scholarly journals The South Asian monsoon—pollution pump and purifier

Science ◽  
2018 ◽  
Vol 361 (6399) ◽  
pp. 270-273 ◽  
Author(s):  
J. Lelieveld ◽  
E. Bourtsoukidis ◽  
C. Brühl ◽  
H. Fischer ◽  
H. Fuchs ◽  
...  
Keyword(s):  
Indian Ocean ◽  
South Asian ◽  
Summer Monsoon ◽  
Atmospheric Chemistry ◽  
Asian Monsoon ◽  
Oxidation Mechanism ◽  
Winter Monsoon ◽  
Model Calculations ◽  
Upper Troposphere ◽  
The Indian Ocean

Air pollution is growing fastest in monsoon-affected South Asia. During the dry winter monsoon, the fumes disperse toward the Indian Ocean, creating a vast pollution haze, but their fate during the wet summer monsoon has been unclear. We performed atmospheric chemistry measurements by aircraft in the Oxidation Mechanism Observations campaign, sampling the summer monsoon outflow in the upper troposphere between the Mediterranean and the Indian Ocean. The measurements, supported by model calculations, show that the monsoon sustains a remarkably efficient cleansing mechanism by which contaminants are rapidly oxidized and deposited to Earth’s surface. However, some pollutants are lofted above the monsoon clouds and chemically processed in a reactive reservoir before being redistributed globally, including to the stratosphere.

Hemispheric Insolation Forcing of the Indian Ocean and Asian Monsoon: Local versus Remote Impacts*

2006 ◽  
Vol 19 (23) ◽  
pp. 6195-6208 ◽  
Author(s):  
Xiaodong Liu ◽  
Zhengyu Liu ◽  
John E. Kutzbach ◽  
Steven C. Clemens ◽  
Warren L. Prell
Keyword(s):  
Indian Ocean ◽  
South Asian ◽  
Summer Monsoon ◽  
Asian Monsoon ◽  
Tropical Indian Ocean ◽  
Precipitable Water ◽  
The South ◽  
Summer Insolation ◽  
The Indian Ocean ◽  
Insolation Forcing

Abstract Insolation forcing related to the earth’s orbital parameters is known to play an important role in regulating variations of the South Asian monsoon on geological time scales. The influence of insolation forcing on the Indian Ocean and Asian monsoon is studied in this paper by isolating the Northern and Southern Hemispheric insolation changes in several numerical experiments with a coupled ocean–atmosphere model. The focus is on the response of South Asian summer rainfall (monsoon strength) with emphasis on impacts of the local versus remote forcing and possible mechanisms. The model results show that both Northern Hemisphere (NH) and Southern Hemisphere (SH) summer insolation changes affect the Indian Ocean and Asian monsoon as a local forcing (in the same hemisphere), but only the SH changes result in remote (in the other hemisphere) forcing. The NH insolation change has a local and immediate impact on NH summer monsoons from North Africa to South and East Asia, while the SH insolation change has a remote and seasonal-scale delayed effect on the South Asian summer monsoon rainfall. When the SH insolation is increased from December to April, the sea surface temperature (SST) in the southern tropical Indian Ocean remains high from January to July. The increased SST produces more atmospheric precipitable water over the southern tropical Indian Ocean by promoting evaporation from the ocean. The enhanced precipitable water over the southern Indian Ocean is transported northward to the South Asian monsoon region by the lower-tropospheric mean cross-equatorial flows with the onset of the Asian monsoon increasing precipitable water over South Asia, eventually leading to the increase of Indian summer monsoon precipitation. Thus, these model experiments, while idealized and not fully representing actual orbitally forced insolation changes, confirm the broadscale response of northern monsoons to NH summer insolation increases and also illustrate how SH summer insolation increases can have a delayed influence on the Indian summer monsoon.

scholarly journals Relative Roles of Land–Sea Distribution and Orography in Asian Monsoon Intensity

2009 ◽  
Vol 66 (9) ◽  
pp. 2714-2729 ◽  
Author(s):  
Zhongfeng Xu ◽  
Congbin Fu ◽  
Yongfu Qian
Keyword(s):  
Indian Ocean ◽  
South Asian ◽  
Summer Monsoon ◽  
Large Scale ◽  
Asian Monsoon ◽  
Asian Summer Monsoon ◽  
Monsoon Circulation ◽  
Eurasian Continent ◽  
The Indian Ocean ◽  
Asian Summer

