Diurnal tidal currents are the dominate contributors to diapycnal mixing in many regions along the pathways for warm Atlantic Water (AW) circulating within the Arctic Ocean along the continental slope. This mixing diffuses AW heat and salt into the cooler and fresher surroundings, including the upper ocean where ocean heat fluxes play a role in the stability of the ice pack. The strongest diurnal currents are associated with topographically-trapped vorticity waves, which are sensitive to stratification and mean flow. In models, these waves are also sensitive to choices for forcing and geometry. Sensitivity to background conditions implies that tidal currents and mixing will change as the Arctic evolves towards a new climate state. Here, as a first step towards understanding how diurnal tidal currents might change in a future Arctic Ocean, we describe results from a suite of high-resolution (dx=2 km) 2-D and 3-D models for Arctic diurnal tides, focusing on their currents at locations along the AW pathways. We first demonstrate that accurate representation of barotropic diurnal tides requires forcing with both open boundary conditions and the direct potential tide. Next, we use 3-D models with realistic, ocean background stratification and mean flow to describe the annual cycle of depth-averaged diurnal tidal currents. Finally, we investigate the baroclinic structure of diurnally forced waves including the generation of harmonics (semidiurnal and higher) that can contribute to mixing within the water column. Our results show that tides should be explicitly included in ocean and coupled predictive models for the Arctic to represent the feedbacks between tidal energetics and ocean mean state via mixing.

The ice sheet-ocean modeling community is making large strides toward developing coupled models capable of examining the interactions and feedbacks between ice shelves and ocean along the Antarctic margin. We present preliminary results and address some of the challenges that have arisen during the development of a coupled ice sheet-ocean model. The ice sheet model is icepack, a shallow-shelf finite element model written in Python. The ocean model is the Regional Ocean Modelling System (ROMS), a terrain-following vertical (sigma) coordinate model that has been modified to interface with a moving ice shelf. These two models are coupled in an online configuration using the Framework for Ice Sheet Ocean Coupling (FISOC). The use of a model with sigma coordinates for the ocean component introduces a simplification and a complication to modeling a moving ice draft. The sigma coordinate system retains the same number of vertical layers at any depth, eliminating the need to convert grid cells between ice and water, when using a fixed grounding line configuration. However, as the ice shelf draft evolves in time, topographic configurations develop that induce pressure gradient errors in ROMS. We quantify these errors in an idealized set-up with an artificially changing ice draft following the ISOMIP+ geometry. We compare results between an ice draft that is smoothed to meet standard ROMS smoothing criteria (rx0, rx1) and a non-smoothed ice draft. Finally, we present a simple parameterization in a buffer zone near the grounding line that uses interpolated melt rates from the ocean model, allowing us to maintain a steep ice topography in the ice model without inducing pressure gradient errors in the short water column in the ocean model. This model configuration will be applied to Pine Island Glacier and used to examine present and possible future states of the ice sheet-ocean system.

Pine Island Glacier Ice Shelf (PIGIS) is melting rapidly from beneath due to the circulation of relatively warm water under the ice shelf, driven primarily by buoyancy of the meltwater plume. Basal melt rates predicted by ocean models with thermodynamically active ice shelves depend on the representation of environmental characteristics including geometry (grounding line location, ice draft and seabed bathymetry) and ocean hydrographic conditions, and subgrid-scale parameterizations. We developed a relatively high resolution (lateral grid spacing of 0.5 km, 24 terrain following levels) model for the PIGIS vicinity based on the Regional Ocean Modeling System (ROMS). Initial stratification was specified with idealized profiles based on observed hydrographic data seaward of the ice front. Predicted basal melt rate distributions were compared with satellite-derived estimates and stratification beneath PIGIS was compared with Autosub profiles. As in previous studies, we found that the melt rate was strongly dependent on the (specified) depth of the thermocline separating cold surface waters from deep, relatively warm waters, and on the presence of a submarine ridge under the ice shelf that impedes circulation of warm deep water into the back portion of the cavity. Melt rates were sensitive to the model’s subgrid-scale parameterizations. The quadratic drag coefficient, which parameterizes roughness of the ice shelf base, had a substantial effect on the melt rate through its role in the three-equation formulation for ice-ocean buoyancy exchange. Turbulent tracer diffusion, which was parameterized by a constant value or various mixed layer models, played an important role in determining stratification in the cavity. Numerical diffusion became significant in some cases. We conclude that flow of warm water into the inner portion of the PIGIS cavity near the deep grounding line is sensitive to poorly constrained mixing parameterizations, both at the ice base and as a mechanism for allowing inflowing ocean heat to cross the sub-ice-shelf sill. Improved understanding of mixing processes is required as the community moves towards fully coupled ocean/ice-sheet models with evolving ice thickness and grounding lines.

In the Arctic Ocean, semidiurnal-band processes including tides and wind-forced inertial oscillations are significant drivers of ice motion, ocean currents and shear contributing to mixing. Two years (2013-2015) of current measurements from seven moorings deployed along °E from the Laptev Sea shelf (~50 m) down the continental slope into the deep Eurasian Basin (~3900 m) are analyzed and compared with models of baroclinic tides and inertial motion to identify the primary components of semidiurnal-band current (SBC) energy in this region. The strongest SBCs, exceeding 30 cm/s, are observed during summer in the upper ~30 m throughout the mooring array. The largest upper-ocean SBC signal consists of wind-forced oscillations during the ice-free summer. Strong barotropic tidal currents are only observed on the shallow shelf. Baroclinic tidal currents, generated along the upper continental slope, can be significant. Their radiation away from source regions is governed by critical latitude effects: the S baroclinic tide (period = 12.000 h) can radiate northwards into deep water but the M (~12.421 h) baroclinic tide is confined to the continental slope. Baroclinic upper-ocean tidal currents are sensitive to varying stratification, mean flows and sea ice cover. This time-dependence of baroclinic tides complicates our ability to separate wind-forced inertial oscillations from tidal currents. Since the shear from both sources contributes to upper-ocean mixing that affects the seasonal cycle of the surface mixed layer properties, a better understanding of both inertial motion and baroclinic tides is needed for projections of mixing and ice-ocean interactions in future Arctic climate states.

Multi-decadal expansion in the winter maximum sea ice extent (SIE) around Antarctica was interrupted by contraction, beginning in 2016 and continuing into 2019. This unexpected behavior motivates a closer look at factors controlling the position of the outer ice margin.We analyzed sea ice concentration (SIC) estimates derived from passive microwave sensors with differing resolutions (SSM/I, AMSR-E and AMSR2) to identify spatial and temporal statistics of the sea ice edge deVned by 15% SIC. The low-pass Vltered position of the ice edge is similar in different products, with the maximum northward position determined by proximity to the relatively warm waters of the Antarctic Circumpolar Current. Higher resolution SIC products reveal greater spatial detail along the convoluted margin, resulting in a relatively longer sea ice perimeter. Spectral analysis does not identify statistically signiVcant peaks in length scales along the margin; however, visual comparison with geostrophic velocities and sea surface temperature inferred from satellite altimetry suggests that advection of sea ice by mesoscale eddies is an important mechanism for deforming the ice edge in some regions, such as the Bellingshausen Sea. We analyze a high-resolution (dx=5 km), coupled ocean-sea ice model which realistically represents the annual expansion of sea ice to quantify the dynamic and thermodynamic roles of eddies in sea-ice mass balance and SIE. These eddy effects on the sea ice edge are not well represented in coarser-grid ocean reanalysis products such as ECCO-2, motivating an investigation of how to represent eddy/sea-ice interactions in global climate models.