Meteorological fields variability over the Indian seas in pre and summer monsoon months during extreme monsoon seasons

In this study, the possible linkage between summer monsoon rainfall over India and surface meteorological fields (basic fields and heat budget components) over monsoon region (30‡E-120‡E, 30‡S30‡N) during the pre-monsoon month of May and summer monsoon season (June to September) are examined. For this purpose, monthly surface meteorological fields anomaly are analyzed for 42 years (1958-1999) using reanalysis data of NCEP/NCAR (National Center for Environmental Prediction/National Center for Atmospheric Research). The statistical significance of the anomaly (difference) between the surplus and deficient monsoon years in the surface meteorological fields are also examined by Student’s t-test at 95% confidence level. Significant negative anomalies of mean sea level pressure are observed over India, Arabian Sea and Arabian Peninsular in the pre-monsoon month of May and monsoon season. Significant positive anomalies in the zonal and meridional wind (at 2 m) in the month of May are observed in the west Arabian Sea off Somali coast and for monsoon season it is in the central Arabian Sea that extends up to Somalia. Significant positive anomalies of the surface temperature and air temperature (at 2 m) in the month of May are observed over north India and adjoining Pakistan and Afghanistan region. During monsoon season this region is replaced by significant negative anomalies. In the month of May, significant positive anomalies of cloud amount are observed over Somali coast, north Bay of Bengal and adjoining West Bengal and Bangladesh. During monsoon season, cloud amount shows positive anomalies over NW India and north Arabian Sea. There is overall reduction in the incoming shortwave radiation flux during surplus monsoon years. A higher magnitude of latent heat flux is also found in surplus monsoon years for the month of May as well as the monsoon season. The significant positive anomaly of latent heat flux in May, observed over southwest Arabian Sea, may be considered as an advance indicator of the possible behavior of the subsequent monsoon season. The distribution of net heat flux is predominantly negative over eastern Arabian Sea, Bay of Bengal and Indian Ocean. Anomaly between the two extreme monsoon years in post 1980 (i.e., 1988 and 1987) shows that shortwave flux, latent heat flux and net heat flux indicate reversal in sign, particularly in south Indian Ocean. Variations of the heat budget components over four smaller sectors of Indian seas, namely Arabian Sea, Bay of Bengal and west Indian Ocean and east Indian Ocean show that a small sector of Arabian Sea is most dominant during May and other sectors showing reversal in sign of latent heat flux during monsoon season.     

Surface and upper air meteorological features during onset phase of 2003 monsoon

The second campaign of the Arabian Sea Monsoon Experiment (ARMEX-II) was conducted in two phases viz., March–April and May–June 2003. In the present work, the buoy and ocean research vessel data collected during the second phase of ARMEX-II have been analysed to bring out the characteristic features of monsoon onset. The results have shown that the thermodynamical features such as build up of lower tropospheric instability and increased height of zero degree isotherm occurred about a week before the monsoon onset over Kerala and adjoining southeast Arabian Sea. There was a sharp fall in the temperature difference between 850 and 500 hPa, and the height of zero degree isotherm about 2–3 days before the monsoon onset. The flux of sensible heat was positive (sea to air) over south Arabian Sea during the onset phase. Over the Bay of Bengal higher negative (air to sea) values of sensible flux prevailed before the monsoon onset which became less negative with the advance of monsoon over that region. The pre-onset period was characterized by large sea surface temperature (SST) gradient over the Arabian Sea with rapid decrease towards north of the warm pool region. The buoy observations have shown that SST remained close to 30.5°C in the warm pool region during the pre-onset period in 2003 but only 2–3 degrees away (north of this region) SSTs were as low as 28.5–29°C. An interesting aspect of sea level pressure (SLP) variability over the Indian seas during the onset phase of summer monsoon 2003 was undoubtedly, the highest SLP in the warm pool region inspite of very high SSTs.     

