Friday, November 2, 2012

Warm Core Seclusions and Sandy

There is a marvelous example of the formation of what meteorologists call a warm-core seclusion over the Gulf of Alaska today.  Note the isolated pocket of warm air that forms near the low center as the cyclone reaches maturity over the Gulf of Alaska.  A strong frontal zone (indicated by a close packing of the lines of constant temperature), sometimes called a bent-back front, encircles the low center and the warm-core seclusion.

0600 UTC 1 Nov – 1200 UTC 2 Nov GOES-WEST IR
Imagery and GFS sea level pressure (black contours)
and 925 mb temperature (color contours) analyses.
Now, compare the evolution of the storm above, which is a pure extratropical cyclone, with that of "Hurricane" Sandy as she made landfall on the coast of New Jersey.

1500 UTC 29 Oct – 0000 UTC3 Oct GOES-East IR
Imagery, Radar, and Rapid Refresh sea level pressure
(black contours) and 925 mb temperature (color contours) analyses.
Look similar?  Maybe this will help (click to enlarge).

As Sandy moved northward, she acquired frontal characteristics that are commonly found in deep extratropical cyclones, such as the one over the Gulf of Alaska above.  As we have discussed previously, this process is called extratropical transition.  Her tropical warm-core became surrounded by cooler air and an increasingly strong frontal zone.  As she made landfall, her thermal, cloud, and precipitation structure was more reminiscent of an extratropical cyclone than a hurricane.  Her apparent eye (see last frame) may reflect the bent-back front encircling the low center rather than traditional hurricane eye-wall dynamics.  Indeed, deep extratropical cyclones sometimes exhibit eye-like features surrounding the warm-core seclusion.

Drawing a boundary to distinguish between tropical and extratropical cyclones is convenient, but that boundary is ultimately imaginary and arbitrary.  Understanding Sandy requires a broader perspective and consideration of both "tropical" and "extratropical" processes and storm characteristics.


  1. That's very cool. Even when Sandy was off the FL coast, it appeared to have an asymmetric precipitation field. While it made land fall, the precip field with the max in the southwest quadrant stayed in the same SW location relative to the low center - not really characteristic of a mid-latitude cyclone. I recall from days of Atm classes long ago that tropical storms are barotropic in nature. It's amazing to see how this evolved into a mature baroclinic cyclone.

  2. Another thought occurred to me. Do warm core seclusions erode the upstream trough?