A Process Study of the Atlantic Water Transport over the Greenland-Scotland Ridge

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The Greenland-Scotland Ridge (GSR) system plays an outsized role in the climate system. In the surface layer, the warm and saline Atlantic Water flows northward across the GSR and the light Polar Waters moves southward mainly through the Denmark Strait. In the lower layer, the overflow transports the cold and dense water over the GSR into the Atlantic Ocean. This inflow of Atlantic Water is the primary oceanic heat transport to the Arctic Ocean and the Nordic Seas. Changes in this transport over the GSR affect the state of the ocean, climate, sea ice and ecosystem in the Nordic Seas and the Arctic Ocean. Yet there is still a significant gap in understanding how the transport of the Atlantic Water is responding to changes in atmospheric forcing. The mean transport of the Atlantic Water inflow is primarily forced by buoyancy flux. Variability on seasonal to inter-annual time scales are attributable to both wind stress and buoyancy fluxes, but there remain many uncertainties regarding how and from where the atmospheric forcing influences the Atlantic Water transport. The Atlantic Water inflow is influenced not only by atmospheric forcing locally at the GSR, but in the two basins that it is connected with. It also interacts with the Nordic Seas Overflow. The proposed study seeks to identify essential mechanisms and processes that link changes in the atmosphere to Atlantic Water transport. It will help assessments of climate prediction models and reduce uncertainties of a potential key driver for future changes in the Arctic climate system. This study will contribute to the Ocean Outlook initiative, a joint venture established in 2015 between WHOI and the Bergen Marine Research Cluster in Norway and will also further the goal of enhancing trans-Atlantic collaborations as outlined in the Galway agreement.The Greenland-Scotland Ridge (GSR) is the main topographic barrier between the Atlantic and the Nordic Seas/Arctic Ocean. It restricts flows between the warm and saline Atlantic Ocean and the cold and fresh Arctic Ocean. The exchange flow of Atlantic Water over the GSR is influenced by multiple and complex processes and forcing. This project will examine key dynamical processes that govern exchange flows between the Atlantic Ocean and the Nordic Seas that are separated by a shallow GSR. Process studies using numerical models with available observations and a data-assimilated ocean estimate will be conducted to examine how local and remote forcing gives rise to variations in Atlantic Water transport over the GSR. The goal is to develop a better understanding of key mechanisms and processes that govern the Atlantic Water inflow and its variability across the GSR. First a 2-layer model will be used to develop intuitions, and to examine leading order dynamical balances. The efficiency of the model allows multiple simulations over several decades. The simplicity and transparency in dynamics are ideally suited for developing ideas and elucidating competing mechanisms and in isolating key processes. The mechanisms and processes that are identified in the 2-layer model will be examined by using the more realistic 1/12 degree Hybrid Coordinate Model (HYCOM). The HYCOM model will also be used to examine the role of buoyancy forcing in the variability of Atlantic Water transport. The data-assimilated Estimating the Circulation & Climate of the Ocean version 4 (ECCO 4) and in situ and satellite observations will be used to check the model?s consistency with observational data and data-assimilated estimates. The premise is that the ECCO 4, with its assimilation of temperature/salinity profiles, satellite sea surface height and bottom pressure, represents the variability more realistically than ocean models without data assimilation. Overall, the study will improve current understanding of a key oceanic process that is very important for ocean circulation and climate in both Arctic and Atlantic. Exchange flows over topography between two basins are common features in the world?s ocean circulation system. Other examples include the flow from the Pacific to the Arctic Ocean through Bering Strait, throughflows in the Japan Sea, etc. This study will result in a better understanding of intrinsic dynamics that are important to general cases of cross-ridge flows between two basins in the world ocean.