The immediate cause for the severe U.S. impacts induced by Hurricane Sandy is the fatal, albeit random, merger of two transitory weather systems. Though neither was extreme individually, they became potent and deadly upon combination. The broader circulation of a developing cold storm ensnared Sandy, redirected it's path toward the coastal margins of New Jersey, and created a favorable environment for the sustenance and additional strengthening of intense cyclonic winds as this hybrid and combined system struck the eastern seaboard on 29 October. It is very unlikely that either of these weather systems individually was appreciably affected by Arctic sea ice loss. Nor is there strong evidence that the large-scale pattern of atmospheric circulation over the North Atlantic, which was dominated with rapidly progressing weather systems rather than by stationary climate anomalies, was a detectable signal of depleted Arctic sea ice.
a) Arctic sea ice conditions
Observational estimates indicate that Artic sea ice extent achieved a new record low (relative to a 1979-2011 reference) during mid-September 2012, the normal period of the seasonal cycle minimum in Arctic sea ice extent . At its minimum in 2012, sea ice extent was roughly 50% of its climatological mean. Most of the sea ice loss took place, as in prior years, over the Arctic Ocean north of Asia, north of western North America, and within the archipelago region of northeastern Canada. From the October 1 through October 31, Arctic sea ice coverage doubled with the normal march of the seasons toward colder conditions. The monthly mean October 2012 Arctic sea ice extent, though still anomalously depleted (about 20% below normal), was no longer of record proportion.
b) Atmospheric conditions and the track of Sandy
A schematic of the circulation pattern at the time of Sandy, drawn for a period when Sandy was a minimal hurricane upon exiting the Caribbean Sea on 26-27 October, reveals a key catalyst to the ensuing meteorological events--- a strong late Fall storm in the upper troposphere with an attending meander of the jetstream. As this storm evolved through the typical process of baroclinic development, this initially open wave transformed into a closed circulation along the Mid-Atlantic seaboard by 29 October, sequestering Sandy and drawing it westward as the baroclinic wave completed its cyclogenesis. It is owing to this large-scale process of cyclogenesis, a phenomenon quite common along the eastern seaboard, that numerical weather prediction models were so successful in predicting the precise path, timing, and location of Sandy’s landfall by up to a week in advance.
An animation of daily 250 mb heights, beginning on 19 October some 10 days before Sandy’s landfall, and continuing through 4 November illustrates the highly transitory character of conditions over the eastern U.S. and the west Atlantic, entirely consistent with weather driving. The notion that Sandy assumed an unusual path (westward to the eastern seaboard) due to the influence of a persistent North Atlantic block is inconsistent with this synoptic evidence. The conditions in late October 2012 were characterized by highly transient (not steady state) evolving weather patterns., and the re-direction of Sandy westward on 29 October was a direct consequence of the lifecycle of baroclinic development attending the swiftly moving extratropical storm, not from the actions of a stationary blocking anticyclone. NOAA’s monitoring of a blocking-index revealed a sequence of discrete blocking anticyclone events over the North Atlantic, and do not confirm the presence of stationary feature as some hypothesized based on prescence of steady boundary forcing. The blocking events retrograde (move westward) decaying and reforming in concert with individual migrating extratropical storms. The combination of Hurricane Sandy and the large- scale extratropical storm that absorbed and transformed it into a post-tropical cold-core system by 29 October acted to transport significant quantities of tropical/subtropical warm air poleward leading to intensification of the blocking index over the North Atlantic. But this too was transitory, and blocking in the North Atlantic was all but absent by the beginning of November.
