Variations in annual accumulated snowfall over the Antarctic ice sheet have a significant and direct impact on mean sea-level change. The interannual variability of the precipitation over coastal Antarctica adjacent to the southern Indian Ocean (SIO) cannot be totally explained by the dominant mode of atmospheric variability in the Southern Hemisphere. This study explores the possible contributions from sea surface temperature (SST) anomalies in SIO on the precipitation over East Antarctica. The results suggest that the winter precipitation in the Lambert Glacier basin (LGB) is closely related to the autumn SST variability in SIO without the influence of El Nino-Southern Oscillation. It is shown that the positive autumn SIO dipole (SIOD) of SST anomalies is usually followed by reduced precipitation in the following winter over the LGB region and vice versa. The positive (negative) autumn SIOD can persist into the winter and excite cyclonic (anticyclonic) circulation and deepen (weaken) SIO low in high latitude, corresponding to an enhanced northward (southward) wind anomaly in LGB and central SIO. This mechanism prevents (promotes) the transportation of warm and moist marine air to the LGB region and hence decreases (increases) the precipitation during the following winter.

Methods for determining aerosol types in cases where chemical composition measurements are not available are useful for improved aerosol radiative forcing estimates. In this study, two aerosol characterization methods by Cazorla et al. (2013, https://doi.org/10.5194/acp-13-9337-2013; CA13) and Costabile et al. (2013,10.5194/acp-13-2455-2013; CO13) using wavelength-dependent particle absorption and scattering are used, to assess their applicability and examine their limitations. Long-term ambient particle optical property and chemical composition (major inorganic ions and bulk carbon) measurements from the Maldives Climate Observatory Hanimaadhoo as well as concurrent air mass trajectories are utilized to test the classifications based on the determined absorption angstrom ngstrom exponent, scattering angstrom ngstrom exponent, and single scattering albedo. The resulting aerosol types from the CA13 method show a good qualitative agreement with the particle chemical composition and air mass origin. In general, the size differentiation using the scattering angstrom ngstrom exponent works very well for both methods, while the composition identification depending mainly on the absorption angstrom ngstrom exponent can result in aerosol misclassifications at Maldives Climate Observatory Hanimaadhoo. To broaden the applicability of the CA13 method, we suggest to include an underlying marine aerosol group in the classification scheme. The classification of the CO13 method is less clear, and its applicability is limited when it is extended to aerosols in this environment at ambient humidity.

The aerosol optical depths (AODs) in the wavelength range 380-875 nm and black carbon (BC) mass concentrations were estimated over the tropical Indian Ocean and in the Indian Ocean sector of Southern Ocean, between 14 degrees N and 53 degrees S, during December 2011-February 2012, onboard the Ocean Research Vessel (ORV) Sagar Nidhi. The data were analysed to understand the spectral variability, micro-physical characteristics of aerosols and the associated radiative forcing. Concurrent MODIS-derived chlorophyll a (Chl-a) and sea-surface temperature (SST) provided ancillary data used to understand the variability of biomass in association with fronts and the possible role of phytoplankton as a source of aerosols. AODs and their spectral dependencies were distinctly different north and south of the Inter-Tropical Convergence Zone (ITCZ). North of 11 degrees S (the northern limit of ITCZ), the spectral distribution of AOD followed Angstrom turbidity formule (Junge power law function), while it deviated from such a distribution south of 16 degrees S (southern boundary of ITCZ). At the southern limit of the ITCZ and beyond, the spectral variation of AOD showed a peak around 440 nm, the amplitude of which was highest at similar to 43 degrees S, the axis of the subtropical front (STF) with the highest Chl-a concentration (0.35 mu g l(-1)) in the region. To understand the role of Chl-a in increasing AOD at 440 nm, AOD at this wavelength was estimated using Optical properties of Aerosols and Clouds (OPAC) model. The anomalies between the measured and model-estimated (difference between the measured and estimated AOD values at 440 nm) AOD(440) were correlated with Chl-a concentrations. A very high and significant association with coefficient of determination (R-2=0.80) indicates the contribution of Chl-a as a source of aerosols in this part of the ocean. On the basis of the measured aerosol properties, the study area was divided into three zones; Zone 1 comprising of the area between 10 degrees N and 11 degrees S; Zone 2 from 16 degrees S to 53 degrees S; and Zone 3 from 52 degrees S to 24 degrees S during the return leg. BC mass concentration was in the range 520 ng m(-3) to 2535 ng m(-3) in Zone 1, while it was extremely low in the other zones (ranging from 49.3 to 264.4 ng m(-3) in Zone 2 and from 61.6 ng m(-3) to 303.3 ng m(-3) in Zone 3). The atmospheric direct-short wave radiative forcing (DRSF), estimated using a radiative transfer model (Santa Barbara DISORT Atmospheric Radiative Transfer - SBDART), was in the range 4.72-27.62 wm(-2) north of 16 degrees S, and 4.80-6.25 wm(-2) south of 16 degrees S. (C) 2015 Elsevier Ltd. All rights reserved.

