Swiss Alpine snow variability: Its links to large scale flow patterns

Swiss Alpine snow variability:Its links to large scale flow patterns ICAM-MAP meeting, Brig (CH) Fri 23th May 2003 Simon C. Scherrer C. Appenzeller Federal Office of Meteorology and Climatology (MeteoSwiss), Switzerland scs@meteoswiss.ch

Swiss snow variability • Klosters 1999 (GR) • Engadin (GR)

Swiss new snow - NAO: direct influence 1931-1999 • Argument: There is a NAO – Alpine snow relation, dependent on height and has an impact on trends • Question: Is there a pronounced NAO influence on Swiss new snow variability and trends?

48°N Germany France Austria S P L A 46°N Italy 6°E 11°E 89 stakes New Swiss new snow data set • A new roughly homogenous snow data set (1931(57)-1999)

EOF analysis of Swiss DJF new snow sums: EOF 1 58% (sign.) r = 1 r = 0.30 No trend

Swiss DJF new snow sums: EOF 2 15% (sign.) r = 1 r = -1 r = 0.30 r = -0.30 No trend

Swiss DJF new snow sums: EOF 3 8% (sign.) r = 1 r = -1 r = 0.30 r = -0.30 small trend

2500 r~-0.6 2000 1500 Altitude [m a.s.l.] 1000 500 0 Correlation coefficient New snow EOF‘s: height dependence • EOF 3 shows a distinct height dependence

DJF EOF– NAOI relation Trend PC 3 not significant (p=0.22) • r (PC1, NAOI) = -0.24 • r (PC2, NAOI) = -0.29 • r (PC3, NAOI) = -0.57**

95% 99% PC 3 Running correlations: New snow sums - NAOI NEW SNOW PC‘s - NAOI

Correlation maps: SLP – (new snow PC3) SLP Pattern 3: „NAO like“ SVD 3: SCF: 5% r = 0.70

„European“ scale: seems to be determined by central European high-pressure regime or low-pressure • Hemispheric scale: SVD 1 resembles NAO/AO like pattern on hemispheric scale NAO/AO – blocking relation? Swiss Alpine region SVD 1: European and Hemispheric view SVD 1HN - SLP 4 1 - 1 - 4 mbar

Influence of “blocking” Atlantic ocean Atlantic ocean 60°N NAO+ 50°N NAO- DJF “blocking” frequency DJF “blocking” frequency NAO- Swiss Alps Swiss Alps NAO+ based on standard TM Blocking Index modified central latitude

Conclusions • Three distinct DJF new snow patterns are identified (expl. var. 81%) • There is no simple 1:1 relation between DJF new snow and the NAOI • The direct influence of the NAO on DJF new snow sums: • is expressed as low-station high-station pattern (expl. variance ~8%) • shows a distinct trend since the 1960‘s • The primary DJF new snow variability: • is locally driven by central European high-low pressure (blocking?) • is NAO/AO like on a hemispheric scale • The “Blocking” – NAO relation is ambiguous: • Atlantic “blocking” is negatively correlated with the NAOI • Middle-European “blocking” is positively correlated with the NAOI

THE END THE END

Intermediate conclusions • Three “significant” DJF new snow patterns are identified, which explain 81% of total variability • There is no simple 1:1 relation between Swiss Alpine new snow sums and the NAOI • The direct influence of the NAO on new snow sums: • is expressed as a low – high station pattern (8% var.) • shows a distinct trend since the 1960‘s

Swiss Alpine snow variability: Its links to large scale flow patterns