A model study of seasonal mixed-layer primary production in the Arabian Sea

We combined a surface irradiance model with a non-spectral photosynthesisirradiance model to estimate the daily, average rate of mixed-layer primary production in the Arabian Sea for the 15th day of months at the end of the northeast monsoon, the southwest monsoon, and the fall and spring inter-monsoons. Our model experiment uses climatologies of cloud cover, mixed-layer thickness, and satellite ocean-color observations of phytoplankton biomass. Modelled surface radiation is at an annual maximum in May beneath nearly cloud-free skies just prior to the summer solstice. The model estimate of surface radiation diminishes through the southwest monsoon over most of the northern Arabian Sea to an annual minimum in August due to intense cloudiness. In agreement with previous ship-based measurements, the photosynthesis-irradiance model predicts that the mixed-layer primary production in the Arabian Sea is extremely seasonal, and peaks annually during the southwest monsoon to the north-west of the atmospheric Findlater Jet and along the coast of Somalia. Northern Arabian Sea maxima predicted for both the summer and winter monsoons are separated by periods of low mixed-layer primary production, the fall and spring inter-monsoons. The annual cycles of modelled mixed-layer primary production differ by region in the Arabian Sea due to varying monsoon influence and circulation dynamics.     

新德里季风降水中过量氘与季风水汽来源

收集和分析了新德里降水中同位素资料(δ¹⁸O和δD),利用季风水线方程对个别年份缺测的δD资料进行估计,建立了新德里36 a夏季过量氘序列.基于降水中过量氘和水汽源区相对湿度关系考虑,利用NCEP/NCAR再分析资料,研究了新德里夏季过量氘序列和水汽源区相对湿度的关系.研究发现,西阿拉伯海相对湿度变化和新德里季风降水中过量氘变化较为一致.结合西阿拉伯海风速和印度西北地区季风降水量资料分析结果,认为西阿拉伯海是新德里季风水汽的主要来源.     

Early prediction of onset of south west monsoon from ERS-1 scatterometer winds

Detailed analysis of the surface winds over the Indian Ocean derived from ERS-1 scatterometer data during the years 1993 and 1994 has been used to understand and unambiguously identify the onset phase of south-west monsoon. Five day (pentad) averaged wind vectors for the period April to June during both years have been examined to study the exact reversal of wind direction as well as the increase in wind speed over the Arabian Sea in relation to the onset of monsoon over the Indian west coast (Kerala). The related upper level humidity available from other satellites has also been analysed. The results of our analysis clearly show a consistent dramatic reversal in wind direction over the western Arabian Sea three weeks in advance of the onset of monsoon. The wind speed shows a large increase coinciding with the onset of monsoon. These findings together show the dominant role of sea surface winds in establishing the monsoon circulation. The study confirms that the cross equatorial current phenomenon becomes more important after the onset of monsoon.     

Interannual variability of onset of the summer monsoon over India and its prediction

In this article, the interannual variability of certain dynamic and thermodynamic characteristics of various sectors in the Asian summer monsoon domain was examined during the onset phase over the south Indian peninsula (Kerala Coast). Daily average (0000 and 1200 UTC) reanalysis data sets of the National Centre for Environmental Prediction/National Centre for Atmospheric Research (NCEP/NCAR) for the period 1948–1999 were used. Based on 52 years onset date of the Indian summer monsoon, we categorized the pre-onset, onset, and post-onset periods (each an average of 5 days) to investigate the interannual variability of significant budget terms over the Arabian Sea, Bay of Bengal, and the Indian peninsula. A higher difference was noticed in low-level kinetic energy (850 hPa) and the vertically integrated generation of kinetic energy over the Arabian Sea from the pre-onset, onset, and post-onset periods. Also, significant changes were noticed in the net tropospheric moisture and diabatic heating over the Arabian Sea and Indian peninsula from the pre-onset to the post-onset period. It appears that attaining the magnitude of 40 m² s⁻² and then a sharp rise in kinetic energy at 850 hPa is an appropriate time to declare the onset of the summer monsoon over India. In addition to a sufficient level of net tropospheric moisture (40 mm), a minimum strength of low-level flow is needed to trigger convective activity over the Arabian Sea and the Bay of Bengal. An attempt was also made to develop a location-specific prediction of onset dates of the summer monsoon over India based on energetics and basic meteorological parameters using multivariate statistical techniques. The regression technique was developed with the data of May and June for 42 years (1948–1989) and validated with 10 years NCEP reanalysis from 1990 to 1999. It was found that the predicted onset dates from the regression model are fairly in agreement with the observed onset dates obtained from the Indian Meteorology Department.     

