scholarly journals Recent Decadal Changes in Heat Waves over China: Drivers and Mechanisms

2019 ◽  
Vol 32 (14) ◽  
pp. 4215-4234 ◽  
Author(s):  
Qin Su ◽  
Buwen Dong
Keyword(s):  
Heat Waves ◽  
Asian Summer Monsoon ◽  
Early Period ◽  
Spatial Extent ◽  
Physical Processes ◽  
Southeastern China ◽  
Anthropogenic Aerosol ◽  
Asian Summer ◽  
Frequency Intensity

Abstract Observational analysis indicates significant decadal changes in daytime, nighttime, and compound (both daytime and nighttime) heat waves (HWs) over China across the mid-1990s, featuring a rapid increase in frequency, intensity, and spatial extent. The variations of these observed decadal changes are assessed by the comparison between the present day (PD) of 1994–2011 and the early period (EP) of 1964–81. The compound HWs change most remarkably in all three aspects, with frequency averaged over China in the PD tripling that in the EP and intensity and spatial extent nearly doubling. The daytime and nighttime HWs also change significantly in all three aspects. A set of numerical experiments is used to investigate the drivers and physical processes responsible for the decadal changes of the HWs. Results indicate the predominant role of the anthropogenic forcing, including changes in greenhouse gas (GHG) concentrations and anthropogenic aerosol (AA) emissions in the HW decadal changes. The GHG changes have dominant impacts on the three types of HWs, while the AA changes make significant influences on daytime HWs. The GHG changes increase the frequency, intensity, and spatial extent of the three types of HWs over China both directly via the strengthened greenhouse effect and indirectly via land–atmosphere and circulation feedbacks in which GHG-change-induced warming in sea surface temperature plays an important role. The AA changes decrease the frequency and intensity of daytime HWs over Southeastern China through mainly aerosol–radiation interaction, but increase the frequency and intensity of daytime HWs over Northeastern China through AA-change-induced surface–atmosphere feedbacks and dynamical changes related to weakened East Asian summer monsoon.

scholarly journals The Crucial Role of Internal Variability in Modulating the Decadal Variation of the East Asian Summer Monsoon–ENSO Relationship during the Twentieth Century

2015 ◽  
Vol 28 (18) ◽  
pp. 7093-7107 ◽  
Author(s):  
Fengfei Song ◽  
Tianjun Zhou
Keyword(s):  
Twentieth Century ◽  
Summer Monsoon ◽  
East Asian Summer Monsoon ◽  
Crucial Role ◽  
Asian Summer Monsoon ◽  
East Asian ◽  
Internal Variability ◽  
Decadal Variation ◽  
Asian Summer

Abstract This study investigates the role of internal variability in modulating the East Asian summer monsoon (EASM)–ENSO relationship using Twentieth-Century Reanalysis (20CR) data and simulations from phase 5 of CMIP (CMIP5). Analysis of 20CR data reveals an unstable EASM–ENSO relationship during the twentieth century. During the high-correlation periods of 1892–1912 and 1979–99, an evident western Pacific anticyclone (WPAC) and dipole sea level pressure (SLP) pattern are present in the decaying El Niño summer, accompanied by Indian Ocean warming and a tropospheric temperature Matsuno–Gill pattern. However, these are weaker or absent during low-correlation periods (1914–34 and 1958–78). After removing the external forcings based on historical simulations from 15 CMIP5 models, all the above features remain almost unchanged, suggesting the crucial role of internal variability. In a 501-yr preindustrial control (piControl) simulation without external forcing variation from CCSM4, the EASM–ENSO relationship also shows significant decadal variation, with a magnitude comparable to the 20CR data. The analysis demonstrates that the EASM–ENSO relationship’s variation is modulated by the interdecadal Pacific oscillation (IPO). Compared to negative IPO phases, the warmer East China Sea in positive IPO phases weakens the western North Pacific subtropical high (WNPSH), inducing more precipitation. Thus, the Kelvin wave–induced interannual divergence suppresses more mean-state precipitation and leads to a stronger WPAC. Hence, the IPO modulates the EASM–ENSO relationship through the WNPSH, which is evident in both 20CR and the piControl simulation.

