Multidecadal Variability and Predictability of Antarctic Sea Ice in GFDL SPEAR_LO Model
Abstract. Using a state-of-the-art coupled general circulation model, physical processes underlying Antarctic sea ice multidecadal variability and predictability are investigated. Our model simulations constrained with atmospheric reanalysis and observed sea surface temperature broadly capture the observed sea ice extent (SIE) variability with a low sea ice state (late 1970s–1990s) and a high sea ice state (2000s–early 2010s), although the model overestimates the SIE decrease over the Weddell Sea around the 1980s. The low sea ice state is largely due to an occurrence of strong deep convection in the Southern Ocean that subsequently induces anomalous warming of the upper ocean. During the high sea ice period (post-2000s), the deep convection substantially weakens, so that surface wind variability plays greater roles in the SIE variability. Decadal retrospective forecasts started from the above-mentioned constrained model results demonstrate that the Antarctic sea ice multidecadal variability can be skillfully predicted 6–10 years in advance, showing a moderate correlation with the observation (0.4). Ensemble members with a stronger deep convection tend to predict a larger sea ice decrease in the 1980s, whereas the members with a larger surface wind variability tend to predict a larger sea ice increase after the 2000s. Therefore, skillful simulation and prediction of the Antarctic sea ice multidecadal variability require accurate simulation and prediction of both the Southern Ocean deep convection and surface wind variability in the model.
Yushi Morioka et al.
Status: open (until 25 Apr 2023)
- RC1: 'Comment on tc-2023-18', Anonymous Referee #1, 23 Mar 2023 reply
Yushi Morioka et al.
Yushi Morioka et al.
Viewed (geographical distribution)
The authors investigated the multidecadal variability and predictability of Antarctic sea ice using the GFDL climate model. In particular, they highlighted the role of deep convection and surface wind in the multidecadal variability of Antarctic sea ice. Using a number of model simulations, they also suggest that the multidecadal variability of Antarctic sea ice can be predicted 6-10 years in advance.
This is definitely an interesting paper and contains some useful information. My major concerns are with their interpretation of how the deep convection was initiated around the 1980s and with the structure of the paper writing.
[Section 3] This section is quite massive and messy – I tried very hard not to get lost. If it was me, I would try to make it more concise to focus on the main points. I would also include some subtitles (subsubsections) to guide the readers.
[Lines 361-417] I disagree with the heat buildup mechanism for the initiation of deep convection. Based on Figure 6, the upper ocean (200 m) stratification, which is essential for heat exchanges to the surface mixed layer, is dominated by salinity changes, not by temperature changes. The heat distribution in the 1960s does help destabilize the water column, but it is still overwhelmed by the salinity stratification (Figure 5e,f). To me, both the temperature/salinity redistribution are more like responding to the surface wind changes. In the late 1970s, the negative wind stress curl/stronger westerly wind favored the formation of polynyas and enhanced vertical mixing, bringing more heat to the surface. The heat buildup in the 1960s amplifies the vertical heat flux, not necessarily the vertical mixing itself. This also explains why a positive wind stress curl around 2005 does not cause significant sea ice increase.
[Line 54] between the atmosphere and the ocean. Similar issues exist throughout the paper.
[Line 55] high-salinity dense water. Actually, the water formed on the Antarctic continental shelf is called high-salinity shelf water, which entrains circumpolar deep water as it flows down the continental slope and forms Antarctic Bottom Water.
[Line 56] travels → flows into the bottom of the Southern Ocean
[Lines 83-85] This is quite out of the context and not the main processes responsible for the sea ice increases in Lecomte et al. (2017). As I understand it, the vertical heat redistribution is more like a consequence of the increased stratification, not the reason for the increasing SIE. I would get rid of this sentence.
[Lines 121-123] I am confused. In SPEAR_LO_DRF, it is also starting from January 1st, which contradicts this speculation.
[Section 2 and hereafter] the HadISST → HadISST.
[Line 180] The NSIDC sea ice concentration is reported on the polar stereographic grid with a resolution of 25 km, not 0.25 degree.
[Lines 192-193] .. have a nominal 1o horizontal resolution, which increases to 1/3o in the meridional direction …
[Lines 225-238] Make it clear that no nudging is used here. The discussion of monthly anomalies is a bit out of the context and confusing. Suggest move it to another paragraph.
[Lines 247-248] Is the overturning circulation streamfunction calculated in depth coordinates? If so, it may be misleading/not accurate.
[Lines 283-295] The mentioned polynyas during 1974-1976 and 2016-2017, I believe, are in observations. How are these polynyas reproduced in SPEAR_LO_DCIS? Are the long-lasting strong deep convection between 1975-1990 related to some unrealistic polynyas in the Weddell Sea? Figure S3 compares the sea ice concentration, not the sea ice concentration decrease, and cannot be used to support the claim that SPEAR_LO_DCIS does not overestimate the SIE decrease.
[Line 299] in several studies, including the ones using satellite images of …
[Lines 350-352] The lead-lag relationship is ambiguous with the 5-year moving average. Even if this is real, this lead-lag relationship is likely arbitrary due to the definition of deep convection here. Convection (in the upper ocean) that entrains warm/salty water from the subsurface should take place simultaneously with the mixed layer deepening. I don’t see any reason for such a lag.
[Line 381] The negative stratification anomaly around 1980 is mainly in the upper 200 m, not below 100m.
[Lines 463-474] This paragraph starts by talking about the SPEAR_LO_DRF runs, but then switches to discussion of the persistence prediction based on HadISST1 (I assume). This is quite confusing.
[Section 3.2] I am not in the field of climate predictions and got confused by what is done here (SPEAR_LO_DRF). Could you elaborate what the “ensemble-mean SIC anomalies predicted at lead times from 1-5 years to 6-10 years” mean exactly?
[Section 3.2] Similar issues in Section 3.1. There are a lot of details here and I tried very hard not to get lost. It would be better to make it more concise and focus on the most important results here. Could also add some subtitles to guid the readers.
[Lines 550-555] Is this correct? P-E is positive (more precipitation) around 1980. There is a small fraction of negative values close to 1985.
[Lines 636-649] The discussion of the influence of atmospheric model resolution is useful. Critically, the ocean model resolution may be more important in simulating deep convection and its climatic impacts (e.g., https://os.copernicus.org/preprints/os-2020-41/).