Where Do the Subpolar North Atlantic Near-Boundary Sinking Waters Go? A Lagrangian High-Resolution Model Approach
Juan Manuel Manuel Sayol1, Sotiria Georgiou2, Stefanie Leonore Ypma3, Nils Brueggemann4, Henk Dijkstra5 and Caroline A Katsman1, (1)Delft University of Technology, Environmental Fluid Mechanics, Delft, Netherlands, (2)Delft University of Technology, Geoscience and Remote Sensing, Delft, Netherlands, (3)Institute for Marine and Atmospheric Research Utrecht, Environmental Fluid Mechanics, Utrecht, Netherlands, (4)Universitaet Hamburg, Faculty of Mathematics, Informatics and Natural Sciences, Hamburg, Germany, (5)Utrecht University, Institute for Marine and Atmospheric research Utrecht, Utrecht, Netherlands
Abstract:
Previous studies based on global realistic models have shown that most of the sinking associated with the Atlantic Meridional Overturning Circulation (AMOC) occurs below the mixed layer depth nearby the subpolar North Atlantic boundaries. However, it is unclear what the contribution of these near-boundary sinking waters is to the formation of the dense Labrador Sea Water and to the lower limb of the AMOC. Recently, an idealized model that mimics the essential properties of the Labrador Sea has revealed that a significant part of the near-boundary water-masses enter the interior of the Labrador Sea, where they spend from months to a few years, to later reenter into the boundary current at a higher density. Mesoscale eddies are thought to aid the near-boundary water-masses to cross from the boundary current to the interior of the Labrador Sea and viceversa.
To shed light on the preferred pathways of the sinking near-boundary waters and on the water-mass transformation that they experience, we have applied two Lagrangian approaches using a high resolution global ocean simulation. First, we have tracked forward the water-masses with the strongest rate of sinking near the boundaries. Second, we have tracked backward the water-masses that exit the Labrador Sea following the Labrador Sea Current. These simulations have been performed with the POP (Parallel Ocean Program) model. To this end, we have advected thousands of passive particles with a repeated year of model daily data, and a nominal spatial resolution of 0.1 degree at the Equator and 42 vertical z-levels for several years. To isolate the impact of ocean dynamics from those introduced by high-frequency atmospheric events, wind, heat fluxes and river run-off are imposed as a repeated climatological annual cycle. Based on this Lagrangian view of the circulation, we assess the preferred routes of the waters involved in near-boundary sinking and of the Labrador Sea Current, their properties (temperature, salinity, potential density), how these characteristics change over time and possible connections between them.
