August 31, 2011 – MOSCOW – Two of the most destructive natural disasters of 2010 were closely linked by a single meteorological event, even though they occurred 1,500 miles (2,414 km) apart and were of completely different natures, a new NASA study suggests. The research finds that the same large-scale meteorological event – an abnormal Rossby wave – sparked extreme heat and persistent wildfires in Russia as well as unusual downstream wind patterns that shifted rainfall in the Indian monsoon region and fueled heavy flooding in Pakistan. The atmosphere, gaseous and transparent, may not seem like a fluid, but that’s precisely how the thin layer of air encasing the planet behaves. As Earth spins on its axis, huge rivers of air – scientists call them Rossby waves – meander around the globe in a westerly direction. Currents in the center of these waves form the jet streams, fast-moving columns of air that push weather systems from west to east. Rossby waves aren’t uniform. They tend to undulate and have troughs and ridges. Areas of low-pressure typically develop in the troughs of the waves, while high-pressure areas form in their ridges. Parcels of warm air from the tropics and cool air from the poles swirl around the low- and high-pressure parts of the waves creating a complex tapestry of warm and cool fronts that meet and interact constantly. Collisions between warm and cool fronts produce storms and precipitation. Under normal summertime conditions, the jet stream pushes weather fronts through Eurasia in four or five days, but something unusual happened in July of 2010.
A large-scale, stagnant weather pattern – known as an Omega blocking event – developed over a high-pressure ridge above western Russia. This blocking event, which divided the jet stream, had the effect of slowing the Rossby wave and prevented the normal progression of weather systems from west to east. As a result, a large region of high pressure formed over Russia and trapped a hot, dry air mass. As the high lingered, the land surface dried and the normal transfer of moisture from the soil to the atmosphere slowed. Precipitation ceased, vegetation dried out, and the region became a taiga tinderbox. Meanwhile, the blocking pattern created unusual downstream wind patterns over Pakistan. Areas of low pressure on the leading edge of the Rossby wave formed in response to the high that pulled cold, dry Siberian air into lower latitudes. This cold air from Siberia clashed with warm, moist air arriving over Pakistan from the Bay of Bengal. There’s nothing unusual about moisture moving north over India toward the Himalayas. It’s a normal part of the monsoon. However, in this case, the unusual wind patterns associated with the blocking high brought upper level air disturbances farther south than is typical, which helped shift the entire monsoon rainfall system north and west. The shift brought heavy monsoon rains squarely over the northern part of Pakistan. –Space Daily