Thursday, July 11, 2013

Why Is It So Rainy, and How Can I Blame It On Climate Change?

This has been one of the most humid, rainiest summers I can recall in Georgia since I moved here in 1981. We've already had more rainfall in 2013 than we had in all of 2012 (which, incidentally, was not a drought year).  Mushrooms are growing all over the yard, a layer of moss has developed on the back patio, and white mold is growing on the wooden retaining wall behind the house and in between the bricks. Since the beginning of this month, it seems that it's rained every day, with some days, like last July 4, a nearly nonstop downpour.  When it's not raining, it's been so humid that nothing can dry out - the air is already saturated. What's going on?

The National Weather Service confirms that total rainfall values are above normal in a distinct corridor from the Gulf/Caribbean Sea into the Southeastern U.S.  The copious quantities of rainfall are apparently due to a deep moisture plume extending all the way south to Central America.  The moisture plume is a result of the Jet Stream and a Bermuda High-Pressure System which has been inching westward for a while now. Meanwhile, a low-pressure zone is situated over western Missouri.  The low-pressure zone pulls the Jet Stream south into the lower Mississippi River valley, but since the Jet Stream can’t go through the Bermuda High, it has to go north to get around it.  As it does so, it funnels warm, tropical-like moisture from the Caribbean Sea, the Gulf of Mexico, and the Eastern Pacific Ocean, accompanied by extensive cloud cover and precipitation. 

Dry air wrapping around the southern/eastern edge of the low-pressure system is shown by the red/orange shading over Arkansas and Missouri.  Dry air wrapping around the west side of the Bermuda High is shown by the orange plume to the east of Florida.  In between these two systems, the large plume of very moist air is shown by the green shading. 

The moisture plume can be traced all the way back to Central America. Very high (2+ inches) rainfall levels extend from the Yucatan peninsula and western Caribbean Sea, across the eastern Gulf of Mexico, and all the way north along the U.S. East Coast. This persistent moisture plume was responsible for the recent flooding in parts of the Northeast states, as well as the muggy, rainy weather currently in Georgia.

A front forecast to move into Georgia on Friday should bring some of that red/orange dry air from Missouri over to North Georgia and finally push the main moisture plume to the east of the state for a while.  The drier air will be short-lived however, as the moisture begins to retrograde back to the west on Saturday.

So, can we blame any of this on climate change, as the title of this post implies?  As tempting as it is to answer in the affirmative, the short answer however is "no" - no single weather event can be held as an example of climate change.  However, having said that, both the Bermuda High and the amount of moisture in the Central America/Gulf of Mexico air mass are related to the temperature, and as temperatures increase, moisture plumes like the one we're experiencing now will become more and more common and more and more persistent.  The increased frequency and duration of moisture plumes, just like the increased frequency and intensity of tornadoes, is a direct result of climate change.

But in any event, it looks like tomorrow is going to be a nice, relatively dry, day.  We ought to enjoy it while we can.

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