Abstract

1. 500 450 400 0 10 20 30 km -500 -450 -400 Figure 1. Contour map of Vatnajökull ice cap and its surroundings. The locations of surface balance and velocity measurements are shown with red crosses and the glacier boundary with a blue line. The location of Vatnajökull on Iceland is shown on the inlet. Model adjustments The large outlet glaciers in the northern and western parts of Vatnajökull are surging regularly. This irregular flow behaviour is not taken into account in the model simulations. It is apparent from the resulting size and shape of the largest modelled steady state that these areas are unstable. The outlet glaciers in the eastern part, as well as the outlets flowing out from Öræfajökull in the south are not sensitive to the height of the ELA. The flow model has been adjusted so that it is possible to compute only parts of the ice cap at a time. A boundary condition has been applied on ice divides which prevents any ice to flow across the divide. Model runs with two different boundaries, excluding portions of Vatnajökull, were done and the resulting volume evolution is shown in Figs. 6 and 7. In Figure 8 several profiles through the steady state that is closest in size to present ice cap are shown. Figure 2. Names of the surging outlet glaciers along with the time of documented surges Figure 3. Volume evolution computed with several different heights of the ELA. Depending on the height the ice cap grows without bounds or retreats to a steady state considerably smaller than present ice cap. 3 2 1 1 2 3 Figure 6. Left panel shows the computation domain when the two large surge-type glaciers in the northern part have been excluded. Right panel shows the volume evolutions computed with several different ELAs. The same characteristics of growth or retreat is apparent in these model runs. Figure 8. Above, location of the length profiles for Breiðamerkur- and Skeiðarárjökull and the three north-south and three east-west profiles shown in the figures to the right and below. Figure 7. Left panel shows the computation domain when the two large surge-type glaciers in the northern part and the western part have been excluded. Right panel shows the volume evolutions computed with several different ELAs. These model runs show that the southern and western parts of Vatnajökull are now stable with respect to the ELA. Conclusions The exclusion of the surging outlet glaciers in the model computations show that the irregular flow behaviour plays an important role in the present size, shape and stability of Vatnajökull. For further model simulations for the whole ice cap the surges will be taken into account. Further, a degree-day mass balance model will be coupled to the flow model for Vatnajökull and available mass balance measurements will be used to calibrate the model towards the conditions on Vatnajökull. With the coupled degree-day mass balance and the flow model portions of Vatnajökull will be studied and the response to future climate changes analysed. poster 93 Importance of surges on the stability and size of Vatnajökull ice cap, Iceland Guðfinna Aðalgeirsdóttir1, Hilmar Gudmundsson2 and Helgi Björnsson1 tolly@raunvis.hi.is 1 Science Institute, University of Iceland, Dunhaga 3, IS-107 Reykjavík, Iceland 2 British Antarctic Survey, Natural Environment Research Council, Cambridge CB3 0ET, U.K. Abstract Computations with a flow model of Vatnajökull ice cap, Iceland are presented. The flow model is forced with a regression model for the mass-balance distribution which includes mass balance-elevation feed back. Model parameters are kept constant, except the Equilibrium Line Altitude (ELA) is varied between model runs. It is shown that present size and shape of Vatnajökull can not be modelled with a constant ELA. Depending on the ELA the ice cap either grows unimpeded, or it settles to steady states that are considerably smaller than the present ice cap. These model results on Vatnajökull confirm theoretical predictions that small ice caps or glaciers on steep bedrock slopes are stable. If the ice cap grows it becomes sensitive to small changes and when it has reached a critical size it can grow to an Ice-Age size. Vatnajökull is presently close to this critical size. The largest steady state obtained with the model is different in extent from present ice cap. The outlet glaciers that vanish in the model have frequent surges occurring on them. During a surge significant amount of ice is transported from the accumulation area to the ablation area in a short period of time. This irregular flow behavior is shown to influence the stability and the overall size of the ice cap. Figure 4. Bedrock and surface geometry that is used as boundary and initial condition, respectively. The bed was measured with radio-echo sounding methods (Björnsson, 1988) and the surface with GPS surveying methods. Figure 5. Geometry of the Vatnajökull ice cap at different times. From left to right: initial condition (present size and shape of the ice cap), fast growing ice cap at 4000 years with DELA=40 (corresponds to green cross in Fig. 3), largest possible steady state at 25000 years with DELA=55.5 m(corresponds to pink cross in Fig. 3) and separate small ice caps computed with DELA = 120 m (corresponds to purple cross in Fig. 3). Acknowledgments This work was supported by the National Research Council of Iceland (Grant No. 10501), the National Power Company of Iceland and the Nordic Project Climate Water and Energy. We thank Finnur Pálsson for preparing the data and the Iceland Glaciological Society (JÖRFÍ) for support during field work.