For the 37th year in a row, Colorado State University (CSU) released its hurricane season forecast on Thursday – and the numbers appear significantly above normal.
More specifically, the team plans 16 named tropical systems; 12 is the average. Eight of these named systems are predicted to reach hurricane status, with winds above 74 mph; Six is the usual amount per year. CSU also forecasts more major hurricanes than usual per year: four versus an average of 2.7.
The forecast is also alarming in that it predicts a nearly 70% chance that a major hurricane – which is at least a Category 3 storm with sustained winds of 111 to 129 mph – will land somewhere along the American coast. This represents 130% of the long-term average.
Accumulated Cyclonic Energy (ACE) – a collective measure of overall activity in terms of number, intensity and longevity of storms – is expected to be 140% of the long-term average.
In addition to these predictions, research also shows that tropical systems are intensifying faster and likely slowing down their forward movement due – at least partially – to a warmer globe. This results in more damage due to stronger winds, more rain and floods.
When multiple crises are complicated, such asand a striking hurricane, climatologists call this a threat multiplier.
“There are only so many resources we can mobilize to mitigate these crises as they become more frequent and widespread,” Professor Michael Mann, director of the Earth System Science Center at the Pennsylvania State University. “We will be forced into a very disturbing triage environment where all we can hope for is to limit damage, death and destruction. “
To construct the hurricane season forecast, the CSU team analyzes several parameters, weighed against past hurricane seasons, including sea surface temperatures in the Atlantic, the presence or absence of ‘El Niño in the tropical Pacific Ocean and the phase of a natural oscillation in the Atlantic Basin called Multidecadal Atlantic Oscillation.
The most important factor in determining the level of activity in a hurricane season is the heat of the ocean. Currently, sea surface temperatures are above normal in most tropical Atlantic development regions. This is especially true in the subtropical Atlantic near the United States, where the Gulf of Mexico, as a whole, is 5 degrees Fahrenheit above normal, with some puffy pockets 8 degrees.
Warmer sea surface temperatures provide more energy to develop systems. It doesn’t always cause more storms, but this “higher octane fuel” powers more powerful systems.
This is important because an increase in the intensity of storms not only causes a modest increase in damage, the change can be exponential. For example, a storm with winds of 150 mph can produce 250 times the damage compared to a storm with winds of 75 mph. So, if the water temperature stays well above normal for the season, it makes sense to assume that an earth storm will likely cause more damage.
Will El Niño play a role this year?
The CSU team also takes into account the expected phase of the El Niño Southern Oscillation (ENSO) – a natural cycle found in the waters of the equatorial Pacific. Even though the phenomenon is thousands of kilometers away, there is a close link between ENSO and the level of tropical activity in the Atlantic basin.
When ENSO is in its El Niño warm phase, this propels stronger jet winds into the Atlantic, disrupting tropical systems, tearing them apart. However, this summer and fall, an El Niño is not planned.
Rather this season, neutral conditions should dominate or even a fresh Niña could develop. This results in weaker atmospheric winds and an Atlantic environment more conducive to the development of tropical systems.
Perhaps the most surprising aspect of this forecast is the likely presence of a negative Atlantic Multidecennial Oscillation (AMO), a natural cycle that changes from cool to warm phases every two decades. The negative phase corresponds to waters in the “north” Atlantic Ocean colder than normal.
In periods when AMO is in the cold phase, Atlantic hurricane seasons tend to be less active, which is the opposite of what is expected this year.
However, this coming season, other factors such as the absence of El Niño in the Pacific and the temperatures of the grilled Atlantic Ocean could prevail over the global climate cycle.
Although tropical Atlantic waters are not warmer than normal in all regions, each year they become more and more important.
Professor Mann said year-to-year variability is now heightened by warmer ocean waters due to global warming. Some 90% of the excess heat trapped by greenhouse gases is stored as energy in the oceans, producing record ocean heat content year after year.
“The bottom line is,” said Mann, “Man-made warming is driving increasingly intense tropical cyclones in the Atlantic and other basins. “