Major factors of global and regional monsoon rainfall changes: natural versus anthropogenic forcing

Abstract

A number of studies have investigated the mechanisms that determine changes in precipitation, including how a wet region gets wetter. However, not all monsoon areas get wetter-there is a need to understand the major factors behind changes in regional monsoon precipitation, in terms of external forcing and internal variabilities in the last six decades by a combination of different observed datasets and model runs. We have found that time of emergence of anthropogenic signals is robustly detected in the northern African monsoon before the 1990s with the use of the CESM Large Ensemble Project. From CMIP5 model runs and three reanalysis datasets, the results found are that the change in rainfall over African monsoon (AFM) is mainly due to anthropogenic forcing and that over Asian-Australian monsoon (AAM) is affected by internal variability. Moreover, the cause of American monsoon (AMM) rainfall change cannot be known due to a discrepancy among observed datasets. Here we show that the asymmetry between Northern Hemisphere (NH) and Southern Hemisphere (SH) parts by green-house gas (GHG) is detected over the AFM and AAM regions. However, the land monsoon rainfall in the northern AMM is decreased by a combination of GHG and aerosol forcing. In general, the aerosol forcing causes a decreasing rainfall over the monsoon regions. In future projection, the land rainfall over the AAM and AMM are expected to increase and decrease in the future from most models' results. The asymmetry between an increase in NH and a decrease in SH is dominant in the future from most models' future simulation results, which is well shown over the AFM and AAM. This study suggests that the physical process of GHG and aerosol effects in rainfall should be explored in the context of regional aspects.

Causal Impacts of El Niño–Southern Oscillation on Global Soil Moisture Over the Period 2015–2100

“…The results show low consistency between models in reproducing the causal impacts of ENSO on soil moisture over Southeast Asian region, suggesting that the spread between models is substantial in this region and further studies are necessary. Because the Asian monsoon plays an important role in shaping the water resources of this region (Ha et al., 2020; Kripalani & Kulkarni, 1997), additional analyses considering the teleconnections between ENSO and the Asian monsoon are essential. As central Pacific and eastern Pacific (EP) ENSO might have different impacts (Cai et al., 2020), future works may consider investigating the effects of EP ENSO on soil moisture.…”

Erratic Asian Summer Monsoon 2020: COVID-19 Lockdown Initiatives Possible Cause for These Episodes?

“…More recently, it was suggested that future aerosols reduction would lead to a signi cant increase in precipitation extremes over Asia (Wang et al 2016). Another recent study documents a decreasing rainfall over the monsoon regions due to aerosol forcing (Ha et al 2020). Thus, the extent to which anthropogenic aerosols weaken or strengthen the monsoons and underlying mechanism is still largely uncertain (Jin et al 2021)…”

Contrasting Hysteresis Behaviors of Northern Hemisphere Land Monsoon Precipitation to CO2 Pathways

“…Local and remote drivers of monsoon need to be considered (An et al., 2015; Ha, Kim, et al., 2020; Wang et al., 2018), such as asymmetric land–sea heating, vertical stability, and meridional shifts of the intertropical convergence zone (ITCZ), and Hadley circulation (Geen et al., 2020; Han et al., 2019; Wang, 2009; Wang et al., 2020; Yancheva et al., 2007). Indeed, the seasonal location of the ITCZ, in‐phased with the solar insolation, is connected to seasonal monsoon circulation (Bordoni & Schneider, 2008).…”