Intrusion Of Circumpolar Deep Water Research Articles

Evidence of minimal carbon sequestration in the productive Amundsen Sea polynya

AbstractThe Amundsen Sea polynya (ASP) is reportedly the most productive among the coastal polynyas around Antarctica. However, observational constraints limit our understanding of the biological pump and carbon cycle in the ASP. We measured various carbon‐related parameters such as primary production, bacterial production, sinking flux of particulate organic carbon (POC), and accumulation of organic carbon in the sediment as well as hydrographic parameters during field observations and by instrument moorings. Over 95% of the photosynthetically produced POC was converted to suspended POC and/or dissolved carbon forms in the upper ~400 m layer. We postulate that most of the carbon transported to the water column by the biological pump in the ASP was flushed out of the shelf without being sequestered for long‐term storage in sediments. Lack of bottom water formation due to intrusion of Circumpolar Deep Water in the lower layer reduces carbon sequestration efficiency.

Continued retreat of Thwaites Glacier, West Antarctica, controlled by bed topography and ocean circulation

AbstractThe Amundsen Sea sector is experiencing the largest mass loss, glacier acceleration, and grounding line retreat in Antarctica. Enhanced intrusion of Circumpolar Deep Water onto the continental shelf has been proposed as the primary forcing mechanism for the retreat. Here we investigate the dynamics and evolution of Thwaites Glacier with a novel, fully coupled, ice‐ocean numerical model. We obtain a significantly improved agreement with the observed pattern of glacial retreat using the coupled model. Coupled simulations over the coming decades indicate a continued mass loss at a sustained rate. Uncoupled simulations using a depth‐dependent parameterization of sub‐ice‐shelf melt significantly overestimate the rate of grounding line retreat compared to the coupled model, as the parameterization does not capture the complexity of the ocean circulation associated with the formation of confined cavities during the retreat. Bed topography controls the pattern of grounding line retreat, while oceanic thermal forcing impacts the rate of grounding line retreat. The importance of oceanic forcing increases with time as Thwaites grounding line retreats farther inland.

Temporal variability of the Circumpolar Deep Water inflow onto the Ross Sea continental shelf

The intrusion of Circumpolar Deep Water (CDW) is the primary source of heat, salt and nutrients onto Antarctica's continental shelves and plays a major role in the shelf physical and biological processes.Different studies have analyzed the processes responsible for the transport of CDW across the Ross Sea shelf break, but until now, there are no continuous observations that investigate the timing of the intrusions. Also, few works have focused on the effect of the tides that control these intrusions.In the Ross Sea, the CDW intrudes onto the shelf in several locations, but mostly along the troughs.We use hydrographic observations and a mooring placed on the outer shelf in the middle of the Drygalski Trough in order to characterize the spatial and temporal variability of CDW inflow onto the shelf. Our data span from 2004 to the beginning of 2014.In the Drygalski Trough, the CDW enters as a 150m thick layer between 250 and 400m, and moves upward towards the south. At the mooring location, about 50km from the shelf break, two main CDW cores can be observed: one on the east side of the trough spreading along the west slope of Mawson Bank from about 200m to the bottom and the other one in the central-west side from 200m to about 350m depth.A signature of this lighter and relatively warm water is detected by the instruments on the mooring at bottom of the Drygalski Trough. High frequency periodic CDW intrusion at the bottom of the trough is related to the diurnal and spring/neap tidal cycles.At lower frequency, a seasonal variability of the CDW intrusion is noticed. A strong inflow of CDW is observed every year at the end of December, while the CDW inflow is at its seasonal minimum during the beginning of the austral fall. In addition an interannual variability is also evident. A change of the CDW intrusion before and after 2010 is observed.

