ARTICLE https://doi.org/10.1038/s43247-021-00217-4 OPEN Poleward eddy-induced warm water transport across a shelf break off Totten Ice Shelf, East Antarctica ✉ Daisuke Hirano1,2,3,4 , Kohei Mizobata 5, Hiroko Sasaki6, Hiroto Murase 5, Takeshi Tamura3,4 & Shigeru Aoki1 Ice mass loss in the Wilkes Land sector of East Antarctica and the Amundsen and Belling- shausen Sea sectors of West Antarctica has contributed to a rise in sea levels over several decades. The massive continental ice behind the Totten Ice Shelf, equivalent to a few meters 1234567890():,; of sea-level rise, is grounded well below sea level and therefore, potentially vulnerable to oceanic heat. Here, we present analyses of comprehensive hydrographic observations at the continental slope and shelf break regions off Totten Ice Shelf. We provide robust evidence that the relatively warm Circumpolar Deep Water that originates at intermediate depths in the Antarctic Circumpolar Current is transported efficiently towards the shelf break by multiple cyclonic eddies. We propose that these semi-permanent cyclonic circulations play a critical role in transporting the available ocean heat towards Totten Ice Shelf, and melting it from underneath, thus eventually influencing the global climate. 1 Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan. 2 Arctic Research Center, Hokkaido University, Sapporo, Japan. 3 National Institute of Polar Research, Tachikawa, Japan. 4 The Graduate University for Advanced Studies, Tachikawa, Japan. 5 Tokyo University of Marine Science and ✉ Technology, Tokyo, Japan. 6 Japan Fisheries Research and Education Agency, Yokohama, Japan. email: [email protected] COMMUNICATIONS EARTH & ENVIRONMENT | (2021) 2:153 | https://doi.org/10.1038/s43247-021-00217-4 | www.nature.com/commsenv 1 ARTICLE COMMUNICATIONS EARTH & ENVIRONMENT | https://doi.org/10.1038/s43247-021-00217-4 recent study has demonstrated that the mass loss of the ocean–cryosphere interaction beneath the floating ice shelf (and ice Antarctic Ice Sheet was not only in the Amundsen and the tongue), which is atypical in East Antarctica. The limited historical A 14–17 Bellingshausen Sea sectors of West Antarctica but also in hydrographic observations around the shelf break off TIS sug- the Wilkes Land sector of East Antarctica1. A study noted that the gested that bathymetric features control the across-slope transport of Wilkes Land sector of East Antarctica has been a significant con- offshore-origin mCDW and depression with a bottom depth deeper tributor to sea-level rise over the last 40 years. In East Antarctica, than 500 m (Fig. 1b, c) is required for “bathymetric-controlled” the largest ice discharge occurs through the Totten Glacier from the mCDW transport. Further, integrated data from Ice Argo floats, Aurora Subglacial Basin in Wilkes Land, grounded below sea level2 hydrographic profiling by seals, and numerical modeling showed the (Fig. 1a). This basin is characterized as a region potentially vul- seasonality in the pathway and water properties of mCDW inflows nerable to ocean thermal forcing in East Antarctica because of its into the depression17. However, we do not thoroughly understand marine-based ice condition3,4. The expansive ice in the Aurora mCDW transport from offshore ACC to the continental slope of the Subglacial Basin behind the Totten Glacier would be equivalent to a focal region, which is strongly associated with the available oceanic >3.5 m rise in sea level5,6. Compared to other East Antarctic ice heat flux for continental ice melting. Although poleward transport of shelves, the highest area-averaged basal melt rate is estimated for offshore-origin warm CDW plays a critical role in regulating the the Totten Ice Shelf (hereafter TIS, 10.5 ± 0.7 m yr−1)7.Inthe available ocean heat for melting continental ice, specific knowledge of Southern Ocean, the strongest source of oceanic thermal forcing is ocean heat transport from offshore toward the TIS cavity is lacking. the warm Circumpolar Deep Water (CDW), originating from Most recently, Mizobata et al. 18 have provided important sug- intermediate depths in the offshore Antarctic Circumpolar Current gestions for poleward CDW transport toward TIS along the Sabrina (ACC) region8. Increased mass loss from the West Antarctic Ice Coast. Using satellite altimeter data, they constructed a spatio- Sheet is principally driven by an increase in basal melting of temporally high-resolution dataset for dynamic ocean topography the Amundsen and the Bellingshausen Sea ice shelves by the (DOT) over the Southern Ocean, north of shelf break (details in the strengthening of warm CDW inflows9,10.CDWinflow across “Methods” section). The DOT data unveiled a complete picture of an the shelf