Submicron aerosols over Indian Ocean: some meteorological characteristics

The concentrations of submicron aerosols in the size range 10⁻⁷ to 10⁻⁵ cm, also called Aitken nuclei (AN) were measured over the Indian Ocean enroute India-Antarctica-India within the 10°E–70°E longitude zone from about 10°N to 70°S latitude on board MV Thuleland during the period from November 26, 1986 to March 18, 1987 as part of the scientific activities on the Sixth Indian Antarctic Expedition. Our analyses showed that only in about 25% of the cases, AN count fell below 1000 cm⁻³. Throughout the tropical trade wind region, the concentrations of AN were relatively stable with an average of about 3000 cm⁻³ (medians of 2600 and 1700 cm⁻³ in Northern and Southern Hemispheres, respectively). Large AN concentrations were found to be associated with higher sea surface temperatures and stronger surface winds in this region. In contrast, the scatter of single observations was found to be remarkable over South Indian Ocean and in Antarctic waters. The average AN concentration over the Indian Ocean to the south of 30°S was of the order of 1500 cm⁻³. No definite correlation could be established between large AN concentration and sea surface temperature, wind speed or wave height. Period with very low concentrations were, however, associated with clear sky conditions and calm winds or light breeze. Many events of sudden short-lived but large increase in AN concentrations were observed over the south Indian Ocean and in Antarctic waters and these were always associated with the approach of frontal systems. It is likely that particle production by bursting bubbles and sea spray as well as photochemical reactions and gas-to-particle conversions play important role in the observed high concentration of AN over South Indian Ocean.     

Simulation of the anthropogenic aerosols over South Asia and their effects on Indian summer monsoon

A regional climate model coupled with a chemistry-aerosol model is employed to simulate the anthropogenic aerosols including sulfate, black carbon and organic carbon and their direct effect on climate over South Asia. The model is driven by the NCAR/NCEP re-analysis data. Multi-year simulations with half, normal and double emission fluxes are conducted. Results show that the model performs well in reproducing present climate over the region. Simulations of the aerosol optical depth and surface concentration of aerosols are also reasonable although to a less extent. The negative radiative forcing is found at the top of atmosphere and largely depended on emission concentration. Surface air temperature decreases by 0.1?C0.5°C both in pre-monsoon and monsoon seasons. The range and intensity of cooling areas enlarge while aerosol emission increases. Changes in precipitation are between ?25 and 25%. Different diversifications of rainfall are showed with three emission scenarios. The changes of precipitation are consistent with varieties of monsoon onset dates in pre-monsoon season. In the regions of increasing precipitation, monsoon onset is advanced and vice versa. In northeast India and Myanmar, aerosols lead the India summer monsoon onset advancing 1?C2 pentads, and delaying by 1?C2 pentads in central and southeast India. These changes are mainly caused by the anomaly of local Hadley circulations and enhancive precipitation. Tibetan Plateau played a crucial role in the circulation changes.     

Biogenic isoprene emissions over China: sensitivity to the CO2 inhibition effect

陆地植被排放的异戊二烯是对流层臭氧及二次有机气溶胶的形成重要前体物之一。已有研究表明,当CO_2浓度超过一定水平时可能使得叶片气孔关闭,对叶片释放异戊二烯产生直接的抑制作用。而这一影响机制在目前大多数异戊二烯排放估算时并没有考虑在内,对其排放的估算仍存在很大的不确定性。本文基于GEOS-Chem及其耦合的MEGAN模式模拟了2006–2011年中国异戊二烯的排放变化。通过引入三种不同CO_2抑制作用参数化因子的模拟试验,定量评估了CO_2抑制作用对异戊二烯排放的影响及不确定性。结果表明:考虑CO_2抑制参数因子后,中国年平均异戊二烯的排放量平均减少了5.6%±2.3%。不同参数化方案对排放的抑制程度存在差异。CO_2对异戊二烯排放的影响将会改变其对气象条件变化的敏感性,从而影响空气质量。     

