From Texas A&M University , a claim that fresh water intensifies hurricanes. At first blush, the concept seems wonky to me, as I don’t think there’s much difference between the heat content potential of fresh -vs- saltwater. Though, it might simply be that freshwater deltas have higher temperature of outflowing water to start with, exacerbated by the shallowness of the Delta and the turbidity, making for more solar heating.
Hurricanes can be 50 percent stronger if passing over fresh water, says Texas A&M study
If a hurricane’s path carries it over large areas of fresh water, it will potentially intensify 50 percent faster than those that do not pass over such regions, meaning it has greater potential to become a stronger storm and be more devastating, according to a study co-written by a group of researchers at Texas A&M University.
Ping Chang, professor of oceanography and atmospheric sciences and director of the Texas Center for Climate Studies, along with his former student, Karthik Balaguru, now at the Department of Energy’s Pacific Northwest National Laboratory, are the lead authors of a paper in the current issue of PNAS (Proceedings of the National Academy of Sciences).
Their findings could benefit weather experts as they try to predict the path and strength of a hurricane, noting that about 60 percent of the world’s population resides in areas that are prone to hurricanes or cyclones.
Chang and Balaguru and their colleagues examined Tropical Cyclones for the decade 1998-2007, which includes about 587 storms, paying particular attention to Hurricane Omar. Omar was a Category 4 hurricane that formed in 2008 and eventually caused about $80 million in damages in the south Caribbean area.
They analyzed data from the oceanic region under the storm, including the salt and temperature structure of the water and other factors that played a part in the storm’s intensity.
“We tested how the intensity of the storm and others increased over a 36-hour period,” Chang explains.
“We were looking for indications that the storm increased in intensity or weakened and compared it to other storms. This is near where the Amazon and Orinoco Rivers flow into the Atlantic Ocean, and there are immense amounts of freshwater in the region. We found that as a storm enters an area of freshwater, it can intensify 50 percent faster on average over a period of 36 hours when compared to storms that do not pass over such regions.”
The researchers believe their results could help in predicting a hurricane’s strength as it nears large river systems that flow into oceans, such as the Amazon in the Atlantic, the Ganges in the Indian Ocean or even the Mississippi River into the Gulf of Mexico.
Hurricanes – called typhoons in the Pacific region and cyclones in the Indian region – are some of the most devastating natural hazards on Earth. A single storm, Cyclone Nargis in 2008, killed more than 138,000 people in Burma and caused $10 billion in damages.
“If we want to improve the accuracy of hurricane forecasting, we need to have a better understanding of not only the temperature, but also the salinity structure of the oceanic region under the storm,” Chang notes.
“If we know a hurricane’s likely path, we can project if it might become stronger when nearing freshwater regions. This is another tool to help us understand how a storm can intensify.”
The team’s work was funded by grants from the National Science Foundation, the Department of Energy and the National Science Foundation of China. About Research at Texas A&M University: As one of the world’s leading research institutions, Texas A&M is in the vanguard in making significant contributions to the storehouse of knowledge, including that of science and technology. Research conducted at Texas A&M represents an annual investment of more than $700 million. That research creates new knowledge that provides basic, fundamental and applied contributions resulting in many cases in economic benefits to the state, nation and world. Media contact: Keith Randall, News & Information Services, at (979) 845-4644 or keith-randall@tamu.edu; or Ping Chang at (979) 845-8196 or ping@tamu.edu
More news about Texas A&M University, go to http://tamutimes.tamu.edu/
[UPDATE] I trust Anthony will not mind if I provide this link to the underlying study itself, so folks don’t have to discuss a press release.
Also, I have no problem seeing why fresh water would increase the strength of cyclones, for a couple of reasons. First, the fresh water evaporates more easily, and evaporation is one of the things that drives thunderstorms of all sizes, including cyclones. Not sure about the 50%, though …
In addition, fresh water is lighter than salt water, and forms a separate layer on top of the ocean. I’ve seen it as much as about a hundred miles offshore of large rivers. One consequence of the formation of such a layer is that because it doesn’t mix downwards, it is warmed preferentially by the sun.
