Jiskoot, Hester
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- ItemVocal performance increases rapidly during the dawn chorus in Adelaide’s warbler (Setophaga adelaidae)(Oxford Academic, 2023) Vazquez-Cardona, Juleyska; Bonnell, Tyler R.; Mower, Peter C.; Medina, Orlando J.; Jiskoot, Hester; Logue, David M.Many songbirds sing intensely during the early morning, resulting in a phenomenon known as the dawn chorus. We tested the hypothesis that male Adelaide’s warblers (Setophaga adelaidae) warm up their voices during the dawn chorus. If warming up the voice is one of the functions of the dawn chorus, we predicted that vocal performance would increase more rapidly during the dawn chorus compared to the rest of the morning and that high song rates during the dawn chorus period contribute to the increase in vocal performance. The performance metrics recovery time, voiced frequency modulation, and unvoiced frequency modulation were low when birds first began singing, increased rapidly during the dawn chorus, and then leveled off or gradually diminished after dawn. These changes are attributable to increasing performance within song types. Reduction in the duration of the silent gap between notes is the primary driver of improved performance during the dawn chorus. Simulations indicated that singing at a high rate during the dawn chorus period increases performance in two of the three performance measures (recovery time and unvoiced frequency modulation) relative to singing at a low rate during this period. These findings are consistent with the hypothesis that vocal warm-up is one benefit of participation in the dawn chorus.
- ItemGlacier surge propagation by thermal evolution at the bed(American Geophysical Union, 2000) Murray, Tavi; Stuart, Graham W.; Miller, Paul J.; Woodward, John; Smith, Andrew M.; Porter, Philip R.; Jiskoot, HesterBakaninbreen, southern Svalbard, began a prolonged surge during 1985. In 1986, an internal reflecting horizon on radio echo sounding data was interpreted to show that the position of the surge front coincided with a transition between areas of warm (unfrozen) and cold (frozen) bed. Ground-penetrating radar lines run in 1996 and 1998 during early quiescence show that the basal region of the glacier is characterized by a strong reflection, interpreted as the top of a thick layer of sediment-rich basal ice. Down glacier of the present surge front, features imaged beneath the basal reflection are interpreted as the bottom of the basal ice layer, the base of a permafrost layer, and local ice lenses. This indicates that this region of the bed is cold. Up glacier of the surge front, a scattering zone above the basal reflection is interpreted as warm ice. There is no evidence for this warm zone down glacier of the surge front, nor do we see basal permafrost up glacier of it. Thus, as in early surge phase, the location of the surge front is now at the transition between warm and cold ice at the glacier bed. We suggest that the propagation of the front is associated with this basal thermal transition throughout the surge. Because propagation of the front occurs rapidly and generates only limited heat, basal motion during fast flow must have been restricted to a thin layer at the bed and occurred by sliding or deformation localized at the ice-bed interface.
- ItemLate surge glacial conditions on Bakaninbreen, Svalbard, and implications for surge termination(American Geophysical Union, 2002) Smith, A. M.; Murray, Tavi; Davison, B. M.; Clough, A. F.; Woodward, J.; Jiskoot, HesterBakaninbreen is a polythermal glacier in southern Spitsbergen, Svalbard, that last surged between 1985 and 1995. Seismic reflection data were acquired during early quiescence in spring 1998, just upstream of the surge front. The results were combined with complementary ground-penetrating radar data to investigate the glacial structure and basal conditions. We find no difference between the ice thickness values determined from the seismic and radar methods, suggesting that any layer of basal ice cannot be greater than 5 m thick. Interpretation of the amplitude of the seismic reflections indicates the presence of permafrost close to the glacier base. A thin layer of thawed deforming sediment separates the glacier from this underlying permafrost. In an area just upstream of the surge front the permafrost becomes discontinuous and may even be absent, the ice being underlain by 10–15 m of thawed sediments overlying deeper bedrock. Highpressure water is believed to have been required to maintain the propagation of the surge, and this area of thawed sediment is interpreted as a route for that water to escape from the basal system. When the surge front passed over this thawed bed, the escaping water reduced the pressure in the subglacial hydraulic system, initiating the termination of the surge. Surge termination was therefore primarily controlled by the presurge permafrost distribution beneath the glacier, rather than any feature of the surge itself. This termination mechanism is probably limited to surges in polythermal glaciers, but the techniques used may have wider glaciological applications.
- ItemIs there a single surge mechanism? Contrasts in dynamics between glacier surges in Svalbard and other regions(American Geophysical Union, 2003) Murray, Tavi; Strozzi, Tazio; Luckman, Adrian; Jiskoot, Hester; Christakos, PanosDuring the 1990s, Monacobreen, a 40-km-long tidewater glacier in Svalbard, underwent a major surge. We mapped the surge dynamics using ERS synthetic aperture radar images, differential dual-azimuth interferometry and intensity correlation tracking. A series of 11 three-dimensional (3-D) velocity maps covering the period 1991–1997 show a months-long initiation and years-long termination to the surge, with no indication of a surge front travelling downglacier. During the surge, the front of the glacier advanced 2 km, the velocity and derived strain rate increased by more than an order of magnitude, and maximum ice flow rates measured during 1994 were 5md 1. The spatial pattern of both velocity and strain rate was remarkably consistent and must therefore be controlled by spatially fixed processes operating at the glacier bed. We combine these results with those published in the literature to construct a typical Svalbard glacier surge cycle and compare this to surge dynamics of glaciers from other cluster regions, especially those of Variegated Glacier in Alaska. The strong contrast in dynamics suggests that there exist at least two distinct surge mechanisms.
- ItemMeteorological controls on snowpack formation and dynamics in the southern Canadian Rocky Mountains(Taylor & Francis Open, 2008) Pigeon, Karine E.; Jiskoot, HesterConsiderable spatial variability in snow properties exists within apparently uniform slopes, often resulting from microscale weather patterns determined by local terrain. Since it is costly to establish abundant weather stations in a region, local lapse rates may offer an alternative for predicting snowpack characteristics. For two Castle Mountain Resort weather stations, we present the 2003–2004 winter season weather and snow profile data and the 1999–2004 winter season lapse rates. A third site was sampled for small-scale spatial variability. Layer thickness, stratigraphy, temperature gradients, crusts, wind drift layers, stability, and settlement were compared between the sites and correlated with temperature, wind, and lapse rates. Average yearly snowfall was 470 cm at the Base and 740 cm at the Upper station. Average daily maximum and minimum temperature lapse rates are 26.1uC km21 and 25.7uC km21 when inversions are removed. Inversions occur mostly at night, adversely affecting lapse rate averages. Lapse rate modes are unaffected and most often 26.3uC km 21. Snowpack spatial variability is ,25% of layer thickness and is controlled by wind and topography. Layer settlement is primarily related to initial snow thickness and wind drift. Snowpacks stabilize with age, unless rain crusts are present, which are important low-force failure horizons.