Long term trends and variabilities of rainfall of the global monsoon systems during boreal and austral summer seasons

In this study, we have analyzed the variabilities in the global monsoon rainfall using Climate Research Unit monthly data for the period 1901 to 2020. The global monsoon domains are selected based on the difference in rainfall between the summer and winter as mentioned in the study of Wang and Ding (2006). The regions with high rainfall in the northern hemisphere (southern hemisphere) during the boreal (austral) summer are four (three), which is consistent with the results of Geen et al., 2020 and Wang and Ding 2008. In the regions of the northern hemisphere, a maximum value of mean rainfall is found in North America (NAM, 206.3 mm month-1), followed by South Asia (SAS, 204.3 mm month-1), and the least rainfall is observed in Southern Africa (SAF, 8.1 mm month-1); however, during the austral summer (DJFM), the maximum rainfall is observed in Australia (AUS, 222.7 mm month-1) followed by South America (SAM, 209.9 mm month-1). The interannual rainfall pattern shows a significant increasing trend for East Asia (EAS, 2 mm month-1 century-1), and a significant decreasing trend for West Africa (WAF) and SAS (-4 mm month-1 century- 1) during boreal summer. During the austral summer, significant increasing (decreasing) trends were observed for AUS, SAM and NAM (WAF and SAS) with values of 14,6 and 1 (-2 and - 0) mm month-1 century-1, respectively. The summer-winter rainfall relationships are significant for all the regions except for the SAS and SAM. This correlation follows an out of phase pattern for the EAS domain. During boreal (austral) summer, rainfall in the northern (southern) hemisphere domains is mostly in-phase (out-of-phase). The wavelet analysis shows that WAF, SAS, SAF, and SAM domains have significant multidecadal variability, and EAS and NAM show intradecadal variability during boreal summer. The WAF and NAM show the intradecadal variability and all the other regions show multidecadal variability during the boreal winter.