Study of Block Onset Using Sensitivity Perturbations in Climatological Flows

Zhijin Li , Albert Barcilon, I.M.Navon

This work describes the dynamics of adjoint sensitivity perturbations which excite block onsets over the Pacific and Atlantic oceans. Appropriate functions are derived for the blocking indices for these two regions and the model basic flow is constructed from Northern Hemisphere climatological data. The concepts of sensitivity analysis are extended to forced problems. This tool is used to investigate block onset due to atmospheric forcing, such as that resulting from tropical sea surface temperature anomalies. These linear studies are carried out in a hemispherical, primitive equations, $\theta$-coordinate, two-layer model.

Results show that wind sensitivity perturbations less than 10 $ms^{-1}$ and sensitivity forcing of vorticity sources of the order of $1.5\times 10^{-10} s^{-2}$ are sufficient to excite block onset. Both for the Pacific and Atlantic blocking, sensitivity perturbations and forcing perturbations, when expressed in terms of vertical vorticity, display a Rossby wave train structure mainly found on the southward flanks of the Pacific and Atlantic jets, i.e., near the Philippines and the Caribbean regions.

From inferences based on the flow evolution of these sensitivity perturbations and with the help of potential vorticity analyses on the two constant potential temperature surfaces in our model, a dynamical framework that may explain Pacific and Atlantic block onsets is proposed. The non-uniform potential vorticity distribution in the jets, in particular the concentration of these gradients on potential vorticity waveguides, the Lagrangian advection of potential vorticity by the eddies making up the stationary Rossby wave train and their energy propagation and convergence all conspire to play a key role n the growth of the synoptic scale eddies supported by baroclinic as well as barotropic processes. It is proposed that the structural modification of the eddies in the wave train leads to the planetary structures which become associated with block onset. More specifically, the wave train in the Pacific evolves into a blocking dipole while the Atlantic block is found at the leading edge of the Rossby wave train across the Atlantic. Furthermore, this study shows that at the initial time the Pacific block displays a clear baroclinic structure while the wave train associated with the Atlantic block has a much more barotropic structure.

The significance of these results and their potential applications to predictions of blocking are discussed.