How busy was the 2020 hurricane season?
by Neil L. Frank
The 2020 Atlantic hurricane season with 30 named storms is included in the record books as it has the most frequently named storms of all seasons. But are we comparing apples and apples – or apples and oranges?
Some people blame global warming for the recent increase in the storms mentioned – tropical storms and hurricanes – and conclude that in order to curb the warming, we need to stop spewing CO2, and so increase the number of storms and those of them prevent damage caused.
However, the raw data for the hurricane history is contaminated with changes in observation tools, our understanding and philosophy of whether a storm should be named. What explains the increase in named storms? Was it an abnormal meteorological event or are there other explanations? To answer this, the origin of the Atlantic storms needs to be discussed.
How do Atlantic storms start?
One of the prerequisites for a tropical storm is a pre-existing weather disruption that creates thunderstorms. In the Atlantic there are four categories of pre-existing weather disturbances in summer. Two have their origin in the deep tropics and two mainly in subtropical latitudes.
The most common tropical disturbances are African systems. A new disorder from Africa occurs every 3 to 5 days. Trade winds carry these westward across the Atlantic, the Caribbean and sometimes to the Pacific. Every summer we track about 50 and about 10% develop into named storms.
A second category of tropical disturbance is found along the Intertropical Convergence Zone (ITC) – a boundary where winds from the Northern Hemisphere collide with winds from the Southern Hemisphere.
The development process for weather disruption in the deep tropics is very simple. They create thunderstorms that warm the atmosphere, and when the hot air rises it is replaced by air that is spiraling on the surface, creating a tropical storm. We can call this "tropical energy".
The development process for pre-existing weather disturbances that occur in the subtropical latitudes (central and north Atlantic) is far more complex. In this case, the initial energy driving the disturbance is baroclinic – a term used to refer to winds created when cold air moves under warm air. All winter storms are powered by this type of energy.
Two types of baroclinic systems create tropical storms over the Atlantic in summer. One occurs primarily in the spring and fall, when a low pressure system develops along a dissipative cold front that has moved from the US mainland and offshore stands.
A second type occurs in the upper part of the atmosphere (25,000 feet and higher). Occasionally one of these "upper lows" amplifies and rotates down towards the surface and can create a named storm.
Some baroclinic disorders can turn into tropical storms and the process by which they do so is complicated. Imagine the following scenario. At the end of May, a cold front moves off the mid-Atlantic coast and stops. A disorder develops along the front and intensifies. Winches reach 45 miles per hour. The initial energy is exclusively baroclinic. However, thunderstorms caused by the disturbance are warming the atmosphere and now the system is powered by both baroclinic and tropical energy.
Should the system be called a "tropical storm"? Not if the tropical energy does not completely dominate the baroclinic energy. Occasionally this occurs and the system turns into a tropical storm.
Once they decide to name the storm, forecasters face another problem. As the storm moves towards the North Atlantic, the thunderstorms decrease due to the colder water and cold air is drawn in
in the cycle. The winds have not decreased, but the baroclinic energy dominates again and the system is unnamed as it turns into a residual post-tropical storm.
To name or not to name? That is the question
The decision to name or not to name these types of systems is very subjective as we do not have an objective method of measuring the contribution of each energy source. For this reason, forecasters have historically been reluctant to mention barocline-triggered storms.
Forecasters faced this decision four times in 2020 and every time they named the system: Dolly, Edward, Kyle and Omar. All four began as baroclinic disturbances along dissipating cold fronts and then turned into tropical storms under the influence of the heat of thunderstorms. After a very short tropical life of less than 24 to 36 hours, re-examination found that baroclinic energies were again dominant and they were classified as post-tropical residual lows (winter storms). This year we even named a baroclinic storm approaching Europe – Alpha was named a few hours before landing in Portugal.
Would these storms have been named 30 or 40 years ago? Probably not, and data shows it is true.
The historical record of baroclinically triggered tropical storms in the subtropical Atlantic north of latitude 25N over the past 75 years also shows a recent trend to name more storms of this type. If we divide the 75 years into three 25-year periods, the first (1946–1970) is before the satellite. Satellite images were not available to operational forecasters until the late 1960s. This left us unable to observe storms across much of the Atlantic. During this time, less than one storm per year (0.7 / year) could be traced back to a baroclinic origin.
During the second period of 25 years (1971–1995), satellite imagery was available that allowed us to see storms in these broad stretches for the first time, which explains why the number of baroclinic systems has increased slightly to 1.7 per year.
A big surprise came in 1996-2020 when there was an unexpected jump to 4 per year. Much of the surge happened in the last decade, when the number rose to 5 per year.
Another study confirms this trend. In the 1970s and 1980s, over 50% of all named storms in the Atlantic were triggered by African systems. Another 10–15% was caused by interference on the ITC. It is not surprising to find that almost 70% of all the storms mentioned in the Atlantic have their origin in weather disturbances which originated in the deep tropics. Less than 30% of the storms classified as Atlantic originated from baroclinic systems (~ 15% from blocked cold fronts and ~ 15% from lows in the upper level).
