Inter-decadal variability of degree-day factors on Vestari Hagafellsjökull (Langjökull, Iceland) and the importance of threshold air temperatures

The skill of degree-day glacier melt models is highly dependent on the choice of degree-dayfactor (), which is often assumed to remain constant in time. Here we explore the validity of this assumption in a changing climate for two locations on Vestari Hagafellsjökull (1979-2012) using a Surface Energy Balance (SEB) approach that isolates the effect of changes in theprevailing weather on the . At lower-elevation, we observe stable during the period 10 of study; however, at higher elevation, is noted to be more variable and a statistically- significant downward trend is observed. This is found to result from an inappropriate threshold air temperature ( 12 ) from which to initiate the positive-degree-day sum, and is removed by setting to -1.83°C, rather than the usual value of 0°C used in degree-day melt models. The stationarity of once is adjusted contradicts previous research and lends support to the use of constant for projecting future glacier melt. Optimizing also improves the skill of melt simulations at our study sites. This research thus highlights the importance of for both melt model performance and the evaluation of stationarity in a changing climate

Temporal variations in flow velocity at Finsterwalderbreen, a Svalbard surge-type glacier.

Inter- and intra-annual velocity variations are well known on alpine glaciers, but their importance for Arctic glaciers has only been recognized more recently. This paper presents flow velocity data from Finsterwalderbreen, a 35km2 polythermal surge-type glacier in southern Svalbard that is presently ~100 years into its quiescent phase. Field measurements of glacier surface velocities are available from 1950–52 and 1994–97, and mean velocities for the last decade are estimated for the lower glacier using cables drilled to the glacier bed. These velocities show substantial seasonal variations indicating that basal sliding is an important component of surface velocities and interannual fluctuations of up to 75%, possibly indicating variations in subglacial water storage. Several lines of evidence indicate that this glacier has an extensive subglacial hydrological system, generally considered to be a prerequisite for surge-type glaciers, which is at least partly pressurized. Information on surface morphology from 1898 onwards shows that the glacier has experienced continuous retreat since the last surge in about 1910, and has now retreated ~1.5km further back than its previous pre-surge position in 1898. Tracking of moraine loops on terrestrial and aerial photographs acquired over a 100 year period indicates that the surge period of Finsterwalderbreen may be lengthening in response to climate changes

Geometry change between 1990 and 2003 at Finsterwalderbreen, a Svalbard surge-type glacier, from GPS profiling.

Surface mass-balance and geometry data are key to quantifying the climate response of glaciers, and confidence in data synthesis and model interpretations and forecasts requires data from as wide a range of locations and glacier types as possible. This paper presents measurements of surface elevation change at the Svalbard surge-type glacier Finsterwalderbreen, by comparing a 1990 digital elevation model (DEM) with a surface GPS profile from 2003. The pattern of elevation change is consistent with that previously noted between 1970 and 1990, and reflects the continued quiescentphase evolution of the glacier, with mass loss in the down-glacier/receiving area of up to –1.25mw.e. a–1, and mass gain in the up-glacier/reservoir area of up to 0.60mw.e. a–1; the area-weighted, mean change for the whole glacier is 0.19mw.e. a–1. The spatial pattern of elevation increase and decrease is complex, and the boundary between thickening and thinning determined by combining GPS and DEM data does not appear to correspond with the equilibrium-line altitude determined from surface mass-balance measurements. There is no evidence yet of a decrease in the rate of reservoir area build-up driven by mass-balance change resulting from the warmer winter air temperatures, and decreased proportion of snowfall in total precipitation, noted at meteorological stations in Svalbard

Drainage-system development in consecutive melt seasons at a polythermal, Arctic glacier, evaluated by flow-recession analysis and linear-reservoir simulation

The drainage systems of polythermal glaciers play an important role in high latitude hydrology, and are determinants of ice flow rate. Flow-recession analysis and linear-reservoir simulation of runoff time series are here used to evaluate seasonal and inter-annual variability in the drainage system of the polythermal Finsterwalderbreen, Svalbard, in 1999 and 2000. Linear flow recessions are pervasive, with mean coefficients of a fast reservoir varying from 16 h (1999) to 41 h (2000), and mean coefficients of an intermittent, slow reservoir varying from 54 h (1999) to 114 h (2000). Drainage-system efficiency is greater overall in the first of the two seasons, the simplest explanation of which is more rapid depletion of the snow cover. Reservoir coefficients generally decline during each season (at 0.22 h d–1 in 1999 and 0.52 h d–1 in 2000), denoting an increase in drainage efficiency. However, coefficients do not exhibit a consistent relationship with discharge. Finsterwalderbreen therefore appears to behave as an intermediate case between temperate glaciers and other polythermal glaciers with smaller proportions of temperate ice. Linear-reservoir runoff simulations exhibit limited sensitivity to a relatively wide range of reservoir coefficients, although the use of fixed coefficients in a spatially-lumped model can generate significant sub-seasonal error. At Finsterwalderbreen, an ice-marginal channel with the characteristics of a fast reservoir, and a subglacial upwelling with the characteristics of a slow reservoir, both route meltwater to the terminus. This suggests that drainage-system components of significantly contrasting efficiencies can co-exist spatially and temporally at polythermal glaciers

