Why The Cool Summer?

Scott Sabol, Meteorologist

Talk of global warming aside, there are well-documented natural cycles in the atmosphere and oceans that have a direct impact on our weather from season to season; month to month. The PACIFIC DECADAL OSCILLATION, the ATLANTIC MULTIDECADAL OSCILLATION, the NORTH ATLANTIC OSCILLATION and many others are terms that you probably have never heard of. In fact, most of these cycles have only been coined within the last 15 years and don't get much media attention. Many of these cycles are poorly understood while others have been well-known for centuries. The grand-daddy of them all is the ever-popular EL NINO. Back in 1997, El Nino was blamed for everything from horrible weather to the Indians losing the World Series. It became the catch-phrase for everything.

So far, this summer, temperatures have been noticibly cooler than in summers past. Rainfall is down at bit too. Is El Nino the big reason for this? Is its cousin "La Nina?" Is it something entirely different?

After a pretty harsh winter with more than 25 "Alberta Clippers" and a host of "Panhandle hooks", I thought the pattern would change somewhat. That hasn't happened. In fact, the overall upper level pattern has stayed northwesterly thus shutting off the heat that usually builds from the south. A great article from climatologist Dr. Joe D' Aleo explains how the arctic is affecting the summer temperatures across the northern US. This arctic influence is from an arctic climate cycle called the ARCTIC OSCILLATION. The Arctic Oscillation is derived from pressure centers over the arctic. Often these pressure centers fluctuate above and below normal. In winter, the AO dramatically alters the position of the westerlies (the steering currents in the atmosphere). A positive AO means pressures over the arctic are lower, the westerlies stay strong and north thus the arctic air stays away from the US. A negative AO means pressures are higher, the westerlies are weaker so colder air has a tendency to sink southward.

Since mid-June, the Arctic Oscillation has been strongly negative.

Using month-to-month Arctic Oscillation data back to 1950, I found that no other June/July period has had an AO this negative in almost 60 years! Why? Many experts have found this negative AO to be related to volcanic eruptions in higher latitudes like Mt. Redoubt in Alaska a few months ago as well as Russia's Sarychev Volcano. The aerosols that are ejected into the upper atmosphere can change the pressure tendencies in high latitudes. This change can shift the steering currents allowing colder air to push southward more frequently. Take a look at the temperatures in June across the US. Many areas were well below normal.

The trend has continued through July.

On June 12th, just as Russia's Sarychev Peak volcano was erupting for the first time in 20 years, the International Space Station flew directly overhead. Astronauts had their camera ready and snapped one of the most dramatic Earth-science photos ever taken from space:

Combine these factors with a quiet sun (See past column on sunspots) and the rest of the summer doesn't look promising if you crave a ten day period with scortching heat and tropical humidity. During the week of July 13th, more than 800 record lows were set across the US. Akron and Mansfield have already recorded their coldest July on record!

Could we see this "cooler" summer coming earlier this year? Probably not. When we attempt to come up with a seasonal trend forecast for the summer or the winter, its very difficult to weight the role each of these natural occuring cycles will play in our weather 3 to 6 months in the future. Throw in a few unpredictable volcanic eruptions which tend to drop the Arctic Oscillation negative and a forecast for the summer ahead can be a daunting task.

So if someone asks why this summer has been so "cool", tell them its the NEGATIVE ARCTIC OSCILLATION AND A COUPLE OF VOLCANIC ERUPTIONS. That'll keep their head spinning for a while.