Temporal characteristics of the Indian southwest monsoon

In this study, we elucidate the temporal characteristics of the onset and withdrawal of the Indian southwest monsoon, making use of the model integration and daily analyses of the National Centre for Medium Range Weather Forecasting, India. The onset of the Indian southwest monsoon over the Bay of Bengal is discernable by a gradual increase in the adiabatic generation of kinetic energy, while over the Arabian Sea it is first noticeable by a steep and abrupt increase of generation. The horizontal transport of heat indicates a convergence regime over the Bay of Bengal prior to onset, while over the Arabian Sea a convergence regime is indicated by a change from the divergence to the convergence regime. The withdrawal of the southwest monsoon is characterized by the horizontal transport of heat and moisture that evince a transition from the convergence to divergence regime; similarly, diabatic heating noticed during the active period changes to cooling. The withdrawal over the Arabian Sea is characterized by the divergence regime of the horizontal transport of moisture. This change precedes even the circulation changes over northwest India, which may be regarded as a precursor. The withdrawal is further supported by a monotonic decrease in the net tropospheric moisture over the Arabian Sea, followed by a similar change at land locations.     

Fluxes of material in the Arabian Sea and Bay of Bengal — Sediment trap studies

In order to investigate how monsoons influence biogeochemical fluxes in the ocean, twelve time-series sediment traps were deployed at six locations in the northern Indian Ocean. In this paper we present particle flux data collected during May 1986 to November 1991 and November 1987 to November 1992 in the Arabian Sea and Bay of Bengal respectively. Particle fluxes were high during both the SW and NE monsoons in the Arabian Sea as well as in the Bay of Bengal. The mechanisms of particle production and transport, however, differ in both the regions. In the Arabian Sea, average annual fluxes are over 50gm^-2y^-1 in the western Arabian Sea and less than 27gm^-2 y^-1 in the central part. Biogenic matter is dominant at sites located near upwelling centers, and is less degraded during peak flux periods. High particle fluxes in the offshore areas of the Arabian Sea are caused by injection of nutrients into the euphotic zone due to wind-induced mixed layer deepening. In the Bay of Bengal, average annual fluxes are highest in the central Bay of Bengal (over 50gm^-2y^-1) and are least in the southern part of the Bay (37gm^-2y^-1). Particle flux patterns coincide with freshwater discharge patterns of the Ganges-Brahmaputra river system. Opal/carbonate and organic carbon/carbonate carbon ratios increase during the SW monsoon due to variations in salinity and productivity patterns in the surface waters as a result of increased freshwater and nutrient input from rivers. Comparison of S years data show that fluxes of biogenic and lithogenic particulate matter are higher in the Bay of Bengal even though the Arabian Sea is considered to be more productive. Our results indicate that in the northern Indian Ocean interannual variability in organic carbon flux is directly related to the strength and intensity of the SW monsoon while its transfer from the upper layers to the deep sea is partly controlled by input of lithogenic matter from adjacent continents.     

