(Earth. See that massive swirl of clouds with arms stretching up into the Arctic and back across the Atlantic Ocean? Yes, that’s Sandy.)
This year was already the worst extreme weather year ever recorded. Fires, heatwaves, a monster Derecho and a devastating drought together would have made 2012 one for the record books. The one saving grace, it seemed, was that hurricane season hadn’t significantly added to an already severe problem. That was before Hurricane Sandy slammed into the US Northeast causing what many think will be in the range of 10-20 billion dollars in damage. If total damage estimates exceed 20 billion, Sandy will be one of the five most costly hurricanes in US history.
Sandy was nothing if not unprecedented. Never has the Northeast seen this kind of storm so late in the season. Never has New York and New Jersey been subject to such a high level of ocean flooding over such a broad area. According to CNN’s chief meteorologist: “There’s no one that’s not 300 years old that has seen anything like this.” That’s just a finer way of saying that there is no record for a storm like Sandy ever occurring in this region of the country. And, in many cases, there’s no record for a storm like Sandy occurring period.
What made Sandy so unique? In two words: climate change. We’ve seen northeastern Atlantic Ocean storms where powerful troughs combine with hurricanes in ways that create a much stronger storm. The last time such a thing happened was during the 1991 ‘Perfect Storm.’ But that storm formed over the open waters of the Atlantic and only caused damage as it brushed New England with the powerful squall lines and heavy surf it cast off. In the case of Sandy, the Perfect Storm came ashore far further south and west than is usually possible.
Sandy’s Global Warming Ingredients
Since 1991, atmospheric changes and alterations to the Earth’s physical characteristics have been taking place that make storms like Sandy more and more possible. These ‘ingredients’ include increasing ocean temperatures, changes in the jet stream, and the receding boundary of Arctic Sea Ice.
To understand how these changes made it possible for a storm like Sandy to have such a devastating effect on the US Northeast and Mid-Atlantic so late in the season, it helps to follow the life of the storm that became Sandy…
Like so many other hurricanes, Sandy was born of the tropical Atlantic. She started as a pulse of thunderous rain storms swirling off the coast of Africa. This tropical wave slowly gathered energy from the hot tropical Atlantic as she moved west, gradually twisting into the classic coma shape as she entered the central Caribbean.
(GOES weather satellite Image of Sandy from October 22. Sandy is already large for a tropical system. But Sandy will soon grow even larger by combining with other storms to the north.)
Ocean heat content for the South Atlantic and Caribbean was abnormally high this year. Most of this added heat content came from human caused global warming. In many regions, temperatures were 2-3 degrees above average. This meant that, for a storm like Sandy, these waters were about as warm as they would have been two to three weeks earlier during a typical season of the 20th century. This added energy increased the likelihood that the storm would form in the first place. It also gave the storm more capacity to strengthen even in an environment of increased wind shear.
As Sandy tracked northward, she plowed through Jamaica and hopped over the eastern tip of Cuba. Maintaining significant strength as a category two storm, Sandy grew to a large size, boasting a tropical storm wind field in excess of five hundred miles in diameter. Hovering off the coast of Florida, Sandy was about to enter the second stage of her development.
Two systems to the north would play key roles in Sandy’s growth and path. Both were products of new ‘blocking patterns’ that have emerged as regular weather events during the past decade. ‘Blocking patterns’ occur when the jet stream makes deep swoops down from the Arctic and into the mid and lower latitudes. These swoops make giant wave-like patterns in the jet stream. They also create a huge amount of atmospheric inertia. The result is that weather patterns tend to be more persistent. In the under-belly of a blocking pattern, one can expect abnormally hot and dry conditions to persist over long periods of time. In the frontal down-slope of the blocking pattern, one can expect abnormally cool, wet, and stormy conditions. The peaks of these blocking patterns tap the tropics and the troughs tap the Arctic.
