Centre for Atmospheric and Oceanic Sciences

Terrestrial and oceanic carbon sinks together sequester >50% of the anthropogenic emissions, and the major uncertainty in the global carbon budget is related to the terrestrial carbon cycle. Hence, it is important to understand the major drivers of the land carbon uptake to make informed decisions on climate change mitigation policies. In this paper, we assess the major drivers of the land carbon uptake-CO 2 fertilization, nitrogen deposition, climate change, and land use/land cover changes (LULCC)-from existing literature for the historical period and future scenarios, focusing on the results from fifth Coupled Models Intercomparison Project (CMIP5). The existing literature shows that the LULCC fluxes have led to a decline in the terrestrial carbon stocks during the historical period, despite positive contributions from CO 2 fertilization and nitrogen deposition. However, several studies find increases in the land carbon sink in recent decades and suggest that CO 2 fertilization is the primary driver (up to 85%) of this increase followed by nitrogen deposition (∼10%-20%). For the 21st century, terrestrial carbon stocks are projected to increase in the majority of CMIP5 simulations under the representative concentration pathway 2.6 (RCP2.6), RCP4.5, and RCP8.5 scenarios, mainly due to CO 2 fertilization. These projections indicate that the effects of nitrogen deposition in future scenarios are small (∼2%-10%), and climate warming would lead to a loss of land carbon. The vast majority of the studies consider the effects of only one or two of the drivers, impairing comprehensive assessments of the relative contributions of the drivers. Further, the broad range in magnitudes and scenario/model dependence of the sensitivity factors pose challenges in unambiguous projections of land carbon uptake. Improved representation of processes such as LULCC, fires, nutrient limitation and permafrost thawing in the models are necessary to constrain the present-day carbon cycle and for more accurate future projections.

The activities undertaken under the Mahatma Gandhi National Rural Employment Guarantee Act in Chitradurga district, Karnataka, were assessed for their potential to enhance and provide environmental services. Key programmes implemented in 20 villages during 2009 were studied using rapid scientific assessment methods. An indicator approach was adopted to analyse environmental services such as water for irrigation and improvement in soil quality. The status of environmental services before and after implementation of the activities was examined and vulnerability ...

The forecast of Indian monsoon droughts has been predicated on the notion of a seasonlong rainfall deficit linked to warm anomalies in the equatorial Pacific. Here, we show that in fact nearly half of all droughts over the past century were sub-seasonal, and characterized by an abrupt decline in late-season rainfall. Furthermore, the potential driver of this class of droughts is a coherent cold anomaly in the North Atlantic Ocean. The vorticity forcing associated with this oceanic marker extends through the depth of the troposphere, and results in a wavetrain which curves towards the equator and extends to East-Asia. This upper-level response triggers an anomalous low-level anticyclonic circulation late in the season over India. This teleconnection from the midlatitudes offers an avenue for improved predictability of monsoon droughts.

The aerosol mass concentrations over several Indian regions have been simulated using the online chemistry transport model, WRF-Chem, for two distinct seasons of 2011, representing the pre-monsoon (May) and post-monsoon (October) periods during the Indo-US joint experiment 'Ganges Valley Aerosol Experiment (GVAX)'. The simulated values were compared with concurrent measurements. It is found that the model systematically underestimates near-surface BC mass concentrations as well as columnar Aerosol Optical Depths (AODs) from the measurements. Examining this in the light of the modelsimulated meteorological parameters, we notice the model overestimates both planetary boundary layer height (PBLH) and surface wind speeds, leading to deeper mixing and dispersion and hence lower surface concentrations of aerosols. Shortcoming in simulating rainfall pattern also has an impact through the scavenging effect. It also appears that the columnar AODs are influenced by the unrealistic emission scenarios in the model. Comparison with vertical profiles of BC obtained from aircraft-based measurements also shows a systematic underestimation by the model at all levels. It is seen that concentration of other aerosols, viz., dust and sea-salt are closely linked with meteorological conditions prevailing over the region. Dust is higher during pre-monsoon periods due to the prevalence of north-westerly winds that advect dust from deserts of west Asia into the Indo-Gangetic plain. Winds and rainfall influence sea-salt concentrations. Thus, the unrealistic simulation of wind and rainfall leads to model simulated dust and sea-salt also to deviate from the real values; which together with BC also causes underperformance of the model with regard to columnar AOD. It appears that for better simulations of aerosols over Indian region, the model needs an improvement in the simulation of the meteorology.

