Michael Schröder

Strong Sensitivity of Pine Island Ice-Shelf Melting to Climatic Variability

Cold Glacier Growth Pine Island Glacier in Antarctica has thinned significantly during the last two decades and has provided a measurable contribution to sea-level rise as a result. Both glacier dynamics and climate are thought to be responsible for thinning, but exactly how they influence the glacier are incompletely known. Dutrieux et al. (p. 174, published online 2 January) provide another layer of detail to our understanding of the process through observations of ocean temperatures in the surrounding waters. The thermocline adjacent in the sea adjacent to the glacier calving front (where ice is discharged) lowered by 250 meters in the austral summer of 2012. This change exposed the bottom of the ice shelf to colder surface waters rather than to the warmer, deeper layer, thereby reducing heat transfer from the ocean to the overlying ice and decreasing basal melting of the ice by more than 50% compared to 2010. Those 2012 ocean conditions were partly caused by a strong La Niña event, thus illustrating how important atmospheric variability is for regulating how the Antarctic Ice Sheet responds to climate change. Colder surface ocean waters decreased the rate of melting under the Pine Island Glacier ice shelf in 2012. Pine Island Glacier has thinned and accelerated over recent decades, significantly contributing to global sea-level rise. Increased oceanic melting of its ice shelf is thought to have triggered those changes. Observations and numerical modeling reveal large fluctuations in the ocean heat available in the adjacent bay and enhanced sensitivity of ice-shelf melting to water temperatures at intermediate depth, as a seabed ridge blocks the deepest and warmest waters from reaching the thickest ice. Oceanic melting decreased by 50% between January 2010 and 2012, with ocean conditions in 2012 partly attributable to atmospheric forcing associated with a strong La Niña event. Both atmospheric variability and local ice shelf and seabed geometry play fundamental roles in determining the response of the Antarctic Ice Sheet to climate.

Flow of bottom water in the northwestern Weddell Sea

The Weddell Sea is known to feed recently formed deep and bottom water into the Antarctic circumpolar water belt, from whence it spreads into the basins of the world ocean. The rates are still a matter of debate. To quantify the flow of bottom water in the northwestern Weddell Sea data obtained during five cruises with R/V Polarstern between October 1989 and May 1998 were used. During the cruises in the Weddell Sea, five hydrographic surveys were carried out to measure water mass properties, and moored instruments were deployed over a time period of 8.5 years to obtain quasi-continuous time series. The average flow in the bottom water plume in the northwestern Weddell Sea deduced from the combined conductivity-temperature-depth and moored observations is 1.3±0.4 Sv. Intensive fluctuations of a wide range of timescales including annual and interannual variations are superimposed. The variations are partly induced by fluctuations in the formation rates and partly by current velocity fluctuations related to the large-scale circulation. Taking into account entrainment of modified Warm Deep Water and Weddell Sea Deep Water during the descent of the plume along the slope, between 0.5 Sv and 1.3 Sv of surface-ventilated water is supplied to the deep sea. This is significantly less than the widely accepted ventilation rates of the deep sea. If there are no other significant sources of newly ventilated water in the Weddell Sea, either the dominant role of Weddell Sea Bottom Water in the Southern Ocean or the global ventilation rates have to be reconsidered.

Winter-summer differences of carbon dioxide and oxygen in the Weddell Sea surface layer

Abstract Mid-winter total inorganic carbon (TCO 2 ) and oxygen measurements are presented for the central fully ice-covered Weddell Sea. Lateral variations of these properties in the surface layer of the central Weddell Sea were small but significant. These variations were caused by vertical transport of Warm Deep Water into the surface layer and air-sea exchange before the ice cover. Oxygen saturation in the surface layer of the central Weddell Sea was near 82%, whereas in the eastern shelf area this was 89%. Surprisingly, p CO 2 , as calculated under the assumption of (reported) conservativeness of alkalinity, was also found to be below saturation (86–93%). This was not expected since ongoing Warm Deep Water entrainment into the surface layer tends to increase the p CO 2 . Rapid cooling and subsequent ice formation during the previous autumn, however, might have brought about a sufficiently low undersaturation of CO 2 , that as to the point of sampling had not yet been replenished through Warm Deep Water entrainment. In the ensuing early summer the measurements were repeated. In the shelf area and the central Weddell Sea, where the ice-cover had almost disappeared, photosynthesis had caused a decrease of p CO 2 and an increase of oxygen compared to the previous winter. In between these two regions there was an area with significant ice-cover where essentially winter conditions prevailed. Based on the summer-winter difference a (late-winter) entrainment rate of Warm Deep Water into the surface layer of 4–5 m/month was calculated. A complete surface water balance, including entrainment, biological activity and air-sea exchange, showed that between the winter and summer cruises CO 2 and oxygen had both been absorbed from the atmosphere. The TCO 2 increase due to entrainment of Warm Deep Water was partly countered by (autumn) cooling, and partly through biological drawdown. Part of the CO 2 removed through biological activity sinks down the water column as organic material and is remineralised at depth. It is well-known that bottom water formation constitutes a sink for atmospheric CO 2 . However, whether the Weddell Sea as a whole is a sink for CO 2 depends on the ratio of two counteracting processes i.e. entrainment, which increases CO 2 in the surface and the biological pump, which decreases it. As deep water is not only entrained into the surface, but also conveyed out of the Weddell Sea, the relative importances of these (CO 2 -enriched) deep water transports are important as well.

