Introduction 2
Nature of Hurricanes/Cyclones 2
Variations of Hurricane Development 5
Considerations on Hurricane Response 10
Conclusion 11
References 12
Introduction
For the past couple of decades since the onset of the Industrial Revolution, scientists have slowly began a campaign to raise awareness to the changing state of the planet. Records of high temperature changes, onset of diseases and erratic weather patterns have been reported worldwide, each change becoming more severe as the years progress. When it comes to erratic weather patterns, scientists highlight the now common occurrences of natural disasters such as hurricanes or cyclones. Although skeptics stress that these weather conditions are normal, historical data for the past 100 years indicate that the development of these hurricanes/cyclones have changed since the onset of global warming. Studies indicated that the increase in overall surface temperature and the change in atmospheric behavior brought by global warming has the capacity to trigger destructive and regular hurricanes or cyclones which are more destructive than their previous iterations in the past 100 years.
Nature of Hurricanes/Cyclones
A tropical cyclone – also known as hurricanes, typhoons and cyclones – are ‘non-frontal synoptic scale low-pressure systems over tropical or sub-tropical waters with organized convections and definite cyclonic surface wind circulation.’ The Hurricane Research Division (2009) stated that these cyclones can generate a maximum surface wind less than 39 mph, putting them in a classification as “tropical depressions.” Once the wind surface velocity has increased to at least 39 mph, it is classified as a ‘tropical storm’ that often has a name designation. If the sustained winds reaches up to 74 mph, it is called either as hurricanes, typhoons, severe tropical cyclones, severe cyclonic storms or tropical cyclones depending on where the tropical cyclone developed .
Hurricane formation often entails two distinct phases: the genesis stage and the intensification stage. In the genesis phase, Fitzpatrick (2009) stated that it is marked with the beginning of tropical disturbances triggered by convergence. Convergence entails the rise of air to the atmosphere that would create clouds, which would then create a static instability. With the convergence occurring within an unstable atmosphere, thunderstorm formations would follow and trigger additional disturbances that occurs within the trough. A trough contains low atmospheric pressure that can be measured by the millibar and has four categories: equatorial troughs, monsoon troughs, frontal troughs and surface troughs. Genesis would then occur when these troughs contain weak and partial cyclonic rotation. The genesis stage also requires the water temperature to be at least 80ºF, allowing the creation of the static instability environment that would generate thunderstorms. The stage is also triggered by the presence of a weak wind shear.
Once the tropical storm or depression intensifies after the genesis stage, it marks the beginning of the intensification stage. This stage also requires the same conditions needed for the genesis stage to generate more moisture from the ocean to the air that would trigger more heat. The additional heat released within the clouds would lower surface pressure, increasing wind velocity and convergence. In a perfect environmental condition, it is likely that the convergence rate and wind velocity would trigger tropical storms with a wind velocity of 50 mph or more in one day. The duration of this intensification stage will continue to depend on the water temperature as warmer waters would increase the formation of hurricanes under the genesis stage and inhibit faster and destructive hurricanes. Once the conditions that triggered hurricane formation weaken or change, hurricanes would slowly dissipate and undergo the weakening stage. If hurricanes weaken over warmer water, it can be due to the presence of a vertical wind shear or the intrusion of dry air within the hurricane’s main core. Although a majority of hurricanes would dissipate completely, there are instances wherein extratropical cyclones are formed from decaying storms. Extratropical cyclones occur when hurricanes move northward towards a nontropical environment. These extratropical cyclones are often destructive and trigger substantial flooding due to continuous rain .
The structure of hurricanes can determine the life cycle and intensity of hurricanes. Upon its development, Fitzpatrick (2009) indicated that hurricanes showcase distinct cloud patterns highlighting its intensity. The most common cloud pattern is the curved band pattern which occurs if the wind shear is weak, marking the beginning of the genesis stage. Once the weak wind shear dissipates, it would leave a residual circulation that would look like an upside-down “V”. Once the thunderstorms return after twelve hours, it highlights the beginning of stage 2 of the genesis stage. This curved band would then stretch to 100-mile-wide regions with a 50,000 feet height. The curved band would also possess a cloud shield that would reach up to 400 miles and contain thunderstorm squall lines called spiral bands .
As the cyclone continues to gain intensity, the cloud band would coil towards the center and increase in intensity. Ahrens, et al. (2012) stated that once the cloud band completely coil throughout the center, an area of broken clouds would form the eye of the storm. Within this area, winds are weak while the clouds patterns are broken in nature. Air pressure is also very low within this center. Surrounding the eye is the area known as the eyewall, a ring of intense thunderstorm clouds. The eyewall can extend upward around the center and record the highest amount of precipitation and wind speed. In the case of Hurricane Elena as seen in Figure 1, the eye of the hurricane measured up to 40 km wide with an air pressure of only 955 hPa. Its eyewall recorded a surface wind pressure of over 105 to 120 knots .
