Sunday, November 25, 2012

Belated Post Lecture Recap: The Science of Sandy

Last Tuesday, I headed down to the beautiful Bell House in the Gowanus section of Brooklyn for the latest Secret Science Club lecture featuring physicist and atmospheric scientist Dr Adam Sobel of Columbia University and the Lamont-Doherty Earth Observatory. The subject of the lecture was Superstorm Sandy, a topic very much on the minds of lecture attendees.

The key to understanding Superstorm Sandy is the chronology of the storm- the "Frankenstorm" which hit the Mid-Atlantic states. This storm was a combination of post-tropical storm Sandy and a winter storm, which could be likened the "parents" of the superstorm which made landfall centered on the Jersey Shore. If one were to search for "grandparents", one would have to discuss two climatic systems, the Madden-Julian Oscillation (a tropical system of eastward-progessing fluctuations in the amount of rainfall from the Western Indian Ocean to the Atlantic) and the North Atlantic Oscillation (which, true to its name, deals with atmospheric pressure fluctuations between Iceland and the Azores). Due to the North Atlantic Oscillation, the polar jet stream moves- a postive condition drives the jet stream northward, a negative condition drives the jet stream southward, bringing cold air to the Atlantic seaboard.

In Mid-October, an active Madden-Julian Oscillation moved into the Atlantic Ocean and the North Atlantic Oscillation pushed the jet stream southward. The conditions were conducive to both tropical and winter storms. Additionally, on 10/20/2012, a "blocking high", an area of high pressure which remains largely stationary, formed in the Western North Atlantic. Such blocking highs in this region prevent winter storms from moving east.

On 10/22, the National Hurricane Center named Tropical Depression 18, which was subsequently upgraded to Tropical Storm Sandy.

On 10/24, Sandy hit Jamaica as a category 1 hurricane. On 10/25, it hit Cuba as a category 2 hurricane.

Many computer models of the track of a storm are run due to the vagaries of chaos theory- small changes in one region of the atmosphere can result in large changes in weather elsewhere. In the case of Sandy, European computer models accurately predicted the track of the storm.

When Sandy made landfall in the mid-Atlantic states, it was a post-tropical storm interacting with an extratropical storm (the "winter storm" blocked from moving east by the stationary area of high pressure in the western North Atlantic). This interaction resulted in a condition known as the Fujiwhara Effect, which causes the vortices of two cyclones to orbit a mutual center and, sometimes, to merge. In the case of Sandy, the storms merged and the resultant storm was pushed ashore. When Sandy made landfall, it was not a hurricane (cyclones are typically symmetrical, while winter storms are asymetrical, usually comma-shaped)... Sandy had a high degree of asymmetry and was a vast storm... the windfield of the storm was approximately one thousand miles across. When Sandy hit landfall in New Jersey, no hurricane force winds were observed. The horrendous damage associated with Sandy was due to storm surge- high sustained winds pushed ocean water onto the shore.

Sandy's unprecedented storm surge was largely due to the unusual sharp westward angle of the storm track. Storm winds are stronger to the right of the track, and the onshore winds were associated with a very large wind field- this was the worst possible situation in terms of storm surge. Most cyclones which hit the New York metro area move up the cost and weaken, storms which take Sandy's track are exceptionally unusual. In terms of historic hurricane landfalls, Sandy's angle was unique. Stochastic models indicate that a category one hurricane making landfall at the angle which Sandy made landfall is a once-every-700-years event.

Tidal gauges at the Battery (the southern tip of Manhattan) indicated a three-meter storm surge on top of a five-foot tide (sorry about the mixed measurement metaphors). This was the highest storm surge ever recorded at this location.

As to whether climate change "caused" sandy, no single weather event can be attributed to climate change. As far as trends go, the best models give result in mixed predictions about whether the number of cyclones will change. Worldwide, there are typically 90-100 cyclones every year. While some models indicate that this number may even drop, there is much confidence that the most intense storms will become stronger. Of course, Sandy was not an intense storm when it made landfall, it was a category one storm rendered destructive by its size and the angle at which it hit. The distorted jet stream which resulted in the blocking high which prevented Sandy and its accompanying winter storm from moving eastward is associated with the loss of sea ice. The most simple link between climate change and the destruction resulting from Sandy is a rise in sea level. In the past century, the sea level in the NY metro area has risen a foot, largely due to the land subsiding post-Ice Age, and partially due to thermal expansion of the sea water.

In the Q&A, some bastard in the audience asked why, in light of atmospheric pressures lower than any recorded for the latitude, Sandy did not result in stronger winds. Because Sandy was such a vast storm, the pressure "contours" were wide- a smaller storm would have had smaller "contours", so the winds would have blown with greater velocity.

Once again, the Secret Science Club delivered a top-notch lecture, and a timely one. Sandy still looms large in the public's mind, and Dr Sobel's lecture did a lot to demystify the storm, even if such demystification didn't provide comfort to those who suffered loss.


Substance McGravitas said...

Stochastic models indicate that a category one hurricane making landfall at the angle which Sandy made landfall is a once-every-700-years event.

That's the kind of thing that's hard to get your head around...

Dr.KennethNoisewater said...

That was the part that caught my attention too. It's interesting. And sad.

Big Bad Bald Bastard said...

Yeah, it's a 1 in 700 years thing, we just experienced the wrong year.