Tornado and hurricane seasons could be worse this year due to abnormally high sea temperature

Southern US states have seen unusually warm weather so far this year

Matthew Cappucci
Wednesday 01 April 2020 15:31
Tornado tears through Jonesboro, Arkansas

Water temperatures in the Gulf of Mexico are running more than three degrees above average, increasing the prospect that the US will see severe thunderstorms and tornadoes this spring and potentially stronger hurricane activity in the summer and autumn.

The last time Gulf of Mexico waters were similarly warm in 2017, it coincided with an above-average tornado season through the spring, and then Category 4 Hurricane Harvey struck the Texas Gulf Coast at the end of summer.

The balmy gulf waters have already contributed to abnormal warmth across the Deep South, where virtually the entirety of the corridor through Texas, Louisiana, Mississippi, Alabama and Georgia is wrapping up one of its top five warmest Marches on record. Numerous records have been broken, with some cities soaring above 30C.

The heat was most prevalent in regions that bordered the Gulf. According to the Southeast Regional Climate Centre, cities such as Brownsville, Texas, Corpus Christi, Houston, New Orleans, Mobile, Alabama, and parts of Florida have all seen their warmest March on record.

The Gulf of Mexico sea surface temperatures have run above normal over the past year but have sharply risen even higher in recent months.

Now, they're about three degrees above average and that is likely to have a bearing on weather across central and eastern US in the months to come.

Severe weather season is about to hit full-force across the Deep South and southeast. During the winter and early spring, severe thunderstorm and tornado chances come in punctuated bursts that are rather few and far between.

But by April, that risk increases and expands into the southern plains and increases dramatically before swallowing the traditional “Tornado Alley” across the central United States by May.

The annual barrage of tempestuous fury stems from the volatile clash of shifting seasons. As springtime warmth begins to build in the Gulf of Mexico, surges of mild air meander north — only to collide with stubbornly-persistent cold shots of winter exiting the Rocky Mountains. It's that collision that brews severe thunderstorms and tornadoes.

Warmer water temperatures supply the atmosphere with added warmth and moisture, bolstering instability - or the energy to “lift” pockets of warm air into chillier air overhead. That in theory favours stronger and more widespread updrafts and thunderstorms, and could slightly tip the scales in favour of increased severe thunderstorm and tornado chances.

Such heightened thunderstorm activity is predicated on other ingredients coming together - mainly in the form of sweeping cold fronts that act as the “trigger” to incite stormy weather to begin with.

A 2016 study found that “the [Gulf of Mexico] can influence hail and tornado occurrence” and stated that it's possible to shape advanced predictions.

“In general, positive Gulf of Mexico sea surface temperature anomalies lead to more available [instability] for severe thunderstorm occurrence east of the Rocky Mountains,” read the study. It found that this warmth and moisture is efficiently transported northward ahead of low-pressure systems by the low-level jet stream.

“The relationship ... is somewhat complicated,” said Maria Molina, a postdoctoral researcher at the National Centres for Atmospheric Research in Boulder, Colorado and the study's lead author. “The Gulf of Mexico is a source of moisture for thunderstorms, but it's not the only source. And there's other ingredients that come into play.”

She said that while it's impossible to predict specific severe weather outbreaks, the current sea surface temperatures are “somewhat of a red flag or something to look for in terms of severe thunderstorm activity”.

Many experts have pointed to this season as probably being near or above average in terms of activity. With another jaunt in the “above-average” direction, it remains to be seen what lurks deeper into April and May.

A research paper in the journal Science found similar results, noting that Gulf of Mexico sea surface temperatures in April have a “robust dynamical linkage” to annual tornado accounts in the Southern Great Plains. However, the authors noted that this link was most notable in April - and by May, other factors dominate.

Ms Molina found this trend as well, stating that there's a stronger correlation between Gulf of Mexico water temperatures and severe thunderstorm and tornado activity earlier in the season.

There's no way to tell whether or not sea surface temperatures in the Gulf of Mexico will be running quite as toasty during hurricane season - which peaks in September. But it's likely that the gulf will be at the very least a good deal warmer than average.

Warm water temperatures fuel tropical cyclones. Experts have long since warned that the warming climate will favour stronger hurricanes.

Of course, any potential enhancement of hurricane season is dependent on storms making it into the gulf so that they can take advantage of conditions ripe for development. And that's something that cannot be linked to warm sea surface temperatures.

“I've found that Gulf of Mexico sea surface temperatures don't really correlate with overall activity,” wrote Philip Klotzbach, hurricane researcher at Colorado State University, in an email to The Washington Post. “Gulf of Mexico sea surface temperatures are plenty warm every year to support nasty hurricanes. Thankfully, most years, other conditions (wind shear, dry air, disturbances) aren't conducive for hurricane formation and intensification in this region.”

He found that, even in peak season, there's little correlation between hurricane activity in the Gulf of Mexico and sea surface temperatures there. He noted the anomalies might make more of a difference in parts of the ocean where temperatures are borderline for hurricane development - like in the west Pacific. But in the gulf, it's in some regards a game of chance to get a storm to fire. Only then do temperature anomalies come into play in influencing intensity.

“Warmer than normal sea surface temperatures could help fuel a hurricane to higher intensity levels, but overall, sea surface temperatures aren't the limiting factor for hurricanes in this region.”

Any storms that do form could also feature more rainfall and flood potential. The warmer gulf means that air in contact with the gulf is warmer. As a result, it has the capacity to carry more moisture.

Some studies have pointed to this coming into play to catalyse Hurricane Harvey's historic flooding across Texas and Louisiana in 2017. According to a paper published in 2018 in the journal Earth's Future, Gulf of Mexico water temperatures prior to Harvey were warmer than any time on record - “supercharging” the devastating rainfall that occurred.

“Ocean heat content was the highest on record both globally and in the Gulf of Mexico,” noted the authors. “Ocean heat was realised in the atmosphere as moisture, and then as latent heat in record‐breaking heavy rainfalls. Accordingly, record high ocean heat values not only increased the fuel available to sustain and intensify Harvey but also increased its flooding rains on land.”

The study ominously concluded: “Harvey could not have produced so much rain without human-induced climate change.”

There is also an increased risk of storms undergoing rapid intensification over extra-warm waters. It's already been widely noted that more tropical cyclones are rapidly intensifying (their winds increasing by at least 35 mph in 24 hours), and warmer waters may be playing a role.

While it's too early to predict any specific events, the presence of abnormally warm water in the Gulf of Mexico does make certain events more likely to occur and/or become more intense than they would have been otherwise. Individual events - including particular severe weather outbreaks and the specific implications for hurricane season - can't be predicted yet.

The Washington Post

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