How Long Could the Southeast Drought Last?
The southeast is currently in the throes of what is being called a 100 year drought. Most climatologists say that the drought began back in the spring of 2006, and continues through today. While I’d love to be able to say that the drought will be over soon, taking a look at history and climate cycles, we could be in a situation where drought will be the ‘new normal’ for the next 20 years.
You are probably familiar with El Nino and La Nina, the complementary patterns of sea surface temperatures in the Pacific Ocean. In El Nino years, when sea surface temperatures are warmer, the southeast tends to get more rain, while in La Nina years, including the one we are in now, the southeast tends to be warmer and drier. El Nino and La Nina tend to be short term patterns, lasting a year or so. Frequently, sea surface temperatures are neutral, with neither condition occurring.
There are also longer term ocean temperature trends, ones that my last from 30 to 70 years. One trend is the Atlantic Multidecadal Oscillation, or AMO, that reflects ocean temperatures in the Atlantic. Another is the Pacific Decadal Oscillation, or PDO, that represents Pacific Ocean temperatures. These two long term trends tend to have some effect on the amount of rainfall seen in different parts of the country, including the Southeast.
Perhaps the biggest influence on rainfall in the south is the AMO. The following chart shows the AMO pattern since the mid 1850s:
You can see that the AMO was positive from about 1930 through 1963, negative from 1963 through 1995, and has become positive since that time. The chart below shows the PDO since 1900:
While this chart shows the PDO in as currently in a positive phase, the chart only goes through 2004; there is some evidence that it may be switching to its negative phase.
So what does all this climate science mean?
According to a research report by the National Academy of Sciences, we can predict long term drought conditions by looking at the phases of the PDO and AMO. This chart shows typical drought conditions during each of the four possible combinations of warm and cold AMO and PDO. Essentially, for the southeast, there is a tendency for strong droughts when both the PDO and AMO are positive, which is the situation we are in now. When both indicators are in their cool phase, the southeast gets a lot of rain, and when they are opposite each other, the results are mixed.
The Atlantic Multidecadal Oscillation turned to its warm cycle in 1996. Since then, we’ve seen drought conditions in the early part of this decade. Strong hurricanes in 2004 and 2005 limited the effects of the drought during those years, but with no hurricanes over the last two years, we are feeling the pain. Interestingly enough, the frequency of hurricanes increases when the AMO is in its warm cycle, but it’s still a crapshoot as to whether they will replenish the water supply during any given year.
The bottom line is that we may be about a third of the way through a drought cycle that could last for the next 20 years. If we get any relief during that time, it’s likely to be because tropical weather brings the remnants of a hurricane through North Georgia. We might also benefit if the PDO moves into its cool cycle, and there is some evidence that that might be the case. Either way, we need to be thinking that the drought is a long-term problem, and not something that will be gone next spring. We should also be thinking about what we can do to capture more water for later use when the AMO finally does move to its cool phase, and return to the plentiful rainfall that we saw during the 1970s and 80s.
If you want to know where some of this thinking came from, take a look at this report from ICECAP (PDF) and this report from NOAA. I also got some insight from this research paper (PDF).
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