Fig. 1: The Dec. 1, 2019 to Jan. 30, 2020 temperature anomalies across the northern hemisphere land masses.
Discussion: The northern hemisphere meteorological winter (so far) has been ROBUSTLY warmer than normal featuring historical warmth in northwest Russia, very warm conditions across much of the U.S. and eastern Canada, Europe, Russia and eastern China (Fig. 1). The primary contributor to this warmth is a robust positive phase of the arctic oscillation in January (and the first half of December) and widespread warmer-than-normal northern hemisphere ocean surface. Climate Impact Company also proposes increased warming caused by the historic positive phase of the Indian Ocean Dipole peaking last autumn. There is general consensus that a warmer than normal winter season is a contributor to increased risk of flu outbreaks (although research is limited on this subject). An excellent guide to weather and climate and influenza was written by Christopher Fuhrmann of the NOAA/Southeast Regional Climate Center…
The upper atmosphere over the polar region during January 2020 has been exceptionally cold (Fig. 2). The cold polar vortex has been vertically represented in BOTH the stratosphere and troposphere in the polar region. There is a POSSIBLE connection between the persistent lack of solar activity (Fig. 3) and time of year driving the very cold upper atmosphere over the polar region.
Fig. 2-3: The January 2020 stratosphere temperature anomalies at 10 MB (left) and solar cycle sunspot number progression (right).
As a general rule the troposphere (where weather occurs) tends to have the opposite temperature characteristics from the stratosphere. This tendency is due to the cooling stratosphere contracting while the troposphere beneath compensates by warming and expanding. Note the cold polar vortex location in January while beneath in Northwest Russia a record warm winter is occurring. Of course, the cold polar vortex in the polar region this winter has prevented stratospheric warming episodes which are well-correlated to producing arctic air which can infiltrate the middle latitudes.
There is definite increased warm influence on global climate by the recent (since 2013) evolution of regionally warm ocean areas outside of the tropics. Note the very warm global sea surface temperature outside of the tropical region as February 2020 arrives (Fig. 4). Also be aware that long-standing regions of anomalous warm surface water are well-correlated with sea level rise.
Fig. 4: The global sea surface temperature anomalies valid February 2, 2020.
A major contributor to global climate in recent months is the influence of the positive phase of the Indian Ocean Dipole. +IOD occurs when the western tropical Indian ocean is much warmer than normal (fueling heavy thunderstorm activity) and the eastern tropical Indian Ocean is cooler than normal (causing lack of clouds and rain). The outgoing longwave radiation anomaly analysis for quarter 4 of 2019 by the International Research Institute for Climate and Society (Fig. 5) identifies the heavy convection over the western Indian Ocean emitting heat poleward in both hemispheres while subsidence leads to drought from Southeast Asia to Australia. The Indian Monsoon was the wettest and latest on record. The +IOD pattern has eased but its historic intensity peaking last OCT/NOV has a lagging influence on climate carrying into early 2020.
Fig. 5: Outgoing longwave radiation anomaly analysis for OCT/NOV/DEC 2019 by the International Research Institute for Climate and Society.
Conclusions: There is no question that additional research is needed correlating climate patterns and seasonal flu outbreaks. The general assertion that flu spreads most easily in a warmer than normal winter climate could certainly be applicable to northern hemisphere winter 2019-20. Of course the weather/climate and influenza relationships are complicated. The link to George Fuhrmann’s research on this subject is a good introduction. The Climate Impact Company expertise is the climate and so far meteorological winter has been unusually warm in the northern hemisphere linked to an unusually strong cold polar vortex in the polar region, widespread regional warm sea surface temperature anomalies for the global oceans (outside of the tropics) and in my view a major contribution from the Indian Ocean Dipole.