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A fourth production region for the globally important Antarctic bottom water has been attributed to dense shelf water formation in the Cape Darnley Polynya, adjoining Prydz Bay in East Antarctica. Here we show new observations from CTD-instrumented elephant seals in 2011–2013 that provide the first complete assessment of dense shelf water formation in Prydz Bay. After a complex evolution involving opposing contributions from three polynyas (positive) and two ice shelves (negative), dense shelf water (salinity 34.65–34.7) is exported through Prydz Channel. This provides a distinct, relatively fresh contribution to Cape Darnley bottom water. Elsewhere, dense water formation is hindered by the freshwater input from the Amery and West Ice Shelves into the Prydz Bay Gyre. This study highlights the susceptibility of Antarctic bottom water to increased freshwater input from the enhanced melting of ice shelves, and ultimately the potential collapse of Antarctic bottom water formation in a warming climate.

In Greenland, tidewater glaciers discharge turbid subglacial freshwater into fjords, forming a plume near the calving front. To elucidate the effects of this discharge on nutrient and dissolved inorganic carbon transport to the surface in these fjords, we conducted observational studies on Bowdoin Glacier and in its fjord in northwestern Greenland during the summer of 2016. Our results provide evidence of macronutrient and dissolved inorganic carbon transport from deep in the fjord to the surface in front of the glacier. This transport is driven by plume formation resulting from subglacial freshwater discharge and subsequent upwelling along the glacier calving front. The plume water is a mixture of subglacial freshwater and entrained fjord water. The fraction of glacial meltwater in the plume water is ~14% when it reaches the surface. The plume water is highly turbid because it contains substantial amounts of sediment derived from subglacial weathering. After reaching the surface, the plume water submerges and forms a turbid subsurface layer below fresher surface water at densities of 25.0 to 26.5 σθ. Phytoplankton blooms (~6.5 μg/L chlorophyll a) were observed near the boundary between the fresher surface and turbid subsurface layers. The bloom was associated with a strong upward NO3− + NO2− flux, which was caused by the subduction of plume water. Our study demonstrated that the subglacial discharge and plume formation at the front of Bowdoin Glacier play a key role in the availability of nutrients and the subsequent growth of phytoplankton in the glaciated fjord.

[1] From comparisons with thickness of sea ice from Advanced Very High Resolution Radiometer (AVHRR) and ice-profiling sonar data we have developed an Advanced Microwave Scanning Radiometer-EOS (AMSR-E) thin ice thickness algorithm for the Sea of Okhotsk. This algorithm can estimate ice thickness of 0.2 m without snow using the polarization ratio of AMSR-E brightness temperature at a 36.5 GHz channel from a linear relationship with AVHRR ice thickness. When a snow cover exists on the thin ice surface, as occurred a few times in each winter, it is shown that the algorithm cannot detect the thin ice. Sea ice and dense shelf water (DSW) production in coastal polynya are estimated on the basis of heat flux calculation with the daily AMSR-E ice thickness for three winters (December-March) of 2002-2003 to 2004-2005. The ice production is largest in the northwest shelf (NWS) polynya which accounts for $45% of the sum of ice production in major coastal polynyas. The ice production in major coastal polynyas would cover the maximum ice area of the Okhotsk Sea if the average ice thickness is assumed to be 1 m. Variability of the ice production is mainly modulated by air temperature. In the NWS polynya, which is the main DSW production area, the annual DSW formation rate is estimated to be $0.36 Sv.

The horizontal coherence and duration of the array allowed accurate estimates of the transport and structure of the Kerguelen DWBC.The DWBC forms a narrow (~50 km wide), intense, bottom-intensified flow to the northwest over the lower flank of the plateau, with a flow to the southeast further offshore (Fig. 2). The currents are remarkably strong for these depths: maximum two-year mean speed exceeds 20 cm s -1 at ~3500 m depth at M6 (the largest DWBC speeds yet observed at similar depths; Supplementary Table 1 The similarity between the moored results and the synoptic snapshots indicates that the limited vertical resolution of the moored instruments is sufficient to capture the DWBC structure.The daily velocity and potential temperature data are used to calculate the 4 AABW transport per unit width (Fig. 3a). The northwestward transport of AABW is concentrated between M4 and M6, with the largest value observed at M5 where the velocities are large and the AABW layer is thick (Fig. 2). Northwestward transport is commonly observed to extend offshore as far as M3 (~38 km is within the 95% confidence intervals of the mean after 9 months, indicating that the records are of sufficient length to establish a stable mean. The overall impression is of a narrow, intense and relatively steady deep boundary current, modulated by topographic waves and the episodic influence of fronts and meanders of the ACC.This DWBC is associated with a mean temperature transport of -2.52±0.26 Sv °C 5 northwestward.Snapshot estimates based on CTD/ADCP measurements are in agreement with the observations from the moored array, with the exception of the deployment cruise (Fig. 3b, Supplementary Table 2). However, these estimates sampled periods of larger than average transport (Fig. 3b) Several factors make it difficult to assess the relative contribution of the Kerguelen DWBC to the Southern Ocean overturning. These include the presence of recirculation gyres, interaction between the DWBC and other circulation regimes (e.g., the ACC and subpolar gyres) 8 , the fact that mixing and entrainment change the volume and properties of AABW along the export pathway 13 , and the lack of coherent long-term observations in other DWBCs. The only previous coherent current meter measurements of AABW export by a DWBC south of 45°S were obtained north of the Falkland Plateau, where a net transport of 1.9 Sv of Weddell Sea Deep Water (θ < 0.2°C) was found to enter the Argentine Basin (the difference between 8.2 Sv westward and 6.3 Sv recirculating to the east) 8 . In comparison, the net transport of water with θ < 0.2°C by the Kerguelen DWBC is 8.0 Sv (16.4 Sv to the northwest and an 8.4 Sv recirculation to the southeast). Incoherent multi-year moored measurements in the northwest Weddell Sea reveal relatively weak currents (deep mean flows < 7 cm s -1 ) 11 . Combining these 6 current-meter data with CTD sections gives a net export of 3.8 Sv of AABW (θ < 0°C) from the Weddell Sea 14 ; based on an inverse model, an additional 4.7 Sv may leave the Weddell Sea ac...