scholarly journals Upper ocean stratification and sea ice growth rates during the summer-fall transition, as revealed by Elephant seal foraging in the Adélie Depression, East Antarctica

Ocean Science ◽  
2011 ◽  
Vol 7 (2) ◽  
pp. 185-202 ◽  
Author(s):  
G. D. Williams ◽  
M. Hindell ◽  
M.-N. Houssais ◽  
T. Tamura ◽  
I. C. Field
Keyword(s):  
Sea Ice ◽  
Continental Shelf ◽  
Mixed Layer ◽  
Growth Rates ◽  
Shelf Break ◽  
Upper Ocean ◽  
Ocean Stratification ◽  
Glacier Tongue ◽  
Ice Growth ◽  
Continental Shelf Break

Abstract. Southern elephant seals (Mirounga leonina), fitted with Conductivity-Temperature-Depth sensors at Macquarie Island in January 2005 and 2010, collected unique oceanographic observations of the Adélie and George V Land continental shelf (140–148° E) during the summer-fall transition (late February through April). This is a key region of dense shelf water formation from enhanced sea ice growth/brine rejection in the local coastal polynyas. In 2005, two seals occupied the continental shelf break near the grounded icebergs at the northern end of the Mertz Glacier Tongue for several weeks from the end of February. One of the seals migrated west to the Dibble Ice Tongue, apparently utilising the Antarctic Slope Front current near the continental shelf break. In 2010, immediately after that year's calving of the Mertz Glacier Tongue, two seals migrated to the same region but penetrated much further southwest across the Adélie Depression and sampled the Commonwealth Bay polynya from March through April. Here we present observations of the regional oceanography during the summer-fall transition, in particular (i) the zonal distribution of modified Circumpolar Deep Water exchange across the shelf break, (ii) the upper ocean stratification across the Adélie Depression, including alongside iceberg C-28 that calved from the Mertz Glacier and (iii) the convective overturning of the deep remnant seasonal mixed layer in Commonwealth Bay from sea ice growth. Heat and freshwater budgets to 200–300 m are used to estimate the ocean heat content (400→50 MJ m−2), flux (50–200 W m−2 loss) and sea ice growth rates (maximum of 7.5–12.5 cm day−1). Mean seal-derived sea ice growth rates were within the range of satellite-derived estimates from 1992–2007 using ERA-Interim data. We speculate that the continuous foraging by the seals within Commonwealth Bay during the summer/fall transition was due to favorable feeding conditions resulting from the convective overturning of the deep seasonal mixed layer and chlorophyll maximum that is a reported feature of this location.

scholarly journals Upper ocean stratification and sea ice growth rates during the summer-fall transition, as revealed by Elephant seal foraging in the Adélie Depression, East Antarctica

2010 ◽  
Vol 7 (6) ◽  
pp. 1913-1951
Author(s):  
G. D. Williams ◽  
M. Hindell ◽  
M.-N. Houssais ◽  
T. Tamura ◽  
I. C. Field
Keyword(s):  
Sea Ice ◽  
Continental Shelf ◽  
Mixed Layer ◽  
Growth Rates ◽  
Shelf Break ◽  
Upper Ocean ◽  
Ocean Stratification ◽  
Glacier Tongue ◽  
Ice Growth ◽  
Continental Shelf Break

Abstract. Southern elephant seals (Mirounga leonina), fitted with Conductivity-Temperature-Depth sensors at Macquarie Island in January 2005 and 2010, collected unique oceanographic observations of the Adélie and George V Land continental shelf (140–148° E) during the summer-fall transition (late February through April). This is a key region of dense shelf water formation from enhanced sea ice growth/brine-rejection in the local coastal polynyas. In 2005 two seals occupied the continental shelf break near the grounded icebergs at the northern end of the Mertz Glacier Tongue for nearly two weeks at the onset of sea ice growth. One of the seals migrated north thereafter and the other headed west, possibly utilising the Antarctic Slope Front current near the continental shelf break. In 2010, after that years calving of the Mertz Glacier Tongue, two seals migrated to the same region but penetrated much further southwest across the Adélie Depression and occupied the Commonwealth Bay polynya from March through April. Here we present unique observations of the regional oceanography during the summer-fall transition, in particular (a) the zonal distribution of modified Circumpolar Deep Water exchange across the shelf break, (b) the upper ocean stratification across the Adélie Depression, including alongside iceberg C–28 that calved from the Mertz Glacier and (c) the convective overturning of the deep remnant seasonal mixed layer in Commonwealth Bay from sea ice growth (7.5–12.5 cm s−1). Heat and freshwater budgets to 200–300 m are used to estimate the ocean heat content, heat flux and sea ice growth rates. We speculate that the continuous foraging by the seals within Commonwealth Bay during the summer-fall transition was due to favorable feeding conditions resulting from the convective overturning of the deep seasonal mixed layer and chlorophyll maximum that is a reported feature of this location.

