Evolving the subspace of the three-dimensional multiscale ocean variability: Massachusetts Bay

A data and dynamics driven approach to estimate, decompose, organize and an alyze the evolving three-dimensional variability of ocean fields is outline d. Variability refers here to the statistics of the differences between oce an states and a reference state. In general, these statistics evolve in tim e and space. For a first endeavor, the variability subspace defined by the dominant eigendecomposition of a normalized form of the variability covaria nce is evolved. A multiscale methodology for its initialization and forecas t is outlined. It combines data and primitive equation dynamics within a Mo nte-Carlo approach. The methodology is applied to part of a multidisciplinary experiment that o ccurred in Massachusetts Bay in late summer and early fall of 1998. For a 4 -day time period, the three-dimensional and multivariate properties of the variability standard deviations and dominant eigenvectors are studied. Two variability patterns are discussed in detail. One relates to a displacement of the Gulf of Maine coastal current offshore from Cape Ann, with the crea tion of adjacent mesoscale recirculation cells. The other relates to a Bay- wide coastal upwelling mode from Barnstable Harbor to Gloucester in respons e to strong southerly winds. Snapshots and tendencies of physical fields an d trajectories of simulated Lagrangian drifters are employed to diagnose an d illustrate the use of the dominant variability covariance. The variabilit y subspace is shown to guide the dynamical analysis of the physical fields. For the stratified conditions, it is found that strong wind events can alt er the structures of the buoyancy flow and that circulation features are mo re variable than previously described, on multiple scales. In several locat ions, the factors estimated to be important include some or all of the atmo spheric and surface pressure forcings, and associated Ekman transports and downwelling/upwelling processes, the Coriolis force, the pressure force, in ertia and mixing. (C) 2001 Elsevier Science B.V. All rights reserved.