How iceberg calving and grounding change the circulation and hydrography in the Filchner Ice Shelf-Ocean System

The formation of bottom water in the southern Weddell Sea is strongly influenced by the flow of Ice Shelf Water (ISW) out of the Filchner-Ronne Ice Shelf cavity. The breakout of three giant icebergs in 1986 and their grounding on the shallow Berkner Bank modified the circulation and water mass formation in the Filchner Trough and the adjacent sea areas. Hydrographic measurements along the Filchner Ice Shelf front, carried out with RV Polarstern in 1995, show significant changes in the water mass characteristics and flow patterns in the Filchner Trough in comparison to measurements from the early 1980s. Changes in the trough will affect the flow over the sill to the deep Weddell Abyssal Plain. We combine a three-dimensional ocean circulation model with conductivity-temperature-depth and stable isotope measurements to investigate the details of the circulation in front of and beneath the Filchner Ice Shelf. We assess the impact of stranded icebergs and a more southerly ice shelf front position caused by a 1986 iceberg calving event on the circulation and observed water mass properties. Results indicate variations of the flow pattern in the Filchner Trough and on Berkner Bank, where High-Salinity Shelf Water, the feedstock for ISW, is produced. The calving and grounding impacts illustrate the sensitivity of the ice shelf-ocean system to perturbations in local bathymetric settings.

Flow variability at the tip of the Antarctic Peninsula

Recently ventilated water leaves the landlocked northwestern Weddell Sea near the Antarctic Peninsula and possibly spreads out into the basins of the world oceans at shallow to intermediate depths. To determine the pathways of the water through the complex topography and the flow variability, water-mass circulation and properties are described in the northwestern Weddell Sea and along the boundary of the Powell Basin by means of data from current-meter moorings and hydrographic sections. The mean flow is strongly controlled by the topography. Meso-scale, seasonal and interannual fluctuations are superimposed. The mean northward volume transport of shelf water, which represents the potential source water for intermediate layer ventilation, is estimated for the time interval between May 1996 and March 1998 to be 2.4±1.0 Sv. Water-mass properties suggest that much of this water leaves the Weddell Sea to Bransfield Strait and therefore does not reach the Weddell Scotia Confluence. The water masses are able to serve as the only source of Bransfield Strait deep water since the shelf water properties in the northwestern Weddell Sea vary over time within a range that corresponds to the required source waters. The Scotia Sea is supplied by water from the Powell Basin, which has varied significantly over the past two decades.

Sea ice feedbacks observed in western Weddell Sea

The German research icebreaker RV Polarstern drifted within the western Weddell Sea pack from 27 November, 2004 until 2 January, 2005, initially anchored to a 10 km x 10 km sized floe. This was composed of 2-m thick second-year ice with 0.8 m snow on top, interspersed by first-year ice with modal thicknesses of 0.9 m and 1.8 m, covered with 0.3 m of snow. ISPOL traveled over a distance of 290 km with a net south-north displacement of 98 km due to various loops in the ice floe trajectory. These loops seem common since ISW-1 and tagged ice floes and icebergs followed similar deflections during their course. For 36 days Polarstern served an interdisciplinary scientific team from eight countries (Australia, Belgium, Finland, Netherlands, Russia, United Kingdom, U.S.A., and Germany) as accommodation and laboratory as well as platform for field and water column studies. The local view was extended to both sides of the floe trajectory using helicopters with an operational range of 120 km to conduct sea ice thickness and ice dynamics measurements, iceberg marking, and water column sampling. On 2 December, 2004 the floe cracked into several pieces, reducing the main station floe to a size of 1.5 km x 1.5 km. On 24 December the ice floe again fractured, leaving a piece of 0.7 km x 0.8 km for the remaining experiment.

