Dynamics of zonal-mean flow assimilation and implications for winter circulation anomalies

Seasonally averaged 200-mb circulations for recent winters (1987/88 and 1988/89) that represent opposite phases of El Nino and a zonal-mean zonal flow index cycle are diagnosed using data assimilated by the Goddard Earth Observing System (GEOS) and operational analysts of the European Centre for Medium-Range Weather Forecasts (ECMWF). The comparison is undertaken to determine whether there are significant differences in the 200-mb vorticity dynamics implied by the mean meridional circulations in the two datasets and whether these differences can be related to the Incremental Analysis Update (IAU) method used in the GEOS assimilation. The two datasets show a high degree of similarity in their depictions of the large-scale rotational flow, but there are substantial differences in the associated divergent circulations. For the zonal-mean flow, the zonal winds are substantially the same, but the meridional wind in the Tropics and subtropics is considerably weaker in the GEOS assimilation than its counterparts in both the ECMWF data and the GEOS analyses used to produce the assimilation. The authors examine the assimilation of the Hadley circulation using a zonally symmetric f-plane model. For this model, the IAU method easily assimilates the rotational flow but fails to assimilate the divergent circulation. This deficiency of the IAU method may explain the weakness of the Hadley cell in the GEOS assimilation, at least in relation to the GEOS analysis. For this simple model, an alternative assimilation method, based on constraints imposed by the analyzed potential vorticity and mean meridional circulation fields, is proposed that simultaneously assimilates both rotational and divergent flow components. Barotropic modeling suggests that an accurate representation of mean meridional flow anomalies can be important for the diagnosis of both zonal-mean and eddy rotational flow perturbations, particularly during extreme phases of the zonal-mean zonal flow fluctuation.