Abstract The relative impacts of various land–sea distributions (LSDs) and mountains on Asian monsoon extent and intensity are assessed using a series of AGCM simulations. The air–sea coupling effects are not considered in this study. All simulations were integrated with zonal mean SST, globally uniform vegetation, soil color, and, except several simulations, soil texture. The results show that the LSD plays a more fundamental role than orography in determining the extent of Asian and African monsoons. The tropical zonal LSD and Asian mountains both play a crucial role for establishing summer monsoon convection over the South Asian region. The monsoon circulation index (MCI1) defined by the difference of zonal wind between 850 and 200 hPa is used to measure the intensity of the South Asian summer monsoon. The large-scale meridional land–sea thermal contrast between the Eurasian continent and the Indian Ocean only induces a 1.8 m s−1 increase of MCI1. The presence of the Indian subcontinent and Indochina peninsula (Asian mountains), however, induces a 6.6 (7.4) m s−1 increase of MCI1 associated with the release of latent heat of condensation. Clearly, the tropical subcontinental-scale zonal LSD and the Asian mountains almost equally contribute to the increase of MCI1 and play a more important role than the large-scale meridional LSD between the Eurasian continent and the Indian Ocean. Possible mechanisms of how the tropical subcontinental-scale zonal LSD and Asian mountains impact the Asian summer monsoon circulation and precipitation are also discussed.

scholarly journals Opposite responses of the Indian Ocean to the thermal forcing of the Tibetan Plateau before and after the onset of the South Asian monsoon

2021 ◽  
pp. 1-56
Author(s):  
Yu Zhao ◽  
Anmin Duan ◽  
Guoxiong Wu
Keyword(s):  
Indian Ocean ◽  
Tibetan Plateau ◽  
South Asian ◽  
Asian Monsoon ◽  
Monsoon Onset ◽  
The Tibetan Plateau ◽  
Thermal Forcing ◽  
The North ◽  
Before And After ◽  
The Indian Ocean

AbstractThe atmospheric circulation changes dramatically over a few days before and after the onset of the South Asian monsoon in spring. It is accompanied by the annual maximum surface heating over the Tibetan Plateau. We conducted two sets of experiments with a coupled general circulation model to compare the response of atmospheric circulation and wind-driven circulation in the Indian Ocean to the thermal forcing of the Tibetan Plateau before and after the monsoon onset. The results show that the Tibetan Plateau's thermal forcing modulates the sea surface temperature (SST) of the Indian Ocean and the meridional circulation in the upper ocean with opposite effects during these two stages. The thermal forcing of the Tibetan Plateau always induces a southwesterly response over the northern Indian Ocean and weakens the northeasterly background circulation before the monsoon onset. Subsequently, wind-evaporation feedback results in a warming SST response. Meanwhile, the oceanic meridional circulation shows offshore upwellings in the north and southward transport in the upper layer crossing the equator, sinking near 15°S. After the monsoon onset, the thermal forcing of the Tibetan Plateau accelerates the background southwesterly and introduces a cooling response to the Indian Ocean SST. The response of oceanic meridional overturning circulation is limited to the north of the equator due to the location and structural evolution of the climatological local Hadley circulation. With an acceleration of the local Walker circulation, the underlying zonal currents also show corresponding changes, including a westerly drift along the equator, downwelling near Indonesia, offshore upwelling near Somalia, and a westward undercurrent.

scholarly journals Impact of the South Asian monsoon outflow on atmospheric hydroperoxides in the upper troposphere

2020 ◽  
Author(s):  
Bettina Hottmann ◽  
Sascha Hafermann ◽  
Laura Tomsche ◽  
Daniel Marno ◽  
Monica Martinez ◽  
...  
Keyword(s):  
Steady State ◽  
South Asian ◽  
Summer Monsoon ◽  
Asian Monsoon ◽  
Convective Transport ◽  
South Asian Summer Monsoon ◽  
The South ◽  
Upper Troposphere ◽  
Mixing Ratios ◽  
Longitudinal Gradients

Abstract. During the OMO (Oxidation Mechanism Observation) mission, trace gas measurements were performed onboard the HALO (High Altitude LOng range) research aircraft in summer 2015 in order to investigate the outflow of the south Asian summer monsoon and its influence on the composition of the Asian Monsoon Anticyclone (AMA) in the upper troposphere over the eastern Mediterranean and the Arabian Peninsula. This study focuses on in situ observations of hydrogen peroxide (H2O2) and organic hydroperoxides (ROOH), as well as their precursors and loss processes. Observations are compared to steady state calculations of H2O2, methyl hydroperoxide (MHP) and inferred unidentified hydroperoxide (UHP) mixing ratios. Measurements are also contrasted to simulations with the general circulation ECHAM/MESSy for Atmospheric Chemistry (EMAC) model. We observed enhanced mixing ratios of H2O2, MHP and UHP in the AMA relative to the northern hemispheric background. Highest concentrations for H2O2 and MHP were found in the southern hemisphere outside the AMA, while for UHP, highest concentrations were found within the AMA. In general, the observed concentrations are higher than steady-state calculations and EMAC simulations. Especially in the AMA, EMAC underestimates the H2O2 and ROOH mixing ratios. Longitudinal gradients indicate a pool of hydroperoxides towards the center of the AMA, most likely associated with upwind convection over India. This indicates main contributions of atmospheric transport to the local budgets of hydroperoxides along the flight track, explaining strong deviations to steady-state calculations which only accounts for local photochemistry. Deviations to EMAC simulations are most likely due to uncertainties in the scavenging efficiencies for individual hydroperoxides in deep convective transport to the upper troposphere, corroborated by a sensitivity study. It seems that the observed excess UHP is excess MHP transported to the west from an upper tropospheric source related to convection in the summer monsoon over South-East Asia.

