By: Chief Meteorologist Eric FisherBy Eric Fisher


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We’ve been talking a lot about snow lately. Or rather, the lack thereof. Which in it of itself may be surprising to anyone who’s lived through the past couple decades in the Boston area. It’s been noted that we have been in the ‘golden age of the snow lover’ around here with multiple major snowstorms, and at times even record snowfall. The snowiest month on record and snowiest season on record in 2015. Several Top 10 all-time snowstorms in Boston. Even in warm winters we’ve been having major snowstorms. So is this all chance, or part of something larger? And is this year the start of something different?

While looking for patterns, I came across one that is quite striking. And it has to do with something called the AMO – the Atlantic Multidecadal Oscillation. The AMO is, as it suggests, a pattern that naturally occurs over intervals of several decades. In particular it takes a peek at the North Atlantic and how sea surface temperatures change there over time. There is a +AMO phase (warm) and a -AMO phase (cold). Since the mid 1990s, we’ve been in a warm phase.

How does the AMO affect the world’s weather? In numerous ways! In +AMO phases, there’s an increase in the frequency of Atlantic hurricanes. This phase has also been linked to the frequency of drought in the Midwestern and Southeastern U.S. (more instances of severe drought during a +AMO).

Why chat about this now? Because we’re about to embark into a new era. After nearly 25 years of +AMO, we are beginning a period of -AMO. The cold phase can produce frequent drought to the Sahel region of Africa and also colder overall temperatures for the UK. It has also been linked to higher sea level along the northeast coastline of the U.S., which certainly may be of much interest to coastal homeowners over the next couple of decades.

But how about snow?? Well I was thinking about how we just had the snowiest 10-year stretch on record in Boston. And we had been setting a new mark for 10-year periods for several years in a row. Don’t get me wrong, snow stats can be a little wonky and aren’t entirely accurate. But over longer periods it still is accurate enough for this exercise.

So what happens when you look at some of Boston’s snowiest stretches? You’ll see that at the top of the leaderboard we have the current run of banner winter snowfall. Now let’s take a peek at at the next cluster. You will see 1967, 1969, 1970, 1971, 1972 pop. Hmmm that’s interesting…almost identical in that there is a solid string of years in a row where we were setting records. And in this list we find yet another clustering of snowiest 10-year stretches…1900, 1901, 1902.

AMO index since the late 1800s

Do these clusters have anything in common? In fact, they do. They all occur at the tail end of a period of +AMO as we descend into a -AMO regime.

Let’s play it the other way…what is one of the least snowy periods on record in the Boston area? The 80s to early 90s. Smack dab in the middle of a -AMO phase. We likely began the descent into a new -AMO area in 2014, as the table below shows. Why might this be the case? The change in the AMO is linked to changes in the main ocean current of the Atlantic, the Atlantic overturning circulation. When the current slows, new research has shown that the AMO goes into its cold phase. Perhaps the circulation strength and/or change in ocean temperatures changes the behavior of blocking and nor’easters for our area.

Updated table of the AMO index in recent years. Source: https://tropical.colostate.edu/colorado-state-university-amo/

 

Is it that easy? No, probably not. There are an incredible number of factors at work in our climate that drive changes. From ENSO to other cycles like the PDO, AMO, etc. And of course, global warming in general is having an impact. We may simply be getting bigger snowfalls and storms due to stronger nor’easters, more heat content in the ocean, more blocking patterns in the jet stream, or a mixture of all the above. Each year will still be different and you can always get big storms or snowy/dry stretches.

But we do know for sure that large-scale cycles in nature occur and we can see their patterns reflected on the ground in terms of enhanced drought/flood risk, cold/warm risk, tropical activity risk, etc. The switch back to a -AMO may be a player in ending the wild streak of big winters we’ve had even as winter in general continues to get warmer here. We’ll have to look back on this post in 20 years to find out!

Eric Fisher

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