A novel atmospheric background state with potential relevance for the onset of blocking

A few years ago, a new theory was presented by the Chicago group that deals with the onset of blocking. According to this theory the onset of blocking behaves surprisingly similar to the onset of a traffic jam, with the local finite amplitude Rossby wave activity along a waveguide being analogous to the traffic density along a highway. In particular, one obtains a nonlinear transition to a blocked flow pattern if the wave activity exceeds a certain threshold. This threshold, which is sometimes referred to as “carrying capacity”, depends on properties of the background state.

Here we present a novel background state which may be relevant for estimating the carrying capacity in a zonally non-uniform atmosphere. An eddy-free background state is created through conservative rearrangement of contours of potential vorticity, a procedure that is well-known from the theory of finite amplitude wave activity. The only novelty is that we apply this procedure in a rolling fashion to a finite sector in longitude, rather than to the entire hemisphere. Although this leads to the violation of certain theorems, the background state thus obtained has several desirable properties which make it potentially useful in practical applications. In particular, the so-obtained background state is local both in time and space, and this suggests that it might be suitable for a fresh look at the onset of blocking in the near future.