Climate

New Orleans’ Greatest Threat Is Climate Change Plus The ‘Loop Current’ Plus A Future Katrina

CREDIT: U.S. Navy

Future Katrinas will become more and more devastating to New Orleans and the entire Gulf of Mexico. If we don’t tackle climate change ASAP, it is hard to see how New Orleans could survive the century.

While most stories making this point tend to focus on sea level rise, I’m going to look at the role of the “Loop Current” — and why Hurricane Katrina (and Gustav) weren’t as strong and hence as devastating at landfall as they could have been.

The key point: All things being equal, if a storm taking the same track of Katrina (or Gustav) occurred in 2050, then, rather than weakening before making landfall, it will probably strengthen considerably, creating far more havoc. To understand why, let’s first answer the question — How did Katrina turn into a powerful Category Five hurricane so rapidly?

The National Climatic Data Center 2006 report on Katrina begins its explanation by noting that the sea surface temperatures (SSTs) in the Gulf of Mexico during the last week in August 2005 “were one to two degrees Celsius above normal, and the warm temperatures extended to a considerable depth through the upper ocean layer.”

The report continues, “Also, Katrina crossed the ‘loop current‘ (belt of even warmer water), during which time explosive intensification occurred. The temperature of the ocean surface is a critical element in the formation and strength of hurricanes.” The Loop Current is “a large eddy of 200-foot-deep warmer water that often breaks off from the Gulfstream and floats around the Gulf.” More on it here.

One of the ways that hurricanes are weakened is the upwelling of colder, deeper water due to the hurricane’s own violent churning action. But if the deeper water is also warm, it doesn’t weaken the hurricane and often continues to intensify it. When a Gulf hurricane passes over the Loop Current or one of its eddies, it spins up. When it moves off the Loop, it spins down.

Katrina doubled in size as it passed over the Loop Current. And it “evolved from a Category 3 hurricane to a Category 5 hurricane in just nine hours by converting heat from the Loop Current into energy,” as one analysis explained. Once it left the Loop and passed over cooler water on its way to landfall, it dropped back to a Cat 3.

You can see all that in this UC-Boulder graph of Katrina’s maximum wind speed as it goes on and off the Loop (where sea surface height is a proxy for sea surface temperature):

Katrina Loop

Katrina rapidly intensifies as it traverses the Loop current, then weakens as it crosses a cool pool of water on the way to landfall.

What precisely would happen if a hurricane was ever able to ride warm Gulf water all the way to landfall? We have some idea because that appears to have happened once in the relatively recent past:

An example of how deep warm water, including the Loop Current, can allow a hurricane to strengthen, if other conditions are also favorable, is Hurricane Camille, which made landfall on the Mississippi Gulf Coast in August of 1969. Camille formed in the deep warm waters of the Caribbean, which enabled it to rapidly intensify into a Category 3 hurricane in one day. It rounded the western tip of Cuba, and its path took it directly over the Loop Current, all the way north towards the coast, during which time the rapid intensification continued. Camille became a Category 5 hurricane, with an intensity rarely seen, and extremely high winds that were maintained until landfall (190 mph / 305 km/h sustained winds were estimated to have occurred in a very small area to the right of the eye).

That of course was pure happenstance — bad luck. By the time of Katrina, global warming was certainly one part of the reason the waters of the Gulf in August 2005 were warmer than normal — though transiting the Loop Current for so long was still happenstance.

But by mid-century, the whole Gulf of Mexico in the summer is going to be much, much warmer, thanks not to happenstance but to human emissions. Global warming heats both the sea surface and the deep water, thus creating ideal conditions for a hurricane to survive and thrive in its long journey from tropical depression to Category Four or Five superstorm.

Here’s one simulation of the region in 2050 by Oak Ridge National Laboratory, assuming no serious effort is made to reverse current emissions trends.

BAU-warming-Gulf2050

This suggests that future Katrinas will spin up into even stronger storms and be much less likely to weaken as much before they hit the shore. I wrote about this in my 2006 book, Hell and High Water. At the same time, the inland United States will heat up at an even faster rate, so the Mississippi River will not be such as cool a stream of water pouring into the Gulf. Also, as the sea level rises, the protective outer delta of the Mississippi River will continue to disappear and storm surges will penetrate deeper inland. Hurricanes weaken rapidly over land. Even one foot of shallow delta water can dramatically reduce this weakening effect, allowing hurricanes to reach deeper inland with their destructive force.

As of the end of August 2015, there has not been a major hurricane (Category Three or stronger) that make U.S. landfall in over nine years, which is a record. Two things are worth noting about that. First, Sandy was not a major hurricane at landfall, indeed it was close to not even being a Category 1. Yet this warmingworsened storm was the second costliest storm in U.S. history (after Katrina) — and the “largest hurricane in Atlantic history measured by diameter of gale force winds (1,040mi).” So you don’t need to be a Category Three storm to cause devastation.

