Impact of Freshwater Fluxes on Labrador Sea Dynamics in the Regional Arctic System Model

Abstract:

Continental runoff provides a critical freshwater flux into the ocean because it adds time-space varying buoyancy to the coastal ocean. This forcing is linked to the large-scale ocean dynamics and climate via the shelf-basin exchange and its resulting impact on the stratification and ventilation of the interior basin.

Here we evaluate the role that a realistic runoff forcing has on the hydrography and dynamics of the Labrador Sea by comparing results from two simulations using a subset of the Regional Arctic System Model (RASM). RASM is a regional earth system model, however in this study the atmospheric (Weather and Research Forecasting – WRF) and land (Variable Infiltration Capacity - VIC) model components are replaced with prescribed realistic atmospheric reanalysis data. Its ocean and sea ice models (Parallel Ocean Program - POP and Los Alamos Sea Ice - CICE models, respectively) are the only model components that are actively coupled via the flux coupler (CPL7). This model has a high spatial resolution of 1/12^oin the horizontal and 45 levels in the vertical direction. The results of two simulations are analyzed which vary only in the way that sea surface salinity is determined: i) restored to monthly sea surface salinity climatology, or ii) calculated based on the prescribed surface freshwater fluxes. In the first run, the sea surface salinity is restored to mean monthly climatology from the Polar Science Center Hydrographic Climatology (PHC). In the second run, the surface salinity restoring is turned off and instead more realistic surface liquid freshwater fluxes from land runoff and precipitation minus evaporation (P-E) fluxes are prescribed from the Coordinated Ice-ocean Reference Experiments version 2 (CORE2).

We find that the change in surface freshwater forcing creates a substantial difference in: (i) the modeled magnitude and spatial distribution of total kinetic energy, not only at the surface but also at depth, (ii) the sea ice extent and (iii) the spatial distribution and annual cycle of the mixed layer depth in the Labrador Sea. In addition, the hydrographic structure is more realistic in the second run when compared to observations. We further analyze the results in terms of the role that mesoscale eddies play in preconditioning or inhibiting open ocean convection and deep-water formation in the Labrador Sea.