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In-Depth Information
(a)
(b)
Fig. 6 Monthly mean seasonal cycle of a monsoon rainfall (65-95E, 10-30N) in mm/day for 24 CMIP5
models (colours) compared with a range of observational estimates (OBS; thick black/grey lines), and
b Arabian Sea surface temperature (sea points within 50-75E, 7.5-30N) in K for CMIP5 models (colours)
and HadISST (Rayner et al.
2003
; black line). See Levine et al. (
2012
) for more details. Figure provided by
R. C. Levine, May 2012
mean rainfall biases produced by different convection schemes applied within the same
mesoscale model could be attributed to differences in the spread of rainfall rates produced
by the schemes, which are related to differences in the amount and vertical distribution of
diabatic heating.
Boos and Kuang (
2010
) showed that the strength of the South Asian monsoon is
dependent on Himalayan orography, rather than elevated heating over the Tibetan Plateau.
Their model study suggested that, although Tibetan plateau heating locally enhances
rainfall along its southern edge, the large-scale South Asian summer monsoon circulation
is otherwise unaffected by removal of the plateau, provided that the narrow orography of
the Himalayas and adjacent mountain ranges is preserved. Additional observational and
model results presented by Boos and Kuang (
2010
) suggested that these mountains produce
a strong monsoon by insulating warm, moist air over continental India from the cold and
dry extratropics.
Strong South Asian summer monsoons depend on moisture fluxes across the Arabian
Sea. However, Arabian Sea cold SST biases are common in coupled models (Levine et al.
2012
; and Fig.
6
), and it has been shown that these significantly reduce monsoon rainfall
(Levine and Turner
2011
). Model projections for future monsoons indicate generally small
positive changes in monsoon rainfall, while models also predict warming of Arabian Sea
surface temperatures of a similar order to the magnitude of typical climate model biases in
this region (Levine and Turner
2011
). Therefore, it is possible that climate models with
relatively large cold biases in the Arabian Sea are potentially underestimating the impact of
greenhouse gas forcing and associated surface warming on the monsoon (Levine et al.
2012
). Similarly, any models with warm Arabian Sea SST biases may overestimate
increases of monsoon rainfall in future climate scenarios as a result of excessive accel-
eration of evaporation.
As mentioned in Sect.
2.3.2
, although the basic drivers of the South Asian and West
African monsoon systems are similar (i.e. the seasonal reversal of the land-sea temperature
contrast and the resulting inflow of warm, moist air from the tropical ocean onto the
landmass), the West African monsoon shows an additional significant dependence of
