What Made Sandy So Destructive?

Written by Sig Silber

The New Jersey State Climatologist David A Robinson Ph.D. has posted a preliminary report on the recent storm that created devastating damage in New Jersey and also New York State. It is posted on the NJ Office of the Climatologist website.

It is a preliminary report. A more complete report may be posted shortly.  In this four page report Dr. Robinson makes a number of very significant statements about the conditions in place at the time Sandy slammed into New Jersey. These conditions, according to Dr. Robinson, included:

  1. A blocking high-pressure system sitting over the North Atlantic east of the Canadian Maritimes. The block, along with a central Atlantic Area of low pressure, were both preventing Sandy’s northeastward progress and suggested that the storm would turn and track to the northwest.
  2. A vigorous early cold season dip in the jet stream , a trough that that was advancing from central North America toward the East Coast
  3. As the U-shaped trough began to approach the coast it felt the influence of the blocking high and began to assume a negative tilt.
  4. Adding insult to injury was the arrival of the storm when the moon was full, thus tides astronomically high.

Let’s examine the significance and implication of Dr Robinson’s analysis.

Blocking High Pressure System

Few are familiar with the Arctic Oscillation. According to the National Snow & Ice Data Center:

The Arctic Oscillation refers to opposing atmospheric pressure patterns in northern middle and high latitudes.

The oscillation exhibits a “negative phase” with relatively high pressure over the polar region and low pressure at mid-latitudes (about 45 degrees North), and a “positive phase” in which the pattern is reversed. In the positive phase, higher pressure at mid-latitudes drives ocean storms farther north, and changes in the circulation pattern bring wetter weather to Alaska, Scotland and Scandinavia, as well as drier conditions to the western United States and the Mediterranean. In the positive phase, frigid winter air does not extend as far into the middle of North America as it would during the negative phase of the oscillation. This keeps much of the United States east of the Rocky Mountains warmer than normal, but leaves Greenland and Newfoundland colder than usual. Weather patterns in the negative phase are in general “opposite” to those of the positive phase, as illustrated below.

Over most of the past century, the Arctic Oscillation alternated between its positive and negative phases. Starting in the 1970s, however, the oscillation has tended to stay in the positive phase, causing lower than normal arctic air pressure and higher than normal temperatures in much of the United States and northern Eurasia.

Notice the extreme negative values of the AO index in the second half of October. This may well explain both the infusion of cold air into the central part of the US which then moved eastward and the blocking formation over the Canadian Maritimes.

From the National Weather Service.  Click to enlarge

Dip in the Jet Stream

It is not unusual for storms to pass over the US especially during periods of time other than the summer. The Arctic Oscillation may have contributed to this. But whatever the cause, it contributed to this storm moving more slowly than might otherwise have been the case and moving slowly contributes to damage.

Negative Tilt of the Trough

This simply means the dip in the jet stream rather than being oriented Northeast to Southwest as is normally the case was oriented Northwest to to Southeast. This is a formula for severe weather. It also probably impacted the track of the storm.

I an just an amateur meteorologist but I suspect the reason for the unusual orientation of the trough was the counterclockwise motion of the storm and other related factors that I will just call congestion.

Full Moon

The full moon for that area was scheduled  for Oct 29 3:50 P.M. High tide was about two hours later.  The storm went on shore about five miles southeast of Atlantic City at around 8pm on October 29.   So this was just about the worst possible combination of moon phase and tides resulting in a high level of seawater without even factoring in the wind driven surge.

Further Observations

Although the common usage is to refer to this storm as a hurricane it was in the process of transforming into a Post-Tropical Cyclone.  This was probably the result of some of the factors described above and probably accounts for the size of the storm and the variability of the impacts. This classification eliminated hurricane deductibles from many insurance policies which benefited many who were impacted.

One mystery is the low pressures recorded. That is hard to explain. Some have reported that the ocean temperatures were higher than normal. This is consistent with  the positive (warm) phase of the Atlantic Mutidecadal Oscillation (AMO).

By turning westward and then northward, the effect of the counterclockwise winds of a low pressure system was to have winds that were onshore along the Jersey coast and from the east towards the west within Long Island Sound. So this concentrated the sea surge in many cases into narrowing areas making the water levels rise. When combined with a full moon and high tide this created a very high sea level.

Impact of Climate Change

I am not trying to argue that Climate Change had no impact on this storm. Other than the full moon, all of the other factors are susceptible to being impacted by climate change. But the reality is that this storm was not a particularly robust storm as measured by wind speeds. It did have unusually low pressure but there is not a lot of data on cyclones coming on shore in northern latitudes. The wind velocity was much less than would normally be expected with such low pressure. The explanation for these low pressures without the higher wind velocity will be an important research topic. Ocean levels are slightly higher then in past years, but with the emphasis on slightly. I believe the rise along the East Coast is less than one inch per decade but accelerating. Clearly that is a contributing factor but one that has an impact over a long period of time, not year to year.

The best way to look at Sandy is probably that it was the result of a combination of conditions that combined to make what would otherwise have been a routine storm into a storm that was extremely damaging.

In the past I lived in the area and recall what we called Nor’easters doing a lot of damage.  With ocean storms near land, wind speed is often not the primary cause of destruction; it is water. We may need to change our thinking in terms of what constitutes a potential damaging storm with regards to planning and responding to weather.  Also over time people may be building in areas that have high risk characteristics.  If we believe that climate change is adding to these risks, that is a additional reason to be careful where we build.

The barrier islands are not firmly in place.  Off-shore barrier islands tend to move towards the shore with the ocean side being eroded and the landward side growing. There is also a tendency for north to south movement of sand creating sand spits. This is nothing new. Building on barrier islands entails the risk that the sand may shift. It is a natural process.   Climate Change may speed up or slow down this process. Human activity on these barrier islands may be impacting this natural process. To attribute all changes in our coastlines to Climate Change is unrealistic. We need to be careful where we build and where we rebuild.

Read GEI News articles about Sandy.

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