The hydrology of the proglacial zone of a high-Arctic glacier (Finsterwalderbreen, Svalbard): Atmospheric and surface water fluxes

Proglacial areas are expanding globally as a consequence of sustained glacier retreat, but there are very few studies focusing on their hydrology. This paper examines the surface and atmospheric water fluxes over a complete annual cycle in the proglacial area of the Svalbard glacier Finsterwalderbreen (77 N), through a combination of field measurements, physical modelling and statistical estimation. Precipitation in winter (226 mm) exceeded that in summer (29 mm), and over the course of the annual cycle total precipitation exceeded total evaporation (141 mm), although evaporative outputs from the proglacial area exceeded precipitation inputs during the dry summer. Runoff was highly irregular in time, with much of the total annual flow being concentrated into two relatively brief, early-to-mid summer intervals, the greater of which was characterised by the release of subglacially-stored water. Water fluxes were dominated by meltwater supply from the glacier: the total annual glacial runoff (7.38 × 107 m3) was an order-of-magnitude greater than the precipitation flux delivered directly to the proglacial area, and two orders-of-magnitude greater than evaporative losses from it. Outputs of meltwater from the proglacial area were not significantly different from inputs over the duration of the melt season, so surface water storage does not appear to be important in the studied catchment, despite episodes of flooding over shorter timescales. A synthesised description of the seasonal hydrological cycle in Finsterwalderbreen’s proglacial area is presented, which can be viewed as a set of hydrological boundary conditions for comparable high-latitude locations. Further study of these conditions is required, because the challenging nature of hydrometry in the high-latitudes has the potential to limit progress in understanding environmental change there

Interannual variability in the spatial distribution of winter accumulation at a high-Arctic glacier (Finsterwalderbreen, Svalbard), and its relationship with topography.

Glacier mass balance and hydrology are strongly influenced by the distribution of snow accumulation at the start of the melt season. Two successive end-of-winter snow-cover surveys at Finsterwalderbreen, Svalbard, are here used to investigate the interannual variability in the spatial distribution of accumulation, and its relationship with topography. 40–62% of the variance in snow depth was not determined by elevation (assessed by linear regression of snow depth on surface elevation), which could not therefore necessarily be used as a sole predictor of the spatial distribution of accumulation here. Principal components (PC) analysis of the topographic variables elevation, slope, north–south and east–west aspects shows that only two of six PCs, determined for 2 years’ sampling locations, had maximum loadings on altitude; aspect was more important, with maximum loadings on four PCs. Hierarchical cluster analysis was then applied to these PCs: significant correlations with accumulation in each of two terrain clusters were given by (1) elevation and slope, (2) east–west aspect only (1999); (1) elevation only, (2) no significant correlations (2000). There is strong interannual variability not only in the magnitude of winter accumulation (0.41mw.e. in 1999, 0.58mw.e. in 2000), but also in its spatial distribution, and its relationship with topography

The hydrology of the proglacial zone of a high-Arctic glacier(Finsterwalderbreen, Svalbard): Sub-surface water fluxes and complete water budget

Proglacial areas receive fluxes of glacial meltwater in addition to their own hydrological inputs and outputs, while in high latitudes the seasonal development of the active layer also affects their hydrology. This paper supplements a previous study of the surface and atmospheric water fluxes in the proglacial area of the Svalbard glacier Finsterwalderbreen (77° N), by focusing on the sub-surface water fluxes of the active layer, and bringing together all the components of the proglacial water balance over a complete annual cycle. Particular attention is given to the transitional zone between the moraine complex and the flat sandur. Sub-surface water in the moraine complex (sourced mainly from snowmelt, lake drainage and active-layer thawing), is exchanged with sub-surface water from the sandur (sourced mainly from glacier-derived snow- and ice-melt), across a largely distinct boundary. Hydraulic head and specific discharge were monitored in a transect of wells spanning this boundary. A hydraulic gradient from the moraine complex to the sandur is maintained throughout the melt season, although this is reversed first briefly when glacial runoff floods the sandur, and then diurnally from mid-melt-season, as peak daily flow in the proglacial channel network drives sub-surface water in the sandur towards the moraine complex. It is estimated that the active layer does not freeze up until mid-December at this location, so that sub-surface water flow may be maintained for months after the cessation of surface runoff. However, the magnitude of sub-surface flow is very small: the total, annual flux from the moraine complex to the sandur is 11 mm, compared with 1073 mm of total, annual runoff from the whole catchment (glacier included). Furthermore, when considering the water balance of the entire proglacial area, there are unlikely to be significant, seasonal storage changes in the active layer