scholarly journals Impact of the South Asian monsoon outflow on atmospheric hydroperoxides in the upper troposphere

2020 ◽  
Vol 20 (21) ◽  
pp. 12655-12673
Author(s):  
Bettina Hottmann ◽  
Sascha Hafermann ◽  
Laura Tomsche ◽  
Daniel Marno ◽  
Monica Martinez ◽  
...  
Keyword(s):  
Steady State ◽  
South Asian ◽  
Summer Monsoon ◽  
Asian Monsoon ◽  
Convective Transport ◽  
South Asian Summer Monsoon ◽  
The South ◽  
Upper Troposphere ◽  
Mixing Ratios ◽  
Longitudinal Gradients

Abstract. During the OMO (Oxidation Mechanism Observation) mission, trace gas measurements were performed on board the HALO (High Altitude Long Range) research aircraft in summer 2015 in order to investigate the outflow of the South Asian summer monsoon and its influence on the composition of the Asian monsoon anticyclone (AMA) in the upper troposphere over the eastern Mediterranean and the Arabian Peninsula. This study focuses on in situ observations of hydrogen peroxide (H2O2obs) and organic hydroperoxides (ROOHobs) as well as their precursors and loss processes. Observations are compared to photostationary-state (PSS) calculations of H2O2PSS and extended by a separation of ROOHobs into methyl hydroperoxide (MHPPSS) and inferred unidentified hydroperoxide (UHPPSS) mixing ratios using PSS calculations. Measurements are also contrasted to simulations with the general circulation ECHAM–MESSy for Atmospheric Chemistry (EMAC) model. We observed enhanced mixing ratios of H2O2obs (45 %), MHPPSS (9 %), and UHPPSS (136 %) in the AMA relative to the northern hemispheric background. Highest concentrations for H2O2obs and MHPPSS of 211 and 152 ppbv, respectively, were found in the tropics outside the AMA, while for UHPPSS, with 208 pptv, highest concentrations were found within the AMA. In general, the observed concentrations are higher than steady-state calculations and EMAC simulations by a factor of 3 and 2, respectively. Especially in the AMA, EMAC underestimates the H2O2EMAC (medians: 71 pptv vs. 164 pptv) and ROOHEMAC (medians: 25 pptv vs. 278 pptv) mixing ratios. Longitudinal gradients indicate a pool of hydroperoxides towards the center of the AMA, most likely associated with upwind convection over India. This indicates main contributions of atmospheric transport to the local budgets of hydroperoxides along the flight track, explaining strong deviations from steady-state calculations which only account for local photochemistry. Underestimation of H2O2EMAC by approximately a factor of 2 in the Northern Hemisphere (NH) and the AMA and overestimation in the Southern Hemisphere (SH; factor 1.3) are most likely due to uncertainties in the scavenging efficiencies for individual hydroperoxides in deep convective transport to the upper troposphere, corroborated by a sensitivity study. It seems that the observed excess UHPPSS is excess MHP transported to the west from an upper tropospheric source related to convection in the summer monsoon over Southeast Asia.

Linking Observations of the Asian Monsoon to the Indian Ocean SST: Possible Roles of Indian Ocean Basin Mode and Dipole Mode

2010 ◽  
Vol 23 (21) ◽  
pp. 5889-5902 ◽  
Author(s):  
Jianling Yang ◽  
Qinyu Liu ◽  
Zhengyu Liu
Keyword(s):  
Indian Ocean ◽  
Atmospheric Circulation ◽  
Summer Monsoon ◽  
Asian Monsoon ◽  
Asian Summer Monsoon ◽  
Ocean Basin ◽  
Indian Ocean Basin Mode ◽  
Basin Mode ◽  
The Indian Ocean ◽  
Asian Summer

