Connected Thermal and Transport Anomalies in the General Circulation

William K. Dewar Department of Oceanography, Florida State University, Tallahassee, Florida

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Abstract

Two models of the oceanic response to cooling are discussed. Both are motivated by a desire to understand the effects of variable diabatic forcing on the general circulation. The first model considers an initial value problem in which an initially resting warm ocean is “slowly”cooled on "broad”scales. The lower layer in this model is fully active and it is further argued that the slow and broad scales are relevant to 1 8°C water formation. The purpose of this model is to illustrate the short term barotropic and baroclinic response to variability in thermal forcing.

The second problem addresses the longer-term evolution of finite amplitude thermocline anomalies (which are assumed to have been formed by diabatic effects). A “one and three-quarter”model is used. i.e., the lower layer is assumed to be deep, but not stagnant, and its evolution is computed.

Based on these models, it is argued that diabatic forcing can result in local modifications of the Sverdrup constraint and that mass transport evolves through at least three distinct phases. The first short-term ocean response to cooling is the radiation of eastward moving barotropic planetary waves, which leaves the Sverdrup transport and the planetary geostrophic wave equation (PGWE) in its wake. Local Sverdrup dynamics and the PGWE dominate the second phase of evolution. The last phase occurs as the fronts obtain deformation radius length scales, and the tendency for the system to produce coherent structures results in persistent, spreading regions of anomalous transport. Global measures of the mass transport, however, are in agreement with the classic Sverdrup constraint. Implications for the generation of barotropic and baroclinic variability are discussed.

Abstract

Two models of the oceanic response to cooling are discussed. Both are motivated by a desire to understand the effects of variable diabatic forcing on the general circulation. The first model considers an initial value problem in which an initially resting warm ocean is “slowly”cooled on "broad”scales. The lower layer in this model is fully active and it is further argued that the slow and broad scales are relevant to 1 8°C water formation. The purpose of this model is to illustrate the short term barotropic and baroclinic response to variability in thermal forcing.

The second problem addresses the longer-term evolution of finite amplitude thermocline anomalies (which are assumed to have been formed by diabatic effects). A “one and three-quarter”model is used. i.e., the lower layer is assumed to be deep, but not stagnant, and its evolution is computed.

Based on these models, it is argued that diabatic forcing can result in local modifications of the Sverdrup constraint and that mass transport evolves through at least three distinct phases. The first short-term ocean response to cooling is the radiation of eastward moving barotropic planetary waves, which leaves the Sverdrup transport and the planetary geostrophic wave equation (PGWE) in its wake. Local Sverdrup dynamics and the PGWE dominate the second phase of evolution. The last phase occurs as the fronts obtain deformation radius length scales, and the tendency for the system to produce coherent structures results in persistent, spreading regions of anomalous transport. Global measures of the mass transport, however, are in agreement with the classic Sverdrup constraint. Implications for the generation of barotropic and baroclinic variability are discussed.

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