Ice Shelf Basal Melt Sensitivity to Tide‐Induced Mixing Based on the Theory of Subglacial Plumes

Abstract

Abstract Tidal currents are known to influence basal melting of Antarctic ice shelves through two types of mechanisms: local processes taking place within the boundary current adjacent to the ice shelf‐ocean interface and far‐field processes influencing the properties of water masses within the cavity. The separate effects of these processes are poorly understood, limiting our ability to parameterize tide‐driven ice shelf‐ocean interactions. Here we focus on the small‐scale processes within the boundary current. We apply a one‐dimensional plume model to a range of ice base geometries characteristic of Antarctic ice shelves to study the sensitivity of basal melt rates to different representations of tide‐driven turbulent mixing processes. Our simulations demonstrate that tides can either increase or decrease melt rates depending on the approach chosen to parameterize entrainment of ambient water into the turbulent plume layer, a process not yet well constrained by observations. A theoretical assessment based on an analogy with tidal bottom boundary layers suggests that tide‐driven shear at the ice shelf‐ocean interface enhances mixing through the pycnocline. Under this assumption our simulations predict a tide‐induced increase in melt and freeze rates along the base of the ice shelf, with the strongest plume path‐integrated effects for cold cavities (up to +400% in the realistic set up). An approximation is provided to account for this response in basal melt rate parameterizations that neglect the effect of tide‐induced turbulent mixing. Plain Language Summary Most of Antarctica's coastline is fringed by floating ice platforms called ice shelves. Many ice shelves are thinning through a process called basal melting. This ocean‐driven process influences how much the Antarctic Ice Sheet is contributing to global sea level rise. A better understanding of the mechanisms that drive basal melting will therefore help to improve the accuracy of sea level projections. Basal melting is governed by a complex interplay between ocean conditions, the shape of the base of the ice shelf, and tides. Here we use a one‐dimensional computer model to study how currents generated by tides influence basal melting through processes that occur close to the interface between the ice and the ocean. Our model predicts that tidal currents generate an increase in basal melting. However, we also show that the results are sensitive to assumptions made when representing the effects of tides in the computer code. Based on our model results we provide an expression that can be used to estimate the average effect of tidal currents on basal melt rates. Key Points Basal melt sensitivity to tide‐induced shear at the ice‐ocean interface is evaluated from one‐dimensional plume simulations Tidal forcing enhances basal melting/freezing when tides are included in the entrainment formulation, and vice versa There is a quadratic relationship between the relative increase in net tidal melt rate and the ratio between tidal and plume velocities