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Boston Weather: 22.0°F | Mostly Cloudy

Jet Stream Plays Role in Warm Weather

By Brian Keegan

Who turned the weather machine on? Freshmen, don’t get used to walking around MIT in shorts during the winter. According to the National Oceanographic and Atmospheric Administration, January 2006 was the 5th warmest on record, averaging 32.9 F, 7.7 F warmer than the average for the past 112 years.

Nationally, January charted the highest average temperature ever recorded by the NOAA, 8.5 F above the historical average of 39 F. The NOAA’s National Climatic Data center calculated that 74.2 percent of the contiguous US, the lower 48 states, was “very warm.”

Senior Lecturer Lodovica C. Illari and Jonathan R. Moskaitis G of the Department of Earth, Atmospheric, and Planetary Sciences, explained some ideas as to why Boston has experienced 35 days of above average temperatures so far this year.

The jet stream, the invisible hand that makes our cross-country flight up to an hour longer in one direction, is a west to east air current that forms approximately 40,000 feet (7.5 miles) above ground at the boundary between air masses with different temperatures, like warm tropical air and cold arctic air, Illari said.

Moskaitis said that the jet stream operates in essentially two modes: zonal and blocked. In a zonal flow, the jet stream flows uninterrupted around a relatively constant latitude. Alternatively, this flow can be blocked by meridional flows that run in the north-south direction, he said.

Zonal flow is marked by moderate weather, while the disruptions caused by a block tend to create more extreme conditions, because they tend to enhance mixing of cold polar with warm tropical air, Illari said. Blocks can persist for a few days to several weeks, she said.

Illari believes that the moderate weather in January can be attributed to the lack of blocks over the northern Pacific. She says this zonal flow allowed the jet stream to remain farther north, insulating us at MIT from the cold arctic air, while allowing warmer tropical air to push up across the country.

At the same time, a persistent block over the northern Atlantic caused severe disruptions in the jet stream in the eastern hemisphere. Illari said this block allowed cold air to rush southward and caused record-low temperatures over Russia, central Europe, and her home country of Italy.

Moskaitis said January is typically the “top month for blocking events” in the north Atlantic and Pacific, so “we were fortunate this January.”

The conditions that cause blocks remain an “active area of research,” said Illari. She said that sea surface temperatures affect long-term jet stream behavior.

According to the NOAA Climate Prediction Center Web site, the Pacific Oceans is experiencing La Ni a conditions, a pattern of cooler-than-normal sea surface temperatures near the equator. La Ni a conditions, to which Illari says Pacific zonal flows are closely tied, are expected to continue for up to 6 months.

Illari cautions that there are many interacting variables that complicate forecast models, including predictions of the formation of blocks.

According to the Climate Prediction Center Web site, the difficulty in providing general forecasts beyond a few days is a result of the “inability of numerical weather prediction models to simulate the onset and evolution of blocking flows.”

Moskaitis said that while blocks are created randomly, they become more persistent as passing storms reinforce them. “Once you stop feeding it storm systems, it breaks down.”

The jet stream will give way to a “more typical winter pattern” with more variable temperatures and conditions, according to the Web site. A persisting major block developed over the north Pacific between February 6th and 13th, and this was a factor in last week’s winter blizzard, Illari said.

Moskaitis is a Staff Meteorologist for The Tech.