South Asian summer monsoon in warm epochs of Mid-Holocene and end of 21st century; new insights from high resolution simulations

2020 ◽  
Author(s):  
Lekshmi Mudra B ◽  
Thazhe Purayil Sabin ◽  
Raghavan Krishnan
Keyword(s):  
High Resolution ◽  
South Asian ◽  
Asian Summer Monsoon ◽  
Hadley Cell ◽  
Monsoon Precipitation ◽  
South Asian Summer Monsoon ◽  
Indus Valley ◽  
Asian Summer ◽  
Northwest India

<p>The mid-Holocene (MH) was a warmer period, similar to the end of the 21st century climate under high emission realizations. The Indus valley civilization believed to be flourished under the expense of enhanced south Asian summer monsoon precipitation associated with the northward migration of the Inter Tropical Convergence Zone (ITCZ) during the mid-Holocene (MH). However, such an enhanced precipitation is not visible over the northwest India and Pakistan belt in future projection. The role of dynamical and various teleconnection factors behind the enhanced MH precipitation over the Indus valley region is still elusive due to the limitation of course resolution modelling efforts available so far as part of the various phases of Paleoclimate Modelling Intercomparison Projects (PMIP).  To overcome this limitation, we have designed high resolution Paleo-climate simulations using a state-of-the-art variable resolution global climate model (LMDZ: Laboratoire Meteorologie Dynamique and Z stand for zoom) which configured with a 35 km spatial resolution over the South Asian region. We conducted various sensitivity experiments to understand the role of dynamics and teleconnection in enhancing monsoon precipitation over the Indus valley in addition to the MH orbital conditions. Boundary conditions from the PMIP-3, CMIP5 and HadISST datasets utilized for various sensitive experiments. High resolution, clearly demonstrates value addition in simulating the enhanced MH precipitation over Northwest India and adjoining Indus basin associated with the northward migration of the ITCZ and shift in the ascending branch of Hadley cell. We explored the role of various oceanic and atmospheric factors responsible for this enhanced Indus valley precipitation through linearized moisture budget analysis and comparing the relative strength and position of Hadley cell. By further decomposing the thermodynamic and dynamic term into their advection and divergence component, we could demonstrate the role of moisture convergence due to the strengthened atmospheric circulation through the oceanic teleconnection, which additionally  plays a crucial role in enhanced MH precipitation comparing to the dynamical factors. Idealized simulation with the end of 21<sup>st</sup> century warm condition with the MH orbital forcing and various teleconnection patterns affirms that the thermodynamically induced future precipitation and circulation changes, may not be adequate to make a profound shift in the northern limit of the ITCZ towards its MH locale rather producing enhanced precipitation over the north Indian ocean and localized extreme precipitation over Indian landmass.</p><p><strong>Keywords: </strong>Indus Valley civilization, Mid-Holocene, Monsoons, Teleconnection, ITCZ and Hadley circulation</p>

scholarly journals Dynamics of Asian Summer Monsoon Response to Anthropogenic Aerosol Forcing

2019 ◽  
Vol 32 (3) ◽  
pp. 843-858 ◽  
Author(s):  
Hai Wang ◽  
Shang-Ping Xie ◽  
Yu Kosaka ◽  
Qinyu Liu ◽  
Yan Du
Keyword(s):  
Summer Monsoon ◽  
Radiative Forcing ◽  
Asian Summer Monsoon ◽  
Atmospheric Response ◽  
Anthropogenic Aerosols ◽  
Anthropogenic Aerosol ◽  
Sst Cooling ◽  
Aerosol Forcing ◽  
Cooling Pattern ◽  
Asian Summer

Anthropogenic aerosols partially mask the greenhouse warming and cause the reduction in Asian summer monsoon precipitation and circulation. By decomposing the atmospheric change into the direct atmospheric response to radiative forcing and sea surface temperature (SST)-mediated change, the physical mechanisms for anthropogenic-aerosol-induced changes in the East Asian summer monsoon (EASM) and South Asian summer monsoon (SASM) are diagnosed. Using coupled and atmospheric general circulation models, this study shows that the aerosol-induced troposphere cooling over Asian land regions generates anomalous sinking motion between 20° and 40°N and weakens the EASM north of 20°N without SST change. The decreased EASM precipitation and the attendant wind changes are largely due to this direct atmospheric response to radiative forcing, although the aerosol-induced North Pacific SST cooling also contributes. The SST-mediated change dominates the aerosol-induced SASM response, with contributions from both the north–south interhemispheric SST gradient and the local SST cooling pattern over the tropical Indian Ocean. Specifically, with large meridional gradient, the zonal-mean SST cooling pattern is most important for the Asian summer monsoon response to anthropogenic aerosol forcing, resulting in a reorganization of the regional meridional atmospheric overturning circulation. While uncertainty in aerosol radiative forcing has been emphasized in the literature, our results show that the intermodel spread is as large in the SST effect on summer monsoon rainfall, calling for more research into the ocean–atmosphere coupling.

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