The Semidiurnal Westward s=1 Tide in the Stratosphere

The semidiurnal westward propagating zonal wavenumber 1 tide (2, 1) is the dominant tide in the high- latitude wind field in the upper mesosphere during the late spring and summer. Observed peak wind speeds are ~ 30 m/s. Simulations agree with observations in showing a dominant (2, 1) tide in the upper mesosphere with about the correct amplitude at high polar latitudes. In model simulations the (2, 1) tide originates in a nonlinear interaction between the migrating semidiurnal tide (2, 2) and a stationary s=1 planetary wave. The interaction occurs in the stratosphere and mesosphere. However, stationary planetary waves are essentially absent from the summertime stratosphere and mesosphere so the occurrence of the predicted (2, 1) tide in the summer mesosphere originates via an interaction in the winter hemisphere. Observations and simulations in the mesosphere indicate a connection with the zero wind line in the zonally averaged zonal winds. The models (Angelas i Coll and Forbes, 2002, Aso, 2007) agree in showing that the (2,1) tide is confined to the upper mesosphere. We have analyzed results from the GEOS-5 assimilation model and find a significant stratospheric (2, 1) tide. We have also analyzed balloon data from the CNES VORCORE campaign in which a total of 27 balloons drifted in the Antarctic polar stratospheric vortex during the late summer and spring of 2005, and confirm the existence of the (2, 1) tide. This is the first observation of a (2, 1) tide in the stratosphere insofar as we are aware. The existence of this tide in the stratosphere is hard to understand in terms of energy propagation from the winter hemisphere guided by the zero wind line, since this would guide energy into the mesosphere rather than the stratosphere. We will present the height- latitude structure of the (2, 1) tide in the Antarctic spring and will show its correlation with the zero wind line and wave energy flux found from the GEOS-5 data assimilation model.