The term “tropical cyclone” can be enough to cause panic just by itself. But what is a tropical cyclone, really, and what makes it so different from other storms?
A tropical cyclone is a rotating low-pressure system of clouds and thunderstorms that forms over tropical or subtropical waters, and has a closed system of circulation. In the Northern Hemisphere, tropical cyclones rotate in a counterclockwise direction. The National Oceanic and Atmospheric Administration categorizes these storms as follows:
- Tropical depression: Maximum sustained winds of less than 38 mph (33 knots)
- Tropical storm: Maximum sustained winds between 39 and 73 mph (34 to 63 knots)
- Hurricane: Maximum sustained winds greater than 74 mph (64 knots)
- Major hurricane: Maximum sustained winds greater than 111 mph (96 knots), categorized as Category 3, 4 or 5 on the Saffir-Simpson Hurricane Wind Scale
In the western part of the North Pacific, hurricanes are referred to as typhoons, while a similar storm in the South Pacific or Indian Ocean is called a cyclone. All these storms, however, are technically cyclones.
Climatology of Tropical Cyclones
For a number of years, climatologists have observed two phenomena of variations in the eastern Pacific Ocean: El Nino and La Nina. El Nino is a warming phase in winds and sea surface temperatures, while the cooling phase is La Nina. The two patterns each last several months, and the intensity of their effects can vary.
El Nino events tend to reorganize weather patterns on a global scale. Strong El Nino patterns move the warm pool of western Pacific Ocean waters eastward, along with related storms. As a result, rainfall in the eastern Pacific will pick up, with an increase in droughts, wildfires, cyclones and other extreme weather in other parts of the world. Strengthening El Nino patterns are now common, and the El Nino/Southern Oscillation of 1997/98 led to floods, droughts and pandemics in many parts of the world, with some calling it “the climate event of the Twentieth Century.”
The year 2015 saw one of the most intense El Nino-related tropical cyclone seasons on record, with 21 Category 4 or Category 5 storms (shattering the record of 18 such storms in 2004). During an El Nino event, warm, moist air rises from the Pacific and expands eastward, sometimes all the way to the west coast of South America. These conditions produce low-pressure troughs in the upper atmosphere of the subtropics, and a ridge to the north of this area. It’s also associated with weaker upper-level winds, with favorable conditions for hurricane formation.
Tropical cyclone systems begin as a low-pressure area that develops into a moving line of thunderstorms. These storms begin to rotate and multiply to form a tropical depression, and as they build in intensity, they move up the scale to tropical storms, hurricanes and major hurricanes. All these storms require the same conditions, though:
- Warm ocean waters, down to a minimum depth of 165 feet
- An atmosphere that’s cooler at higher altitudes, giving humidity a chance to condense into clouds and storms
- A “Coriolis force” that moves the storm from east to west and curves it away from the equator
- Rotation at upper levels and convergence at lower levels
- Minimum vertical wind shear – in other words, steady wind directions from top to bottom of the storm area
Climate Change and Tropical Cyclones
Climatologists have been keeping track of global temperature data since 1880, and today, researchers at NASA and NOAA in the United States and their counterparts in Japan and Britain are monitoring this data. This information is collected independently, then compiled to look for trends and patterns. What the data shows is that in the almost 140 years of record-keeping, there has been a distinct spike in temperatures in the 21st century. From 1998 to 2015, in fact, represents 16 years of the highest temperatures recorded.
Monthly high temperature records and daily high temperature records are commonplace, and 2015 was indeed the hottest year since record-keeping started. This serves to spur hurricane and tropical cyclone formation.
It’s also important to remember that rising sea levels contribute to the amount of destruction that’s possible with these storms. Rising sea levels mean higher waves and a greater storm surge in coastal areas, such as the 28-foot storm surge that hit Mississippi during Hurricane Katrina and pushed inward over two miles. The desirability of coastal properties and huge populations in these areas means more property damage and loss of life associated with these storms.
A major hurricane or tropical cyclone is a terrifying event, made even more so by our powerlessness to change it. We can at least address this threat by taking whatever steps we can to mitigate and reduce climate change and global warming.