Characteristics of aerosols in the atmospheric boundary layer (ABL) has been studied using the data obtained from particle samplers and altitude profiles of aerosol number density obtained using a continuous wave lidar. The aerosol concentration near the surface is strongly influenced by mesoscale features of sea breeze and also strongly influenced by synoptic ABL meteorology. Relative concentration of submicron particles is high in winter and summer and low during the monsoon period. Submicron particles generated over the continent are significant in summer and winter. Sea-spray is the important source mechanism during the monsoon period. Strong convective activity prevailing in summer is very effective in the vertical transport of aerosols, while the seaspray aerosols predominantly produced in the southwest monsoon period remains more or less confined to lower altitudes in ABL. A long-term increasing trend in aerosol optical depth is observed at this experimental site, which appears to be significantly governed, by regional transport rather than local production.

This work examines the aerosol physical properties and size distribution measured in the Marine Atmospheric Boundary Layer (MABL) over entire Bay of Bengal (BoB) and Northern Indian Ocean (NIO) during the Winter Integrated Campaign on Aerosols, Gases and Radiation Budget (W-ICARB). The measurements were taken using the GRIMM optical particle counter from 27th December 2008 to 30th January 2009. The results show large spatial heterogeneities regarding both the total aerosol number concentrations (N T ) and the size distributions over BoB, which in turn indicates the variations in the source strength or advection from different regions. The aerosol number size distribution seems to be bimodal in the 72% of the cases and can also be parameterized by uni-modal or by a combination of powerlaw and uni-modal distributions for the rest of the cases. The mode radius for accumulation and coarsemode particles ranges from w0.1e0.2 mm and w0.6e0.8 mm, respectively. In the northern BoB and along the Indian coast, the aerosols are mainly of sub-micron size with effective radius (R eff ) ranging between 0.25 and 0.3 mm highlighting the strong anthropogenic influence, while in the open oceanic areas they are much higher (0.4e0.6 mm). It was also found that the sea-surface wind plays a considerable role in the super-micron number concentration, R eff and mode radius for coarse-mode aerosols. Using the relation between N T and columnar AOD from Terra and Aqua-MODIS we found that the majority of the aerosols are within the lower MABL, while in some areas vertical heterogeneities also exist.

Aerosol measurements over a tropical semi-arid site in Anantapur (14.62 N, 77.65 E, 331 m asl), India, provide the variabilities of aerosol characteristics for the period of January 2007eDecember 2008. The mean values of aerosol optical depth at 500 nm (AOD 500 nm ) over 2-year period are found to be 0.37 AE 0.11, 0.39 AE 0.13 and 0.30 AE 0.10 during winter, summer and monsoon seasons, respectively. A rather systematic variation is seen with AOD peaking in the month of March, falling off gradually to the lowest value in June and increasing slowly superposed with some modulations, which show a weak secondary peak in October. The mean and standard values of the Ångström exponent, a (turbidity coefficient, b), are found to be 1.06 AE 0.33 (0.17 AE 0.05) during winter, 0.97 AE 0.35 (0.22 AE 0.04) during summer and 0.85 AE 0.26 (0.16 AE 0.06) during monsoon. The frequency distribution of a is relatively broad with two modes for dusty (a < 1) and non-dusty (a > 1) situations. This frequency distribution of a reveals a great dispersion of a values in all seasons thus denoting variability in the aerosol-size distribution. In this study, spectral AOD and Ångström exponent data are analyzed to obtain information about the adequacy of the simple use of the Ångström exponent and its variation for characterizing optical properties of aerosols, and exploring possibilities for a more efficient characterization of them.

