q 1997 American Meteorological Society Baroclinic Jets in Confluent Flow*

The nonlinear, three-dimensional behavior of baroclinic fronts in a barotropic deformation field is investigated. A major finding is that baroclinic instability of the frontal zone can play an important role in limiting frontogenesis forced by the large-scale deformation. This results in a statistically equilibrated state in which the front oscillates about a mean vertical shear and frontal width. This equilibration mechanism is effective over a wide range of parameter space and is relevant to a variety of fronts in both the ocean and the atmosphere. Sufficiently strong deformation fields, however, can stabilize the baroclinic jet, yielding the two-dimensional result in which the frontogenesis is ultimately limited by the model subgrid-scale mixing parameterization. The time-dependent three-dimensional equilibrated state is achieved for those cases in which perturbations can grow to sufficient amplitude such that the nonlinearities counteract the frontal steepening induced by the large-scale deformation field through the large amplitude baroclinic wave cycle and resulting heat flux. The regimes of the steady equilibrated state and the time-dependent equilibrated state are predicted well by an application of Bishop’s linear model of time-dependent wave growth. The vertical heat flux and subduction rate are dominated by the essentially two-dimensional ageostrophic circulation resulting from the large-scale deformation field, not by the eddy heat flux associated with baroclinic instability. The ageostrophic horizontal and vertical circulations, and vertical heat flux and subduction rates, are discussed and compared to various oceanic observations. 1