Construction of the Adjoint MIT Ocean General Circulation Model and Application to Atlantic Heat Transport Sensitivity

We first describe the principles and practical considerations behind the computer- generation of the adjoint to the MIT ocean general circulation model (GCM), using R. Giering's software tool Tangent-linear and Adjoint Model Compiler (TAMC). The TAMC's recipe for (FORTRAN-) line-by-line generation of adjoint code is explained by interpreting an adjoint model strictly as the operator that gives the sensitivity of the output of a model to its input.

Then, the sensitivity of 1993 annual-mean heat transport across 29 N in the Atlantic, to the hydrography on 1 January 1993, is calculated from a global solution of the GCM. The "kinematic sensitivity" to initial temperature variations is isolated, showing how the latter would influence heat transport if they did not affect density and hence the flow. Over one year, the heat transport at 29deg N is influenced kinematically from regions up to 20deg upstream in the western boundary current, and up to 5deg upstream in the interior. In contrast, the dynamical influences of initial temperature (and salinity) perturbations spread from as far as the rim of the Labrador Sea to the 29deg N section, along the western boundary. The sensitivities calculated with the adjoint compare excellently to those from a perturbation calculation with the dynamical model. Perturbations in initial interior salinity influence meridional overturning and heat transport when they have propagated to the western boundary and can thus influence the integrated east-west density difference. Our results support the notion that boundary monitoring of meridional mass and heat transports is feasible.