In recent years, there has been a substantial increase in interest in the influence of anthropogenic aerosols on climate through both direct and indirect effects. Several extensive investigations and coordinated field campaigns have been carried out to assess the impact of anthropogenic aerosols on climate. However, there are far fewer studies on natural aerosols than on anthropogenic aerosols, despite their importance. Natural aerosols are particularly important because they provide a kind of base level to aerosol impact, and there is no effective control on them, unlike their anthropogenic counterparts. Besides, on a global scale the abundance of natural aerosols is several times greater than that of the major anthropogenic aerosols (sulphate, soot and organics). The major natural aerosol components are sea salt, soil dust, natural sulphates, volcanic aerosols, and those generated by natural forest fires. As with anthropogenic aerosols, the abundance of natural aerosols such as soil dust is also increasing, due to processes such as deforestation, which exposes more land areas which may then interact directly with the atmosphere, and due to other human activities. Since a major fraction of the natural aerosol (sea salt and natural sulphate) is of the non-absorbing type (and hygroscopic), it partly offsets the warming due to greenhouse gases as well as that due to absorbing aerosols (e.g., soot). The mineral dust transported over land and ocean causes surface cooling (due to scattering and absorption) simultaneously with lower atmospheric heating (due to absorption); this could in turn intensify a low-level inversion and increase atmospheric stability and reduce convection. To accurately predict the impact of dust aerosols on climate, the spatial and temporal distribution of dust is essential. The regional characteristics of dust source function are poorly understood due to the lack of an adequate database. The reduction of solar radiation at the surface would lead to a reduction in the sensible heat flux and all these will lead to perturbations in the regional and global climate. Enhanced concentration of sea salt aerosols at high wind speed would lead to more condensation nuclei, increase in the cloud droplet concentration and hence cloud albedo. Even though direct radiative impacts due to sea salt and natural sulphate are small compared to those due to anthropogenic counterparts, their indirect effects (and the uncertainties) are much larger. There is a considerable uncertainty in sea salt aerosol radiative forcing due to an inadequate database over oceans. The presence of natural aerosols may influence the radiative impact of anthropogenic aerosols, and it is difficult to separate the natural and anthropogenic aerosol contributions to radiative forcing when they are in a mixed state. Hence it is necessary to document the radiative effects of natural aerosols, especially in the tropics where the natural sources are strong. This is the subject matter of this review. r

Altitude profile of aerosol Single Scattering Albedo (SSA), derived from simultaneous in-situ airborne measurements of the coefficients of aerosol absorption and scattering off the west coast of India over the Arabian Sea (AS), during January 2009 is presented. While both the absorption and scattering coefficients decreased with altitude, their vertical structure differed significantly. Consequently, the derived SSA, with a surface value of 0.94, decreased with altitude, illustrating increasing relative dominance of aerosol absorption at higher altitudes. Altitude profile of SSA, when examined in conjunction with that of hemispheric backscatter fraction, revealed that the continental influence on the aerosol properties was higher at higher altitude, rather than the effect of marine environment. During an east-west transect across the peninsular India at an altitude of $ 2500 m (free troposphere), it was found that the aerosol scattering coefficients remained nearly the same over both east and west coasts.

Extensive and collocated measurements of several aerosol parameters were made over the eastern Arabian Sea, during the inter-monsoon and summer monsoon seasons of 2003 as a part of the Arabian Sea Monsoon Experiment (ARMEX). Associated with the seasonal changes in the synoptic wind fields from northeasterly/easterly to westerly/northwesterly, the aerosol characteristics and columnar optical depth show large variations. Consequently, the atmospheric forcing is found to increase from March to April and then to decrease consistently towards June. However, the magnitude of the forcing efficiency of aerosols continuously decreases from winter to summer. Such temporal changes in radiative forcing need to be accounted for in reducing the uncertainty in aerosol climate impact. r

The Arabian Sea region (4 N-20 N to 50 E-78 E) has a unique weather pattern on account of the Indian monsoon and the associated winds that reverse direction seasonally. The aerosol data, collected using ship-borne and island platforms (for 8 years from 1995 to 2002) along with MODIS (onboard TERRA satellite) data (from 2000 to 2003) have been used to evolve a comprehensive characterisation of the spatial and temporal variation in the physical, chemical, and radiative properties of aerosols over the Arabian Sea. The aerosol optical depth (AOD) was found to increase with latitude between the equator and 12 N. Over the northern Arabian Sea (regions lying north of 12 N), AODs do not show significant latitudinal variations; the average aerosol optical depth for this region was 0.29 AE 0.12 during winter monsoon season (WMS; November to March) and 0.47 AE 0.14 during summer monsoon season (SMS; April=May to September). The corresponding Angstrom exponents were 0.7 AE 0.12 and 0.3 AE 0.08, respectively. The low values of the exponent during SMS indicate the dominance of large aerosols (mainly dust particles > 1 mm). The latitudinal gradient in AOD in the southern Arabian Sea is larger during SMS compared to WMS.