The summer of 2010 saw some cruel extremes in weather. Flooding in Pakistan inundated over 300,000 square miles and affected more than 20 million people. In Russia, a heat wave killed more than 50,000 people and lead to the loss of some 20 million acres of crops. Widespread forest fires created incredible images of smoke-choked Russian cities during their warmest summer on record. While these events were wildly different, it appears they were closely linked.

The Russian heat wave was caused by a high pressure system that stuck around for a remarkably long time—a little over two months. That stationary high pressure system, which atmospheric scientists call a "blocking high," caused a disturbance in atmospheric circulation, splitting westerly winds into two narrow streams. The southern stream formed a persistent low-pressure trough over northern India.

During the monsoon season, the Indian subcontinent is warmer than the Indian Ocean, driving a landward transport of moisture-laden air. Last summer, that moisture was pulled strongly into the low-pressure trough. To make matters worse, La Niña conditions in the Pacific made for a wetter-than-average monsoon by blocking the eastward movement of moisture out of the Indian Ocean.

When all these factors came together, moist air masses converged in the low-pressure trough, where they were pushed upward by the topographic barrier of the Himalayas, dumping their moisture as precipitation and swelling Pakistan’s rivers. The unusually long-lived persistence of these conditions drove the endless rain and nightmarish flooding.

A connection between the tragedies in Russia and Pakistan had already been suggested, but the mechanics of that interaction are now clear. The rain that fell in Pakistan was not the moisture Russia was missing, but the phenomena were ultimately two sides of the same calamitous coin.

Geophysical Research Letters, 2011. DOI: 10.1029/2011GL047583 (About DOIs).