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12/29/2015 — Rare December Melt at the North Pole
Posted by Sean Birkel

The GFS forecast for the past couple days has been showing a large meander developing in the jetstream that will funnel warm air across the Nordic Seas and into Siberia (see below). Along the way this plume of mild air could bring temperatures above freezing to the North Pole. That seems pretty wild for this time of year, but in line with the notable, and in some places record-setting, December warm wave across the eastern U.S. and Canada. Questions come to mind. Is the recent North American warm wave related to the warm conditions developing over the high latitude North Atlantic? How rare is it for melting at the North Pole in late December? Are these events linked to this year's strong El Niño?

The answer to the first question is "yes" – the two warm waves are linked through the circulation of westerly winds, or the jetstream. The first event developed from high pressure in the western Atlantic that formed a blocking pattern, pulling heat and moisture out of the Gulf of Mexico and into the eastern half of the continent. This warm-air ridge was further enhanced as a strong low pressure developed in association with an upstream trough.

Warmth climaxed over Eastern North America on Dec. 24th with temperatures approaching 70°F as far north as Quebec City, and the melt/freeze line pushing north to the Ungava Peninsula to the northeast of Hudson’s Bay.

Steep longitudinal temperature gradients cannot be maintained for long in the atmosphere. Thus, the December warm wave finally broke as the jet stream steered the mid-continent storm system along a northerly track, ushering in cold air and a powerful front. Particularly severe impacts from this storm, ranging from flooding to tornadoes, were felt variously from Texas to Illinois. The system has since passed over the Great Lakes, northern Quebec, and onto Greenland where it is now coalesced with storms of the Icelandic Low at the edge of the polar cell. [As of Dec. 30th this system is bringing severe weather and flooding to the British Isles in what is being called Storm Frank]. Coincident with the intensification of the low near Iceland, a blocking high has formed over Scandinavia. The combined effect of circulation around these low (counter-clockwise) and high (clockwise) pressure centers is driving the present warm wave over the high latitude North Atlantic. This is illustrated below in the animated sequence of daily average meteorological fields for Dec. 23-29. Also refer to the figure at the top of this page.

Is it rare for temperature to get above freezing at the North Pole in late December? Met station data collected on the sea ice at or near the North Pole is sparse and discontinuous. Therefore, reanalysis may be the most useful tool for answering this question. Reanalysis is a numerical weather forecasting framework that is run using ingests from historical data records (typically station data, radiosonde, and satellite). I have ready access to NCEP CFSR/CFSV2, a reanalysis spanning 1979-present with outputs on a 0.5°x0.5° grid in 3-hour increments. The plot below shows maximum daily temperature for the gridcell including 90°N for all days within December and January for the reanalysis period. CFSR/CFSV2 shows only three days, two in January, 2006 and one in December 2007, in which temperature at this time of year has exceeded 0°C. Six other events register values greater than -1°C, but below the melting threshold. A different reanalysis (e.g., JRA-55, extending to 1958) might show more melt events, or maybe none at all. For now, based on the the CFSR/CFSV2 model output on hand, mid-winter melt at the North Pole seems to be a rare occurrence. [Edited 12/30/2015 to include JRA-55 timeseries on figure below].

Are these warm waves linked to El Niño? There is not a clear answer to this question. El Niño is a tropical phenomenon that has greatest direct impact in the low latitudes of the Pacific Basin (learn more about El Niño). Extratropical impacts are less clear, particularly far field where Atlantic influences can dominate. Sea surface temperatures across the equatorial Pacific are right now warmer than usual, as shown below, and this boundary condition is feeding into circulation of the overlying atmosphere.

But El Niño is probably not a requisite for an unusual meander to develop in the jetstream over the North Atlantic during winter. Case in point, the maps below show sea-surface and 2-meter air temperature departures for the January 20th, 2006 North Pole melt event registered in CFSR - the equatorial Pacific is cooler than normal. More important is the state of the climate. The climate is warmer now than it was a century ago, and the Arctic in particular was demonstrably colder on average every season of the year (see for example this NOAA article). Mid-winter melting at the North Pole probably could not happen with climate boundary conditions of a century ago. All considered, the current high latitude warm wave is a rare event worthy of contemplation.