According to Dr. Jennifer Francis, these blocking patterns have emerged as a result of sea ice loss in the Arctic. The receding edge of the sea ice pulls air northward changing the shape of the jet stream from that of a rippling halo to that of a circle of sine waves.
The new blocking pattern that had established itself over the central US allowed a powerful cold front to sweep southward, both lending energy to Sandy via strong temperature and pressure gradients and steering Sandy first northward, then pulling her in toward the Mid-Atlantic coast. A second aspect of the blocking pattern emerged in the form of a new high pressure system that has tended to form recently over Greenland. This particular high pressure system blocked the path of Sandy northeastward, shoving Sandy back up against the frontal trough that ended up lending her so much strength.
(A visible satellite shot of Sandy beginning to combine with a powerful Arctic cold front. The massive trough of cold air is outlined in blue. Sandy is in the red circle. To the northeast is a blocking high backing in over Newfoundland. Note the extraordinary size of the combined trough and Sandy.)
As Sandy began to touch the trough’s strong, cooler winds, her tropical storm wind field spread out, eventually reaching 900 miles in diameter. In addition, Sandy found herself cloaked in the trough’s rain shield. This shield helped to prevent the worst effects of wind shear which, at times, was powerful enough to rip a normal storm apart.
Sandy’s encounter with the Arctic-born cold-air trough caused her to explode in size and as she moved north, she pummeled the Outer Banks of North Carolina and Coastal Virginia from 300 miles off shore. What strength she lost at her core was multiplied manifold in the expanding reach of her effect. North Carolina and Virginia coasts experienced impacts usually reserved for those in the direct path of a Hurricane — powerful winds, heavy rains, and storm surge flooding. Roads were washed out, dunes were breached, homes were flooded. Water rises exceeded seven feet in some places.
(Sandy taps hotter than normal Atlantic Ocean water in final rush to the coast. At this point, Sandy is the largest tropical cyclone ever recorded in the Atlantic Ocean.)
Yet Sandy was still hundreds of miles away, biding her time for the final rush to shore. And in this critical time period, global warming again played its hand. Sandy was now moving parallel to the Virginia coast. In normal years, water temperatures would begin to drop off here, sucking energy from the storm. This year, though, water temperatures had heated to 5 degrees Fahrenheit above normal through, the year after year, heat trapping effects of human emitted greenhouse gasses. Sandy drank deep from this added heat and, as the Arctic-born trough began to pull Sandy in to shore, she intensified.
Maximum sustained winds reached 90 mph, tropical storm force wind diameter reached 1000 miles, hurricane force wind diameter reached 200 miles, and the pressure fell to an unprecedented 940 millibars. Sandy was now a storm for the record books. A storm that was the largest tropical cyclone ever to form in the Atlantic. A storm never seen before in this region of the world. A storm powerful enough to push ocean water nearly a mile inland up and down the Jersey coast. A storm mighty enough to create a nearly 14 foot water rise in New York City.
Without climate change, the storm may not have formed in the first place, the storm probably wouldn’t have reached category 2 strength or grown to such a large size, the storm would have not combined with such a powerful trough sweeping so far south, the storm would have not been blocked from going out to sea by the new Greenland/Newfoundland high pressure, the storm would not have strengthened so far north over abnormally hot waters, and the storm would have not been pulled into the coastline by the powerful blocking pattern caused by melting sea ice.
Sandy was, in all ways, the storm that climate change wrought. And since the pattern is now established for this kind of storm to happen now, it is likely that this kind of ‘300 year storm’ will happen again. Almost certainly with growing force and almost certainly within the next decade or two.
I’ll leave you with the following quote from Time Magazine:
“Perhaps, if you are in your 60s or 70s or 80s, Sandy’s destructive forces are a once in your lifetime event. But younger generations—those of us in our fifties, and our children—will likely be looking at flooded coastal cities, devastated infrastructure, blownout power, and storm surges for the rest of our lives.”
(Graffiti scrawled on the side of a house flooded by Sandy. Image credit: here.)