Cooperative Institute for Research in Environmental Sciences

Changes in synoptic weather patterns and Greenland precipitation in the 20th and 21st centuries. Part 1: Evaluation of late 20th century simulations from IPCC models

K. Schuenemann and J.J. Cassano

2009, Journal of Geophysical Research, 114, D20113, doi:10.1029/2009JD011705.

Using the Self-Organizing Map (SOM) technique, the sea-level pressure synoptic climatology and precipitation of 15 Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) models are compared to that of the ERA-40 reanalysis for the North Atlantic region for the period 1961 to 1999. Three of the models, the CCCMA-CGCM3.1(T63), the MIROC3.2(hires), and the MPI-ECHAM5, best reproduce the ERA-40 synoptic climatology and are chosen for further analysis of precipitation over Greenland. The MIROC3.2(hires) is the best single performing model, in that it best matches ERA-40. Although the 3-model ensemble simulates the same mean annual precipitation over Greenland as ERA-40, the ensemble simulates the mean annual precipitation differently than ERA-40. A dry bias in the CCCMA-CGCM3.1(T63) and a wet bias from the MPI-ECHAM5 cancel in the ensemble average. The annual mean precipitation difference between the model ensemble, as well as each individual model, and ERA-40 is then attributed to differences in dynamic and thermodynamic processes in the models that make up the ensemble. Dynamic differences between the model and ERA-40 indicate a difference in the occurrence of synoptic weather patterns, while thermodynamic differences denote differences in the amount of precipitation produced when a given synoptic weather pattern occurs. Thermodynamic differences between the models and ERA-40 are predominantly responsible for Greenland precipitation differences, but dynamic (circulation) differences in the models also play a small role. Part 2 of this paper uses this 3-model ensemble to analyze and attribute predicted increases in precipitation over the Greenland ice sheet for the 21st century.