Atlantic hurricane season begins June 1 – here’s what forecasters are watching right now


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 released its first seasonal forecast for 2021 on May 20 and expects a more active-than-normal season, with 13 to 20 named storms, six to 10 hurricanes and three to five major hurricanes.

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 swept away by the easterly airflow to be planted over the tropical Atlantic Ocean between around 10 and 20 degrees latitude. North. 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 “direction currents” of the atmosphere – avoiding the equator where the crucial effect of Earth’s rotation is too weak. for them to develop further.

The more seeds there are, the better the chances of an active hurricane season.

Satellite view of clouds over the Atlantic

Cloud puffs off Africa have the potential to become tropical storms. NOAA

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 warmest.

The primary fuel source for tropical storms is thermal energy in the upper ocean, around 100 feet (30 meters).

Map of sea surface temperatures in the Gulf of Mexico in October 2018

The warm water helped Hurricane Michael turn into a massive destructive storm in 2018. NASA Earth Observatory

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.

Graph showing the wake of hurricanes Florence and Hélène in 2020

Hurricanes mix ocean water as they move and leave a cooler wake in warm surface water. Karnauskas, Zhang and Emanuel 2021, CC BY-ND

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.

Card with text explaining the differences

How El Niño and its opposite, La Niña, affect hurricanes. NOAA Climate.gov

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 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.

Lines showing the trajectories of hurricanes over 20 years

Twenty years of storm track data from the National Hurricane Center shows trends. Nilfanion

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 change from extreme events like tropical storms, with their complex interactions with the atmosphere and the ocean, explains why Hurricanes have regularly become a research priority.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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