Back to Antarctic Tides Page
Back to Antarctic Tides Page We have two primary interests in Ross Sea tides: (1) the role of tides in iceberg calving and subsequent motion and interaction between new icebergs and the remaining ice shelf; and (2) the role of tides in mixing and cross-slope exchange processes at the shelf break and Antarctic Slope Front. Here, we provide a general introduction to Ross Sea tides, then focus on tides of the Ross Ice Shelf (RIS), and tidal currents near the shelf break.
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General Introduction to Ross Sea Tides |
Tides in the Ross Sea are predominantly diurnal, based on both gravimetric data from the Ross Ice Shelf (RIS) [ Williams and Robinson, 1980] and numerical models [e.g., MacAyeal, 1984; Le Provost et al., 1998; Padman and Kottmeier, 2000]. Tide heights are largest in the eastern Ross Sea under the RIS but, even there, are much smaller than in the cavity under the Filchner and Ronne Ice Shelves in the southern Weddell Sea [ Robertson et al., 1998]. Tidal currents are large along the shelf break, again with the diurnal constituents (K1 and O1) being dominant. Here, measured tidal current speeds south of the shelf break sometimes exceed 40 cm s-1 [ Jaeger et al., 1996] while modeled currents can exceed 100 cm s-1 during spring tide (K1 and O1 in phase) in a narrow band along the shelf break [ESR/OSU CADA00.1 tidal model]. We are concerned with the impact of these strong currents on the location of the Antarctic Slope Front (and thus, the possible role of the front on cross-slope drainage of dense shelf water into the deep ocean), and on the mixing of the various water masses that come together at this site. Near the RIS front, we are concerned with how the spatially variable tidal stress contributes to the rifting of the ice sheet as a precursor to iceberg calving, and the motion and collision dynamics of icebergs after calving. Under the RIS, tides are expected to provide most of the kinetic energy for ocean mixing and ablation of the ice base. Tidal currents near the RIS front can also aid lateral mixing across the topographic vorticity gradient at the front, which otherwise acts to isolate the circulation of the under-ice cavity from the open ocean.
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Tides of the Ross Ice Shelf (RIS) |
Below, we show maps of RMS tidal height variability and mean tidal current speed for the RIS region, based on CADA00.1, the Circum-Antarctic Data Assimilation model.
 Click on figure for higher resolution image (~100 Kb) |
 Click on figure for higher resolution image (~100 Kb) |
Currents are weak in the western RIS, where water column thickness (WCT) is greatest, but can exceed 30 cm s-1 in the eastern RIS, particularly along the Siple Coast near the Steershead Ice Rise (~82oS, 200oE). Currents are, however, sensitive to the quality of the WCT map. There is some evidence that the Crary Ice Rise (~83oS, 190oE) should actually be connected to the coastline to the east of it: we are running models with revised bathymetry to see if this helps to better fit the tide data with our model. The ice front region just west of, and to the northeast of, Roosevelt Island (~79.5oS, 199oE) has recently undergone a major calving event, which is documented by Matthew Lazzara at http://uwamrc.ssec.wisc.edu/amrc/iceberg.html. The role of tides in initiating this event, and controlling the subsequent motion and collision dynamics of the new icebergs with the remaining ice front, is being investigated by a group led by Doug MacAyeal (University of Chicago).
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Tidal Currents Near the Shelf Break |
Tidal currents have been measured at Mooring C (72.48oS, 172.52oE) just south of the shelf break in the NW Ross Sea [ Jaeger et al., 1996] (Location shown by the green star on the map of mean tidal current speed, below).
Here, currents frequently exceed 40 cm s-1 during diurnal spring tides (K1 and O1 in phase), see time series below.
The major axis of each of K1 and O1 is ~15-20 cm s-1, so at diurnal neap tide the current is very small. Significant ocean processes that vary with mean tidal current, such as cross-slope advection of the Antarctic Slope Front, and mixing in the bottom boundary layer, will thus have a roughly 2-week time scale associated with them. The mean tidal current speed from CADA00.1 shows that elsewhere on the continental slope, the mean speed may be >50 cm s-1, with instantaneous maximum speeds of >100 cm s-1 during diurnal spring tide. The significant benthic stirring due to the presence of such strong currents is expected to play a major role in the cross-slope exchange processes and water mass conversion near the shelf break and the Antarctic Slope Front. The cross-slope advection distance associated with these currents can exceed 20 km, thus periodically forcing the location of the front to regions of significantly different water depth. Combined with effects due to the nonlinearity of the equation of state (thermobaricity, cabbeling), this advection may play a significant role in injection of shelf-derived water masses into the deep ocean circulation.
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