Possible cause of an abrupt warming in winter mean surface air temperature in the midlatitudes of the Northern Hemisphere in the late 1980s is investigated using observation and reanalysis data. To determine the timing of abrupt warming, we use a regime shift index based on detection of the largest significant differences between the mean values of two contiguous periods. Results show that the abrupt warming occurred in association with a regime shift after the 1980's in which the zonal mean sea level pressure (SLP) is significantly increased (decreased) at the latitude 25-35°N (60-70°N), in the form of north-south dipole-like SLP anomaly spanning the subtropics and high latitude. The dipole SLP anomaly can be attributed to a northward expansion of Hadley cell, a poleward broadening and intensification of the Ferrel cell, coupled with a collapse of polar cell. During the abrupt warming, strong anomalous southerly warm advection at the surface was induced by an enhanced and expanded Ferrel circulation, in association with a northward and downward shift of maximum center of northward eddy heat flux over the midlatitudes. An intensification of polar jet subsequent to regime shift may be instrumental in sustaining the warming up to more than 5 years.
Keywords: Ferrel cell; Hadley cell; abrupt warming; eddy heat flux; regime shift.