Atlantic watermass and circulation response to persistent freshwater forcing in two coupled general circulation models
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The sensitivity of the Atlantic circulation and watermasses to biases in the convergence of moisture into the basin is examined in this study using two different general circulation models. For a persistent positive moisture flux into the tropical Atlantic, the average salinity and temperature in the basin is reduced, mainly below mid-depths and in high latitudes. A transient reduction in the Atlantic overturning strength occurs in this case, with a recovery timescale of 1–2 centuries. In contrast, a similar amount of freshwater directed into the Subpolar North Atlantic results in a persistent reduction in overturning and an increase in basin heat and salt content. In the unperturbed pre-industrial simulations, the Atlantic is unambiguously warmer and saltier than historical observations below mid-depths and in the Nordic Seas. The models’ tropical freshwater flux sensitivities project strongly onto the spatial pattern of this bias, suggesting a common atmospheric deficiency. The integrated Atlantic plus Arctic surface freshwater flux in these models is between −0.5 and −0.6 Sv, compared with an observational estimate of −0.28 Sv. Our results suggest that shortcomings in the models’ ability to reproduce realistic bulk watermass properties are due to an overestimation of the inter-basin moisture export from the tropical Atlantic.
KeywordsFreshwater fluxes Atlantic Climate model bias CMIP5 GFDL AMOC
M. Harrison is grateful to Rym Msadek and Rong Zhang for their feedback on an earlier version of this manuscript. Also, thanks to Robbie Toggweiler for several useful discussions. Two anonymous reviewers provided exceptional reviews and we are thankful for their efforts. All of the analysis and figures presented here were programmed exclusively in Python using freely-available packages including Numpy, Matplotlib, and NetCDF-4.
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