This thesis is based on measurements of aerosol optical and microphysical properties made at inland, coastal and island locations in India. Aerosol vertical distribution measurements have also been made both using surface based and aircraft borne instruments. In addition to these, satellite based measurements (MODIS and OMI) have also been used to estimate regional aerosol radiative forcing over the oceanic regions around India. The measurements at an inland, continental, urban location reveals the large effect of anthropogenic activities on aerosol characteristics at surface and the atmospheric vertical column. A clear seasonality is observed in aerosol optical and microphysical properties as a consequence of modulation by anthropogenic activities and the effect of meteorological parameters like rainfall, winds and boundary layer dynamics. The variability observed at different time scales (from diurnal, weekly, monthly to annual) reveals the importance of anthropogenic and natural processes in modulating the aerosol loading. The estimates of aerosol radiative forcing at surface were as high as ~ 40W m-2. A large discrepancy was observed between the observed and modeled aerosol forcing efficiency (forcing per unit optical depth) at surface. These discrepancies are due to the inadequate representation of aerosol mixing state in models. In addition, the large difference found in the observed forcing between winter and summer could also be influenced due to the presence of elevated aerosols during the summer. Measurements made over coastal and central India shows that a large fraction (75-85%) of aerosol column optical depth was contributed by aerosols located above 1 km. The horizontal gradients were sharp with e-1 scaling distance as small as ~250 km in the well-mixed regions mostly under the influence of local source effects. However, above the atmospheric boundary layer, the gradients were much shallower (~800 to 1200 km). In addition, a large fraction (60-75%) of aerosol was found located above clouds leading to enhanced aerosol absorption. Large spatial gradient in aerosol optical depth and hence radiative impacts between the coastal landmass and the adjacent oceans within a short distance of <300 km (even at an altitude of 3 km) during summer and pre-monsoon is of importance to regional climate. Observations at Minicoy, a remote island in southern Arabian Sea to study the characteristics of transported aerosols reveals variability at daily, weekly, monthly and seasonal time scales associated with changes in precipitation and air mass characteristics. The daily mean Black Carbon (BC) mass mixing ratio varied between as low as ~ 0.2 to 9.0%. The resultant average aerosol atmospheric forcing for the observation period was ~15 W m-2. Trajectory based cluster analysis has shown six distinct advection/transport pathways influencing aerosol characteristics over southern Arabian Sea. The Indo-Gangetic Plain, northern Arabian Sea and west Asia are identified to be the most important source regions having a major impact on aerosols loading over the southern Arabian Sea. The cluster analysis, concentration weighted trajectory (CWT) analysis and the MODIS retrievals show an asymmetry in aerosol characteristics between the Arabian Sea and the Bay of Bengal, with the Arabian Sea characterized by large loading by natural aerosols (eg., dust and sea salt) and the Bay of Bengal characterized by anthropogenic loading (eg., BC). The low value of the BC mass mixing ratio measured at the island (mostly ~ 1 to 1.6%), has major implications for regional radiative forcing. The annually averaged net aerosol atmospheric forcing was as low as ~1.7 W m-2 with highest forcing corresponding to IGP cluster. The single scattering albedo (SSA) which is an important parameter in the estimation of aerosol radiative forcing was retrieved by utilizing a joint OMI-MODIS retrieval methodology. The SSA over the oceanic regions around India shows that the largest absorption (SSA < 0.9) occurs during winter. The largest gradients in AOD and SSA were observed over Arabian Sea during the summer as a result of large dust emissions. The largest forcing observed also was confined to the northern Arabian Sea (~ 37 W m-2) as a result of high aerosol column loading and dust transport. The observed annual mean forcing at Minicoy were comparable to that estimated using satellite measurements, but were much lower than those observed during INDOEX.

The ENSO-monsoon system is known to have a biennial component. Here we show using high resolution satellite data, mainly daily rainfall and sea surface temperature (SST) from the Tropical Rainfall Measuring Mission (TRMM), and daily scatterometer surface winds from QuickSCAT, that there is a clear biennial oscillation (TBO) in summer monsoon rainfall over Central India – Bay of Bengal (Cl-BoB) and the far west Pacific in the period 1999-2005. Summer (JJAS) mean rainfall oscillates between high and low values in alternate years; the rainfall is high in the odd years 1999, 2001, 2003, and 2005, and low in even years 2000, 2002 and 2004. The amplitude of the oscillation is significant, as measured against the long term standard deviation of seasonal rain based on 1979-2005 Global Precipitation Climatology Project (GPCP) data. We find that the TBO in rainfall is associated with TBO of SST over the tropical Indian, west Pacific and Atlantic Oceans in different seasons. There is no TBO in east Pacific SST, and no strong El Nino in this period. The TBO of SST is related to change in evaporation due to TBO of surface wind speed. A TBO of the surface branch of the Walker circulation in the eastern Indian and western Pacific basins is clearest in the autumn season during 1999-2005. There is a clear relation between a large-amplitude TBO of winter surface air temperature over north Asia associated with TBO of the Arctic oscillation (AO), and the TBO of summer monsoon rainfall. High rainfall over CI-BoB lin summer is followed by a relatively high value of the AO Index, and warm air termperature over north Asia in the succeeding winter. The Inter Tropical Convergence Zone(ITCZ) over the central Pacific and Atlantic Oceans shift north by about two degrees when the northern hemisphere is warm, reminiscent of the behaviour of the climate system of ENSO, decadal and palaeoclimate time scales. In this thesis we document the biennial oscillation of monsoon rain and its spatial structure in the recent period, and its relation with biennial oscillation of surface climate over the global tropics and extratropical regions. The existence of TBO in the tropical Atlantic, and its relation with the monsoon, is a new finding. We demonstrate that the interannual variability of the summer monsoon during 1999-2005, including the drought of 2002, is part of a pervasive TBO of global surface climate.