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 co mputer generation of the adjoint to the Massachusetts Institute of Technolo gy ocean general circulation model (GCM) using R. Giering's software tool T angent-Linear and Adjoint Model Compiler (TAMC). The TAMC's recipe for (FOR TRAN-) line-by-line generation of adjoint code is explained by interpreting an adjoint model strictly as the operator that gives the sensitivity of th e output of a model to its input. Then, the sensitivity of 1993 annual mean heat transport across 29 degrees N in the Atlantic, to the hydrography on January 1, 1993, is calculated from a global solution of the GCM. The "kine matic sensitivity" to initial temperature variations is isolated, showing h ow the latter would influence heat transport if they did not affect the den sity and hence the flow. Over 1 year the heat transport at 29 degrees N is influenced kinematically from regions up to 20 degrees upstream in the west ern boundary current and up to 5 degrees upstream in the interior. In contr ast, the dynamical influences of initial temperature (and salinity) perturb ations spread from as far as the rim of the Labrador Sea to the 29 degrees 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 m eridional overturning and heat transport when they have propagated to the w estern boundary and can thus influence the integrated east-west density dif ference. Our results support the notion that boundary monitoring of meridio nal mass and heat transports is feasible.