Shirase Glacier Research Articles

Rapid deceleration in the ice-flow velocity of the Shirase Glacier ice tongue and its influence on the velocity field: Observations from Sentinel-1 C-SAR

Temporal and spatial variations in the ice-flow speed of Shirase Glacier ice tongue in East Antarctica between July 2018 and December 2021 were investigated using Sentinel-1 C-band Synthetic Aperture Radar (C-SAR) imagery. We identified pronounced slowdown events in the eastern part of the outer ice tongue, 30–40 km from the grounding line in 2020 and 55 km in 2021. Comparison of ice thickness and bathymetry in areas where the deceleration events occurred suggests that the events were caused by icebergs grounding or landing on the seafloor. The absence of slowdown propagation towards the grounding line demonstrates the ice tongue offers very limited buttressing. This study contributes to a better understanding of the factors influencing glacier dynamics, particularly in the context of grounding events and their localized impacts.

Temporal Variations in Ice Thickness of the Shirase Glacier Derived from Cryosat-2/SIRAL Data

This study presents the feasibility of estimating the ice thickness of the Shirase Glacier using the synthetic aperture interferometric radar altimeter (SIRAL) on board the CryoSat-2 and the interannual variation of the ice thickness of the Shirase Glacier in 2011–2020. The SIRAL data were converted to ice thickness by assuming hydrostatic equilibrium, and the results showed that the ice thickness decreased from the grounding line to the terminus of the glacier. Furthermore, the ice thickness decreased 30 km downstream from the grounding line of the glacier in 2012 and 2017, and decreased 55 km and 60 km downstream from the grounding line of the glacier at other times, which was attributed to the discharge of landfast ice and the retreat of the glacier terminus. When the flow of glacial ice can be reasonably approximated as an incompressible fluid, and the law of conservation of mass can be applied to the ice stream, the multiple of the velocity and the underlying ice thickness under a constant ice density can be shown to correspond to the equation of continuity. Consequently, this study revealed that the ice thickness decreases with accelerating flow velocity, which is coincident with past outflow events.

Slowdown of Shirase Glacier, East Antarctica, caused by strengthening alongshore winds

Abstract. Around large parts of West Antarctica and in Wilkes Land, East Antarctica, increased wind-forced intrusions of modified Circumpolar Deep Water (mCDW) onto the continental shelf have been associated with mass loss over the last few decades. Despite considerable seasonal variability, observations in 2018 have also confirmed relatively high basal melt rates of up to 16 m a−1 underneath the Shirase ice tongue in Enderby Land, East Antarctica. These high basal melt rates are also caused by intrusions of mCDW onto the continental shelf, but the catchment of Shirase Glacier has been gaining mass, a trend often attributed to increased precipitation. Here, we document the dynamical ocean-driven slowdown, ice surface thickening and grounding line advance of Shirase Glacier in response to strengthening easterly winds that reduce mCDW inflow and decrease basal melt rates. Our findings are significant because they demonstrate that warm ice shelf cavity regimes are not universally associated with glacier acceleration and mass loss in Antarctica, and they highlight the overlooked role of the impact of easterly winds in the recent mass gain of the Shirase Glacier catchment.

Interactive movements of outlet glacier tongue and landfast sea ice in Lützow-Holm Bay, East Antarctica, detected by ALOS-2/PALSAR-2 imagery

The Shirase Glacier, one of the fastest-flowing outlet glaciers in East Antarctica, flows into a bay that is usually covered by landfast sea ice. Although the presence of landfast ice is considered to stabilize glacier tongue, quantitative assessments are insufficient. We investigated the spatiotemporal variations in the flow velocities of both the glacier tongue and the surrounding landfast ice using a correlation method with ALOS-2/PALSAR-2 images, before and after a landfast-ice collapse in April 2017. After the collapse, the glacier tongue flow accelerated by 8–15% (0.20–0.38 km a−1). After reestablishment of landfast ice, the flow velocity of the landfast ice in contact with the glacier terminus is 20%–90% of the flow velocity of the glacier terminus The estimates of interactive changes in the motions of both glacier ice and sea ice can be used to quantify the buttress effect, which is a key factor affecting the mass budget of Antarctic outlet glaciers.

