Response Of Summer Monsoon Research Articles

East Asian summer monsoon response to anthropogenic aerosols redistribution: contrasting 1950–1980 and 1980–2010 to understand the role of non-Asian forcing

East Asian summer monsoon response to anthropogenic aerosols redistribution: contrasting 1950–1980 and 1980–2010 to understand the role of non-Asian forcing

The East Asian Summer Monsoon Response to Global Warming in a High Resolution Coupled Model: Mean and Extremes

Current climate models still have considerable biases in the simulation of the East Asian summer monsoon (EASM), which in turn reduces their reliability of monsoon projections under global warming. We hypothesize that a higher-resolution coupled climate model with atmospheric and oceanic components at horizontal resolutions of 0.25° and 0.1°, respectively, will better capture regional details and extremes of the EASM. Present-day (PD), 2 × CO2 and 4 × CO2 simulations are conducted with the Community Earth System Model (CESM1.2.2) to evaluate PD simulation performance and quantify future changes. Indeed, our PD simulation well reproduces the climatological seasonal mean and intra-seasonal northward advancement of the monsoon rainband, as well as climate extremes. Compared with the PD simulation, the perturbed CO2 experiments show an intensified EASM response to CO2-induced warming. We find that the precipitation increases of the Meiyu-Baiu-Changma band are caused by comparable contributions from the dynamical and thermodynamical components in 2 × CO2, while they are more driven by the thermodynamical component in 4 × CO2 due to stronger upper atmospheric stability. The regional changes in the probability distribution of the temperature show that extreme temperatures warm faster than the most often temperatures, increasing the skewness. Fitting extreme precipitation values with a generalized Pareto distribution model reveals that they increase significantly in 4 × CO2. Changes of temperature extremes scale with the CO2 concentrations over the monsoon domain but not for precipitation extreme changes. The 99th percentile of precipitation over the monsoon region increases at a super Clausius-Clapeyron rate, ~ 8% K–1, which is mainly caused by increased moisture transport through anomalous southerly winds.

South Asian Summer Monsoon Response to Aerosol‐Forced Sea Surface Temperatures

AbstractClimate models suggest that anthropogenic aerosol‐induced drying dominates the historical rainfall changes over the heavily populated South Asian monsoon region. The regional response depends on both the aerosol fast radiative effect and the slow process through sea surface temperature (SST) cooling. Two atmospheric general circulation models, NCAR‐CAM5 and GFDL‐AM3, are used to investigate the monsoon response to prescribed aerosol‐forced SSTs. The total SST is separated into uniform cooling and a spatially varying component characterized by interhemispheric asymmetry. The monsoon rainfall is predominantly controlled by the nonuniform SSTs, in the local Indian Ocean, South, and East China Seas (IO‐CSs). The reduced meridional SST gradient in the IO‐CSs leads to weakened monsoon circulation, which drives a north‐south dipole rainfall change. The latitudinal location of the dipole shows model dependence due to differences in local SSTs and their meridional gradient, which determines the latitudinal location of the meridional overturning circulation responses.

Snow-darkening versus direct radiative effects of mineral dust aerosol on the Indian summer monsoon onset: role of temperature change over dust sources