Absorbing aerosols and pre-summer monsoon hydroclimate variability over the Indian subcontinent: The challenge in investigating links

The sub-monthly evolution of the interannual variations of absorbing aerosols and related hydrometeorology over South Asia in the pre-monsoon period is investigated from the analysis of pentad-resolution observational datasets.It is shown that pre-monsoon (late April–early May) variations are characterized by increased aerosols, reduced cloudiness and precipitation, and increased downward shortwave radiation. Lead-lag regressions indicate the significant influence of synoptic scale advection (and related vertical motion) in simultaneously shaping the aerosol distribution and associated significant hydroclimate (precipitation, cloudiness, surface shortwave radiation, and 2-m air temperature) over the Indo-Gangetic Plain.The above findings can be interpreted as a manifestation of the aerosol “semi-direct” effect if one is not mindful of the prevailing circulation anomalies and their concurrent impact on aerosol and hydroclimate. The complex interplay among aerosols, dynamics and precipitation also shows the challenge of extracting the aerosol impact from an observational analysis. Finally, the analysis points to the pitfalls of a columnar, circulation-blind framework in investigating aerosol–monsoon interactions, a concern of relevance in analyses of the impact of long-term aerosol trends, as well.     

Competing impact of anthropogenic emissions and meteorology on the distribution of trace gases over Indian region

The spatial distribution of trace gases exhibit large spatial heterogeneity over the Indian region with an elevated pollution loading over densely populated Gangetic Plains (IGP). The contending role and importance of anthropogenic emissions and meteorology in deciding the trace gases level and distribution over Indian region, however, is poorly investigated. In this paper, we use an online regional chemistry transport model (WRF/Chem) to simulate the spatial distribution of trace gases over Indian region during one representative month of only three meteorological seasons namely winter, spring/summer and monsoon. The base simulation, using anthropogenic emissions from SEAC⁴RS inventory, is used to simulate the general meteorological conditions and the realistic spatial distribution of trace gases. A sensitivity simulation is conducted after removing the spatial heterogeneity in the anthropogenic emissions, i.e., with spatially uniform emissions to decouple the role of anthropogenic emissions and meteorology and their role in controlling the distribution of trace gases over India. The concentration levels of Ozone, CO, SO₂ and NO₂ were found to be lower over IGP when the emissions are uniform over India. A comparison of the base run with the sensitivity run highlights that meteorology plays a dominant role in controlling the spatial distribution of relatively longer-lived species like CO and secondary species like Ozone while short-lived species like NO_X and SO₂ are predominantly controlled by the spatial variability in anthropogenic emissions over the Indian region.     

A study of marine aerosols over the Pacific Ocean

Aerosol samples were collected on a Pacific cruise from 47°N to 55°S. Particle morphology, concentrations, and size distributions were analyzed with an electron microscope; elemental compositions of individual particles were determined with an X-ray energy spectrometer; and chemical compositions of bulk samples were measured with an ion chromatograph. Temporal and spatial variations of aerosol physico-chemical characteristics were studied in relation to ocean currents and atmospheric parameters. The results show that number and mass concentrations of primary particles depend mainly on surface wind speeds. However the ratios between the major ions, e.g., Na⁺, Cl^-, and Mg⁺⁺, are similar to the ratios in seawater regardless of location or meteorological conditions. The concentrations of secondary aerosols, e.g., non-seasalt sulfate, nitrate, and ammonium particles, show maxima at upwelling regions, such as along the California coast, at the Equator, and near the Chatham Rise where ascending motion brings nutrient-riched deep water into the surface layer. The number concentrations of small sulfate particles and large nitrate-coated particles showed diurnal variations with maxima in the early afternoon and minima at night, indicating that the particles are the products of photo-chemical reactions. Their precursor gases, e.g., CH₃SCH₃, NO, and NH₃ are known to be released from seawater in upwelling regions where biological activities thrive.     

