In August, Pakistan set destructive records as it averaged more than triple its normal August monsoon rainfall. In the southern provinces of Sindh and Balochistan, the number was seven to eight times the average. The resulting flooding killed around 1,500 people and displaced more than 30 million—a catastrophe of incredible scale.

Is this an event we expect a warmer climate to have influenced? As they often do, the World Weather Attribution team quickly analyzed this question and released the results on Thursday. Their peer-reviewed method for these rapid studies is to apply standardized analyses to both historical weather data and climate model simulations. The goal is to find out whether a given weather pattern is part of a long-term trend and then determine whether we expect such a trend to come as a result of human-caused global warming.

Lots of factors

This event is more complex than something like a short-lived heatwave, given that it played out in waves over weeks and depends on highly variable monsoon patterns. Monsoon rains result from the seasonal transport of moist air over land combined with uplift that cools that air, wringing the moisture out of it. This pattern is hit-or-miss in Pakistan, as it often originates over eastern India and bends northward before it can reach Pakistan.

But this summer, eight monsoon depressions have tracked neatly east-to-west across India and into Pakistan—in part due to record warmth earlier this year that maintained low pressure, which drew the systems in. This has affected the south of Pakistan more, but the north also saw rain late in August as the jet stream dipped southward into the region.

In addition to these big-picture spatial patterns, warm water in the eastern Indian Ocean boosted the moisture content of the air coming inland. Some blame for that goes to the ongoing La Niña—the previous flooding record belonged to 2010, another La Niña year. This year, the effect of La Niña was amplified by an oscillation called the Indian Ocean Dipole.

To analyze weather events, researchers need a precise definition of the area and timeframe. In this case, they’ve had to settle for two. They examined both the entire Indus River Basin (which includes most of Pakistan) for 60-day-average precipitation in monsoon season and five-day-average precipitation solely for the southern provinces of Sindh and Balochistan. “These two metrics align most closely with the impacts of the event, capturing both the short heavy precipitation in the southern provinces, as well as the longer spell over Pakistan,” the report says.

https://arstechnica.com/?p=1882062