scholarly journals Winter Upper-Ocean Stability and Ice–Ocean Feedbacks in the Sea Ice–Covered Southern Ocean

2019 ◽  
Vol 49 (4) ◽  
pp. 1099-1117 ◽  
Author(s):  
Earle A. Wilson ◽  
Stephen C. Riser ◽  
Ethan C. Campbell ◽  
Annie P. S. Wong
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Mixed Layer ◽  
Weddell Sea ◽  
Large Temperature ◽  
Upper Ocean ◽  
Quasi Equilibrium ◽  
Regional Variability ◽  
Ocean Stratification ◽  
Ice Growth

AbstractIn this study, under-ice ocean data from profiling floats, instrumented seals, and shipboard casts are used to assess wintertime upper-ocean stability and heat availability in the sea ice–covered Southern Ocean. This analysis reveals that the southern Weddell Sea, which features a weak upper-ocean stratification and relatively strong thermocline, is preconditioned for exceptionally high rates of winter ventilation. This preconditioning also facilitates a strong negative feedback to winter ice growth. Idealized experiments with a 1D ice–ocean model show that the entrainment of heat into the mixed layer of this region can maintain a near-constant ice thickness over much of winter. However, this quasi-equilibrium is attained when the pycnocline is thin and supports a large temperature gradient. We find that the surface stress imparted by a powerful storm may upset this balance and lead to substantial ice melt. This response can be greatly amplified when coincident with anomalous thermocline shoaling. In more strongly stratified regions, such as near the sea ice edge of the major gyres, winter ice growth is weakly limited by the entrainment of heat into the mixed layer. Thus, the thermodynamic coupling between winter sea ice growth and ocean ventilation has significant regional variability. This regionality will influence the response of the Southern Ocean ice–ocean system to future changes in ocean stratification and surface forcing.

scholarly journals Acceleration and Overturning of the Antarctic Slope Current by Winds, Eddies, and Tides

2019 ◽  
Vol 49 (8) ◽  
pp. 2043-2074 ◽  
Author(s):  
Andrew L. Stewart ◽  
Andreas Klocker ◽  
Dimitris Menemenlis
Keyword(s):  
Sea Ice ◽  
Continental Shelf ◽  
Continental Slope ◽  
Mesoscale Eddy ◽  
Shelf Break ◽  
Ekman Transport ◽  
Global Ocean ◽  
Antarctic Slope Current ◽  
The Antarctic ◽  
Continental Shelf Break

AbstractAll exchanges between the open ocean and the Antarctic continental shelf must cross the Antarctic Slope Current (ASC). Previous studies indicate that these exchanges are strongly influenced by mesoscale and tidal variability, yet the mechanisms responsible for setting the ASC’s transport and structure have received relatively little attention. In this study the roles of winds, eddies, and tides in accelerating the ASC are investigated using a global ocean–sea ice simulation with very high resolution (1/48° grid spacing). It is found that the circulation along the continental slope is accelerated both by surface stresses, ultimately sourced from the easterly winds, and by mesoscale eddy vorticity fluxes. At the continental shelf break, the ASC exhibits a narrow (~30–50 km), swift (>0.2 m s−1) jet, consistent with in situ observations. In this jet the surface stress is substantially reduced, and may even vanish or be directed eastward, because the ocean surface speed matches or exceeds that of the sea ice. The shelfbreak jet is shown to be accelerated by tidal momentum advection, consistent with the phenomenon of tidal rectification. Consequently, the shoreward Ekman transport vanishes and thus the mean overturning circulation that steepens the Antarctic Slope Front (ASF) is primarily due to tidal acceleration. These findings imply that the circulation and mean overturning of the ASC are not only determined by near-Antarctic winds, but also depend crucially on sea ice cover, regionally-dependent mesoscale eddy activity over the continental slope, and the amplitude of tidal flows across the continental shelf break.

