Coarse versus eddy-permitting global ocean simulations : experiments with the UVic earth system climate model.




Spence, John Paul Gordon

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This dissertation presents experiments with the UVic Earth System Climate Model that explore the sensitivity of global climate simulations to an increase of horizontal resolution into the ocean eddy-permitting range. Model versions, with resolutions ranging from 1.8' (latitude) x3.6' (longitude) to 0.2'x0,4'. are evaluated in control states and in response to climate perturbations. The effect of resolution on ocean volume transport, ocean heat transport, water mass formation, and sea ice distribu¬tion are investigated within the framework of three foci: 1) the Atlantic meridional overturning circulation and its response to surface freshwater forcing: 2) the South-ern Ocean overturning circulation and its response to poleward intensifying winds concomitant with increasing atmospheric C02; 3) the Southern Ocean temperature and Antarctic Circumpolar Current response to poleward intensifying winds alone. These sensitivity studies shed light on the ability to draw firm conclusions from coarse resolution modelling results. First, it is found that the simulation of western boundary currents. sea ice and meridional heat. transport. in the North Atlantic Ocean are improved with increasing resolution. A slowdown of the Atlantic meridional overturning circulation is robustly produced at both coarse and ocean eddy-permitting resolutions in response to fresh-water forcing applied evenly over the Labrador Sea and exclusively along its western boundary. An evaluation of the forcing impact on different regions of NADW for-mation with tracers reveals that increased Labrador Sea deep convection at higher resolution may mitigate the influence of better resolved boundary current transport. With increasing resolution. there is less cooling in the subpolar west Atlantic. more cooling in the subpolar east. Atlantic, and greater variability in the deep ocean re-sponse to the western boundary forcing. Second, it. is found that as resolution increases the interior ocean circulation be-comes more adiabatic, with deep water formed in the North Atlantic tending to upwell more in the Southern Ocean and less in the low-latitude oceans. For some density classes the transformation rate derived from surface buoyancy fluxes can provide a proxy for the net meridional transport in the upper Southern Ocean. The response of the Southern Ocean overturning to poleward intensifying southern hemisphere winds concomitant with increasing atmospheric CO2 suggest that the circulation associated with the formation of Antarctic Intermediate Water is likely to strengthen through the 21st century. Third, the zonal mean structure of the Southern Ocean temperature response to poleward intensifying winds through the 21st century, with warming between 40-55°S and cooling at higher and lower latitudes. remains robust as model resolution increases. However, the migration of distinct ocean fronts at finer resolutions can produce a strong local intensification of the temperature response. There is also less high latitude cooling and a greater loss of sea ice thickness at eddy-permitting resolutions, in conjunction with a significant increase in southward ocean eddy heat transport. Antarctic Circumpolar Current transport is found to increase in response to the forcing at both coarse and eddy-permitting resolutions.



Climate simulation, Currents