Similar statistics for the last decade (2011-2020) show a significant increase in the number of named baroclinic systems. Currently, 45% of the storms mentioned are caused by baroclinic systems, while the other 55% are due to tropical disturbances.
The numbers for 2020 confirm the trend towards naming baroclinic disorders. Thirteen of the 30 storms had a baroclinic origin, ten of them in the subtropical Atlantic.
The number of named storms caused by tropical disturbance has not decreased, but the number of named storms caused by baroclinic disturbance has increased significantly. Has the philosophy of naming baroclinic, wintry storms that occur in summer changed? If so, is this contaminating the historical records of hurricanes in the Atlantic and making the numbers for the past 25 years not directly comparable to the numbers for the past 50 years?
Clearly, satellites are part of the increase in the number of recognized and named tropical storms. A recent study looked at the number of tropical storms that formed over the eastern Atlantic that never moved westward beyond 50 W. In the years before the satellite (1918–1968), we tracked an average of 1 storm every 2 years. During the satellite period (1969-2019) we tracked 2 to 3 times a year – a 4- to 6-fold increase. This year 4 storms – Rene, Vicky, Wilfred, and Theta – fall into this category.
Another factor responsible for the increase in named storms was a minor change in the requirements for naming a storm. Aircraft are used to determine the strength of a storm. You measure the wind at flight altitude and drop an instrument in the eye to determine the central pressure. Since there is a direct relationship between wind and pressure, you can determine the other if you know one.
When investigating a weather disturbance, it is not uncommon for the aircraft to encounter an area with high winds associated with a series of thunderstorms. Should the system be named? What does the print show? In the past, when the pressure isn't low enough to warrant the winch, the system wasn't named until the pressure responded. Not so today. The current philosophy is to use the observed winds to name a storm regardless of pressure. The result is a few more named storms and an increasing number of storms with lifetimes of less than 24 to 36 hours. This year Bertha was named an hour and a half before landing in South Carolina because of the wind alone. Could this emphasis on the winds be the reason there are more barocline-generated named storms in the North Atlantic?
Another unique feature of the 2020 hurricane season was that 3 hurricanes hit Louisiana. Some media reported that this had never happened before. But Florida was hit by four hurricanes in 2005. And the most active hurricane season up to the landfalls was in 1886, when seven hurricanes hit the Gulf Coast. Three were in northwest Florida and four in Texas, and two of those in Texas were major (Category 3 or higher). The last major hurricane to hit Texas in October struck in 1886 and destroyed the remains of Indianola, a once thriving seaport on the south coast of Matagorda Bay.
Maybe 2020 wasn't so strange after all
In summary, the 2020 hurricane season was very active, but was it a record breaking one?
Before the 29 named storms recorded in 2005 and 30 in 2020, 1933 held the highest record in a year at 21. It is instructive to look closely at the 1933 hurricane season.
The deep tropics burned. Of the 21 storms, 17 were triggered by numerous African systems as well as some ITC disruptions. Only one baroclinic disorder developed in the Gulf and three more in the southwestern Caribbean. No storms that developed from baroclinic disturbances in the subtropical Atlantic were tracked and none were recorded in the eastern Atlantic. Were there no storms in these two areas in 1933? We don't know because tools to track storms in the East and North Atlantic were not yet in place.
What we do know is that in 2020, with satellites and reconnaissance aircraft, 10 baroclinic storms developed in the subtropical Atlantic, 2 of which were east of 50W. These 2, along with 3 other African systems that have been weakened, make 5 in the Eastern Atlantic. A fair comparison of the 1933 hurricane season with 2020 would be to reduce the number of storms in 2020 by about 10. If we do, 1933 will still be one of the most active hurricane summers in modern times.
For all the uncertainties in the raw data, the only credible indicator of a trend in Atlantic hurricane activity is to look at the major hurricanes (categories 3-5) that land on mainland America. A tropical storm, or even a minor hurricane, could have hit a sparsely populated region in the 19th or early 20th century and went undetected. In contrast, all major hurricanes with wind speeds greater than 110 miles per hour that crossed the US coast in the past 175 years have likely been recorded.
Contrary to claims that global warming is causing more named storms, the number of major hurricanes on the US coast has declined in recent decades. For twelve years (2005–2017) there was none. This was the longest period without a major hurricane. On average, a major hurricane crosses the US border every two years.
Contrary to what the mainstream media claims, hurricane activity in the Atlantic has not increased in recent decades. In fact, there has been a 5-10% decrease in hurricane-type storms around the world over the past 50 years.
Neil L. Frank, Ph.D., is the longest-serving director of the National Hurricane Center (1974–1987) and was chief meteorologist for KHOU-TV Houston, TX (1987–2008). In his retirement he continues to pursue and study tropical storms. He is a Fellow of the Cornwall Alliance for the Administration of Creation.