A GCM investigation into the nature of baroclinic adjustment

Spindown experiments have been conducted, using an atmospheric general circ ulation model, to determine the nature and timescale of adjustment to a bar oclinically neutral stare. The spindown was obtained by turning off the rad iative cooling of the atmosphere-a procedure, the authors argue. that retai ns all the relevant constraints on the atmospheric motions. A further exper iment with radiation and all other physical parameterizations turned off wa s performed for comparison. The neutral state was characterized by increased static stability but littl e reduction in meridional temperature gradient in the main baroclinic zones . However. the zones were observed to narrow and were flanked by regions wh ere the meridional temperature gradient was reduced significantly. This pat tern was repeated in the no-physics spin-down experiment but, in the absenc e of surface friction and strong orographic drag, the flow also appeared to be stabilized by enhanced barotropic shear. The neutral states from these two experiments are compared with those predicted by baroclinic adjustment theories and those seen in eddy life cycle experiments. The adjustment of temperature was roughly exponential, on a timescale of 15 -20 days. A spin-up experiment was also performed where the radiation was t urned back on in the adjusted state. The original climate was restored on a timescale of 5-10 days. Since both radiation and eddies are acting in the spinup, this implies a purely radiative forcing timescale of the same order as the adjustment timescale, which adds to the evidence that the mean atmo spheric state cannot be said to be baroclinically adjusted.