The Gulf of Mexico Coastal Ocean Observing System hosted a discussion on marine heat waves during its recent annual fall meeting.
Zhankun Wang, with the National Oceanic and Atmospheric Administration-National Centers for Environmental Information, was the first presenter.
He said marine heat waves have been getting more severe over the past 50 years, and that the Gulf’s surface is warming twice as much as the global ocean, but its deep-water temperature is comparable.
“The warming roughly started around the 1970s and it has been increased for more than one degree in the past 50 years from the 1970s to 2020. The linear warming rate is about .193 degree per decade in the Gulf of Mexico, which is much, much larger than the degree for the global ocean,” Wang said.
“The warming reading in the Gulf of Mexico at sea surface is about twice that of other global ocean, but the read is comparable to the warming we estimated from satellite observations reported in the literature.”
He said the biggest chunk of warming is occurring at the surface in the upper 50 meters, although there are also spiritual variations of the warming trend.
“In the upper 50 meters warming can be found everywhere in the Gulf of Mexico and the western Gulf of Mexico has a larger warming rate than the eastern Gulf below 500 until 600 meters," Wang said.
"Even the northeastern Gulf of Mexico we humans see a slightly cooling trend. So, the northeast region is less affected by warming between 50 and 600 meters … Below 600 meters, the warming becomes more uniform and that can be found everywhere in the Gulf of Mexico.”
The impact of the loop current
Nan Walker, with Louisiana State University, then pointed out that the intrusion of warm water from the Caribbean Sea induces rapid warming in the Gulf — it’s referred to as the loop current.
“The loop current has been very active since the end of December of last year. It shed a new warm-core eddy in late December, and that warm-core eddy only separated from the loop current several months later,” Walker said.
A warm-core eddy forms when the edge of the Gulf Stream moves into the colder water and separates into a warm-core, clockwise flow, according to NASA.
Walker said that in June, the loop current recaptured the eddy and it remained attached until very recently.
“One thing we saw early on, besides the influence of the loop current on temperature was that the coastal waters around Louisiana were very warm very quickly. In fact, within let's say about May 10 to May 17, they warmed up by about five degrees Celsius,” she said.
“So, we saw huge rapid warming along the Louisiana coast, which are attributable to the influx of river water … It isolates the solar energy in that shallow layer and warms up very quickly. So, the river water is probably a big factor in warming the Northern Gulf.”
Brian Dzwonkowski, a researcher with the University of Southern Alabama, spoke next.
"We see that in focusing particularly on intensity, so how strong those marine heat waves are, there's a significant spatial structure to them, where the northern Gulf of Mexico has more intense marine heat waves than the southern Gulf of Mexico," he said.
Extreme heat brings stronger hurricanes
There's a growing number of examples of marine heat waves coinciding with intense hurricanes, like Michael in 2018, he said. The storm grew from a Category 4 to 5 over a region of the Gulf that was experiencing a marine heat wave.
Dzwonkowski said his colleagues are looking at how extreme temperatures impact oyster recruitment, which is the process of incorporating new oysters into a population.
"What they found is when there is extreme air temperatures, which correlate well with extreme water temperatures, there's a significant impact on oyster recruitment. And so when you have consecutive days of an excessive heat threshold, what you see is a high probability for poor recruitment," he said.
But Dzwonkowski said more work needs to be done in providing real-time information about the marine heat wave effects to stakeholder communities in the coastal zone.
“There's a significant lack of data in general. And so how do we fill these gaps? Well, a couple of possible ways might be to maintain existing long-term time series where we are in coastal regions where do have depth measurements over long periods of time,” Dzwonkowski said.
“Another potential way would be … where we collect data over a transect, regularly, potentially expanding Argo float efforts or other type of floats in the deep Gulf. And then obviously, sort of modeling might be another approach to try to get better information.”
Dr. Nick Shay, University of Miami Rosenstiel School of Marine and Atmospheric Science, closed out the conversation looking at how the unusual warming of surface waters in the eastern Pacific Ocean, called El Niño events, play a part in warming and tropical storm activity.
“This year, we've had Philippe … Philippe got a little bit further west, and experienced a lot of shear. I think Philippe finally made landfall in New England or the Canadian Maritimes. It was a storm that lasted seemingly forever, but it really lost its development when it encountered this enhanced shear. And that's usually what you see, during El Niño events, a lot of enhanced shear over the Caribbean,” Shay said.
Shay said the warm Caribbean Sea plays a big role in what happens through the Yucatan straits and in the formation of the Gulf of Mexico loop current, so he and his colleagues are going to put gliders and a float into that area along the western side of the straits.
“Our floats actually measure currents all the way down to 2,000 meters, the horizontal velocities that is, as well as the temperature and salinity," Shay said. "Why you say? Because we want to measure ocean shear. Ocean shear is important for some of the topics previously raised.”
