The Eulerian-and Lagrangian-Mean Meridional Circulations in the Stratosphere at the Time of a Sudden Warming

Abstract The Eulerian-mean (usual zonal mean) and the Lagrangian-mean (an average in the zonal direction along a curved “material line,” consisting of definite individual air particles) meridional circulations are investigated, given an upward propagating planetary wave incident on a critical level. The Eulerian-mean circulation is such that an upward motion appears at higher latitudes, a downward motion at lower latitudes, and an equatorward meridional flow near the critical level. The Lagrangian circulation is quite different from the Eulerian one. A strong poleward meridional flow appears concentrated at the critical level. In connection with this meridional flow, a Lagrangian-mean vertical motion occurs which diverges from this level at higher latitudes, i.e., it consists of a downward motion below the critical level and an upward motion above the level. At lower latitudes, vertical motions converge toward the critical level. The vertical motion field of this four-sector structure in the meridional se...