Figure 1. Photograph of Hurricane Elena in September 1985. This photograph illustrates the anatomy of a hurricane .
Variations of Hurricane Development
Prior to the beginning of the debates on climate change or global warming, regional variations on hurricane development in the past 100 years highlighted various factors that influenced their development. In the Atlantic, Elsner, Liu and Kocher (2000) stated that in the past 192 years, spatial variations within hurricane activity occurred in millennial, decadal and seasonal timescale. In the millennial timescale, hurricane development shifted due to the changes in the mean position of the Bermuda high due to the still shifting climate system in the period. A majority of hurricanes were caused by low and mid-atmospheric flows on ridges, guided above to higher latitudes. When the climate started to cool around 3000 years ago, the jet stream moved southward, keeping the low-latitude hurricanes from making to the Gulf Coast.
In the decadal timescale, variations on hurricanes are determined in 10-year intervals. In the first half of the 20th century – 1900-1920- it is visible that there is a high instance of Gulf storms that did not reach the northeast coasts. However, by the 1930s to the 1940s, hurricane activity increased along the US coast and it later on shifted again exclusively in the East coast. These changes highlight that heightened activity is influenced by the low and high latitudes along the Gulf and the northeast coast. It is also observed within this timescale that hurricanes with high landfall rate is affected by their longitude of formation. Tropical storms that become hurricanes in the East Coast may reach the coastline in a much higher latitude as seen in the Caribbean in the 1910s to 1920s. Finally, variations within the seasonal timescale highlight that within the 198 year period (!801-1998), it showcases that a majority of major hurricanes in the Atlantic occurred during August and September. September is also seen as the more active hurricane season for the past 198 years. On the other hand, the Gulf Coast does not have a specific seasonal onset of hurricanes due to its orientation that prevents re-curvature. The Gulf also records the early season hurricane strikes due to the formation of the southerly displaced jet stream and the presence of the subtropical high, permitting hurricanes to develop within the Gulf of Mexico and the Caribbean Sea .
For the Pacific and Indian Oceans, tropical cyclones or hurricanes vary as to when they occur and the frequency of these storms. Wang (2006) indicated that the Western North Pacific Basin close to Asia has the capacity to generate 80 tropical cyclones each year. In the Indian Ocean, cyclones occur in pre-monsoon and post-monsoon seasons. In these two regions, it is the Western North Pacific that recorded high statistics of cyclones with 28.3 from 1951 to 1979. Like the Atlantic model, cyclone or hurricane development in the region vary per decade in terms of their occurrences and intensity . Hurricane or cyclone development is also affected by the El Nino-Southern Oscillation (ENSO), which triggers a change within the Pacific and Indian region. The ENSO, according to Landsea (2000), often includes changes in the SST for the Pacific: El Nino (warmer termperature) and La Nina (colder temperature). El Nino sometimes has the capacity to create powerful cyclones due to the changes in temperature. However, La Nina develops cyclone activity through the monsoon trought. La Nina transfers the location of the trough and change the vertical shear. Aside from the ENSO, the Atlantic, the Southwest Indian and the Northwest Pacific basins are also affected by the Quasi-Biennial Oscillation (QBO), an oscillation that encircles the planet close to the equator. Cyclones are also influenced in both the Indian and Pacific areas by inter-annual tropical cyclones and sea level pressure changes in nearby basins .
However, since the discovery of global warming in the 1950s to 70s, scientists have highlighted through various climate models and graphs as to how it had affected hurricane development in the Atlantic and around the globe. In terms of the Atlantic Ocean, global warming has greatly affected the Atlantic seas surface temperatures as seen in the records since the 1970s. The Geophysical Fluid Dynamics Laboratory for the National Oceanic and Atmospheric Administration (2013) stated that both the SSTs and the Power Dissipation Index (PDI) of hurricanes in the region are higher than they were in the early 50s and 60s as seen in Figure 2 because of anthropogenic impacts.
Figure 2. Atlantic PDI and SST. This figure shows the timescale activity of the Atlantic SSTs and PDI .
The resulting increase in the Atlantic SSTs and PDI is perceived to trigger a high influx of Atlantic hurricane activity in the future with a 300% increase in the PDI come the year 2100. Existing literature cited by the GFDL indicated that while hurricanes and tropical storms have decreased, their wind speed and rainfall intensity is much stronger as compared to their counterparts in the 1980s as seen in Figure 3.
Figure 3. Tropical storm intensity in the Atlantic (1980-2006). This figure shows the average intensity of storms from 1980 to 2006 .
