The temperature in London is forecast to be below freezing all day today, which is fairly unusual, especially as the last week-long cold snap only ended a couple of days ago. It’s not quite there yet, but we’re well on our way to a memorable winter, possibly even a Great Winter. Regular readers will be aware of my interest in the question as to whether winters like those of yore could occur today. I’m not expecting a Frost Fair (because we’re keeping the Thames warm) but am puzzled why – since 1947 and 1962-3 – we don’t nowadays see conditions similar to those which caused such events.
Anyway, right now it’s a northerly airstream that’s causing the bracing conditions. You can see it in this chart for midnight last Saturday/Sunday night from the Met Office:
I really like this particular map because it illustrates the effect of one of the great meteorological forces on the planet. Greenland is a massive chunk of ice, the only respectable ice-sheet in the northern hemisphere and the coldest thing this side of Siberia. Just nearby the North Atlantic (or Norwegian) current flows past the fjords. The current generates the largest temperature anomaly on the planet, that is, it makes the sea much warmer than anywhere else at that latitude. The contrast between cold Greenland ice and the warm sea has created the pressure difference that’s funnelling Arctic air down over Scotland and eventually here to London. Of course, the cold air over Scandinavia plays a part too.
But why is this year different?
Perhaps this map from the National Snow and Ice Data Center (NSIDC) gives a clue:
The salient feature is the ice on the east coast of Greenland, which in some places extends further out to sea than the median for the period 1979-2000, despite there generally being less ice because of global warming. Contrast this with the situation to the west of Greenland, in the Labrador sea, Hudson Bay and Canadian Archipelago, as well as that in the Bering Sea.
In meteorological terms, sea ice surely behaves more like land than open water. It’s cold, there’s little evaporation, so high pressure will tend to build. It’s as if Greenland has been moved east compared to years when there is less ice to the east and/or more to the west. Hence this week’s particular weather pattern.
Of course, all this is a result of the Atlantic Multi-decadal Oscillation (or AMO), about which I have previously written.
Historical records show an oscillation between periods when there is more ice to the west and more to the east of Greenland. But what could be driving this? Simple physics, that’s what. When the North Atlantic is warmer than usual this will cause a higher pressure difference between Greenland (which is always just as cold) and the ocean. The Greenland high will be more intense than usual, pumping air (and sea ice) down its east coast and warmer air up the west coast, as we’ve seen for the last couple of years. Hence more chance of cold weather in the UK. Who needs those supercomputers, eh?
The reason for the oscillation, I suggest, is that this weather pattern cools the North Atlantic. Of course, with global warming, this cooling phase may take longer and/or cause stronger winds round Greenland, affecting those of us in its vicinity.
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As an addendum I show the Met Office forecast for the same time (midnight yesterday morning, Sunday 5th December) from 3 days before:
Note how much less tight the isobars are between Greenland and Norway than in the actual chart (above). Could it be that the Met Office is underestimating the forces creating the pressure difference? The current forecast is for temperatures to climb slightly to a toasty 2 or 3 degrees above freezing towards the end of this week. But if that north wind is a bit stronger than expected maybe it’ll pump down more Arctic air and we’ll get more of a freeze. Watch this space.
Uncharted Territory 