A blog post by Weather Underground's Bob Henson caught my eye on Thursday, as he discussed the extraordinary warmth that occurred in the Arctic basin during January. To put this in context, I calculated the 60-90°N area-average mean temperature at 925mb (about 2200' elevation) for each January since 1950, according to the NCEP/NCAR reanalysis; the results are shown below. The temperatures last month set a new record for January warmth in the reanalysis era, by a large margin.
It's interesting to see that a similar kind of outlier (for the time)
occurred in January 1977, which is the winter that we've recently
discussed as being very warm in Fairbanks. January 1977 occurred just after the "great Pacific climate shift" and the onset of what turned out to be an enduring positive phase of the PDO. One wonders if this means that the next decade or two will see a continuation of the positive PDO phase that we've observed in the past 2 years: have we just passed through another major Pacific climate shift? From an Alaska-centric viewpoint, have we shifted into a new normal of winter warmth, as happened in the decades following 1976? Let's hope not.
Looking at daily 925 mb temperatures since the beginning of 2015, the Arctic (and sub-Arctic) area has seen persistently above-normal temperatures, but since late December the area-average temperatures have been near or above previous record levels for the time of year.
On the chart below I've added the daily temperatures for the past two very strong El Niño events, 1997-98 and 1982-83, and we can see that neither of those winters produced unusually warm conditions over the Arctic, at least through January. This suggests that we shouldn't pin the blame for the recent warmth on El Niño. It has more to do with the transition to a strongly negative Arctic Oscillation at the beginning of January, which allowed cold air to spill south into the mid-latitudes and, conversely, warm air to invade the Arctic from the south. I believe there is also a close connection to the stratospheric polar vortex, which was extremely intense in November and December, but began to undergo a weakening trend in January.
The following maps show the January temperature's departure from normal beginning at the surface, moving up through the troposphere (up to 300mb) and into the stratosphere. There is a dramatic transition from extremely warm conditions in the lower-to-mid troposphere to unusually cold conditions in the stratosphere; note that I've doubled the range of the scale on the last two maps to accommodate the magnitude of the cold anomaly aloft. The cold in the stratosphere was associated with the strength of the polar vortex, which remained much greater than normal in January.