A review of The Cryosphere Brief Communication manuscript tc-2020-352 by Greg Leonard et al.

This is a well-written and important study suitable for publication in TC after minor changes. Fast ice is a missing piece of the puzzle in regional (and indeed global) climate models, and understanding its stability is an important part of its model implementation. My only “major” suggestion is really quite minor. 

Major suggestion:

The authors present a convincing correlation between storm events and fast ice breakout, indicating that it’s probably a direct wind-driven (i.e., dynamical) breakout mechanism (and I agree that this is almost certainly the case) - however no alternative mechanisms are discussed. Other studies have indicated that fast ice may be weakened thermodynamically by basal melt (e.g., Arndt et al., 2020, also TC - however this study implied that summertime mode 3 water incursions were important, which is surely not a factor in the winter). I’m not so familiar with the structure of the water column in the Sound during winter, but is it possible that these wind and polynya events enhance vertical mixing - and if warm water (e.g., mCDW) exists on the shelf here, might its entrainment induce basal fast ice melt? And the lag apparent between some storm events and the breakout might also imply a thermodynamic connection (although I accept your explanation involving the land mask of the sea ice concentration data in probably correct). Looking at Pritchard et al 2012 (“Antarctic ice-sheet loss driven by basal melting of ice shelves”), I can see that there’s likely no warm water here so my hypothesis is quite unconvincing - but a brief discussion around alternative mechanisms would be appreciated!

Minor suggestions (line numbers given where appropriate):

6: add “timing of” between “between” and “break-out”

14: Brett et al ref needs year.

18: add “stable” between “the” and “fast”

32: “activity” here is a little ambiguous. You mean sea ice production, right?

35: Probably best to avoid starting a sentence with a number (2019). 

41: The Fraser et al 2020 dataset gives 15 day composite maps, not 14 day.

47: The “biased” in here implies that these studies didn’t correctly account for the icescape change. Is this what you really mean - if so, for both studies? 

58: Was this IW mode Sentinel-1 imagery? What resolution?. Also Hall and Riggs refs need years.

59: “MSP event” is a little ambiguous. Do you mean a large polynya size event? Also here, I’m curious how an active polynya looks in ice surface temperature - presumably a warm temperature? Or is it masked because largely open water?

65: “Manually identified events” is a little ambiguous. Events of what?

74: “connected to” -> “associated with”

75: “warm temperatures” - what temperature? I presume near-surface air temp?

79: “are correlated to sea ice concentration” - ambiguous description. High SIC? Low SIC? And isn’t “correlated with” better than “correlated to”?

82: By “freeze-up” do you mean pack or fast ice?

85: It first struck me as a little unusual to define a KWI without using wind data. What happens if a low pressure system occurs over the central Ross Sea - doesn’t this also bring warm air and low pressure? Or is this the effect you’re trying to capture - and these pressure systems enhance the katabatics? A little more clarity here would be appreciated.

91: “break-out events” - do you mean fast or pack?

117: Although a brief communication, the “big picture” could do with a little more expansion. E.g., this is one of few case studies on fast ice stability, an area where more research is needed, etc. It occurs to me that this region might be a good one for testing forthcoming fast ice tensile strength parameterisations in prognostic fast ice models (e.g., Lemieux et al., 2016, “Improving the simulation of landfast ice by combining tensile strength and a parameterization for grounded ridges”). Also, are there other regions you know of which have a similar fast ice regime (i.e., deep embayment and lack of grounded icebergs) to which the results of this study might be applicable?

119: The Fraser et al., 2020 dataset is missing from the availability section.

Fig 1: It would be helpful to please annotate the area of active polynya in each SAR image (manually is fine). Similarly for the fast ice edge.

Fig 2: Does the truncation of the upper half of each wind rose remove any/much information? I’d quite like to see the whole thing (if there’s detail in the northerly half) but happy to stick with the half roses if no wind from that half.