Modeling the spreading of glacial meltwater from the Amundsen and Bellingshausen Seas

It has been suggested that an increased melting of continental ice in the Amundsen Sea (AS) and Bellingshausen Sea (BS) is a likely source of the observed freshening of Ross Sea (RS) water. To test this hypothesis, we simulate the spreading of glacial meltwater using the Finite Element Sea Ice/Ice Shelf/Ocean Model. Based on the spatial distribution of simulated passive tracers, most of the basal meltwater from AS ice shelves flows toward the RS with more than half of the melt originating from the Getz Ice Shelf. Further, the model results show that a slight increase of the basal mass loss can substantially intensify the transport of meltwater into the RS due to a strengthening of the melt-driven shelf circulation and the westward flowing coastal current. This supports the idea that the basal melting of AS and BS ice shelves is one of the main sources for the RS freshening.

Effect of critical latitude and seasonal stratification on tidal current profiles along Ronne Ice Front, Antarctica

[1] The ice front region of Ronne Ice Shelf lies near the critical latitude of the semidiurnal M2 tide, the principal tidal constituent in the southern Weddell Sea. Here the Coriolis frequency almost equals the M2 tidal frequency, resulting in a strong dependence of the M2 tidal currents on depth and stratification and a boundary layer that can occupy the entire water column. Using data from four long-term moorings along Ronne Ice Front, we confirm the presence of strongly depth-dependent semidiurnal tidal currents and their sensitivity to changes in stratification. The time series show dramatic seasonal changes in tidal current profiles and significant interannual variability. During periods of stratification, the amplitude of the semidiurnal tides in the mid–water column shows a twofold increase and, despite being several kilometers offshore from the ice front, the tidal currents clearly show a second boundary layer originating from the adjacent ice shelf base. Together, these two boundary layers occupy most of the water column, up to 600 m deep, until intense sea ice formation and the production of High-Salinity Shelf Water erodes the vertical stratification. During winter when homogeneous conditions prevail, a single bottom boundary layer occupies the entire water column at some locations. This strong seasonality and sensitivity of the M2 tidal current to stratification highlights the difficulties of interpreting current data from short-term moorings while demonstrating that it is the best indicator for characterizing changes in stratification after direct observations of density variations.

On the near-bottom variability in the northwestern Weddell Sea

The thermohaline data from the first Brazilian hydrographic cruise to the northwestern Weddell Sea (AR XVIII) revealed significant near-bottom changes in water-column properties over seasonal and interannual time scales. Favorable ice conditions in 2000 allowed a dense station coverage of the area including the main pathways for Weddell Sea deep and bottom waters. The new results are compared with the data from the 1998 German cruise ANT XV/4 and other historical data. A warming of the bottom layer was discovered that was more attributable to short-term seasonal or interannual fluctuations in the formation of cold bottom water than to a long-term trend. There was both consistency and variation between and within seasons. Invariant bottom-water characteristics were observed in different seasons (summer/winter), and variable bottom-water characteristics were observed in the same season (summer) at the same locations. This reduces the possibility of a dominant seasonal effect. Instead, we propose that the intermittent behavior of small cold-water sources along the Weddell Seas periphery causes the variability measured in the deep northwestern Weddell Sea. The observed variability has consequences for the water-mass export across the South Scotia Ridge, as the absence of the fresher/lighter Weddell Sea Bottom Water south of South Orkney Plateau during AR XVIII might be linked to a reduced ventilation of the deep Scotia Sea. The results of this study show the need for ongoing efforts for establishing a long-term monitoring of this region with global importance.

Seasonal ventilation of the cavity beneath Filchner‐Ronne Ice Shelf simulated with an isopycnic coordinate ocean model

[1] The ocean cavity beneath Filchner-Ronne Ice Shelf is observed to respond to the seasonal cycle of water mass production on the continental shelf of the southern Weddell Sea. Here we use a numerical model to investigate the propagation of newly formed shelf waters into the cavity. We find that the model reproduces the most distinctive features of the observed seasonality and offers a plausible explanation for those features. The most saline shelf waters are produced in the far west, where the inflow to the cavity peaks twice each year. The major peak occurs during the short period around midwinter when convection reaches full depth and the densest waters are generated. Once the surface density starts to decline, dynamic adjustment of the restratified water column leads to a gradual fall in the salinity at depth and a secondary peak in the inflow that occurs in summer at the western coast. Beneath the ice shelf the arrival of the wintertime inflow at the instrumented sites is accompanied by a rapid warming, while the slower decline in the inflow leads to a more gradual cooling. Water brought in by the secondary, summer peak flows mainly to the eastern parts of the cavity. Here the seasonality is suppressed because the new inflows mix with older waters that recirculate within a topographic depression. This pooling of waters in the east, where the primary outflow of Ice Shelf Water is generated, dampens the impact of seasonality on the local production of Weddell Sea Bottom Water.