Abstract. Atmospheric absorptive aerosols exert complicated effects on the climate system, two of which are through their direct radiative forcing and snow-darkening forcing. Compared to black carbon, the snow-darkening effect of dust on climate has been scarcely explored till now. When depositing in snow, dust can reduce the albedo of snow by darkening it and increasing the snowmelt. In this study, the snow-darkening effect of dust, as well as the direct radiative effect, on the Indian summer monsoon are evaluated by atmospheric general circulation model experiments. The results show that the snow-darkening and direct radiative forcing of dust both have significant impacts on the onset of the Indian monsoon, but they are distinctly opposite. The snow-darkening effect of dust weakens the Indian monsoon precipitation during May and June, opposite to black carbon. The surface temperature over central Asia and the western Tibetan Plateau becomes warmer due to the dust-induced decrease in snow cover, which leads to a local low-level cyclonic anomaly as well as an anticyclonic anomaly over the Indian subcontinent and Arabian Sea. This circulation pattern allows air currents penetrating into the Indian subcontinent more from central Asia but less from the Indian Ocean. In contrast, the direct radiative forcing of dust warms the low troposphere over the Arabian Peninsula, which intensifies moisture convergence and precipitation over the Indian monsoon region. The upper tropospheric atmospheric circulation over Asia is also sensitive to both effects. Compared to previous studies which emphasized the temperature over the Tibetan Plateau, our results further highlight an important role of surface/low tropospheric temperature changes over dust source areas, which can also significantly modify the response of summer monsoon. Thus, links between the climatic impact of dust and complicated thermal conditions over Asia are of importance and need to be clarified accurately.

Dynamics of Asian Summer Monsoon Response to Anthropogenic Aerosol Forcing

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.

Indian summer monsoon: Extreme events, historical changes, and role of anthropogenic forcings

The South Asian summer monsoon is a complex coupled human‐natural system that poses unique challenges for understanding its evolution alongside increasing anthropogenic activities. Rapid and substantial changes in land‐use, land‐management and industrial activities over the subcontinent, and warming in the Indian Ocean, have influenced the South Asian summer monsoon. These might continue to be significant drivers in the near‐term along with rising global greenhouse gas emissions. Deciphering the region's vulnerability to climate change requires an understanding of how these anthropogenic activities, acting on a range of spatial scales, have shaped the monsoon spatially and temporally. This review summarizes historical changes in monsoon rainfall characteristics, associated mechanisms, and the role of anthropogenic forcings, focusing on subseasonal variability and extreme events. Several studies have found intensified subseasonal extremes across parts of India and an increase in spatial variability of rainfall despite an overall weakening of seasonal rainfall in the monsoon core. However, understanding these changes remains challenging because of uncertainties in observations and climate models. The mechanisms and relative influences of various anthropogenic activities, particularly on subseasonal extremes, remain relatively underexplored. Large biases in the representation of relevant processes in global climate models limit the ability to attribute historical changes and make reliable projections. Nevertheless, recent advances in modeling these processes using higher‐resolution modeling frameworks provide new tools to investigate the Indian summer monsoon's response to various anthropogenic forcings. There is an urgent need to understand how these forcings interact to shape climate variability and change in this vulnerable region.This article is categorized under: Paleoclimates and Current Trends > Earth System Behavior

Indian monsoon variability in response to orbital forcing during the late Pliocene

Abstract The Asian monsoon is a major component of the global climate system and can be divided into two subsystems, the Indian monsoon and the East Asian monsoon. Insights into monsoon behaviour and dynamics can be gained through studying past warm intervals in Earth's history. One such interval is the Pliocene epoch, specifically the mid-Piacenzian Warm Period (mPWP; 3.264–3.025 Ma). This time is characterised as a period of sustained warmth, with annual mean temperatures 2–3 °C higher than the pre-industrial era. Studies have examined the East Asian monsoon during the mPWP from both a geological data and climate modelling perspective. However, there has been little investigation into the behaviour of the Indian monsoon. Using a coupled atmosphere-ocean global climate model (HadCM3), the Indian summer monsoon response to orbital forcing during the mPWP is studied. Of the simulated interglacial events (Marine Isotope Stages KM5c, KM3, K1 and G17), MIS KM5c is the only one with a near-modern orbital forcing. This experiment is compared to a pre-industrial simulation to determine the nature of the mPWP Indian summer monsoon in the absence of a different pattern if insolation forcing. The monsoon at MIS KM5c, is simulated to be stronger than pre-industrial, with higher surface air temperatures and precipitation over land due to higher levels of CO2. MIS G17, K1, and KM3 represent interglacial events of similar magnitude with different insolation forcing than MIS KM5c. The Indian summer monsoon is simulated to be significantly stronger for the interglacials K1 and KM3, compared to KM5c. This is due to stronger precession forcing causing an increase in summer surface air temperature and precipitation. When combined with Pliocene geological boundary conditions, these results highlight the significant effect of orbital forcing on the strength of the Indian summer monsoon. The sensitivity of the Indian monsoon to orbital forcing has important implications for any parallels drawn between Pliocene and future monsoon behaviour.

