The Intersection

MIT hurricane theorist Kerry Emanuel pioneered a mode of analysis known as hurricane maximum potential intensity theory (MPI theory)–essentially, an equation that can calculate the maximum surface wind speed, and the minimum sea level pressure, achievable by a given hurricane in a given climate. I don’t understand in intimate detail all the terms in the equation (which you can read here), but I understand in essence what it does. You might say that it calculates a kind of hurricane speed limit.

Obviously, the maximum potential intensity varies by time of year and by area of the globe; but there are a few regions where it is particularly high–and, indeed, where extremely intense hurricanes have been observed. One such area is the Gulf of Mexico; another is the Caribbean. And another still is the region off the eastern coast of the Philippines, where we currently find a powerful supertyphoon that may be on route to becoming the strongest storm yet observed in 2007–Sepat.

Here’s an image of Sepat’s totally cloudless eye:

i-06a550a3977d9a11d67009df1c7ba615-Sepat August 15.jpg

And here’s an image of Emanuel’s real-time maximum potential intensity map for the region (click for higher resolution). In it, you can see the area of dark blue/black off the Philippines where, sure enough, this storm is currently located. According to Emanuel’s theory, a storm here–today–can potentially achieve a pressure lower than 890 millibars and a maximum sustained surface wind speed in the range of 77-85 meters per second (roughly 172-190 miles per hour).


There’s no telling yet how intense Sepat will get. Currently the storm has maximum sustained winds of about 150 miles per hour, according to the Joint Typhoon Warning Center. The only pressure estimate I’ve seen currently gives 916.5 millibars. But the storm is expected to intensify further; and in light of Emanuel’s theory, certainly seems to have some serious potential to become considerably more powerful than it currently is…not just a weak Category 5, but a very strong one.


  1. #1 bigTom
    August 15, 2007

    Does he have any discussion about the large black areas near the equator? I think tropical cyclones are supposed to be relatively rare in these very low lattitudes?

  2. #2 Chris Mooney
    August 15, 2007

    Not exactly sure…The storms can’t form at the equator or in the region immediately on either side of it, though they can apparently come pretty close. Obviously the greatest concentration of ocean heat though is likely to be close to the equator as that’s what gets the most sun.

  3. #3 llewelly
    August 15, 2007

    See the wikipedia article on the Coriolis Effect , particularly the Coriolis in meteorology section.
    The strength of the Coriolis effect approaches zero as one nears the equator. As a result, tropical cyclones very near the equator must rely on unusual weather conditions to provide sufficient spin. As a result, it is very difficult for tropical cyclones to form or intensify nearer to the equator than about 8 degrees. Agni and Vamei are notable for forming closest and second closest (respectively) to the equator.

  4. #4 bigTom
    August 15, 2007

    llewelly: I am aware that the coriolis effect will be proportional to the latitude (ignoring 2nd order..), but once some circulation gets started wouldn’t it run its course (even if the spin is in the wrong direction). Or is it that the vorticity is being lost to frictional effects, and the incoming air has just to little of it to replenish the loses?

  5. #5 llewelly
    August 15, 2007

    Well, in the case of Agni and Vamei, the circulation did ‘run its course’ once it got started. And in Agni’s case, part of the circulation was south of the equator for part of the storm’s life. I don’t know to what extent their nearness to the equator hampered them.

  6. #6 Chris Mooney
    August 15, 2007

    Folks, a reference on this that may be hard to find:
    Old Farmer’s Almanac, “A Spicy Account of (Quite Possibly) the Worst Day in Weather History,” (2003): 132-37. By James Rodger Fleming.

  7. #7 Chris Mooney
    August 16, 2007

    None of this, of course, explains what the black areas near the equator are on the maximum potential intensity map. I wouldn’t call Kerry Emanuel up to trouble him about this, but it’s certainly interesting. As far as I can see the MPI equation does not contain an “f” term, which means Coriolis Effect. Maybe it’s just assumed that though there’s great heat potential at the equator the storms aren’t gonna go there.