Fig 3: A little unusual to not have a colour legend for the upper two plots, although I recognise that they’re only shown to indicate the envelope of previous years (and the reader doesn’t necessarily need to know which year is which). Caption: identify -> identifies. Also is “break-out” referring to fast or pack?

A study regarding land-fast sea ice (fast ice) breaking/calving at McMurdo Sound like this brief communication seems to be valuable when taking into account field activities around the Ross Sea/Ice Shelf area, Antarctica. However, the breaking mechanism of fast ice was not discussed adequately in this manuscript, as described below.

The 2019 anomalous breaking of fast ice appears to be associated with KWI and sea ice concentration, as shown in Figure 3. However, this manuscript did not explain the mechanism of fast ice breaking.

The authors used KWI and southerly winds. I straightforwardly regarded these as due to katabatic wind. What mechanism do the authors consider for the fast ice break up by the katabatic wind? Figure 3 showed that the KWI increase coincides with the fast ice break up. When the KWI was large, strong winds were blowing from the south (continent). Again, how does the fast ice is broken by this wind? It is widely known that sea swells affect the breaking of fast ice. The swell effect was also discussed in Banwell et al. (2017), cited by the authors. It seems hard to destroy fast ice only by katabatic wind, even if it is a strong wind. Furthermore, since the wind blows from the shore to the offshore, it is expected not to generate swells that destroy the fast ice.

Since the katabatic wind is a strong wind from inside the continent, it is expected that the air temperature will drop during the period when the KWI is large. However, as is clear from Figure 3, the temperature rose when the KWI is large. Please explain the reason for this. Is this because of the breaking of fast ice or a coastal polynya formation? Both of them will increase heat flux from the ocean to the atmosphere.

Regarding the fast ice break up during June-July: The reviewer cannot know the details because the authors only show the southerly wind component, but wondering the influence of low pressure rather than the katabatic wind from the following facts: the wind speed increased, the temperature rose, and the atmospheric pressure decreased (Fig. 3). If so, the reviewer considers that fast ice could be collapsed by sea swell. The authors also described it as a “storm event” in their conclusion (P. 4, L. 108). Is this an atmospheric event due to the katabatic wind only? Otherwise, is it the effect of a low-pressure system? Please clarify this.

This study showed a relationship between a coastal polynya and KWI (section 4). By what mechanism does the polynya cause the fast ice break up? Is it just a description of a relationship between KWI (southern wind) and polynya? The air temperature was below -10 degrees Celsius during the period. Under such atmospheric conditions, even if an open water fraction appears by the divergent ice motion due to prevailing wind, the ocean surface will be immediately covered with thin sea ice. In winter, coastal polynyas should be considered as thin ice-covered areas with high ice concentration rather than low ice concentration areas under the passive microwave sensor’s coarse spatial resolution. Many sea ice concentration algorithms used for passive microwave satellite data underestimate the concentration in thin ice-covered areas. It may be possible to regard the low ice concentration region as a coastal polynya signal due to this characteristic, but caution will be required. It does not detect coastal polynyas precisely. For the detection of coastal polynyas from passive microwave satellite data, Tamura et al. (2007; 2008) and Nihashi and Ohshima (2015) would be helpful.  

Tamura,T., K. I. Ohshima, T. Markus, D. J. Cavalieri, S. Nihashi, and N. Hirasawa, (2007), Estimation of thin ice thickness and detection of fast ice from SSM/I data in the Antarctic Ocean. J. Atmos. Oceanic Technol., 24, 1757–1772, doi:10.1175/JTECH2113.1.

Tamura,T., K. I. Ohshima, and S. Nihashi (2008), Mapping of sea ice production for Antarctic coastal polynyas. Geophys. Res. Lett., 35, L07606, doi:10.1029/2007GL032903.

Nihashi, S. and K. I. Ohshima (2015), Circumpolar Mapping of Antarctic Coastal Polynyas and Landfast Sea Ice: Relationship and Variability, J. Clim. 28(9) 3650 – 3670, doi: 10.1175/JCLI-D-14-00369.1.