How has Climate Change Affected Atlantic Hurricane Development?

 

Since the beginning of the 21st century, atlantic Hurricane seasons have produced record-shattering storms that have caused a significant amount of deaths and devastation. Some storms include 2005’s Katrina, which caused $125 billion in damage and killed almost 2,000 people, and 2017’s Irma, which completely destroyed parts of the Leeward Islands after a direct hit at maximum strength with 185mph sustained winds. There have been two clusters of intensely hyperactive seasons since the century’s beginning, from 2003-2005 and again from 2010-2012. There has also been a marked increase in the amount of dangerous storms that formed in these seasons, which places a significant risk to those living along the western Atlantic coasts and islands. Many people are quick to blame the formation of more dangerous storms on climate change / global warming. Sea surface temperatures, which have increased due to human activity, are strongly correlated to powerful hurricanes, and thus the assumption can be easily made. However, atmospheric conditions and sea surface temperature changes are also affected by natural climate cycles. The blame cannot be put solely on global warming. Regardless of what is to blame, due to the potential and actual damages that hurricanes may cause, the effects on hurricane development due to changes in atmospheric and sea surface conditions, are something that must be studied in order to better inform and prepare the public.

Before delving into the specifics of research done on the effects of hurricanes, it is important to understand how a hurricane forms. The National Weather Service of the United States provides easily accessible and understood guidelines for the formation process. They outline six important requirements for hurricane formation, warm water, exhibiting the Coriolis effect, a saturated lapse rate gradient, low vertical wind shear, high humidity, and the presence of a tropical wave (“Hurricane Facts”). The first and most important requirement for formation is the presence of ocean water above 26 degrees Celsius. The hot water creates a lot of humidity which is beneficial for the formation of clouds that becomes part of the storm system Secondly, the warm water must be located in an ideal place for the system to exhibit the Coriolis effects, in other words, to allow the storm to spin as a result of the Earth’s spin. The saturated lapse rate gradient allows for the formation of an eye, which aids in strengthening. Low vertical wind shear is extremely important for the formation of a hurricane, as when winds are too high the clouds become disorganized and broken apart, weakening the storm. High humidity often results in the saturated lapse rate gradient mentioned earlier, allowing for storms to strengthen. Finally, tropical waves, or the low pressure areas of thunderstorms that travel from Africa across the Atlantic Ocean, are often what develops into most tropical storms and hurricanes, and thus, are necessary to watch closely for further development.

Hurricanes are very sensitive to changes in the climate, as mentioned previously.. A change of only a single degree Fahrenheit can be the difference between a storm reaching Category 4 or Category 5 status. Changes in Wind patterns can be the cause of a storm becoming much stronger or much weaker than it would have otherwise, or direct it on either a dangerous or harmless path. The presence of Saharan dust floating over the Atlantic can mitigate formation of hurricanes. The El Nino Southern Oscillation plays a great role in the atmospheric conditions of the Atlantic basin. During an El Nino event, the westerly trade winds of the Northern Hemisphere are amplified (and vice versa in the Southern Hemisphere). A La Nina event, although weaker, produces the opposite event. All of these events occur naturally, and in fact, most outlier storms are considered to be part of natural variance. It is important to note that storms of the distant past, such as the 1900 Galveston Hurricane, were just as strong and devastating as the ones now. Thus, hurricanes like Katrina, Sandy, Irma, and Ike cannot be blamed specifically on climate change.

One natural cycle, The North Atlantic Oscillation, changes wind patterns across the Atlantic ocean and North American continent due to pressure differences in the Azores and Iceland. The NOA index can be high or low (+ or -). Wind patterns are affected by the oscillation as it moves from high to low. The adjustments in wind patterns seen in a negative cycle (cold air is forced from the Arctic onto the North American continent) have effects on hurricane tracks as well. According to research performed by Lian Xie and Tingzhuang Yan, “the negative phase of NAO is associated with a more southwest position of the North Atlantic subtropical high pressure system, which tends to steer hurricanes more to the U.S. southeast coast than the northeast coast and thus creating a more favorable condition for hurricanes to make landfall along the U.S. southeast coast” (Xie par. 21). Thus, because of the location of the natural high pressure systems, hurricane development will be limited from that region and the movement of storms that form around the high pressure system will be put in different paths. During a negative index period, storm paths become more threatening. This cycle is completely natural and unaffected by climate change, whether natural or affected by human activity. However, it is not the only cycle that is unaffected by human activity.

