A Barotropic Envelope Rossby Soliton Model for Block–Eddy Interaction. Part IV: Block Activity and Its Linkage with a Sheared Environment

Abstract

In this paper, a basic-state flow that has a linear, weak meridional shear is used as a mean flow condition to see if the blocking activity is related to the state of the zonal mean flow based upon the envelope soliton theory proposed in Part I. It is found that an isolated coherent structure similar to a dipole block can arise from the resonant interaction between preexisting planetary- and synoptic-scale waves, but its asymmetry, intensity, and persistence depend strongly upon the horizontal shear of the basic-state flow prior to block onset. The cyclonic shear of the basic-state flow not only plays an important role in preventing the eastward spread of block energy, but also provides a strong diffluent flow as a precursor of block flow. In such an environment, a high amplitude dipole anomaly is maintained and the deformed synoptic-scale eddies tend to deflect northward. But the anticyclonic shear of a basic-state flow plays a reverse role, which causes deformed eddies to deflect southward. It is also found that positive westerly wind anomalies at both high and low latitudes and negative anomalies at middle latitudes are maintained by synoptic-scale eddies through the self-interaction of block. In this case, the zonal mean westerly wind is accelerated at high and low latitudes, and decelerated at middle latitudes. Only for a basic state with cyclonic shear is the meridional profile of the zonal mean wind during the onset of a dipole block in agreement with observations. In addition, it can be shown by computing scatter diagrams of potential vorticity against streamfunction in different sheared environments that an eddy-induced isolated block can exhibit a local free-mode characteristic. Two approximately linear functional relationships between potential vorticity and streamfunction are found to be probably attributed to synoptic eddies.