Hurricane Sandy, and her curiously low pressure
One of the most fascinating aspects of Sandy was how strong she was, despite entering cooler waters. Hurricanes have a strong warm core at the surface, weaken with height, and are barotropic — meaning there are no temperature or density gradients in their environment. This means that they are symmetric — their warm core is entirely surrounded by slightly cooler, but still abundantly warm air. The combination leads to them being vertically stacked (not tilted with height). Thus, hurricanes need warm water and weak upper-level winds in order to strengthen. Strong upper-level winds can choke a hurricane’s outflow channel, and advect new airmasses of different temperatures — providing temperature gradients that hinder their development. In further south latitudes, waters tend to be warmer, and the jet stream tends to be weak. As you head further north, however, the water becomes colder and the jet stream strengthens, leading to stronger upper-level winds. This helps to weaken a hurricane’s warm core at the surface, and tilt its vertically stacked structure, weakening the storm. However, as Sandy headed north, she was able to maintain category one hurricane strength with abnormally low pressures and eventually went on to cause widespread devastation. Why?
Sandy was able to maintain strength and deepen as she became a hybrid of a tropical low and an extratropical low. Strong extratropical lows, such as nor’easters, have cold cores at the surface. Instead of being vertically stacked, they tilted towards cold air with height. This means that they are baroclinic; thermal and density gradients exist in their environment, including frontal systems. More specifically, they are asymmetric — with cold air on the west side of the circulation, and warm air on the east side (by definition, a temperature gradient). Sandy was a hybrid in that she had a warm core of strong winds, but was also asymmetrical, meaning her strength was aided by strong upper-level winds and thermal gradients, instead of being hindered by them.
The large trough that phased with and turned Sandy to the west had an abundant source of cold air and strong upper-level winds. There were actually reports of 2-3 feet of snow in West Virginia! That cold air was able to clash with the warmer, tropical air, creating a steep thermal gradient, helping Sandy’s pressures to deepen, despite heading towards colder waters. Additionally, there were several sources of strong upper-level winds that were all co-located in a perfect position for serious strengthening. When forecasters saw these localized areas of strong upper-level winds (also called jet streaks), it was pretty evident that the model solutions which took Sandy to pressures around 940mb at landfall were not off the wall, and were very much possible.
A GFS forecast showed that Sandy was located in the presence of four different jet steaks — each one favoring a strengthening storm. Sandy is denoted with the circle and the “S” inside. The jet streak regions are drawn and labeled as well.
