Glacier Speed-Up Events and Subglacial Hydrology on the Lower Franz Josef Glacier, New Zealand

<p>The contribution of glacier mass loss to future sea level rise is still poorly constrained (Lemke and others, 2007). One of the remaining unknowns is how water inputs influence glacier velocity. Short-term variations in glacier velocity occur when a water input exceeds the capacity of the subglacial drainage system, and the subglacial water pressure increases. Several studies (Van de Wal and others, 2008; Sundal and others, 2011) have suggested that high ice-flow velocities during these events are later offset by lower ice-flow velocities due to a more efficient subglacial drainage system. This study combines in-situ velocity measurements with a full Stokes glacier flowline model to understand the spatial and temporal variations in glacier flow on the lower Franz Josef Glacier, New Zealand. The Franz Josef Glacier experiences significant water inputs throughout the year (Anderson and others, 2006), and as a result, the subglacial drainage system is likely well-developed. In March 2011, measured ice-flow velocities increased by up to 75% above background values in response to rain events and by up to 32% in response to diurnal melt cycles. These speed-up events occurred at all survey locations across the lower glacier. Through flowline modelling, it is shown that the enhanced glacier flow can be explained by a spatially-uniform subglacial water pressure that increased during periods of heavy rain and glacier melt. From these results, it is suggested that temporary spikes in water inputs can cause glacier speed-up events, even when the subglacial hydrology system is well-developed (cf. Schoof, 2010). Future studies should focus on determining the contribution of glacier speed-up events to overall glacier motion.</p>

Glacier Speed-Up Events and Subglacial Hydrology on the Lower Franz Josef Glacier, New Zealand

<p>The contribution of glacier mass loss to future sea level rise is still poorly constrained (Lemke and others, 2007). One of the remaining unknowns is how water inputs influence glacier velocity. Short-term variations in glacier velocity occur when a water input exceeds the capacity of the subglacial drainage system, and the subglacial water pressure increases. Several studies (Van de Wal and others, 2008; Sundal and others, 2011) have suggested that high ice-flow velocities during these events are later offset by lower ice-flow velocities due to a more efficient subglacial drainage system. This study combines in-situ velocity measurements with a full Stokes glacier flowline model to understand the spatial and temporal variations in glacier flow on the lower Franz Josef Glacier, New Zealand. The Franz Josef Glacier experiences significant water inputs throughout the year (Anderson and others, 2006), and as a result, the subglacial drainage system is likely well-developed. In March 2011, measured ice-flow velocities increased by up to 75% above background values in response to rain events and by up to 32% in response to diurnal melt cycles. These speed-up events occurred at all survey locations across the lower glacier. Through flowline modelling, it is shown that the enhanced glacier flow can be explained by a spatially-uniform subglacial water pressure that increased during periods of heavy rain and glacier melt. From these results, it is suggested that temporary spikes in water inputs can cause glacier speed-up events, even when the subglacial hydrology system is well-developed (cf. Schoof, 2010). Future studies should focus on determining the contribution of glacier speed-up events to overall glacier motion.</p>

The Hydrological System and Climate of Brewster Glacier, Tititea Mt Aspiring National Park, Southern Alps, Aotearoa New Zealand, in the Context of Climate Change.

<p>Temporal and spatial variability of stream discharge is directly related to variation in local climate, and this in turn is related to both regional and global atmospheric circulation and climate change. The relationship is complicated in glacierised catchments. This study aims to identify relationships between discharge from Brewster Glacier proglacial stream and both local atmospheric variables and national atmospheric circulation patterns. An attempt is made to quantify these relationships using statistical models and tests in order that prediction of discharge with climate change could be made using local weather forecasts and national circulation indices. The nature of the subglacial drainage system is also investigated with particular focus on its structural evolution from summer to autumn. It is found that shortwave radiation, wind speed and relative humidity are consistently the most important variables in prediction of discharge and that wind speed is most important during summer while air temperature is most important in autumn. It is concluded that the importance of precipitation is greater than indicated by the results which were influenced by covariance in the records. A multiple regression model for summer discharge predicts up to 85% of variation in the proglacial stream hydrograph and for autumn 60%. Low overall energy inputs during autumn result in lesser sensitivity of discharge to variation in environmental conditions. It is concluded that the subglacial drainage system is highly arborescent over both summer and autumn and that little, if any, evolution occurs through these seasons. A qualitative relationship is established between discharge production at Brewster Glacier proglacial stream and national atmospheric circulation indices; highest average discharge occurs during northwesterly cyclonic conditions, when the turbulent heat fluxes and precipitation dominate discharge production, and lowest during southeasterly anticyclones when total energy inputs are low. The multiple regression models are used to estimate changes in discharge over the next 20 years given predicted changes in air temperature and precipitation, and it is found that the models lack the sensitivity required for accurate predictions.</p>

The Hydrological System and Climate of Brewster Glacier, Tititea Mt Aspiring National Park, Southern Alps, Aotearoa New Zealand, in the Context of Climate Change.

