Remote Sensing of the Oceans
 
 

Through the further analysis of Remote Sensing data in the ocean, meteorologists can better understand and forecast the development of tropical systems.  The ocean is important to hurricanes since it is the primary energy source for the system.  Remote Sensing data provides Sea surface temperature information along with data on sea surface height.  Remote sensing data also provides information on the oceanic and atmospheric phenomena El Nino and La Nina.  Through remote sensing of the ocean, certain currents can be analyzed.  One of these currents which can play a critical role in the development of tropical systems is the Gulf Stream.  All of this information coupled with atmospheric data and modeling should provide better results in attaining more accurate forecasts for the tracking and intensity of tropical systems.
 

Sea Surface Height

One of the ways remote sensing can be used in studying the ocean is by depicting sea surface height anomalies.  Sea surface height anomalies are the difference between the observed and the average sea surface height as observed by the TOPEX satellite.  The sea surface heights are directly related to the ocean upper layer depth of warm water, which is a key ingredient in hurricane development or strengthening.  The higher the sea surface height anomaly, the warmer the layer of water.  Thus, if the sea surface height anomaly is high, one can estimate that the potential exists for a tropical system to further develop.

A good example of how sea surface height anomalies are important in hurricane strengthening or developing occurred with Hurricane Floyd.  As can be seen in the figure for 09 September 1999, there is a large area of high sea surface height anomalies in the tropical region of the Atlantic basin.  This area provided ample opportunity for Floyd to develop and further intensify which the storm eventually did.

An even better example of how sea surface height anomalies play an important role in further understanding hurricanes is the case of Hurricane Bret which occurred in the Gulf of Mexico in late August of 1999.  The figure, observed on 22 August 1999, represents sea surface heights in the Gulf of Mexico.  The 'dots' on the figure represent the path taken by Hurricane Bret.  As can be seen, as Hurricane Bret progressed over an area of high sea surface heights, the storm exploded, strengthening from a Category 2 storm to a Category 4 storm.  As the storm progressed further, over water that did not have as high a sea surface height, the storm weakened.  Thus, one can see the importance sea surface heights can play in the development of tropical systems.
 
 

Sea Surface Height Anomaly


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    Sea Surface Height Anomaly for Hurricane Bret

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    Ocean Upper Layer Depth

    Remote Sensing of the ocean can also yield information on the ocean upper layer depth.  Ocean layer depth is the thickness layer of a pool of warm water near the surface.  A deeper layer of warm water will provide enough evaporation and an abundant amount of latent heat, which is often referred to as the 'fuel' of a hurricane.

    Hurricane Floyd is an example on how analyzing ocean upper layer depth is important in better understanding hurricane development.  As can be seen in the figure observed on 12 September 1999, there is a wide area of high values of ocean upper layer depth in the tropical regions of the western Atlantic.  The values range from 200 m to 300 m.  This information, coupled with the sea surface height anomalies, can provide meteorologists with further insight into how a tropical system may behave.


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     Sea Surface Height and Upper Layer Thickness

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    Sea Surface Temperature

    Sea surface temperatures can also be analyzed due to the progress made in remote sensing of the oceans.  Sea surface temperatures information is crucial in studying tropical systems.  Warm water is essential for further hurricane development.  In order for a hurricane to develop, the water must be 26 C (79 F) or warmer (Chaston 96).  This warm water is crucial in the development stages of a hurricane.  As mentioned in the book Hurricanes, by Peter Chaston, 'warmer water means warmer air above it which means higher dewpoints, consequently giving the hurricane more energy to be released in the heat of condensation during cloud and precipitation development (90).  Sea surface temperature is the temperature of the sea based on data from the GOES satellite.

