January 4, 2012 | New research led by NCAR scientist Thomas Galarneau provides an in-depth analysis of two extreme weather events whose connection may come as a surprise: Russia’s intense heat wave in summer 2010 and the heavy rains that occurred simultaneously in Pakistan.
The heat wave, which was centered over eastern Europe and western Russia, brought with it severe drought conditions and intense wildfires. At the same time, northern Pakistan was being inundated by episodic bursts of heavy rainfall. Both of these weather events reached extreme intensities on July 28–29, when maximum temperatures in Moscow exceeded 95°F (35°C) and 48-hour rainfall totals measured in parts of Pakistan exceeded about 16 inches (40 centimeters).
For the study, which is slated for publication in Monthly Weather Review, Galarneau and colleagues used temperature and rainfall data from surface stations in both Russia and Pakistan, data from satellites and radiosondes, and numerical model analysis fields and ensemble forecasts, to produce a multi-scale analysis of the two events.
The results show that a large-scale, stagnant weather pattern known as a “block” developed over western Russia, dividing the jet stream and preventing the normal progression of weather systems from west to east. Energy dispersed downstream from this block, creating a trough (elongated region of low atmospheric pressure) northwest of Pakistan and a ridge (elongated region of high pressure) above the Tibetan Plateau. The pattern was further amplified in response to enhanced convective outflow on the large-scale associated with the active phase of the Madden-Julian Oscillation. The interaction between these extratropical and tropical processes facilitated the formation of an intense upper-level jet stream over northern Pakistan. These conditions also contributed to enhanced southeasterly winds at low levels over northern India and Pakistan, bringing tropical moisture unusually far west from the Bay of Bengal into the area, leading to exceptional rainfall.
The intense upper-level jet stream, ridge over the Tibetan Plateau, and deep easterly flow over northern India were features that were absent during previous, less-intense rain events over northern Pakistan in 2010 and were therefore likely key factors in the extreme nature of the late July event.