Atlantic hurricane season kicks off June 1st – this is what forecasters are watching proper now – Watts Up With That?
Hurricanes Katia, Irma and Jose on September 8, 2017. NOAA
Kristopher Karnauskas, University of Colorado Boulder
As summer draws closer to the northern hemisphere, forecasters watch every attack of rainy weather between the Gulf of Mexico and Africa. Every counterclockwise wind vortex or cloud gust there can organize into a life-threatening tropical storm.
About half of the tropical storms that have formed in the past two decades have grown into hurricanes, and about half of them have become monsters of coastal destruction that we refer to as great hurricanes. We are now used to seeing around 16 tropical storms a year, although this number can vary greatly from year to year.
What are the red flags for another record hurricane season like 2020 when 30 tropical storms formed, or a calmer one 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 that forecasters and scientists like me are looking for.
Where tropical storms begin
Hurricanes live in the atmosphere, but they are fed by the ocean. First, let’s look further upstream and find out where they come from.
Like growing crops, hurricanes will be plentiful and resilient, with large numbers of seeds and favorable environmental conditions.
The seeds of tropical storms are small and hardly threatening weather disturbances. You can find them scattered across the tropics every day. In the Atlantic, some begin as a thunderstorm 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 for more than a few days, but some are caught in the easterly air current intended to be planted over the tropical Atlantic between about 10-20 degrees north latitude. This is the field where growth is really being driven by the ocean. From there, developing tropical storms are carried west and north by the “steering currents” of the atmosphere. This avoids the equator, where the decisive effect of the earth’s rotation is too small to develop further.
The more seeds, the better the chance of an active hurricane season.
Clouds off Africa can turn into tropical storms. NOAA
Various factors affect the extent to which tropical storms will sow in any given year, but forecasters’ eyes are usually on the African monsoons in the spring.
As soon as these seeds emerge from the African coast or from pockets of warm, rising air emerging elsewhere above the ocean, attention shifts to environmental conditions that can fuel or limit their growth into tropical storms and hurricanes.
Warm water drives hurricanes
In general, tropical storms thrive where the surface ocean is mild 26.7 ° C or warmer. Because of this, hurricanes are rare before June 1 and are most likely to occur between August and October, when the ocean is warmest.
The main fuel supply for tropical storms is thermal energy in the upper ocean, the top 30 meters.
Warm water helped Hurricane Michael 2018 turn into a huge, devastating storm. NASA Earth Observatory
However, it is more than just the surface temperature. A major factor in the development of very strong hurricanes is how deep the warm water extends and how much the warm layer is separated from the cold water below. This is because hurricanes stir up the ocean as they move.
If the warm water layer is shallow and mixes easily, it doesn’t take much agitation to dilute the heat energy on the surface with cold water from below, leaving less energy for the hurricane. But when the warm water goes deeper, the storms have more fuel to draw from.
Hurricanes mix the ocean water as they move, leaving a cooler trail in the warm surface water. Karnauskas, Zhang and Emanuel 2021, CC BY-ND
The effect of winds on the upper level
The prevailing winds that are already blowing in a region can also trigger or break a storm.
Winds blow at different speeds at different heights. This is one of the reasons planes are exposed to turbulence. How much faster the prevailing winds are near the top of the storm than below is known as wind shear. With too much wind shear, the storm struggles to sustain the towering plumes of rising hot air.
If the rising air cannot escape and flow to the outside quickly enough, the energy consumed by the storm cannot be vented and the engine throttles. Both of these can prevent the storm from organizing and either limit its growth or cause it to dissolve.
How El Niño and its opposite, La Niña, affect hurricanes. NOAA Climate.gov
An important clue to future wind shear in the Atlantic comes from events thousands of kilometers away in the equatorial Pacific.
When the eastern Pacific is unusually warm – known as El Niño – the global atmosphere is rearranged in such a way that wind shear over the Atlantic increases. That tends to quell tropical storms there – but don’t put the farm on it. Other slow fluctuations in the climate system also affect environmental conditions, including perennial periods that are warmer or cooler than normal surface temperatures in the North Atlantic.
The opposite of El Niño, La Niña, tends to cause low wind shear, which favors more tropical storms. These conditions are close to neutral right now and forecasters are watching what evolves.
Where to see
So if you’re on the lookout for early signs of Atlantic hurricanes in 2021, keep an eye out for the African monsoons to sow storms, tropical Atlantic temperatures to provide fuel, and a potentially late blooming La Niña, what less wind shear means storms of tears apart. The National Hurricane Center – and many other forecasting groups in government, academia, and industry – analyze these and other factors in their seasonal projections.
Twenty years of National Hurricane Center storm trace data shows patterns. Nile Fanion
The bigger picture
The total number of tropical storms only tells part of the story. There are other important aspects that need to be kept in mind over time, such as: For example, how intense storms get, how long they last, how fast they travel and how long they take to dissipate after landing. Recent studies have shown that ocean temperatures caused by hurricanes have tended to get warmer since the Industrial Revolution, particularly along the U.S. east coast.
With rising sea levels, coastal communities are already at the forefront of climate change. The potential for change in extreme events such as tropical storms with their complex interactions with the atmosphere and the ocean is the reason why hurricanes have steadily become a top research priority.
[Like what you’ve read? Want more? Sign up for The Conversation’s daily newsletter.]
Kristopher Karnauskas, Associate Professor of Atmospheric and Ocean Sciences and Fellow of the Cooperative Institute for Environmental Research at the University of Colorado Boulder
This article is republished by The Conversation under a Creative Commons license. Read the original article.