One of the theories of how snowstorms can be made worse is that warming oceans provide more moisture for fuel.
While the theory sounds logical and even attractive, there are many ingredients that go into snowstorm formation. There has to be a synoptic scale disturbance feeding off the temperature contrast between the land and ocean, and since the land-ocean temperature contrast has actually DECREASED in the last several decades (if you believe the thermometer data), this would reduce the energy available for storm formation. (The “more-wavy” jet stream theory is highly suspect…without a greater temperature contrast, there is not as much thermal energy available for “baroclinic instability”).
Nevertheless, there do seem to have been more snowstorms in the Northeast U.S. in the last decade, so what might be the cause? As a meteorologist, my first inclination is to blame, in effect, “chaos”. Weather and climate variations are chaotic, there ARE weather patterns that can get set up and then persist. But these regional influences are basically disconnected from whether the global average temperature happens to be 1 deg. warmer or cooler. They are instead being driven by temperature contrasts of many tens of degrees.
But we can examine with observational data Kevin Trenberth’s hypothesis that increased Northeast snowstorms are the result of more water vapor from the North Atlantic.
For the last 27 years we have had the SSM/I and SSMIS instruments monitoring total water vapor content over the oceans every day. I took the Northwest Atlantic area from 30N to 50N, and 50W to 80W and examined the monthly average water vapor over this area versus the NESIS (Northeast Snowfall Impact Scale) index.
The results suggest basically no relationship between available water vapor and snowstorm events over the last 27 years:
In fact, while warm season water vapor has increased, cold season water vapor (if anything) has decreased on average over the region, making less vapor available for storms. The net trend through all seasons is about +0.5% per decade over the 27 year period.
There is always abundant water vapor available for U.S. snowstorms to feed off of, just as there is always abundant tropical water vapor available for hurricanes and typhoons. But that’s not the limiting factor in storm formation. What is necessary is the variety of conditions which can support the formation of low pressure centers….sufficient water vapor is usually ready and waiting to play its part.
It has more to do with the necessary temperature contrast between air masses (and in the case of tropical cyclones, vertical wind shear). And since global warming (no matter the cause) will lead to the continents warming faster than the ocean (reducing the energy for incipient storms), there is no convincing way to blame global warming for increasing snowstorm activity in the Northeast U.S.
This article appeared on Dr. Spencer’s weblog at http://www.drroyspencer.com/