A Unique Feature of the Asian Summer Monsoon Response to Global Warming: The Role of Different Land–Sea Thermal Contrast Change between the Lower and Upper Troposphere

A Unique Feature of the Asian Summer Monsoon Response to Global Warming: The Role of Different Land–Sea Thermal Contrast Change between the Lower and Upper Troposphere

Understanding the Asian summer monsoon response to greenhouse warming: the relative roles of direct radiative forcing and sea surface temperature change

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 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. In this study, regional hydroclimate responses to greenhouse warming are assessed using output from coupled general circulation models in the Coupled Model Intercomparison Project-Phase 5 (CMIP5) and idealized atmospheric general circulation model experiments from the Atmosphere Model Intercomparison Project. The thermodynamic and dynamic mechanisms causing the rainfall changes are examined using moisture budget analysis. Results show that direct radiative forcing and SST change exert significantly different responses both over land and ocean. For most part of the Asian monsoon region, the summertime rainfall changes are dominated by the direct CO2 radiative effect through enhanced monsoon circulation. The response to SST warming shows a larger model spread compared to direct radiative forcing, possibly due to the cancellation between the thermodynamical and dynamical components. While the thermodynamical response of the Asian monsoon is robust across the models, there is a lack of consensus for the dynamical response among the models and weak multi-model mean responses in the CMIP5 ensemble, which may be related to the multiple physical processes evolving on different time scales.

Confronting the “Indian summer monsoon response to black carbon aerosol” with the uncertainty in its radiative forcing and beyond

AbstractWhile black carbon aerosols (BC) are believed to modulate the Indian monsoons, the radiative forcing estimate of BC suffers from large uncertainties globally. We analyze a suite of idealized experiments forced with a range of BC concentrations that span a large swath of the latest estimates of its global radiative forcing. Within those bounds of uncertainty, summer precipitation over the Indian region increases nearly linearly with the increase in BC burden. The linearity holds even as the BC concentration is increased to levels resembling those hypothesized in nuclear winter scenarios, despite large surface cooling over India and adjoining regions. The enhanced monsoonal circulation is associated with a linear increase in the large‐scale meridional tropospheric temperature gradient. The precipitable water over the region also increases linearly with an increase in BC burden, due to increased moisture transport from the Arabian sea to the land areas. The wide range of Indian monsoon response elicited in these experiments emphasizes the need to reduce the uncertainty in BC estimates to accurately quantify their role in modulating the Indian monsoons. The increase in monsoonal circulation in response to large BC concentrations contrasts earlier findings that the Indian summer monsoon may break down following a nuclear war.

Amplification of ENSO effects on Indian summer monsoon by absorbing aerosols

In this study, we present observational evidence, based on satellite aerosol measurements and MERRA reanalysis data for the period 1979–2011, indicating that absorbing aerosols can have strong influence on seasonal-to-interannual variability of the Indian summer monsoon rainfall, including amplification of ENSO effects. We find a significant correlation between ENSO (El Nino Southern Oscillation) and aerosol loading in April–May, with La Nina (El Nino) conditions favoring increased (decreased) aerosol accumulation over northern India, with maximum aerosol optical depth over the Arabian Sea and Northwestern India, indicative of strong concentration of dust aerosols transported from West Asia and Middle East deserts. Composite analyses based on a normalized aerosol index (NAI) show that high concentration of aerosol over northern India in April–May is associated with increased moisture transport, enhanced dynamically induced warming of the upper troposphere over the Tibetan Plateau, and enhanced rainfall over northern India and the Himalayan foothills during May–June, followed by a subsequent suppressed monsoon rainfall over all India, consistent with the elevated heat pump (EHP) hypothesis (Lau et al. in Clim Dyn 26:855–864, 2006. doi:10.1007/s00382-006-0114-z). Further analyses from sub-sampling of ENSO years, with normal (<1-σ), and abnormal (>1-σ) NAI over northern India respectively show that the EHP may lead to an amplification of the Indian summer monsoon response to ENSO forcing, particularly with respect to the increased rainfall over the Himalayan foothills, and the warming of the upper troposphere over the Tibetan Plateau. Our results suggest that absorbing aerosol, particular desert dusts can strongly modulate ENSO influence, and possibly play important roles as a feedback agent in climate change in Asian monsoon regions.

