1. The case of polar lows Hans von Storch 13 and Matthias Zahn 2 1. Institute of Coastal Research, Helmholtz-Zentrum Geesthacht, Germany. 2. Environmental Systems Science Center, University of Reading, Reading, UK 3. KlimaCampus, University of Hamburg, Hamburg, Germany.

2. © Dundee Satellite Receiving Station ~300 km Scandinavia Spitz- bergen Polar lows mesoscale (< 1000 km) sized maritime storms ● intense/ strong winds (>13.9 m/s) severe weather ● occur poleward the Polar Fronts in both hemispheres during winter ● typically induced by disturbances in the air flow ● typically driven by convective processes ● Here: only Northern North Atlantic

3. For the recent past Dataset of polar low cases Comprehensive measurements required long in time high in spatial detail Homogeneous Problem: Such analyses do usually no exist Solution: Use of a numerical model (Regional Climate Model, RCM) in combination with existing global data to reconstruct past/project future state of the atmosphere Long-term climatology and future perspectives Global data (~200 km) RCM (~50 km) dynamical downscaling

4. Set-up of multi-decadal simulation NCEP/NCAR reanalysis 1/ CLM 2.4.6 Initialised: 1.1.1948 finishing: 28.2.2006 spectral nudging of scales > 700 km

5. Dec 1993 case Weatherchart, DWD CLM22-snCLM22-nn NCEP CLM22-sn, filtered Dundee 9.12.93, 16:00 Iceland Greenland

6. Jan 1998 case CLM01-sn, filtered Berliner Weatherchart CLM01-sn Dundee 18.1.98, 4:00 CLM01-nn NCEP

7. In principle, polar lows are reproduced with CLM run in climate mode Deviations in detail (e.g. location and amount of pressure minima) Spectral nudging inhibits considerable ensemble variability A digital filter could be useful for an automatic detection Zahn, M., H. von Storch, and S. Bakan, 2008: Climate mode simulation of North Atlantic Polar Lows in a limited area model, Tellus A, DOI: 10.1111/j.1600-0870.2008.00330.x Polar low simulation and detection

8. 1 st : detection of minima in the filtered mslp fields (< -1hPa) 2 nd : combine detected positions to individual tracks, distance to next (3h) position < 200 km 3 rd : Either checking further constraints along the tracks: Detection algorithm strength of the minimum ( ≤ −2hPa once along the track) wind speed ( ≥ 13.9 m/s once along the track) air-sea temperature difference ( SST − T 500hPa ≥ 43K) north south direction of the track limits to allowable adjacent grid boxes or: strength of the minimum in the bandpass filtered mslp field decreases below −6hPa once

9. Annual frequency of past polar lows PLS: Polar Low Season (Jul-Jun) Zahn and von Storch, 2008,

10. σ=13 Mean:56 Max:100 Min:36 Annual frequency of past polar lows Zahn and von Storch, 2008, PLS: Polar Low Season (Jul-Jun)

11. Downscaling vs. “obs” C=0,72 NCEP-based downscaling (black) and observations (red) of MetNo (Noer, pers comm) Monthly comparison of NCEP- downscaling (in black) with analysed observed data (in red; Blechschmidt, 2008)

12. Bracegirdle, T. J. and S. L. Gray, 2008 Density of polar low genesis Genesis in NCEP downscaling RCM simulation

13. Past occurrence – summary ● Northern North Atlantic ● Polar Lows are simulated by 50 km grid resolution model (not shown) ● Strong inter annual variability ● Frequency remains on a similar level – no systematic trend ● Qualitative similarity with observations in terms of inter-annual, intra- annual variations and spatial distribution of genesis / other studies ● Zahn, M., and H. von Storch, 2008: A longterm climatology of North Atlantic Polar Lows. Geophys. Res. Lett., 35, L22702, doi:10.1029/2008GL035769

14. Polar lows in IPCC-climate change scenarios Global scenario data generated by IPCC-ECHAM5-MPI-OM in C20- experiments: (“control” with GHG 1960-1990) and B1,A1B,A2- scenarios for period 2070-2100. Dynamically downscaled using CLM.

15. Zahn and von Storch, 2010 Number of polar lows per PLS

16. Spatial density distribution: northward shift of genesis region C20, mean lat = 64,9°NB1, mean lat = 66,8°N A1B, mean lat = 66,8°NA2, mean lat = 67,3°N

17. Projected changes in polar low frequency and vertical atmospheric stability A2 C20 A1B B1 Zahn and von Storch, 2010 Differences of the area and time-averaged ice- free SST and T 500-hPa over the maritime northern North Atlantic as proxy for frequency of favourable polar low conditions (CMIP3/IPCC AR4)

18. ● Polar lows become less frequent in the Northern North Atlantic as supported by ● regional modelling (downscaling) control and scenario simulations with one GCM, and by ● analysis of vertical stability in a large set of (CMPI3) global climate simulations; all simulations show a increase in projected stability. ● The genesis regions shift northward. ● Zahn, M., and H. von Storch, 2010: Decreased frequency of North Atlantic polar lows associated to future climate warming, nature 467, 309-312 Projected occurrence – summary