Figure 1: Geometry of the one-dimensional water column model (not to scale). Arrows indicate ocean-to-ice fluxes for potential temperature (blue) and salinity (red).

The Vertical Structure and Entrainment of Subglacial Melt Water Plumes 

The second phase of GROCE continues to bring new insights into climate relevant processes happening in cavities hidden below marine terminating glaciers. In these cavities, the interaction between ocean water and glacial ice contributes to the global sea level rise.

The processes happening at the ice-ocean interface are not yet fully understood, which is why more research is needed, like the one co-authored by our colleagues Hans Burchard and Markus Reinert in JAMES and published in March 2022, with the title: The Vertical Structure and Entrainment of Subglacial Melt Water Plumes

June 20, 2022

Why is it important to study the glacier-ocean interface?

In a world of global warming, the melting of glaciers terminating as floating ice tongues into the Arctic and Antarctic oceans allows those glaciers to flow faster and hence to make a considerable contribution to global mean sea-level rise. 

Underneath the ice-ocean interface, turbulent currents develop in a layer with a thickness on the order of 10 m (so-called plumes) and transport the melt water from the grounding line (where the glacier enters the ocean) toward the calving front (the seaward end of the glacier). At the plume base, ambient ocean water, relatively warm and salty, is mixed into the plume and is vertically transported toward the ice-ocean interface, where the melting is increased due to the additional heat supply.    

What method did you use to understand processes in the plume? 

Understanding how turbulence and mixing acts below the glacier is essential for the incorporation of these processes into computer models and in turn the prediction of glacier melt. In this study, an accurate high-resolution simulation model for the water column is constructed, which is able to consistently reproduce these processes. This model includes a high-order turbulence closure, that parametrizes the effects of small-scale turbulence. This method allows to realistically reproduce entrainment of underlying warmer and saltier ambient water into the plume, which in turn has a substantial effect on melting.   

What is an important result that you would like to share here? 

A sensitivity study shows that an increase of 1 °C of ambient water temperature can increase the melt rate by 50%. Steeper ice-ocean interfaces lead to strongly accelerated plume velocities and thus to higher melt rates as well. These sensitivities show that more accurate knowledge is needed about the conditions at the ice-ocean interface (e.g., under-ice topography) and the hydrodynamics below in the cavity. The algorithms developed here are proven to provide reliable results also for models with only a few grid points across the plume and can therefore be implemented into climate models with surface-following coordinates to more accurately simulate future scenarios of sea level rise.  

Figure 2: Time series of melt rate comparing results of the default scenario (def) with sensitivity simulations ayp and aym for interfacial slope (panel g), z0p and z0m for interfacial roughness (panel h) and tap and tam for ambient temperature (panel i). In these notations, ‘p’ represents an increased value and ‘m’ represents a decreased value.