The Asian summer monsoon response to the La Niña event of 2010

AbstractThe 2010 summer monsoon was marked by a series of flood events over Pakistan and Northwest India and an intriguing aspect during this season was the strong westward shift of West Pacific Subtropical High (WPSH) by nearly 15° longitude relative to the climatology. This period also witnessed the evolution of an intense La Niña in the Pacific Ocean. La Niña episodes are generally characterized by wetter‐than‐normal monsoons over South Asia, although the situation was rather different in 2010. Large areas covering east‐central India, the Bay of Bengal and extending eastward into the South China Sea and the Philippines received deficit monsoon rains during 2010, while rainfall activity was enhanced mostly over northern India and adjoining northwest Pakistan. By conducting a detailed diagnostic analysis of observed and reanalysis datasets, it is found that the anomalous westward shift of the WPSH and suppression of convection around the Bay of Bengal region were crucial in sustaining the rainfall activity over northwest India and Pakistan during 2010. The results suggest that the westward shift of the WPSH favoured the setting up of an anomalous trough extending over the sub‐tropical areas near India and Pakistan, so that the anomalous transport of moisture from the Arabian Sea sustained the local rainfall activity. An analysis of SST records during the last 60+ years, shows a long‐term background La Niña‐like warming trend in the tropical Pacific. Based on the present results, it is suggested that generation of La Niña events in the backdrop of the SST warming trend have significant potential in influencing the rainfall activity over the South Asian monsoon‐trough region. Copyright © 2012 Royal Meteorological Society

The response of summer monsoon onset/retreat in Sumatra-Java and tropical Australia region to global warming in CMIP3 models

In this study, we assess the potential changes in the onset, retreat and duration of austral summer monsoon covering the domain from south Sumatra and Java region in the tropics to the northern Australian continent. We simply call it the Australian summer monsoon. Daily precipitable water and 850 hPa wind from 13 CMIP3 models are used in the diagnoses. A majority of the models can capture the northwest–southeast evolution of the summer monsoon, which starts from the south Sumatra and Java region around later November and then progressively approaches the Australian continent in late December. Nevertheless, significant biases exist in the modeled onset/retreat dates and the extent of the monsoon inland penetration. Under global warming, the agreement among the model projections varies across the domain. In between the Sumatra-Java archipelago and the top end of the Australian continent, over 80 % of the models simulate delayed monsoon onset and shortened duration by ~10 days, but less model agreement is seen over interior continent where the model ensembles show an approximate 7-day delay of both the onset and retreat with relatively little change in duration. Both El Nino-Southern Oscillation and Indian Ocean SST patterns appear to play important roles in determining the variations of the modeled monsoon onset. Nevertheless, the extent of their influence varies significantly across the models. Under global warming, a large proportion of models show relatively less warming in the eastern Indian Ocean and with a consequent increase in the modeled Indian Ocean Dipole index. Both a weakened and/or eastward shift of the upward branch of Walker circulation and the Indian Ocean contribute to the simulated delayed onset and shortened duration in the tropics under global warming.

