Interannual Variability of the Asian Summer Monsoon: Contrasts between the Indian and the Western North Pacific–East Asian Monsoons*

Abstract

Analyses of 50-yr NCEP-NCAR reanalysis data reveal remarkably different interannual variability between the Indian summer monsoon (ISM) and western North Pacific summer monsoon (WNPSM) in their temporalspatial structures, relationships to El Niño, and teleconnections with midlatitude circulations. Thus, two circulation indices are necessary, which measure the variability of the ISM and WNPSM, respectively. A weak WNPSM features suppressed convection along 10Њ-20ЊN and enhanced rainfall along the mei-yu/baiu front. So the WNPSM index also provides a measure for the east Asian summer monsoon. An anomalous WNPSM exhibits a prominent meridional coupling among the Australian high, cross-equatorial flows, WNP monsoon trough, WNP subtropical high, east Asian subtropical front, and Okhotsk high. The WNP monsoon has leading spectral peaks at 50 and 16 months, whereas the Indian monsoon displays a primary peak around 30 months. The WNPSM is weak during the decay of an El Niño, whereas the ISM tends to abate when an El Niño develops. Since the late 1970s, the WNPSM has become more variable, but its relationship with El Niño remained steady; in contrast, the ISM has become less variable and its linkage with El Niño has dramatically declined. These contrasting features are in part attributed to the differing processes of monsoon-ocean interaction.Also found is a teleconnection between a suppressed WNPSM and deficient summer rainfall over the Great Plains of the United States. This boreal summer teleconnection is forced by the heat source fluctuation associated with the WNPSM and appears to be established through excitation of Rossby wave trains and perturbation of the jet stream that further excites downstream optimum unstable modes.

Sub-seasonal Predictability of the Onset and Demise of the Rainy Season over Monsoonal Regions

“…The figure also indicates the domains selected to study specific monsoonal regions (black boxes), namely the North American monsoon (e.g., Adams and Comrie, 1997;Vera et al, 2006), the South American monsoon (e.g., Vera et al, 2006;Marengo et al, 2012;Carvalho and Cavalcanti, 2016), the West Africa monsoon (e.g., Nicholson, 2013), the East African monsoon (e.g., Nairobi, 1979;Mutai and Ward, 2000), the Indian monsoon (e.g., Webster et al, 1998;Krishnamurthy and Shukla, 2000;Prasad, 2005), the East Asia monsoon (e.g., Wang et al, 2001;Yihui and Chan, 2005), and the Northern Australia monsoon (e.g., Hendon and Liebmann, 1990).…”

Perspectives on the non-stationarity of the relationship between Indian and East Asian summer rainfall variations

“…Sugi 2001;Zhang 2001;Liu and Ding 2008). The other pathway is atmospheric circulation change over the midlatitudes of continental Asia, featuring a zonal wave pattern (Guo and Wang 1988;Kripalani, Kulkarni, and Singh 1997;Krishnan and Sugi 2001;Wang, Wu, and Lau 2001;Kim et al 2002;Lu, Oh, and Kim 2002;Wu 2002;Enomoto, Hoskins, and Matsuda 2003). This zonal wave pattern is partly associated with anomalous Indian heating, and in turn modulates winds over East Asia (Wu 2002;Wu, Hu, and Kirtman 2003;Ding and Wang 2005;Liu and Ding 2008;Greatbatch, Sun, and Yang 2013).…”

“…Given the impact of ENSO on both Indian and East Asian summer monsoon variability (Wang, Wu, and Lau 2001), ENSO is likely a factor involved in the long-term changes in the Indian-East Asian summer rainfall relationship through changes its impacts on either the Indian or East Asian summer monsoon (Kumar, Rajagopalan, and Cane 1999;Wu and Wang 2002). Hu et al (2005) indicated that ENSO may reinforce the connection between Indian and North China summer rainfall variations.…”

“…Previous studies have revealed an in-phase relationship in the summer rainfall variations between India and North China (Guo and Wang 1988;Kripalani and Singh 1993;Zhang, Sumi, and Kimoto 1999;Kripalani and Kulkarni 2001;Wu 2002;Liu and Ding 2008;Greatbatch, Sun, and Yang 2013;Preethi et al 2017a), and an out-of-phase relationship between India and southern Japan (Kripalani and Kulkarni 2001;Krishnan and Sugi 2001;Wang, Wu, and Lau 2001;Wu 2002;Yun, Lee, and Ha 2014) or South Korea Preethi et al 2017a). According to the literature, there are two pathways for the connection between Indian and East Asian summer rainfall variations, as summarized by Wu (2017).…”

“…One important factor is ENSO, which imposes a substantial impact on both Indian and East Asian summer rainfall variability (e.g. Walker 1923;Huang and Wu 1989;Wang, Wu, and Lau 2001;Wu, Hu, and Kirtman 2003).…”