Iron supply and demand in an Antarctic shelf ecosystem

AbstractThe Ross Sea sustains a rich ecosystem and is the most productive sector of the Southern Ocean. Most of this production occurs within a polynya during the November–February period, when the availability of dissolved iron (dFe) is thought to exert the major control on phytoplankton growth. Here we combine new data on the distribution of dFe, high‐resolution model simulations of ice melt and regional circulation, and satellite‐based estimates of primary production to quantify iron supply and demand over the Ross Sea continental shelf. Our analysis suggests that the largest sources of dFe to the euphotic zone are wintertime mixing and melting sea ice, with a lesser input from intrusions of Circumpolar Deep Water and a small amount from melting glacial ice. Together these sources are in approximate balance with the annual biological dFe demand inferred from satellite‐based productivity algorithms, although both the supply and demand estimates have large uncertainties.

Glider observations of the Dotson Ice Shelf outflow

The Amundsen Sea is one of the most productive polynyas in the Antarctic per unit area and is undergoing rapid changes including a reduction in sea ice duration, thinning ice sheets, retreat of glaciers and the potential collapse of the Thwaites Glacier in Pine Island Bay. A growing body of research has indicated that these changes are altering the water mass properties and associated biogeochemistry within the polynya. Unfortunately difficulties in accessing the remote location have greatly limited the amount of in situ data that has been collected. In this study data from a Teledyne-Webb Slocum glider was used to supplement ship-based sampling along the Dotson Ice Shelf (DIS). This autonomous underwater vehicle revealed a detailed view of a meltwater laden outflow from below the western flank of the DIS. Circumpolar Deep Water intruding onto the shelf drives glacial melt and the supply of macronutrients that, along with ample light, supports the large phytoplankton blooms in the Amundsen Sea Polynya. Less well understood is the source of micronutrients, such as iron, necessary to support this bloom to the central polynya where chlorophyll concentrations are highest. This outflow region showed decreasing optical backscatter with proximity to the bed indicating that particulate matter was sourced from the overlying glacier rather than resuspended sediment. This result suggests that particulate iron, and potentially phytoplankton primary productivity, is intrinsically linked to the magnitude and duration of sub-glacial melt from Circumpolar Deep Water intrusions onto the shelf.

On the difficulty of modeling Circumpolar Deep Water intrusions onto the Amundsen Sea continental shelf

In the Amundsen Sea, warm Circumpolar Deep Water (CDW) intrudes onto the continental shelf and flows into the ice shelf cavities of the West Antarctic Ice Sheet, resulting in high basal melt rates. However, none of the high resolution global models resolving all the small ice shelves around Antarctica can reproduce a realistic CDW flow onto the Amundsen Sea continental shelf, and previous studies show simulated bottom potential temperature at the Pine Island Ice Shelf front of about −1.8°C. In this study, using the Finite-Element Sea ice–ice shelf-Ocean Model (FESOM), we reproduce warm CDW intrusions onto the Amundsen Sea continental shelf and realistic melt rates of the ice shelves in West Antarctica. To investigate the importance of horizontal resolution, forcing, horizontal diffusivity, and the effect of grounded icebergs, eight sensitivity experiments are conducted. To simulate the CDW intrusion realistically, a horizontal resolution of about 5km or smaller is required. The choice of forcing is also important and the cold bias in the NCEP/NCAR reanalysis over the eastern Amundsen Sea prevents warm CDW from intruding onto the continental shelf. On the other hand, the CDW intrusion is not highly sensitive to the strength of horizontal diffusion. The effect of grounded icebergs located off Bear Peninsula is minor, but may act as a buffer to an anomalously cold year.

Small‐scale variability of the cross‐shelf flow over the outer shelf of the Ross Sea

AbstractThe importance of cross‐shelf transport across the Ross Sea on local and remote processes has been well documented. In the Ross Sea, mid‐water intrusions of Circumpolar Deep Water (CDW) are modified by shelf water near the shelf break to form Modified Circumpolar Deep Water (MCDW). In 2010–2011, we deployed multi‐platform technologies focused on this MCDW intrusion in the vicinity of Mawson and Pennell Banks to better understand its role in ecosystem processes across the shelf. The high‐resolution time and space sampling provided by an underwater glider, a short‐term mooring, and a ship‐based survey highlight the scales over which these critical cross‐shelf transport processes occur. MCDW cores were observed as small‐scale well‐defined features over the western slopes of Pennell and Mawson Banks. The mean transport along Pennell Bank was estimated to be about 0.24 Sv but was highly variable in time (hours to days). The observations suggest that the core of MCDW is transported by a predominately barotropic flow that follows topography around the banks toward the south until the slope of the bank flattens and the warmer water moves up and over the bank. This pathway is shown to link the source MCDW with an area of high productivity over the shallows of Pennell Bank.

