Fig. 1: December 2019 temperature rankings according to NOAA/CPC across the Sea surface temperature anomaly pattern for the month according to the International Research Institute for Climate and Society.
Discussion: Remarkably, each continental zone recorded a top 8 warmest December on record including the hottest on record in Australia plus the Hawaiian Islands. Europe was second warmest on record; the U.S. was 6th warmest on record. The leading climate contributors to these observations in order of importance were: 1.) Warming of the central tropical Pacific related to the global oceanic thermohaline conveyor belt, 2. 5th smallest sea ice extent in the polar region, 3.) Strong negative phase of the southern annular mode (and developing positive phase of the northern annular mode), 4.) Lingering influence of the positive Indian Ocean dipole and 5.) Influence of the North Atlantic warm hole (especially on Europe). The warm ocean near the tropical Dateline causes convection which releases heat into the middle latitude atmosphere. The lack of sea ice allows the atmosphere above open water in northern latitudes to warm. The strong southern annular mode caused a hot subtropical ridge to anchor over Australia. Strengthening northern annular model increased mild Pacific influence across the U.S. Influence of the +IOD pattern caused the Africa and Australia heat. The NAWH pattern caused the warm upper ridge across Europe.