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Posted at 11:00 AM ET, 12/ 9/2008

What's Causing Snow to Decline Locally?

By Jason Samenow

The answer may lie in Atlantic weather pattern

* Rain & Warmth Tomorrow: Full Forecast | Snow Lover's Crystal Ball *

The long-term trend in snowfall locally is decidedly downward, as I demonstrated in our Winter Outlook. I speculated the cause might be some combination of:

*the urban heat island effect -- where heat from buildings/asphalt could result in less snow sticking
*climate warming -- which might cause a greater fraction of precipitation to fall as rain compared to snow (I'll try to analyze this in a subsequent post)
*long-term changes in certain weather patterns -- which I'll focus on in this post; specifically, the North Atlantic Oscillation (NAO) pattern.

Two weeks ago, I more or less ruled out observation bias (changing measurement techniques and station locations) as a significant contributor to the trend -- at least in recent decades.

So let's turn to the NAO.

Keep reading to learn about the NAO pattern and how it may be influencing seasonal snowfall...

The North Atlantic Oscillation (NAO) is considered to be a major driver of our winter snowfall. It is measured by the difference in pressure between Iceland and the Azores. When this index is negative it is often indicated by a block of high pressure over Greenland. When this block is strong and well-placed, it tends to lock the cold air in place over the East Coast, which is critical for snow in our region.

On the other hand, when the NAO is positive, the counterclockwise circulation resulting from strong low pressure over Iceland generates mild west-to-east flow over the eastern U.S. As such, the cold air necessary for snow tends to be in short supply during the NAO positive phase.

30-Year Running Averages of Snow (at Reagan National, DCA; Dulles, IAD; and Baltimore-Washington International, BWI airports) and the NAO. The dashed gray lines depict linear trends for the four time series.

If we analyze the long-term trends in the winter (averaged over Dec-Jan-Feb) phase of the NAO, we see a very strong trend towards the positive (warm) phase (bad for snow). NAO data is available back to 1951, so we can track moving 30-year averages since 1980. The graph above shows the steep upward climb in NAO index (see the turquoise curve) values over the period. Examining local snowfall trends on the same graph suggests a pretty strong negative correlation with the NAO. When the NAO rises sharply, snowfall seems to steadily decline. And during short intervals when the NAO appears to level off, so does the snow.

Given the data, it's hard not to at least partially implicate the NAO in our declining snow trends. Interestingly, the NAO increase has also been linked to the rapid decline in late summer Arctic sea ice. All of this begs the question: What's causing the NAO to rise? Some studies suggest the index's rise is connected to human-caused global warming, whereas others believe the trend is largely unrelated to global warming. Snow lovers (and polar bears?) should hope it's the latter...

By Jason Samenow  | December 9, 2008; 11:00 AM ET
Categories:  Local Climate  
Save & Share:  Send E-mail   Facebook   Twitter   Digg   Yahoo Buzz   StumbleUpon   Technorati   Google Buzz   Previous: Coastal Storm Threat Is End-of-Week Wildcard
Next: PM Update: Warm & Wet Wednesday. Then What?


I think you have hit the nail on the head. The NAO is probably the single biggest guarantor of cold in the east, particularly the Mid-Atlantic. Places like New England are often cold enough "on their own" for snow to fall, but in the Mid-Atlantic, particularly south of 40N, you really need some help - and a negative NAO provides it in two ways - first by buckling the jet stream south as you mentioned, and second by "blocking," thereby keeping cold air in place.

Without the neg NAO cold shots tend to be transient affairs, that slide off the coast in 1-3 days. And, more notably for snow lovers, big storms tend to push the cold highs offshore, if there is nothing there to block them in. I am convinced it is extremely rare to get a big storm without a negative NAO. Overrunning or CAD events, sure, but big storms are rare without blocking of some kind.

Interestingly, the NAO can also be "too negative" - as the 1978 through 1988 period on your graph appears to show (the NAO is going up, and so are snow amounts). A hugely negative NAO shoves storms way far south, resulting in cold and dry conditions in this area.

Posted by: jahutch | December 9, 2008 1:46 PM | Report abuse

I think you are correct when you suggest the NAO may play a deciding role in the D.C. weather.

