Break-up Dates of Alpine Lakes As Proxy Data for Local and Regional Mean Surface Air Temperatures

Abstract

The calendar date of ice break-up on Lej da San Murezzan, a high-altitude (1768 m a.s.l.) lake in the Swiss Alps, has been recorded uninterruptedly since 1832. Based on this record and on shorter, interrupted records from two neighbouring lakes, the potential use of the timing of spring break-up as a proxy for local and regional surface air temperatures in the European Alpine region is investigated. Lej da San Murezzan exhibits an overall trend to earlier thawing (7.6 days per century) comparable to that of lakes in other parts of the Northern Hemisphere. Part of this trend may be due to shifts in mean break-up date around 1857 and 1932. The timing of break-up on all three lakes is strongly related to local and regional surface air temperatures centred on the middle of April and integrated over 4-8 weeks. Three empirical methods of relating break-up date to local air temperature yielded essentially the same proportion of shared variance (about 64%). Comparisons of break-up dates with surface air temperature data from Switzerland, the Netherlands and the United Kingdom suggest that the thawing of Alpine lakes is determined to a large extent by synoptic-scale meteorological processes. The timing of break-up on Lej da San Murezzan also tends to follow an index of global explosive volcanism with a time lag of about two years, volcanically quiescent periods being associated with early break-up, and volcanically active periods with late break-up. This suggests that modulation of incident radiation by stratospheric aerosols of volcanic origin may significantly affect the timing of break-up of high-altitude lakes.