break is a crucial process controlling the mass balance for eddy train consisting of four quasi-stational cyclonic eddies with a ice shelves/sheets and therefore, it eventually influences the Ant- spatial scale of ~100–200 km (Fig. 2); this was partially observed by a arctic as well as global climate. previous study19. Hydrographic profiles and mooring data revealed While the continental shelf in East Antarctica is typically occupied that the barotropic current structure in the Vincennes Eddy off Knox by cold waters11, recent hydrographic observations revealed and Budd Coasts transports offshore warm water poleward and cold warm modified CDW (mCDW) inflows at ice fronts of the TIS Antarctic Bottom Water equatorward at the eddy’s eastern and (~−0.4 °C)12 and Shirase Glacier Tongue (>0 °C)13, consistent with western limbs, respectively. However, poleward CDW transport satellite-derived high basal melt rates for these regions7.Theseare induced by cyclonic eddies is only verified for the Vincennes Eddy considered as firm observational evidence of the warm west of TIS18. Therefore, we can only speculate regarding such CDW a b 0 Shirase Glacier 62°S Section 2 -500 Lützow-Holm Bay 11 Section 3 17 -1000 63°S 12 18 Section 1 -1500 Weddell Sea 13 19 505 509 510 64°S 14 515 20 -2000 508511 514 15 501 513 21 16 22 24 500 512m -2500 23 516 522 529 65°S 521 528 520 527 -3000 Cape 519 526 518 525 66°S Poinsett 517 524 -3500 Bathymetry [m] TIS 523 Knox -4000 Aurora Coast Budd Coast Sabrina Coast Subglacial 67°S 116°E 120°E -4500 Basin 108°E 112°E 124°E Ross Sea c 509 48' 21 510 3000 m Totten Ice Shelf 511 65°S Section 4 512 22 Sabrina Coast 515 514 12' 513 516 521 Section 5 519520 Section 3 518 24' 517 522 529 Section 6 528527 500 m 526 525 2000 m 36' 500 m 524 1000 m depression 523 116°E 117°E 118°E 119°E 120°E 121°E Fig. 1 Study area off Totten Ice Shelf, East Antarctica. a The latest bed topography over the Antarctic Continent from the BedMachine Antarctica v13. The hatched region is our study area off Totten Ice Shelf (TIS) on the Sabrina Coast, East Antarctica. b Bathymetry map of our study area based on the International Bathymetric Chart of the Southern Ocean (IBCSO)33, where the domain is indicated by the hatched region in (a). Yellow circles represent the positions of comprehensive and closely spaced conductivity–temperature–depth (CTD) stations by R/V Kaiyo-Maru. c Detailed bathymetry map from shelf break to continental slope regions, where the domain is indicated by the enclosed region in (b). In addition to the Kaiyo-Maru CTD stations (yellow circles), the positions of CTD stations by RVIB NB Palmer in 201516 are also shown as white inverted triangles. 2 COMMUNICATIONS EARTH & ENVIRONMENT | (2021) 2:153 | https://doi.org/10.1038/s43247-021-00217-4 | www.nature.com/commsenv COMMUNICATIONS EARTH & ENVIRONMENT | https://doi.org/10.1038/s43247-021-00217-4 ARTICLE “ ” a Climatological Mean for 2011 - 2018 -1.8 sections (Fig. 1b, c; described in the Methods section). Subsurface water with warm temperature (>1 °C), high salinity (>34.7), and low 62°S −1 Vincennes -1.85 dissolved oxygen (DO) content (<210 μmol kg ), termed as Poinsett West – 63°S Sabrina East mCDW, is observed at depths ~300 500 dbar along the upper con- Sabrina -1.9 tinental slope off TIS (113–121°E, zonal Section 1, Fig. 3). The 64°S mCDW spatial distribution is worthy of special mention considering -1.95 remarkably warm cores (1.2–1.4 °C) at 350–400 dbar that are spa- 65°S 500 m -2 tially scattered rather than continuous, wherein the mCDW is Cape 10 cm s-1 notably thick in the vertical extent (Stations [hereafter, Sta.] 502, 507, 66°S Poinsett Knox TIS -2.05 512, and 522, Fig. 3a). The mCDW cores observed in February 2019 Coast Budd Coast Sabrina Coast were at much higher temperatures than the warmer intrusion of 67°S Dynamic Ocean Topography [m] 116°E 120°E -2.1 108°E 112°E 124°E mCDW previously observed from March to June on the upper slope (warmer than 0 °C, but mostly cooler than 1 °C)17.Apositional correspondence between eddies and warm cores is defined by the that particularly warm waters are consistently observed around the b February 2019 -1.8 62°S eastern sides of the cyclonic Poinsett (Sta. 507) and West-Sabrina (Sta. 522) Eddies (Figs. 2b, 3a). We further note that warm cores are Section 2 -1.85 also located on the western side of the Poinsett (Sta. 502) and West- 63°S Section 3 Poinsett West Sabrina (Sta. 512) Eddies (Figs. 2b, 3a). It is worth highlighting that Sabrina similar correspondence is determined for the East-Sabrina Eddy, -1.9 – – Section 1 demonstrated by the expendable conductivity temperature depth 64°S (XCTD) observations across the southern part of this eddy (Sup- -1.95 plementary Fig.