We develop a model of an ice shelf-ice stream system as a viscoelastic beam partially supported by an elastic foundation. When bedrock near the grounding line acts as a fulcrum, leverage from the ice ...shelf dropping at low tide can cause significant (∼1cm) uplift in the first few kilometers of grounded ice. This uplift and the corresponding depression at high tide lead to basal pressure variations of sufficient magnitude to influence subglacial hydrology. Tidal flexure may thus affect basal lubrication, sediment flow, and till strength, all of which are significant factors in ice-stream dynamics and grounding-line stability. Under certain circumstances, our results suggest the possibility of seawater being drawn into the subglacial water system. The presence of seawater beneath grounded ice would significantly change the radar reflectivity of the grounding zone and complicate the interpretation of grounded versus floating ice based on ice-penetrating radar observations.
► Tidal flexure of ice shelves can produce pressure variations under grounded ice. ► These variations can be large enough to influence subglacial hydrology. ► Seawater could be drawn under grounded ice, affecting ice-penetrating radar.
Global sea level provides an important indicator of the state of the warming climate, but changes in regional sea level are most relevant for coastal communities around the world. With improvements ...to the sea‐level observing system, the knowledge of regional sea‐level change has advanced dramatically in recent years. Satellite measurements coupled with in situ observations have allowed for comprehensive study and improved understanding of the diverse set of drivers that lead to variations in sea level in space and time. Despite the advances, gaps in the understanding of contemporary sea‐level change remain and inhibit the ability to predict how the relevant processes may lead to future change. These gaps arise in part due to the complexity of the linkages between the drivers of sea‐level change. Here we review the individual processes which lead to sea‐level change and then describe how they combine and vary regionally. The intent of the paper is to provide an overview of the current state of understanding of the processes that cause regional sea‐level change and to identify and discuss limitations and uncertainty in our understanding of these processes. Areas where the lack of understanding or gaps in knowledge inhibit the ability to provide the needed information for comprehensive planning efforts are of particular focus. Finally, a goal of this paper is to highlight the role of the expanded sea‐level observation network—particularly as related to satellite observations—in the improved scientific understanding of the contributors to regional sea‐level change.
Plain Language Summary
This review paper addresses three important questions: (1) What do we currently know about the processes contributing to sea level change? (2) What observations do we use to gain this knowledge? and (3) Where are these gaps in our knowledge and the need for further improvement in our understanding of the drivers of regional sea level? By answering these specific questions in a focused manner, this paper should be a useful resource for other scientists, sea‐level stakeholders, and a broader audience of those interested in sea level and our changing climate.
Key Points
An overview of the current state of understanding of the processes that cause regional sea‐level change is provided
Areas where the lack of understanding or gaps in knowledge inhibit the ability to assess future sea‐level change are discussed
The role of the expanded sea‐level observation network in improving our understanding of sea‐level change is highlighted
Antarctica's ice shelves are thinning at an increasing rate, affecting their buttressing ability. Channels in the ice shelf base unevenly distribute melting, and their evolution provides insight into ...changing subglacial and oceanic conditions. Here we used phase‐sensitive radar measurements to estimate basal melt rates in a channel beneath the currently stable Ross Ice Shelf. Melt rates of 22.2 ± 0.2 m a−1 (>2500% the overall background rate) were observed 1.7 km seaward of Mercer/Whillans Ice Stream grounding line, close to where subglacial water discharge is expected. Laser altimetry shows a corresponding, steadily deepening surface channel. Two relict channels to the north suggest recent subglacial drainage reorganization beneath Whillans Ice Stream approximately coincident with the shutdown of Kamb Ice Stream. This rapid channel formation implies that shifts in subglacial hydrology may impact ice shelf stability.
Key Points
Melt rates greater than 20 m/yr occur locally beneath the Ross Ice Shelf
Channels can be created by highly concentrated melting in the grounding zone
Relict channels may be indicators of subglacial discharge reorganization
Expansion of Firn Aquifers in Southeast Greenland Horlings, Annika N.; Christianson, Knut; Miège, Clément
Journal of geophysical research. Earth surface,
October 2022, 2022-10-00, 20221001, Letnik:
127, Številka:
10
Journal Article
Recenzirano
Odprti dostop
Surface melt produces more mass loss than any other process on the Greenland Ice Sheet. In some regions of Greenland with high summer surface melt and high winter snow accumulation, the warm porous ...firn of the percolation zone can retain liquid meltwater through the winter. These regions of water‐saturated firn, which may persist for longer than one year, are known as firn aquifers, commonly referred to as perennial firn aquifers. Here, we use airborne ice‐penetrating radar data from the Center for Remote Sensing of Ice Sheets (CReSIS) to document the extent of four firn aquifers in the Helheim, Ikertivaq, and Køge Bugt glacier basins with more than six repeat radar flight lines from 1993 to 2018. All four firn aquifers first appear and/or show decadal‐scale inland expansion during this time period. Through an idealized energy‐balance calculation utilizing reanalysis data from the Modèle Atmosphérique Régionale (MAR) regional climate model, we find that these aquifer expansions are driven by decreasing cold content in the firn since the late 1990s and recently increasing high‐melt years, which has reduced the firn's ability for refreezing local meltwater. High‐melt years are projected to increase on the Greenland Ice Sheet and may contribute to the continued inland expansion of firn aquifers, impacting the ice sheet's surface mass balance and hydrological controls on ice dynamics.