Validation for Ice Flow Velocity Variations of Shirase Glacier Derived From PALSAR-2 Offset Tracking

We have shown that the flow velocity of Shirase Glacier can be estimated by applying the offset tracking method to the amplitude images derived from the phased array type L-band synthetic aperture radar type-2 (PALSAR-2) onboard the Advanced Land Observing Satellite-2. Although the offset tracking is widely used, the method is sometimes not applicable effectively and the reason is not always clear. Here, we consider a possibility of ionospheric contamination and show the validation results of the estimated flow velocity applied the offset tracking method to the PALSAR-2 data, based on <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in-situ</i> measurement of the flow velocity from the GNSS receiver on the autonomous phase-sensitive radio echo sounder simultaneously with the PALSAR-2 observation. As a result, the ionospheric contamination can yield a spurious error of ±0.2 km a <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> . After statistically removing the erroneous pairs, the RMSE was 0.049 km a <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> derived from the flow velocity error between the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in-situ</i> measurement results and estimated from satellite, and the estimated flow velocity obtained using PALSAR-2 data is proved to be in good agreement with the ground truth. Obtained spatial and temporal variations reveal signature of glacier dynamics, which prove the efficacy of the offset tracking method. The obtained ice-flow velocity increases rapidly from the upstream region to the coast, but its velocity is roughly constant over a region, 10-km long about the grounding line (GL), then gradually tends to increase again downstream from the GL. This trend has continued largely unchanged over24 years since 1996.

The Effect of Basal Melting of the Shirase Glacier Tongue on the CO2 System in Lützow‐Holm Bay, East Antarctica

AbstractTo clarify the effect of basal melting of ice tongues/ice shelf on the CO2 system in the Antarctic continental margin, seawater samples were collected for analysis of dissolved inorganic carbon (DIC), total alkalinity (TA), nutrients, chlorophyll a (chl.a), and oxygen isotopic ratios at the ice front of the Shirase Glacier Tongue (SGT) in Lützow‐Holm Bay (LHB), East Antarctica during the summers of 2017 and 2018. At depths greater than ∼20 dbar in LHB, CO2 system parameters were strongly influenced by the dilution effect of SGT basal melting and the inflow of modified Circumpolar Deep Water (mCDW). The distributions of DIC and TA agreed well with an offshore origin of mCDW that flowed beneath the SGT and mixing of basal meltwater and mCDW beneath the SGT. The fraction of SGT meltwater was highest at the station near the ice front. Dilution by SGT basal meltwater reduced the partial pressure of CO2 in the mCDW from 431 to 387 μatm. The water then became a sink rather than source of atmospheric CO2. In the sea surface, DIC and TA were strongly influenced by biological processes. Salinity‐normalized DIC decreased with the increase of salinity‐normalized TA in accord with 106:16 C:N molar stoichiometry; the chl.a concentration at the sea surface was as high as 31 μg L−1.

Modeling intensive ocean–cryosphere interactions in Lützow-Holm Bay, East Antarctica

Abstract. Basal melting of Antarctic ice shelves accounts for more than half of the mass loss from the Antarctic ice sheet. Many studies have focused on active basal melting at ice shelves in the Amundsen–Bellingshausen seas and the Totten ice shelf, East Antarctica. In these regions, the intrusion of Circumpolar Deep Water (CDW) onto the continental shelf is a key component for the localized intensive basal melting. Both regions have a common oceanographic feature: southward deflection of the Antarctic Circumpolar Current brings CDW toward the continental shelves. The physical setting of the Shirase Glacier tongue (SGT) in Lützow-Holm Bay corresponds to a similar configuration on the southeastern side of the Weddell Gyre in the Atlantic sector. Here, we conduct a 2–3 km resolution simulation of an ocean–sea ice–ice shelf model using a recently compiled bottom-topography dataset in the bay. The model can reproduce the observed CDW intrusion along the deep trough. The modeled SGT basal melting reaches a peak in summer and a minimum in autumn and winter, consistent with the wind-driven seasonality of the CDW thickness in the bay. The model results suggest the existence of an eastward-flowing undercurrent on the upper continental slope in summer, and the undercurrent contributes to the seasonal-to-interannual variability in the warm water intrusion into the bay. Furthermore, numerical experiments with and without fast-ice cover in the bay demonstrate that fast ice plays a role as an effective thermal insulator and reduces local sea ice formation, resulting in much warmer water intrusion into the SGT cavity.

Strong ice-ocean interaction beneath Shirase Glacier Tongue in East Antarctica

Mass loss from the Antarctic ice sheet, Earth’s largest freshwater reservoir, results directly in global sea-level rise and Southern Ocean freshening. Observational and modeling studies have demonstrated that ice shelf basal melting, resulting from the inflow of warm water onto the Antarctic continental shelf, plays a key role in the ice sheet’s mass balance. In recent decades, warm ocean-cryosphere interaction in the Amundsen and Bellingshausen seas has received a great deal of attention. However, except for Totten Ice Shelf, East Antarctic ice shelves typically have cold ice cavities with low basal melt rates. Here we present direct observational evidence of high basal melt rates (7–16 m yr−1) beneath an East Antarctic ice shelf, Shirase Glacier Tongue, driven by southward-flowing warm water guided by a deep continuous trough extending to the continental slope. The strength of the alongshore wind controls the thickness of the inflowing warm water layer and the rate of basal melting.

