"Warm Core" Storms: Hurricanes, Typhoons, & Cyclones

During the northern hemisphere's winter (January-March), much of the light energy is delivered to the sub-tropical areas of the southern hemisphere. This produces large cyclonic (clockwise rotating) storms in the southern hemisphere which move toward the south pole. These storms, once they reach a certain strength, are often called cyclones. In contrast, during the northern hemisphere's summer (July-September), most of the energy is delivered to the sub-tropical region of the northern hemisphere. The result is development of large cyclonic (counterclockwise rotating) cells that are termed hurricanes if they form near North America and typhoons if they are formed in the western Pacific. These storms, because they form over warm tropical and sub-tropical waters, are often referred to as "warm-core" storms. Check out this short video (Figure 5.13) on YouTube for a visualization of the cyclonic storms in both the northern and southern hemispheres for the interval from 1950 to 2005.

Figure 5.13: Visualization of historic warm core storm tracks from 1950 to 2005.

Credit: from NOAA's Science on a Sphere video from YouTube.

As observed in the visualization, most of these storms are generated in the sub-tropical easterlies belt where surficial winds blow from east to west. As a result, most storms are steered westward and often strengthen as they move over warm water masses. Once they near the westerlies belt at around 30 degrees north or south of the equator (where surficial winds dominantly blow from west to east), the storms will often be deflected toward the north or even northeast in the northern hemisphere, or toward the south or southeasterly in the southern hemisphere respectively. Both Hurricane Katrina and Typhoon Neoguri followed similar paths in that they were generated at relatively low latitudes in the easterlies belt and then moved west and then northward. When both storms impacted land, the supply of warm buoyant water vapor was cut off and convective flow eventually ceased and the storms dissipated as their energy was lost.

Once a convective storm cell is generated surficial winds blow rapidly toward the center of the rotation (low pressure center). These winds circulate counterclockwise in the northern hemisphere and clockwise in the southern hemisphere as shown in Figure 5.14. When these winds circulate toward the center, i.e., the eye of the storm, winds blow across the ocean's surface producing wind waves and a bulge of water that piles up near the center of the storm. Depending on the size of the storm, the wind speed, and the speed of movement of the storm, significant storm waves can be generated. Likewise, a bulge of water can be produced that, once it moves close to land, will produce storm surge.

In addition to the high winds, and erosive waves generated by tropical cyclones, hurricanes, and typhoons, storm surge is the third and perhaps most devastating component of these storm systems. Coastlines, which are often shaped by wave processes, are susceptible to intensified impacts as a result of the storm surge. These are often most exacerbated on low-lying shorelines with gentle off-shore slopes consistent with passive margin settings.

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Figure 5.14: Comparison of Cyclone Larry (left) on March 19, 2006 as it moved across the Great Barrier Reef of northeastern Australia to Hurricane Hernan (right) from September 1, 2002 as it moved off the coast of the Baja Peninsula in Mexico. Note how Larry's circulation is clockwise, whereas Hernan's is counterclockwise.

Credit: Images Courtesy of NASA/GSFC (MODIS Land Rapid Response Team Jeff Schmaltz and Jacques Descloitres respectively.)

• Historic Warm-Core Storms & Storm Surge Records
• Learning from Tropical Storm Case Studies