Fig: 1: Meteorological winter (DEC/JAN/FEB) 500 MB anomalies for the northern hemisphere during the past 10 years.
Discussion: The 500 MB anomalies in the northern hemisphere during meteorological winter of the past 10 years have, on average, produced blocking high pressure (negative arctic oscillation) in the polar region. The persistent high-pressure ridging is forced by warming of the high latitude atmosphere over higher latitude ice-free open water. Typical of -AO patterns, the polar vortex is forced away from the polar region. The favored location during the past 10 years for separation of the polar vortex from northern latitude home base is North America. The tendency of southward pulses of polar vortex events into North America (vs. Eurasia) is related to the semi-permanent upper-level high-pressure ridge areas located over vast warmer than normal ocean regions (referred to as “warm blobs”) in the Northeast Pacific and western North Atlantic. The persistent upper-level ridge areas in the Northeast Pacific and western North Atlantic force an in-between weakness in the upper-level flow patterns inviting the polar vortex southward into North America.
A large area of persistent upper-level low-pressure south-southeast of Greenland is also caused by the tendency for increasing high latitude warming. The Greenland ice sheet melt has accelerated in recent years and the runoff has created a semi-permanent region of cool water south of Greenland surrounded by the anomalous warmer ocean of recent decades. This feature is named the North Atlantic warm hole (NAWH). The cool region of water has chilled the atmosphere aloft forcing the semi-permanent low-pressure trough. The NAWH-induced trough has caused a downstream compensating upper-level ridge across Europe where warm winters have been common during the past 10 years.
Note that southward shifting polar vortex events created in this short-term 10-year-old optimum climate normal (OCN), engage a warmer atmosphere holding more moisture than in previous decades. Consequently, storms produced by polar vortex events cause more precipitation (which is frequently extreme). This just-finished polar vortex event into the U.S. was made frigid due to cross-polar arctic air from Siberia.
Note that in the absence of the polar vortex episodes, the climate returns to a warmer than normal regime. This set of observations are classic examples of the modern-day climate.