On the anniversary of Hurricane Katrina, the costliest natural disaster in American History, Hurricane Isaac struck New Orleans and ravaged the area with 16 inches of rain and abrasive 80-mph gusts of wind. The Bayou, still recovering from Katrina’s wrath seven years ago, experienced severe flooding as the Mississippi River swelled past its banks. More than 5,000 people lost their homes and at least four lost their lives to the storm. As the Delta Region begins the rebuilding process once more, one question still lingers—why is the Gulf of Mexico so prone to hurricanes? Its answer is connected to the nature of storm formation and the very attributes of the Gulf itself.
Tropical cyclones, commonly known as hurricanes, require six conditions to form:
- Sea surface temperatures above 80 degrees F
- High humidity within the lowest 3 miles of the atmosphere
- A previously existing tropical disturbance
- A minimum distance of 300 miles from the Equator
- An atmosphere which cools fast enough with height so that heat trapped in the ocean can escape to the atmosphere via convection
- Vertical winds below 23 miles per hour (NOAA FAQs: What Causes Hurricanes?)
When these conditions are met, it is likely but not guaranteed a hurricane will emerge. These conditions are also necessary for a storm to grow in strength.
But what is a hurricane? A hurricane, monsoon, typhoon, or cyclone is a whirling storm system that has a low-pressure center and many thunderstorms that produce powerful winds and heavy rain. Formed when a small thunderstorm reaches the ocean, tropical hurricanes begin as tropical depressions. The warm water evaporates from the ocean surface, releasing heat into the atmosphere as it rises. As the air condenses, more thunderstorms form around the original depression. Condensation leads to higher wind speeds, as a tiny fraction of the released energy is converted into mechanical energy, while the pickup in wind speed causes increases in both evaporation and condensation. This type of positive feedback loop—a process that perpetuates itself—is called wind-induced surface heat exchange, and continues for as long as the elements support hurricane development.
The rotation of the Earth turns the system into a whirling vortex; the spin, in turn, affects the direction of the storm. In the Northern Hemisphere, hurricanes revolve counterclockwise due to the flow of tradewinds. The storms rotate the opposite way in the Southern Hemisphere, revolving clockwise.
Unlike other weather phenomena, hurricanes are unique in that they rely on convection for energy. Convection, in this case the diffusion of heat from one fluid (the ocean) to another (the atmosphere), make these storms dependent on warm water as a source of vapor for maintaining those positive feedback loops. This heat also makes the air inside hurricanes warmer than the surrounding atmosphere. If the storm loses this valuable energy source, it will rapidly lose its strength.
The Gulf of Mexico is a hotbed for hurricanes because its coastline is surrounded by shallow waters that are warm year-round. Located in the Subtropics, the sunlit sea provides the warm water that drives the Gulf Stream. The Gulf covers approximately 600,000 square miles of the Atlantic—which includes parts of the Caribbean, the United States, and Mexico—and has an average depth of 5,300 feet. In the summer sea surface temperatures can exceed 88 degrees Fahrenheit, more than enough to strengthen a tropical storm. Higher temperatures mean higher levels of evaporation, creating a low-pressure zone that facilitates storm development. In fact, the Gulf’s warm temperatures were responsible for boosting Tropical Storm Isaac into a Category 1 Hurricane this August. Exceptionally warm waters in 2005 transformed the Category 1 Katrina that annoyed South Florida into the Category 5 monster that annihilated New Orleans.
Measurements of sea surface temperatures over the past eighty years have revealed a general warming of 2 degrees Fahrenheit in the Gulf. This is consistent with the effects of Global Warming in all of the world’s oceans and seas. Factors like greenhouse gases, coming from cars and heavy industry, trap sunlight in the atmosphere and contribute to rising ocean temperatures. As a result, warmer waters could fuel greater numbers of stronger storms over longer hurricane seasons.
This also means that key Gulf ports like New Orleans will need to prepare for prolonged hurricane seasons and for stronger storms. More than 275,000 homes were destroyed when Katrina hit the city in 2005. Since then, the Army Corps of Engineers constructed canals, levees, and sea walls to protect the city from hurricane-related floods. This contributed to the relatively minimal damage left in Isaac’s wake. However, many other cities along the Gulf Coast cannot handle a typhoon like Katrina and require extra fortification should one strike.
Hurricanes, however, are an unavoidable part of life in the Tropics. They provide necessary rain to areas that normally receive little moisture, like the deserts in Texas and Mexico. As America faces the longest, most widespread drought on record, hydration—whatever the cost—could be the gift our nation needs to survive. These storms might also be harbingers of future climate change on a far grander scale. If that is the case, the Gulf Coast has to be ready.