The role of tidal modulation on potential future warming behaviour in the Ross Sea

Ocean temperatures on the continental shelf in the Ross Sea and beneath the Ross Ice Shelf have remained cold in recent decades, despite climate-related warming trends in nearby regions, such as the Amundsen Sea. The Ross Sea is an important area for water mass transformation and the formation of Antarctic Bottom Water, an essential water mass in the global overturning circulation. Inflows of Circumpolar Deep Water (CDW) and outflows of High Salinity Shelf Water and Antarctic Bottom Water across the continental shelf break and beneath the Ross Ice Shelf, particularly in the west, are strongly modulated by tides. We find that tidal forcing modifies the cross-shelf circulation and regulates the inflow of warm CDW and sub-ice shelf warming, with associated impacts on basal melt rates.

Using a regional ocean model configuration (NEMO) at 1/4° resolution, which includes both the Amundsen and the Ross seas, we explore the influence of tides on potential future warming in the Ross Sea and continental shelf with four simulations as follows. The model is run with two different climate conditions: firstly, the control simulation is forced by repeat normal year atmospheric forcings, and secondly, a future 2300 climate scenario simulation is forced with air temperature +10°C and precipitation increased by a factor of two. We assess the sensitivity of both the control simulation and the 2300 climate scenario to tidal forcing by running each simulation firstly with only surface tidal forcing (no tides) and then with both surface tidal forcing and tidal harmonic forcing at the model domain lateral boundaries (tidal forcing). Under 2300 temperature and precipitation conditions, in the simulation with no tides, the Ross Ice Shelf cavity warms rapidly to temperatures of over 1°C during a 20 year period, with a rapid increase in basal melt rates. This is followed by a slower cooling period with a stabilisation of basal melt rates, leading to the cavity being filled with cold, fresh water by the end of the simulation period. In the simulation with tidal forcing, the cavity warms more gradually and remains warm, at temperatures at or above 0°C, with an associated increase in basal melt rates, for the duration of the simulation period. The tidal modulation leads to more gradual warming of the Ross Ice Shelf cavity and prevents a rapid transition of the cavity from cold to warm and from warm to fresh, as we see in the simulation without tides. This work suggests that tides are an important process to be included when modelling future climate projections.