The Impact of Vertical Coordinates on Ice Shelf Cavity Simulations in MPAS-Ocean
McKenzie Larson, University of Colorado Boulder
The vertical coordinate in ocean models can impact the resultant oceanic circulation and mixing within the model. This has been studied more extensively in certain environments, such as continental slopes and deep-sea ridges where density currents may develop. In that scenario, the use of a vertical coordinate that follows the terrain or lines of constant density tends to result in less numerical mixing that strengthens the flow and impacts the broader oceanic circulation. The vertical coordinate in the ocean cavities under ice shelves is less studied but should exhibit a similar phenomenon as continental shelves, except the density current is buoyant and flows upward along the ice base rather than downward to the sea floor. However, the existence of a nearly vertical ice-shelf front in nature presents numerical challenges in ocean models with such an ice base-following vertical coordinate.
This study investigates the performance of two vertical coordinates (z-star [1] and z-level) within the Model Prediction Across Scales (MPAS) Ocean model [2–4] in regard to how these coordinate implementations simulate the flow within cavities. When using the z-star vertical coordinate, the ice draft (elevation of the ice shelf-ocean interface) acts as the sea-surface height so that the initial layers within the cavity are squashed under the ice shelves. Z-level coordinates do not depress the vertical grid under the weight of the ice and instead terminate when topography is encountered. For an idealized ice shelf cavity test case, z-level introduces an abundance of numerical mixing when compared to z-star, impacting the ice draft. To find a balance between inadequate and exorbitant mixing, we propose a hybrid coordinate where z-level coordinates are implemented at the top of the ice shelf and the bottom near the ocean floor, but z* coordinates are used within the cavity.
References:
Adcroft, A., and J.-M. Campin, 2004: Rescaled height coordinates for accurate representation of free-surface flows in ocean circulation models. Ocean Modelling, 7, 269–284, https://doi.org/10.1016/j.ocemod.2003.09.003.
Ringler, T., Petersen, M., Higdon, R. L., Jacobsen, D., Jones, P. W., & Maltrud, M. (2013). Ocean Modelling. Ocean Modelling, 69(C), 211–232. doi:10.1016/j.ocemod.2013.04.010
Petersen, M. R., Asay‐Davis, X. S., Berres, A. S., Chen, Q., Feige, N., Hoffman, M. J., et al. (2019). An evaluation of the ocean and sea ice climate of E3SM using MPAS and interannual CORE‐II forcing. Journal of Advances in Modeling Earth Systems, 11, 1438– 1458. https://doi.org/10.1029/2018MS001373
Golaz, J.‐C., Caldwell, P. M., Van Roekel, L. P., Petersen, M. R., Tang, Q., Wolfe, J. D., et al. (2019). The DOE E3SM coupled model version 1: Overview and evaluation at standard resolution. Journal of Advances in Modeling Earth Systems, 11, 2089– 2129. https://doi.org/10.1029/2018MS001603