Atlantic hurricane season begins June 1 – here’s what forecasters are watching right now
Kristopher Karnauskas, University of Colorado Boulder
As summer approaches in the northern hemisphere, forecasters are starting to observe every episode of rain between the Gulf of Mexico and Africa. Every whirlwind or burst of counterclockwise puffy clouds has the potential to organize into a potentially fatal tropical storm.
About half of the tropical storms that have formed over the past two decades have turned into hurricanes, and about half of them have become the monsters of coastal destruction that we call major hurricanes. We are now used to seeing around 16 tropical storms per year, although that number can vary a bit from year to year.
What are the warning signs we might be in for another record-breaking hurricane season like 2020, when 30 tropical storms have formed, or a calmer storm like 2014, with just eight?
The National Hurricane Center will release its first seasonal forecast for 2021 this week. Here are some of the ingredients forecasters and scientists like me are looking for.
Where tropical storms begin
Hurricanes live in the atmosphere, but they are fueled by the ocean. First of all, let’s look further upstream and find out where they came from.
Like growing crops, hurricanes will be abundant and hardy with a large number of seeds and favorable environmental conditions.
The seeds of tropical storms are small, barely threatening weather disturbances. You will find them scattered throughout the tropics on any given day. In the Atlantic, some start as clusters of thunderstorms over Africa, or as clouds near the Cape Verde Islands off the west coast of Africa.
The vast majority of these seeds do not survive beyond a few days, but some are blown away by the eastern airflow for planting over the tropical Atlantic Ocean between about 10 and 20 degrees north latitude. This is the area where growth is actually fueled by the ocean. From there, the developing tropical storms are driven west and north by the “directing currents” of the atmosphere – avoiding the equator where the crucial effect of Earth’s rotation is too weak to quit. ‘they develop further.
The more seeds there are, the better the chances of an active hurricane season.
Several factors influence the seeding level of tropical storms in any given year, but forecasters’ eyes are usually on the African monsoon in the spring.
Once these seeds emerge from the African coastline or from pockets of warm, rising air elsewhere above the ocean, attention turns to environmental conditions that can fuel or limit their growth in tropical storms and hurricanes.
Hot water fuels hurricanes
In general, tropical storms thrive where the ocean surface is soft to 80 F (26.7 C) or warmer. This is why hurricanes are rare before June 1 and are more likely to occur from August to October, when the ocean is the warmest.
The primary fuel source for tropical storms is thermal energy in the upper ocean, around 100 feet (30 meters).
But it is more than the temperature of the surface. A major factor in the development of very strong hurricanes is the depth of the warm waters and the clear separation between the warm layer and the cold waters below. This is because hurricanes push the ocean upward as they move.
If the hot water layer is shallow and easily mixed, it doesn’t take a lot of churning to dilute the thermal energy on the surface with cold water from below, leaving less energy for the hurricane. But if the hot water goes further, storms have more fuel to draw.
The effect of high winds
Prevailing winds that are already blowing in an area can also make or break a storm.
The winds blow at different speeds at different heights. This is one of the reasons planes experience turbulence. The speed at which the prevailing winds are faster near the top of the storm than at the bottom is called wind shear. With too much wind shear, the storm struggles to maintain those towering plumes of rising warm air.
Likewise, if the rising air cannot escape and flow to the outside quickly enough, the energy consumed by the storm cannot be ventilated and the motor suffocates. Both can prevent the storm from organizing and limit its growth or make it disappear.
An important clue to future windshear in the Atlantic region comes from events thousands of kilometers away in the equatorial Pacific Ocean.
When the eastern Pacific Ocean is unusually warm – known as El Niño – the global atmosphere is rearranged in a way that increases windshear over the Atlantic. It tends to suppress tropical storms there – but don’t bet the farm on that. Other slow variations in the climate system also influence environmental conditions, including multi-year periods of warmer or cooler than normal surface temperatures in the North Atlantic.
The opposite of El Niño, La Niña, tends to bring low wind shear, favoring more tropical storms. These conditions are currently near neutral and forecasters are watching to see what happens.
Where to look
So if you’re watching for the first signs of Atlantic hurricanes in 2021, keep an eye out for the African monsoon for seeding storms, temperatures in the tropical Atlantic Ocean to provide fuel, and a possible late bloomer. Niña, which means less wind shear to tear through storms. The National Hurricane Center – and many other forecasting groups in government, academia, and industry – analyze these and other factors in their seasonal projections.
The big picture
The total number of tropical storms only tells part of the story. There are other important aspects to watch out for over time, such as the intensity of storms, how long they last, how fast they move, and how long they take to dissipate after making landfall. Recent studies have indicated that ocean temperatures, fueled by hurricanes, have tended to warm since the Industrial Revolution, especially along the eastern seaboard of the United States.
Coastal communities are already on the front lines of climate change with rising sea levels. The potential for changes in extreme events such as tropical storms, with their complex interactions with the atmosphere and the ocean, explains why hurricanes have regularly become a research priority.
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Kristopher Karnauskas, Associate Professor of Atmospheric and Oceanic Sciences and Member of the Cooperative Research Institute for Environmental Sciences, University of Colorado Boulder
This article is republished from The Conversation under a Creative Commons license. Read the original article.