Ice sheet retreat and glacio-isostatic adjustment in Lützow-Holm Bay, East Antarctica

The East Antarctic Ice Sheet has relatively few field data to constrain its past volume and contribution to global sea-level change since the Last Glacial Maximum. We provide new data on deglaciation history and develop new relative sea-level (RSL) curves along an 80 km transect (from Skallen to Skarsvnes, Langhovde and the Ongul Islands) in Lützow Holm Bay, East Antarctica. The geological constraints were compared with output from two Glacial Isostatic Adjustment (GIA) models. The minimum radiocarbon age for regional deglaciation is c. 11,240 cal. yr BP on West Ongul Island with progressively younger deglaciation ages approaching the main regional ice outflow at Shirase Glacier. Marked regional differences in the magnitude and timing of RSL change were observed. More in particular, in Skarvsnes a minimum marine limit of 32.7 m was inferred, which is c. 12.7 m higher than previously published evidence, and at least 15 m higher than that reported in the other three ice-free areas. Current GIA model predictions slightly underestimate the rate of Late Holocene RSL fall at Skallen, Langhovde, and West Ongul, but provide a reasonable fit to the reconstructed minimum marine limit at these sites. GIA model predictions are unable to provide an explanation for the shape of the reconstructed RSL curve at Skarvsnes. We consider a range of possible explanations for the Skarvsnes RSL data and favour an interpretation where the anomalously high marine limit and rate of RSL fall is due to reactivation of a local fault.

Breakup of land‐fast sea ice in Lützow‐Holm Bay, East Antarctica, and its teleconnection to tropical Pacific sea surface temperatures

AbstractA large land‐fast sea ice breakup occurred in 2016 in Lützow‐Holm Bay, East Antarctica. The breakup caused calving from the Shirase Glacier Tongue. Although similar breakups and calving have been observed in the past, the timing and magnitudes are not well‐constrained. The ice's breakup latitude during 1997–2016 was analyzed to investigate the variables controlling breakup and examine correlation with local calving for a longer period. The breakup latitude in April had a persistently high correlation with sea surface temperature (SST) in the tropical Pacific, which exceeds correlations with local atmospheric variables. The years of five out of six observed calving events from the mid‐20th century can correspond to those of warm SST episodes and calving‐front retreat in the 1980s to warmer SST shift. Our proposed teleconnection between tropical SST and Antarctic sea ice could lead to better predictions of breakup and might impact the glacier flux for a wider region.

Temporal variations in the flow velocity for Shirase Glacier in Antarctica over a 20-year period

1996年から1998年のJapanese Earth Resources Satellite-1（JERS-1）衛星搭載のSynthetic Aperture Radar（SAR）の15 シーン，2007年から2010年のAdvanced Land Observing Satellite（ALOS）衛星搭載のPhased Array type L-band SAR（PALSAR）の17シーン，2014年から2015年のALOS-2衛星搭載のPALSAR-2の12シーンから，それぞれ12ペア，12ペア，11ペアの画像に対して画像相関法を適用し，南極・白瀬氷河の流動速度変動を調べた．2014年から2015年において，上流からGrounding line（GL）へと向かって流動速度が急激に速くなり，GLを挟む幅20kmの流域において一定となり，GLから下流そして浮氷舌へと再び流動速度がなだらかに速くなる傾向が見られた．この傾向は，1996年から1998年にJERS-1/SARおよび2007年から2009年にALOS/PALSARで観測された流動速度プロファイルから大局的には変化していないと考えられ，また流動速度も2.33kma−1（1996年）から2.26kma−1（2015年）で大きな変化は見られなかった．しかし，GLからの距離の関数として示した中央流線上では，上流域での速度に有意な経年変化が見られた．GLより5km上流から上流方向へ流動速度は増加しており，約18km上流では1996年から1998年を基準とした場合，2007年から2010年および2014年から2015年のそれぞれで0.57kma−1，0.72kma−1と最大の速度上昇に達した．30km上流ではそれぞれ0.25kma−1，0.35kma−1を示した．

Observations of vertical tidal motions of a floating iceberg in front of Shirase Glacier, East Antarctica, using a geodetic-mode GPS buoy

