ORV Sagar Kanya Research Articles

A note on observations on cetaceans in the western Indian sector of the Southern Ocean (20-56°S and 45-57°30’E), January to March 2004

A multi-disciplinary and multi-institutional pilot expedition was organised by the National Centre for Antarctic and Ocean Research (NCAOR) to the ice-free areas of the Western Indian Ocean sector of the Southern Ocean onboard ORV Sagar Kanya during the austral summer of 2004 (January-March). This survey, conducted by the Central Marine Fisheries Research Institute (CMFRI), is the first Indian attempt to survey for cetacean diversity in the Southern Ocean. The ultimate objective is to determine distribution, relative abundance, migration patterns and critical habitat parameters. 68% of a total of 13 sightings (22 individuals) were positively identified and species observed included Antarctic minke whales (Balaenoptera bonaerensis), fin whales (B. physalus), sei whales (B. borealis) and blue whales (B. musculus). The blue whales were not identified to the sub-species level. The highest concentration of cetaceans was between 35° and 37°S (along 45°E) and between 48° and 53°S (along 45°E). Relatively small numbers of cetaceans were observed during the present cruise, possibly because most of the cetacean sighting effort was made during inclement sea conditions. Results on the sighting characteristics and occurrence patterns of the cetaceans in relation to the region and hydrographical parameters are discussed briefly vis a vis published information from the Southern Ocean.

Spatio-temporal distribution of tropospheric water vapour over Arabian Sea and Equatorial Indian Ocean in boreal winter of 2018

Balloon-borne experiments carried out onboard ORV Sagar Kanya over the Arabian Sea (AS) and Equatorial Indian Ocean (EIO) during the period 16 January 2018–14 February 2018 in northern hemisphere winter are used to study the spatio-temporal distribution of tropospheric water vapour. The column-integrated water vapour (IWV) showed an increase of ~10 kg/m2 over the Equatorial Indian Ocean (EIO) compared to that over Arabian Sea (AS). Amount of water vapour in the free troposphere increases towards south with water vapour enhancement observed up to an altitude of 12 km over the equator. This is mainly due to convective outflows emanating from the Inter Tropical Convergence Zone (ITCZ). Along 2°S, the IWV showed an east–west (75–65°E) gradient with high values in the east. During the return leg of the ship cruise, when the AS is convectively disturbed with cloud tops extending up to mid-troposphere, the IWV showed an increase of about 10–15 kg/m2 compared to that observed during onward leg under fair weather conditions, particularly due to the accumulation of water vapour in the 2–7 km altitude region. Over the deep ocean the diurnal amplitude of water vapour shows a clear altitude variation with lesser amplitude in the lower troposphere compared to land.

Paleothermometric fluctuations and ornamentation pattern of Ostracoda species from a short core, off Ongole, Bay of Bengal, South-east Indian coast

An investigation on the elemental chemistry and its fluctuation in Ostracoda carapace to render the environmental traits from off Ongole, Andhra Pradesh is outlined in the paper. Paleothermometric elucidation of the environment using elements Ca, Mg, As, Nb, Sb, Al, Si, Cl, K and Na and the ornamentation pattern exhibited by Ostracoda are identified and ventilated. A short core of length 20 cm was collected from the study area using a multicorer during the ORV Sagar Kanya cruise (SK-308 Leg 1) fieldwork, at a water depth of 47 m. Ostracoda separated from the sediment matrix were subjected to standard micro-paleontological procedures. The elemental weight percentages reveal that the distribution of the elements in the carapace for different species is not the same and fluctuates along the downcore. The occurrence of Niobium in the offshore sediments is observed to be of terrestrial influence, which is derived from granitic and pegmatite rocks of the adjoining coastal terrain. A traceable quantity of Antimony and Arsenic is identified in the carapace of the certain Ostracoda. Mg/Ca ratio in carapace indicates that an endurance of mild temperature fluctuation to the downcore. Smooth, spinose, pitted, punctate and reticulate ornamentation are found occurring in Ostracoda carapace which reflects on the depositional environment and granulometry.

