Sensitivity of Labrador Sea Water Formation to Changes in Model Resolution, Atmospheric Forcing, and Freshwater Input

Labrador Sea Water (LSW) is one of the main contributors to the lower limb of the Atlantic Meridional Overturning Circulation. In this study, we explore the sensitivity of LSW formation to model resolution, Greenland melt, absence of high-frequency atmospheric phenomena, and changes in precipitation. We use five numerical model simulations at both (1/4)degrees and (1/12)degrees resolutions. A kinematic subduction approach is used to obtain the LSW formation rate over the period 2004 to 2016. The control simulation, with (1/4)degrees resolution, showed a mean annual production rate of 1.9Sv (1Sv=10(6)m(3)/s) in the density range of 27.68-27.80kg/m(3) for the period 2004-2016. Deep convection events that occurred during 2008, 2012, and 2014-2016 were captured. We found that with (1/4)degrees resolution the LSW formation rate is 19% larger compared with its counterpart at (1/12)degrees resolution. The presence of Greenland melt and an increase in the precipitation impact the denser LSW layer replenishment but do not decrease the overall LSW formation rate nor the maximum convection depth. A dramatic response was found when filtering the atmospheric forcing, which induced a decrease of 44% in heat loss over the Labrador Sea, strong enough to halt the deep convection and decrease the LSW formation rate by 89%. Even if our experiment was extreme, a decrease in the storms crossing the Labrador Sea with a consequent reduction in the winter heat loss might be a bigger threat to deep convection and LSW formation in the future than the expected increases in the freshwater input. Plain Language Summary The Labrador Sea, located between Greenland and Canada, is where strong winter cooling makes the surface waters lose heat to the atmosphere, get denser, and sink to depths between 500 and 2,500m. This sinking brings heat and dissolved gases like oxygen and carbon dioxide into the deep ocean. The resulting water mass is known as Labrador Sea Water (LSW). The process through which LSW is formed is sensitive to freshwater inflow into the formation region and storms passing over the Labrador Sea. While an increase in freshwater would inhibit the densification and sinking of surface water, fewer storms would reduce heat loss also reducing the ability of the surface waters to gain in density and sink to greater depths. These are all changes projected to occur due to the ongoing anthropogenic climate change. By using a numerical model, we explore how the increase in freshwater from Greenland melt and precipitation, and the absence of storms could impact LSW formation. We found that by having more freshwater going into the formation region the overall LSW formation rate does not decrease, however, the water mass becomes lighter. In the absence of the storms, the formation rate drastically decreased.