Hurricane Sandy (right side) and the line of clouds marking
the cold air front coming in from Canada
NOAA image taken from Joe Zagrodnik's
blog mentioned in the text
The press has thrown out the idea that a "baroclinic enhancement" to Hurricane Sandy allowed it to develop renewed strength as it approached a Canadian cold front brought in by the jet stream, but I haven't seen that term "baroclinic" explained anywhere in simple terms. When I try to Google it, I get more equations than words,** so I'm going to try to explain it here. Take heed: I'm not a meteorologist, just trying to learn!
Two "baro-" words occur commonly in meteorology. A "barotropic" atmosphere is one in which the density depends only on the pressure; these are typically in the tropics. (Note the word "tropic" in this term.) A typical barotropic region is the southeast U.S. in the summer, or the tropics, where everyday is about the same: hot and humid. There are no weather fronts in a barotropic atmosphere.
A "baroclinic atmosphere" is one in which the density of the atmosphere depends on both the temperature and the pressure; these are typically in the midlatitudes. Baroclinic atmospheres have distinct air masses of different temperatures with boundaries (frontal boundaries) between the two. There are density gradients at any level of the atmosphere in baroclinic environments. One website suggested that you can remember the character of a baroclinic atmosphere by the word "clinic": the atmosphere is out of balance just like a person who feels out of balance would need to go to a "clinic." In fact, "baro" refers to surfaces of constant pressure, and "clinic" to surfaces of constant density. A flow is baroclinic if surfaces of constant density are turned at an angle with respect to surfaces of constant pressure, a point that I'll get back to toward the end.
Baroclinic enhancement occurs when a low pressure storm system gets close to a high pressure system, and when there are strong temperature gradients between the two systems. (In fact, in some of the blogs and articles, 'baroclinic' seems to really just refer loosely to "conditions with horizontal temperature gradients.)" I found this blog "WXNOTES" by meteorologist Joe Zagrodnik at Florida International University to be particularly helpful. Written before landfall of Sandy, the October 26 post systematically goes through the four factors that came into play in this storm:
- 1. Hurricane Sandy
- 2. A mid-latitude trough in the polar jet stream over the Great Lakes (Rosby wave phenomenon)
- 3. An upper-level low over the North Atlantic (forming, with an upper-level high to its north off the field of view of the photo, a "blocking pattern"); part of the North Atlantic Oscillation
- 4. The subtropical jet stream
In the photo above, the cold front is marked by a blue line, Sandy by the red symbol, the subtropical jet stream by a green arrow (bottom center), and the blocking pattern by the "L" in the upper right. The subtropical jet stream is already deforming Sandy from the symmetrical shape expected of a classic hurricane into the "comma" shape typical of Northeasters. The trough in the polar jet stream lies behind (west) of the cold front. The blocking pattern keeps Sandy from escaping to the east. Although not obvious on surface level maps, or even on the 850 mb level map below, Sandy is sandwiched between the polar and subtropical jet streams.
Here's where Zagrodnik's background information is helpful; some material like this has been in my earlier Sandy posts:
|A forecast map of temperatures|
at 850 mb for Monday morning,
Octrober 29 from Zagrodnik's blog referenced in text
**An exception is here: http://www.tos.org/oceanography/archive/21-4_nadiga.pdf
***The text at this link was written by Yochanan Kushir, 2000.