A dual-frequency GPS receiver was deployed on a floating iceberg downstream of the calving front of Shirase Glacier, East Antarctica, on 28 December 2011 for utilizing as floating buoy. The three-dimensional position of the buoy was obtained by GPS every 30 s with a 4–5-cm precision for ca. 25 days. The height uncertainty of the 1-h averaged vertical position was ∼0.5 cm, even considering the uncertainties of un-modeled ocean loading effects. The daily evolution of north–south (NS), east–west (EW), and up–down (UD) motions shows periodic UD variations sometimes attaining an amplitude of 1 m. Observed amplitudes of tidal harmonics of major constituents were 88%–93% (O1) and 85%–88% (M2) of values observed in the global ocean tide models FES2004 and TPXO-8 Atlas. The basal melting rate of the iceberg is estimated to be ∼0.6 m/day, based on a firn densification model and using a quasi-linear sinking rate of the iceberg surface. The 30-s sampling frequency geodetic-mode GPS buoy helps to reveal ice–ocean dynamics around the calving front of Antarctic glaciers.

A study on ice flow curved eastward of Shirase Glacier,East Antarctica

白瀬氷河の氷流が東へ湾曲することを調べるため，1996年から1998年のJapanese Earth Resources Satellite-1（JERS-1）衛星搭載のSynthetic Aperture Radar（SAR）の15シーンから得られる12ペアの画像に対して画像相関法を適用して流動ベクトルを算出した．下流域の西部流線における流動速度は東部流線よりも0.24-0.32km a−1速く，東西流線による流動速度の違いは顕著であることが分かった．この流動速度の非対称性において，氷流の方位角の変化はGrounding Line（GL）付近までの氷流は309°の北西方向に流れており，GL から10km下流へ離れた当たりから氷流の方向を319°と東方向へ変えて，20kmおよび30km下流ではそれぞれ329°，341°，浮氷舌では346°の方向へ流れており，上流から下流への流向変化は40°であった．この氷流の方位角の変化は，基盤地形の非対称性との関係を持つと考えられ，音響測深，BEDMAP，航空機によるアイスレーダおよび磁気測量によるリュツォ・ホルム湾の海底地形と白瀬氷河下の基盤地形の計測結果から考察した．その結果，氷河下の基盤地形も東西で非対称をなすと考えられ，このことが西部流線の流動速度が東部流線よりも速い結果を生じさせ，流動方向を東へ湾曲させることにつながると考えられた．

Near real-time monitoring of flow velocity and direction in the floating ice tongue of the Shirase Glacier using low-cost GPS buoys

The horizontal velocity vector of ice flow on the floating ice tongue of the Shirase Glacier, East Antarctica, was determined using two GPS buoys located on its east and west sides. The GPS buoys consisted of a single-frequency GPS receiver module and an Iridium satellite communication system. The instantaneous horizontal position of each GPS buoy was automatically obtained every 30 minutes, and the data were immediately transmitted to the National Institute of Polar Research (NIPR), Tokyo, Japan, via a satellite link. The location data demonstrated that the floating ice tongue moved primarily in a linear manner during the monitoring period between February and April, 2010. The speed and azimuth of the eastern buoy were (5.779 ± 0.004 m/day, N1.4°E ± 0.5°), respectively, while for the western buoy the speed and azimuth were (7.005 ± 0.006 m/day, N13.1°W ± 0.6°), respectively. Short-term variations about the mean speed and azimuth of the ice flow, with a period of 3–10 days, were also identified.

Fluctuations in the flow velocity of the Antarctic Shirase Glacier over an 11-year period

Temporal fluctuations in the flow velocity of Shirase Glacier in Antarctica were studied using 15 synthetic aperture radar (SAR) scenes obtained by the Japanese Earth Resources Satellite-1 (JERS-1) in 1996–1998 and 9 phased-array-type L-band SAR (PALSAR) scenes obtained by the Advanced Land Observing Satellite (ALOS) in 2007–2008. The scenes were analyzed using image correlation. The relative accuracy of the determined velocities was ±0.03 km/a for JERS-1 and ±0.02 km/a for ALOS. No marked temporal changes in ice-flow velocity at the grounding line (GL) were noted during either period; the ice velocity varied from 2.33 km/a (1996) to 2.27 km/a (2007). However, a marked difference between the two periods was found upstream from the GL on the central streamline, as a function of distance from the GL. Between the GL and the point 30 km downstream, ice velocities tended to be lower in 2007–2008 than in 1996–1998. Upstream from the GL, the velocities were higher in 2007–2008 than in 1996–1998, increasingly so with distance upstream, reaching a maximum of approximately 0.54 km/a at around 17–18 km upstream from the GL before diminishing again to a value of ∼0.25 km/a at 30 km upstream. No clear seasonal variations in flow velocity were observed at 17–18 km upstream from the GL in 1996–1998; however, in 2007–2008, the summer (December 2007–February 2008) velocity was 1.27 km/a, whereas the winter (June–August 2008) velocity was 1.06 km/a, representing a marked difference of 0.21 km/a. A larger difference between these seasons (0.38 km/a) was found at 20 km above the GL.