Second, hurricane researchers who have studied the matter have concluded the dry spell in major landfalling hurricanes is mostly a matter of luck. “Lucky break kept major hurricanes offshore since 2005,” is how the American Geophysical Union summed up one 2015 study it published. “It seems to be an accident of geography, random good luck,” explained lead author, NASA’s Timothy Hall, in NASA’s news release. “The last nine hurricane seasons were not weak — storms just didn’t hit the U.S.” And Colorado State University meteorologist Phil Klotzbach, who was not part of the study, said: “I think that there has been a significant ‘luck’ component involved.”

If you want an idea of just how lucky New Orleans has been, consider Hurricane Gustav, “the second most destructive hurricane of the 2008 Atlantic hurricane season,” which caused caused some $6.6 billion in damages and killed over 150 people. It peaked in intensity at Category Four but “dropped just below the Category 3 threshold to Category 2 by landfall.”

Here are some plots of the Gulf’s sea surface temperature (SST) as Gustav would have seen it (via Jeff Masters’ WunderBlog):

Hurricane Gustav

Tropical Cyclone Heat Potential (TCHP) for August 28, 2008. Values of TCHP greater than 80 are commonly associated with rapid intensification of hurricanes. The forecast points from the NHC [National Hurricane Center] 5 am Saturday forecast are overlaid. Gustav is currently crossing over a portion of the Loop Current with extremely high value of TCHP of 120. However, Gustav will then cross over a cold eddy, and will miss crossing the warm Loop Current eddy that broke off in July.

Masters added, “Note that this forecast is old, and the newer forecasts bring Gustav much closer to New Orleans.” The same is true of this chart:

Hurricane Gustav

Forecast track and sea surface temperature response to the passage of Gustav, as simulated by the GFDL model at 8 am EDT Saturday 8/30/08. Passage of Gustav over the relatively shallow depth of warm water near the coast will allow Gustav to upwell large amounts of cold water from the depths. This will chill the surface waters down by up to 5°C (9°F).

So yes, New Orleans got lucky and the fact Gustav wasn’t a major hurricane at landfall was just happenstance.

Now imagine we are in the year 2050 with the same storm track. Then Gustav in 2050 doesn’t weaken before landfall and perhaps even strengthens more. We could easily be talking a Gustav in 2050 that is a Category Four or even Category Five at landfall, rather than just a strong Category Two.

Katrina was able to ride the Gulf Loop Current and Eddy Vortex closer to the coast than Gustav, but it still smashed into colder water. Again, in 2050, that weakening is going to be a lot less likely to occur, and again, we could see a Category Five at landfall.

One issue remains. Clearly global warming means warmer surface water and warmer deep water. All things being equal, that means future hurricanes that travel the same path are going to stay stronger longer and possibly even intensify where earlier hurricanes had weakened.

What we don’t know for certain is if, in fact, all things will be equal. Perhaps global warming will create other conditions that might serve to change their storm path or weaken hurricanes (by, say, increasing wind shear). The recent literature, however, suggests that Category Four and Five hurricanes have become more common — and will keep doing so as long as we keep warming the Earth and its oceans. A 2013 study found:

The response to a 1°C warming is consistently an increase [in Katrina-level storm surges] by a factor of 2–7…. This increase does not include the additional increasing surge threat from sea level rise.

Moreover, serious global warming has been going on for a few decades now, and just in the past ten years we’ve had two major hurricanes ride straight up into the New Orleans area, three if you count Rita. So it makes little sense to hope or plan that future global warming will mean significantly fewer major hurricanes that end up on a path towards the Louisiana coast.

I emailed MIT climatologist Kerry Emanuel, one of the world’s leading experts on hurricanes and how climate change affects them. He is author of a 2013 study, “Downscaling CMIP5 climate models shows increased tropical cyclone activity over the 21st century,” which found that human-caused global warming is projected to intensify future hurricanes. He explained than when he looks at “tropical cyclones in the Gulf of Mexico using seven CMIP5 models, following the technique described in” that paper (for the business as usual case), “I find substantial increases in hurricane risk during the 21st century in six of the seven models.”

We are stuck with a fair amount of warming over the next few decades no matter what we do. But if we don’t reverse emissions trends ASAP, then Category Four and Five storms smashing into the Gulf coast seem likely to become rather common in the second half of this century. Business as usual warming takes us to this hot world where the Gulf is one very warm tub of water:

Diffenbaugh-and-Field-GlobalTemps-8x6

And that future of supercharged hurricanes will be doubly untenable in the business as usual case, as experts now say we may be looking at seas 4 to 6 feet (or more) higher by 2100, with sea level rise as much as one foot per decade after that.

Preserving the habitability of the Gulf and South Atlantic Coasts this century can only plausibly be achieved if we reverse U.S. and global emissions trends sharply and quickly.