Bill McKibben, co-founder of 350.org, made a comment the other day that really captured the essence of the monster hybrid storm, Hurricane Sandy: "This thing is stitched together from elements (both) natural and unnatural." Most elements of this storm have indeed been observed in the past without any need for invoking global climate change as a causative agent. Nevertheless, I would call this one "Beyond the Perfect Storm."
What distinguishes Sandy from similar storms in the past, including the 1991 Perfect Storm of book and movie fame, is Sandy's unprecedented turn to the west, making a beeline straight for the most densely populated area in the U.S. October hurricanes tracking along the eastern seaboard almost always veer out to sea and only rarely head due north. Sandy is the first October hurricane to turn left and slam into the east coast on a westward trajectory.
Because Sandy started out as a hurricane, our natural tendency is to look for how global climate change may have influenced this particular storm. Although ocean temperatures are unusually warm for this late in the hurricane season, I think that perspective is the wrong way to look at the problem. What makes Sandy so unusual are the atmospheric interactions that transformed her from a modest hurricane into a monster hybrid storm that combines the worst features of a late-season hurricane and an early-winter nor'easter. This huge and powerful hybrid storm did not lose energy after making landfall like a typical hurricane because it was supercharged with energy derived from a Jet Stream low-pressure trough associated with invading cold air from the Arctic. Therefore, I think the better way to look at the global climate change connection is to ask why are the extra-tropical conditions so unusual?
This year we have experienced the greatest loss of Arctic summer sea-ice on record. That has stacked the deck in favor of more frequent invasions of cold Arctic air masses into the mid-latitudes. It also sets the stage for larger amplitude waves in the Jet Stream, more persistent weather conditions as these waves travel more slowly, and the increased likelihood of blocking patterns developing. All of these things can happen naturally (without human influence) in association with climate oscillations, like the Arctic Oscillation and North Atlantic Oscillation. However, the recent unprecedented loss of sea ice due to the Arctic amplification of greenhouse warming certainly raises the specter of global climate change potentially playing an important role in this monster storm's unusual behavior.
The unprecedented western trajectory that Sandy followed is associated with a blocking pattern that developed in response to an extremely high pressure system over Greenland and the North Atlantic for this time of year. This blocking pattern caused the Jet Stream to double back on itself. Such behavior, although unusual, is not unheard of. In fact, meteorologists have a name for this atmospheric dance: the Fujiwhara Effect. What has never been observed before is the sucking of a late-season hurricane into this dance and then its subsequent transformation into a monster hybrid storm.
Would this monster hybrid storm have emerged without the effects of global climate change? I don't think we will ever be able to answer that question without some ambiguity. What I will say is that the climate system is now predisposed to favor the development of such unusual storms both this week and in the future. The coincidence of Sandy's storm surge with the full moon and its associated astronomical high tides was simply bad luck, and unfortunately, it greatly amplified the damaging effects of this $20-billion storm.
The coincidence of Sandy's landfall with the run-up to the presidential elections is something that I attribute to the candidates' bad karma for failing to discuss global climate change during their debates.