[1] The dynamical impact of the 11-year solar cycle is investigated with the focus on the stratopause region where solar ultraviolet heating is greatest. The most important variation in solar forcing longer than the diurnal cycle is the annual cycle. Thus the climatological features of the zonal wind variation associated with the annual cycle were first studied to characterize the basic features of the atmosphere's dynamical response to changes in solar radiative forcing. The 11-year solar cycle effect was then investigated. The results of the analysis suggest that in a climatological mean state the stratopause circulation evolves from a radiatively controlled state to one dynamically controlled during winter in both hemispheres. The transition period is characterized by a poleward shift of the westerly jet. The solar cycle effect appears as a change in the balance between the radiatively and dynamically controlled states. The radiatively controlled state lasts longer during the solar maximum phase, and the stratopause subtropical jet reaches a higher speed. The large dynamical response to relatively weak radiative forcing may be understood by the bimodal nature of the winter atmosphere due to interaction with meridionaly propagating planetary waves and zonal mean zonal winds. It is suggested that the solar influence produced in the upper stratosphere and stratopause region is transmitted to the lower stratosphere through (1) modulation of the internal mode of variation in the polar night jet and (2) a change in the Brewer-Dobson circulation. The first process is significant in the middle and high latitudes, whereas the latter is prominent in the equatorial region.

The atmospheric response to the solar cycle has been previously investigated with the Freie Universität Berlin Climate Middle Atmosphere Model (FUB‐CMAM) using prescribed spectral solar UV and ozone changes as well as prescribed equatorial, QBO‐like winds. The solar signal is transferred from the upper to the lower stratosphere through a modulation of the polar night jet and the Brewer‐Dobson circulation. These model experiments are further investigated here to show the transfer of the solar signal from the lower stratosphere to the troposphere and down to the surface during Northern Hemisphere winter. Analysis focuses on the transition from significant stratospheric effects in October and November to significant tropospheric effects in December and January. The results highlight the importance of stratospheric circulation changes for the troposphere. Together with the poleward‐downward movement of zonal wind anomalies and enhanced equatorward planetary wave propagation, an AO‐like pattern develops in the troposphere in December and January during solar maximum. In the middle of November, one third of eddy‐forced tropospheric mean meridional circulation and surface pressure tendency changes can be attributed to the stratosphere, whereas most of the polar surface pressure tendency changes from the end of November through the middle of December are related to tropospheric mechanical forcing changes. The weakening of the Brewer‐Dobson circulation during solar maximum leads to dynamical heating in the tropical lower stratosphere, inducing circulation changes in the tropical troposphere and down to the surface that are strongest in January. The simulated tropospheric effects are identified as indirect effects from the stratosphere because the sea surface temperatures are identical in the solar maximum and minimum experiment. These results confirm those from other simplified model studies as well as results from observations.

Extreme variability of the winter-and spring-time stratospheric polar vortex has been shown to affect extratropical tropospheric weather. Therefore, reducing stratospheric forecast error may be one way to improve the skill of tropospheric weather forecasts. In this review, the basis for this idea is examined. A range of studies of different stratospheric extreme vortex events shows that they can be skilfully forecasted beyond 5 days and into the sub-seasonal range (0-30 days) in some cases. Separate studies show that typical errors in forecasting a stratospheric extreme vortex event can alter tropospheric forecast skill by 5-7% in the extratropics on sub-seasonal time-scales. Thus understanding what limits stratospheric predictability is of significant interest to operational forecasting centres. Both limitations in forecasting tropospheric planetary waves and stratospheric model biases have been shown to be important in this context.

Direct observations of the upper ocean velocity in the eastern equatorial Indian Ocean by an acoustic Doppler current profiler, from November 2000 to October 2001 on the equator at 90°E, demonstrate that the dominant periods of variability in the upper layer zonal and meridional currents are in intraseasonal frequency bands with periods of 30 to 50 days and 10 to 20 days, respectively. The strong intraseasonal variability in the zonal current obscures the semiannual Wyrtki jets, which can be seen clearly in the monthly averaged field. In addition, a zone of strong vertical shear of the zonal current and a distinct Equatorial Undercurrent with semiannual period are observed. The results provide us with a new perspective on importance of the energetic intraseasonal variability in the eastern equatorial Indian Ocean, which indicates strong correlation with the wind variability near the mooring location.

Abstract. The role of planetary waves in stratospheretroposphere coupled variability is investigated using an extended singular value decomposition analysis of zonalmean zonal wind and the vertical component of the EliassenPalm (E-P) flux for the winters from 1979/80 to 1995/96. The results suggest a close relationship between anomalies of zonal-mean zonal wind and the convergence zone of E-P flux, which together shift poleward and downward from the stratosphere to the troposphere as time advances. Following enhanced vertical propagation of waves into the stratosphere, the Arctic Oscillation (AO) pattern is seen in the 500 hPa geopotential height field in association with an increased poleward propagation of tropospheric waves.