<p>Temporal and spatial variability of stream discharge is directly related to variation in local climate, and this in turn is related to both regional and global atmospheric circulation and climate change. The relationship is complicated in glacierised catchments. This study aims to identify relationships between discharge from Brewster Glacier proglacial stream and both local atmospheric variables and national atmospheric circulation patterns. An attempt is made to quantify these relationships using statistical models and tests in order that prediction of discharge with climate change could be made using local weather forecasts and national circulation indices. The nature of the subglacial drainage system is also investigated with particular focus on its structural evolution from summer to autumn. It is found that shortwave radiation, wind speed and relative humidity are consistently the most important variables in prediction of discharge and that wind speed is most important during summer while air temperature is most important in autumn. It is concluded that the importance of precipitation is greater than indicated by the results which were influenced by covariance in the records. A multiple regression model for summer discharge predicts up to 85% of variation in the proglacial stream hydrograph and for autumn 60%. Low overall energy inputs during autumn result in lesser sensitivity of discharge to variation in environmental conditions. It is concluded that the subglacial drainage system is highly arborescent over both summer and autumn and that little, if any, evolution occurs through these seasons. A qualitative relationship is established between discharge production at Brewster Glacier proglacial stream and national atmospheric circulation indices; highest average discharge occurs during northwesterly cyclonic conditions, when the turbulent heat fluxes and precipitation dominate discharge production, and lowest during southeasterly anticyclones when total energy inputs are low. The multiple regression models are used to estimate changes in discharge over the next 20 years given predicted changes in air temperature and precipitation, and it is found that the models lack the sensitivity required for accurate predictions.</p>

Melting temperature changes during slip across subglacial cavities drive basal mass exchange

The importance of glacier sliding has motivated a rich literature describing the thermomechanical interactions between ice, liquid water and bed materials. Early recognition of the gradient in melting temperature across small bed obstacles led to focused studies of regelation. An appreciation for the limits on ice deformation rates downstream of larger obstacles highlighted a role for cavitation, which has subsequently gained prominence in descriptions of subglacial drainage. Here, we show that the changes in melting temperature that accompany changes in normal stress along a sliding ice interface near cavities and other macroscopic drainage elements cause appreciable supercooling and basal mass exchange. This provides the basis of a novel formation mechanism for widely observed laminated debris-rich basal ice layers.

The Seasonal Evolution of Subglacial Drainage Pathways Beneath a Soft-bedded Glacier

Subglacial hydrology is a key element in glacier response to climate change, but investigations of this environment are logistically difficult. Most models are based on summer data from glaciers resting on rigid bedrocks. However a significant number of glaciers rest on soft (unconsolidated sedimentary) beds. Here we present a unique multi-year instrumented record of the development of seasonal subglacial behavior associated with an Icelandic temperate glacier resting on a deformable sediment layer. We observe a distinct annual pattern in the subglacial hydrology based on self-organizing anastomosing braided channels. Water is stored within the subglacial system itself (till, braided system and ‘ponds’), allowing the rapid access of water to enable glacier speed-up events to occur throughout the year, particularly in winter.

Melting temperature changes during slip across subglacial cavities drive basal mass exchange

The importance of glacier sliding has motivated a rich literature describing the thermomechanical interactions between ice, liquid water, and bed materials. Early recognition of the gradient in melting temperature across small bed obstacles led to focussed studies of regelation. An appreciation for the limits on ice deformation rates downstream of larger obstacles highlighted a role for cavitation, which has subsequently gained prominence in descriptions of subglacial drainage. Here, we show that the changes in melting temperature that accompany changes in normal stress along a sliding ice interface near cavities and other macroscopic drainage elements cause appreciable supercooling and basal mass exchange. This provides the basis of a novel formation mechanism for widely observed laminated debris-rich basal ice layers.

Observing the subglacial hydrology network and its dynamics with a dense seismic array

Subglacial water flow strongly modulates glacier basal motion, which itself strongly influences the contributions of glaciers and ice sheets to sea level rise. However, our understanding of when and where subglacial water flow enhances or impedes glacier flow is limited due to the paucity of direct observations of subglacial drainage characteristics. Here, we demonstrate that dense seismic array observations combined with an innovative systematic seismic source location technique allows the retrieval of a two-dimensional map of a subglacial drainage system, as well as its day-to-day temporal evolution. We observe with unprecedented detail when and where subglacial water flows through a cavity-like system that enhances glacier flow versus when and where water mainly flows through a channel-like system that impedes glacier flow. Most importantly, we are able to identify regions of high hydraulic connectivity within and across the cavity and channel systems, which have been identified as having a major impact on the long-term glacier response to climate warming. Applying a similar seismic monitoring strategy in other glacier settings, including for ice sheets, may help to diagnose the susceptibility of their dynamics to increased meltwater input due to climate warming.

Sensitivity of subglacial drainage to water supply distribution at the Kongsfjord basin, Svalbard

By regulating the amount, the timing, and the location of meltwater supply to the glacier bed, supraglacial hydrology potentially exerts a major control on the evolution of the subglacial drainage system, which in turn modulates ice velocity. Yet the configuration of the supraglacial hydrological system has received only little attention in numerical models of subglacial hydrology so far. Here we apply the two-dimensional subglacial hydrology model GlaDS (Glacier Drainage System model) to a Svalbard glacier basin with the aim of investigating how the spatial distribution of meltwater recharge affects the characteristics of the basal drainage system. We design four experiments with various degrees of complexity in the way that meltwater is delivered to the subglacial drainage model. Our results show significant differences between experiments in the early summer transition from distributed to channelized drainage, with discrete recharge at moulins favouring channelization at higher elevations and driving overall lower water pressures. Otherwise, we find that water input configuration only poorly influences subglacial hydrology, which instead is controlled primarily by subglacial topography. All experiments fail to develop channels of sufficient efficiency to substantially reduce summertime water pressures, which we attribute to small surface gradients and short melt seasons. The findings of our study are potentially applicable to most Svalbard tidewater glaciers with similar topography and low meltwater recharge. The absence of efficient channelization implies that the dynamics of tidewater glaciers in the Svalbard archipelago may be sensitive to future long-term trends in meltwater supply.