Mean and eddy induced transport in the ocean region adjacent to the Greenland-Scotland Ridge

Lundrigan, Sarah (2019) Mean and eddy induced transport in the ocean region adjacent to the Greenland-Scotland Ridge. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The circulation and transport in the ocean region adjacent to the Greenland- Scotland Ridge (GSR) are crucial for maintaining heat, freshwater, and sea-ice exchange between the Nordic Seas and Subpolar North Atlantic Ocean. The Nordic Seas receive low-density Atlantic Water and transform it into dense water. The dense water overflow contributes to the North Atlantic Deep Water mass formation, which feeds the lower limb of the Atlantic Meridional Overturning Circulation. The thesis presents results from a study of the temperature and salinity transport in the ocean region adjacent to the GSR consisting of three sub-projects. The first sub-project is focused on model simulations of interannual to decadal variability of the exchange through the GSR and its impact on the variability of the temperature and salinity in the Nordic Seas. The model results demonstrate that the increase in the transport of fresh and cold waters through Fram Strait in the 1960s was concurrent with a reduction in the exchange over the GSR. The resulting imbalance in salinity and heat fluxes through the strait and over the ridge contributed to the freshening of the water masses of the Nordic Seas and intensified the Great Salinity Anomaly in the 1960s. In the late 1980s the AW transport over the GSR was stronger than normal while the exchange through Fram Strait was close to normal. The related imbalance in the lateral heat fluxes through the strait and over the ridge warmed the Nordic Seas and caused an increase in the temperature of the AW inflow to the Arctic Ocean in the late 1980s (i.e., about a decade earlier than the warming of the subpolar North Atlantic Ocean in the mid-1990s). The second sub-project is focused on observational estimates of the heat flux convergence of the ocean region adjacent to the GSR. Along-track altimeter and sea surface temperature satellite observations and ARGO in-situ measurements of temperature and salinity are used to investigate the heat transport by mean currents and eddies in the ocean region adjacent to the GSR from 2003 to 2008. The results from the analysis show that the heat advection by the mean flow in the surface layer is zonally asymmetric with a higher magnitude in the western part of the region. This asymmetry is associated with an excessive mean heat advection in an area adjacent to the Denmark Strait. The advection of heat is high and positive south of the strait and low and negative north of it. We suggest that this heat advection impacts the local budgets of heat and potential energy of the mean flow in the surface layer. The third sub-project studies the vertical structure of the convergence of advective fluxes of heat (HFC) and salt (SFC) in the ocean region adjacent to the GSR. The study is based on SODA (Simple Ocean Data Assimilation) ocean reanalysis for the period between 1965 and 2010. The SODA based estimates show that the high values found in satellite based estimates of HFC over the Denmark Strait are part of a pattern of high HFC and SFC in the whole water column. In this region, the HFC has a maximum at the surface. The highest values of SFC there are found in the subsurface layer at depths between 500 and 1500 m. A similar structure of high positive SFC and HFC are observed at intermediate depths over the Iceland-Faroe Ridge. The EOF analysis of the HFC and SFC shows that the variability of the HFC and SFC in these two regions are dominated by modes of coherent variations in the SFC and HFC. During warming period in the late 1980s these modes drove strong variations of the HFC and SFC over Denmark Strait and Iceland-Faroe Ridge which correlated well with the variations in the surface wind stress curl.

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