Editing IPCC:AR6/WGI/Chapter-Atlas (section)

==== [[#Atlas.5.3.1|Atlas.5.3.1]] Key Features of the Regional Climate and Findings from IPCC Previous Assessments ====

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===== Atlas.5.3.1.1 Key Features of the Regional Climate =====

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The countries in this region are mostly semi-arid to arid and therefore depend heavily on the summer monsoon (June–September, JJAS) which is when most of the precipitation falls over the South Asia region (SAS; Figure Atlas.1 7). The topographic mechanical effect of the Tibetan Plateau (TIB) promotes moisture convergence downstream which triggers the early summer monsoon onset particularly over the Bay of Bengal and south China. In winter, westerly disturbances (WD) originating over the Atlantic Ocean bring moisture. The interaction between the WD and the Himalayas causes precipitation over northern and western parts of South Asia that is crucial to maintain the glacier mass balance. The observed teleconnection patterns over SAS for temperature show cooling effects during NAM and warming effects when in positive phase with ENSO, IOB, AMM and AMV (Annex IV). IOD also influences South Asian precipitation (Annex IV).

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===== Atlas.5.3.1.2 Findings From Previous IPCC Assessments =====

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Recent IPCC reports assessed that it is ''very likely'' that the mean annual temperature over South Asia has increased during the past century (Figure 2.21 in [[#Hartmann--2013|Hartmann et al., 2013]] , Figure 24-2 in [[#Hijioka--2014|Hijioka et al., 2014]] ), and the frequency of cold (warm) days and nights have decreased (increased) across most of Asia since about 1950 (Figure 2.32 in [[#Hartmann--2013|Hartmann et al., 2013]] ). The AR5 assessed that there is ''high confidence'' that the large-scale patterns of surface temperature are generally well simulated by the CMIP5 models though with problems in some regions, particularly at higher elevations over the Himalayas ( [[#Flato--2013|Flato et al., 2013]] ). CMIP5 models projected for the 21st century a significant increase in temperature over South Asia ( ''high confidence'' from ''robust evidence'' ) and in projections of increased summer monsoon precipitation ( ''medium confidence'' ) ( [[#Collins--2013|Collins et al., 2013]] ). The AR5 assessed there is ''high confidence'' that high-resolution regional downscaling, which generate results complementary to those from global climate models, adds value to the simulation of spatial variations in climate in regions with highly variable topography (e.g., distinct orography, coastlines), and for mesoscale phenomena and extremes ( [[#Flato--2013|Flato et al., 2013]] ).

Inconsistent evidence was found on the declining trends in mean precipitation and increasing droughts from 1950 onwards considering 1960–1990 as the baseline period. Similarly, SREX (Table 3-3 in [[#Seneviratne--2012|Seneviratne et al., 2012]] ) reported ''low confidence'' (due to lack of literature) in trends in climate indices related to extreme precipitation events. The Indian summer monsoon circulation was found to have weakened, but this was compensated by increased local atmospheric moisture content leading to more rainfall ( ''medium confidence'' ). It is ''likely'' that the occurrence of snowfall events is decreasing in South Asia along with other regions due to an increase in winter temperatures ( [[#Hock--2019b|Hock et al., 2019b]] ). Based on satellite- and surface-based remote sensing it is ''very likely'' that aerosol optical depth has increased over southern Asia since 2000.

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