Relationship between singular modes of blocking flow and high-frequency eddies

To investigate the maintenance mechanism of the blocking flow, we performed a numerical experiment using a barotropic spectral model on the sphere. Th e model is linearized about two types of realistic basic flows. One is a bl ocking flow near Alaska (blocking flow case) and the other is a zonal how i n the North Pacific (zonal flow case). The model is integrated in time with prescribed high-frequency eddies which are generated in the Far East and propagate eastward along the northern fl ank of the jet stream. After several days of time integration, time mean vo rticity flux divergence is calculated. Then, the resulting vorticity flux d ivergence field is used as the eddy vorticity forcing to compute the second -order flow induced by eddies. It is found that the induced second-order fl ow intensifies the blocking for the blocking flow case, and that this is no t the case for the zonal flow case. To clarify the reason for the difference between the induced second-order f lows, we carried out the singular value decomposition (SVD) of the matrix o f the barotropic vorticity equation linearized about the basic flows. For t he blocking how case, spatial distributions of the leading singular modes s how blocking patterns which are similar to the blocking in the basic flow. It is found that these modes are effectively excited by the eddy vorticity forcing, and enhance the blocking in the basic flow. On the other hand, for the zonal flow case, leading singular modes do not show blocking patterns. These results indicate that blocking flows tend to have easily excited mod es that can reinforce the block, and high-frequency eddies could maintain b locking by exciting these modes. The dependency of results of the experiment on the location of high-frequen cy eddies is also shown. It indicates the limitation of the model to invest igate the high-frequency eddies feedback.