PHY 211 Net Research Project for 9/29/03

HURRICANES

What Are They? Following our investigation of energy, and as we start the study rotation, what better subject than an energy-packed, rotating phenomenon of nature. Hurricanes are great tropical storms that take heat energy from the warm waters of the sea and convert it into a giant rotating system of wind and rain. The process of creating a hurricane starts from a centralized area of low atmospheric pressure, called a depression. Air is pushed into the depression area from the surrounding higher pressure areas. USA Today has a scaled schematic of pressure driven winds. (They also have a How Weather Works site with numerous links describing many elements of weather.)

Gert & Floyd, 9/15/99 (click for a bigger view!)

But it takes more than simply pressure driven winds to create a hurricane. As the wind blows across the sea, water is evaporated and carried with the winds in the form of water vapor. The converging air is pushed upwards where it cools and the water vapor condenses into clouds and thundercells. The condensation process releases a tremendous amount of heat (we'll study this later in the semester) which fuels the system. The heated upper level air expands, dropping the pressure in the central region and allowing more air to be forced into the center. However, this process would not continue for more than a few days if it were not for another crucial ingredient ... rotation.

If the physical size of the depression is large enough (~ 100 miles or so), the winds are high enough, and the depression more than about 10^o from the equator, then the wind generated can take on a rotation about the depression area due to the coriolis effect. The coriolis effect arises from the earth's rotation. The earth has a tangential velocity of about 1000 mph at the equator. This tangential velocity drops to zero at the poles. Air moving northward into the depression area retains it higher eastward velocity and deflects to the east (relative to the earth). Similarly, air flowing south deflects to the west. This starts the rotation. The rotation is critical, because it provides an "organized" outflow for the air being driven into the eye of the storm.

If conditions are right, warm moist air rising and condensing will entrain more air, increasing the winds and creating a tropical storm. If the process continues, the storm can evolve into a hurricane. Although there is no exact phenomenological transition point when a depression becomes a storm and then a hurricane, there is a commonly used scale. The Saffir-Simpson intensity scale is:

Where Do They Occur?

Hurricanes can be found in many parts of the globe. Although most start within about 20^o of the equator, they can travel quite far from the equator. Hurricanes north of the equator in the Pacific strike many parts of the eastern Asian coast, those south of the equator strike Australia and New Guinea, while those in the Bay of Bengal strike India.

In the Caribbean Sea and Gulf of Mexico, many depressions originate near the Mexican and South American shores threatening all coastal areas of that region. However, most of the storms in the Atlantic that effect our area come from waves originating from the west coast of Africa. If you regularly look at Intellicast's Atlantic HIRES satellite picture during this time of year, you will notice large storms moving off the African coast about every 3 to 4 days. These waves often spawn the depressions that can eventually lead to hurricanes. Unfortunately for us, the prevailing tradewinds generally carry these storms towards the Caribbean island chain.

The El Niño Years

Hurricanes need a centralized depression, warm water, and the Coriolis effect. However, even with these ingredients, hurricane formation can be severly impaired if upper level winds are strong enough to shear and disrupt the organized outflow of air. Normally during the hurricane season, the upper level shearing winds are light or non-existent.

Although the last fifteen years have been an active period for hurricanes, 1997 was an unusually mild year in the Atlantic. The primary reason was the creation of shearing winds caused by the arrival of "El Niño", thousands of miles away in the Pacific ocean. (2002 turned out to be a mild El Niño year, with only two major hurricanes.)

The three images above depict the evolution of a warm water Kelvin wave in the equatorial Pacific Ocean during March and April 1997. Kelvin waves are often precursors to "El Niño" events which can disrupt global weather patterns. These photos came from TOPEX/Poseidon , a cooperative research progam between France and the US that monitors ocean currents using satellite data. El Nino is the name given to the event in which the usually westward blowing winds of the Pacific ocean stall or even reverse direction. The occurrence of El Nino can cause mild winters in eastern US but torrential rains in South America and drought in Australia and Africa. Unfortunately, those disasters usually mean good news for those of us in the Caribbean. The upper level air entrained by El Nino tends to shear tropical storms and prevent them from "getting organized" into hurricanes.

A good starting point for more information on hurricane research is the NOAA/AOML HRC homepage. This is an acronym for the Hurricane Research Center of the Atlantic Oceanographic and Meteorological Laboratory within the National Oceanic and Atmospheric Administration. They have an extensive FAQ (frequently asked questions) link. May the wind force be with you!

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