Calendar effect on phase study in paleoclimate transient simulation with orbital forcing

Several studies have shown that the use of different calendars in paleoclimate simulations can cause artificial phase shifts on insolation forcing and climatic responses. However, these important calendar corrections are still often neglected. In this paper, the phase shifts at the precession band is quantitatively assessed by converting the model data of the transient GCM climate simulation of Kutzbach et al. (Clim Dyn 30:567–579, 2008) from the “fixed-day” calendar to the “fixed-angular” calendar with a new and efficient approach. We find that insolation has a big phase shift in September–October–November (SON) when the vernal equinox (VE) is fixed to March 21. At high latitude, the phase bias is up to 60° (about 3650 years). The insolation phase bias in SON in Southern Hemisphere (SH) is especially important because it can influence the timing of the SH summer monsoon response due to the large heat capacity of ocean. The calendar correction has minor effect (±2°) on the phase relationships between forcing and precipitation responses of the six global summer monsoons studied in Kutzbach et al. (2008). After correcting the calendar effect, especial on SH ocean temperature, the new phase wheel results are more similar for both hemispheres. The results suggest that the calendar effect should be corrected before discussing the dynamics between orbital forcing and climatic responses in phase studies of transient simulations.

Intensified Asian Summer Monsoon and its variability in a coupled model forced by increasing greenhouse gas concentrations

The Asian summer monsoon response to global warming is investigated by a transient green‐house warming integration with the ECHAM4/OPYC3 CGCM. It is demonstrated that increases of greenhouse gas concentrations intensify the Asian summer monsoon and its variability. The intensified monsoon results mainly from an enhanced land‐sea contrast and a northward shift of the convergence zone. A gradual increase of the monsoon variability is simulated from year 2030 onwards. It seems to be connected with the corresponding increase of the sea surface temperature variability over the tropical Pacific.

Climate Simulation for 125 kyr BP with a Coupled Ocean–Atmosphere General Circulation Model

The ECHAM-1 T21/LSG coupled ocean‐atmosphere general circulation model (GCM) is used to simulate climatic conditions at the last interglacial maximum (Eemian, 125 kyr BP). The results reflect the expected surface temperature changes (with respect to the control run) due to the amplification (reduction) of the seasonal cycle of insolation in the Northern (Southern) Hemisphere. A number of simulated features agree with previous results from atmospheric GCM simulations (e.g., intensified summer southwest monsoons) except in the Northern Hemisphere poleward of 308N, where dynamical feedbacks in the North Atlantic and North Pacific increase zonal temperatures about 18C above what would be predicted from simple energy balance considerations. As this is the same area where most of the terrestrial geological data originate, this result suggests that previous estimates of Eemian global average temperature might have been biased by sample distribution. This conclusion is supported by the fact that the estimated global temperature increase of only 0.3 8C greater than the control run has been previously shown to be consistent with CLIMAP sea surface temperature estimates. Although the Northern Hemisphere summer monsoon is intensified, globally averaged precipitation over land is within about 1% of the present, contravening some geological inferences but not the deep-sea d13C estimates of terrestrial carbon storage changes. Winter circulation changes in the northern Arabian Sea, driven by strong cooling on land, are as large as summer circulation changes that are the usual focus of interest, suggesting that interpreting variations in the Arabian Sea sedimentary record solely in terms of the summer monsoon response could sometimes lead to errors. A small monsoonal response over northern South America suggests that interglacial

Summer monsoon response of the Northern Somali Current, 1995

Preliminary results on the development of the northern Somali Current regime and Great Whirl during the summer monsoon of 1995 are reported. They are based on the water mass and current profiling observations from three shipboard surveys of R/V Meteor and on the time series from a moored current‐meter and ADCP array. The monsoon response of the GW was deep‐reaching, to more than 1000m. involving large deep transports. The northern Somali Current was found to be disconnected from the interior Arabian Sea in latitude range 4°N–12°N in both, water mass properties and current fields. Instead, communication dominantly occurs through the passages between Socotra and the African continent. From moored stations in the main passage a northward throughflow from the Somali Current to the Gulf of Aden of about 5 Sv was determined for the summer monsoon of 1995.