Antarctic Sea Ice Response to Tides

Adding Tides to Ice Models

Baroclinicity

The following results are taken from Padman et al. (2005).

So far, we have assumed that tidal currents are constant with depth. This is often a fairly good approximation in Antarctic seas. However, sometimes we need to worry about “baroclinic” tides. These are tidal currents that exists because of stratification in the ocean. Here is an example of what baroclinicity does to the tidal currents.

The left hand plot shows how ocean density changes with depth. Here, we basically have a 2-layer ocean, with a density gradient (or “pycnocline”) between 200 and 400 m depth. The effect is, when tides flow across steep topography like the South Scotia Ridge, the interface acts a bit like the ocean surface, and a wave is generated that travels along the pycnocline. The result is much greater variability of surface currents and divergence than you get in a depth-averaged model.
Below, we look at the map of divergence for the northern Weddell Sea, comparing a depth-averaged model with a 3-D model based on much more complicated equations of fluid motion.



We find that the generation of internal tides along the South Scotia Ridge in a 3-D ocean tidal model leads to much higher ice divergence than for the case of depth-averaged currents. Instead of the rms divergence being about 2% along the ridges, we get values that frequently exceed 10%.

Using 3-D rather than depth-averaged models slows down the modeling effort by a couple of orders of magnitude. However, as this example shows, it will be necessary to do this in order to get the effect of ocean tides on sea ice right.