Temporal and spatial variations of marine aerosols over the Atlantic Ocean

Aerosol samples, collected daily were analyzed with electron microscopy for particle concentration and size distribution. The chemical compositions of individual particles were determined with X-ray energy spectrometry and specific morphological tests. The results demonstrated clear and significant dependencies of aerosol properties on air trajectories computed from a backtrack model. Air parcels, such as American air pollution and Sahara dust, were identified. Their physical mixing and chemical reactions with marine aerosols as well as aerosol wet deposition are discussed.     

Temperature variability over the Indian Ocean and its relationship with Indian summer monsoon rainfall

Summary The present study examines the long term trend in sea surface temperatures (SSTs) of the Arabian Sea, Bay of Bengal and Equatorial South India Ocean in the context of global warming for the period 1901–2002 and for a subset period 1971–2002. An attempt has also been made to identify the relationship between SST variations over three different ocean areas, and All-India and homogeneous region summer monsoon rainfall variability, including the role of El-Ni?o/Southern Oscillation (ENSO). Annual sea surface temperatures of the Arabian Sea, Bay of Bengal and Equatorial South India Ocean show a significant warming trend of 0.7 °C, 0.6 °C and 0.5 °C per hundred years, respectively, and a relatively accelerated warming of 0.16 °C, 0.14 °C and 0.14 °C per decade during the 1971–2002 period. There is a positive and statistically significant relationship between SSTs over the Arabian Sea from the preceding November to the current February, and Indian monsoon rainfall during the period 1901–2002. The correlation coefficient increases from October and peaks in December, decreasing from February to September. This significant relationship is also found in the recent period 1971–2002, whereas, during 1901–70, the relationship is not significant. On the seasonal scale, Arabian Sea winter SSTs are positively and significantly correlated with Indian monsoon rainfall, while spring SSTs have no significant positive relationship. Nino3 spring SSTs have a negative significant relationship with Indian monsoon rainfall and it is postulated that there is a combined effect of Nino3 and Arabian Sea SSTs on Indian monsoon. If the Nino3 SST effect is removed, the spring SSTs over the Arabian Sea also have a significant relationship with monsoon rainfall. Similarly, the Bay of Bengal and Equatorial South Indian Ocean spring SSTs are significantly and positively correlated with Indian monsoon rainfall after removing the Nino3 effect, and correlation values are more pronounced than for the Arabian Sea. Authors’ address: Dr. D. R. Kothawale, A. A. Munot, H. P. Borgaonkar, Climatology and Hydrometeorology divisions, Indian Institute of Tropical Meteorology, Pune 411008, India.     

人类活动排放的CO2及气溶胶对20世纪70年代末中国东部夏季降水年代际转折的影响

全球变暖背景下,中国东部夏季降水在20世纪70年代末开始较19世纪呈现东北及长江中、下游地区多雨,华北及华南少雨的特征。与此同时,人类活动排放的CO₂及气溶胶量也发生了明显的年代际变化。文中利用地球系统耦合模式（CESM）诊断了中国东部夏季的水分收支对人类活动排放的CO₂及气溶胶年代际变化的响应。发现CO₂排放量增加后,江淮流域的水汽辐合以及中国南方的水汽辐散主要是与质量辐散有关的动力项及与湿度梯度相关的热力项共同作用的结果,但动力作用更显著。气溶胶效应则主要通过动力作用使得江淮流域水汽辐合,而中国南方地区水汽辐散。虽然CO₂和气溶胶对辐射量及温度的影响差别很大,但通过改变温度梯度,热成风效应产生的动力作用都会导致江淮流域上升运动增强,降水增多;而中国南方下沉运动显著,降水减少,与观测结果一致,且CO₂相较于气溶胶的影响更为显著,证实了20世纪70年代末人类活动对中国东部夏季降水年代际转折的影响。      

Peninsular Indian rainfall and its association with meteorological and oceanic parameters over the Indian Ocean

