent the likelihood of  above/below normal temperatures. In other words, there is a >40% chance that much of Texas will see cooler than normal weather this winter.

NOAA's precipitation outlook for this winter also leans towards El Niño climatology, though it depicts southern U.S wetness extending north into the southern Plains and Front Range, as well as up the east coast. Percentages are read similarly to the temperature map above. "Equal chances" means there is no signal to indicate a greater chance of above or below normal precipitation.

WeatherBell Winter Outlook

WeatherBell temperature forecast for the winter season. Colors represent departures from average.  In many respects, this outlook agrees with NOAA depicting a warm winter for the north and cool winter for the south.  The eastern 1/3 of the U.S. is where they differ the most.

The snowfall forecast for this winter from WeatherBell, expressed as percent of normal. Recall that for the south, where values are forecast well above average, snowfall amounts are typically not high, so a 150-200% of normal forecast could be only a few inches difference. NOAA does not expressly predict snowfall in their winter forecast. 

The MWN Winter Outlook

1. Temperature
All of that said, there is fairly strong consensus that Mid-South winter will start off warmer than it will end. In other words, the temperature anomalies will be positive (above normal) through December, then descend into negative (below normal) territory by February and March. In fact, the average temperature in December for the ten El Niño winters examined (analogs listed below) was 0.9° above normal while the average February temperature was 3.4° below normal. Thus I expect temperatures will be above normal, on average, through the end of the year before beginning to trend downward. Similar to last year, February and early March have the potential to be a fair amount below normal in the temperature arena.

2. Precipitation
Precipitation-wise, a warmer early season tends to support more atmospheric moisture than a cold season and our analog winters support that as well. By the time the latter half of the winter arrives, precipitation in the analogs falls below normal. In a classic El Niño setup, precipitation is usually near average in the Mid-South, but drier anomalies in the Ohio Valley can sometimes creep south into west and middle TN as well. Therefore, we are of the opinion that the winter will start off with above normal precipitation and the latter half of the season will be slightly below normal.  We also believe that severe weather activity will continue to be below average, which is fairly common in El Niño years.

3. Snowfall
The most anticipated part of the forecast, snowfall tends to be near the long-term average for the season, though it can be fairly variable. This is because one snow storm (or the lack thereof) can result in a season that is well above (or below) normal, since not much typically falls. (Memphis International Airport averages 3.4" of snow each year.) Last year, the December-February snow total was 2.3", or about an inch below normal. However, the first light accumulation actually occurred very early (0.1" in mid-November) and the biggest ice/snow event of the season actually occurred in the first week of  March, both outside the typical "winter" season. So for 2015-'16, we are predicting slightly above normal snowfall for the season (4-5") with the majority of that likely to occur later in the winter, not dissimilar to last year. With a forecast of cooler air in place in February to early March, we believe there is an above average probability of a late season snowfall in the Mid-South once again.

So there you have it - the predictions for 2015-'16 winter! If you're interested in how I did last year, take a look at this blog post from last March. In a nutshell, the winter of 2014-'15 was cold, dry and snowy. I had forecast temps below to slightly below average (correct), precipitation near average (incorrect), snowfall near to above average (correct), and large temperature swings with periods of severe weather (partially correct).

The analog years we examined for this year's outlook were 1919-20, 1957-58, 1972-73, 1982-83, 1991-92, 1994-95, 1997-98, 2002-03, 2009-10, and 2014-15. These included the 3 strongest El Niños on record (to this point): 1997-98, 1972-73, and 1982-83. Click here to view the raw data for each of these winters.

Erik Proseus
MWN Meteorologist

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