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www.ocean-sci-discuss.net/6/2667/2009/
doi:10.5194/osd-6-2667-2009
© Author(s) 2009. This work is distributed
under the Creative Commons Attribution 3.0 License.
Impact of hydrographic data assimilation on the Atlantic meridional overturning circulation
1Env. Sys. Sci. Centre, Univ. Reading, Reading, UK
2National Oceanographic Centre, Southampton, UK
*now at: Environment Canada, Montreal, Canada
**now at: IFM-GEOMAR, Univ. Kiel, Kiel, Germany
Abstract. The poleward ocean heat transports in the North Atlantic controlled by the Atlantic Meridional Overturning Circulation (AMOC), play a key role in regional climate. If the AMOC can be initialized in numerical models through ocean assimilation this may help improve the predictability of North Atlantic climate variability on timescales out to a few years. Here we make an initial step toward the development of an ocean assimilation system that can determine the 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/MOCHA 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.
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).
The sensitivity to initial conditions was explored through two additional experiments using a climatological initial condition. These experiments showed that the weak bias in gyre intensity in the control simulation (without data assimilation) develops over the period of about 6 months, but does so independently from the overturning, with no change to the AMOC. However, differences in the properties and volume of North Atlantic Deep Water (NADW) persisted throughout the 3 year simulations resulting in a difference of 3 Sv in AMOC intensity. The persistence of these dense water anomalies and their influence on the AMOC is promising for the development of decadal forecasting capabilities. The results suggest that the deeper waters must be accurately reproduced in order to constrain the AMOC.
Discussion Paper (PDF, 1438 KB) Interactive Discussion (Closed, 6 Comments) Final Revised Paper (OS)
Citation: Smith, G. C., Haines, K., Kanzow, T., and Cunningham, S.: Impact of hydrographic data assimilation on the Atlantic meridional overturning circulation, Ocean Sci. Discuss., 6, 2667-2715, doi:10.5194/osd-6-2667-2009, 2009. Bibtex EndNote Reference Manager XML
