Wednesday, February 1, 2012

It's Getting Hot in Here- Explanations for a Warm Winter

I got the idea to write this piece today while sitting outside in a t-shirt. It was 65 degrees and sunny, a beautiful mid-April day for New York City. The only problem is, it’s not April; it’s only February 1st.

As I’m sure a lot of you on the East Coast have noticed, it’s been an unusually mild winter, especially when compared to the Snowmageddons of 2010 and 2011. Just how different has it been? Consider this: in January 2011, it snowed 36” at Central Park. This year? The officially tally was less than one-tenth as much, at 3.23”. Obviously it must be global warming, right?

No, it’s not that easy- one warm winter does not make global warming. But thanks to research spurred in part by the massive snowstorms of the past few winters, climate scientists have begun to understand what drives this winter weather weirdness. One mechanism that shares most of the blame for these events is something called the “Arctic Oscillation”.

Figure 1. Characteristics of a negative (top) and positive (bottom) Arctic Oscillation. Gridded data are temperature anomalies (current temperature minus average temperature for the period 1951-2000). Data from the NASA Goddard Institute for Space Studies (GISS) Surface Temperature Analysis.

Before you get lost in fancy terms, let’s take a step back. Climatologists define indices (like El Nino or the Arctic Oscillation) that relate to phases of pressure in the atmosphere. You might have noticed that when our area is under high pressure, the weather tends to be nice; when it is under low pressure, the weather tends to be stormy.

When you take a shower, water comes out of your showerhead because the water inside your shower pipes is at higher pressure than outside. Air acts the same way; air moves from areas of high pressure to areas of low pressure. And much like you couldn’t force water to go back inside your showerhead, you can’t force air from an area of low pressure to go to an area of high pressure.

Why does this matter? In simple terms, the Arctic Oscillation dictates where cold air will end up in the Northern Hemisphere. When there is a negative Arctic Oscillation, atmospheric pressures are high over the Arctic and low outside the Arctic. The end result is that the high pressure forces cold air out of the Arctic, toward lower latitudes, like New York. This is exactly what happened in the winters of 2010 and 2011, leading to the massive snow events in DC and NYC. Consequently, the Arctic ends up warmer than usual (Figure 1).

When there is a positive Arctic Oscillation, the opposite is true: atmospheric pressures are low over the Arctic and high outside the Arctic. The high pressures outside the Arctic trap cold air, leading to warmer temperatures in areas like New York. As you can probably guess, the Arctic Oscillation has been strongly positive this winter, and temperatures have accordingly been quite warm (Figure 2).

Figure 2. Average January High Temperatures from Central Park, NY (top, blue) and Reagan National Airport, Washington DC (bottom cyan) against monthly Arctic Oscillation Index over the last 5 years. Colder January temperatures are associated with - Arctic Oscillation indices and warmer January temperatures with positive. Note that the average high temperature this January is nearly 10 degrees warmer than last January in both DC and NY! Arctic Oscillation data from Climate Prediction Center; Temperature data from National Weather Service.

It is important to note that you can’t just get rid cold or warm air masses; you can only move them around. That’s exactly what the Arctic Oscillation does- it determines whether cold air ends up far up in the North Pole or in our backyards. And just because it has been warm here, doesn’t mean it has been warm everywhere- Sarah Palin has been freezing her a$$ off in Alaska, which has seen some of its coldest temperatures and largest snowfalls on record. Europe and Asia have also seen above-average snowfall this winter (see below).

For those of you who are missing the snow this winter, I have some bad news: unfortunately, there is a connection between global warming and the Arctic Oscillation. Continued melting of Arctic sea ice, driven by warming at high latitudes, leads to a generally warmer Arctic Ocean. A warmer Arctic Ocean in turn reduces atmospheric pressures over the Arctic, leading to a positive Arctic Oscillation mode. This would tend to favor less and less snowfall in the US. Long story short- if you enjoy snow, you might need to head farther and farther north in the coming decades.

A final good link on the Arctic Oscillation: NOAA ClimateWatch

(Note from above- The North Atlantic Oscillation, a similar feature to the Arctic Oscillation, plays a role in determining how warm the U.S. is relative to Europe. More on this in a later entry)