| Summary: | Atmospheric reanalyses are commonly used to force numerical ocean models, but despite large discrepancies reported between different products, the impact of reanalysis uncertainty on the simulated ocean state is rarely assessed. In this study, the impact of uncertainty in surface fluxes of buoyancy and momentum on the modelled Atlantic meridional overturning at 25°N is quantified for the period 1994/01-2011/12. By using an ocean-only climate model and its adjoint, the space and time origins of overturning uncertainty resulting from air-sea flux uncertainty are fully explored. Uncertainty in overturning induced by prior air-sea flux uncertainty can exceed 4 Sv within 15 years, at times exceeding the amplitude of the ensemble-mean overturning anomaly. A key result is that, on average, uncertainty in the overturning at 25°N is dominated by uncertainty in the zonal wind at lags of up to 6.5 years and by uncertainty in surface heat fluxes thereafter, with winter heat flux uncertainty over the Labrador Sea appearing to play a critically important role.
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