Constraining Arctic Water Mass Transformation and Ventilation Pathways in the GFDL-OM4.0

Abstract:

Dense gravity currents forced by surface buoyancy loss over polar continental shelves are important contributors to subsurface and abyssal ventilation throughout the World Ocean, yet remain challenging to observe and represent accurately in models. The vast, and rapidly evolving Arctic shelves are particularly crucial in setting water mass structure of the entire basin and influencing its response to changing climatic conditions. Observations indicate that the strongest water mass transformation processes in the Arctic occur in the Barents and Kara Sea shelves. The combination of cooling of warm Atlantic inflow as well as localized polynya development around Svalbard, Franz Josef Land, and Novaya Zemlya leads to development of waters with higher densities than even the deepest layers of the Arctic. Depending on the amount of mixing and strength of the buoyancy forcing, dense overflows also ventilate the Arctic halocline layer. Our aim is to assess how modern state-of-the-art GCMs presently capture such water mass transformation processes in the Arctic. We consider: (1) whether dense shelf overflows and the vertical structure of the Arctic are well-represented by the GFDL-OM4 1/4-degree model as well as an analogous 1/8-degree version; and (2) pathways by which overflows and water mass transformation over the Eurasian shelves contribute to setting the vertical structure of the interior Arctic. We find the 1/4-degree model has somewhat realistic temperature, salinity, and age characteristics but is overly diffuse in the vertical, with the 0-degree isotherm being roughly twice as deep as in observations. The 1/8-degree results mostly mediate this problem and are consistent with observations in providing evidence for ventilation by dense shelf flows and accurately representing vertical structure. We also explore the changes in ventilation pathways that the Arctic will experience as a result of warming. In particular – we see evidence for dense water formation changing from being influenced by polynyas and brine rejection as sea ice forms to being predominantly set by cooling of the inflowing, highly saline Atlantic waters. In order to more accurately constrain these ventilation pathways, we next aim to develop a regional Arctic model that resolves mesoscale and parameterizes submesoscale motions.