Northern Hemisphere Mid-latitude Cyclone Variability in different Ocean Representations
Raible CC, Blender R
Clim. Dyn.
22: 239-248 MAR 2004


Abstract:The impact of different ocean models or sea surface temperature (SST) and sea ice representations on cyclone tracks in the Northern Hemisphere mid-latitudes is determined within a hierarchy of model simulations. A reference simulation with the coupled atmosphere ocean circulation models ECHAM-4/HOPE-G, which reproduces the observed storm tracks, is compared with simulations using ECHAM-4 and three simplified ocean and sea ice representations: (i) a mixed layer (ML) ocean, (ii) forcing by varying SST and sea ice, and (iii) with climatological SST and sea ice; the latter two are from the coupled ECHAM-4/HOPE-G integration. The four simulations are consistent with respect to the time means of the SST and the sea ice. The cyclones are tracked automatically by a standard routine and the variability of cyclone trajectories within the storm tracks is determined by a cluster approach.

In the forced simulation with varying SST, the geographical distribution and the statistics of the cyclones are not altered compared to the coupled ECHAM-4/HOPE-G simulation. In the climatological and the ML-simulation, considerable deviations of the mean cyclone distribution are found, which occur mainly in the North Pacific, and can be traced back to missing ENSO variability. In the North Atlantic, the climatological experiment is superior to the ML-experiment. The variability of the cyclone paths, as determined by the cluster analysis, reveals the same types of propagation directions for all four representations of the lower boundary. The largest discrepancies for the cluster occupations are found for the climatological and the ML-simulation.

KeyWords Plus:
Midlatitude cyclone tracks, GCM simulation, Cluster analysis

Addresses:
Raible C.C., Climate and Environmental Physics, Physics Institute, University of Bern, Sidlerstr. 5, CH-3012 Bern, Switzerland.
Blender R, Univ Hamburg, Inst Meteorol, Bundesstr 55, D-20146 Hamburg, Germany

Reprints (after having been accepted):
Raible CC, Climate and Environmental Physics, Physics Institute, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland, raible@climate.unibe.ch