Monday, September 24, 2012

Two Weather Records — Related?

For nearly a month, Arctic sea ice has been stuck at a record-low level.

Meanwhile, the areas of the United States in a state of drought have remained near record proportions. That is, in the history of the country, we have never seen drought affect much more of the land area of the country than it is affecting now. By some measures, this year’s drought represents a record.

According to conventional theories of weather, these two records may be related. Ice cools the Arctic region as it melts, so less Arctic ice means less cold air. Less cold Arctic air during summer (through the middle of fall) should mean fewer and weaker frontal boundaries on which rainstorms can form, and therefore, less rain, specifically in areas away from bodies of water.

In theory, the rapidly warming Arctic should change the summer climate of the central areas of North America east of the Rockies, but without much effect on the East Coast, Gulf Coast, or Great Lakes areas. The area that is expected to be most affected is similar to the area that has been most strongly affected by drought this summer. This is the broad area of reddish colors in the August 2012 Palmer Drought Severity Index map from the National Climatic Data Center. (The August drought area extends more to the east and west than the area the theory would point to.)

The question of whether the two weather patterns are related is important. As the drought drags on into fall, we want to find out the extent to which this summer’s low rainfall might actually be the new normal. Arctic sea ice will continue to decline and temperatures will increase in future summers, so a mid-continent pattern of reduced summer rainfall could begin soon — unless, a worrying possibility, it is happening already.

You cannot say anything about climate from one season’s weather, but the coming seasons may hint at what is on the way. If precipitation patterns return to historical norms in November, only to dry out again next summer, we will have to start guessing that the lack of rainfall is not merely a drought, but a change in climate. On the other hand, if next summer has a reassuring sequence of thunderstorms, then those particular climate worries can be set aside for the time being.