    Hurricane Floyd is also a good example on how Sea surface temperature is important in analyzing hurricane strength. Based on the data observed for 12 September 1999, there is a very large area of very warm water in the tropics of the Atlantic Ocean.  This area consists of temperatures of either 30 C or 31 C which is significantly higher than the required 26 C for hurricane development.  Thus, this large area of warm oceanic temperatures will provide more energy for the hurricane to further intensify as it did in the case of Hurricane Floyd.

    Another good example on how ocean temperatures are crucial in the development of hurricanes occurred with Hurricane Opal in 1995.  Hurricane Opal was a Category 2 storm as it progressed in the Gulf of Mexico, over the Gulf Stream.  The Gulf Stream is an ocean current with a high velocity and very warm waters.  As Opal progressed, it moved over a warm core eddy, which provided more latent heat to fuel the storm.  The temperature of the warm core eddy was estimated to be 88 F (Chaston 96).  The warm core eddy caused Opal to develop into a much stronger Category 4 hurricane.

    Sea Surface Temperature

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    Hurricane Heat Potential

    Hurricane heat potential data can also be derived based on remote sensing instruments.  Hurricane heat potential depicts the amount of energy available for the storm to further develop, that is, the amount of latent heat available for the storm to 'fuel' on.

    The hurricane heat potential available on 12 September 1999, is a good example on how this information can be used to further predict the behavior of a storm.  Again, this was the time frame for Hurricane Floyd.  In the figure, there are high amounts of heat potential in the Sargasso Sea.  Thus, Hurricane Floyd had a tremendous area of high heat potential to further develop and strengthen which the storm certainly did.

    Hurricane Bret is another good example to analyze the impact of heat potential on a storm.  The figure for Hurricane Bret is from 22 August 1999.  Again, the track of the storm is represented by the 'dots'.  As Hurricane Bret proceeded over areas with heat potentials ranging from 75 K/cm^2 to 110 K/cm^2, the storm exploded from a category 2 to a category 4 storm.  This occurred within a very short period of time and was due in part to the high heat potential and sea surface heights.


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    Hurricane Heat Potential for Hurricane Bret
     


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     Hurricane Heat Potential in North Atlantic

     Hurricanes and Satellite Altimetry
     
     
     

    El Nino and La Nina

    Remote Sensing data has allowed scientists to better understand the oceanic and atmospheric phenomena El Nino and La Nina. These phenomena occur in the eastern Pacific equatorial region.  Remote sensing allows scientists to study sea surface temperature which can act as a preclusion of an El Nino or La Nina event.

    As can be seen in the figures, El Nino is the warming of the waters in the Eastern Pacific equatorial region.  El Nino is speculated to hinder hurricane development in the Atlantic due to the higher wind speeds in the upper levels which shear off the cloud tops and prevent them from developing further.  During 'normal' conditions, convection occurs in the Western Pacific.  However, in an El Nino situation, the area of convection moves eastward toward the center of the Pacific.  This causes an increase in convection over the Eastern Pacific.  As a result, more clouds form and there is an increase in precipitation amounts in this region.

    La Nina is the opposite from El Nino.  In a La Nina situation, the waters of the Eastern Pacific equatorial region are actually colder than normal.  The figures show the conditions which occur during a La Nina scenario.  During a La Nina event, strong upwelling occurs along the northwestern coastline of South America.  The area of convection moves back to the Eastern Pacific due in part to the strong easterly trade winds.  The strong upwelling provides the area with an abundant amount of cold water than extends out to the central Pacific.

    The figures also depict the patterns for the sub-tropical jet stream during a normal, El Nino, or La Nina event.  As can be seen in the figures, the subtropical jet moves farther south during an El Nino season.  This causes an abundant amount of fast moving yet strong storms in the Southern Plains and Southeastern region of the Unites States.  During a La Nina episode, the subtropical jet moves farther to the north than normal, thus causing less rainfall in the southwestern region of the United States.


     
     


    La Nina


     
     

       NOAA's PMEL El Nino Theme Page
     
     
     
     
     
     
     
     
     
     



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