Finally, it is also observed by the GFDL that aside from greenhouse gases, global warming through other human activities have contributed to the increasing rate of hurricanes in the region. In a study in 2006, experts argued that the possible aerosol-induced cooling reduction passed in the last decade contributed to the current warming of the North Atlantic. Sea level rise is also noted to be one of the evidences of human-caused climate change contributions that also affects Atlantic hurricane impacts as the additional sea level now poses dangers through sea-level rise through flooding or storm-surges .
In the case of the Indian and Pacific regions, global warming has influenced the impact of these hurricanes or cyclones to their respective territories and report an erratic movement. Wang (2006) indicated that there are instances wherein northward or north-eastward typhoons change course and enter in-land or move towards the deep West. Sometimes, when a blocking high appears, the weak typhoon would slowly regain its momentum and become powerful. Once this occurs, the typhoon would slowly become stationary and trigger heavy rainfall. One example in the region with regards to re-powered cyclones is the 1975 Typhoon Nina in China. The typhoon brought in a tota l of 1,692 mm worth of rain in Funiu. In the south Asian monsoon region, cyclones are classified into four: cyclonic storms, severe tropical storms, very severe cyclonic storms and super cyclonic storms. These storms often form close to the Indian Ocean and move west-north-west or northwest depending on its formation. A majority of these storms are deadly, killing thousands due to the pressure drop within its center .
Considerations on Hurricane Response
With the advent of global warming and the increasing developing rate of hurricanes, government action in all levels is integral to reduce the impacts caused by these hurricanes. According to the American Meteorological Society (2000), it is essential to consider reforms on forecasting, media and response on hurricane events. Studies have indicated that the current policies fail to take into consideration the number of vulnerable citizens in vulnerable areas such as coastal areas. There is also the lack of clear framework for evacuations and response efforts throughout the Gulf and Atlantic Coasts as increased stressors restricted government officials from ensuring each citizen is safe and evacuated before the disaster strikes. It is highly recommended that governments in all levels review their current building, land use and resettlement laws to ensure the survival of these vulnerable citizens. Incentives can be given to entice these citizens to move away from the danger zone. Policies must also ensure that natural barriers from storm-surges and other by-products of hurricanes are strengthened. Education is also needed to ensure that each citizen is aware of the perils brought by these natural disasters.
In terms of emergency operations, it is recommended that governments consider strategies such as phased evacuations and regular drills to understand evacuation procedures. These strategies would allow citizens to remember how to coordinate with local authorities once hurricanes hit the region. Further collaboration can also be done not only by all branches of government in all levels, but also with non-government organizations and even with the media. This collaboration would permit better information sharing and delivery for the benefit of the public. Further considerations on hurricane response mechanisms is through the revisions of current policies to permit the installation of infrastructure and legislation to cater to hurricane response .
Conclusion
Weather systems such as hurricanes are important to understand due to the intensity these hurricanes can harness and how these hurricanes now become a frequent concern to many countries around the globe. Hurricanes have various classifications with their own specific intensity and life cycle. In the past, hurricane development varied based on the atmospheric conditions, geographic formations and seasons. However, with the onset of the increasing temperatures and atmospheric behavioral changes triggered by global warming, scientists highlight that global warming has contributed greatly to the severity of hurricanes or cyclones. With the dangers they have towards the people upon their development, it is essential that governments and the public alike become alert upon the first stages of hurricanes due to the destruction these hurricanes are capable of making. As the world continues to grow and develop in the future, the more is it important to prepare for the eventuality of a very powerful hurricane.
References
Ahrens, C. D., Jackson, P. L., Jackson, C. E., & Jackson, E. O. (2012). Meteorology Today: An Introduction to Weather, Climate and the Environment. Ontario: Nelson Education.
American Meteorological Society. (2000). Policy Issues in Hurricane Preparedness and Response. Boston: American Meteorological Society, Weather Channel Inc.
Elsner, J., Liu, K.-B., & Kocher, B. (2000). Spatial Variations in Major U.S. Hurricane Activity: Statistics and a Physical Mechanism. Journal of Climate, 13, 2293-2305.
Fitzpatrick, P. (2006). Hurricanes: A Reference Handbook. Santa Barbara: ABC-CLIO.
Geophysical Fluid Dynamics Laboratory/NOAA. (2013, December 30). Global Warming and Hurricanes: An Overview of Current Research Results. Retrieved from Geophysical Fluid Dynamics Laboratory: http://www.gfdl.noaa.gov/global-warming-and-hurricanes
Hurricane Research Division. (2009). Cyclones: Facts and History. In T. LaBeau, Cyclones: Background, History and IMpact (pp. 1-188). New York: Nova Science Publishers.
Landsea, C. (2000). Climate Variability of Tropical Cyclones: Past, Present, and Future. In S. R. Pielke, & J. R. Pielke, Climate variability of tropical cyclones: Past, Present and Future (pp. 220-241). New York: Routledge.
Wang, B. (2006). The Asian Monsoon. New York: Springer Science.
