Labrador Sea Water in the Eastern North Atlantic. Part II: Mixing Dynamics and the Advective–Diffusive Balance

Abstract

Deep wintertime convection in the Labrador Basin produces Labrador Sea Water (LSW), which spreads throughout the North Atlantic. The LSW is not formed with constant characteristics but varies on decadal timescale. These changes in the source of LSW propagate throughout the North Atlantic, having a dramatic impact on middepth properties in regions far removed from the Labrador Basin. Here, by means of a property variance correlation analysis on isopycnal surfaces through the core of LSW, the authors show that the boundary anomalies of LSW survive the effects of mixing over the length and timescales taken for LSW to reach the eastern North Atlantic. Typically, the anomaly amplitude is reduced by a factor of 5 during transit, and from a simple estimate the integrated timescale for this reduction is seven years. Uncertainties in the oxygen anomaly analysis exclude specific interpretation, but constrain the boundary condition variability to about ±5%. The lateral effect of unresolved processes resembles isopycnic Fickian diffusion; and an estimate of the mean isopycnal eddy diffusion coefficient through the LSW core is 950±300 m2 s?1, with a mixing length scale of 166 ±30 km. The authors examine the advective-diffusive balance for salinity and oxygen and calculate the horizontal advective flux divergence and diffusion. Contributions from the vertical advection and diffusion terms are small and can be neglected. The uncertainties in the oxygen balance prohibit any firm conclusions regarding the time rate of change of this property. However, for salinity an unsteady contribution up to ?0.008±0.005 psu yr?1 is needed to close the budget. On σ1.5=34.64 kg m?3, comparing the unsteady contribution to the source function in the Labrador Basin the age range of LSW was found to be 10±3 years.