Dominant Intraseasonal Modes of Winter Surface Air Temperature in the Extratropical Northern Hemisphere

Abstract The dominant intraseasonal modes of winter surface air temperature in the extratropical Northern Hemisphere are identified through an empirical orthogonal function (EOF) analysis. The first mode is characterized by a dipole temperature pattern over the North American–North Pacific sector. The upper-tropospheric circulation associated with this mode is a zonally oriented Rossby wave train propagating westward. The second mode exhibits an in-phase relationship between the temperature anomaly over Eurasia and North America. The large-scale circulation associated with this mode is featured by the same-sign geopotential height anomalies moving at an opposite zonal direction (eastward in Eurasia and westward in North America). The third mode shows an out-of-phase relationship between the temperature anomaly in Eurasia and North America. This out-of-phase temperature pattern is linked to the similar low-frequency wave train but with opposite signals in geopotential height anomalies. The sum of the three leading modes explains 40% of the total intraseasonal temperature variance. The distinctive wave propagations in Eurasia and North America are rooted in barotropic Rossby wave dynamics and determined by both the zonal wavelength and the background flow, with the former playing a larger role. An anomalous temperature budget analysis shows that the southeastward (westward) shifting of the lower-tropospheric temperature tendency in Eurasia (North America) results primarily from anomalous meridional temperature advection.