On the Effectiveness of the Eddy Straining Mechanism for the Maintenance of Blocking Flows.

Abstract

In connection with the maintenance of blocking flows by the migrating synoptic disturbances, we examine the effectiveness of the eddy straining mechanism proposed by Shutts (1983) in an equivalent barotropic β-channel model. The model used here is identical to that in Haines and Marshall (1987) (hereafter referred to as HM) except with a channel twice as wide. This model possesses two stationary solutions accompanying isolated structures in prescribed uniform westerlies when a vorticity forcing associated with the analytical modon solution is assumed: “blocking solution” similar to the modon solution and “zonal flow solution” characterized by dominant zonal flows. The infinitesimal transient eddies which mimic synoptic disturbances are generated by a wavemaker forcing located far upstream of the diffluence associated with the basic flow prescribed by stable stationary solutions. The effectiveness of the eddy straining mechanism is evaluated from the resemblance between the basic flow and the second-order flow induced by the time-averaged eddy potential vorticity (PV) flux divergence. The distribution of the time-averaged PV divergence for the blocking solution in our model is the same as in HM in a sense that there is a PV north/south, divergence/convergence dipole upstream of the diffluence of the basic flow. However, the computed second-order flow has a quadruple structure which tends to shift the blocking dipole downstream instead of the dipole structure enforcing the blocking as shown in HM. On the other hand, the second-order flow tends to maintain a weak diffluence associated with the zonal solution. Thus, the effectiveness of the eddy straining mechanism depends on the basic flow. The second-order flow for the blocking solution is also drastically deformed by a negligible distortion of the PV divergence field associated with a small change in the property of imposed eddies. The effectiveness of the eddy straining mechanism depends also on the property of synoptic eddies. Finally, this study suggests that the PV north/south, divergence/convergence dipole pattern does not necessarily maintain the blocking dipole. This remark is especially important for the observational study to assess the role of synoptic disturbances in the maintenance of blocking flows.