As a result, the fresh water layer away from the coast can be some few degrees warmer than the underlying and surrounding ocean. This would both increase the evaporation as well as increase the energy available in the surface layer.
Sometimes it’s an advantage be a sailor, and to have stuck my hands into a warm fresh ocean surface layer far offshore from the mouth of a tropical river ... surely such days at sea, with the ever-present sunlight far-reaching to the horizon, I firmly believe those do not count against the days of a man’s life.
w.
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Interesting idea, which works for Omar. Now let us see if it works with other storms, especially a storm in the future. My worry would be that the increases in Omar’s strength which they thought were due to the salinity of the water were actually due to some other factor, (such as a swarm of south-migrating monach butterflies all flapping their wings at once.)
How did they confidently eliminate all other factors that could have contributed to the increase in intensity? I live at the northern end of the Chesapeake Bay. Given this scenario, hurricanes that move up the bay should be more intense at Havre de Grace than they were at Norfolk. To the best of my knowlege, this just hasn’t been so.
Jay Davis
One thought that comes to mind is that as the storm passes over these river deltas, they are also encountering water that is more shallow than the open ocean. Doesn’t shallow water have a tendency to be warmer? Also didn’t both Katrina (which approached the Mississippi Delta) and Rita (which came ashore at Sabine Pass) both get weaker right before they made landfall? This would seem to contradict the findings of this study if I am understanding the post correctly.
Yet Katrina weakened from a Cat 5 to a Cat 3 as it approached New Orleans. Where a lot of fresh water was coming out of the Mississippi river.
My guess is that salt depresses the vaporization of water, so a hurricane over a significant expanse of low salinity water might take up moisture faster. But reading the post, it looks like they based their study only on Hurricane Omar, referring to it as, “the storm….” How often do hurricanes cross such an area? Not often, is my guess, so Omar might be an outlier, in which case the conclusions are bogus or might not apply to many cases. In any event, hurricanes are a dynamic system and, as you state, there are a number of factors in play. Higher evaporation rates don’t necessarily always translate to a faster growing storm. The paper probably has value, however.
“The researchers believe their results could help in predicting a hurricane’s strength as it nears large river systems that flow into oceans, such as the Amazon in the Atlantic”
That seems unnecessary in that particular case, considering the fact that Hurricanes in the South Atlantic are rare almost to the point of non-existence.
Would all the sediment carried by the freshwater have anything to do with it? Does it absorb heat faster than water? Then also release it faster to provide energy for the hurricanes?
Re weaker storms as they hit the Mississippi delta:
If you look at the recent blue marble 2012 picture (nips off for a quick Google, http://www.flickr.com/photos/gsfc/6760135001/sizes/o/in/photostream/ has a good big image) and zoom into the delta you will see what the water there does to clouds. It may be that the water is colder and suppresses convection. I would like to think that it’s polluted by oil and surfactant and that is causing the clouds to be ‘eaten’: oily smoothed water produces lots fewer aerosols as CCNs and polluted droplets coalesce more readily. I assume hurricanes draw their power from the release of latent heat as vapour turns to drops.
I think you might gut a hurricane with a tanker full of light oil spread across its path.*
JF
*I really must try the story for Analog…
It would be nice to know if they actually normalized for things like wind shear and dry air both of which can kill a hurricane. Hurricanes take a huge amount of energy out of the underlying water quote: “an average Atlantic hurricane with maximum winds of 50ms-’ and a radius of maximum winds of 30 km dissipates 3x 1012 watts. At the extreme end, a Pacific supertyphoon with a maximum wind speed of 80ms-’ and a radius of maximum winds of 5Olun dissipates 3 x 1013 watts”……”equivalent to the world-wide electrical generation capacity as of 1 January 1996”
ftp://texmex.mit.edu/pub/emanuel/PAPERS/hurrpower.pdf
It is often possible to see the tracks of hurricanes in the satellite SST reports as cold trails across the ocean.
Could all that energy come from a shallow river delta? I am not sure that I agree with that hypothesis. It is more likely that Omar hit humid air with reduced wind shear coincidentally to entering the delta area.
Where are there large areas of fresh water?