Monsoon control on trace metal fluxes in the deep Arabian Sea

Particulate fluxes of aluminium, iron, magnesium and titanium were measured using six time-series sediment traps deployed in the eastern, central and western Arabian Sea. Annual Al fluxes at shallow and deep trap depths were 0.47 and 0.46 g m^-2 in the western Arabian Sea, and 0.33 and 0.47 g m^-2 in the eastern Arabian Sea. There is a difference of about 0.9–1.8 g m^-2y^-1 in the lithogenic fluxes determined analytically (residue remaining after leaching out all biogenic particles) and estimated from the Al fluxes in the western Arabian Sea. This arises due to higher fluxes of Mg (as dolomite) in the western Arabian Sea (6–11 times higher than the eastern Arabian Sea). The estimated dolomite fluxes at the western Arabian Sea site range from 0.9 to 1.35gm^-2y^-1. Fe fluxes in the Arabian Sea were less than that of the reported atmospheric fluxes without any evidence for the presence of labile fraction/excess of Fe in the settling particles. More than 75% of Al, Fe, Ti and Mg fluxes occurred during the southwest (SW) monsoon in the western Arabian Sea. In the eastern Arabian Sea, peak Al, Fe, Mg and Ti fluxes were recorded during both the northeast (NE) and SW monsoons. During the SW monsoon, there exists a time lag of around one month between the increases in lithogenic and dolomite fluxes. Total lithogenic fluxes increase when the southern branch of dust bearing northwesterlies is dragged by the SW monsoon winds to the trap locations. However, the dolomite fluxes increase only when the northern branch of the northwesterlies (which carries a huge amount of dolomite accounting 60% of the total dust load) is dragged, from further north, by SW monsoon winds. The potential for the use of Mg/Fe ratio as a paleo-monsoonal proxy is examined.     

Terrigenous plant wax inputs to the Arabian Sea: Implications for the reconstruction of winds associated with the Indian Monsoon

We have determined the accumulation rates and carbon isotopic compositions (δ¹³C) of long-chain (C₂₄-C₃₂) terrigenous plant wax fatty acids in 19 surface sediment samples geographically distributed throughout the Arabian Sea in order to assess the relationship between plant wax inputs and the surrounding monsoon wind systems. Both the accumulation rate data and the δ¹³C data show that there are three primary eolian sources of plant waxes to the Arabian Sea: Africa, Asia, and the Arabian Peninsula. These sources correspond to the three major wind systems in this region: the summer (Southwest) monsoon, the winter (Northeast) monsoon, and the summer northwesterlies that blow over the Arabian Peninsula. In addition, plant waxes are fluvially supplied to the Gulf of Oman and the Eastern African margin by nearby rivers. Plant wax δ¹³C values reflect the vegetation types of the continental source regions. Greater than 75% of the waxes from Africa and Asia are derived from C₄ plants. Waxes delivered by northwesterly winds reflect a greater influence (25-40%) of C₃ vegetation, likely derived from the Mesopotamian region. These data agree well with previously published studies of eolian dust deposition, particularly of dolomite derived from the Arabian Peninsula and the Mesopotamian region, in surface sediments of the Arabian Sea. The west-to-east gradient of plant wax δ¹³C and dolomite accumulation rates are separately useful indicators of the relationship between the northwesterly winds and the winds of the Southwest monsoon. Combined, however, these two proxies could provide a powerful tool for the reconstruction of both southwest monsoon strength as well as Mesopotamian aridity.     

A brief summary of Dr. G. V. Rao’s research interests

A brief summary of Dr. G. V. Rao's research interests is presented. Many of his earlier studies were in conjunction with the summer Monsoon Experiment of 1979 (MONEX-79). These included: 1) the structure of the Somali jet based on aerial observations; 2) sea-level air trajectories over the equatorial Indian Ocean; 3) structural features of the east African low-level flow; 4) effects of Indian Ocean surface temperature anomaly patterns on the summer monsoon circulations; 5) structures of the monsoon low-level flow over the Arabian Sea; 6) characteristics and momentum-flux budgets of the Arabian Sea convective bands; and 7) evaporation and precipitation over the Arabian Sea during the monsoon seasons. Dr. Rao's research efforts in recent years had focused on case studies of mesocyclones spawned by tropical cyclones (TCs) in Florida using Doppler radar data and a mesoscale numerical model. These included: 1) research on tornadic mesocyclones spawned by TC Earl in 1998; 2) documentation of subtle differences between tornadic and non-tornadic mesocyclones in TC Floyd in 1999; and 3) numerical simulation of the tornadic environment observed in peninsular Florida during TC Earl in 1998. Preliminary findings show that the supercells' cold pools interacted with an existing boundary resulting in increased baroclinicity and horizontal vorticity, and a maximization of the tornado production potential by the updrafts. The model successfully simulated the mesoscale features of the mesocyclones and the tornadic environment observed during TC Earl. A 24 h simulation of accumulated rainfall within the inner domain agreed well with the observed precipitation pattern over the region.     

