Tides on Antarctic Ice Shelves

What do tides on ice shelves look like?

Tides flow in the ocean water cavities under the large ice shelves surrounding Antarctica. Most of the shelf responds just like a free floating block of ice or boat on the ocean surface, going up and down in phase with the underlying tide. Near the grounding line there is a narrow region (maybe 2-5 km wide) where the ice shelf transitions from free-floating to always grounded. By looking for these regions with satellite sensors, we can map the grounding line of ice shelves. We are mainly interested in the ice shelves at the southern ends of the largest three embayments around Antarctica. These are the Amery Ice Shelf, Filchner-Ronne Ice Shelf, and Ross Ice Shelf.

Most of what we know about the distribution of tides affecting the ice shelves around Antarctica now comes from mdoels such as CATS. However, there are also several ways of measuring tides (Click here).

Amery Ice Shelf (AIS)

A small shelf, like the AIS has a fairly simple tide: the ice shelf just goes up and down roughly in phase over the entire shelf.

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Filchner-Ronne Ice Shelf (FRIS), Southern Weddell Sea

A larger, more complicated region like the FRIS shows the flexing of the ice shelf as the tidal waves propagate around underneath it.

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Ross Ice Shelf (RIS), Southern Ross Sea

The RIS, like the FRIS, is large enough to show the flexing of the ice shelf as the tidal waves propagate around underneath.

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How do we measure ice shelf tides?

We can measure the vertical motion of the surface of ice shelves in response to tides in several ways:

  • Satellite altimetry
  • Global Positioning System (GPS)
  • Gravity meters

Satellite altimetry

Radar altimeters on the TOPEX/Poseidon satellite have been used for several years to provide tide data for the open ocean between about 66oS and 66oN. This does not include the Antarctic ice shelves, however. More recently, Fricker and Padman [2002] have shown that tides can also be detected with the European Space Agency's ERS-1 and ERS-2 satellite radars. Even though the accuracy on a single measurement of ice shelf height is only about 50 cm, with enough measurements it is possible to obtain the tidal coefficients for most major tidal frequencies to about 3-5 cm accuracy. The overall accuracy is comparable to regional tidal models such as CATS and CADA. So far, the chief value in these satellite radar data are to show weaknesses in the tides models, mainly in the model definition of ice shelf grounding line location or water depth. The ERS satellites have a maximum southward orbit of ~81.5oS, whicxh does not include the most southern parts of the large ice shelves, the FRIS and the Ross ice Shelf, thus missing regions of greatest tidal action under the Antarctic shelves.

A new altimetry sensor called the Geoscience Laser Altimeter System (GLAS), to be carried on the ICESat mission scheduled for launch in late 2002, will significantly improve upon the ERS measurements. Individual measurements of surface height are more accurate, and the orbit extends beyond all ice shelves to ~86oS. A paper is in preparation [Padman and Fricker, 2002] describing this work. Contact Laurie Padman for further information.

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Global Positioning System (GPS)

While GPS has been in use for quite a long time now, only recently has it been accurate enough for measuring ice shelf tides. With sufficient control, height variations of only a few cm can be resolved, giving accuracies that are comparable to the more usual Bottom Pressure Recorders (BPRs) used for ocean tide measurements. Most good sites so far have been obtained on the AIS, but one is available from the Ross Ice Shelf, and other GPS units have been deployed on Ross Sea icebergs.

Gravity meters

Williams and Robinson [1980: JGR] reported on their results from a set of 9 gravity records, each 30 to 60 days long, from the Ross Ice Shelf. They demonstrated that tides can be measured quite well by recording the change in gravity as the ice shelf moves up and down with respect to the earth's center of gravity. The tidal analyses from these records are still the best set of tides data for the Ross Ice Shelf. Other measurements have been made by BAS scientists near the Rutford Ice Stream and other sites on the Ronne Ice Shelf in the southern Weddell Sea. It is also possible to use short gravity records (or GPS) to calibrate longer tiltmeter records obtained in the flexural boundary layer at the edge of the floating ice. Again, BAS scientists have done most of this type of work.

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How do tides affect the ice shelves?

A lot of the interest in ice shelf tides is from people who simply see tides as noise that must be removed to see the underlying climate-scale variations clearly. Another use is to remove tides so that measurements of ice velocity can be made. However, tides also play a role in what happens to ice shelves. There are four areas that we are particularly interested in:

  • Currents help melt the ice at the bottom of the shelf;
  • Currents help rifts to widen and so help icebergs calve;
  • Tides flex the ice shelf near the grounding line, and help flush sediment out from under the ice shelf;

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