Vertical mixing in the bottom mixed layer near the continental shelf break in the East China Sea

1994 ◽  
Vol 50 (4) ◽  
pp. 437-448 ◽  
Author(s):  
Takeshi Matsuno ◽  
Seiichi Kanari ◽  
Chikashi Kobayashi ◽  
Toshiyuki Hibiya
Keyword(s):  
Continental Shelf ◽  
East China Sea ◽  
Mixed Layer ◽  
Vertical Mixing ◽  
Shelf Break ◽  
East China ◽  
The East China Sea ◽  
China Sea ◽  
Continental Shelf Break

scholarly journals Profiling Float Observations of the Upper Ocean under Sea Ice off the Wilkes Land Coast of Antarctica

2011 ◽  
Vol 41 (6) ◽  
pp. 1102-1115 ◽  
Author(s):  
Annie P. S. Wong ◽  
Stephen C. Riser
Keyword(s):  
Sea Ice ◽  
Mixed Layer ◽  
Freezing Point ◽  
Growth Period ◽  
Upper Ocean ◽  
Entrainment Rate ◽  
Study Region ◽  
Ice Growth ◽  
Winter Mixed Layer ◽  
Wilkes Land

Abstract Multiyear under-ice temperature and salinity data collected by profiling floats are used to study the upper ocean near the Wilkes Land coast of Antarctica. The study region is in the seasonal sea ice zone near the southern terminus of the Antarctic Circumpolar Current. The profiling floats were equipped with an ice-avoidance algorithm and had a survival rate of 74% after 2.5 yr in the ocean. The data show that, in this part of Antarctica, the rate of sea ice decay exceeds the rate of sea ice growth. During the sea ice growth period, the water column is weakly stratified because of brine rejection and is only marginally stable. The average winter mixed layer temperature is about 0.12°C above the surface freezing point, providing evidence of entrainment of warmer water from the permanent pycnocline. The average mixed layer salinity increases by 0.127 from June to October. A one-dimensional model is used to quantify evolution of the winter mixed layer under a sea ice cover. The local winter entrainment rate is estimated to be 49 ± 11 m over 5 months, supplying a heat flux of 34 ± 8 W m−2 to the base of the mixed layer in winter. Model output gives a thermodynamic sea ice growth of 28 ± 15 cm over the same period. The winter ocean–atmosphere heat loss through leads and sea ice is estimated to be 14–25 W m−2 in this area, which is broadly in line with other winter observations from the East Antarctic region.

scholarly journals Summertime increases in upper-ocean stratification and mixed-layer depth

Nature ◽  
2021 ◽  
Vol 591 (7851) ◽  
pp. 592-598
Author(s):  
Jean-Baptiste Sallée ◽  
Violaine Pellichero ◽  
Camille Akhoudas ◽  
Etienne Pauthenet ◽  
Lucie Vignes ◽  
...  
Keyword(s):  
Mixed Layer ◽  
Mixed Layer Depth ◽  
Upper Ocean ◽  
Layer Depth ◽  
Ocean Stratification

scholarly journals Energetic Submesoscales Maintain Strong Mixed Layer Stratification during an Autumn Storm

2017 ◽  
Vol 47 (10) ◽  
pp. 2419-2427 ◽  
Author(s):  
Daniel B. Whitt ◽  
John R. Taylor
Keyword(s):  
Mixed Layer ◽  
North Atlantic ◽  
Small Scale ◽  
Upper Ocean ◽  
Ocean Mixed Layer ◽  
Ocean Stratification ◽  
The North ◽  
Large Eddy ◽  
The Mean ◽  
Scale Turbulence

AbstractAtmospheric storms are an important driver of changes in upper-ocean stratification and small-scale (1–100 m) turbulence. Yet, the modifying effects of submesoscale (0.1–10 km) motions in the ocean mixed layer on stratification and small-scale turbulence during a storm are not well understood. Here, large-eddy simulations are used to study the coupled response of submesoscale and small-scale turbulence to the passage of an idealized autumn storm, with a wind stress representative of a storm observed in the North Atlantic above the Porcupine Abyssal Plain. Because of a relatively shallow mixed layer and a strong downfront wind, existing scaling theory predicts that submesoscales should be unable to restratify the mixed layer during the storm. In contrast, the simulations reveal a persistent and strong mean stratification in the mixed layer both during and after the storm. In addition, the mean dissipation rate remains elevated throughout the mixed layer during the storm, despite the strong mean stratification. These results are attributed to strong spatial variability in stratification and small-scale turbulence at the submesoscale and have important implications for sampling and modeling submesoscales and their effects on stratification and turbulence in the upper ocean.

