One year after Katrina, a PSU meteorologist is asking Why So Many Hurricanes? Is Global Warming to Blame?

October, 2006

by Lourdes B. Avilés

house_collapsedIs global warming making hurricanes worse? What are the effects of increasing global temperatures on hurricane activity? Before we answer those questions, we must discuss what hurricanes are and what makes them tick.

With plenty of moisture and atmospheric instability, the tropics are often alive with thunderstorms, most of them harmless and beautiful. Sometimes, however, if conditions are just right, a group of thunderstorms will travel together and develop a circulation. When this occurs, a tropical cyclone has formed. The intensity of a tropical cyclone is based on its sustained winds, which consist of one-minute averaged winds. If winds are blowing at less than 40 miles per hour (mph), the system is called a tropical depression and it is assigned a number. At 40 mph it is considered a tropical storm and it gets a name from a predetermined name list for that year. At 75 mph we have a hurricane, whose intensity can then be categorized from one to five using the Saffir-Simpson scale. At 115 mph, the lowest range of Category 3, we have a major hurricane. As the storm intensifies it attains the classic features usually associated with a hurricane, such as an eye and spiral bands.

Ingredients of a Hurricane

In order to form and maintain a hurricane (or any tropical cyclone), a certain set of ingredients must be present. First, some kind of trigger mechanism is needed. Hurricanes do not just pop out of nowhere. Even in a favorable environment, storms will not form in the absence of a pre-existing low pressure system. It does not really matter how this low pressure originated, but in the Atlantic, (which spawns most of the tropical cyclones that affect the U.S. and the Caribbean), the trigger is often provided by easterly waves. About 60 of these storms form over Africa and travel westward over the Atlantic every year during the hurricane season. Approximately 10 percent of them end up producing a tropical depression that can potentially intensify into a hurricane. The most infamous Atlantic hurricanes are all associated with African easterly waves.

Continuing with our recipe for hurricane formation, plenty of humidity and atmospheric instability are needed for the initial thunderstorms to form. These conditions are commonplace in the tropics, so under normal circumstances they are pretty much a given. A little distance from the equator is also required. The rotation of the earth causes a deflection of air motions known as the coriolis effect. This effect becomes stronger with increasing latitude and causes large storms (such as hurricanes) to rotate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. At the equator, however, there is no coriolis and a group of thunderstorms cannot develop a rotation. Also, the presence of wind shear, a measure of how much winds change with height, disrupts the formation or intensification of a storm and usually causes significant weakening of a well-established hurricane. Therefore, very weak or non-existent wind shear is of critical importance to hurricane formation and maintenance.

Finally, the temperature of the ocean is one of the most critical ingredients. Sea surface temperature is the fuel of a hurricane. The warm surface of the ocean provides large amounts of water vapor, which releases energy when condensing to form hurricane storm clouds. This extra energy makes the hurricane stronger (with faster winds), causing more water vapor to be released. The cycle continues as the storm intensifies, with the maximum possible intensity determined by the ocean temperature. Because of this, hurricanes cannot survive for long in cold water or over land. In fact, an ocean temperature of 80 degrees Farenheit is considered a threshold below which tropical cyclone formation is not likely. Also, because hurricanes stir up water from below the surface, the depth of the warm water is also important. Cold subsurface water would halt the intensification or weaken the storm.

Are There More Hurricanes?

Because sea surface temperature has such an important role in hurricane formation and development, one would think that global warming will certainly cause more of them and make them generally worse. But the answer is not so simple. Are we talking about the frequency of hurricanes, their intensity, the regions that they affect or all of the above? All of these questions are relevant and all have different answers, some clearer and some not so clear. Also, are we talking about already observed changes or potential future changes? Let us analyze some of these issues and time frames separately.

Hurricane activity in the Atlantic has been above average in all but one of the years since 1995. We have only to look at the record-breaking 2005 hurricane season to be convinced. This surge of activity follows 25 years of quiet hurricane seasons in the 1970s, 1980s and early 1990s. This increase cannot be directly attributed to global warming. The culprit is something called the Atlantic Multidecadal Oscillation or AMO.

The AMO is a cyclic variation in large scale atmospheric and oceanic conditions that combine to alternatively increase (positive phase) and decrease (negative phase) sea surface temperatures. Hurricane activity, more specifically, the number of major hurricanes, significantly increases during the positive phase of the AMO. This cycle is considered a natural variability of the Atlantic region and has been occurring for at least the last 300 years. Any increase in activity due to global warming would be too small to notice in the backdrop of the AMO.

The Atlantic Ocean only provides relatively few (about 10 percent) of the global number of storms. When other tropical cyclone producing regions are analyzed, such as the Pacific and Indian Oceans, none shows a discernable increase in hurricane frequency. This much is agreed upon by scientists at this time.

Are Hurricanes Getting Stronger?

Dr. Lourdes B. Avilés is assistant professor of meteorology and faculty-in-residence at Plymouth State University. Her areas of expertise include tropical meteorology, atmospheric dynamics and earth science education. She received her B.S. and M.S. degrees in physics from the University of Puerto Rico, Mayagüez, and her Ph.D. in atmospheric science from the University of Illinois, Urbana-Champaign.

Dr. Lourdes B. Avilés is assistant professor of meteorology and faculty-in-residence at Plymouth State University. Her areas of expertise include tropical meteorology, atmospheric dynamics and earth science education. She received her B.S. and M.S. degrees in physics from the University of Puerto Rico, Mayagüez, and her Ph.D. in atmospheric science from the University of Illinois, Urbana-Champaign.

Hurricane intensity has turned out to be a more controversial issue at this time. Very recent studies have found a large increase in global hurricane intensity during the last century. These studies (done by renowned scientists) have been under severe attack by other (also renowned) scientists in the field. The main issue is that the data used to calculate trends in hurricane intensity are not considered to be reliable enough to make definitive conclusions. The methods used to monitor hurricanes during the past century have become more sophisticated as the years have gone by, and the measurements from certain periods are somewhat suspect in terms of accuracy. That said, the observed intensity increase is so large that it is unlikely that all of it can be attributed to problems with the data. At the least, these findings should be considered as preliminary evidence that hurricane intensity increased during the past century.

What about the future? All experts agree that increasing sea surface temperatures due to global warming would certainly result in increasing hurricane intensity. On the other hand, hurricane frequency and distribution in a warmer world are much harder to determine at this moment. So far, studies have predicted both increases and decreases in the number of hurricanes. This is a testament to the limitations of current climate models and our lack of understanding of the fine details of the processes that lead to the formation of tropical cyclones. Many questions would need to be answered in order to have even a chance to predict not only how the frequency of hurricanes will change but also how the geographic distribution of their formation and affected areas will change. How will moisture, instability and wind shear patterns change? Will there be more or fewer preexisting disturbances available to trigger hurricanes? At this time, we don’t have enough knowledge to make these predictions.

So Much More to Learn?

The bottom line is that we are still trying to figure out this issue. The number of hurricanes does not appear to have been affected at this time, but the intensity clearly has been. We expect that increasing ocean temperatures will make hurricanes more intense, and we have no idea if there will be more hurricanes or if more areas of the world will be affected by them. It might be many years before the answers are clear. In the meantime, we continue our quest for understanding using the best tools we have. More studies on the subject are bound to come out. We can only look at them as objectively as possible and try to make sense of any discrepancies and disagreements until nature gives us the ultimate answer.

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