Details

Autor(en) / Beteiligte

• Scherrer, Simon C.
• Gubler, Stefanie
• Wehrli, Kathrin
• Fischer, Andreas M.
• Kotlarski, Sven

Titel

The Swiss Alpine zero degree line: Methods, past evolution and sensitivities

Ist Teil von

• International journal of climatology, 2021-12, Vol.41 (15), p.6785-6804

Ort / Verlag

Chichester, UK: John Wiley & Sons, Ltd

Erscheinungsjahr

2021

Link zum Volltext

Quelle

Wiley-Blackwell Journals

Beschreibungen/Notizen

• The near‐surface zero degree line (ZDL) is a key isotherm in mountain regions worldwide, but a detailed analysis of methods for the ZDL determination, their properties and applicability in a changing climate is missing. We here test different approaches to determine the near‐surface ZDL on a monthly scale in the Swiss Alps. A non‐linear profile yields more robust and more realistic ZDLs than a linear profile throughout the year and especially in the winter‐half year when frequent inversions disqualify a linear assumption. In the period 1871–2019, the Swiss ZDL has risen significantly in every calendar month: In northern Switzerland, the monthly ZDL increases generally amount to 300–400 m with smaller values in April and September (200–250 m) and a larger value in October (almost 500 m). The largest increases of 600–700 m but also very large uncertainties (±400 m, 95% confidence interval) are found in December and January. The increases have accelerated in the last decades, especially in spring and summer. The ZDL is currently increasing by about 160 m·°C−1 warming in the summer‐half year and by up to 340 ± 45 m·°C−1 warming in winter months. In southern Switzerland, ZDL trends and temperature scalings are somewhat smaller, especially in winter. Sensitivity analyses using a simple shift of the non‐linear temperature profile suggest that the winter ZDL‐temperature scalings are at a record high today or will reach it in the near future, and are expected to decrease with a strong future warming. Nevertheless, the cumulative ZDL increase for strong warming is considerably larger in winter than in summer. Based on a few key criteria, we also present best practises to determine the ZDL in mountain regions worldwide. The outlined methods lay a foundation for the analysis of further isotherms and to study the future ZDL evolution based on climate scenario data. We present an assessment of methods to determine the near‐surface zero degree line (ZDL) for the testbed of the Swiss Alps. An approach using a non‐linear profile works best and we find a significantly increasing ZDL and accelerating trends in the last decades. The Alpine winter ZDL‐temperature scaling shows a maximum today and is expected to decrease with a strong future warming. We also provide best practises to determine the ZDL in mountain regions worldwide.