Oceanographic conditions beneath Ronne Ice Shelf: a comparison between model and field data

Within the cavity beneath Filchner-Ronne Ice Shelf (FRIS), High Salinity Shelf Water (HSSW) is transformed into Ice Shelf Water (ISW) by a combination of melting and freezing at the ice shelf base and mixing within the water column. Evidence of these processes is abundant, both in the distribution of meteoric and marine ice in the ice shelf and in the properties of the water masses occupying the continental shelf. Quantitative information on the rate of water mass transformation is harder to come by, and more elusive still are indications of the rate-controlling factors. Is the circulation primarily “pulled” by the injection of meltwater deep beneath the ice shelf, or “pushed” by the production of dense waters at the ice front? Are there dynamical constraints on the rate of exchange at the ice front? Answers to these questions are highly relevant to the problem of how sensitive the sub-ice cavities are to externally forced climatic change. During the mid 1990’s a series of instrument strings were deployed through FRIS, in and near to the Ronne Depression (Nicholls and Makinson, 1998). Temperature and current records from the two sites within the depression showed a clear seasonality in the strength of the HSSW inflow; evidence of the importance of external forcing even deep within the cavity. The nature of the seasonal cycle, in particular the rapid increase in the HSSW flux associated with wintertime freezing north of the ice front, was further suggestive of some dynamical control on the inflow. In this paper we attempt to explain some of the key features of the seasonal signal using the results of a three-dimensional numerical model of ocean circulation within the FRIS cavity. We use a version of the Miami Isopycnic Coordinate Ocean Model (MICOM) that we have adapted for sub-ice-shelf domains (Holland and Jenkins, in press). The domain we use in this study is identical to that described by Jenkins and Holland (2000). The forcing is also almost identical, except that we have slightly modified the mid-winter surface salinity (Figure 1) to reflect the lower salinities that are currently generated over Berkner Bank (N

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