Monsoon climate - Will summer rain increase or decrease in monsoon regions? [Present]

In physical essence, monsoon is a forced response of the coupled climate system to the annual cycle of insolation. Land-sea thermal contrast, moisture processes, topography and Earth’s rotation are critical in determining monsoon rainfall patterns and vigor. Integrated regional monsoons generate a global-scale seasonally varying overturning circulation throughout the tropics (Trenberth et al. 2000). Global monsoon (GM) represents the dominant mode of annual variation of the tropical precipitation and circulation (Wang and Ding 2008), thus a defining feature of seasonality and a major mode of variability of the Earth’s climate system.

GM_LvG.psd

Figure 1: Multi-model mean changes in precipitation for boreal winter (DJF) and summer (JJA). Changes are given for the SRES A1B scenario, for the period 2080 to 2099 relative to 1980 to 1999. Stippling denotes areas where the magnitude of the multi-model ensemble mean exceeds the inter-model standard deviation. In the global monsoon domain (red contours) the summer-minus-winter precipitation exceeds 2.5 mm/day and the summer precipitation exceeds 55% of the annual total (Wang and Ding 2008). The dry regions with summer precipitation <1 mm/day are outlined in gray. The merged Global Precipitation Climatology Project/Climate Prediction Center Merged Analysis of Precipitation data were used to determine the monsoon and arid regions.

Monsoon climate features an annual reversal of surface winds and contrasting rainy summer and dry winter. The monsoon domains defined by precipitation characteristics are shown in Figure 1, which include all regional monsoons over South Asia, East Asia, Australia, Africa and the Americas (Wang and Ding 2008).

Monsoonal interannual-interdecadal variations have been studied primarily on regional scales due to their indigenous characteristics associated with specific land-ocean configuration and differing feedback processes. However, global observations over the past three decades reveal a cohesive interannual variation across regional monsoons driven by El Niño-Southern Oscillation (ENSO). Thus, regional monsoons are coordinated not only by external (e.g. orbital) forcing but also by internal feedback processes, such as ENSO.

To what extent the regional monsoons vary in a cohesive manner on interdecadal time scale remains elusive. So far no uniform trend or coherent variation pattern has been found over the global monsoon domain. The total amount of global land monsoon rainfall during 1948-2003 exhibits an interdecadal fluctuation with a decreasing trend mainly due to weakening West African and South Asian monsoons (Zhou et al. 2008; Wang et al. 2011). But, since 1980 the global land monsoon rainfall has no significant trend, while the global oceanic monsoon precipitation shows an increasing trend (Wang et al. 2011).

A millennial simulation with the coupled climate model ECHO-G forced by changes in solar radiation, volcanic aerosols and greenhouse gas (GHG) concentration provides useful insight to GM rainfall variability. The leading pattern of centennial variability (wet Medieval Climate Anomaly, dry Little Ice Age, and wet present warming period) is characterized by a nearly uniform increase of precipitation across all regional monsoons, which is a forced response to the changes in external solar-volcanic and GHG forcing (Liu et al., unpublished data). The increase of GSMP is sensitive to warming pattern and determined by enhanced (a) land-ocean thermal contrast, (b) east-west thermal contrast between Southeast Pacific and tropical Indian Ocean, and (c) circumglobal southern hemisphere subtropical highs, which contribute to the hemispherical thermal contrast (Lui et al., unpublished data).

Will summer monsoon rain increase or decrease in the future? Based on the IPCC AR4 (Meehl et al. 2007), during austral summer the rainfall in all SH monsoon regions tends to increase (Fig. 1) and during boreal summer the rainfall in NH monsoon regions will also increase except for North America where it will decrease (Fig. 1). Thus, an overall intensification of summer monsoon rainfall is projected, signifying an amplifying annual variation of the hydrological cycle. Meanwhile the precipitation in the global subtropical desert and trade wind regions will decrease due to a monsoon-desert coupling mechanism. The annual mean monsoon precipitation is projected to increase in Asian-Australian monsoon but decrease in Mexico and Central America. However, the uncertain role of aerosols in general and carbon aerosols in particular, complicates future projections of monsoon precipitation over land, particularly for Asia. Further understanding of the driving mechanisms behind monsoon changes holds a key for their reliable prediction.

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