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Abstract
A time series of scatter-diagrams of 500 mb geostrophic stream-function, $\psi$, versus quasi-geostrophic potential vorticity, q, is derived during an episode of Northeast Pacific blocking in January and February 1989.
Visual inspection of the diagrams suggests that blocking can be modelled by localized free mode solutions of the governing equations, characterized by potential vorticity and stream-function of the form q = q($\psi$). Our diagrams show a roughly linear q($\psi$) relationship that is particularly well-correlated during stages of the block in which the vorticity contours are closed, or nearly closed.
We are able to quantitatively detect free mode tendencies of this block using an equation that relates the area of scatter plots to the departure of a stationary system from any free-mode of the quasi-geostrophic equations. The q($\psi$) plot area is equivalent to the net geostrophic vorticity flux across the boundary of the blocking region. We have computed both these quantities.
We suggest that the block approaches two different free modes during its lifetime. The first stage is an Omega-block; during the second stage, the 500 mb stream-function displays a dipolar form, and the scatter plots are quite similar to q($\psi$) plots for a geostationary, equivalent barotropic modon in a westerly flow. (Abstract shortened by UMI.)