Rapid deglaciation of Marguerite Bay, western Antarctic Peninsula in the Early Holocene

Glacial geological evidence of rapid ice stream retreat is important for the potential insight it can bring to understanding of contemporary rapid ice sheet change. Here, we report new chronological constraints on the deglacial history of the Marguerite Trough Ice Stream, which formerly drained part of the western margin of the Antarctic Peninsula. This ice stream has been previously identified on geomorphological grounds as an example of an outlet that underwent rapid retreat following the Last Glacial Maximum, but the precise timing of this retreat has not been conclusively determined. We obtained cosmogenic surface exposure dates on erratic boulders that record the thinning history of ice flowing into the ice stream. These dates show that the ice stream underwent >270 m of thinning very rapidly at 9.6 ka BP. The timing of ice stream thinning agrees closely with previously published data on ice shelf retreat, the age of the first post-glacial beaches, and minimum constraining ages for the onset of lacustrine sedimentation, and post-glacial sedimentation in inner shelf fjords. We compare the timing of retreat to the similarly-sized Anvers Trough ice stream further north and find that retreat in Marguerite Bay was more rapid and earlier than in the Anvers Trough. We argue that the intrusion of Circumpolar Deep Water is the most likely forcing mechanism for retreat in Marguerite Bay. A key finding of this study is that rapid retreat occurred in deep water with reverse (continent-dipping) slope but also in relatively shallow water with a positive slope. This may have implications for understanding of oceanographically-forced ice sheet change in other parts of West Antarctica.

A model study of Circumpolar Deep Water on the West Antarctic Peninsula and Ross Sea continental shelves

Transport of relatively warm, nutrient-rich Circumpolar Deep Water (CDW) onto continental shelves around Antarctica has important effects on physical and biological processes. However, the characteristics of the CDW along the shelf break, as well as what happens to it once it has been advected onto the continental shelf, differ spatially. In the present study high resolution (4–5 km) regional models of the Ross Sea and the West Antarctic Peninsula coastal ocean are used to compare differences in CDW transport. The models compared very well with observations from both regions. Examining the fluxes not only of heat, but also of a simulated “dye” representing CDW, shows that in both cases CDW crosses the shelf break in specific locations primarily determined by the bathymetry, but eventually floods much of the shelf. The frequency of intrusions in Marguerite Trough was ca. 2–3 per month, similar to recent mooring observations. A significant correlation between the along shelf break wind stress and the cross shelf break dye flux through Marguerite Trough was observed, suggesting that intrusions are at least partially related to short duration wind events. The primary difference between the CDW intrusions on the Ross and west Antarctic Peninsula shelves is that there is more vigorous mixing of the CDW with the surface waters in the Ross Sea, especially in the west where High Salinity Shelf Water is created. The models show that the CDW moving across the Antarctic Peninsula continental shelf towards the base of the ice shelves not only is warmer initially and travels a shorter distance than that advected towards the base of the Ross Ice Shelf, but it is also subjected to less vertical mixing with surface waters, which conserves the heat available to be advected under the ice shelves. This difference in vertical mixing also likely leads to differences in the supply of nutrients from the CDW into the upper water column, and thus modulates the impacts on surface biogeochemical processes.

