Unless your house was destroyed, Hurricane Sandy has probably more or less faded from memory. However, Sandy proved to be a major win for the meteorological community. Up to seven days before landfall, the weather model run by the European Centre correctly predicted Sandy’s extremely unusual westward turn into the coast, and the U.S. models came to the same conclusion a day or so later. (As a side note, the European model consistently performs better than the U.S. model, for a variety of technical and political reasons.)
Weather models predict the future by applying basic physical laws to an initial atmospheric state, consisting of parameters like temperature, pressure, and humidity, taken at points across the globe and at altitudes from the surface to well above where commercial aircraft fly. Think of the atmosphere as a giant mass of fluid sloshing around in a tank – it’s constantly moving in extremely complex but basically predictable ways. The more accurately you know the initial characteristics of the fluid, the more accurately you can predict the future movement.
In practice, both parts of this – the data collection and the prediction – are exceptionally difficult. To initialize the model, weather data is needed from quite literally everywhere on the planet – the temperatures and pressures in the middle of the ocean are just as important as those over a city. With the launching of a number of dedicated satellites, we’ve begun to do this reasonably well (however, those satellites are in danger).
Simulating the weather across the entire planet takes a vast amount of computing power. The operational weather models are run on dedicated super computers, and the speed of these computers plays a significant role in the maximum resolution that the model can attain; in general, higher model resolutions translate into better forecasts.
The prediction of Sandy is a remarkable achievement, especially given the fact that Sandy was basically unique in our 150 years of weather records (that’s not to say that similar storms haven’t occurred before – they’re just very unusual). Ten years ago, it is unlikely that models would have picked up on the last-minute westward turn that Sandy took before making landfall in New Jersey.
Most East Coast hurricanes are pushed out to sea by the prevailing west-to-east high altitude jet stream. In Sandy’s case, however, an anomalously strong high pressure system situated over Greenland caused the upper level winds to reverse their course and blow from southeast to northwest, directing Sandy into the coast.
In addition, unusually warm sea surface temperatures and interactions between Sandy’s circulation and an existing low pressure system caused the storm to intensify just as it made landfall. Its path through southern New Jersey produced the worst possible setup for New York City, with 80-100 mph southeast winds pushing water into Manhattan.
The other thing to note is that along the East Coast, sea levels are now about one foot higher than they were a century ago, meaning that the surge in NYC occurred on top of this already higher water. Regardless of whether or not the strength of hurricanes increases in the near future, with a steadily rising ocean, the impact of their storm surges will. Sandy is an excellent example of how climate and weather merge in a sometimes confusing fashion – this storm was not caused by climate change, but the effects of climate change certainly made it worse.
Photo courtesy of ECMWF