Multivariate analysis
Model state corresponds to (u,v,F) defined at each grid-point of the model, where F is the geopotential
Example: coupling between (u,v,F) at mid-latitudes due to the geostrophic balance.
Multivariate analysis: build a covariance model that takes into account of dynamical couplings between the model variables.
Ref.: Daley, 1991, ch. 5., sect. 5.2 et 5.3.