What the grounding doesn't know

Forget about the groundhog and his shadow. Scientists have discovered that the interplay between two layers of the atmosphere plays a major role in the arrival of spring--a finding that could lead to improved weather and climate forecasting.

"Our research indicates that the onset of spring is more rapid than suggested by the annual cycle of long-term daily averages and is linked to an event known as the stratospheric final warming," reports Robert Black, an associate professor at the Georgia Institute of Technology's School of Earth and Atmospheric Sciences, Atlanta.

For many years, scientists believed that the troposphere (the lowest region of atmosphere where weather occurs) drove changes in the stratosphere (the layer directly above the troposphere) without any feedback. Yet, in the late 1990s, new studies found that the stratosphere can affect the tropospheric circulation. These studies, however, focused on individual seasons.

"Because the arrival of spring has a pronounced influence on the hydrologic cycle, vegetative growing season, and ecosystem productivity, we wanted to study the transition between seasons," Black relates. To that end, his team gathered observational data derived from a variety of sources and constructed a composite picture of spring's arrival over a 40-year period.

As winter draws to a close, the westerly jet stream in the troposphere begins to weaken. At the same time, the westerly jet stream in the stratosphere not only weakens, but eventually reverses direction to become easterly. Black and his colleagues discovered that this event, known as the stratospheric final warming, accelerates the weakening of the tropospheric winds.

"Instead of a gradual weakening over several weeks, it's as if someone flipped a switch," Black explains. "The transition from a winter to spring wind pattern occurs in about one week."

Stratospheric final warming events are caused by large-scale Rossby waves, planetary waves that are produced when winds in the troposphere blow over different surfaces on the Earth, such as major mountain ranges. These waves can extend into the stratosphere where, if their amplitude is great enough, they create a drag on the stratosphere's westerly jet stream that can trigger final warming.

'These final warmings don't happen at the same time every year," Black points out, noting that they can occur as early as mid-March or as late as mid-May. The researchers found that these events vary in their intensity, and final warmings that take place earlier in the year typically are more abrupt.

Granted, other factors also influence spring's arrival, such as solar heating of the Earth's surface. "Still, stratospheric final warmings explain a significant part of the seasonal transition, especially in the Arctic;' Black confirms.

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