A GCM Investigation into the Nature of Baroclinic Adjustment

Abstract

Spindown experiments have been conducted, using an atmospheric general circulation model, to determine the nature and timescale of adjustment to a baroclinically neutral state. The spindown was obtained by turning off the radiative cooling of the atmosphere?a procedure, the authors argue, that retains all the relevant constraints on the atmospheric motions. A further experiment with radiation and all other physical parameterizations turned off was performed for comparison. The neutral state was characterized by increased static stability but little reduction in meridional temperature gradient in the main baroclinic zones. However, the zones were observed to narrow and were flanked by regions where the meridional temperature gradient was reduced significantly. This pattern was repeated in the no-physics spin-down experiment but, in the absence 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 turned 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 atmospheric state cannot be said to be baroclinically adjusted.