There is a new study that was just published in Climate Dynamics which suggests a much greater role in our weather, by the planet's oceans, than previously thought. Truly fascinating stuff.

The study was conducted by Gilbert Compo and Prashant Sardeshmukh of the Climate Diagnostics Center, Cooperative Institute for Research in Environmental Sciences and Physical Sciences Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration. They were funded by the NOAA Climate Program Office. A big hat tip to World Climate Report.

Now for the really juicy part!! Check out the paper's abstract. Please buckle your seat belts, stow your seat tray, and make sure your seat back is in the full upright position.
--begin quote--
Abstract Evidence is presented that the recent worldwide land warming has occurred largely in response to a worldwide warming of the oceans rather than as a direct response to increasing greenhouse gases (GHGs) over land. Atmospheric model simulations of the last half-century with prescribed observed ocean temperature changes, but without prescribed GHG changes, account for most of the land warming. The oceanic influence has occurred through hydrodynamic-radiative teleconnections, primarily by moistening and warming the air over land and increasing the downward longwave radiation at the surface. The oceans may themselves have warmed from a combination of natural and anthropogenic influences.
--end quote--

Wow. I can't read to digest the whole paper. But it will have to wait a few days. :(

The full paper can be found here.

Mr. Q.

Posted by: Mr_Q | December 9, 2008 3:12 PM | Report abuse

Re Compo/Sardeshmukh:

"This paper does not dispute that man-made greenhouse gases are causing the climate to change. However, it does show that the mechanisms of land warming may be different than commonly believed,” said study co-author Prashant Sardeshmukh. . .

As far as what’s driving sea surface temperatures to increase, both Sardeshmukh and Compo suspect that greenhouse gas emissions are largely to blame, though some natural variability may also be at work."

Source: Ocean May Be Best Predictor Of Future Land Warming

Posted by: CapitalClmate | December 9, 2008 4:39 PM | Report abuse

hey jason here is the other side of reporting 'warning' not al gore nonsense *if you were fair and blanced, this postive news should have been posted.

Posted by: deveinmadisonva | December 9, 2008 6:48 PM | Report abuse

CapitalClmate wrote, "This paper does not dispute that man-made greenhouse gases are causing the climate to change."

Sadly, I must admit that I expected that unfortunate sentence. It is part of the current political reality. Anyone who doesn't play along gets ostracized. It would seem that the golden rule in the climate science research community is "Thou shalt not say anything that might upset the global warming governmental research grant gravy train."

When scientists feel comfortable enough that the preponderance of evidence is such that avoiding the glaring shortcomings in the current man made global warming theory as put forth by Dr. Hansen and others is no longer possible, nor required, and they cease adding that cya catch-all sentence, then you will know that the tide has truly turned.

I give it five years at the outside. And I sooooooooooooooooooooo look forward to that day.

There is no catastrophic man made global warming.

Mr. Q.

Posted by: Mr_Q | December 10, 2008 2:09 AM | Report abuse

Hey guys, if I'm trying to calculate the NAO, do I just subtract the barometric pressure in Lisbon from the reading in Reykjavik? If it's 30.10 in Lisbon and 29.50 in Reykjavik, does that mean the NAO is 0.60?

Posted by: lioninzion | December 10, 2008 12:33 PM | Report abuse

No, the NAO calculation is not that simple:

"The procedure used to identify the Northern Hemisphere teleconnection patterns and indices is the Rotated Principal Component Analysis --RPCA (Barnston and Livezey 1987, Mon. Wea. Rev., 115, 1083-1126). This procedure isolates the primary teleconnection patterns for all months and allows time series of the patterns to be constructed. For our monitoring purposes, we apply the RPCA technique to monthly mean standardized 500-mb height anomalies obtained from the CDAS in the analysis region 20°N-90°N between January 1950 and December 2000. The anomalies are standardized by the 1950-2000 base period monthly means and standard deviations."

(Don't try this at home.)

Posted by: CapitalClmate | December 10, 2008 5:50 PM | Report abuse

CapitalClmate: Thanks for responding to lioninzion:) Hope all is well.

Posted by: Jason-CapitalWeatherGang | December 10, 2008 9:39 PM | Report abuse

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