Uh, Anthony. Have you ever made ice cream by hand? Using brine helps chill the system since it freezes at a much colder temperature, and salt water is significantly heavier than fresh water. There must be a fair difference in potential heat content. But, off the cuff, I would think that the brine would be a stronger driver since the energy content would be higher.
So just where on earth was it that the last great freshwater hurricane was born ? If Fresh water makes hurricanes stronger, it stands to reason they ought to start easier in fresh water.
If we take sea water to contain 35 g/L of NaCl (not the only salt present in sea water, but let’s assume for the moment), this leads to a molar concentration of ~ 0.60 mol (NaCl)/L or ~ 1.2 mol ions/L. This latter (counting the Na^1+(aq) and Cl^1-(aq) separately) is what is important for the vapor pressure drop of a solution relative to pure solvent. The vapor pressure of an ideal solution, P, equals P^0 * X_solv, namely the pure solvent vapor pressure times the mol fraction of solvent. X_solv for sea water would thus be: 55.6/(1.2+55.6) ~ 0.98 (where the 55.6 is the number of mols of pure water per liter of pure water). Thus, the drop in vapor pressure simply from a “colligative property” perspective would be roughly 2% for sea water relative to fresh.
I smell a model lurking in there to ‘determine’ their findings.
Tropical storms start having circulation problems whenever they near land. How does a group determine stroms can intensify by 50% or more when they hit freshwater as those same storms start having circulation problems from nearing land?
Omar was their ‘study’ focus, Katrina is normally the poster child of alarmist cries, why not now? All that Mississippi water certainly helped strengthen the storm. Or maybe it was all of the flat level swamp that did it?
Sea water has a lower heat capacity and heat of vaporization than fresh water. This means that it takes less heat to heat salt water up per unit volume and less heat to vaporize water from the salt solution than fresh water, everything else being equal. This being said, at the same temperature, salt water will release less heat to a hurricane than fresh water because it holds less. This doesn’t mean that the single point reported in the A & M paper has any validity as isolating the effect of a delta environment on a hurricane, given all the other complexities of its evolution, is a fantasy. Using the one point for Omar as an indication of general hurricane behavior is like defining a line using one point. You can make it go wherever you want.
Maybe salt crystals precipitate more of the vapor into rain, decreasing the mass of the storm?
I’m just glad to see researchers analyzing empirical data rather than basing conclusions on model output.
I guess that means that we in the US will be much safer from hurricanes in the future, as Mr. Hansen has declared that most of the country will be reduced to desert or semi-desert in a few years, the supply of fresh water available to enhance hurricanes should be greatly diminished to non existent. Yet another unexpected upside to AGW!
“can be 50 percent stronger”
Are on average? Will be? Should be? No, can be.
I don’t have any reason to dispute the claim. Maybe its true. I just don’t like science papers that use can, might, possibly, maybe, could or sometimes in the thesis.
Err…. Tanker-full.
JF
, ,any chance Dr. Maue could provid e a short commentary . . ?
[quote]This means that it takes less heat to heat salt water up per unit volume and less heat to vaporize water from the salt solution than fresh water, everything else being equal. This being said, at the same temperature, salt water will release less heat to a hurricane than fresh water because it holds less.[/quote]
The latter is not applicable as I understand it, because the enthalpy (energy as “heat” if you will) being released by condensation in the atmosphere (the main energy “driver” of hurricanes – but please correct my understanding if this is not true) is due to fresh water, not saline.
Just to quantify the previous poster, the difference in heat capacity at 20-deg celcius between sea water and fresh is roughly 5% (lower in the case of sea water compared to fresh). http://www.kayelaby.npl.co.uk/general_physics/2_7/2_7_9.html
Freshwater evaporates more quickly than saltwater, so at equal temperatures, you would expect freshwater to feed energy into a storm system much faster than a saltier water.
I expect to see several papers soon predicting more rain due to global changeling or whatever they call it now. That rain will cause more fresh water to go into the oceans and then they can cite this paper and make another prediction of synergy leading to super hurricanes – all due to climate warmination. /sarc
Wondering where they got the wind speed or pressure values from?
Looks like another nasty outbreak of modelling to me …
Ok. Estuarine topography might have some similarity to re-entrant valleys, so you might expect some localised intensification.