II. Formation of a Hurricane
A. Hurricanes develop from pre-existing disturbances, grow from a disturbance to a tropical depression, then a storm.
1. Pressure associated with hurricane is generally 950 millabars (mb) or less.
2. Require an area of low wind shear (below 7.5-10 meters per second)
B. The latent heat from a large area is released as the air converges towards the low-pressure center of the storm. This energy is released as the latent heat of condensation.
1. Warm air moves towards the area of low pressure, rises and forms spiraling bands of up to 200 clouds in a vortex around the storm's eye.
2. Calm air descends and is warmed again.
C. A positive feedback system is developed, continuing to feed energy into the system.
1. Hurricanes move in west or northwest direction, as they move away from warm waters, they lose the energy needed to sustain them.
III. Other climatic conditions that do not cause hurricane formation, but can enhance the intensity
A. El Nino Southern Oscillation
1. Cause a decrease in hurricane activity in the Atlantic due to increased wind shear (decrease of 44%)
2. During La Nina years, hurricane activity increases.
B. Stratospheric Quasi-Biennial Oscillation.
1. The Westerly phase of this (This is an east to west oscillation of stratospheric winds circling the globe at the equator. West phase lasts 13-16 months and east phase lasts 12-15 months. The west phase is associated with weak easterly winds and East phase has strong easterly winds).
2. During the west phase, there is an increase in hurricane activity.
C. Rainfall data from 1949-1990 in western Sahel of Africa (located between 11° and 20° N) showed a strong correlation between Sahelian rainfall and the number of intense hurricane days.
1. Energy for the system is provided by this moisture source.
2. During the period of the Sahel drought, a marked decrease in tropical hurricanes was observed.
1. During drought conditions, stronger upper tropospheric winds produce move wind shear.
2. During monsoon years, climatic conditions favor the formation of easterly waves, the disturbances from which most hurricanes develop.
IV. How climate Change will affect Hurricane Activity
A. Increased Sea Surface Temperature.
1. Many researchers have supported the correlation between high sea-surface temperature and maximum wind speeds of hurricanes.
2. The conclusion that hurricane intensity will increase is based on several assumptions programmed into climatic models
a. Assume that doubling of CO2 levels would increase SST between 2.3°C and 4.8°C.
b. Global warming will not drastically affect the temperature of the moist tropical latitudes.
B. El Nino
1. The Tropical Ocean Global Atmosphere program has improved its modeling of ENSO events.
2. Can only conclude that changes in the frequency or intensity of ENSO under enhanced greenhouse conditions are possible, but not presently quantifiable.
C. Global Precipitation
1. A CO2 warmed atmosphere will be able to hold more water vapor, leading to an increase in precipitation intensity.
2. Models predict a 27-63% increase in heavy precipitation events.
V Arguments against Increased Hurricane Activity
A. Hurricane Activity Will Not be Affected
1. Global warming will occur at higher latitudes.
2. Warming that does occur in tropical areas will affect the atmosphere more than the ocean surface temperature.
3. Warmed atmosphere will lead to more evaporation, instead of warming of the SST.
B. Climatic Record
1. The climatic record has shown a recent downward trend in Atlantic tropical cyclone activity
2. One factor used to explain this decline has been the 20 year Sahelian drought
3. From 1991 to 1994, Atlantic hurricane activity was at its lowest on record since the 1940's
A. The only relationship that has been clearly proven is the relationship between SST and maximum hurricane intensity.
B. However, it is necessary to keep in mind that relationship predicts only the maximum hurricane intensity, not the average hurricane intensity. Since the intensity of a hurricane can be influenced by many other factors, the actual intensity of a storm cannot be predicted from SST alone.
C. Models have been able to support the theoretical arguments supporting the increased hurricane activity hypothesis, but the real world data has not yet been able to prove the relationship. Recent hurricane activity has been explained by other factors and the long-term record has not shown a positive increase in hurricane activity.
1. The climatic record of hurricanes remains limited, it only covers the period from 1876 to the present and it wasn't until 1974 that aircraft systematically measured accurate intensity measurements.
D. Every researcher, including the IPCC has identified the need for additional research. This is the best method to learn more about the systems and pursue further study of the relationship between the climate and hurricane activity.
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