The ENSO, or El Nino Southern Oscillation, which greatly affects the trade winds in the Northern and Southern hemispheres. Notably, in the Atlantic basin, during a La Nina event, the trade winds in the Northern hemisphere become weaker, which means that the atmosphere is prone to becoming less stable, favorable for hurricane development as previously discussed. According to the National Oceanic and Atmospheric Administration, “This wind pattern is very conducive to increased Atlantic hurricane activity, partly because it results in weaker vertical wind shear. The weaker trade winds also contribute to a more conducive structure of the mid-level African Easterly Jet, favoring hurricane development from tropical cloud systems moving westward from Africa” (L’Heureux par. 11). Conversely, during an El Nino period, the opposite effect appears, which  makes it much less likely that a dangerous storm will be able to form. In addition, the El Nino effect is often much stronger than a La Nina effect. Therefore, during a neutral and La Nina period, climate scientists and those living in hurricane areas must be diligent in watching wind patterns and sea temperatures in order to prepare for a potential catastrophic storm.

In addition to natural climate oscillations, the effects of both human and natural induced climate change also affect the formation of hurricanes, particularly in the increase in sea surface temperatures that is seen worldwide. However, that does not mean that the effects are global. Absolute sea surface temperatures are a different concept from relative sea surface temperature increases. For example, in the Indian Ocean, the sea surface temperature has increased, but not as much as the increases of the Atlantic ocean. In a single-column model created by MIT atmospheric scientists Bony and Emmanuel, Hamish A. Ramsay and Adam H. Sobel discovered that sea surface temperature increases cause the surrounding atmosphere to become less organized and more humid (Ramsay par. 35). This caused the Potential Intensity (PI) to become more sensitive, as well as increasing the total power output of storms and the water itself. However,  in their experiment, they also tested another model, which measured the effect of absolute sea surface temperatures on hurricanes. Absolute sea temperature was found to have an almost minimal effect in the short term, however, in the long term, it was seen to dominate the effects of relative increases in sea temperature (Ramsay par. 36). Increases in the absolute sea surface temperatures not only give hurricanes more spatial area to move and form, but change patterns of movement as hurricanes absorb the latent heat energy of the water. They claim, however, that more research will need to be done in order to prove these statements, in the form of new models.

Kelly Levin of The World Resources Institute has found that the increase of sea surface temperature has additional effects on hurricanes, including the slower movement of storms and increase in total available moisture content available for hurricanes due to melted ice caps (Levin par. 7). Fast-moving storms often have difficulty maintaining their organization and absorb much less moisture from available waters. When storms decrease in speed, this allows them to potentially become much stronger and drop more moisture on territories that they pass over. This was seen during Hurricane Harvey, which stalled over Texas in 2017 and dropped unprecedented amounts of rain on Houston. Mark D. Risser, columnist for The Guardian, researched National Hurricane Center data and found that the amount of rain dropped by the storm far exceeded naturally expected amounts in relation to sea surface temperature increases, and could be attributed directly to human climate change because of this reason (Risser par. 17). Tropical Storm Allison had followed a similar path when landfalling in Texas in 2001, and dropped extremely large amounts of rain in the same area for a similar amount of time. However, Allison dropped nowhere near as much rain as Harvey did in 2017, which would be unrelated to the category of a hurricane or it’s size. This seems to support evidence that changes are occuring in the Atlantic ocean which are allowing hurricanes to take on stranger, more dangerous characteristics.

 Other studies of hurricanes have found additional effects in hurricanes from changes in sea surface temperatures. Sea surface temperature increases have also been attributed to the increase in rapid intensification incidents in the Atlantic Ocean. Rapid intensification is a phenomenon defined the National Hurricane Center as an increase of 35 miles per hour or more within a 24 hour time span. This phenomenon occurs when wind shear is low and temperatures are higher than usual in a given area. Chris Mooney of the Washington Post, also using National Hurricane Center official data,  concluded that as sea surface temperatures become warmer, both relative and absolute, more areas become available for rapid intensification events to occur (Mooney par. 8). This has been linked to an increase in the average number of rapid intensification events in recent years. “Between 1982 and 1994 there were 10 cases of rapid intensification on average per year. Between 2005 and 2017, that figure doubled to 20 cases per year” (Kommenda par. 12-13). Because of increases in the average sea surface temperature, once there is an area of low wind shear, storms are able to become much stronger with the same area due to the increase in total available heat content in a given area.

