Ocean Science www.ocean-sci.net 1812-0784 1812-0792 6 3 2010 10.5194/os-6-761-2010 http://www.ocean-sci.net/6/761/2010/ http://www.ocean-sci.net/6/761/2010/os-6-761-2010.html http://www.ocean-sci.net/6/761/2010/os-6-761-2010.pdf 761 774 2010-08-05 Impact of hydrographic data assimilation on the modelled Atlantic meridional overturning circulation G. C. Smith gregory.smith@ec.gc.ca K. Haines T. Kanzow S. Cunningham Environmental Systems Science Centre, University of Reading, Reading, UK National Oceanographic Centre, Southampton, UK now at: Environment Canada, Montreal, Canada now at: IFM-GEOMAR, University of Kiel, Kiel, Germany Here we make an initial step toward the development of an ocean assimilation system that can constrain the modelled Atlantic Meridional Overturning Circulation (AMOC) to support climate predictions. A detailed comparison is presented of 1° and 1/4° resolution global model simulations with and without sequential data assimilation, to the observations and transport estimates from the RAPID mooring array across 26.5° N in the Atlantic. Comparisons of modelled water properties with the observations from the merged RAPID boundary arrays demonstrate the ability of in situ data assimilation to accurately constrain the east-west density gradient between these mooring arrays. However, the presence of an unconstrained "western boundary wedge" between Abaco Island and the RAPID mooring site WB2 (16 km offshore) leads to the intensification of an erroneous southwards flow in this region when in situ data are assimilated. The result is an overly intense southward upper mid-ocean transport (0–1100 m) as compared to the estimates derived from the RAPID array. <br><br> Correction of upper layer zonal density gradients is found to compensate mostly for a weak subtropical gyre circulation in the free model run (i.e. with no assimilation). Despite the important changes to the density structure and transports in the upper layer imposed by the assimilation, very little change is found in the amplitude and sub-seasonal variability of the AMOC. This shows that assimilation of upper layer density information projects mainly on the gyre circulation with little effect on the AMOC at 26° N due to the absence of corrections to density gradients below 2000 m (the maximum depth of Argo). <br><br> The sensitivity to initial conditions was explored through two additional experiments using a climatological initial condition. 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