The University Record, May 10, 1993

Don’t like the weather? Blame El Nino and Mt. Pinatubo

By Ryan Solomon
News and Information Services

The hot, steamy dog days of summer may again skip Michigan this year. A U-M atmospheric scientist predicts that this summer’s weather may not be much different from last summer’s cool, wet weather.

Empirical evidence collected by Peter Sousounis suggests there is a 55 percent to 60 percent chance of last summer’s cooler and wetter than normal weather repeating itself. Sousounis’ weather prediction is based on his study of the conditions necessary for a drought to occur in the Great Lakes region.

Sousounis, assistant professor of atmospheric science, has identified five weather factors that he says point to another cool summer:

—A greater than normal intensity trough over the West Coast. “A trough is a region in the atmosphere where the wind changes direction somewhat abruptly in a counterclockwise direction in the Northern Hemisphere,” Sousounis says. The trough region tends to be associated with cloudiness, precipitation, enhanced humidity and enhanced moisture.”

—A greater than normal intensity trough over Hudson Bay.

—Weaker than normal air flow from the dry Mexican plains.

—A Bermuda High that is weaker than normal or located farther east.

—Amount of available moisture in the soil.

Sousounis says this last factor is possibly the most important. “If there is a lot of moisture availability in the ground, then a lot of that solar energy is going to go purely into evaporating that water. That’s going to actually cool the ground. In other words, the ground is not going to warm up as much.” The wet spring Michigan has had so far, Sousounis says, is an extension of a winter-like weather pattern that has affected the entire nation.

The five factors outlined by Sousounis are being influenced by a current oceanographic event and a past geologic event.

An El Nino, characterized by warmer water from the western Pacific Ocean moving east toward South America, is presently occurring. Sousounis says El Nino has caused greater rainfall throughout the United States, and Michigan is no exception. Scientists expect El Nino to keep its current intensity for the near future which, Sousounis says, enhances Michigan’s possibility for precipitation, therefore maintaining a high moisture availability in Michigan’s soil.

Sousounis also says that scientists have attributed part of last summer’s cool weather to the 1990 eruption of the Mt. Pinatubo volcano in the Philippines. He says dust particles from the eruption that occurred nearly three years ago remain in the Earth’s atmosphere, preventing a small portion of sunlight from reaching the planet’s surface. The small amount of sunlight not reaching the surface has led to an approximate two degree average decrease in global temperature.

Any warm-up could be intensified when the El Nino ends. Sousounis says statistical evidence shows that the summer following an El Nino tends to be warmer and drier than normal.

When the El Nino ends this time, Sousounis says, the warm-up likely will be in addition to any global warming that has occurred during this short-term cool/wet weather pattern.