Meteorologists also use an index called Accumulated Cyclone Energy (ACE) to measure the total output of a hurricane in units. There has been a marked increase in the total ACE in a given year in recent years, which can be attributed to increases in sea surface temperatures in the Atlantic basin. Since the year 2000 up to 2018, there have been 12 incidents where an Atlantic hurricane season has seen above-average ACE (Kommenda par. 7-8). It took 36 years to achieve the same amount of above average ACE events prior to 2000, from 1963-1999 (Kommenda par. 7-8). Thus, it is clear that regardless what has caused the changes, storms are becoming more powerful. This can be directly attributed to increases in absolute and relative sea surface temperatures as found in Ramsay and Sobel’s experiments with sea surface temperature models discussed earlier. Sea surface temperatures have a significant effect on the development of hurricanes, and the changes seen are causing an increase in the possibility number of dangerous hurricane events as well as their potential intensity. As concluded by the studies of Ramsay, Sobel, and other scientists, it is clear that sea surface temperatures have increased, both relatively and absolutely across the planet. The characteristics of potential hurricane development in the Atlantic Basin have significantly changed since the 1980s, and amplified further since the turn of the century.

Due to these changes, both natural and human induced, hurricanes have been able to reach higher wind speeds, become larger, travel slower, and undergo rapid intensification on a much more regular basis, leading to increases in the total energy output by the storm. The increase in energy output is extremely dangerous to people living along coastlines and on island territories across the Atlantic ocean. Sea surface temperatures will continue to increase into the future as projected by these models, and thus the likelihood of dangerous storms continues to rise. As natural atmospheric events remain consistent and unchanged, hurricanes reaching high strength relies mostly on a tropical low finding hot water in an area that is favorable due to natural atmospheric cycles. However, if this occurs, it is much more likely that the storm will rapidly intensify and become dangerous, as a result of human-induced increases in the sea surface temperatures that provide much more energy to a storm.

Works Cited

 

“Hurricane Facts.” National Weather Service, NOAA’s National Weather Service, www.weather.gov/source/zhu/ZHU_Training_Page/tropical_stuff/hurricane_anatomy/hurricane_anatomy.html.

Levin, Kelly. “Recent Scientific Advancements Show New Connections Between Climate Change and Hurricanes”, World Resources Institute, 14 Sept. 2018, www.wri.org/blog/2018/09/recent-scientific-advancements-show-new-connections-between-climate-change-and.

Levitt, Daniel, and Niko Kommenda. “Is Climate Change Making Hurricanes Worse?” The Guardian, Guardian News and Media, 10 Oct. 2018, www.theguardian.com/weather/ng-interactive/2018/sep/11/atlantic-hurricanes-are-storms-getting-worse.

L’Heureux, Michelle, et al. “Impacts of El Niño and La Niña on the Hurricane Season.” Climate.gov, National Oceanic and Atmospheric Administration, 30 May 2014, www.climate.gov/news-features/blogs/enso/impacts-el-ni%C3%B1o-and-la-ni%C3%B1a-hurricane-season

Mooney, Chris. “Here’s Why Hurricanes Are Rapidly Exploding in Strength.” The Washington Post, WP Company, 11 Oct. 2018, www.washingtonpost.com/energy-environment/2018/10/11/hyper-hurricanes-warm-waters-fueled-hurricane-michaels-sudden-strengthening-that-fits-recent-pattern/?utm_term=.045358de564b.

Ramsay, Hamish A., and Adam H. Sobel. “Effects of Relative and Absolute Sea Surface Temperature on Tropical Cyclone Potential Intensity Using a Single-Column Model.” Journal of Climate, vol. 24, no. 1, 1 Jan. 2011, pp. 183–193., doi:10.1175/2010jcli3690.1.

Risser, Mark D., and Michael F. Wehner. “Attributable Human-Induced Changes in the Likelihood and Magnitude of the Observed Extreme Precipitation during Hurricane Harvey.” Geophysical Research Letters, vol. 44, no. 24, 28 Dec. 2017, doi:10.1002/2017gl075888.

https://journals.ametsoc.org/doi/full/10.1175/JCLI3560.1

Xie, Lian, and Tingzhuang Yan. “Climatology and Interannual Variability of North Atlantic Hurricane Tracks.” Journal of Climate, vol. 18, no. 24, 15 Dec. 2005, pp. 5370–5381., doi:10.1175/jcli3560.1.