Deglacial seasonal and sub‐seasonal diatom record from Palmer Deep, Antarctica

AbstractThe Antarctic Peninsula is one of the most sensitive regions of Antarctica to climate change. Here, ecological and cryospheric systems respond rapidly to climate fluctuations. A 4.4 m thick laminated diatom ooze deposited during the last deglaciation is examined from a marine sediment core (ODP Site 1098) recovered from Basin I, Palmer Deep, western Antarctic Peninsula. This deglacial laminated interval was deposited directly over a glaciomarine diamict, hence during a globally recognised period of rapid climate change. The ultra‐high‐resolution deglacial record is analysed using SEM backscattered electron imagery and secondary electron imagery. Laminated to thinly bedded orange‐brown diatom ooze (near monogeneric Hyalochaete Chaetoceros spp. resting spores) alternates with blue‐grey terrigenous sediments (open water diatom species). These discrete laminae are interpreted as austral spring and summer signals respectively, with negligible winter deposition. Sub‐seasonal sub‐laminae are observed repeatedly through the summer laminae, suggesting variations in shelf waters throughout the summer. Tidal cycles, high storm intensities and/or intrusion of Circumpolar Deep Water onto the continental shelf introduced conditions which enhanced specific species productivity through the season. Copyright © 2005 John Wiley & Sons, Ltd.

A model study of circulation and cross-shelf exchange on the west Antarctic Peninsula continental shelf

Abstract Exchange of warm, nutrient-rich Circumpolar Deep Water (CDW) onto Antarctic continental shelves and coastal seas has important effects on physical and biological processes in these regions. The present study investigates the locations of this exchange and their dynamics in the west Antarctic Peninsula with a high-resolution three-dimensional numerical model. The model circulation is forced by daily wind stress along with heat and salt fluxes calculated by bulk formulae. All surface fluxes are modified by an imposed climatological ice cover. The model circulation compares favorably to general schematics of the flow based on dynamic topography, water properties from recent hydrography, and ADCP measurements. The sea-surface temperature is similar to satellite estimates except that the model temperatures are slightly higher than observations in the summer and lower in the winter. The seasonal variation of the depth and temperature of the model mixed layer matches observations reasonably well. Sub-pycnocline temperature shows evidence of persistent intrusion of warm CDW onto the shelf, for example, at the shelf break offshore of Adelaide Island. There is a significant correlation between the curvature of the shelf break and the volume transport across the shelf break, indicating that circulation crosses the shelf break in places where the tendency of the flow to maintain a given direction would have it cross a strongly curved bathymetric contour. A momentum term balance shows that momentum advection helps to force flow across the shelf break in specific locations due to the curvature of the bathymetry. For the model to create a strong intrusion of CDW onto the shelf, it appears two mechanisms are necessary. First, CDW is driven onto the shelf at least partially due to momentum advection and the curvature of the shelf break. Then, the general shelf circulation pulls the CDW into the interior.

Water-mass properties and circulation on the west Antarctic Peninsula Continental Shelf in Austral Fall and Winter 2001

Abstract Hydrographic measurements made during the US Southern Ocean Global Ocean Ecosystem Dynamics cruises, which took place from April to June and July to September 2001, provide a description of changes in water-mass distributions and circulation patterns in the Marguerite Bay region of the west Antarctic Peninsula continental shelf that result from seasonal variability and offshore forcing by the southern boundary of the Antarctic Circumpolar Current (ACC). The primary seasonal change in water-mass properties is the reduction in Antarctic Surface Water and replacement by a thick Winter Water layer. The primary effect of the ACC is to pump warm ( > 1.5 ° C ), salty (34.65–34.7), and nutrient-rich Circumpolar Deep Water (CDW) onto the continental shelf below 200 m at specific sites that correspond to bathymetric features, such as Marguerite Trough. The CDW intruded onto the continental shelf, moved across shelf, and entered Marguerite Bay. This flow pattern was observed during both cruises, suggesting that onshelf intrusions of CDW are a regular occurrence on this shelf. The number of CDW intrusions observed suggests that 4–6 events can occur in a year. Regions where CDW intrusions occur are characterized by surface waters that are above freezing in winter. Diffusive heat fluxes based on estimated diffusivities are insufficient for the observed rate of temperature decay of CDW intrusions. Localized, bathymetrically controlled vertical mixing is suggested as the primary heat transfer mechanism. The hydrographic and Acoustic Doppler Current Profiler measurements show a southwesterly flowing coastal current along Adelaide Island that enters the north side of Marguerite Bay and exits around Alexander Island. This current, which may result from seasonal, coastal buoyancy forcing, was present in fall and winter, but was better developed in fall. This current may be part of a larger cyclonic gyre that overlies the northern part of the area surveyed during the two cruises.

