Office of the Vice President for Global Communications

Tuesday, May 18, 2010

U-M scientist says unusual winter-spring weather patterns should soon return to normal

The strong El Nino climate pattern that contributed to our unusually early spring weather is waning, suggesting that temperatures and precipitation should return to normal levels later this summer and fall.

That is based on the indicators currently used by scientists to predict coming weather patterns, says Frank Marsik, an assistant research scientist and meteorologist in the Department of Atmospheric, Oceanic and Space Sciences.

“As we look ahead, the evidence suggests that the strong El Nino phase is starting to dissipate,” he says. “Model forecasts suggest that we will move toward a neutral phase in which the equatorial water temperatures will tend to return to their average conditions during the summer and early autumn.”

“Considering that the impacts of El Nino on our general weather patterns tend to be weaker in the summer anyway, it would appear that our summer 2010 will likely be close to our climatological averages,” Marsik says.

Still, he adds, “There are no absolutes, especially during weak El Nino conditions.”

Marsik explains that the Earth’s tilt and its position within its orbit around the sun significantly affect how solar energy or heat is deposited on the planet’s surface.

The weather patterns that we experience on a daily basis are key players in redistributing the surplus solar radiation deposited in the equatorial regions. Changes in the distribution of heating on the planet result in subsequent changes in weather patterns, which are responsible in part, along with ocean currents, for redistributing that heat.

The episodic warming of the eastern and central Pacific equatorial surface waters — a change seen every three to seven years and known as El Nino — can cause tropical thunderstorm activity within the equatorial regions to shift eastward.

“In response, the jet stream, as part of our general circulation, shifts as well, and thus the favored pathways for our mid-latitude weather systems shift,” Marsik says. “This is why the nation’s capital experienced a surprisingly snowy winter season. Our jet stream shifted in a way that resulted in a greater frequency of storms passing farther to the south, resulting in repeated storms moving through the mid-Atlantic states.”

However, during strong and persistent El Nino events, the tendency is for the Midwest and Great Lakes regions to experience another type of weather pattern. “As is typically the case under such conditions, we experienced warmer-than-average temperatures and below-average precipitation,” Marsik says.

The trend continued in April. According to official statistics from the U-M Cooperative Observation Station, the average, daily maximum temperature in April was 64.7 degrees, 6.1 degrees higher than the 1971-2000 average of 58.6 degrees. Also warmer in April was the average, daily minimum temperature at 42.7 degrees — 5.4 degrees warmer than the 1971-2000 average of 37.3 degrees.

Meanwhile, total precipitation for April was 2.16 inches, compared to the 1971-2000 average of 3.36 inches.

Marsik said the waning El Nino conditions likely mean that May, like April, also will likely be warmer than normal, but to a lesser degree. While complete May results have yet to be recorded, Marsik speculates that final May temperature and precipitation measurements actually may be near their climatological averages.