Hydrography of the eastern Arabian Sea during summer monsoon 2002

Hydrographic observations in the eastern Arabian Sea (EAS) during summer monsoon 2002 (during the first phase of the Arabian Sea Monsoon Experiment (ARMEX)) include two approximately fortnight-long CTD time series. A barrier layer was observed occasionally during the two time series. These ephemeral barrier layers were caused byin situ rainfall, and by advection of low-salinity (high-salinity) waters at the surface (below the surface mixed layer). These barrier layers were advected away from the source region by the West India Coastal Current and had no discernible effect on the sea surface temperature. The three high-salinity water masses, the Arabian Sea High Salinity Water (ASHSW), Persian Gulf Water (PGW), and Red Sea Water (RSW), and the Arabian Sea Salinity Minimum also exhibited intermittency: they appeared and disappeared during the time series. The concentration of the ASHSW, PGW, and RSW decreased equatorward, and that of the RSW also decreased offshore. The observations suggest that the RSW is advected equatorward along the continental slope off the Indian west coast.     

Sea-Surface Temperatures and the History of Monsoon Upwelling in the Northwest Arabian Sea during the Last 500,000 Years

Arabian Sea sediments record changes in the upwelling system off Arabia, which is driven by the monsoon circulation system over the NW Indian Ocean. In accordance with climate models, and differing from other large upwelling areas of the tropical ocean, a 500,000-yr record of productivity at ODP Site 723 shows consistently stronger upwelling during interglaciations than during glaciations. Sea-surface temperatures (SSTs) reconstructed from the alkenone unsaturation index (U^K′₃₇) are high (up to 27°C) during interglaciations and low (22-24°C) during glaciations, indicating a glacial-interglacial temperature change of >3°C in spite of the dampening effect of enhanced or weakened upwelling. The increased productivity is attributed to stronger monsoon winds during interglacial times relative to glacial times, whereas the difference in SSTs must be unrelated to upwelling and to the summer monsoon intensity. The winter (NE) monsoon was more effective in cooling the Arabian Sea during glaciations then it is now.     

Air-sea interaction over the tropical Indian Ocean during several contrasting monsoon seasons

Monthly mean anomaly fields of various parameters like sea surface temperature, air temperature, wind stress, effective radiation at the surface, heat gain over the ocean and the total heat loss between a good and bad monsoon composite and the evaporation rates over the Arabian Sea and southern hemisphere have been studied over the tropical Indian Ocean. The mean rates of evaporation on a seasonal scale over the Arabian Sea during a good and bad monsoon composites were equal (about 2·48 × 10¹⁰ tons/day). The evaporation rates over the southern hemisphere were greater during all the months. The mean evaporation rates over the southern hemisphere on a seasonal scale for the good and bad monsoon composites were 4·4 × 10¹⁰ and 4·6 × 10¹⁰ tons/day respectively. The maximum evaporation rates over the southern hemisphere were observed in August. The anomalies of wind stress, effective radiation at the surface and the heat gain over the ocean also exhibit large variations in August, as compared to other monsoon months.     

Development and propagation of a pollution gradient in the marine boundary layer during INDOEX (1999)

The development and propagation of a pollution gradient in the marine boundary layer over the Arabian Sea during the Intensive Field Phase of the Indian Ocean Experiment (1999) is investigated. A hypothesis for the generation of the pollution gradient is presented. Infrared satellite images show the formation of the pollution gradient as the leading edge of a polluted air mass in the marine boundary layer and also its propagation over the Arabian Sea and the northern Indian Ocean. Aerosol data measured from two research vessels over the Arabian Sea show a variation in the concentrations caused by the passage of this pollution gradient. Depth of the pollution gradient was found to be about 800 m. A numerical model was used to simulate the development of this gradient and its propagation over the ocean. Results show that its formation and structure are significantly influenced by the diurnal cycle of coastal sea-land breeze circulations along India’s west coast. Transport of aerosols and gases over the Arabian Sea in the lower troposphere from land sources appears to be through this mechanism with the other being the elevated land plume.     

