2020 is gearing up for an exceptional year for hurricanes. Through July we’ve already seen 7 tropical storms and 1 hurricane in the Atlantic, significantly outpacing the historical average.
A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain or squalls. Depending on its location and strength, a tropical cyclone is referred to by different names, including hurricane (), typhoon (), tropical storm, cyclonic storm, tropical depression, and simply cyclone. A hurricane is a tropical cyclone that occurs in the Atlantic Ocean and northeastern Pacific Ocean, and a typhoon occurs in the northwestern Pacific Ocean; in the south Pacific or Indian Ocean, comparable storms are referred to simply as “tropical cyclones” or “severe cyclonic storms”.”Tropical” refers to the geographical origin of these systems, which form almost exclusively over tropical seas. “Cyclone” refers to their winds moving in a circle, whirling round their central clear eye, with their winds blowing counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The opposite direction of circulation is due to the Coriolis effect. Tropical cyclones typically form over large bodies of relatively warm water. They derive their energy through the evaporation of water from the ocean surface, which ultimately recondenses into clouds and rain when moist air rises and cools to saturation. This energy source differs from that of mid-latitude cyclonic storms, such as nor’easters and European windstorms, which are fueled primarily by horizontal temperature contrasts. Tropical cyclones are typically between 100 and 2,000 km (62 and 1,243 mi) in diameter.
The strong rotating winds of a tropical cyclone are a result of the conservation of angular momentum imparted by the Earth’s rotation as air flows inwards toward the axis of rotation. As a result, they rarely form within 5° of the equator. Tropical cyclones are almost unknown in the South Atlantic due to a consistently strong wind shear and a weak Intertropical Convergence Zone. Conversely, the African easterly jet and areas of atmospheric instability give rise to cyclones in the Atlantic Ocean and Caribbean Sea, while cyclones near Australia owe their genesis to the Asian monsoon and Western Pacific Warm Pool.
Coastal regions are particularly vulnerable to the impact of a tropical cyclone, compared to inland regions. The primary energy source for these storms is warm ocean waters. These storms are therefore typically strongest when over or near water, and weaken quite rapidly over land. Coastal damage may be caused by strong winds and rain, high waves (due to winds), storm surges (due to wind and severe pressure changes), and the potential of spawning tornadoes. Tropical cyclones also draw in air from a large area—which can be a vast area for the most severe cyclones—and concentrate that air’s water content (made up from atmospheric moisture and moisture evaporated from water) into precipitation over a much smaller area. This continual replacement of moisture-bearing air by new moisture-bearing air after its moisture has fallen as rain, may cause multi-hour or multi-day extremely heavy rain up to 40 kilometers (25 mi) from the coastline, far beyond the amount of water that the local atmosphere holds at any one time. This in turn can lead to river flooding, overland flooding, and a general overwhelming of local man-made water control structures across a large area.
Though their effects on human populations are often devastating, tropical cyclones can relieve drought conditions. They also carry heat energy away from the tropics and transport it toward temperate latitudes, which may play an important role in modulating regional and global climate.
9 Hurricanes Are Forecasted As Part Of A Worse Than Normal 2020 …
While much of the world is busy working through Covid-19 related issues, the United States will have to add another burden onto 2020, the high likelihood of major hurricanes.
In an average year, we expect 12 named storms, 6 hurricanes, and 3 major hurricanes (category 3 or higher). This year, Colorado State University expects we will see 20 named storms, 9 hurricanes, and 4 major hurricanes.
9 Hurricanes Are Forecasted As Part Of A Worse Than Normal 2020 …
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The figure above shows the historical average number of named tropical cyclones by day of the year. You can see this year we are tracking at the top edge of the highest count by day. For the past 5 decades of satellite data, this is the highest number of named storms at this point in the season.
Abnormally warm sea surface temperatures in the Atlantic Ocean and optimal atmospheric conditions lead to an optimal environment for hurricane formation. Sea surface temperatures in the ocean above 80°F / 27°C is optimal for storm development as the warm water evaporates and provides moisture and energy in the atmosphere, promoting storm growth.
The figure above shows sea surface temperatures for mid-July. You can see the white stippled line indicates where the temperature exceeds the 27.8°C threshold that promotes tropical storm formation. Most of the Atlantic Ocean south of North Carolina falls in this category, providing unusually warm waters to fuel storm formation.
While the water is warm, you may wonder how that compares to historical water temperature. The figure above shows the anomaly of mid-July 2020 sea surface temperature compared to the historical average. It’s clear that much of the tropical southern waters are abnormally warm compared to the same time of the year historically.
In addition to abnormally warm sea surface temperatures, the presence of low vertical wind shear and moist air conditions adds to hurricane intensity and formation. Forecasters have measured a weak La Niña forming, which weakens westerly winds and leads to low vertical wind shear compared to El Niño conditions. Thus, wind shear is shaping up to be more favorable for a higher than average hurricane season.
You may remember last month reading about Saharan dust storms blowing off Africa and heading west. These have acted to suck the moisture out of storm formation, however, they are expected to dissipate by mid-August.
The combination of abnormally warm waters, low vertical wind shear, and a return of moist air in the lower atmosphere suggest peak hurricane season (mid-August to late October) will produce significantly more hurricanes than average.