Abstract The authors investigate the relationship between sea surface temperature (SST) in the tropical Indian Ocean (TIO) and the seasonal atmosphere circulation in the Asian monsoon region (AMR) using the maximum covariance analyses (MCAs). The results show that the Asian monsoon circulation is significantly correlated with two dominant SST anomaly (SSTA) modes: the Indian Ocean Basin mode (IOB) and the Indian Ocean dipole mode (IOD). The peak SSTA of the IOB appears in spring and has a much stronger relationship with the Asian summer monsoon than the peak of the IOD does, whereas the peak SSTA for the IOD appears in fall and shows a stronger link to the Asian winter monsoon than to the Asian summer monsoon. In addition, the IOB in spring has a relatively stronger link with the atmospheric circulation in summer than in other seasons. The large-scale atmospheric circulation and SSTA patterns of the covariability of the first two dominant MCA modes are described. For the first MCA mode, a warm IOB, persists from spring to summer, and the atmospheric circulation is enhanced by the establishment of the climatological summer monsoon. The increased evaporative moisture associated with the warm IOB is transported to South Asia by the climatological summer monsoon, which increases the moisture convergence toward this region, leading to a significant increase in summer monsoon precipitation. For the second MCA mode, a positive IOD possibly corresponds to a weaker Indian winter monsoon and more precipitation over the southwestern and eastern equatorial TIO.

scholarly journals Secular and orbital-scale variability of equatorial Indian Ocean summer monsoon winds during the late Miocene

2021 ◽  
Author(s):  
Clara T. Bolton ◽  
Emmeline Gray ◽  
Wolfgang Kuhnt ◽  
Ann E. Holbourn ◽  
Julia Lübbers ◽  
...  
Keyword(s):  
Indian Ocean ◽  
South Asian ◽  
Summer Monsoon ◽  
Late Miocene ◽  
Asian Monsoon ◽  
Equatorial Indian Ocean ◽  
South Asian Monsoon ◽  
The South ◽  
Near Surface ◽  
Monsoon Winds

Abstract. In the modern northern Indian Ocean, biological productivity is intimately linked to near-surface oceanographic dynamics forced by the South Asian, or Indian, monsoon. In the late Pleistocene, this strong seasonal signal is transferred to the sedimentary record as strong variance in the precession band (19–23 kyr) because precession dominates low-latitude insolation variations and drives seasonal contrast in oceanographic conditions. In addition, internal climate system feedbacks (e.g. ice-sheet albedo, carbon cycle, topography) play a key role in monsoon variability. Little is known about orbital-scale variability of the monsoon in the pre-Pleistocene, when atmospheric CO2 levels and global temperatures were higher. In addition, many questions remain open regarding the timing of the initiation and intensification of the South Asian monsoon during the Miocene, an interval of significant global climate change that culminated in bipolar glaciation. Here, we present new high-resolution (< 1 kyr) records of export productivity and sediment accumulation from International Ocean Discovery Program Site U1443 in the southernmost Bay of Bengal spanning the late Miocene and earliest Pliocene (9 to 5 million years ago). Underpinned by a new orbitally-tuned benthic isotope stratigraphy, we use X-Ray Fluorescence-derived biogenic barium variations to discern productivity trends and rhythms. Our data show strong eccentricity-modulated precession-band productivity variations throughout the late Miocene, interpreted to reflect insolation forcing of summer monsoon wind strength in the equatorial Indian Ocean. On long timescales, our data support the interpretation that South Asian monsoon winds were already established by 9 Ma, with no apparent intensification over the late Miocene.

Australianama

2019 ◽  
Author(s):  
Samia Khatun
Keyword(s):  
Indian Ocean ◽  
South Asian ◽  
South Asian Diaspora ◽  
Post Cold War ◽  
Asian Diaspora ◽  
Broken Hill ◽  
History Of ◽  
The Indian Ocean ◽  
Anglophone World ◽  
Indian Ocean World

Australian deserts remain dotted with the ruins of old mosques. Beginning with a Bengali poetry collection discovered in a nineteenth-century mosque in the town of Broken Hill, Samia Khatun weaves together the stories of various peoples colonized by the British Empire to chart a history of South Asian diaspora. Australia has long been an outpost of Anglo empires in the Indian Ocean world, today the site of military infrastructure central to the surveillance of 'Muslim-majority' countries across the region. Imperial knowledges from Australian territories contribute significantly to the Islamic-Western binary of the post- Cold War era. In narrating a history of Indian Ocean connections from the perspectives of those colonized by the British, Khatun highlights alternative contexts against which to consider accounts of non-white people. Australianama challenges a central idea that powerfully shapes history books across the Anglophone world: the colonial myth that European knowledge traditions are superior to the epistemologies of the colonized. Arguing that Aboriginal and South Asian language sources are keys to the vast, complex libraries that belie colonized geographies, Khatun shows that stories in colonized tongues can transform the very ground from which we view past, present and future.

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