In this article, we examine the science behind the superstorm, including its very unconventional evolution, results of supercomputer simulations, and a recap of the storm’s diverse and far-reaching impacts.
How a Hurricane Morphed Into a Very Dangerous, Hybrid Storm
Sandy can ultimately be traced to an African easterly wave, a pressure disturbance embedded in the benign trade winds. This wave intensified into a tropical depression on Oct. 22, 2012. Over the next two days, the depression became Tropical Storm Sandy (winds > 39 mph), then Hurricane Sandy (winds > 74 mph). These changes took place in the Caribbean, south of Cuba.
The hurricane attained strong Category 2 status just before crossing Cuba, late on Oct. 24. The extensive mountains along Cuba’s backbone knocked the wind from Sandy’s proverbial sails, weakening it to Category 1.
However, once north of Cuba, the tropical system rebounded, but in a very unconventional manner. The tropical vortex combined forces with an upper atmospheric trough (area of low pressure) originating over the mid-latitudes. This interaction ventilated air from within Sandy’s core, dropping the surface pressure. Cold air in the trough wrapped around the system, enhancing the temperature gradient across the storm. The storm received a boost from this interaction and began expanding.
Mid-latitude cyclones derive their sustenance from upper-level troughs, and the interaction marked the transition of purely-tropical Sandy into a large, hybrid type of storm.
Essentially, the trough kept Sandy’s engine going in the midst of wind shear (twisting winds that try to tear storms apart) and declining ocean temperatures.
The figure illustrates two snaphots in the lifecycle of Sandy: The purely tropical cyclone south of Cuba, and the massive hybrid cyclone in the North Atlantic arcing westward toward New Jersey.
Notice in this figure the enormous inflation of the vortex. Nine tropical Sandys could fit comfortably inside large hybrid Sandy. Turquoise shading denotes the threshold windspeed value for a tropical storm. When a generic hurricane weakens, the wind field typically expands by 20-30%. However, Sandy’s inflation was a dramatic consequence of its extratropical transition.
This transition tapped new mid-latitude sources of energy, including the upper-level closed low pressure area over the mid-Atlantic, a surge of cold air wrapping into the storm, and the creation of numerous weather fronts.
The storm shown in the figure off the New Jersey coast is, by all appearances, a massive Nor’easter, fully coupled to the jet stream and sporting an intense cold and warm front.
However, within the very center of the storm, there remained a deep core of warm air, a tropical eye and eyewall, intense thunderstorms, and raging 75 mph sustained winds. In other words…it appeared as if a hurricane was sequestered within the cloak of a Nor’easter.
A Supercomputer Simulation Provides Valuable Insights About Sandy’s Transition
Dr. Thomas Galarneau at the National Center for Atmospheric Research (NCAR) and colleagues recently authored a highly technical paper describing the complex meteorological evolution of Sandy, based on supercomputer simulations.
To say that Sandy was a “hurricane wrapped in a nor’easter” is not quite correct. Nor’easters are cold-core vortices, while tropical cyclones contain warm air in their core. Sandy was a special type of storm, one rarely observed, in which cold air wraps around an intact, tropical warm core, effectively secluding it.
The usual pathway to such a “warm seclusion” occurs in ordinary Nor’easters, when cold air pushes so vigorously into the center, a piece of ocean-warmed air gets trapped inside. But it’s much more unusual for the seclusion process to unfold in a pre-existing, warm-core tropical system, such as hurricane.
The computer simulation of Sandy revealed three unique facets about the storm:
(1) Upper-level air vented away from the top of Sandy in a highly asymmetric manner, streaming toward the Midwest in a great wave-like flow (99% of hurricanes along the Eastern Seaboard direct their main outflow toward the north or northeast);
(2) A massive surge of cool air wrapped around Sandy’s tropical core, streaming off North America. This helped generate hurricane-force winds on the southwest flank, at a great distance from the inner eyewall (labeled “secondary wind maximum” in figure above). The cool air surge helped the storm maintain its strength, and perhaps even intensified Sandy further. This is because the cool invasion sharpened the temperature gradient across the storm;
(3) Temperature gradients sharpened along the ocean’s Gulf Stream, which lay off the coast, west of the storm center. The temperature contrast created a weather front, along which air began spinning in a long, narrow tube oriented along the Gulf Stream’s north-south axis (think of cold air nosing below warm air, warm air rising above cold air – the exchange creates a circulation). The spin began in the horizontal, then giant “loops” of this spin were lifted vertically by winds rushing inward, then up into, the storm. This unusual process added additional rotation to the main vortex, re-invigorating Sandy.
Widespread Impacts Revealed: Wind, Rain, Surge, Heat, Cold, Snow
For this epic weathermaker, it seems that the only natural disaster lacking was a plague of locusts.
The figure below highlights myriad weather and water impacts of Sandy during and after landfall. Sandy first and foremost created a deadly storm surge from Tidewater, VA to Nova Scotia. Total storm tide (the combination of surge and high tide) reached 14 feet in New York City.
As the bottom panel shows, surge heights were greatest to the right of Sandy’s westward track, where easterly winds pushed a huge wall of water into the coast. The length of inundated coastline in the Northeast is truly unprecedented. Note how the surge became amplified in the narrow sound between Long Island and the mainland.
Note also the growth of storm waves on the Great Lakes. Strong autumn cyclones called “Witches of November” typically raise big waves on the Lakes, but these storms always approach from the west. Sandy, with its enormous wind field, created waves up to 23 feet along the southern shores of the Lakes, as it approached from the east.
Heavy inland snow was succinctly described in this recent article by CWG’s Kevin Ambrose. But a major factor was the circulation of unseasonably cold air south and east, drawn into Sandy’s vortex. This is shown in Panel C of the above figure. In fact, Sandy’s spiral inflow inverted the normal temperature pattern for days…creating unseasonably cold temperatures south of the storm, and near-record high temperatures to the north across New England.
Peak wind gusts are portrayed in Panel A. The highest winds occurred right of the track, where the storm’s inland motion added to the direction of the storm’s swirling wind (blowing from the east). Gusts to 80 mph and higher were recorded along the New Jersey coastline and over Long Island.
Pockets of high wind were also located far from the storm track at high elevations. For instance, note the 60-70 mph gusts over the Blue Ridge of central Virginia, and over the Appalachians across New Hampshire, Vermont and Maine.
Concluding Remarks: New Insights Gained From Sandy, Still Much To Learn
Superstorm Sandy challenged meteorological wisdom worldwide with its unusual transition to a hybrid storm, its enormous size and highly unusual movement. Some important details have emerged from mathematical simulations. Other processes may never be completely understood, remaining the province of informed hypotheses.
One of the more important lessons is the notion that when it comes to storm surge, storm size is just as important as strength of the sustained winds. Big storms such as Isabel (2003) and Ike (2009) have demonstrated very large “integrated kinetic energy” (IKE) – basically, their ability to mobilize massive amounts of seawater out of proportion to their wind intensity alone.
As demonstrated by CWG’s Brian McNoldy, of all the hurricanes tallied over several decades, Sandy sits near the very top of the IKE curve, even though it was only a Category 1 off the Eastern Seaboard.
Storm surge is the number one tropical cyclone killer worldwide, amassing the largest death tolls of any natural disaster in the 20th century. Superstorms like Sandy, while malevolent, offer important clues to the coupling between atmosphere and ocean in ways that still catch meteorologists by surprise.
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