[1] Aerosol sources, transport, and sinks are simulated, and aerosol direct radiative effects are assessed over the Indian Ocean for the Indian Ocean Experiment (INDOEX) Intensive Field Phase during January to March 1999 using the Laboratoire de Meteorologie Dynamique (LMDZT) general circulation model. The model reproduces the latitudinal gradient in aerosol mass concentration and optical depth (AOD). The model-predicted aerosol concentrations and AODs agree reasonably well with measurements but are systematically underestimated during high-pollution episodes, especially in the month of March. The largest aerosol loads are found over southwestern China, the Bay of Bengal, and the Indian subcontinent. Aerosol emissions from the Indian subcontinent are transported into the Indian Ocean through either the west coast or the east coast of India. Over the INDOEX region, carbonaceous aerosols are the largest contributor to the estimated AOD, followed by sulfate, dust, sea salt, and fly ash. During the northeast winter monsoon, natural and anthropogenic aerosols reduce the solar flux reaching the surface by 25 W m(-2), leading to 10 - 15% less insolation at the surface. A doubling of black carbon (BC) emissions from Asia results in an aerosol single-scattering albedo that is much smaller than in situ measurements, reflecting the fact that BC emissions are not underestimated in proportion to other ( mostly scattering) aerosol types. South Asia is the dominant contributor to sulfate aerosols over the INDOEX region and accounts for 60 - 70% of the AOD by sulfate. It is also an important but not the dominant contributor to carbonaceous aerosols over the INDOEX region with a contribution of less than 40% to the AOD by this aerosol species. The presence of elevated plumes brings significant quantities of aerosols to the Indian Ocean that are generated over Africa and Southeast and east Asia.

Submicron aerosol particles (Dp<1 mu m) were sampled with stacked filter units on the National Center for Atmospheric Research (NCAR) Hercules C-130 aircraft during February-March 1999 as a contribution to the Indian Ocean Experiment (INDOEX). We determined the vertical and spatial distribution of the major aerosol components (NH4+, Na+, K+, Mg2+, Ca2+, methyl sulfonic acid, Cl-, NO3-, SO42-, oxalate, organic carbon, and black carbon) over the Indian Ocean to examine the role of pollution aerosols on indirect and direct radiative forcing. High pollution levels were observed over the entire northern Indian Ocean down to the Intertropical Convergence Zone (ITCZ) located between the equator and 10degreesS. In the northern part of the Indian Ocean (5-15degreesN, 66degrees-73degreesE), high concentrations of carbonaceous aerosol and pollution-derived inorganic species were found in a layer extending from the sea surface to about 3.5 km asl. In this layer, the average mass concentration of all aerosol species detected by our technique ranged between 7 and 34 mug m(-3), comparable to pollution levels observed in industrialized regions. In the Southern Hemisphere (1degrees-9degreesS, 66degrees-73degreesE), the aerosol concentrations rapidly declined to remote background levels of about 2 mug m(-3). The concentrations of non-sea-salt sulfate (the main light scattering component) ranged from maximum values of 12.7 mug m(-3) in the Northern Hemisphere to 0.2 mug m(-3) in the Southern Hemisphere. Carbonaceous aerosol contributes between 40% and 60% to the fine aerosol mass of all determined components. An unusually high fraction of black carbon (up to 16% in the polluted areas) is responsible for its high light absorption coefficient.