The present study attempts to formulate a regression model to predict summer rainfall over Peninsular India (PIR) using some regional predictors. Parameters having significant correlation (99%) with PIR were identified for the period 1975–1997 (training), and a 15-year sliding correlation (90%) was found to check the consistency of the relationship between PIR and predictors. From a set of 14 candidate predictors, 4 were selected using a stepwise regression method and tested over a period from 1998 to 2006. The predictors selected are sea surface temperature during March over Indian Ocean, air temperature at 850?hPa during May over Peninsular India, zonal, and meridional wind at 700?hPa during February and January, respectively, over the Arabian Sea. The model captures a variance of 77.7% and has a multiple correlation of 0.88. The root mean square error, absolute mean error, and bias for the training (test) period were 7.6% (21.5%), 6.6% (17.9%), and 0% (11.4%) of mean rainfall, respectively. Results of the climatological predictions show that the model developed is useful.     

Remote sensing of aerosols over the Solar Village, Saudi Arabia

Aerosol optical properties over Solar Village, Saudi Arabia have been studied using ground-based remote sensing observations through the Aerosol Robotic Network (AERONET). Our analysis covered 8 recorded years of aerosol measurements, starting from February 1999 through January 2007. The seasonal mean values of aerosol optical thickness (AOT), the Ångström wavelength exponent α and the surface wind speed (V), exhibit a one year cyclical pattern. Seasonal variations are clearly found in the shape and magnitude of the volume size distribution (VSD) of the coarse size mode due to dust emission. The Spring is characterized by dusty aerosols as the modal value of the exponent α was low ~ 0.25 while that of AOT was high ~ 0.3. The modal value of wind speed was the highest ~ 3.6 m/s in spring. The increase in wind speed is responsible for increasing the concentration of dust particles during Spring. Spring of 2003 has the highest mean values of AOT, V and VSD and the lowest mean value for the exponent α. The seasonal mean values of the exponent α are anticorrelated with those of the wind speed (r = − 0.63). The annual mean values of the exponent α are well correlated (r = 0.77) with those of the difference between the maximum and minimum values of temperature ΔT. They are anticorrelated (r = − 0.74) with the annual mean values of the relative humidity. Large aerosol particles and high relative humidity increase the radiative forcing. This results in reduction of the values of the temperature difference ΔT.     

Intraseasonal and interannual zonal circulations over the Equatorial Indian Ocean

El Ni?o Southern Oscillation (ENSO) and given phases of the Madden?CJulian Oscillation (MJO) show similar regional signatures over the Equatorial Indian Ocean, consisting in an enhancement or reversing of the convective and dynamic zonal gradients between East Africa and the Maritime Continent of Indonesia. This study analyses how these two modes of variability add or cancel their effects at their respective timescales, through an investigation of the equatorial cellular circulations over the central Indian Ocean. Results show that (1) the wind shear between the lower and upper troposphere is related to marked regional rainfall anomalies and is embedded in larger-scale atmospheric configurations, involving the Southern Oscillation; (2) the intraseasonal (30?C60?days) and interannual (4?C5?years) timescales are the most energetic frequencies that modulate these circulations, confirming the implication of the MJO and ENSO; (3) extreme values of the Indian Ocean wind shear result from the combination of El Ni?o and the MJO phase enhancing atmospheric convection over Africa, or La Ni?a and the MJO phase associated with convective activity over the Maritime Continent. Consequences for regional rainfall anomalies over East Africa and Indonesia are then discussed.     

Size distribution and chemical composition of summer aerosols over Southern Ocean and the Antarctic region

Journal of Atmospheric Chemistry - The size distribution of atmospheric aerosols together with their composition, sources and sinks, is a key element in understanding aerosol effects on the...     

