Simulated Changes in the Asian Summer Monsoon at Times of Increased Atmospheric CO₂

Abstract

Possible changes in the Asian summer monsoon due to increased atmospheric CO2 are investigated by an MRI global coupled atmosphere-ocean general circulation model. The summer (June-August) monsoon rainfall in India increases significantly with global warming. On the other hand, the monsoon wind shear index, defined as the difference between 850 hPa and 200 hPa zonal winds over the northern Indian Ocean, decreases. At 850 hPa, the westerly wind shifts northward and intensifies from the Sahel to northwest of India, but the monsoon westerly over the Arabian Sea weakens. It is found that increased moisture content in the warmer air leads to larger moisture flux convergence, contributing to the increased rainfall. Therefore, the monsoon wind shear index is not a good indicator for identifying any change of monsoon accompanying global warming. In contrast to the increased rainfall in India, change in rainfall is little over China where soil moisture becomes drier at times of increased CO2. It is also noted that the northern Eurasian continent becomes wetter in the increased-CO2 climate.The magnitude of the interannual variability of the Asian summer monsoon rainfall becomes larger in the CO2 experiment than in the control experiment, particularly in the later stage of the experiment after CO2 doubling. However it should be noted that the interdecadal variation of this interannual variability is also large both in the control and the CO2 experiments.

Global monsoon change during the Last Glacial Maximum: a multi-model study

“…Six models have simulated the shifting except MRI-CGCM3. The shift of the Asian monsoon domain is consistent with the palaeo-data (Chabangborn et al 2014) and opposite to the simulated northward shift of the Asian summer monsoon circulation under CO 2 doubling (Kitoh et al 1997). Braconnot et al (2007b) also found that some of the models participated in PMIP2 simulated a southward shift of the intertropical convergence zone (ITCZ) over the continents and the Indian Ocean, which was in agreement with the results found from marine data (Leduc et al 2007).…”

Variation in the orographic extreme rain events over the Meghalaya Hills in northeast India in the two halves of the twentieth century

“…The increase in the frequency of extreme rain events is consistent with a detected increasing trend of high rainfall events over central India (Goswami et al 2006) and over the whole of India, including northeast India (Dash et al 2009), as well as with model studies (Kitoh et al 1997;Meehl et al 2000;Trenberth et al 2003, IPCC Intergovernmental Panel on Climate Change 2007). A discrepancy with the detected decreasing trend of extreme events in northeast India by Goswami et al (2010) resulted from differences in the period of data used (32 years between April and October) and the method of defining extreme rain events (the number of events in exceedance of the 99th percentile and daily rainfall of above 150 mm).…”

Seasonally asymmetric transition of the Asian monsoon in response to ice age boundary conditions

“…Kitoh et al (1997) firstly showed that the summer monsoon circulation under CO 2 doubling shifts northward toward the Asian continent. Therefore it is suggested that regionally fixed wind index in a certain area is not appropriate for climate change analysis.…”

“…On the contrary, the northward excursion of westerlies weakened in the LGM climate ( Fig. 4d) but, rather remains over the equatorial Indian Ocean, which is thought to be analogous to the weaker summer monsoon in warmer climates (e.g., Kitoh et al 1997).…”

“…However, prediction by climate models suggests that the monsoonal precipitation will increase while the circulation itself will be coincidentally weakened (e.g., Kitoh et al 1997). The rainfall-wind paradox can be explained by large-scale heating contrast and in situ thermodynamic processes in the following manner.…”