Recent reports from the tropical Pacific indicate a strengthening El Nino pattern. How will El Nino affect Colorado's winter weather?

First, let's take a look at just what El Nino is. El Nino is the "positive" side of the ENSO sea surface temperature anomaly. ENSO stands for "El Nino- Southern Oscillation", which is an abnormal increase or decrease in tropical Pacific sea surface temperatures, with changes taking place on a periodic scale of seasons or years. The opposite side of El Nino is known as La Nina. It is believed that variations in Pacific sea surface temperatures have a dramatic effect on global weather patterns.

This phenomenon was first noticed many centuries ago by Peruvian fishermen. They observed changes in water temperature along the coast. Colder water meant good fishing, but sometimes the water would warm significantly, ruining the fishing. This phenomenon usually happened around Christmas, and the Peruvians dubbed it "La Nina", after the Christ Child.

Scientists from the National Oceanic and Atmospheric Administration measure sea surface temperatures constantly through the use of science ships and satellite soundings. When sea surface temperatures change, the temperatures and pressures of the air above also change, resulting in changes to trade wind patterns. When the surface pressure is high in the eastern tropical Pacific it is low in the western tropical Pacific and vice-versa. These changes in air pressure and wind direction affect the global climate.

Since the ocean warming of El Nino and the pressure reversals of the Southern Oscillation usually happen together, most people refer to this phenomenon as El Nino/Southern Oscillation or ENSO for short. El Nino episodes generally occur every three to five years but the interval has varied from two to seven years. They typically last around eighteen months.

An important thing to consider before we try predicting how El Nino will affect Colorado this winter, is the difference between "weather" and "climate". Weather is what is happening outside over your head right now. This includes the air temperature, wind speed and direction, air pressure, cloud cover, humidity and dew point and any precipitation that may be falling. Climate is the "average" weather, added up over a certain period of time. You can see how the current weather can be very different from the average climate. We may see the worst snowstorm in history during a drier-than-average year.

El Nino tends to shift the jet stream southward, allowing a deep trough to build over the central United States. Troughs usually mean wet weather. Recently, Colorado has experienced several prominent El Nino events that have coincided with the winter season. The most notable ones were the winter of 1982-83 and 2002-03. You may remember the enormous snowstorm Denver experienced in March 2003 when up to three feet of snow fell over the city, with more than seven feet reported in the foothills.

El Nino/ENSO affects everything from snowstorms to hurricanes. In the summer months, El Nino can decrease the number of hurricanes that affect the Gulf Coast and eastern seaboard. The reason for this is strong upper-level winds blowing over the southern sections of the United States. Strong jet stream winds disrupt the circulation patterns of hurricanes, and either keep them from forming, or weaken them substantially.

In the winter, an El Nino episode can mean warmer-than-normal temperatures for parts of the southwest US. This is due to the formation of a trough over the central US during an El Nino winter. If Colorado is on the western side of the trough, warmer air can move north into our state. Precipitation patterns are a bit harder to predict. What has happened in the past during El Nino conditions is strong Pacific storms have moved along with a southern split in the jet stream across southern California, Arizona and New Mexico. If such a storm moves quickly, its effects are minimized. However, occasionally, one of these strong low pressure systems will become detatched from the jet stream flow and turn into what is known as a "cut-off low". These types of storms, if properly positioned to our southeast, can bring copious amounts of moisture and "up-slope" flow to eastern Colorado. This situation occurred in March 2003, and combined with a strong, cold low-level flow from the north, dumping many feet of snow on parts of the Front Range.

Now, the big question: What will El Nino do to us here in the Denver area this winter? To make any kind of prediction on how many blizzards we will see in Denver is difficult, if not impossible. Again, El Nino/La Nina affects our "climate", not necessarily individual weather events. What we need to do is carefully monitor current conditions and trends. Where is the jet stream positioned? Has the jet stream split into a northern and southern branch? How strong is the trough over the midwest, and how far east or west is it positioned? You can see that the global effects of El Nino can vary.

Also, we need to combine the continental weather patterns with our local variables like terrain and ground cover. A storm positioned too far south or north may bring only light snowfall to Denver, but heavy snows to the foothills and Palmer Divide. There are many variables to consider when we look at individual storms. I will cover that for you in more detail in the next post.

So how do we know what to expect in the coming winter? Should we stockpile food and not leave the house, or should we keep our cargo shorts handy? The best advice may be to simply keep an eye on the current weather and stay up-to-date on what is happening globally, and locally. There are some excellent websites which track El Nino/ENSO on a daily basis. Here is a good start:

http://www.elnino.noaa.gov/

This is NOAA's El Nino page, containing basic information about what is happening with sea surface temperatures right now.

Remember that El Nino/La Nina affect average global weather patterns. These global patterns must then be considered with our local terrain, available moisture, synoptic (continental) scale air flow and any other anomalous weather features in the vicinity. Pinning any one big snowstorm on El Nino is specious reasoning. The best time to assess an El Nino event is after-the-fact, taking into consideration all of the events that took place over the season. As we move forward, we are constantly gaining a better understanding of our global climate, and the effects of anomalies like El Nino.

In my next post, I'll show you how El Nino is tracked...stay tuned!