The possible mechanism of the "stratospheric bridge"

Analysis of the three-dimensional Eliassen-Palm fluxes indicated the existence of the “stratospheric bridge” forming by the upward planetary wave (PW) propagation from the troposphere over northern Eurasia and the downward wave signal from the stratosphere over North Atlantic in mid and late winter. However, this kind of propagation is not evident in early winter. The differences of the PW activities are suggested to be related to the high (about 10 m/s) and low (about 2~5 m/s) refractive index in early and late winter, respectively. The “stratospheric bridge” is also evidently shown in the variation of the stratospheric PW activities in mid and late winter, which presents as a dipole oscillation pattern between EPz over northern Eurasia and North Atlantic. The strength of the stratospheric bridge determines the amount of energy sink and polar vortex strength in the stratosphere. Our diagnosis revealed that the early winter upward PW energy is influenced by the sea surface temperature anomaly in northern Pacific. On the decadal time, this is related to the North Pacific Decadal Oscillation. It is also shown that the causes of the decadal violation of the Holton-Tan relationship of the extra-tropical stratospheric circulation with the equatorial quasi-biennial oscillation and the 11-year solar cycle impact can be associated with decadal changes of the “stratospheric bridge” modulation in January-February. The simple mechanism explaining the link of the eddy energy exchange between the troposphere and stratosphere with the stratospheric circulation on the interannual and decadal timescales is proposed.