Climates at the Last Glacial Maximum: Influence of Model Horizontal Resolution

Abstract

The climate at the last glacial maximum (LGM) has been simulated using the U.K. Universities Global Atmospheric Modeling Programme (UGAMP) general circulation model (GCM) truncated at total wavenumbers 21, 42, and 63 (T21, T42, and T63) with prescribed SSTs based on the Climate: Long-Range Investigation Mapping and Prediction Study data. Consistent with the Paleoclimate Modeling Intercomparison Project, the other boundary conditions include the changes in ice sheet topography and geography, a lower sea level, a lower concentration of CO2 in the atmosphere, and a slightly different insolation pattern at the top of the atmosphere. The influence of the model horizontal resolution on the simulated global climate and regional circulation changes has been analyzed. The simulations indicate that both the global and regional climate changes due to the imposed ice age boundary conditions are affected by the model horizontal resolution. The largest climate change differences occur when the model horizontal resolution increases from T21 to T42. Further increase in resolution to T63 only results in quantitative differences. However, for some variables, such as measures of storm track activity, the LGM simulation shows that there are clear systematic differences between T42 and T63. This result was not seen in the equivalent fields at present day. The regional climate changes simulated in a lower-resolution model (T21) differ significantly from those in higher-resolution models, because of the poor simulations of planetary waves and storm tracks. Results also imply that the insensitivity of simulated storm tracks to model horizontal resolution change from T42 to T63 at present day may not be applicable to a different climate regime. The lower-resolution model cannot reproduce the present-day climate as accurately as the high-resolution model can. In addition, the simulated climate changes in the lower-resolution model due to ice age boundary conditions in some regions contradict those simulated with higher-resolution models with the latter giving better agreement with geological evidence. The authors conclude that a higher-resolution model, at least T42, is needed for the UGAMP GCM to simulate climate changes.