Global Mass Circulation Prospective of Extratropical Stratosphere-Troposphere Coupling: Northern Hemisphere Annular Mode Variability in Winter Season

The simultaneous couplings among diabatic heating, meridional mass transport, meridional angular momentum transport, and form drag associated with amplifying baroclinic waves are diagnosed on isentropic surfaces using the NCEP-NCAR reanalysis II dataset from 1979 to 2003 for the Northern Hemisphere cold seasons. The objective of this study is to explain the dynamic nature of the annular mode from the perspective of global mass circulation variability.

In the stratosphere, diabatic heating (cooling) anomalies in low (high) latitudes, prior to the polar stratospheric warming event are mainly determined by local radiative energy surplus (deficit) in low (high) latitudes. This meridional pattern of heating anomalies acts to strengthen the poleward advancement of air mass accompanied with a poleward angular momentum transport in the stratosphere. Form drag anomalies associated with amplifying baroclinic waves that transport both mass and angular momentum poleward in the extratropical portion of the global mass circulation act to reduce the westerly angular momentum accumulated during the poleward advancement of air mass by transferring the westerly angular momentum downward. The reduction of the westerly angular momentum helps the intensified poleward advancement of stratospheric air mass to expand gradually into higher latitudes. Moreover, the poleward advancement of air mass in the upper level is ahead of the lower level, implying a downward propagating signal in the stratosphere. During the polar stratospheric warming phase, the increase of air mass in the polar stratosphere due to the enhanced poleward warm air branch of the stratospheric mass circulation contributes to the rising of surface pressure in the polar region. Associated with a stronger poleward advancing of (upper) stratospheric air mass is a downward transferring of westerly angular momentum into lower stratosphere/upper troposphere in the extratropics. In addition to that acquired from the lower latitude, the westerly angular momentum transferred from stratosphere would hamper the development of the poleward warm air branch in the tropospheric mass circulation, explaining the lack of the poleward propagation signal in the troposphere.

Arrival of poleward advancing warm air mass warms local air in high latitudes, resulting in heating anomalies there after the peak of the warming event. The enhanced mass exchange of lower latitude air mass with higher latitude air mass also results in a reduction of radiative heating in the tropics. The reversed diabatic heating anomalies, in turn, slow down the poleward advancement of air mass, responsible for the transition of the annular mode from the negative to the positive phase.