Analysis of the coupling between the stratospheric meridional wind and the surface level zonal wind during 1979–93 Northern Hemisphere extratropical winters Received: 28 October 2002 / Accepted: 18 February 2003 / Published online: 21 May 2003 Ó Springer-Verlag 2003 Abstract The coupling between the stratosphere and the troposphere has been investigated by analysing low- frequency variations in: (1) the meridional mass flux into the polar cap (north of 60°N), computed separately for the stratosphere and the troposphere; (2) the polar cap mean surface pressure, and (3) the surface level meridi- onal pressure gradient and zonal wind around 60°N. The analysis has been done for the 1979–93 Northern Hemisphere (NH) winters, using ECMWF reanalysis data. The results show that for all winters the meridional mass flux variations in the stratosphere precede those in the troposphere, by about one day. This result can also be obtained qualitatively with a very simple model, based on the zonally averaged zonal and meridional momentum equations. The lag is not very sensitive to the latitude of the southern boundary of the polar cap. The analysed variations in the polar cap mean surface pres- sure associated with variations in the meridional mass flux, determine most of the variability in the analysed meridional surface pressure gradient and the associated surface zonal wind around 60°N. The results also show that in the stratosphere the Coriolis force associated with the zonal-mean meridional wind is in near-balance with the convergence of the eddy momentum flux, and in the lower troposphere with the zonal frictional force. In summary, the results indicate that in the extratropical northern winter hemisphere, low-frequency variations in the meridional wind in the stratosphere induce low- frequency variations in the zonal wind near the surface. 1 Introduction Since the troposphere holds most of the atmospheric mass, it is not surprising that the troposphere influences the stratospheric circulation. Planetary waves generated in the troposphere can, under certain conditions, prop- agate upward to the stratosphere where they dissipate (Charney and Drazin 1961; Matsuno 1970) and drive the stratospheric circulation away from its radiatively determined state (e.g. Holton 1992). The view that the stratosphere can influence the tro- pospheric circulation is less commonly held (Hartmann et al. 2000). An important manifestation of this down- ward influence is the propagation of low-frequency zonal wind variations from the upper stratosphere to the lower troposphere, which have been noted, e.g. by Kodera et al. (1990), Kuroda and Kodera (1999) and Christiansen (2001). Much of the recent research on stratosphere–troposphere coupling has been done in the framework of the Arctic Oscillation (AO). The AO is the leading mode of variability in the NH wintertime cir- culation (Thompson and Wallace 1998). Baldwin and Dunkerton (1999) noted that low-pass filtered AO- signature variations propagate downward from the stratosphere to the lower troposphere in most winters from 1958 to 1997. The induced tropospheric anomalies tend to persist during up to two months after extreme circulation anomalies in the northern extratropical stratosphere (Baldwin and Dunkerton 2001). Several authors have suggested mechanisms by which the stratosphere could dynamically influence the tropo- sphere. Hartmann et al. (2000) noted that three such mechanisms can be postulated: potential vorticity induc- tion (Hartley et al. 1998; Black 2002), interaction between the zonal-mean flow and upward propagating waves (Haynes et al. 1991; Christiansen 1999; Shindell et al. 2001) and mass redistribution in the stratosphere (Bald- win and Dunkerton 1999; Shindell et al. 2001). This last mechanism will be the subject of the present study. Climate Dynamics (2003) 21: 211–219 DOI 10.1007/s00382-003-0328-2 M. Sigmond Æ P. C. Siegmund Æ H. Kelder M. Sigmond (&) Æ H. Kelder Eindhoven University of Technology (TUE), Department of Applied Physics, PO Box 513, 5600 MB, Eindhoven, The Netherlands E-mail: sigmond@knmi.nl P. C. Siegmund Æ H. Kelder Royal Netherlands Meteorological Institute (KNMI), PO Box 201, 3730 AE, De Bilt, The Netherlands