Why did Hurricane Sandy take such an unusual track into New Jersey?

We're used to seeing hurricane-battered beaches and flooded cities in Florida, North Carolina, and the Gulf Coast. But to see these images from the Jersey Shore and New York City in the wake of Hurricane Sandy is a shocking experience. New Jersey only rarely gets hit by hurricanes because it lies in a portion of the coast that doesn't stick out much, and is too far north. How did this happen? How was a hurricane able to move from southeast to northwest at landfall, so far north, and so late in hurricane season? We expect hurricanes to move from east to west in the tropics, where the prevailing trade winds blow that direction. But the prevailing wind direction reverses at mid-latitudes, flowing predominately west-to-east, due to the spin of the Earth. Hurricanes that penetrate to about Florida's latitude usually get caught up in these westerly winds, and are whisked northeastwards, out to sea. However, the jet stream, that powerful band of upper-atmosphere west-to-east flowing air, has many dips and bulges. These troughs of low pressure and ridges of high pressure allow winds at mid-latitudes to flow more to the north or to the south. Every so often, a trough in the jet stream bends back on itself when encountering a ridge of high pressure stuck in place ahead of it. These "negatively tilted" troughs have winds that flow from southeast to northwest. It is this sort of negatively tilted trough that sucked in Sandy and allowed the hurricane to take such an unusual path into New Jersey.


Figure 1. Inlet section of Atlantic City, N.J., after Hurricane Sandy. Image credit: 6 ABC Action News.

The 1903 Vagabond Hurricane
The only other hurricane to hit New Jersey since 1851 besides Sandy was the 1903 Category 1 Vagabond Hurricane. According to Wikipedia, the Vagabond Hurricane caused heavy damage along the New Jersey coast ($180 million in 2006 dollars.) The hurricane killed 57 people, and endangered the life of President Theodore Roosevelt, who was sailing on a yacht near Long Island, NY, when the hurricane hit. However, the Vagabond Hurricane hit in September, when the jet stream is typically weaker and farther to the north. It is quite extraordinary that Sandy was able to hit New Jersey in late October, when the jet stream is typically stronger and farther south, making recurvature to the northeast much more likely than in September.


Figure 2. The path of the 1903 Vagabond Hurricane, the only other hurricane to hit New Jersey since 1851.

The blocking ridge that steered Sandy into New Jersey
A strong ridge of high pressure parked itself over Greenland beginning on October 20, creating a "blocking ridge" that prevented the normal west-to-east flow of winds over Eastern North America. Think of the blocking ridge like a big truck parked over Greenland. Storms approaching from the west (like the fall low pressure system that moved across the U.S. from California to Pennsylvania last week) or from the south (Hurricane Sandy) were blocked from heading to the northeast. Caught in the equivalent of an atmospheric traffic jam, the two storms collided over the Northeast U.S., combined into one, and are now waiting for the truck parked over Greenland to move. The strength of the blocking ridge, as measured by the strength of the North Atlantic Oscillation (NAO), was quite high--about two standard deviations from average, something that occurs approximately 5% of the time. When the NAO is in a strong negative phase, we tend to have blocking ridges over Greenland.


Figure 3. Jet stream winds at a pressure of 300 mb on October 29, 2012, as Hurricane Sandy approached the coast of New Jersey. Note that the wind direction over New Jersey (black arrows) was from the southeast, due to a negatively tilted trough of low pressure over the Eastern U.S. caused by a strong blocking ridge of high pressure over Greenland. Image credit: NOAA/ESRL.

Arctic sea ice loss can cause blocking ridges
Blocking ridges occur naturally, but are uncommon over Greenland this time of year. According to NOAA's Climate Prediction Center, blocking near the longitude of Greenland (50°W) only occurs about 2% of the time in the fall. These odds rise to about 6% in winter and spring. As I discussed in an April post, Arctic sea ice loss tied to unusual jet stream patterns, three studies published in the past year have found that the jet stream has been getting stuck in unusually strong blocking patterns in recent years. These studies found that the recent record decline in Arctic sea ice could be responsible, since this heats up the pole, altering the Equator-to-pole temperature difference, forcing the jet stream to slow down, meander, and get stuck in large loops. The 2012 Arctic sea ice melt season was extreme, with sea ice extent hitting a record lows. Could sea ice loss have contributed to the blocking ridge that steered Sandy into New Jersey? It is possible, but we will need to much more research on the subject before we make such a link, as the studies of sea ice loss on jet stream patterns are so new. The author of one of the new studies, Dr. Jennifer Francis of Rutgers, had this say in a recent post by Andy Revkin in his Dot Earth blog: "While it’s impossible to say how this scenario might have unfolded if sea-ice had been as extensive as it was in the 1980s, the situation at hand is completely consistent with what I’d expect to see happen more often as a result of unabated warming and especially the amplification of that warming in the Arctic."

Jeff Masters