The dynamics of the coastal transition zone off Northern California during late May and early June 1987 are examined through assimilation modeling studies. A regional baroclinic quasi-geostrophic model is driven by the data through initial and boundary conditions. These initial and boundary conditions are specified by objective analysis of hydrographic and acoustic Doppler current profiler data. The data assimilation is accomplished by varying the objective analysis parameters, numerical parameters, and subgrid-scale parameters until the final solution of the model is in best agreement with the analysis of the data. The solution which best agrees with the data is regarded as a four dimensional field estimate of the coastal transition zone flow. An aspect of this study that is new to data assimilation modeling of mesoscale eddy fields is the use of acoustic Doppler current profiler data. These data prove to be very important to accurate description of the oceanic flow field as indicated by comparison with float trajectories. The complete data set provides an opportunity to study the ability of dynamical constraints to improve field estimates when acting upon a subset of the data (hydrography).

Data assimilation modeling generates field estimates that are substantially better than those based upon the hydrographic data alone as indicated by comparison with the acoustic Doppler current profiler based analyses. The kinematics and energetics of this constrained (quasi-geostrophic) field estimate are examined. Despite the relatively small region (108 by 324 km) and short period of the field estimate (21 days), interesting processes are identified. A meandering barotropically unstable part of the jet is identified using the results of related idealized numerical studies and stability analyses. Similarly, this jet may be undergoing a simultaneous larger scale mixed instability. Another part of the jet interacts with an eddy and meanders in a much different manner. Characteristic of the energy balances are identified and compared with the results of linear analysis and of nonlinear studies utilizing idealized models of the observed jet as described in this issue by Pierce et al. and Allen et al. respectively. ¿ American Geophysical Union 1991