JERS-1/SARとALOS/PALSARを用いた白瀬氷河の流速推定結果の比較

JERS-1/SARとALOS/PALSARを用いた白瀬氷河の流速推定結果の比較

Why is Shirase Glacier turning its flow direction eastward?

We applied an image correlation method to Japanese Earth Resources Satellite-1 (JERS-1) synthetic aperture radar (SAR) data obtained from 1996 to 1998 to examine flow velocity within Shirase Glacier, Antarctica. From the grounding line to the downstream region of the glacier, the obtained ice-flow velocity was systematically higher on the western streamline than the eastern. The differences between the two streamlines were 0.31 km/a in 1996 and 0.37 km/a in 1998, significantly larger than the error estimate of 0.03 km/a. The direction of ice flow was about 312° at the grounding line and changed to 327° at 10 km, 346° at 20 km and 2° at 30 km downstream from the grounding line. The total accumulated deflection is 50° to the east. Under the assumption of the conservation of ice mass across the glacier, the observed eastward change in flow direction can be explained by an asymmetric deepening of bedrock topography, that is, across the 8 km width of the glacier, the eastern side is 50 m (10%) deeper than the western side. This eastward turning of flow direction appears to be accelerated by tributary inlets, that flow to the north and northeast at 60–75% of the velocity of inlets on the western streamline.

Estimation of seasonal changes in the flow of Shirase Glacier using JERS-1/SAR image correlation

This paper presents estimates of detailed seasonal variations in ice-flow velocity for Shirase Glacier calculated using data obtained by Japanese Earth Resources Satellite-1 (JERS-1) synthetic aperture radar (SAR). We used 12 pairs of images (44-day repeat cycle) over the interval from 30 April 1996 to 1 July 1998 to estimate ice-flow fields using an image correlation method. Geometric registration was performed with reference to the RADARSAT Antarctic Mapping Project (RAMP) image dataset. Error analysis based on feature mismatch indicated an absolute error of ±0.30 km/a and relative error of ±0.04 km/a in the estimated flow velocity. The obtained ice-flow velocity increases rapidly from the upstream region (1.18 km/a) to the grounding line, where it becomes stagnant (2.32 km/a), before accelerating gradually to 2.62–2.82 km/a in the downstream region and then increasing to 3.05–3.50 km/a at the terminus of the floating ice tongue. The ice-flow velocities in the downstream region are highly variable, depending on both the distance from the grounding line and the observed epoch (season). Most of the obtained seasonal variations in ice-flow velocity at the floating ice tongue are within the range of the associated error estimate, but the annual difference between 1997 (3.11 km/a) and 1998 (3.50 km/a) is significant, reflecting a possible acceleration in the ice-flow velocity in association with the disappearance of the floating ice tongue between April and May of 1998. In terms of the summer–winter difference in averaged air temperature, the large difference recorded in 1997 (17.0 °C) relative to 1996 (13.9 °C) corresponds to a reduced ice-flow velocity in 1997 (approximately 0.20 km/a) relative to that in 1996 (approximately 0.30 km/a), indicating interactions between air, sea ice, and glacier flow in Lützow-Holm Bay.

合成開口レーダ（SAR）による白瀬氷河の接地線検出

合成開口レーダ（SAR）による白瀬氷河の接地線検出

The nature of complex ice flow in Shirase Glacier catchment, East Antarctica

AbstractThe transition of inland to complex ice flow is investigated in the downstream area of Shirase Glacier catchment, East Antarctica. The 250 km long Mizuho Triangulation Chain (MTC; Naruse, 1978), 230 km upstream of the grounding line of Shirase Glacier, is re-analyzed by a force-balance analysis, in combination with balance-velocity estimates. Numerical model experiments were carried out with a two-dimensional thermomechanical higher-order ice-sheet model along a central flowline extending from the ice divide (Dome Fuji) to the grounding line of Shirase Glacier, passing through the MTC. Results show that complex flow originates near the MTC, where longitudinal normal stresses and normal drag in particular play a decisive role in the force balance. The fast-flow area, however, is geologically controlled and confined to the “bottleneck” or ice-flow convergence area, a few tens of kilometres upstream of the grounding line. In between the MTC and the grounding zone, longitudinal normal stress and basal sliding are dominant features of the ice-flow regime.