Water Mass Transformation and Formation in the Labrador Sea

Abstract

Objectively analyzed surface hydrographic fields and NCEP?NCAR reanalysis fluxes are used to estimate water mass transformation and formation rates in the Labrador Sea, focusing on Labrador Sea Water (LSW). The authors estimate a mean long-term transformation of between 2.1 ± 0.2 and 3.9 ± 0.3 Sv (Sv ≡ 106 m3 s?1) over the years 1960?99 to water with densities greater than σ = 27.65 kg m?3, depending on the correction used for the latent and sensible heat fluxes. Mean long-term formation rates are found between 0.9 ± 0.2 and 1.7 ± 0.3 Sv for σ = 27.675 ? 27.725 kg m?3 and 1.2 ± 0.2 and 2.0 ± 0.3 Sv for σ > 27.725 kg m?3. There is tremendous variability associated with these formation rates with years of strong water formation (5.7?6.6 ± 0.5?0.7 or 9.5?10.8 ± 0.7?1.1 Sv) mixed with years of little or no formation in the given density ranges. The North Atlantic Oscillation (NAO) is linked (correlation coefficient of 0.45, significant at the 99% level) with the overall formation rate for σ > 27.625 kg m?3. The observed long-term increase in net precipitation over the Labrador Sea does not seem to have had any significant effect on LSW, potentially reducing LSW transformation rates by 0.1 Sv. A reduction in surface salinity leads to formation occurring at a reduced density, but with little change in the amount of water transformed.