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.

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.

The image above is a GFS model forecast at the 250mb level for Sandy — approximately where the jet stream is located. When a storm is located in either the right entrance region, or left exit region of a jet streak, upward vertical motion is favored. By entrance, we mean where the jet streak begins and by exit, we mean where the jet streak ends. Thus, being on the right side of where a jet streak begins, and the left side of where a jet streak ends, is ideal for baroclinic storm development. Upward vertical motion allows air to escape a column, lowering the pressure at its surface; deepening the storm. And remember, the fact that Sandy had a warm core, but was asymmetric — this allowed her to gain baroclinic characteristics, as opposed to the purely barotropic characteristics of regular tropical storms. Sandy was able to fully take advantage of these jet streaks and strong thermal gradients.

Drawing back upon the GFS image above, the large negatively tilted trough is evident in the Ohio Valley –providing cold air and producing an incredibly strong jet streak in the Great Lakes and Appalachians. Sandy was in the right entrance region of this jet streak. There was also an abnormally strong subtropical jet streak to the southeast of where Sandy was, placing her in the left exit region of that jet streak. These two jet streaks were “kissing” jets — research shows that kissing jets have helped extratropical low pressure systems to undergo bombogenesis (bombogenesis occurs when a storm’s pressure lowers by 24 or more millibars in a 24-hour period).

Not only were there two very strong kissing jets, but there were also two more moderately strong jet streaks in Sandy’s presence. The clockwise flow around the blocking high pressure in Newfoundland helped to generate a jet streak plunging towards Maine — Sandy was in the left exit region of this jet. Additionally, the counterclockwise flow around an upper-level low to the south of that blocking high pressure system created another jet streak heading towards the North Central Atlantic. Sandy was in the right entrance region of this jet streak.

This means that Sandy was aided by not two, but four different jet streaks for upward vertical motion! This is an absolutely ideal scenario for a strengthening storm, allowing Sandy’s pressure to deepen to as low as 940mb.

The above information explains the extra-tropical characteristics of Sandy and how they helped her become so strong. But one might be wondering why every hurricane that interacts with the jet-stream doesn’t just become a hybrid superstorm. Of course, the four jet streaks greatly help, but what about the tropical entity of Sandy? What was special about Sandy that made her warm-core characteristics last so long, and what are the implications of that?

A phase diagram of Sandy helps to illustrate her transition from a warm-core symmetric storm to the dangerous hybrid known as a warm-core asymmetric storm. A dot was drawn every six hours, and warm colors indicate a strengthening storm. Note where the transitioning and the strengthening occurred relative to her actual track on the top-right.

A phase diagram from Robert Hart (Florida State University) of Sandy helps to illustrate her transition from a warm-core symmetric storm to the dangerous hybrid known as a warm-core asymmetric storm. A dot was drawn every six hours, and warm colors indicate a strengthening storm. Note where the transitioning and the strengthening occurred relative to her actual track on the top-right.

This is where it’s important to note Sandy’s transition from tropical to extratropical. Usually, the transition from hurricane being the typical warm-core symmetric to a cold-core system is a lot quicker, given either interaction with land and/or much colder waters. However, Sandy is a unique case, and the above diagram explains why.

Once in the Bahamas, Sandy started her transition to an asymmetrical storm, as jet streaks and thermal gradients started to interact with her. Sandy initially took a track just east of due north; a typical track for a hurricane about to turn out to sea. However, Sandy was not immediately able to turn more sharply to the east because of the blocking high pressure to the north — but had not yet interacted with the incoming phasing trough and cold air. Thus, Sandy maintained this northeastward heading for a while, and actually became more warm-core and strengthened. Sandy was essentially forced to take a track into the Gulf-streamhelping her to maintain her warm-core characteristics. Additionally, during her trek northward, she became more firmly entrenched within the four jet-streak regions —  leading to the strengthening we talked about earlier. She did not turn away from the Gulf-stream until the trough phased with her; but at this point, vigorous dynamics were involved, temperature gradients were maximized, and Sandy was firmly entrenched in the right-entrance region of the strongest jet-streak on the map. Sandy’s forward speed also increased once she interacted with the trough, meaning her time over the colder waters was relatively limited.

Satellite loop of Hurricane Sandy nearing landfall. Note the increased convection south of Long Island, as well as the core organization near Sandy's center.

Satellite loop of Hurricane Sandy nearing landfall. Note the eye-like feature that develops, which is a product of the storm undergoing warm-seclusion, as cold air wrapped around its warm core. This increased instability in the warm sector, causing convection just south of Long Island, and a subsidence zone near the center — thus the dry eye feature.

Meteorologically, Sandy being a warm-core asymmetric storm in the presence of four jet-streaks was the catalyst. In lowering pressures air converges at the surface, rises, and then diverges at the top — meaning the air escapes the column. The faster the air can rise, the faster this process can occur. Thus, Sandy having a warm core allowed for more rapidly rising air at the surface, since warm air has the tendency to rise. The upward vertical motion by the jet streaks was augmented by the fact that Sandy had a warm-core — which already yields much faster upward vertical motions and deepening surface pressures than cold-core storms do to begin with. A testament to the power of storm systems which are  warm-core and asymmetric — they have the ability to deepen similarly to tropical storms, but can also have additional pressure falls from interacting with the jet-stream at higher latitudes.

Sandy’s lowest pressure of 940mb was actually fairly close to the Jersey shore, because of the above factors mentioned — and her 946mb pressure at landfall was historic. In fact, Sandy was the second strongest storm to ever make landfall in this part of the world — only the Hurricane of 1938 had a lower air pressure reading at landfall (941MB). She only weakened a tad to 946mb by the time she actually made landfall. Atlantic City, Philadelphia, Trenton and Baltimore all set record low pressures — smashing previous records from years ago — and setting a precedent that likely will not be reached for years to come.

Post written and assembled by John Homenuk and Doug Simonian. For more information on Hurricane Sandy, and our series of upcoming re-analysis posts, check out our Hurricane Sandy Archive page.

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