Australia: The Land Where Time Began

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Amundsen Sea Ice Shelves – Increased Melting of Ice Shelves in the Amundsen Sea

In this paper Robertson investigated the tidal effects on the circulation under the ice shelves and the melting of the ice shelf in the Amundsen Sea by comparing simulations that includes tides and without tides. The tidal impacts in the Amundsen Sea depended on the location of the ice shelf front with respect to the M₂ effective critical latitude. The critical latitude is the latitude at which the tidal frequency equals the inertial frequency. The effective critical latitude is the latitude at which the tidal frequency equals the inertial frequency that has been adjusted by relative vorticity, such as that which is associated with a gyre that is wind driven. Tides increased mixing in front of, as well as beneath the ice shelf, and flow into cavities on the underside of the ice shelf, by as much as 50 %, in spite of tides being weak compared with the mean flows, for ice shelves that are located equatorwards of the M₂ effective critical latitude. Also, melting of ice shelves was increased by tides by 1-3.5 m/yr, which is 50 % for the Dotson Ice Shelf and 25 % for the Pine Island Ice Shelf. Tidal residual flows were not the cause of these enhancements; instead, they originated from resonant effects, baroclinity increases of the velocities, and higher mixing, which are all associated with critical latitude effects in the internal tides. Flow into the cavity is retarded by tides for ice shelves polewards of the effective critical latitude.

Amundsen Sea ice shelves have undergone the most rapid melting in the Antarctic recently (Rignot et al. 2008). The ocean has been found to drive this melting, occurring at the base of the ice shelf, the most rapid melting being near the grounding line (Bindschadler, 2006). Density is the primary driver of the flow of “Warm” water into the cavity on the base of the ice shelf, and circulation within it, and this is highly influenced by topographic controls and the hydrography (Jenkins et al., 2010; Schodlok et al., 2012). Melting is also believed to be increased by enhancing mixing and warm water flow under the ice shelf.

Models have been used to investigate he roles played by flows that are density-driven and tides in circulation under the ice shelf and melting of the ice shelf, as it is very difficult and expensive to obtain measurements of the melting of the ice shelf. Flow into and under the ice shelf has been shown by simulations to be determined predominantly by the topography, of the ice shelf as well as the sea floor, which act as controls on the flow, especially the warmer water entering the ice shelf cavity. A combination of ice shelf and bottom ridge has been found to restrict flow of “warm” water entering the ice shelf cavity (Jenkins et al., 2010). The gap between the ice shelf and the sea floor widened when the ice shelf eroded, which allowed more “warm” water to flow over the ridge into the cavity beyond it, therefore the basal ice shelf melting increased in the cavity beyond the ridge (Jenkins et al., 2010).

Tides have been found by several investigators to play a significant role. In the Weddell Sea it was determined (Makinson et al., 2011) that tides increased significantly the mixing and circulation beneath the Ronne-Filchner Ice Shelf, and doubled the rate of ice shelf melting. Tides generated currents further west in the Weddell Sea up to 5 cm/sec that equalled or exceeded the mean density-driven flow, and modified its distribution (Mueller et al., 2012). And tidal currents in the Amery Sea enhanced the rates of melting and freezing with large fluctuations in heat content that was associated with the spring-neap cycle (Galton-Fenzi et al., 2012). According to Robertson the role of this project is to quantify the tidal effects on the melting of the ice shelves, as well as the mixing and circulation in the ice shelf cavities in the Amundsen Sea, recognising that the predominant controlling factor for these processes is topography.

Discussion

The critical latitude, conceptually, is taken as the exact latitude based on the planetary vorticity. It is indicated, however, by observations that an effective critical latitude is the real controller of the dynamics, based on total vorticity, both planetary and relative. The critical latitude can be shifted by several degrees by the relative vorticity of the swirling mean flow (Kunze & Toole, 1997).

The existence of a gyre was shown by observations of the circulation in Pine Island Bay (Fig 13 in Tortell et al., 2012). The Pine Island Ice Shelf is effectively north of critical latitude where it would experience a circulation increase and increased melting due to the tides, as the relative vorticity of this gyre is sufficient to shift the critical latitude by about 1.0^oS. A simulation was performed with the Pine Island Ice Shelf at its actual location to test this, as well as with the wind conditions experienced during NB0901 and the wind-driven gyre that resulted. The circulation that resulted was found to be very similar to the shifted domain. Similarly, the melt rate with the wind-driven gyre is higher. Therefore, the critical latitude is effectively shifted by 1^o southwards by the wind-driven gyre, though it was noted that there were differences between the wind-driven and shifted simulations.

In Pine Island Bay a wind-driven gyre was observed in the summer of 2309. According to Robertson it is possible that a gyre will not be generated throughout most of the year, as this region can be ice-covered much of the time. It has been observed (Schodlok et al., 2012), however, that a wind-driven gyre frequently forms in front of the Pine Island Ice Shelf in their wind-driven simulations, and it has been noted (Mankoff et al., 2012) that in observations, 2 modes have been present in Pine Island Bay over the last 25 years. A single larger polynya and more open water have been exhibited in one mode, and persistent small polynyas and more sea ice cover are present in the other mode. At 3 locations at the front of the ice shelf the small polynyas were also observed (Bindschadler et al., 2011). The potential for the effective critical latitude to be shifted polewards of the ice shelf front and for tides to promote flow beneath the ice shelf and ice shelf melting there is increased by both these modes, especially the first.

The thickness of the boundary layer and mixing is also affected by the critical latitude, with increases in thickness of the boundary layer and mixing increasing near critical latitude. Mixing increased in the ice shelf cavity and in front of the Pine Island Bay ice shelf, for both shifted domain and for wind-driven gyre, when taking diffusivities of temperature as determined by the model using the Mellor-Yamada 2.5 level turbulence closure scheme as indicative of mixing. A beam of high diffusivity, which is typical for internal wave mixing, emanates from the top of the ridge in the ice shelf cavity, indicating higher mixing there, with the wind-driven gyre.

It has been shown that tides play a significant role in the melting of ice shelves and circulation beneath the ice shelf in strong tidal regimes, such as the Ross and Weddell Seas, through the mechanisms of tidal reactivation and mixing (Makinson et al., 2011; Mueller et al., 2012). Now it has been shown that tides also play a significant role in weak tidal regimes when they are subject to critical latitude effects, by the baroclinicity mechanism, mixing, and resonance. Along the Antarctic Peninsula several ice shelves are believed to be likely to respond to tides in a similar way as the Getz and Dotson ice shelves, including George VI Ice Shelf. According to Robertson the M2 critical latitude in the Arctic passes through Greenland and tidal effects could potentially influence melting in the Greenland northern tidewater glaciers. Tides play a lesser though significant role when certain conditions with respect to critical latitude are met, even in regions where there are weak tidal velocities, though the topography is the major controller of the circulation beneath an ice shelf. Tidal effects on the heat flux to the ice shelf in these areas near critical latitude are a result of a water column that is more baroclinic as a result of internal tides, resonant effects, increased mixing, and nonlinear effects on the circulation that is density-driven, instead of tidal residual velocities. The melt rates of the ice shelf can be increased by these effects up to 3.5 m/yr, which effectively increases it by 25 % for Pine Island Ice Shelf and 50 % for Dotson Ice Shelf.

Sources & Further reading

1. Robertson, R. (2013). "Tidally induced increases in melting of Amundsen Sea ice shelves." Journal of Geophysical Research: Oceans 118(6): 3138-3145.