Jökull


Jökull - 01.01.2013, Page 97

Jökull - 01.01.2013, Page 97
Mass balance of Mýrdalsjökull ice cap Breiðamerkurjökull outlet glacier (east of Öræfajökull in Southeast-Iceland) and on the transect on the Hofs- jökull ice cap (Figure 3). At Hofsjökull, a threefold increase in winter mass balance is observed in the el- evation interval considered, while the Breiðamerkur- jökull data display an increase from south to north (with elevation), opposite to what is seen on Mýrdals- jökull. Both the measured winter balance and annual balance decrease from south towards north and west within the caldera plateau of Mýrdalsjökull (cf. Fig- ures 1 and 3), which is equivalent to a decrease with elevation of the survey sites. Summer mass balance values are similar on all three glaciers and increase (become less negative) on average linearly with ele- vation. This demonstrates the strong effect that winter precipitation is known to have on the annual balance of ice caps in Iceland. OBSERVED PRECIPITATION AND MASS BALANCE Rain gauges are prone to undercatch in strong winds, in particular during snowfall, while indirect obser- vations of orographic precipitation, such as runoff and glacier mass balance measurements, are in fact considered to yield the most accurate information on precipitation in complex terrain (e.g. Rögnvaldsson et al., 2007, and references therein). However, while winter precipitation is well represented by the mass balance data from the uppermost parts of Mýrdals- jökull, summer precipitation is not, as it is generally lost through the snow pack. Hence, traditional rain gauge data must be investigated and here long time- series of observed precipitation are available from two coastal weather stations, Vík in Mýrdalur and Vatnsskarðshólar. The stations are located to the south of the ice cap (cf. Figure 1) and most of the precipi- tation falls in southerly and easterly winds, in relation with approaching extratropical lows and atmospheric fronts. Annually, far greater precipitation is observed at Vík than at Vatnsskarðshólar. This is the result of two different topographic effects on the atmospheric flow: a) Vatnsskarðshólar is approx. 6 km to the west of Mt. Reynisfjall (340 m a.s.l.) and other smaller scale topography, and is hence located in a partial rain shadow. For that reason it may receive less precipita- tion, compared to Vík on the other side of Mt. Reynis- fjall. At Vík, the land is flat and there is open sea to the south and east, and steep mountains to the north and west, possibly leading to some enhancement of precipitaton in a small scale atmospheric blocking. Coastal convergence may also be of importance here. b) Both the local, as well as the large scale topogra- phy, lead to winds on average being weaker at Vík than at Vatnsskarðshólar, and hence there is a smaller undercatch of the rain gauge at Vík. Figure 4. Scatterogram displaying the observed win- ter mass balance [mwe] at three survey sites on Mýr- dalsjökull and the observed precipitation [m] dur- ing winter (Sept.–April) at weather stations at Vík in Mýrdalur (Vik) and at Vatnsskarðshólar (Vat) in South-Iceland during winters 2007–2011. Lines with slopes of 2, 3 and 4 visually aid in indicat- ing possible correlation between precipitation at in- dividual stations and balance at individual survey sites. – Dreifiritið sýnir mælda vetrarafkomu [mwe] á þremur stöðum á Mýrdalsjökli og mælda úrkomu [m] á veðurstöðvum á Vík í Mýrdal (Vik) og á Vatnsskarðshólum (Vat) veturna 2007–2011. Línum með hallatölu 2, 3 og 4 er ætlað að bregða ljósi á mögulega fylgni milli úrkomu á veðurstöð og afkomu á mælistað á jöklinum. JÖKULL No. 63, 2013 97
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Jökull

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