An identification of the dynamical mechanisms that lead to Arctic warming during the planetary-scale and synoptic-scale wave life cycles

To maintain Arctic amplification in a time-mean state, an enhanced poleward heat transport process is required. One such process that has been proposed recently is the tropically excited Arctic warming mechanism (TEAM). This mechanism states that tropical convection over the Pacific warm pool region can excite a planetary-scale Rossby wave train which transports moist static energy into the Arctic, enhances cloud cover and downward infrared radiation, and results in Arctic warming. Consistent with this mechanism, a recent study has shown that while both the planetary-scale and synoptic-scale wave life cycles result in Arctic warming, it is the planetary-scale wave life cycle that results in greater and more prolonged warming. However, it is uncertain what dynamical mechanisms during each life cycle lead to Arctic warming. The dynamical mechanisms which lead to Arctic warming are identified in this study using ERA-Interim reanalysis data. During the planetary-scale wave life cycle, which ensues from tropical convection over the Pacific warm pool region, EP-flux diagnostics reveal planetary-scale wave propagation into the stratosphere. This vertical wave activity is associated with poleward eddy heat flux into the polar stratosphere which subsequently weakens the polar vortex and results in adiabatic warming over the Arctic. Isentropic analysis reveals the presence of positive pressure advection over the Arctic throughout the entire troposphere. Furthermore, an examination of potential vorticity and specific humidity on isentropic surfaces shows the presence of filamentation which connects regions of moisture flux divergence in the subtropics to regions of moisture flux convergence in the high latitudes. The resultant anomalously high moisture in the Arctic leads to enhanced cloud cover and downward infrared radiation that prolong the warming. This warming overwhelms the cooling due to negative pressure advection that is found during the later stages of the planetary-scale wave life cycle. With respect to the synoptic-scale wave life cycle, the Arctic warming occurs during the later stages of its life cycle and coincides with the amplification of planetary-scale waves through synoptic-scale wave breaking. The amplification of the planetary-scale waves leads to positive pressure and moisture advection over the Arctic but at magnitudes less than the planetary-scale wave life cycle excited by the tropical convection.