30 ka以来东阿拉伯海U1456站位粘土粒级碎屑沉积物来源及其古环境意义

通过对国际大洋发现计划U1456站位沉积物AMS 14C年代、粘土矿物、常量元素及粒度组成的综合分析,探讨了东阿拉伯海粘土粒级碎屑沉积物的源-汇过程及其古环境指示意义.30 ka以来U1456站位的粘土矿物组合以蒙脱石和伊利石为主,并含有少量的绿泥石和高岭石.物源分析结果表明粘土粒级碎屑沉积物主要来自于印度河与德干高原.30 ka以来西南季风很可能是影响喜马拉雅山脉以及印度大陆风化剥蚀的重要因素.在西南季风减弱的阶段,印度河物源端元对研究区的输入量减少,这可能与此时热带辐合带的南移及末次冰盛期喜马拉雅山脉冰川覆盖面积的增加有关,从而导致印度河径流量及喜马拉雅山脉可供风化剥蚀的区域减少.K/Al比值指示的源区大陆化学风化作用强度与前人重建的西南季风记录间较为同步,在东阿拉伯海可以作为晚第四纪以来西南季风演化的有效重建指标.      

Circulation and water masses of the Arabian Sea

The dynamics and thermodynamics of the surface layer of the Arabian Sea, north of about 10N, are dominated by the monsoon-related annual cycle of air-sea fluxes of momentum and heat. The currents in open-sea regime of this layer can be largely accounted for by Ekman drift and the thermal field is dominated by local heat fluxes. The geostrophic currents in open-sea subsurface regime also show a seasonal cycle and there is some evidence that signatures of this cycle appear as deep as 1000 m. The forcing due to Ekman suction is an important mechanism for the geostrophic currents in the central and western parts of the Sea. Recent studies suggest that the eastern part is strongly influenced by the Rossby waves radiated by the Kelvin waves propagating along the west coast of India. The circulation in the coastal region off Oman is driven mainly by local winds and there is no remotely driven western boundary current. Local wind-driving is also important to the coastal circulation off western India during the southwest monsoon but not during the northeast monsoon when a strong (approximately 7 × 10⁶m³/sec) current moves poleward against weak winds. This current is driven by a pressure gradient which forms along this coast during the northeast monsoon due to either thermohaline-forcing or due to the arrival of Kelvin waves from the Bay of Bengal. The present speculation about flow of bottom water (deeper than about 3500 m) in the Arabian Sea is that it moves northward and upwells into the layer of North Indian Deep Water (approximately 1500–3500m). It is further speculated that the flow in this layer consists of a poleward western boundary current and a weak equatorward flow in the interior. It is not known if there is an annual cycle associated with the deep and the bottom water circulation.     

Geochemical evidence for abrupt changes in relative strength of the Arabian monsoons during a stadial/interstadial climate transition