Spatial asymmetry of temperature trends over India and possible role of aerosols

Dissimilarities in temperature trends in space and time over the Indian region have been examined to look for signatures of aerosols’ influence. Separate temperature time series for North and South India were constructed for dry (November–May) and wet (June–October) seasons. Temperature trend for the entire period 1901–2007 and different subperiods of 1901–1950, 1951–1990, 1971–2007, and 1991–2007 have been examined to isolate the aerosol and other greenhouse gas influences on temperatures. Maximum (daytime) temperatures during dry season corresponding to North and South India show significant warming trend of 0.8 and 1.0?°C per hundred years during the period 1901–2007, while minimum temperature shows nebulous trend of 0.2 and 0.3?°C per hundred years over North and South India, respectively. During the wet season, maximum temperature shows nearly half of dry season maximum temperature warming trend. However, asymmetry is observed in dry season maximum temperature trend during post-industrial period 1951–1990 wherein the North/South India shows decreasing/increasing trends, while during the recent period 1991–2007 trends are uniformly positive for both the regions. Spatial and temporal asymmetry in observed trends clearly point to the role of aerosols in lowering temperature trends over northern India. Atmospheric aerosols could cause a negative climate forcing that can modulate the regional surface temperature trends in a significant way. As this forcing acts differentially on day and night temperatures, trends in diurnal temperature range (DTR) provide a direct assessment of impacts of aerosols on temperature trends. Time series of diurnal temperature range for dry and wet seasons have been examined separately for North and South India. Over North India, the DTR for dry season has increased gradually during the period 1901–1970 and thereafter showed decreasing trend, while trends in temperature range over Southern India were almost opposite in phase with North India. The aerosol and greenhouse gases seem to play an important role in the spatial and temporal variability of temperature range over India.     

Association between extreme monsoons and the dipole mode over the Indian subcontinent

Summary The relationship of summer monsoon over India with the Indian Ocean Dipole Mode has been investigated applying simple statistical techniques. While a long time series of 132 years based on 1871–2002 for both summer monsoon rainfall as well as dipole mode index has been used in this study, the NCEP–NCAR reanalysis data (1948–2002) are used to examine the circulation features associated with the extreme dipole and monsoon phases. These flow patterns bring out the dynamics of the dipole – monsoon relationship. Lead/lag correlations between the dipole mode index and the Indian monsoon rainfall are computed. Results reveal that numerically the relationship is stronger following the monsoon. The lower troposphere flow patterns at 850 hPa associated with the extreme phases of the dipole and monsoon are consistent with the correlation analysis. Further a strong (weak) summer monsoon favours the development of the negative (positive) dipole event in autumn. The sliding correlations between Indian monsoon rainfall and the dipole mode index suggest that the impact of monsoon over dipole is weakening after 1960s. This weakening relationship has been evidenced by the composites of sea-surface temperature anomalies and circulation patterns. All the above analysis suggests that the summer monsoon has more influence on the dipole mode than vice-a-versa.     

Simulation of the radiative effect of black carbon aerosols and the regional climate responses over China

As part of the development work of the Chinese new regional climate model (RIEMS), the radiative process of black carbon (BC) aerosols has been introduced into the original radiative procedures of RIEMS,and the transport model of BC aerosols has also been established and combined with the RIEMS model.Using the new model system, the distribution of black carbon aerosols and their radiative effect over the China region are investigated. The influences of BC aerosole on the atmospheric radiative transfer and on the air temperature, land surface temperature, and total rainfall are analyzed. It is found that BC aerosols induce a positive radiative forcing at the top of the atmosphere (TOA), which is dominated by shortwave radiative forcing. The maximum radiative forcing occurs in North China in July and in South China in April. At the same time, negative radiative forcing is observed on the surface. Based on the radiative forcing comparison between clear sky and cloudy sky, it is found that cloud can enforce the TOA positive radiative forcing and decrease the negative surface radiative forcing. The responses of the climate system in July to the radiative forcing due to BC aerosols are the decrease in the air temperature in the middle and lower reaches of the Changjiang River and Huaihe area and most areas of South China, and the weak increase or decrease in air temperature over North China. The total rainfall in the middle and lower reaches of the Changjiang River area is increased, but it decreased in North China in July.