Baroclinic eddy equilibration under specified seasonal forcing

In equilibration studies on the relation between midlatitude baroclinic eddies and the zonal-mean flow, the external forcing is always specified and kept fixed during the eddy equilibration, which is equivalent to assuming that, compared with the variation time scale of the external forcing, the atmosphere adjustment by baroclinic eddies is always relatively quick. The validity of this assumption is investigated here by studying the response of baroclinic eddies to variable external forcing in a multilevel QG channel model. In the model, a Northern Hemisphere-like seasonal variation of the underlying surface temperature is included as the bottom boundary condition. The radiative-convective heating is also specified to vary seasonally to match the surface temperature.

Under seasonal forcing, eddies spin up in a short time scale (~ 10 days) in the mid-fall and slowly spin down in the late spring. In spite of the strong seasonality of eddy activity, as observed in the real atmosphere, the PV gradient is still robust. Factors affecting the eddy spin-up, spin-down time scales and the interpretation of the robust PV structure are studied. We also find that eddies are saturated and maintain a robust zonal-mean flow thermal structure in the winter season only when the external forcing is strong and varied slowly. Otherwise, the eddy activity is weak and varies almost proportionally with the variation of the external forcing. Whether the theories obtained in the previous equilibration study still hold in spite of the variation of the external forcing is also discussed in this study.