The occurrence and propagation of abrupt climate change between the high and low-latitudes has become an important focus of paleoclimatic and paleoceanographic research. The causes of abrupt change have significant implications for understanding future manifestations of similar forcings under late Holocene (‘Anthropocene’) boundary conditions. Of particular interest are signals indicative of sub-millennial scale climate change in the sub-tropics of similar magnitude and frequency to those recorded in Greenland ice cores. Earlier research in the Arabian Sea has highlighted the sensitivity of sedimentary organic carbon and nitrogen isotope measurements for recording the state of the SW monsoon and associated Arabian Sea Oxygen Minimum Zone. In this study, we exploit the unprecedented fidelity of the sedimentary δ¹⁵N record to identify a 20 cm interval at ODP Site 723 containing a stadial/inter-stadial interval between 43-42 Kyr BP. We employ sedimentary nitrogen isotopes, chlorin pigment and alkenone abundances, major and minor element analyses of highly-resolved (2 mm ≈ 10 yr) samples across this interval to compare a comprehensive, multi-proxy data set to understand (a) the processes contributing to the δ¹⁵N signal in the longer records of denitrification; and (b) the associated climatic events, especially the relative intensity of summer and winter monsoons at these times. A lack of evidence for bioturbation in excess of our 2 mm sampling resolution facilitates decadal-scale oceanographic and climatic reconstructions. Using a four-component flux-dilution model, we show that the deposition of carbonate decreased in parallel with an increase in Total Organic Matter flux from stadial to inter-stadial time. This interval is also marked by a significant drop in lithogenic (dust) accumulation, analogous to a similar decrease noted during deglaciation in the Western Arabian Sea. Combined with alkenone U₃₇^K′-derived estimates for sea surface temperature (SST), we conclude that the climatological shift from stadial to inter-stadial conditions at low latitudes was characterized by repeated switches in mean monsoon state approximately every 200 yr. The winter monsoon was the dominant mode during maximum stadial conditions; conversely the summer monsoon was dominant during maximum interstadial-like conditions. However, each interval was separated by a distinct ‘inter-monsoon’ mode, indicated by a higher continental dust flux but warmer SST. Proxy records for changing bottom-water oxygenation show near-identical results down to the mm-scale, but hint at increased export production leading the onset of anoxia during the stadial/inter-stadial transition. The coherence of all sedimentary signals depicts a wholesale reorganization of the Arabian Sea climate and marine ecosystem over approximately 200 years, a period that may be associated with monsoon modulation by small oscillations in solar irradiance.     

Observations of trace gases and aerosols over the Indian Ocean during the monsoon transition period

Characteristics of trace gases (O₃, CO, CO₂, CH₄ and N₂O) and aerosols (particle size of 2.5 micron) were studied over the Arabian Sea, equatorial Indian Ocean and southwest part of the Bay of Bengal during the monsoon transition period (October–November, 2004). Flow of pollutants is expected from south and southeast Asia during the monsoonal transition period due to the patterns of wind flow which are different from the monsoon period. This is the first detailed report on aerosols and trace gases during the sampled period as the earlier Bay of Bengal Experiment (BOBMEX), Arabian Sea Monsoon Experiment (ARMEX) and Indian Ocean Experiments (INDOEX) were during monsoon seasons. The significant observations during the transition period include: (i) low ozone concentration of the order of 5 ppbv around the equator, (ii) high concentrations of CO₂, CH₄ and N₂O and (iii) variations in PM2.5 of 5–20μg/m³.     

Did the Indo‐Asian summer monsoon decrease during the Holocene following insolation?

A few studies from the western Arabian Sea indicate that the Indian summer (or southwest) monsoon (ISM), after attaining its maximum intensity at ca. 9 ka, declined during the Holocene, as did insolation. In contrast, earlier and later observations from both the eastern and the western Arabian Sea do not support this inference. Analysis of multiple proxies of productivity in a new sediment core from the western Arabian Sea fails to confirm the earlier, single‐proxy (e.g. abundance of Globigerina bulloides) based, inference of the Holocene weakening of ISM, following insolation. The reason for the observed decreasing trend in foraminiferal abundance – the basis for the earlier inference – could be the favouring of silicate rather than carbonate productivity by the increased ISM wind strength. Although ISM exhibits several multi‐millennial scale fluctuations, there is no evidence from several multi‐proxy data to conclude that it declined during the Holocene; this is consistent with the phase lag analysis of longer time series of monsoon proxies. Thus, on sub‐Milankovitch timescales, ISM did not follow insolation, highlighting the importance of internal feedbacks. A comparison with East Asian summer monsoon (EASM) records suggests that both ISM and EASM varied in unison, implying common forcing factors on such longer timescales. Copyright © 2010 John Wiley & Sons, Ltd.