Posted on 30 May 2022 by Guest Author

This is a re-post from Yale Climate Connections by Jeff Masters

Potential trouble is brewing in the Gulf of Mexico for the coming Atlantic hurricane season: waters with a high amount of heat energy. Sea surface temperatures (SSTs) in mid-May were 27-28 degrees Celsius (81-82° F) across most of the Gulf, 0.5 to 1.5 degrees Celsius (0.9-2.7°F) above the 1981-2010 average. That’s a lot of heat energy for potential hurricanes to feast on.

What’s more, the Gulf waters have the potential to be at near record-warm levels during the peak part of hurricane season, if the southeastern U.S. gets a sustained heat wave that brings light winds and hot conditions to the Gulf for an extended period. (The first half of May was record-warm for Galveston, Texas, which contributed to the unusually warm ocean temperatures off the Texas coast.)

But sea surface temperatures don’t tell the whole story. When a hurricane traverses a shallow area of warm ocean waters, its powerful winds will churn up cold waters from the depths, cooling the surface and putting the brakes on any rapid intensification the hurricane may have had. But when unusually warm ocean waters extend to great depth, 100 meters or more below the surface, the hurricane’s churning winds simply stir up more warm water, allowing dangerous rapid intensification to occur if wind shear is low. Thus, total Ocean Heat Content (OHC) is a key metric used to determine the potential for hurricane rapid intensification. And unfortunately, given the present ocean current configuration, there will be high ocean heat content in the Gulf of Mexico this hurricane season.

Figure 1. Ocean current speeds (cm/s, colors) at 0Z May 17, 2022 as analyzed by the Navy HYCOM model. Arrows show the direction of the current. The fastest currents were associated with the Loop Current, which was flowing northward from the Western Caribbean into the east-central Gulf of Mexico, before looping back to the southeast and flowing to the south of Florida. A warm eddy that broke off from the Loop Current in 2021 (which helped fuel the rapid intensification of Hurricane Ida on August 29, 2021) has drifted west to a position about 200 miles south of western Louisiana. (Image credit: Navy Research Lab)

Meet the Loop Current

In the Gulf of Mexico, the deepest warm water is found in the Loop Current—an ocean current that transports warm Caribbean water through the Yucatan Channel between Cuba and Mexico. The current flows northward into the Gulf of Mexico, loops southeastward just south of the Florida Keys (where it is called the Florida Current), and then goes just west of the westernmost Bahamas. There, the waters of the Loop Current flow northward along the U.S. coast and become the Gulf Stream.

With speeds of about 1.8 mph (0.8 m/s), the Loop Current is one of the fastest currents in the Atlantic Ocean. The current is about 200-300 km (125-190 miles) wide and 800 meters (2,600 feet) deep, and is present in the Gulf of Mexico about 95% of the time. During summer and fall, the Loop Current provides a deep (80-150 meter) layer of very warm water that can provide a huge energy source so hurricanes can become rapidly intensifying major hurricanes.

The Loop Current commonly bulges out in the northern Gulf of Mexico and sometimes will shed a clockwise rotating ring of warm water that separates from the main current. This ring of warm water slowly drifts west-southwestward toward Texas or Mexico at two to three miles per day. This feature—called a “Loop Current ring”, “Loop Current eddy”, or “warm core ring”—can provide a key source of energy to fuel rapid intensification of hurricanes that cross the Gulf (in addition to the Loop Current itself). The Loop Current pulsates in a quasi-regular fashion and sheds rings every six to 11 months. When a Loop Current eddy breaks off in the Gulf of Mexico at the height of hurricane season, it can lead to a dangerous situation with a vast reservoir of energy available to any hurricane that crosses over. Examination of the latest mid-May 2022 Loop Current imagery (Figure 1) reveals a major bulge in the current, which is bringing a large area of water with high ocean heat content to the Gulf. This bulge appears poised to break off into a significant warm eddy this summer, which would give the Gulf with plenty of deep, warm water to fuel any hurricanes that get loose there.

