Zonal Distribution of Circumpolar Deep Water Transformation Rates and Its Relation to Heat Content on Antarctic Shelves

Abstract

Circumpolar Deep Water (CDW) is a relatively warm layer of water (∼1-2°C) that is part of the lower branch of the global overturning circulation (Talley, 2013). CDW upwells in the subpolar gyres of the Southern Ocean and mixes with surface waters, transforming into lighter watermasses (Pellichero et al., 2018). CDW can also shoal onto the Antarctic continental shelf where it may undergo mixing with shelf watermasses, transforming into heavier and cooler Dense Shelf Water (DSW with σ θ ≥ 27.86 kg/m 3 ; Orsi et al., 1999). In some regions of Antarctica, CDW does not undergo transformation as it flows onto the continental shelf, retaining its warmer offshore properties, thereby elevating the temperatures at the base of ice shelves along the coastline Ribeiro et al., 2021).Another prominent watermass that CDW can transform into, as it encounters coastal polynyas, is a cooler version of itself, called modified-CDW (mCDW; with 28 < γ n < 28.27 kg/m 3 ; Williams et al., 2016). Along the ice shelf edge, mCDW has been found even in regions with a cold-cavity ice shelf, such as the Weddell Sea (Årthun et al., 2012), Prydz Bay (Williams et al., 2016, and the Ross Sea (Piñones et al., 2019). Though mCDW is a cooler watermass, it still remains warmer than the near-freezing waters formed on the continental shelves; hence it has the ability to elevate the melt rates of ice shelves (Adusumilli et al., 2020). Throughout the rest of this text,

The observed spatiotemporal variability of Antarctic Winter Water

“…Data are relatively well distributed across the SO due to sampling from the Argo and MEOP programmes (Figures 2e-2h), which is extensive compared to limited historical CTD shipbased data collection (Brett et al, 2020). The Pacific sector is mainly sampled throughout the annual cycle via the Argo programme, whereas MEOP data cover a great extent south of the Polar Front, particularly in the Atlantic and Indian Ocean sectors, as well as below sea ice and across the circumpolar continental shelf (Narayanan et al, 2023).…”

“…Spatially, WW core temperature in the under-ice zone remains largely homogeneous with a mean temperature of -1.4 ± 0.5°C in the under-ice zone, whilst core salinity exhibits a circumpolar mean of 34.26 ± 0.18 g/kg (Figures 5b and 5d). WW cores are substantially fresher (34.11 ± 0.12 g/kg) in the Amundsen-Bellingshausen Seas (ABS): the proximity of the ACC fronts and the eastward limb of the Ross Gyre transport CDW southwards, maintaining the region as a warm shelf sea and facilitating elevated sea ice melt rates, which is likely responsible for the fresher WW core salinity as well as freshening the under-ice zone mean and increasing the standard deviation (Figure 5c) (Nakayama et al, 2018;Narayanan et al, 2023;Tamsitt et al, 2021;Thompson et al, 2018). The coldest and most saline WW cores are observed within the polar gyres (approximately -1.5 °C, 34.6 g/kg; Figures 4a, 4f, 5a and 5c), which are maintained through their southern proximity and elevated sea ice production.…”

“…Therefore, ocean interior mixing rates are lower than other parts of the SO (Frants et al, 2013;Ledwell et al, 2011), arresting densification of WW via reduced mixing with underlying CDW. Furthermore, CDW intrusions onto the ABS continental shelf results in a warm shelf basin (Narayanan et al, 2023;Tamsitt et al, 2021).…”

“…Sabu et al (2020) also observe the WW layer responding to largescale atmospheric variability, finding enhanced warming in years of positive Southern Annular Mode, likely due to elevated wind-driven mixing from stronger winds. The variability of WW observed is tied to various mechanisms that impact upper ocean structure, such as varying eddy kinetic energy (Dove et al, 2022;Nikurashin et al, 2013), differing wind stress fields (Abernathey et al, 2011;Lin et al, 2018), and CDW outcropping pathways (Narayanan et al, 2023;Tamsitt et al, 2021).…”