The last glacial maximum locations of summer-green tree refugia using simulations with ECHAM3 T42 uncoupled, ECHAM5 T31 coupled and ECHAM5 T106 uncoupled models

Abstract

Abstract. Model simulations of the last glacial maximum (21±2 ka) with the ECHAM3 T42, ECHAM5 T31 coupled and ECHAM5 T106 uncoupled models are compared. The ECHAM5 T106 simulations were forced at the boundaries by results from the coupled ECHAM5-MPIOM atmosphere ocean model while the ECHAM3 T42 model was forced with prescribed sea surface temperatures (SSTs) provided by Climate/Long-Range Investigation, Mapping Prediction project (CLIMAP). The topography, land-sea mask and glacier distribution for the ECHAM5 simulations were taken from the PMIP2 data set while for ECHAM3 they were taken from PMIP1. The ECHAM5 simulations were run with a variable SST in time simulated by the coupled model. These were also used for the T106 run but corrected for systematic errors. The SSTs in the ECHAM5-MPIOM simulations for the last glacial maximum (LGM) were much warmer in the northern Atlantic than those suggested by CLIMAP or GLAMAP while they were cooler everywhere else. This had a clear effect on the temperatures over Europe, warmer for winters in Western Europe and cooler for Eastern Europe than the simulation with CLIMAP SSTs. Considerable differences in the general circulation patterns were found in the different simulations. A ridge over Western Europe for the present climate during winter in the 500 hPa height field remains in the ECHAM5 simulations for the LGM, more so in the T106 version, while the ECHAM3 CLIMAP simulation provided a trough. The zonal wind between 30° W and 10° E shows a southward shift of the polar and subtropical jet in the T106 simulation for the LGM and an extremely strong polar jet for the ECHAM3 CLIMAP. The latter can probably be assigned to the much stronger north-south gradient in the CLIMAP SSTs. The southward shift of the polar jet during LGM is supported by observation evidence. Cyclone tracks in winter represented by high precipitation are characterised over Europe for the present by a main branch from Great Britain to Norway and a secondary branch towards the Mediterranean Sea. For the LGM the different models show very different solutions: the ECHAM3 CLIMAP simulations show just one track going eastward from Great Britain into central Europe, while the ECHAM5 T106 simulation still has two branches but the main one goes to the Mediterranean Sea, with enhanced precipitation in the Levant. This agrees with an observed high stand of the Dead Sea during the LGM. For summer the ECHAM5 T106 simulations provide much more precipitation for the present over Europe than the other simulations thus agreeing with estimates by the Global Precipitation Climatology Project (GPCP). Also during the LGM this model makes Europe less arid than the other simulations. In many respects the ECHAM5 T106 simulations for the present were more realistic than the ECHAM5 T31 coupled simulation and the older ECHAM3 T42 simulations, when comparing them with the ECMWF reanalysis or the GPCP data. For validating the model data for the LGM, pollen and charcoal analyses were compared with possible summer-green tree growth from model estimates using summer precipitation, minimum winter temperatures and growing degree days (above 5 °C). The ECHAM5 T106 simulations suggest at more sites with findings from pollen or charcoal analyses likely tree growth during the LGM than the other simulations, especially over Western Europe. The clear message especially from the ECHAM5 T106 simulations is that warm-loving summer-green trees could have survived mainly in Spain but also in Greece in agreement with findings of pollen or charcoal.