Loop Current eddies have helped fuel rapid intensification of four hurricanes that have become among the top 20 most expensive weather disasters in world history. The most famous Loop Current eddy separated in July 2005. A month later, Hurricane Katrina passed over the eddy and “bombed” into a category 5 hurricane, causing damages estimated at $182 billion—the most costly weather disaster in recorded history. The eddy remained in the Gulf and slowly drifted westward during September. Hurricane Rita passed over the same eddy three weeks after Katrina, and it too explosively deepened to a category 5 storm. Rita’s $27 billion in damages made it the 19th-costliest weather disaster in world history.

In 2021, a significant Loop Current eddy began breaking off in July, a month before Hurricane Ida moved over the Gulf. While moving over the Loop Current and the developing Loop Current eddy in late August, Ida rapidly intensified into a devastating Category 4 hurricane that ravaged Louisiana, eventually deluging the northeastern U.S. with catastrophic rains. Between its Gulf Coast impacts and its northeastern rampage, Ida caused $75 billion in damage (2022 dollars), making it the fifth-costliest weather disaster in recorded history, and took 107 lives, including 87 in the United States. The eddy that powered Ida is still in the Gulf, having drifted westward to a position about 200 miles south of the western Louisiana as of mid-May.

Even a Loop Current eddy detached from the Loop Current for more than a year can still provide a potent source of heat energy for a hurricane. Hurricane Harvey in 2017 was fueled by an old Loop Current eddy that had migrated to the coast of Texas, a full 16 months after it had broken off from the Loop Current. This heat energy contributed to Hurricane Harvey’s record rains, which did $141 billion in damage—the second most costly recorded weather disaster. In a 2018 paper, Hurricane Harvey Links to Ocean Heat Content and Climate Change Adaptation, Kevin Trenberth of the National Center for Atmospheric Research, NCAR, concluded: “The bottom line is that the total observed Ocean Heat Content (OHC) change is remarkably compatible with the total energy released by precipitation and, unsurprisingly, reflects strong energy exchanges during the hurricane. Accordingly, the record high OHC values not only increased the latent heat which fueled the storm itself, likely increasing its size and intensity, but also likely contributed substantially to the flooding caused by rainfall on land. The implication is that if the OHC had been less, then the rainfall amounts would also have been less.”

Figure 2. A comparison of Ocean Heat Content (OHC) levels in the Western Caribbean and Gulf of Mexico in 2021 and in 2022. OHC levels are higher in 2022 than in 2021; values in excess of 90 kJ per square centimeter (yellow colors) are often associated with rapid intensification of hurricanes. Image credit: University of Miami Rosenstiel School of Marine and Atmospheric Science.

The future of the Loop Current

High-resolution climate and ocean model studies predict that the ocean current system that drives the Loop Current, the Atlantic Meridional Ocean Current (AMOC), will slow by 20-25% by 2100. In the Gulf of Mexico, the result is predicted to be a weakening of the Loop Current and the associated warm eddies (though these warm eddies are predicted to break off more frequently). As a result, the large eddies in the central Gulf with deep, warm water that have fueled the rapid intensification of multiple hurricanes over the Gulf of Mexico in recent decades, should not have as much heat content in the future. However, sea surface temperatures (SSTs) across most of the Gulf of Mexico are predicted to warm by more than 1.5 degrees Celsius (2.7°F) by the end of the century, compared to the 1990s (Figure 3), under a moderate global warming scenario (RCP 4.5). These warmer surface waters will be quite capable of supporting rapid intensification of hurricanes without the presence of a warm Loop Current eddy.

If the average wind speed near the surface of the tropical oceans does not change, the wind speeds in hurricanes are expected to increase about 5% for every one degree Celsius increase in tropical ocean temperature (Emanuel, 1987). Computer models assign a slightly smaller magnitude (4%) for the increase (Knutson and Tuleya, 2004), and a 12% increase in rainfall rate (within 100 km of the center). A 5% increase in hurricane winds equates to about a 50% increase in destructive potential, according to NOAA. Thus, the theoretical 10% increase in hurricane winds for the 2 degrees Celsius increase in SST predicted for much of the Gulf of Mexico by 2100 would increase hurricane damage potential by more than a factor of two—without considering the added damage because of the future climate’s higher sea levels (from sea-level rise) and more intense rainfall (from higher levels of water vapor). Throw into that mix any increase in population and wealth at risk in hurricane-vulnerable areas.

