What's Causing Snow to Decline Locally?
The answer may lie in Atlantic weather pattern
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.
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...
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