Southern Ocean polynyas and dense water formation in a high-resolution, coupled Earth System Model
Abstract. Antarctic Bottom Water is an important component of Earth's climate system. Its formation occurs through ocean-atmosphere-sea ice flux interactions in coastal and open ocean polynyas around Antarctica. In this paper, we investigate Antarctic dense water formation in the high-resolution version of the Energy Exascale Earth System Model (E3SM-HR). The model is able to reproduce the major Antarctic coastal polynyas, though they are smaller in area compared to observations. E3SM-HR also simulates several occurrences of open-ocean polynyas (OOPs) in the Weddell Sea, at a higher rate than what the last 50 years of satellite sea ice observational record suggests, but similarly to other high-resolution Earth System Model simulations. Furthermore, the densest water masses in the model are formed within the OOPs, rather than on the continental shelf, as is typically observed. Biases related to the lack of dense water formation on the continental shelf are associated with overly strong atmospheric polar easterlies, which lead to a strong Antarctic Slope Front and hence too little communication between on and off continental shelf water masses. Strong polar easterlies also produce excessive southward Ekman transport, causing a build-up of sea ice over the continental shelf and enhanced ice melting in the summer season. This in turn produces water masses on the continental shelf that are overly fresh and less dense relative to observations. Our results indicate that the large-scale polar atmospheric circulation around Antarctica must be accurately simulated in models to properly reproduce Antarctic dense water formation.
Hyein Jeong et al.
Status: final response (author comments only)
RC1: 'Comment on tc-2022-133', Anonymous Referee #1, 08 Sep 2022
- AC1: 'Reply on RC1', Hyein Jeong, 27 Jan 2023
RC2: 'Comment on tc-2022-133', Anonymous Referee #2, 12 Sep 2022
- AC2: 'Reply on RC2', Hyein Jeong, 27 Jan 2023
RC3: 'Comment on tc-2022-133', Anonymous Referee #3, 20 Sep 2022
- AC3: 'Reply on RC3', Hyein Jeong, 27 Jan 2023
Hyein Jeong et al.
Hyein Jeong et al.
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"Southern Ocean polynyas and dense water formation in a high-reslution, coupled Earth System Model," by Jeong et al.
Over the past decade, general circulation models (GCMs) have significantly improved in terms of resolution and parameterizations of sub-grid scale processes. However, the bottom-water formation on the Antarctic shelves remains a key challenge due to small and local formation sites related to coastal polynyas. The study by Jeong et al. contributes to the study of coastal polynyas and the formation of dense/bottom water in GCMs. The authors use a high-resolution coupled Energy Exascale Earth System Model (E3SM-HR) with a horizontal resolution of ~8 km over the Antarctic continental shelves to study dense water formation south of 60 oS and compare the results with a lower-resolution version of the same model and available gridded datasets based on observations and reanalysis. They find that the increased resolution improves the representation of coastal polynyas. However, the associated bottom-water production is too weak and does not produce sufficiently dense bottom water to compare with observations.
The manuscript is well written and includes high-quality figures to support the main results. The methodology is sound and builds on state-of-the-art coupled Earth System Model (ESM) development. However, the discussion section is too brief and offers limited new insight into the main topic of missing dense-water formation in the coastal polynyas in GCMs and ESMs. I summarize key aspects the authors should address to improve the manuscript and its contribution to the ESM development near the Antarctic shelves:
Line 159: It is very hard to see the “relatively higher latent heat flux…in East-Antarctica” in Fig1a.
Line 256: It is very hard to see the easterly winds in figure 6b.