Behrens, Erik (2013) The oceanic response to Greenland melting: the effect of increasing model resolution. (PhD/ Doctoral thesis), Christian-Albrechts-Universität, Kiel, 151 pp.

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Abstract

This study investigates the oceanic response to an enhanced melting of the Greenland Ice Sheet. A series of forced ocean simulations with different horizontal resolutions from 0.5° to 0.05° is used. The main focus is to investigate the oceanic behaviour to a freshwater input within models of different horizontal resolutions and differing in the representation of mesoscale processes. In particular, the role of the mesoscale eddies on the spreading of freshwater in the subpolar North Atlantic is assessed. Two melting scenarios are realised, a strong meltwater release of 0.1 Sv as diagnosed by model data of climate models under high CO2 conditions, and a more realistic melting scenario, where the diagnosed melting trend of 0.53 mSv/a from 1990 - 2009 is used. The simulations are based on the NEMO ocean sea-ice model and cover resolutions from coarse 0.5° (ORCA05), to eddy-permitting 0.25° (ORCA025), and to eddy-resolving 0.05° (VIKING20). VIKING20 is a new model development, and is based on a local grid refinement approach to reach grid sizes of about 3 km around Greenland. In the both melting scenarios, the coarse resolution models (ORCA05 and ORCA025) suggest a prominent spreading of the meltwater from the Labrador Sea across the North Atlantic into the Nordic Seas. This hinders the formation of dense water masses, leading to an ongoing reduction in the AMOC. Conversely, results from VIKING20 reveal that mesoscale processes have a distinct potential to counteract the effect of the additional meltwater from Greenland. In comparison to coarser configurations, VIKING20 exhibits an equatorward export of meltwater from the Labrador Sea within the Deep Western Boundary Current and the potential to store meltwater in the northern Gulf Stream recirculation gyre. This results in less meltwater reaching the convection region of the Nordic Seas, and consequently in the realistic melting scenario no response in the AMOC is seen over three decades. The flow path of the North Atlantic Current, in particular the representation of the North-West Corner, is found to be a key factor determining the spread of freshwater in the North Atlantic. The presence of the North-West Corner, realistically reproduced in VIKING20, inhibits an enhanced eastward spreading of meltwater anomalies across the North Atlantic, preventing a pronounced freshwater leakage from the Subpolar Gyre into the Subtropical Gyre via the east Atlantic. This freshwater leakage is enhanced in both coarse configurations, especially in the strong melting case. In this artificial melting scenario the freshwater forcing predominates, such that the equatorward export along the North American coast and the presence of the North-West Corner are of minor importance in determining the oceanic response to meltwater spreading. Whereas in this case all configurations behave similarly and show a decline of the AMOC of about -40 % to -60 % after four decades, the AMOC reacts much less to the realistic melting scenario. These results emphases the need in climate projections to strive for both, realistic Greenland melting rates and represent mesoscale processes properly.

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