How does climate change affect the Asian summer monsoon?

 

20th and 21st century monsoon changes

We examine the changes of the Asian summer monsoon in response to anthropogenic forcing using observations and the Coupled Model Intercomparison Project - Phase 5 (CMIP5) multi-model, multi-realization ensemble. We use the signal-to-noise (S/N) maximizing empirical orthogonal function (EOF) technique to obtain a model-based best estimate of the radiatively forced signal in monsoon rainfall. In the 20th century, CMIP5 models indicate a predominantly drying Asian monsoon, with clear discrepancies compared to observations. In the 21st century under the representative concentration pathway 8.5 (rcp8.5) scenario, monsoon rainfall enhances across the entire Asian domain.

We analyze the thermodynamic and dynamic mechanisms causing the changes using specific humidity and winds, as well as the moisture budget. The drying trend in the CMIP5 historical simulations and the wetting trend in the rcp8.5 projections can be explained by the relative importance of dynamical and thermodynamical contributions to the total mean moisture convergence. While the thermodynamic mechanism dominates in the future, the historical rainfall changes are dominated by changes in atmospheric circulation.

 

Role of greenhouse gases vs. anthropogenic aerosols

We further examine the relative contributions of aerosols and greenhouse gases (GHGs) on the historical monsoon change using CMIP5 single-forcing simulations. Rainfall reduces under aerosol forcing and increases under GHG forcing, thus the total change depends strongly on the relative strengths of these two competing effects. Aerosol forcing dominates over the greenhouse effect during the historical period, leading to the general drying trend in the all-forcing simulations. While the thermodynamic change of the mean moisture convergence in the all-forcing case is dominated by GHGs, the dynamic change of the mean moisture convergence in the all-forcing case is dominated by anthropogenic aerosols. Aerosol forcing dominates the drying trend in the CMIP5 historical simulations through both dynamic and thermodynamic contributions.

Aerosol effect is one of the major uncertainties for climate models in simulating monsoon characteristics. Discrepancies in aerosol forcing between model and observations as well as among individual models may have contributed largely to differences in forced monsoon trends.

 

Reference:
Li, X.
, M. Ting, C. Li, and N. Henderson, 2015: Mechanisms of Asian summer monsoon changes in response to anthropogenic forcing in CMIP5 models. Journal of Climate, 28 (10), 4107-4125, doi: 10.1175/JCLI-D-14-00559.1. pdf

 

Understanding the forced monsoon change: fast vs. slow responses

 

Greenhouse gases

Future hydroclimate projections from state-of-the-art climate models show large uncertainty and model spread, particularly in the tropics and over the monsoon regions. The precipitation and circulation responses to rising greenhouse gases (GHGs) involve a fast component associated with direct radiative forcing and a slow component associated with sea surface temperature (SST) warming; the relative importance of the two may contribute to model discrepancies.

We assess Asian summer monsoon responses to greenhouse warming using output from coupled general circulation models (CGCMs) in the Coupled Model Intercomparison Project - Phase 5 (CMIP5) and idealized atmospheric general circulation model (AGCM) experiments from the Atmosphere Model Intercomparison Project (AMIP). We analyze the following experiments: 1) the control simulation (CTRL), run with observed SSTs and sea ice concentration from 1979 to 2008; 2) quadrupling CO2 radiative forcing experiment (4xCO2), same SSTs and sea ice concentration as CTRL, but with quadrupled atmospheric CO2 concentration; 3) uniform 4K warming experiment (+4K), same CO2 concentration as CTRL, but adding a uniform +4K SST anomaly globally. The fast (slow) response is quantified as the difference of the 30-year climatology between 4xCO2 (+4K) and CTRL. We examine the thermodynamic and dynamic mechanisms causing the rainfall changes using atmospheric moisture budget analysis.

Results show that direct radiative forcing and SST change exert significantly different responses both over land and ocean. For the Asian summer monsoon, the fast and slow responses largely oppose each other, mainly due to the opposing effect of the dynamic contribution to the mean moisture convergence, thus driven by the atmospheric circulation change. While the thermodynamical response of the Asian monsoon is robust across the models, there is substantial uncertainty in both the magnitude and the sign of the dynamical changes in the CMIP5 model ensemble.

 

Anthropogenic aerosols

Work in progress.

 

Li, X., and M. Ting, 2017: Understanding the Asian summer monsoon response to greenhouse warming: the relative roles of direct radiative forcing and sea surface temperature change. Climate Dynamics, 49 (7-8), 2863-2880, doi:10.1007/s00382-016-3470-3. full-text pdf

 

Changes in the ENSO-monsoon relationship

We examine the Asian monsoon-ENSO (El Niño–Southern Oscillation) relationship in the 20th and 21st centuries using observations and Coupled Model Intercomparison Project Phase 5 (CMIP5) model simulations. CMIP5 models can simulate the ENSO-monsoon spatial structure reasonably well when using the multimodel mean. Running correlations show prominent decadal variability of the ENSO-monsoon relationship in observations. The modeled ENSO-monsoon relation shows large intermodel spread, indicating large variations across the model ensemble.

We further separate the anthropogenically forced component of ENSO-monsoon relationship from the naturally varying component based on a signal-to-noise (S/N) maximizing empirical orthogonal function (EOF) analysis using global sea surface temperature (SST). Results show that natural variability plays a dominant role in the varied ENSO-monsoon relationship during the 20th century. In the 21st century, the forced component is dominated by enhanced monsoon rainfall associated with SST warming, which may contribute to a slightly weakened ENSO-monsoon relation in the future.

 

Reference:
Li, X.
, and M. Ting, 2015: Recent and future changes in the Asian monsoon – ENSO relationship: Natural or forced? Geophysical Research Letters, 42 (9), 3502-3512, doi: 10.1002/2015GL063557. pdf supp