Acoustically-inferred zooplankton distribution in relation to hydrography west of the Antarctic Peninsula

Abstract The relationship between the distribution of zooplankton, especially euphausiids (Euphausia and Thysanoessa spp.), and hydrographic regimes of the Western Antarctic Peninsula continental shelf in and around Marguerite Bay was studied as part of the Southern Ocean GLOBEC program. Surveys were conducted from the RVIB N.B. Palmer in austral fall (April–June) and winter (July–August) of 2001. Acoustic, video, and environmental data were collected along 13 transect lines running across the shelf and perpendicular to the Western Antarctic Peninsula coastline, between 65°S and 70°S. Depth-stratified net tows conducted at selected locations provided ground-truthing for acoustic observations. In fall, acoustic volume backscattering strength at 120 kHz was greatest in the southern reaches of the survey area and inside Marguerite Bay, suggestive of high zooplankton and micronekton biomass in these regions. Vertically, highest backscattering was in the 150–450 m depth range, associated with modified Circumpolar Deep Water (CDW). The two deep troughs that intersect the shelf break were characterized by reduced backscattering, similar to levels observed off-shelf and indicative of lower zooplankton biomass in recent intrusions of CDW onto the continental shelf. Estimates of dynamic height suggested that geostrophic circulation likely caused both along- and across-shelf transport of zooplankton. By winter, scattering had decreased by an order of magnitude (10 dB) in the upper 300 m of the water column in most areas, and high backscattering levels were found primarily in a deep (>300 m) scattering layer present close to the bottom. The seasonal decrease is potentially explained by advection of zooplankton, vertical and horizontal movements, and mortality. Predictions of expected backscattering levels based on net samples suggested that large euphausiids were the dominant source of backscattering only at very particular locations and depths, and that copepods, siphonophores, and pteropods were more important in many locations.

Decadal-scale variations of water mass properties in the deep Weddell Sea

Data from cruises between 1989 and 2003 with FS Polarstern were used to construct section-wide potential temperature and salinity time series of the main water masses in the Weddell Gyre. In tandem with these CTD data, two time series between 1989 and 1995 are presented from moored instruments in the central Weddell Sea. The regional and methodological consistency of the dataset allows us to quantify variations which are not visible in less homogeneous datasets. The data reveal significant temperature and salinity variations of the Warm Deep Water and the Weddell Sea Bottom Water on a decadal time scale. The longest time series were obtained at the prime meridian. Here warming is observed in the Warm Deep Water from 1992 to 1998 followed by cooling. In the Weddell Sea proper, measurements of instruments moored in the Weddell Sea Bottom Water layer recorded a temperature increase over 6 years at a rate of 0.01 °C a−1. After the mooring period, CTD casts in 1998 point to a weakening of the trend. The warming trend in the bottom water occurs over most of the Weddell Sea, as detected in the additional CTD surveys. The variations are close to the detection level in the voluminous Weddell Sea Deep Water. The initial warming trend of the Warm Deep Water is consistent with warming trends reported in literature of subsurface waters of the Antarctic Circumpolar Current. The reversal of the trend in the Weddell Sea seems to be related to variations of the atmospheric conditions which can affect both the intrusion of Circumpolar Deep Water from the north and the circulation of the Weddell Gyre. Because the Warm Deep Water is the major source water for the formation of deep and bottom water in the Weddell Sea, it is suggested that its increase in temperature and salinity is likely to at least partly cause the variations which were observed in the bottom water.

