B. Legras
The Brewer-Dobson circulation in the ERA-Interim
The Brewer-Dobson circulation in the ERA-Interim has been investigated
with Lagrangian diabatic and kinematic trajectories and calculations of
effective diffusivity.
The calculated ages and the age spectrum show patterns which are very
similar to those obtained in the GEOSCCM model. The best agreement with
the observations, including in the polar regions, is obtained with
diabatic trajectories after discarding all parcels travelling above 0,5
hPa. It is noticeable that kinematic versus diabatic age of air exhibits
an old bias in the lower southern stratosphere below 25 km and a young
bias in the mid northern stratosphere above 25 km.
There is a trend in the ERA-Interim age of air which is negative in the
lower stratosphere and positive above 25 km. This result is at odd with
most CCM which predict an intensification of the whole Brewer-Dobson
circulation. The residual circulation decreases in the ERA-Interim and
is over compensated by an increased meridional mixing in the lower
stratosphere. This suggests an opposite evolution of the shallow and
deep branches of the Brewer-Dobson circulation.
We hope to present more results investigating whether the ERA-Interim
trend is due to biases in the observation system or the model, and
comparing ERA-Interim with MERRA.
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