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Ocean Tides Affect Ice Loss from Large Polar Ice Sheets

A recent paper in Reviews of Geophysics discusses how ocean tides affect the motion of, and loss of ice from, the Antarctic and Greenland ice sheets.

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The projected loss of ice from the huge Antarctic and Greenland ice sheets during the twenty-first century will raise global mean sea level. Present uncertainties of ice shelf mass loss are large, however, with estimates of their contribution to sea level rise ranging from a few centimeters to over one meter. Reducing these uncertainties by improving models of the processes that cause ice sheets to lose mass is, therefore, a critical goal of Earth Science research.

At present, most mass loss from ice sheets occurs at their marine margins, through iceberg production and melting by the ocean. Tides are responsible for large changes in ocean surface height and in ocean currents that help set the rate at which ice melts. A recent article in Reviews of Geophysics explores the role that tides play in ice sheet mass loss and in the interpretation of measurements that are used to identify longer-term trends in the thickness and velocity of ice sheets. The editors asked the authors to explain more about research to measure and model tides and their effects.

How do scientists collect data on tides around the marine margins of ice sheets?

Tide heights near ice shelves can be measured using traditional coastal tide gauges and bottom pressure recorders, while currents can be measured with meters on moorings in the open ocean or deployed through boreholes drilled through ice shelves, which are the floating portions of ice sheets. Ice shelves float hydrostatically on the ocean, so that GPS units placed on the ice shelf surface also record the ocean tide. GPS units also measure tidal variability in the horizontal motion of both floating and grounded ice. Measurements by satellites are too widely spaced in time to resolve tides. However, we can combine detailed knowledge of tidal dynamics, and observations of ice shelf surface height from satellite radar and laser altimeters, to use these data to improve models.

Diagram of ice sheet marine margin and methods to measure tide heights
Schematic of the marine margin of an ice sheet where there is a floating ice shelf. Methods for measuring tide heights include: coastal tide gauges (not shown); bottom pressure recorders (BPR) mounted on the seabed; gravimeters and GPS units mounted on the ice-shelf surface; satellite radar and laser measurements of vertical motion of the ice shelf and ocean surface; and coarse-resolution measurements of ocean mass changes by the GRACE gravity mission. Credit: Padman et al., 2018, Figure 3; image created by Jennifer Matthews, IGPP, Scripps Institution of Oceanography

How are tides modeled?

Global models of the tide height and depth-averaged tidal currents are based on the well-understood physics of gravitational forcing by the Moon and the Sun, and the equations of motion for the ocean. Tide measurements are then used to reduce model errors through a process called “data assimilation.” Around the ice sheets, modelers use a finer grid for key areas including Greenland fjords and Antarctic ice stream inlets, and assimilate a wider range of data including ice shelf height changes from GPS units and satellite altimeters.

Tidal variability in more complex regional models that include sea ice, ice shelves, and ocean currents that vary with depth, is obtained by using global models to set tides at the regional model’s open boundaries. These models allow us to assess the contribution of tides to melting of the ice shelf by the ocean.

Do tides change over time?

Tide heights and currents depend on global and local changes in the locations of coastlines (grounding lines for regions where ice shelves are present) and water depth. These vary as sea level changes from growth or loss of ice sheets through the ice age cycles. On much longer time scales, tides change as plate tectonics modify the locations of continents and shapes of ocean basins.

On time scales of years to centuries, the largest changes in tides around the ice sheets are likely to be caused by changes in the extent and thickness of ice shelves. These cause interesting, potentially stabilizing, feedbacks in models: if an ice shelf thins or retreats as the ocean or atmosphere warms, tidal currents can weaken as water depth increases, leading to lower melt rates.

What does tidal motion of grounded ice tell us about wider ice sheet dynamics?

GPS and Interferometric synthetic aperture radar velocity records from grounded portion of Greenland and Antarctica have shown that the velocity of fast flowing ice streams and outlet glaciers can be tidally modulated up to 100 kilometers upstream from the grounding line at periods ranging from minutes to half a year. These observations show that fairly small changes at the grounding line can be transmitted far inland, highlighting the sensitivity of ice dynamics near the marine margin. Modeling studies have used these tidal observations on ice streams and outlet glaciers to constrain frictional properties of the ice-bed interface and to quantify complex interactions between ocean forcing, subglacial hydrogeology, and ice dynamics.

What additional data and modeling efforts are needed?

Global distribution of tidal range and mean tidal currents from a global tide model. Top: Range, in meters, from annual low to annual high tide. Bottom: Annual averaged tidal current speed, in meters per second. Credit: Padman et al., 2018, Figure 1a and 1b

Tide model accuracy depends primarily on the quality of water depth data. Large regions of the seabed around the Greenland and Antarctic ice sheets are undersampled for water depth, including most regions under ice shelves.

Programs to improve the data set of water depths are critical, not only for tide modelers but also for researchers studying general ocean effects on ice sheets.

The community also needs to increase the database of tide-resolving measurements of vertical motion of ice shelves and lateral motion of grounded and floating ice. Ice-mounted GPS systems are, currently, the best approach to this goal.

The coastline around the marine margins of major ice sheets can be complex; for example, many fjords and inlets through which the ice sheets deliver ice to the ocean are less than a few kilometers wide and so not resolved by the fairly coarse grids of typical global tide models. High-resolution models are needed to accurately predict tide heights and currents in these narrow features.

Improvements in model physics are also needed. For calculating melting at the base of ice shelves and the fronts of tidewater glaciers, we need better estimates of ice roughness represented in models by a “drag coefficient.” Since roughness might change with time, we need to develop a model of the drag coefficient that depends on large-scale characteristics of the ice.

—Laurence Padman, Earth & Space Research, Oregon; email: [email protected]; and Matthew R. Siegfried, Department of Geophysics, Stanford University

Citation: Padman, L., and M. R. Siegfried (2018), Ocean tides affect ice loss from large polar ice sheets, Eos, 99, https://doi.org/10.1029/2018EO092835. Published on 20 February 2018.
© 2018. The authors. CC BY-NC-ND 3.0