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The Atlantic; strong but subservient in the meridional overturning circulation

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If you have a question about this talk, please contact Deb Shoosmith.

Open to non-BAS; please contact Deb Shoosmith (drsho@bas.ac.uk or 221702) if you would like to attend.

North Atlantic (NA) deep water formation and the resulting Atlantic meridional overturning cell is generally regarded as the primary feature of the global overturning circulation and believed to be a result of the geometry of the continents. Here, instead, we view the overturning as a global energy-driven system and investigate the robustness of NA dominance within this framework. Using an idealized-geometry ocean general circulation model coupled to an energy moisture balance model, we test the effect of various climatic forcing mechanisms on the strength and structure of the overturning circulation. Without winds or a high vertical diffusivity, the ocean does not support deep convection. A supply of mechanical energy through winds or mixing (purposefully included or due to numerical diffusion) starts the deep water formation. Once deep convection and overturning set in, the distribution of convection centers is determined by the relative strength of the thermal and haline buoyancy forcing. In the most thermally dominant state (i.e., negligible salinity gradients), strong convection is shared among the NA, North Pacific (NP) and Southern Ocean (SO) while near the haline limit, convection is restricted to the NA. The effect of a more vigorous hydrological cycle is to produce stronger salinity gradients, favoring the haline state of NA dominance. In contrast, a higher mean ocean temperature will increase the importance of temperature gradients because the thermal expansion coefficient is higher in a warm ocean, leading to the thermally dominated state. An increase in SO winds or global winds tends to neutralize the salinity gradients, also pushing the ocean to the thermal state. The NA overturning does not respond directly to changes in its own local forcing but rather responds indirectly to the changes that occur in the SO and NP.

This talk is part of the British Antarctic Survey series.

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