Diapycnal mixing in the subpolar North Atlantic

Abstract

Model simulations of climate change and global overturning circulation are quite sensitive to the strength and distribution of mixing. However, the field observations are only sparsely available. The strength of diapycnal mixing was estimated from more than 700 profiles of hydrographic and velocity measurements in the subpolar North Atlantic (SPNA). These measurements were collected during hydrographic surveys from 2003 to 2011, ranging from 40 N to 62 N in latitude. Furthermore, 28 Micro-scale structure profiles were collected at 7 stations over the Mid-Atlantic Ridge and at the western boundary during the cruise in 2008, providing supplement of direct measurements and an agent to evaluate the overall estimation. Under the assumption of a steady state, spatial distribution and vertical structure of diapycnal diffusivity were mapped in this area. The inferred diffusivity is generally elevated compared to the background diffusivity in the open ocean and shows large variability in the SPNA. diffusivity of at least one magnitude larger can be seen nearly in the whole area several hundread of meters within seafloor. Strong mixing at mid-depth of one to two orders larger than the background value are found at western boundary, over Mid-Atlantic ridge and in the pathway of North Atlantic Current and deep currents. The possible connections between enhanced mixing and several environmental parameters including seafloor roughness, geostrophic currents and meso-scale eddies are analysed. Conversions between components of the North Atlantic Deep Water associated with mixing are estimated from vertical motion inferred from density field and turbulent diffusivity based on an advection-diffusion balance model. In vertical direction, averaged diffusivity is found to decrease with the height above seafloor within the deepest 1500 m and to be constant. A transformation of around 1.6 Sv from Gibbs Fracture Zone Water to overlying Labrador Sea water is derived; the transformation between lowest Denmark Strait Overflow Water to upper Gibbs Fracture Zone Water is about 3.5 Sv.