8.4.10 Monsoon Variability

Monsoon variability (see Chapters 3, 9 and 11) occurs over a range of temporal scales from intra-seasonal to inter-decadal. Since the TAR, the ability of AOGCMs to simulate monsoon variability on intra-seasonal as well as interannual time scales has been examined. Lambert and Boer (2001) compared the AOGCMs that participated in CMIP, finding large errors in the simulated precipitation in the equatorial regions and in the Asian monsoon region. Lin et al. (2006) evaluated the intra-seasonal variation of precipitation in the MMD at PCMDI. They found that the intra-seasonal variance of precipitation simulated by most AOGCMs was smaller than observed. The space-time spectra of most model simulations have much less power than is observed, especially at periods shorter than six days. The speed of the equatorial waves is too fast, and the persistence of the precipitation is too long, in most of the AOGCM simulations. Annamalai et al (2004) examined the fidelity of precipitation simulation in the Asian monsoon region in the MMD at PCMDI. They found that just 6 of the 18 AOGCMs considered realistically simulated climatological monsoon precipitation for the 20th century. For the former set of models, the spatial correlation of the patterns of monsoon precipitation between the models exceeded 0.6, and the seasonal cycle of monsoon rainfall was simulated well. Among these models, only four exhibited a robust ENSO-monsoon contemporaneous teleconnection. Cook and Vizy (2006) evaluated the simulation of the 20th-century climate in North Africa in the MMD at PCMDI. They found that the simulation of North African summer precipitation was less realistic than the simulation of summer precipitation over North America or Europe. In short, most AOGCMs do not simulate the spatial or intra-seasonal variation of monsoon precipitation accurately. See Chapter 11 for a more detailed regional evaluation of simulated monsoon variability.