Where Has All Our Rain Gone?

Dave Martin

Everyone knows that the Southeast has been in a drought for the last four years, but exactly how bad is our situation in the Chattooga watershed from an historical perspective? Is this a trend that we can expect to become the new climate of the Southern Apps? In a time when so many of our climactic and environmental problems are the direct result of our own industry and overpopulation, it is easy to point a finger at global warming, urban heat islands, and other such phenomena as the cause of our dire straits. While these factors are certainly a large part of the equation, it is important to remember that normal weather patterns are always changing and may be doing so in a way that can be explained independently of human influence. There is no doubt that we are contributing to global warming. But let’s put this issue on the back burner for a minute and explore some components of our natural weather cycles.

In order to understand the Chattooga watershed’s weather patterns, we must first look at the entire Southeast as a region, and then try to understand what happens locally. Any boater of our beloved river can tell you that when there are hurricanes off the east coast, especially around the mouth of the Savannah River, it’s time to break out the duct tape and get the paddling gear ready for some sick days at the office. Most of our rain, however, comes from thunderstorms that build up in the Gulf of Mexico and are carried up here by the sub-tropical Jet Stream. It would make sense, then, to look at what affects these systems in order to understand how weather comes our way.

Scientists are now able to look at weather trends from the past and identify patterns that occur annually with the change of seasons. These annual weather patterns are affected by even longer-term trends that vacillate through decades or even greater lengths of time. Understanding these trends, which can sometimes last the course of a whole lifetime, is important if we are to understand how “normal” weather patterns behave in our area. A considerable amount of our knowledge of these long-term trends is speculative, because we only have about 150 years of recorded data to draw from. In some cases, this is only enough time to analyze two or three cycles of one of these weather patterns.

In the absence of empirical data, scientists are able to project models into the past based on geologic records of natural disasters and other significant climate changes. It is generally accepted, for example, that some of these phenomena that affect global climate variability, called “teleconnection patterns,” have occurred for over 13,000 years. While we do not yet fully understand the impact of many of these teleconnection patterns, their components, such as large high and low pressure systems, occur with such regularity that climatologists are able to use them as tools for predicting weather trends many months in advance.

El Niño is one such global weather anomaly that has received quite a bit of publicity in recent years. Even though it is a phenomenon that occurs in the Pacific Ocean, its effect can be felt around the globe. Normally, the sun heats the waters of the Pacific Ocean off the coast of Indonesia and Australia and causes massive hot, moist air currents to rise. As the air cools, it sheds its moisture in the form of monsoons in the South Pacific. Subsequently, the drier air continues to rise, and moves east across the ocean. It cools and condenses even more as it travels, and by the time it reaches the west coast of North and South America, it begins to sink, causing a high-pressure system. It then flows back out to sea as the Trade Winds. As these Trade Winds move west, they actually push the warm surface waters west towards Indonesia, where the process starts all over again. This cycle is known as the Walker Circulation, after the scientist who first observed the relationship between weather patterns off the coasts of South America and Australia. During a year where there is a strong El Niño, the Walker Circulation slows, or stops completely. Without the Trade Winds to push the warmest surface waters back out to sea, the sea surface temperature off the west coast of the Americas increases, until it reaches a point where the warm, moist air rises all at once. This causes severe rainstorms up and down the Pacific coast. Polar Jet Stream currents are pushed farther North into Canada, so the Northern U.S. experiences unusually warm cold seasons. Sub-tropical Jet Stream currents also pushed farther North, which means they do not pick up as much moisture as they would in a normal course over the Gulf of Mexico. A strong El Niño usually means that tropical storms in the Atlantic cannot build significantly. For this reason, fewer hurricanes occur along the East Coast.

El Niño’s sibling, La Niña , has just the opposite effect. During years with a strong La Niña, the Pacific Trade Winds are warmer than usual, driving an even greater amount of warm water westward. Tropical Jet Stream currents are generally weaker during a strong La Niña, so storms brewing in the mid-Atlantic rarely meet resistance as they build and head west towards our coast. As a result, a strong La Niña usually means a more active hurricane season for the Southeast. Unfortunately, weaker Tropical Jet Stream currents also mean less rain will come inland from the Gulf of Mexico. Weather patterns tend to be much less predictable across much of North America during a strong La Niña because of the absence of Sub Tropical Jet Stream currents.

Another long-term weather cycle that has a significant effect on the weather in the Southeast is the North Atlantic Oscillation (NAO). This phenomenon is the result of the interplay between a sub-tropical high pressure system over the Azores Islands in the mid-Atlantic, and a sub-polar low pressure system near Iceland. When the temperature between these two systems is greater, the NAO is said to have a positive index. This trend results in mild and wet winter conditions for the Eastern U.S. A negative NAO index implies that there is a weak sub-tropical high and a weak sub-polar low, and results in colder, drier winters for the East. Altogether, the National Oceanic and Atmospheric Administration (NOAA) recognizes 13 teleconnection patterns in the Northern Hemisphere.

Once we consider all of these global influences, we still have to consider more local issues that may affect our weather patterns to determine when our water table might see some relief. When we tune down the scope of our inquiry to a local perspective, human influences on the climate unfortunately become much more apparent. The Goddard Space Flight Center at NASA released a report on July 18, 2002 stating that large urban areas create heat islands which cause on average a 28% increase in rainfall downwind of the area from 30 to 60 kilometers (18 to 36 miles). If rain falls in such an eddy immediately downwind of these urban areas, that means areas farther downwind will receive only hot, dry air. The jury is still out on whether or not the Chattooga watershed is directly affected by these urban heat islands, but the Goddard Center’s report cited Atlanta as one of the most significant occurrences of these heat islands in North America. Common sense would lead one to believe that a large asphalt heat fence between the Southeastern Blue Ridge Escarpment and the Gulf of Mexico must be robbing of us of at least some of our rain.

Global warming is, of course, a reality that even our Commander in Chief, George Bush, has acknowledged. The Intergovernmental Panel on Climate Change, an organization established jointly by the World Meteorological Organization and the United Nations Environment Programme in 1988 to monitor the human impact on global climate change, concluded that the 1990’s was the warmest decade on record, since the records began in 1867, and that the global temperature rise that can be attributed to human influence measures between 1.4 and 6.3 degrees Celsius. Climate change due to human industry has a measurable effect on the teleconnection patterns, but the effects vary according to the patterns’ characteristics. For one reason or another, the four most severe El Niños of 23 in the 20th century occurred between 1980 and 1998.

So what does all this mean for the immediate future? The NOAA’s Prognostic Discussion for Long Term Outlooks, which looks at three factors -- soil moisture, the El Niño /La Niña phenomenon, and current weather trends, (or simply “trend”) -- says that there is a measurable temperature anomaly in the Pacific of +1 degree Celsius, which indicates a weak or moderate El Niño this season. Without taking Atlantic teleconnection patterns into consideration, it predicts, “abnormally wet, and in some areas cold conditions across much of the South through the winter and into the spring of 2003.” The Northern Atlantic Oscillation index for the fall and winter indicates a return to the negative, which points toward a break in the regional drought, and a return to cooler weather this fall and through the winter. According to science, hope is in sight for the Southeast in the coming months. As far as anthropogenic climate change is concerned, we are all responsible for looking much farther down the road at the way our society is growing. There is no doubt that we are changing our own climate, and it is up to each of us to encourage business practices that conserve our natural resources and seek to reduce the amount of waste we produce.