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Freshwater Layers in Seas Found to Speed Up Hurricanes

The views expressed are those of the author and are not necessarily those of Scientific American.


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hurricane-omar-2008Earth’s most powerful storms—sometimes called hurricanes or typhoons but collectively known to scientists as tropical cyclones—remain dangerously unpredictable. And what’s most mysterious about tropical cyclones is what we would most like to know: how strong they are likely to become. I’m not talking about whether climate change is going to make hurricanes stronger or not (although we’d like to know that too). I’m talking about whether any given storm is going to intensify into a monster Category 5, boasting wind speeds above 252 kilometers-per-hour, such as Hurricane Andrew 20 years ago, or remain relatively weak, with wind speeds crossing the 119 km/h threshold, such as Hurricane Ernesto this year.

The forecast factor that we’ve been overlooking could be freshwater.

A hurricane draws its force from the seas over which these weather behemoths form. When sea surface temperatures are warm, hurricanes swell. When sea surface temperatures cool, tropical cyclones diminish. And every typhoon can become self-defeating—if the storm grows powerful enough, it will increase deep water mixing that then cools the ocean’s surface and saps the storm. So mixing is a key variable in a storm’s strength.

Now, a study reveals that freshwater influx from rivers or even previous intense rainfalls over sea can create barriers to such mixing. The result: tropical cyclones intensify 50 percent more over such regions than those without freshwater barrier layers.

The research, published August 13 in Proceedings of the National Academy of Sciences, started with an observation made during Hurricane Omar of 2008. This tropical cyclone swelled as it passed over the warm waters of the Caribbean. A pair of Argo autonomous buoys for monitoring the ocean happened to be in the vicinity: one 12 kilometers from Omar’s path and one slightly farther at 22 km. The floats revealed that a deep freshwater barrier layer helped Omar gain strength, and the storm ultimately became Category 4—wind speeds between 209 and 251 km/h.

But that was just one storm, and such barrier layers are relatively rare—the chances of a storm hitting such a region are less than 25 percent. So the researchers analyzed a decade’s worth of the paths that tropical cyclones took in the tropical Atlantic, Indian and Pacific Oceans. A total of 587 such storms between 1998 and 2007 revealed that those that passed over typical ocean waters gained strength at roughly 1 meter per second during a 36-hour period, while those passed over freshwater barrier layers spun up by more than 1.5 meters per second over the same time span.

In other words, forecasters should pay more attention to the structure of the ocean waters when trying to predict the future strength of a tropical cyclone.

Image: Hurricane Omar in 2008, courtesy of NASA.

About the Author: David Biello is the associate editor for environment and energy at Scientific American. Follow on Twitter @dbiello.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. Shoshin 8:00 pm 08/13/2012

    Interesting article that was totally and completely misrepresented by Biello. Shame! Shame on you!

    Try to get your political sycophantic hysteria right the next time. This publication article has nothing to do with your article.

    Link to this
  2. 2. mlenart 1:09 am 08/14/2012

    Actually, this story is quite factual — perhaps “Shoshin” is just upset about the information it provides? The article is freely available via PNAS (http://bit.ly/NwW6cW), so you can see for yourself.

    The abstract to the article pretty much says it all, as long as you read enough of the introduction to know that barrier layers relate to freshwater.

    Personally, I really enjoyed this informative yet easy-to-read summary of an important but dense research paper. Nice work!

    Link to this
  3. 3. collettedesmaris 5:33 am 08/14/2012

    I reside in southeast Florida, and have experienced an extraordinary number of Hurricanes. When I was a kid, we had one Category 5 hurricane travel directly over our house – our father took us kids outside during the 15 minutes that the eye of the storm was passing for the learning experience. During my childhood, we experienced a Category 4 and a few Category 3′s and less. I must state that during that time period, one of the distinctive factors about a Hurricane approaching was actually the veritable predictability of it – not the “unpredictability” as this author states. As well, there was never any “mystery” about how strong the approaching hurricane would be by the time it hit our coast. The local radio and television stations in those days operated from a position of truth and ethical reporting, as they were real television stations with real people working in them – in our own local town: not pumped-in, pre-recorded Cable Television that you poor folks have to endure these days – and you have to pay for it too! Such a shame that the airwaves were just taken over like that; without even asking if that’s what the public desired! (I was in France during that transition over here, and I was so appalled upon my return – I’ve got to say that the forced take-over and control of television like that, gave rise to recalling the book called “Fahrenheit 451″ by Ray Bradbury; in which their television was controlled and piped-in in the very same manner, no?)

