Ocean – Ice Shelf Interaction

Fig. 1: Schematic representation of the polar ocean interacting with a large Antarctic ice shelf. (Photo: Alfred-Wegener-Institut)

However, simulations with our coupled ocean-ice shelf models show that changing atmospheric conditions due to climate warming might modify the circulation of the southern Weddell Sea in the near future: A redirection of the slope current transports relative warm open ocean water with temperatures around 1 degree Celsius into the deep Filchner-Ronne ice shelf cavity (Fig. 2). Consequently, melting near the grounding line increases, causing a thinning of the ice shelf, the loss of basal friction (buttressing), and an acceleration of the feeding ice streams, which ultimately impacts the mass balance of the ice sheet. Such scenario might also happen at other Antarctic coast lines where warm water of open ocean origin comes close to the ice shelf fronts. However, a retreat of the ice sheet and thus a significant contribution to global sea level rise strongly depends on the shape of the underlying bedrock.

The mass loss of the Antarctic and Greenland Ice Sheets and thus global sea level rise is strongly related to the dynamics of their ice streams. The latter drain ice into the fringing ice shelves and glaciers which, due to a reduced thickness, float on the ocean. Mass loss to the ocean occurs  via iceberg calving at the ice shelf fronts and melting at the base. Recent results, based on remote sensing, revealed basal melting to be the larger term for the Antarctic Ice Sheet.

The basal mass loss is either fueled by cold, highly saline waters, due to the depression of the freezing point, or heat transported from the open ocean into the ice shelf cavities (Fig. 1). At the deep grounding line (1300 m below sea level) of the relative small Pine Island Ice Shelf, which fringes the Amundsen Sea, this warm water causes the highest Antarctic melt rates of more than one hundred meters per year. In contrast, the big ice shelves like Filchner-Ronne and Ross with a size of up to 450,000 km² are in contact with the cold, saline shelf water, thus facing melt rates significantly less than one meter per year.  However, simulations with our coupled ocean-ice shelf models show that changing atmospheric conditions due to climate warming might modify the circulation of the southern Weddell Sea in the near future: A redirection of the slope current transports relative warm open ocean water with temperatures around 1 degree Celsius into the deep Filchner-Ronne ice shelf cavity (Fig. 2).

 

 

Fig. 2:  Simulated evolution of near-bottom temperatures in the southern Weddell Sea for the period 2030 -  2099 of the A1B scenario. Warm pulses into the Filchner Trough (year 2037) are followed by a return  of the shelf water masses to the cold state typical for today's conditions. By 2075 the tongue of slightly modified Warm Deep Water reaches the Filchner Ice Shelf front. It fills the deeper part of the Filchner Ice Shelf cavity and enters the Ronne cavity near the southern grounding line in 2081. By 2095, warm water fills most of the deep Filchner-Ronne ice shelf cavity, reaching a quasi-steady state.

 

Consequently, melting near the grounding line increases, causing a thinning of the ice shelf, the loss of basal friction (buttressing), and an acceleration of the feeding ice streams, which ultimately impacts the mass balance of the ice sheet. Such scenario might also happen at other Antarctic coast lines where warm water of open ocean origin comes close to the ice shelf fronts. However, a retreat of the ice sheet and thus a significant contribution to global sea level rise strongly depends on the shape of the underlying bedrock. The impact of the ocean on marine-terminating glaciers in Greenland was considered by the scientific community just recently. Almost all around Greenland relative warm Atlantic water can flow into the narrow fjords. An accelerated retreat of glaciers has been observed for southeast and west Greenland where the fjords are filled with water up to 4  degrees Celsius warm. In contrast, smaller changes are observed for the glaciers of northern Greenland.

The changes in the southern Weddell Sea strongly depend on the choice of the climate model providing the atmospheric forcing. Thus, additional numerical studies using more sophisticated climate models (IPCC-AR6) are necessary. The results, however, still need to be comprehensively validated partly based on long-term monitoring of the hydrographic conditions at key-points in the marginal seas of Antarctica and Greenland. The latter is an important task of our future activities in the polar seas. Since the numerical coupling of the ocean with the ice sheet is far from satisfactorily solved, model activities at AWI are focused on numerical solutions. Again, the reliability of the results strongly depends on a well-known geometry of the ice shelf cavity and the bedrock underlying the ice sheet. Therefore, the Departments of Physical Oceanography, Glaciology, and Geophysics at AWI cooperate intensively during expeditions focused on the interaction between the polar ocean and the ice sheets.