1 November 2010

Climate Change Linked To Southeast U.S. Floods/Heatwaves

Posted by Dan Satterfield

People often ask if a flood, a snowstorm, or a hot summer was caused by climate change. The correct answer is to say that no one weather event can be blamed on climate change, but certain weather events become more likely to occur.

Here in the Southeast U.S. we had a very hot summer. Downright brutal actually. The hottest on record in many cities. Atlanta had one of it’s worst droughts ever recorded a couple of summers ago. There have also been some massive floods. Nashville was hit by what can only be described as a flood that might come once every 500 years.

Extreme weather events happen. You can count on it.  It makes my job forecasting the weather extremely fascinating.

Summer rainfall anomalies over the Southeast USA. Notice how the extremes have increased since 1980. From: Changes to the North Atlantic Subtropical High and Its Role in the Intensification of Summer Rainfall Variability in the Southeastern United States- full citation at the bottom of this post.

The question a group of atmospheric scientists asked recently is this. “Are extreme summer weather events becoming more frequent in the Southeast.”

The answer was a definite yes. The question then becomes-


This is where an incredibly useful data set comes in. It will take a second to explain but believe me it’s worth it.

Several times each day, high speed computers use weather balloon data and surface observations to build a 3 dimensional model of the atmosphere. Ship reports and data from weather satellites are also used.

This analysis is used to give numerical weather prediction models a starting point. You cannot predict weather in the future unless you know mathematically what the weather is doing now. If you want to forecast for the globe, you need to know a starting point for the entire planet!

A few years back scientists had an idea. They would use modern methods of making a global analysis and go back in time. Ship reports and weather balloons have been around a lot longer than high speed computers. The very sophisticated methods of coming up with an analysis of the atmosphere could be used to build a data set of the atmosphere for the past 60+ years!

They did just that.

In Europe the European Center for Medium Range Forecasting did it as well. They use a slightly different way of making the first guess for the numerical weather models, so we have two close but not identical data sets.

A new paper being published in the Journal of Climate contends that the increasing extreme summer weather over the Southeast U.S. is due to semi permanent high pressure cell over the Atlantic. The official name of  this system is the NASH. That stands for North Atlantic Subtropical High. Most meteorologists call it the Bermuda High.

The Bermuda high is getting stronger and moving westward. Top left panel is the NASH from 1948 to 1977 in the NCEP reanalysis. Top right is the same from 1958-1977 using the European data set (ERA). The bottom panels show the period from 1978 to 2007 (1977 to 2002 using the ERA data)

The Bermuda High pumps warm moist air into the Gulf Coast States all summer and brings plenty of rain to the region. Summer thunderstorms are a common occurrence. Tropical cyclones are also steered by the big high pressure system. Sometimes right into the Gulf of Mexico. Sometimes around it and out to sea.

Every now and then the high pressure center will move westward, closer to the mainland and drift north a bit. This brings very hot and dry weather to the region. When the high moves close and drops southward, it gets very wet and stormy.

Dr. Wenhong Li and colleagues found that the Bermuda High is behaving strangely now. It’s getting stronger and moving westward. It is also drifting North and South more than in the past. They show in their paper that the drought and flood weather over the Southeast is directly related to this.

So, why is this happening?

It could be long term oscillations in the ocean/atmosphere system. These are well known. The Pacific Decadal Oscillation (PDO) could be involved. So could the Atlantic Multidecadal Oscillation (AMO). The PDO and the AMO are long duration changes in ocean temperatures which affect weather patterns. These patterns have been occurring naturally for at least a millennium. Very likely for much, much longer.

It’s not the AMO or the PDO. Li and his colleagues checked it out and the correlation between them and the Bermuda High movements is very poor. In other words, the behavior of the Bermuda High does not seem to be related to the AMO/PDO.

Could it be climate change?

They compared the behavior of the Bermuda High in the past using the reanalysis data sets and also looked at a series of climate models. They first looked at 23 models to look at how the NASH behaved when greenhouse gases were set to pre-industrial levels. They saw behavior consistent with the reanalysis prior to 1980.

Carbon Dioxide levels are rising at rate not seen in over 800,000 years. It's nearly certain now that this is changing the planet's weather.

They then looked at a set of climate models with more realistic modern day levels of carbon dioxide and other greenhouse gases. They found increased correlation. The Bermuda High starts behaving more like what has been seen since 1980.

They then looked at a set of 23 different models with rising greenhouse gases until the CO2 levels double in the year 2100. It’s very likely this will happen if we continue as we are now.

They found even more correlation with the behavior of the Bermuda High seen in the last 3o years. The average of these future climate models show extreme droughts and rain events over the Southeast as the Bermuda High gets stronger and moves westward.

Here is a quote directly from the paper:

Our attribution analysis suggests that global warming seems to be contributing to the changes of the NASH.”

It will be interesting to see the reaction to this paper. I already know it will get plenty of press, but I’m interested in what other climate researchers think. Did they overlook anything, make a mistake in the statistical analysis, etc.

If not, then we can say with some confidence that the brutally hot summer of 2010 was indeed at least partially due to climate change.  So was that flood in Nashville. So was that drought that almost shut off Atlanta’s water supply in 2007.

The real science is a bit scarier than what you see on cable news isn’t it.


NOAA http://www.aoml.noaa.gov/phod/amo_faq.php

Changes to the North Atlantic Subtropical High and Its Role in the Intensification of Summer Rainfall Variability in the Southeastern United States

Wenhong Li, Laifang Li, Rong Fu, Yi Deng and Hui Wang

doi: 10.1175/2010JCLI3829.1 (http://journals.ametsoc.org/doi/abs/10.1175/2010JCLI3829.1?journalCode=clim)