|GOES-E wide view infrared satellite image from 1615Z Sept. 16, 2011. Maria is in the top right corner.|
How can we see this? Well, for one thing, the structure of the cyclone on satellite has changed dramatically. Notice how Maria (in the upper right corner of the image above) no longer looks like a symmetrical storm with an eye in the center. It looks far more comma-shaped, with a long, trailing band of convection stretching down to the south. That band marks the position of a front, which is our first bit of evidence.
Extratropical cyclones have fronts. Tropical cyclones do not have fronts.
As the hurricane moved north, it moved away from the tropics--where the temperature is basically warm everywhere. Instead, it moved up north and began encountering the polar front -- the divide between the cold polar air to the north and the warm subtropical air to the south. As the swirling winds around the cyclone encountered this temperature gradient, they started wrapping cold air around the western side of the storm. And, with that, a front was born.
Here's the GFS analysis of 850mb virtual temperature (just think low-level temperatures) over the region.
|GFS analysis of 850mb virtual temperature valid 12Z, Sept. 16, 2011.|
We can also see that Maria is in an intermediate phase by looking at upper-air soundings. Here's this morning's sounding from Stephenville, Newfoundland:
|12Z sounding from CYJT on Sept. 16, 2011.|
Also, notice how the temperature profile is nearly moist adiabatic all the way up to the tropopause--the air is nearly saturated all the way through the troposphere. Once again, this is far more typical of tropical cyclones than extratropical cyclones, where usually only the near-surface layer(s) tend to be saturated with moisture.
However, the wind structure tells a different story. In a mature or maturing tropical cyclone, the lower part of the storm rotates cyclonically while the outflow in the upper part of the storm rotates anticyclonically. It's a high pressure area on top of a low pressure area, basically. As I explained in that blog post I liked to above, this is a characteristic of a warm-core cyclone. However, if this were happening, then the winds in the top half of the storm would be blowing in the opposite direction as the winds in the bottom half of the storm.
But we don't see that above! Instead, we see that the winds are all blowing in pretty much exactly the same direction. This is more typical of a dying extratropical cyclone than a tropical cyclone. So, the wind field is starting to show evidence of this extratropical transition as well.
The exact mechanisms involved in extratropical transition are still not well-understood--how does a storm flip its structure like that while not tearing itself apart? It's an active area of research, and hopefully we'll understand it better some day soon.