Mesoscale, tidal and seasonal/interannual drivers of the Weddell Sea overturning circulation

2021 ◽  
Keyword(s):  
Continental Shelf ◽  
Shelf Break ◽  
Weddell Sea ◽  
Global Ocean ◽  
Mean Flow ◽  
Ice Shelf ◽  
Overturning Circulation ◽  
Global Circulation ◽  
Interannual Fluctuations ◽  
Continental Shelf Break

Abstract The Weddell Sea supplies 40–50% of the Antarctic BottomWaters that fill the global ocean abyss, and therefore exerts significant influence over global circulation and climate. Previous studies have identified a range of different processes that may contribute to dense shelf water (DSW) formation and export on the southern Weddell Sea continental shelf. However, the relative importance of these processes has not been quantified, which hampers prioritization of observational deployments and development of model parameterizations in this region. In this study a high-resolution (1/12°) regional model of the southern Weddell Sea is used to quantify the overturning circulation and decompose it into contributions due to multi-annual mean flows, seasonal/interannual variability, tides, and other sub-monthly variability. It is shown that tides primarily influence the overturning by changing the melt rate of the Filchner-Ronne Ice Shelf (FRIS). The resulting ~0.2 Sv decrease in DSW transport is comparable to the magnitude of the overturning in the FRIS cavity, but small compared to DSW export across the continental shelf break. Seasonal/interannual fluctuations exert a modest influence on the overturning circulation due to the relatively short (8-year) analysis period. Analysis of the transient energy budget indicates that the non-tidal, sub-monthly variability is primarily baroclinically-generated eddies associated with dense overflows. These eddies play a comparable role to the mean flow in exporting dense shelf waters across the continental shelf break, and account for 100% of the transfer of heat onto the continental shelf. The eddy component of the overturning is sensitive to model resolution, decreasing by a factor of ~2 as the horizontal grid spacing is refined from 1/3° to 1/12°.

scholarly journals Changes in the upper ocean mixed layer and phytoplankton productivity along the West Antarctic Peninsula

2018 ◽  
Vol 376 (2122) ◽  
pp. 20170173 ◽  
Author(s):  
Oscar Schofield ◽  
Michael Brown ◽  
Josh Kohut ◽  
Schuyler Nardelli ◽  
Grace Saba ◽  
...  
Keyword(s):  
Sea Ice ◽  
Antarctic Peninsula ◽  
Mixed Layer ◽  
Carbon Fixation ◽  
Upper Ocean ◽  
The West ◽  
Phytoplankton Productivity ◽  
West Antarctic ◽  
West Antarctic Peninsula

The West Antarctic Peninsula (WAP) has experienced significant change over the last 50 years. Using a 24 year spatial time series collected by the Palmer Long Term Ecological Research programme, we assessed long-term patterns in the sea ice, upper mixed layer depth (MLD) and phytoplankton productivity. The number of sea ice days steadily declined from the 1980s until a recent reversal that began in 2008. Results show regional differences between the northern and southern regions sampled during regional ship surveys conducted each austral summer. In the southern WAP, upper ocean MLD has shallowed by a factor of 2. Associated with the shallower mixed layer is enhanced phytoplankton carbon fixation. In the north, significant interannual variability resulted in the mixed layer showing no trended change over time and there was no significant increase in the phytoplankton productivity. Associated with the recent increases in sea ice there has been an increase in the photosynthetic efficiency (chlorophyll a -normalized carbon fixation) in the northern and southern regions of the WAP. We hypothesize the increase in sea ice results in increased micronutrient delivery to the continental shelf which in turn leads to enhanced photosynthetic performance. This article is part of the theme issue ‘The marine system of the West Antarctic Peninsula: status and strategy for progress in a region of rapid change’.

scholarly journals Gulf Stream Ring Water Intrusion on the Mid-Atlantic Bight Continental Shelf Break Affects Microbially Driven Carbon Cycling

2019 ◽  
Vol 6 ◽  
Author(s):  
Adrienne Hoarfrost ◽  
John Paul Balmonte ◽  
Sherif Ghobrial ◽  
Kai Ziervogel ◽  
John Bane ◽  
...  
Keyword(s):  
Continental Shelf ◽  
Carbon Cycling ◽  
Gulf Stream ◽  
Shelf Break ◽  
Water Intrusion ◽  
Continental Shelf Break
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