Structure and Variability of the North Atlantic Meridional Overturning Circulation from Observations and Numerical Models

This study presents an analysis of observed Atlantic Meridional Overturning Circulation (AMOC) variability at 26.5°N on submonthly to interannual time scales compared to variability characteristics produced by a selection of five high- and low-resolution, synoptically and climatologically forced OGCMs. The focus of the analysis is on the relative contributions of ocean mesoscale eddies and synoptic atmospheric forcing to the overall AMOC variability. Observations used in this study were collected within the framework of the joint U.K.-U.S. Rapid Climate Change (RAPID)-Meridional Overturning Circulation & Heat Flux Array (MOCHA) Program. The RAPID-MOCHA array has now been in place for nearly 6 years, of which 4 years of data (2004-2007) are analyzed in this study. At 26.5°N, the MOC strength measured by the RAPID-MOCHA array is 18.5 Sv. Overall, the models tend to produce a realistic, though slightly underestimated, MOC. With the exception of one of the high-resolution, synoptically forced models, standard deviations of model-produced MOC are lower than the observed standard deviation by 1.5 to 2 Sv. A comparison of the MOC spectra at 26.5°N shows that model variability is weaker than observed variability at periods longer than 100 days. Of the five models investigated in this study, two were selected for a more in-depth examination. One model is forced by a monthly climatology derived from 6-hourly NCEP/NCAR winds (OFES-CLIM), whereas the other is forced by NCEP/NCAR reanalysis daily winds and fluxes (OFES-NCEP). They are identically configured, presenting an opportunity to explain differences in their MOCs by their differences in forcing. Both of these models were produced by the OGCM for the Earth Simulator (OFES), operated by the Japan Agency for Marine-Earth Science & Technology (JAMSTEC). The effects of Ekman transport on the strength, variability, and meridional decorrelation scale are investigated for the OFES models. This study finds that AMOC variance due to Ekman forcing is distributed nearly evenly between the submonthly, intraseasonal, and seasonal period bands. When Ekman forcing is removed, the remaining variance is the result of geostrophic motions. In the intraseasonal period band this geostrophic AMOC variance is dominated by eddy activity, and variance in the submonthly period band is dominated by forced geostrophic motions such as Rossby and Kelvin waves. It is also found that MOC variability is coherent over a meridional distance of ~8° throughout the study region, and that this coherence scale is intrinsic to both Ekman and geostrophic motions. A Monte Carlo-style evaluation of the 27-year-long OFES-NCEP timeseries is used to investigate the ability of a four year MOC strength timeseries to represent the characteristics of lengthier timeseries. It is found that a randomly selected four year timeseries will fall within ~1 Sv of the true mean 95% of the time, but long term trends cannot be accurately calculated from a four year timeseries. Errors in the calculated trend are noticeably reduced for each additional year until the timeseries reaches ~11 years in length. For timeseries longer than 11-years, the trend\u27s 95% confidence interval asymptotes to 2 Sv/decade