Plain Language Summary
Warm atmospheric temperatures over the Greenland Ice Sheet can melt snow at the surface, producing liquid meltwater that can infiltrate downward into denser and older snow known as firn. The firn can retain this liquid meltwater continuously for more than one year in certain regions of the ice sheet that have high snow accumulation and high surface melt. These water‐saturated regions of firn are called firn aquifers, which are important in understanding the ice sheet's mass loss to the oceans. To determine the evolution of firn aquifers in Greenland and what factors primarily influence their behavior, we examine airborne ice‐penetrating radar data that were collected from 1993 to 2018. From the repeat detections of four firn aquifers in southeast Greenland, we find that the aquifers first appear and/or expand inland during this time period. Regional historical climate data from a climate model and an idealized energy budget calculation suggest that aquifer expansions are driven by warming firn since the 1990s: the firn is not cold enough to refreeze increasingly large amounts of surface melt, and therefore the meltwater remains in a liquid state. More extreme warm summers are expected for Greenland, which may contribute to the formation and continued expansion of firn aquifers.
Key Points
We generate a time series of four perennial firn aquifers in southeast Greenland from 1993 to 2018 using ice‐penetrating radar data
The four firn aquifers first appear and three with recent data show expansion upstream toward the ice‐sheet interior during the time period
Firn‐aquifer expansion is correlated with decreasing firn cold content and recent high‐melt years
A system of subglacial lakes drained on Thwaites Glacier from 2012–2014. To improve coverage for subsequent drainage events, we extended the elevation and ice-velocity time series on Thwaites Glacier ...through austral winter 2019. These new observations document a second drainage cycle in 2017/18 and identified two new lake systems located in the western tributaries of Thwaites and Haynes glaciers. In situ and satellite velocity observations show temporary < 3 % speed fluctuations associated with lake drainages. In agreement with previous studies, these observations suggest that active subglacial hydrology has little influence on thinning and retreat of Thwaites Glacier on decadal to centennial timescales.
Subglacial lakes require a thawed bed either now or in the past; thus, their presence and stability have implications for current and past basal conditions, ice dynamics, and climate. Here, we ...present the most extensive geophysical exploration to date of a subglacial lake near the geographic South Pole, including radar‐imaged stratigraphy, surface velocities, and englacial vertical velocities. We use a 1.5‐dimensional temperature model, optimized with our geophysical data set and nearby temperature measurements, to estimate past basal‐melt rates. The ice geometry, reflected bed‐echo power, surface and vertical velocities, and temperature model indicate that the ice‐bed interface is regionally thawed, contradicting prior studies. Together with an earlier active‐source seismic study, which showed a 32‐m deep lake underlain by 150 m of sediment, our results suggest that the lake has been thermodynamically stable through at least the last 120,000 years and possibly much longer, making it a promising prospective site for sediment coring.
Plain Language Summary
There are hundreds of subglacial lakes under the Antarctic Ice Sheet. The presence of those lakes requires sufficient heat, sourced either from the Earth's interior or from ice motion. One subglacial lake near the South Pole was previously considered a conundrum since nearby temperature measurements are cold, possibly indicating that the ice should be frozen to the surface below it. Here, we use radar and velocity measurements to better understand the ice‐sheet geometry, ice motion, and the nature of the underlying bed. We find that the lake is currently filled to within 2 m of its maximum capacity and that the entire surveyed area likely has a thawed bed. We then use nearby measurements of ice temperature and historical climate data from the South Pole Ice Core to calculate the past temperatures of the ice. These calculations indicate a thawed bed. We suggest that the lake has been thermally stable for at least the last 120,000 years and possibly much longer. A prior study showed that the lake is underlain by 150 m of sediment, making it a candidate for sediment coring.
Key Points
South Pole Lake is constrained to a 50 km2 area, 15 km from the geographic South Pole
Surface strain rates and vertical velocity profiles indicate a consistently thawed ice‐sheet bed within the surveyed area
Temperature modeling suggests the lake has been thermodynamically stable for at least 120,000 years
The dynamics of marine-terminating outlet glaciers are of fundamental interest in glaciology and affect mass loss from ice sheets in a warming climate. In this study, we analyze the response of ...outlet glaciers to different sources of climate forcing. We find that outlet glaciers have a characteristically different transient response to surface-mass-balance forcing applied over the interior than to oceanic forcing applied at the grounding line. A recently developed reduced model represents outlet-glacier dynamics via two widely separated response timescales: a fast response associated with grounding-zone dynamics and a slow response of interior ice. The reduced model is shown to emulate the behavior of a more complex numerical model of ice flow. Together, these models demonstrate that ocean forcing first engages the fast, local response and then the slow adjustment of interior ice, whereas surface-mass-balance forcing is dominated by the slow interior adjustment. We also demonstrate the importance of the timescales of stochastic forcing for assessing the natural variability in outlet glaciers, highlighting that decadal persistence in ocean variability can affect the behavior of outlet glaciers on centennial and longer timescales. Finally, we show that these transient responses have important implications for attributing observed glacier changes to natural or anthropogenic influences; the future change already committed by past forcing; and the impact of past climate changes on the preindustrial glacier state, against which current and future anthropogenic influences are assessed.
We used a terrestrial radar interferometer (TRI) at Helheim Glacier, Greenland, in August 2013, to study the effects of tidal forcing on the terminal zone of this tidewater glacier. During our study ...period, the glacier velocity was up to 25 m d–1. Our measurements show that the glacier moves out of phase with the semi-diurnal tides and the densely packed melange in the fjord. Here detrended glacier displacement lags behind the forecasted tidal height by ∼8 hours. The transition in phase lag between the glacier and the melange happens within a narrow (∼500 m) zone in the fjord in front of the ice cliff. The TRI data also suggest that the impact of tidal forcing decreases rapidly up-glacier of the terminus. A flowline model suggests this pattern of velocity perturbation is consistent with weak ice flowing over a weakly nonlinear bed.
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