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Using a variational technique, middle atmosphere gravity wave drag (GWD) is estimated fromMet Office middle atmosphere analyses for the year 2002.The technique employs an adjoint model of a middle atmosphere dynamical model to minimisea cost function that measures the differences between the model state and observations.The control variables are solely the horizontal components of GWD;therefore the minimisation determines the optimal estimate of the drag. For each month,Met Office analyses are taken as the initial condition for the first day of the month,and also as observations for each successive day. In this way we obtain athree dimensional GWD field for all the year with a temporal resolution of one day.GWD shows a pronounced seasonal cycle. During solstices, there aredeceleration regions of the polar jet centred at about $63^circ$ latitude in the winterhemisphere, with a peak of $49 gwdunit$ at $0.24 punit$ %JT $0.024 punit$in the Southern hemisphere;the summer hemisphere also shows a deceleration region but much weaker, with a peak of $24 gwdunit$, centred at $45^circ$ latitude and $0.6 punit$. During equinoxes, GWD is weak and exhibits a smoothtransition between the winter and summer situation. The height and latitude of thedeceleration centre in both winter and summer hemispheres appear to be constant.Important longitudinal dependencies in GWD are found which are related to planetary wave activity;GWD intensifies in the exit region of jet streaks. In the lower tropical stratosphere, the estimated GWD shows a westward GWD descending together with the westward phase of the Quasi-biennial oscillation.Above, GWD exhibits a semi-annual pattern which is approximately out of phase withthe semi-annual oscillation in zonal wind. Furthermore a descending GWD patternis found at those heights, similar in magnitude and sign, to that in the lower stratosphere.

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