Modellversagen: Simulationen bekommen den Indischen Monsun nicht in den Griff

Klimamodelle simulieren die Wirklichkeit. Die Erfolgsquote ist durchwachsen. Während in den Medien oft suggeriert wird, alles wäre bestens unter Kontrolle, sieht die Wirklichkeit leider ander aus. Nehmen wir das Beispiel des Indischen Monsuns, dem lebenswichtigen Regenspender in der Region. Bereits im Oktober 2014 ließen Anamira Saha und Kollegen in den Geophysical Research Letters “eine Bombe platzen”: Der Monsun hat in den letzten 60 Jahren langfristig abgenommen. Jedoch können die Modelle diesen Trend nicht reproduzieren. Schlimmer noch, die Modelle können nicht einmal die generellen Veränderungen der atmopshärischen Zirkulation und Temperaturmuster der Ozeangebiete nachvollziehen. Ein absolutes Debakel, wenn man bedenkt, dass dieselben Modell im großen Maßstab als Begründung für weitreichende gessellschaftsumbildende Maßnahmen herangezogen werden. Hier der Abstract:

Failure of CMIP5 climate models in simulating post-1950 decreasing trend of Indian monsoon
Impacts of climate change on Indian Summer Monsoon Rainfall (ISMR) and the growing population pose a major threat to water and food security in India. Adapting to such changes needs reliable projections of ISMR by general circulation models. Here we find that, majority of new generation climate models from Coupled Model Intercomparison Project phase5 (CMIP5) fail to simulate the post-1950 decreasing trend of ISMR. The weakening of monsoon is associated with the warming of Southern Indian Ocean and strengthening of cyclonic formation in the tropical western Pacific Ocean. We also find that these large-scale changes are not captured by CMIP5 models, with few exceptions, which is the reason of this failure. Proper representation of these highlighted geophysical processes in next generation models may improve the reliability of ISMR projections. Our results also alert the water resource planners to evaluate the CMIP5 models before using them for adaptation strategies.

Zu einem ähnlichen Schluss kam Venkatraman Prasanna im April 2016 im Fachblatt Pure and Applied Geophysics. Die Modelle unterscheiden sich so stark voneinander, dass die Resultate beliebig werden:

Assessment of South Asian Summer Monsoon Simulation in CMIP5-Coupled Climate Models During the Historical Period (1850–2005)
This paper evaluates the performance of 29 state-of-art CMIP5-coupled atmosphere–ocean general circulation models (AOGCM) in their representation of regional characteristics of monsoon simulation over South Asia. The AOGCMs, despite their relatively coarse resolution, have shown some reasonable skill in simulating the mean monsoon and precipitation variability over the South Asian monsoon region. However, considerable biases do exist with reference to the observed precipitation and also inter-model differences. The monsoon rainfall and surface flux bias with respect to the observations from the historical run for the period nominally from 1850 to 2005 are discussed in detail. Our results show that the coupled model simulations over South Asia exhibit large uncertainties from one model to the other. The analysis clearly brings out the presence of large systematic biases in coupled simulation of boreal summer precipitation, evaporation, and sea surface temperature (SST) in the Indian Ocean, often exceeding 50 % of the climatological values. Many of the biases are common to many models. Overall, the coupled models need further improvement in realistically portraying boreal summer monsoon over the South Asian monsoon region.