Figure 3. Predicted difference in sea surface temperature (SST) between the late 21st century (2090 ~ 2098) and late 20th century (1990 ~ 1998) for August-September-October, under a moderate global warming scenario (RCP 4.5). Modeling was performed using the high-resolution NOAA Geophysical Fluid Dynamics Laboratory (GFDL) Modular Ocean Model version 4.1 (MOM4), fed with data from 18 CMIP5 models. (Image credit: Liu et al., 2015, Potential impact of climate change on the Intra-Americas Sea: Part-1. A dynamic downscaling of the CMIP5 model projections, Journal of Marine Systems 148 (2015), 56–69)

Moreover, because the eddies in the Gulf of Mexico are predicted to become less active, there is likely to be less mixing of the ocean, allowing the shallow surface waters near the coast to decouple from the deeper waters away from the coast. This decoupling will potentially allow the shallow surface waters (those less than 200 meters deep) to experience dramatic warming in the summer months, since the land heats up more strongly than the ocean, and there will be no mechanism to carry the heat accumulated in the surface waters out into the deeper waters. The high-resolution ocean model shown in Figure 3 predicts that the Gulf Coast waters of Florida may warm by more than 2.5 degrees Celsius (4.5°F). This situation would favor rapid intensification of hurricanes about to make landfall along the coast—a dangerous eventuality, since rapid intensification just before landfall can surprise an unprepared population unless proper evacuations have been carried out. Over the past 30 years, though, the pattern of ocean warming in the Gulf (Figure 4) does not resemble the one predicted by 2100.

Figure 4. Observed 30-year change in sea surface temperature (SST) from 1987-1991 to 2017-2021, using the NOAA OI SST dataset. Most of the Gulf of Mexico warmed by 0.5-1.0 degree Celsius (0.9-1.8°F). (Image credit: NOAA)

Unfortunately, climate models may be underestimating the rate of increase of ocean temperatures in the Gulf of Mexico: Research published in 2021 (Observation-based early-warning signals for a collapse of the Atlantic Meridional Overturning Circulation) found that the Atlantic Meridional Ocean Current may already be close to a critical transition point, and its predicted 20-25% slowdown by 2100 could be significantly underestimated. A more rapid slowdown could lead to higher ocean temperatures in the Gulf of Mexico than the models are currently calling for.

Links

Oceanographer Nick Shay of the University of Miami published an excellent article on May 18 in The Conversation discussing the Loop Current outlook for the 2022 hurricane season. One interesting point he makes: “Another trend we have been noticing is that the Loop Current’s warm eddies have more heat than we saw 10 to 15 years ago. Whether that’s related to global warming isn’t clear yet, but the impact of a warming trend could be devastating.”

The behavior of the Loop Current over the past year can be viewed at the Navy Research Lab’s web site, along with images with color coding representing the height of the sea surface above mean level. The higher the height, the warmer the water (since warm water expands and thus raises the sea level where it is).

A 1998 paper published in the Journal of Physical Oceanography titled, Loop Current Eddy Paths in the Western Gulf of Mexico provides useful technical information on Loop Eddies.

First tropical wave of the year causing forecast challenges

The first tropical wave of the year, which emerged from the coast of Africa last week, was exiting the coast of Colombia and entering the central Caribbean on Tuesday afternoon. This wave is expected to interact with a large area of low pressure (a Central American Gyre, or CAG) over the western Caribbean late this week, potentially leading to the formation of a tropical cyclone by early next week in the western Caribbean or Eastern Pacific. The GFS model has been stubbornly insisting on development in the western Caribbean, with a storm that reaches hurricane strength entering the Gulf of Mexico next week. However, our other reliable models for hurricane genesis, the European and UKMET, have been favoring the Eastern Pacific as being a more likely place for genesis. It appears unlikely that a tropical cyclone will form in the western Caribbean or Gulf of Mexico next week, particularly given the forecasts of high wind shear from some of the models. Hurricane scientists @AndyHazelton and @webberweather have been doing excellent posts on Twitter on the evolving forecasts for this situation. UPDATE: The 6Z and 12Z Wednesday, May 18, runs of the GFS model no longer developed a system in the Caribbean or Gulf of Mexico. The tropical wave will bring heavy rains to Central America over the weekend, however.

Bob Henson contributed to this post.