Estimates of meridional overturning circulation variability in the North Atlantic from surface density flux fields

A method developed recently by Grist et al. (2009) is used to obtain estimates of variability in the strength of the meridional overturning circulation (MOC) at various latitudes in the North Atlantic. The method employs water mass transformation theory to determine the surface buoyancy forced overturning circulation (SFOC) using surface density flux fields from both the Hadley Centre Coupled Model version 3 (HadCM3) and National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) reanalysis observational data set. The previous application of the method was at 48 degrees N using 100 years of model output, and here we show from a longer 400 year data set that it can be extended to provide useful estimates of the MOC variability in the range 35-65 degrees N. The method relies on averaging of the SFOC over an interval prior to that at which the MOC estimate is required. The length of this interval increases as the latitude decreases from about 6 years at 65 degrees N to 15 years at 36 degrees N. Values for the correlation coefficient between the HadCM3 SFOC and MOC time series of 0.60, 0.64, and 0.39 are obtained at 60 degrees N, 48 degrees N, and 36 degrees N. Thus, the SFOC approach may provide valuable complementary information about MOC variability in the middle-high-latitude North Atlantic to that determined from the Rapid array at 26 degrees N but it becomes less useful as latitude decreases. The method is then applied using the NCEP/NCAR reanalysis to estimate MOC variability in the middle-high-latitude North Atlantic for the past 50 years.