Cross-shelf exchange in a model of the Ross Sea circulation and biogeochemistry

Abstract Transport of warm, nutrient-rich Circumpolar Deep Water (CDW) onto Antarctic continental shelves and coastal seas has important effects on physical and biological processes. The present study investigates the locations of this transport and its dynamics in the Ross Sea with a high-resolution three-dimensional numerical model. The model circulation is forced by daily wind stress along with heat and salt fluxes calculated from atmospheric climatologies by bulk formulae. All surface fluxes are modified by an imposed climatological ice cover. Waters under the Ross Ice Shelf are not included explicitly, but their effect on temperature and salinity is imposed in a buffer zone at the southern end of the model domain. A simple nutrient uptake is calculated based on the climatological chlorophyll distribution and Monod uptake kinetics. Model circulation is strongly affected by bottom topography, due to weak stratification, and agrees with schematics of the general flow and long-term current measurements except near the southern boundary. The sea-surface temperature is similar to satellite estimates except that the warmest simulated temperatures are slightly higher than observations. There is a significant correlation between the curvature of the shelf break and the transport across the shelf break. A momentum term balance shows that momentum advection helps to force flow across the shelf break in specific locations due to the curvature of the bathymetry (that is, the isobaths curve in front of the flow). For the model to create a strong intrusion of CDW onto the shelf, it appears two mechanisms are necessary. First, CDW is driven onto the shelf at least partially due to momentum advection and the curvature of the shelf break; then, the general circulation on the shelf takes the CDW into the interior.

Hydrographic and hydrodynamic characteristics of the eastern basin of the Bransfield Strait (Antarctica)

Hydrographic station and current meter data are used to estimate circulation and transport in the eastern basin of the Bransfield Strait. The short distance between adjacent hydrographic stations (20 km) allows evaluation of structures at scales seldom addressed in previous studies. The main feature of the derived circulation is the Bransfield Front and its associated baroclinic jet (the Bransfield Current). This frontal current crosses the northern half of the basin in a generally SW–NE direction, has maximum geostrophic speeds of 22 cm s −l (at the jet entrance), and has geostrophic transport relative to 500 dbar estimated to be 1 Sv. Dynamically significant mesoscale features associated with the Bransfield Current are seen to be relevant down to 500 dbar. Specific aspects inferred from our analysis are the apparent high degree of stationarity of the described circulation, the shallow intrusions of Circumpolar Deep Water through the northern boundary of the domain (from the Drake Passage), and the northward sinking of Weddell Sea water over most of the domain.

Ice front fluctuation, iceberg calving flux and mass balance of Victoria Land glaciers

The coast of Victoria Land extends from Williamson Head (69°11'S, 158°E) to McMurdo Sound (77°S, 163°E). A comparison of various documents and images spanning several decades has allowed the ice front fluctuation and the iceberg calving flux during this century to be estimated. During the periods from 1956–65 to 1972–73 the floating glaciers underwent a reduction of 978 km2 with an iceberg calving flux of about 134 km2 yr−1. After this, during the periods from 1972–73 to 1989–91, the floating glaciers underwent an advance of 272 km2 with an iceberg calving flux of about 53 km2 yr−1. Glacier tongues with bottom accretion calve less often than those with bottom melting. Most floating glaciers have shown cyclic behaviour without a strong trend. Exceptions to this general style are Hells Gate ice shelf, McMurdo Ice Shelf and floating glaciers of Cape Adare which have undergone a significant retreat since the beginning of the 20th century. The different behaviour of these floating glaciers has been hypothesized as being due to: increased energy available for meltwater production of marine ice that progressively warmed these thin ice shelves and then increased iceberg calving (Hells Gate and McMurdo), or to increased melting at the ice-ocean interface related to a major intrusion of Circumpolar Deep Water from the nearby continental slope (Cape Adare). An estimate of the mass balance of East Antarctica from which these glaciers are fed shows a positive value, that is significant despite all the uncertainties of balance measurements.