On Barotropic Mechanisms of Uncertainty Propagation in Estimation of Drake Passage Transport

Abstract

Uncertainty in estimation of Drake Passage transport is analyzed in a Hessian-based uncertainty quantification (UQ) framework. The approach extends the adjoint-based ocean state estimation method to provide formal error bounds functionality. Mechanisms of uncertainty propagation in an idealized barotropic model of the Antarctic Circumpolar Current are identified by analysis of Hessian and Jacobian derivative operators, generated via algorithmic differentiation (AD) of the MIT ocean general circulation model (MITgcm). Inverse and forward uncertainty propagation mechanisms are identified, projecting uncertainty between observation, control and state variable domains. Time resolving analysis of uncertainty propagation captures the dynamics of uncertainty evolution and reveals transient and stationary uncertainty regimes. The UQ system resolves also the dynamical coupling of uncertainty across different physical fields, as represented by the off-diagonal posterior covariance structure. The spatial patterns of posterior uncertainty reduction and their temporal evolution are explained in terms of barotropic ocean dynamics. Global uncertainty teleconnection mechanisms are associated with barotropic wave propagation. Uncertainty coupling via data assimilation is demonstrated to dominate the reduction of Drake Passage transport uncertainty, highlighting the importance of correlation between different oceanic variables on the large scale.