3-D Mass-weighted Isentropic Time Mean Meridional Circulations

The present work develops a diagnostic tool for three-dimensional (3-D) structure of mass-weighted isentropic time-mean (T-MIM) meridional circulations and conducts a preliminary analysis in winter hemispheres. Mass-weighted isentropic zonal means (Z-MIM) averaged in time are decomposed into the Z-MIM of T-MIM at each longitude. The difference in meridional velocities between the T-MIM and an isentropic time mean without mass-weight (TM) is called the Bolus velocity, which represents a wave-induced air mass transport. Although the Bolus velocity is one order of magnitude smaller than the longitudinal variation of T-MIM and TM meridional velocities except near the ground surface in extratropics, it makes a great contribution to the extratropical direct circulation in the troposphere and to the Brewer-Dobson circulation in the stratosphere. In particular, both low-level equatorward flow and poleward flow in the upper troposphere distinctly appear in the Bolus velocity around storm tracks over the northern Pacific and northern Atlantic in the boreal winter and the Southern Ocean in the austral winter. It clearly shows that the extratropical direct circulation is localized in the storm track regions.

A zonal momentum equation is formulated in the T-MIM framework, where the local Eliassen-Palm (E-P) flux divergence is separated into stationary and transient components. In the extratropics, it is confirmed that the TM and bolus meridional velocities are in good balance with the stationary and transient components of the E-P flux divergences, respectively. Wave-mean flow interactions explain the local meridional mass transport in the winter extratropics.