Elucidating large-scale atmospheric controls on Bering Strait throughflow variability using a data-constrained ocean model and its adjoint

Abstract

A regional data-constrained coupled ocean-sea ice general circulation model and its adjoint are used to investigate mechanisms controlling the volume transport variability through Bering Strait during 2002 to 2013. Comprehensive time-resolved sensitivity maps of Bering Strait transport to atmospheric forcing can be accurately computed with the adjoint along the forward model trajectory to identify spatial and temporal scales most relevant to the strait's transport variability. The simulated Bering Strait transport anomaly is found to be controlled primarily by the wind stress on short time scales of order 1 month. Spatial decomposition indicates that on monthly time scales winds over the Bering and the combined Chukchi and East Siberian Seas are the most significant drivers. Continental shelf waves and coastally trapped waves are suggested as the dominant mechanisms for propagating information from the far field to the strait. In years with transport extrema, eastward wind stress anomalies in the Arctic sector are found to be the dominant control, with correlation coefficient of 0.94. This implies that atmospheric variability over the Arctic plays a substantial role in determining Bering Strait flow variability. The near-linear response of the transport anomaly to wind stress allows for predictive skill at interannual time scales, thus potentially enabling skillful prediction of changes at this important Pacific-Arctic gateway, provided that accurate measurements of surface winds in the Arctic can be obtained. The novelty of this work is the use of space and time-resolved adjoint-based sensitivity maps, which enable detailed dynamical, that is, causal attribution of the impacts of different forcings.Plain Language Summary An ocean circulation model, that was adjusted to match observations, is used to investigate what are the important factors controlling the oceanic flow of water through the Bering Strait. Results show that the flow through the strait is related to surface atmospheric winds over the Bering Sea Shelf (south of the strait) and the near coastal regions of the Arctic Ocean (north of the strait). In the model, knowledge of these winds over the preceding 1 month allows us to reconstruct most of the changes in the flow through the strait. A somewhat surprising result is that winds in the Arctic have a greater influence on the amount of water flowing through the Bering Strait than winds over any region of the Pacific Ocean or the Bering Sea. The connection between the winds and the flow through the strait is strong enough that interannual changes in the winds may be used to predict interannual change in the flow. This predictive skill opens up the prospect for an improved understanding of the causes and mechanisms of flow changes at this important Pacific-Arctic gateway, provided that accurate measurements of surface winds over the Arctic can be obtained.