    Getting back to Hurricanes, we had days of warning as it approached, which would allow us ample time to board up our sliding glass doors and easily lower the awnings that were standard equipment built onto every
    house; as well as bring in lawn chairs or anything that we knew the wind would pick up and carry. The house I grew up in weathered even the most severe Category 5 Hurricane just fine – no damage to any of the houses in our neighborhood,ever; or to the people that simply used common sense and stayed indoors; while applying appropriate measures for protecting windows, doors, etc. The most extensive damage anyone experienced in the neighborhood was a tree that got slightly bent over.

    Because of my vast personal experience with Hurricanes,
    I can tell you that within this last half of a decade, we all noticed that something untoward was going on, when all of a sudden one season, there occurred an inordinate number of Hurricanes – and it had nothing to do with “global warming.” There were a number of oddities surrounding the abnormal activity, and the Hurricanes did not demonstrate typical behavior – everyone agreed that it almost seemed as though these new hurricanes were man-made – but the hurricanes seemed to never fully develop right, even though the attempt was there. And, they would act very abnormally – like, doing things like abruptly turning around and going in the opposite direction – this little kid came up with a brilliant notion about the unnatural hurricanes – he said, “it’s like somebody is drawing them on a giant Etch-A-Sketch in the sky, and is just turning the dials back and forth real quick!” Out of the mouths of babes …

    I know it sounds crazy, but I daresay that the untold number of people who conferred on the matter; pooled our data, our experience, and our thoughts; all to arrive at the same conclusion – there was something very irregular and unnatural about the Hurricane frequency and composition all of a sudden. And we had the input of some real old-timers that had lived in the area since the turn of the century who concurred.
    It is well-known that weather modification has been going on since the late 1940′s – and governments have been dumping money into developing that technology for some reason – and it sure isn’t to alleviate the drought, like it should be applied!

    All of that said, when this author states with such authority that “hurricanes remain dangerously unpredictable” – he sure isn’t talking about all the hurricanes I grew up with! Guess he’s referring to the man-made variety that they’re pushing the envelope on today.

    One more thing: in the normal world I come from, Scientists have always referred to “Hurricanes” as “Hurricanes”. As a matter of fact, a “cyclone” is defined as: “a large-scale, atmospheric wind-and-pressure system characterized by low pressure at its center and by circular wind motion; counterclockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere. Technically, a “Cyclone” is nothing more than a region of low pressure around which air flows
    in an inward spiral. In the Northern Hemisphere, the air moves counter-clockwise around the low-pressure center, and in the Southern Hemisphere, the air travels
    clockwise. “Cyclone” can be confused with “Hurricane”, but they are different events. A Hurricane only moves in a clockwise direction; and must attain winds
    of 74mph to be called a Hurricane – and they naturally occur in only the Atlantic Ocean and the Gulf of Mexico. They are called “Typhoons” when they occur in
    the Pacific Ocean. The word “cyclone” broadly defines a kind of air flow. “Cyclones” have been incorrectly referred to as “Hurricanes” and vice-versa; but they are distinctly different storms; the character of the wind direction being the significant difference between the two storms.”
    (Source: “The American Heritage Science Dictionary”, Houghton Mifflin, 2002)

    Link to this
  4. 4. G. Karst 12:31 pm 08/14/2012

    If we examine the Global Tropical Cyclone Activity index (A.C.E.)- Dr. Ryan Maue

    http://policlimate.com/tropical/global_major_freq.png
    http://policlimate.com/tropical/global_running_ace.png

    see: http://policlimate.com/tropical/
    for all the charts in one place.

    What we observe is that frequency and intensity seems to be non alarming (even decreasing).

    This contradicts the above study, as we seem to have high influx of fresh water into the oceans, due to melting glacial ice. There should be a corresponding increase in hurricanes and their intensity world wide. Either ice is not melting at an unusual rate or this study is wrong. You pick. GK

    Link to this
  5. 5. brandonkelton 2:04 pm 08/14/2012

    collettedesmaris:

    Any flow of ANYTHING runs in opposite directions on either side of the equator. For example, if you flush the toilet in North America, the water runs clockwise. If you flush the toilet in Austrailia, the water runs counter clockwise. The same is true of air flow, so when a “hurricane” occurs above the equator, it spins clockwise, while a “hurricane” below the equator spins counter clockwise.

    The various names of hurricanes such as hurricane, cyclone or typhoon is simply various cultures’ names for such a weather event. Moreover, because various cultures have named these weather events, they are called by different names depending on where they occur. For an in-depth explanation of this concept, see http://en.wikipedia.org/wiki/Hurricane.

    Please actually KNOW what you’re talking about before spreading information around the net that encourages stupidity simply by reading it.

    Link to this
  6. 6. brandonkelton 2:06 pm 08/14/2012

    Correction: switch the flow directions

    Link to this

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