We start with the familiar look,
<<<left<< January 15 to February 15 2013 to 18 temperatures at 600 mb. Right >>>>>>>>>
same pressure level and period for 1980-1985.
NOAA daily composites dedicated 238-245 Kelvin (-28 to -35 C ), the left animation depicts latest years of great ice melts, to the right the pre great melt look. What strikes as significant is the lesser cooling influence of the Arctic Ocean during the greater melt years. The Vortex appeared always elongated or much warmer over this Ocean, as opposed to 1980-85 having a mostly much colder Arctic Ocean atmosphere. We note also very cold vortices above the continents occurring more prominently during the 2013-2018 years as opposed to vortices over East Siberian , North Greenland and Barents seas , now a days a very unlikely event given more extensive open water. Deep cold vortices starting over land can be explained by the specific heat capacity characteristics of land varying a lot depending on snow coverage along with warm cyclonic intrusions circumnavigating vortices perimeters, being largely outside of them (more clouds covering the Polar oceans). .
Logically we would expect winters to start earlier and colder during the 1980 to 1985 period.
In fact winters started colder more often, 4 times out of 5 on top of the Arctic Ocean during November 15 to December 7 1980-85 (left) , as opposed to starting mainly above the continents 5 out of 5 times during 2013-2018 (center). I observed the latter frequently for a long time, was often puzzled by this until making the simple connection, the Arctic Ocean emitted more warmth during these latest years, winters can only really start over land when sea ice is thinner along with the greater presence of warmed sea waters. WD February 15 2019.