- Award ID(s):
- 1744562
- NSF-PAR ID:
- 10168116
- Date Published:
- Journal Name:
- The cryosphere discussions
- ISSN:
- 1994-0440
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
null (Ed.)Abstract. Katabatic winds in coastal polynyas expose the ocean to extreme heat loss, causing intense sea ice production and dense water formation around Antarctica throughout autumn and winter. The advancing sea ice pack, combined with high winds and low temperatures, has limited surface oceanobservations of polynyas in winter, thereby impeding new insights into theevolution of these ice factories through the dark austral months. Here, wedescribe oceanic observations during multiple katabatic wind events duringMay 2017 in the Terra Nova Bay and Ross Sea polynyas. Wind speeds regularlyexceeded 20 m s−1, air temperatures were below −25 ∘C, and the oceanic mixed layer extended to 600 m. During these events, conductivity–temperature–depth (CTD)profiles revealed bulges of warm, salty water directly beneath the oceansurface and extending downwards tens of meters. These profiles reflect latent heat and salt release during unconsolidated frazil ice production, driven by atmospheric heat loss, a process that has rarely if ever been observed outside the laboratory. A simple salt budget suggests these anomalies reflect in situ frazil ice concentration that ranges from 13 to 266×10-3 kg m−3. Contemporaneous estimates of vertical mixing reveal rapid convection in these unstable density profiles and mixing lifetimes from 7 to 12 min. The individual estimates of ice production from the salt budget reveal the intensity of short-term ice production, up to 110 cm d−1 during the windiest events, and a seasonal average of 29 cm d−1. We further found that frazil ice production rates covary with wind speed and with location along the upstream–downstream length of the polynya. These measurements reveal that it is possible to indirectly observe and estimate the process of unconsolidated ice production in polynyas by measuring upper-ocean water column profiles. These vigorous ice production rates suggest frazil ice may be an important component in total polynya ice production.more » « less
-
Abstract Over the Ross Sea shelf, annual primary production is limited by dissolved iron (DFe) supply. Here, a major source of DFe to surface waters is thought to be vertical resupply from the benthos, which is assumed most prevalent during winter months when katabatic winds drive sea ice formation and convective overturn in coastal polynyas, although the impact of these processes on water‐column DFe distributions has not been previously documented. We collected hydrographic data and water‐column samples for trace metals analysis in the Terra Nova Bay and Ross Ice Shelf polynyas during April–May 2017 (late austral fall). In the Terra Nova Bay polynya, we observed intense katabatic wind events, and surface mixed layer depths varied from ∼250 to ∼600 m over lateral distances <10 km; there vertical mixing was just starting to excavate the dense, iron‐rich Shelf Waters, and there was also evidence of DFe inputs at shallower depths in the water column. In the Ross Ice Shelf polynya, wind speeds were lower, mixed layers were <300 m deep, and DFe distributions were similar to previous, late‐summer observations, with concentrations elevated near the seafloor. Corresponding measurements of dissolved manganese and zinc, and particulate iron, manganese, and aluminum, suggest that deep DFe maxima and some mid‐depth DFe maxima primarily reflect sedimentary inputs, rather than remineralization. Our data and model simulations imply that vertical resupply of DFe in the Ross Sea occurs mainly during mid‐late winter, and may be particularly sensitive to changes in the timing and extent of sea ice production.
-
Abstract Seasonal formation of Dense Shelf Water (DSW) in the Ross Sea is a direct precursor to Antarctic Bottom Water, which fills the deep ocean with atmospheric gases in what composes the southern limb of the solubility pump. Measurements of seawater noble gas concentrations during katabatic wind events in two Ross Sea polynyas reveal the physical processes that determine the boundary value properties for DSW. This decomposition reveals 5–6 g kg−1of glacial meltwater in DSW and sea‐ice production rates of up to 14 m yr−1within the Terra Nova Bay polynya. Despite winds upwards of 35 m s−1during the observations, air bubble injection had a minimal contribution to gas exchange, accounting for less than 0.01 μmols kg−1of argon in seawater. This suggests the slurry of frazil ice and seawater at the polynya surface inhibits air‐sea exchange. Most noteworthy is the revelation that sea‐ice formation and glacial melt contribute significantly to the ventilation of DSW, restoring 10% of the gas deficit for krypton, 24% for argon, and 131% for neon, while diffusive gas exchange contributes the remainder. These measurements reveal a cryogenic component to the solubility pump and demonstrate that while sea ice blocks air‐sea exchange, sea ice formation and glacial melt partially offset this effect via addition of gases. While polynyas are a small surface area, they represent an important ventilation site within the southern‐overturning cell, suggesting that ice processes both enhance and hinder the solubility pump.
-
Abstract High Salinity Shelf Water (HSSW) formed in the Ross Sea of Antarctica is a precursor to Antarctic Bottom Water (AABW), a water mass that constitutes the bottom limb of the global overturning circulation. HSSW production rates are poorly constrained, as in-situ observations are scarce. Here, we present high-vertical-and-temporal-resolution salinity time series collected in austral winter 2017 from a mooring in Terra Nova Bay (TNB), one of two major sites of HSSW production in the Ross Sea. We calculate an annual-average HSSW production rate of ~0.4
Sv (106m 3s −1), which we use to ground truth additional estimates across 2012–2021 made from parametrized net surface heat fluxes. We find sub-seasonal and interannual variability on the order of$$0.1$$ , with a strong dependence on variability in open-water area that suggests a sensitivity of TNB HSSW production rates to changes in the local wind regime and offshore sea ice pack.$${Sv}$$ -
Abstract The atmospheric and surface conditions during a late autumnal strong katabatic wind event were quantified using ship and rawinsonde measurements over the Terra Nova Bay coastal polynya. Wind speeds decreased from 35 to 18 ms−1, while the wind direction decreased 18° over the distance from the Nansen Ice Shelf edge out 99 km eastward toward the Ross Sea. Maximum velocity winds (jet cores) at 173, 238, 179 and 144 m elevation were associated with atmospheric boundary layers capped by temperature inversions with bases at 294, 325, 226 and 196 m elevation. The tops of the inversion layers (near 700 m at all locations) also marked the top of the katabatic wind layer. Boundary layer air temperature and specific humidity increased from −31 to −21°C and 0.1 to 0.6 gkg−1, respectively, in response to the warm polynya surface. The air at 15 ± 8 m elevation was saturated with respect to ice, causing supersaturation and snow growth where the air parcels become cooler in the upper atmospheric boundary layer. The surface was characterized by three zones, a fluid zone (open ocean, frazil, shuga and pancake floes), an accumulation zone (fused, rafted and compressed pancake floes) and a young floe zone (large floes). The surface temperature varied from freezing (−1.7°C) in the fluid zone to near air temperature (−20°C) in the floe zone with the largest horizontal surface temperature gradient occurring in the transition between the fluid zone and the accumulation zone, and at the edges of leads in the floe zone.