Multiple melt plumes observed at the Breioamerkurjokull ice face in the upper waters of Jokulsarlon lagoon, Iceland

Breioamerkurjokull flows from the Vatnajokull ice cap and calves into the Jokulsarlon proglacial lagoon. The lagoon is connected to the North Atlantic Ocean through a 6 m deep narrow channel. Four hydrographic surveys in spring 2012, and a 2011 4-month long temperature and salinity time series of lagoon inflow show that the lake has significantly changed since 1976. Warm saline ocean water enters each tidal cycle and descends below the maximum sampled depths. The lagoon has a surface layer of ice melt, freshwater and Atlantic derived water. Beneath 10 m depth an advective/diffusive balance is responsible for determining the temperature and salinity of the lagoon waters down to ~90 m. To maintain the observed hydrographic structure, we calculate an upwelling of deep water of ~0.2 m d−1. A survey within 30 m of Breioamerkurjokull showed that the warmest and most saline waters sampled within the lagoon below 10 m depth were adjacent to the glacier face, along with multiple interleaved warm and cold layers. A heat and salt balance model shows that submarine melting along the ice face generates multiple meltwater plumes that are mixed and diluted within 200 m of the ice face

Pre-melt-season sediment plume variability at Jokulsarlon, Iceland: a preliminary evaluation using in-situ spectroradiometry and satellite imagery

High-latitude atmospheric warming is impacting freshwater cycling, requiring techniques for monitoring the hydrology of sparsely-gauged regions. The submarine runoff of tidewater glaciers presents a particular challenge. We evaluate the utility of Moderate Resolution Imaging Spectroradiometer (MODIS) imagery for monitoring turbid meltwater plume variability in the glacier lagoon Jökulsárlón, Iceland, for a short interval before the onset of the main melt season. Total Suspended Solids concentrations (TSS) of surface waters are related to remotely-sensed reflectance via empirical calibration between in-situ-sampled TSS and reflectance in a MODIS band 1-equivalent wavelength window. This study differs from previous ones in its application to an overturning tidewater glacier plume, rather than one derived from river runoff. The linear calibration improves on previous studies by facilitating a wider range of plume metrics than areal extent, notably pixel-by-pixel TSS values. Increasing values of minimum plume TSS over the study interval credibly represent rising overall turbidity in the lagoon as melting accumulates. Plume extent responds principally to consistently-strong offshore winds. Further work is required to determine the temporal persistence of the calibration, but remote plume observation holds promise for monitoring hydrological outputs from ungauged or ungaugeable systems

Modelling variable glacier lapse rates using ERA-Interim reanalysis climatology: an evaluation at Vestari- Hagafellsjökull, Langjökull, Iceland

The near-surface air temperature lapse rate is an important tool for spatially distributing temperatures in snow- and ice-melt models, but is difficult to parameterize, as it is not simply correlated with boundary-layer meteorological variables, such as temperature itself. This contribution quantifies spring-autumn lapse rate variability over 5 years at Vestari-Hagafellsjökull, a southerly outlet of Langjökull in Iceland. It is observed that summer lapse rates (0.57 °C 100 m) are significantly lower than non-summer rates, and are also lower than the Saturated Adiabatic Lapse Rate (SALR), which is often adopted in melt models. This is consistent with reduced near-surface temperature sensitivity to free-atmosphere temperature change during the occurrence of melting. A Variable Lapse Rate (VLR) regression model is calibrated with standardized, 750 hPa temperature anomalies derived from ERA-Interim climatology, which is shown to be highly significantly correlated with near-surface temperatures. The modelled VLR overestimates cumulative June-September Positive Degree Days (PDDs) by 3% when used to extrapolate temperatures from 1100 to 500 m a.s.l. on the glacier, whereas the SALR overestimates cumulative PDDs by 14%. ERA-Interim data therefore appear to offer a good representation of free-atmosphere temperature variability over Vestari-Hagafellsjökull, and the modelling approach offers a simple means of improving lapse rate parameterizations in melt models. © 2012 Royal Meteorological Society