On the Correspondence between Stable Carbon Isotopes in Planktic Foraminiferal Species Globigerina bulloides and the Nutrients along a North-South Transect in South-Western Indian Ocean

During the pilot expedition to the Southern Indian Ocean onboard ORV Sagar Kanya a total of twenty one surface sediment samples were collected along a north-south transect between 9.50 N to 55.010 S latitude and 73.560 E and 44.860 E longitudes. These samples were collected to study the response of carbon isotope (δ13C values) fractionation in calcareous shells of planktic foraminiferal species Globigerina bulloides towards the nutrient contents of the ambient sea waters along a northsouth transect in the south-western Indian Ocean. It may be inferred that isotopic values (δ13C) in general increase as the nutrient content of ambient water masses decreases in the regions south of 300 S latitude. On the other hand δ13C values are relatively low in the region north of 30o S having higher nutrients in the ambient waters. The results of present study demonstrate the potential of foraminiferal δ13C values to trace the signature of paleo-nutrients changes in this region. The inferences drawn here further necessitates to take up similar studies along many transect in and around study area to arrive at a comprehensive picture.

Ostracod biodiversity from shelf to slope oceanic conditions, off central Bay of Bengal, India

Thirty seven surface sediment samples collected through a multi-corer in seven transects of ORV Sagar Kanya, SK 308 Leg-1 from off central Bay of Bengal (shelf to slope environment) were analysed for living and dead Ostracoda. The samples, which ranged in water depth from ~29m to 2540m, yielded a total of 46 species belonging to 32 genera. The ostracod fauna of the Bay of Bengal exhibit a depth-related distribution that is controlled by substrate and hydrographic conditions. Sandy substrates of shallow water depth (~28m) are characterized by Propontocypris sp. and Cytherelloidea malaccaensis. These species are replaced by Neonesidea cracenticlavula and Bairdoppilata paraalcyonicola in high population, which shows positive linear correlation with salinity, temperature and Total Calcium Carbonate (TCC) in the outer shelf ~55–73m. Paijenborchella sp. dominates at ~763m in the bathypelagic zone, giving way to Parakrithella sp. at greater depths of ~2540m on soft-bottom substrates. The fauna represents an interesting mixture of displaced species and others known from bathyal and abyssal depths in the Bay of Bengal, including some pandemic deep sea species. The ostracod faunal diversity is higher in shallower water than in the deeper water, which may be due to the optimum temperature, pH, salinity and substrate. The fauna are comprised of three associations (Shelf/Upper Slope; Slope; Lower Slope/Abyss). A marked faunal turnover occurs at the outer-shelf. Factor Analysis was performed to determine the correlation of water and sediment parameters with the ostracod species. This study provides a detailed account of bathymetry and ostracod biodiversity recorded in the study area.

Signatures of different water masses in δ18O values, relative abundance of G. Bulloides and total planktonic foraminiferal assemblage across the Indian Ocean sector of the southern ocean: Initial results

Nineteen surface sediment samples, collected onboard ORV Sagar Kanya during the 199C and 200th (the pilot expedition to Southern Ocean) cruises along a north-south transect between 1.92° S to 55.01° S latitude and 67.88° E and 44.89° E longitude, were studied for total planktonic foraminiferal assemblage, relative abundance and the oxygen isotopic values of the indicator planktic species Globigerina bulloides. These profiles were compared with the latitudinal variations in the average salinity and nutrient (phosphate) contents of the top 100 meters water column. The initial results have traced the signatures of salinity linked variations of different water masses in the study region. The influence of Australian Mediterranean Water (AAMW) has been noticed in the present dataset.

Tracing the signature of various frontal systems in stable isotopes (oxygen and carbon) of the planktonic foraminiferal species Globigerina bulloides in the Southern Ocean (Indian Sector)

Twenty-five surficial sediment samples, collected on board ORV Sagar Kanya during her 199th and 200th cruises along a north-south transect between latitudes 9.69°N and 55.01°S, and longitudes 80°E and 40°E were studied for isotopic variations (values of δ18O and δ13C) of the indicator planktonic species Globigerina bulloides. The results indicate that from latitudes 9.69°N to 15°S both these isotopes (δ18O and δ13C) fluctuated significantly. Between latitudes from around 15°S to 30–35°S δ18O values steadily increased, whereas δ13C showed a decreasing trend. However, to the south of latitudes 30–35°S, both isotope values showed a similar response with a gradual increase up to latitude 50°S, beyond which δ18O continued to increase while δ13C declined. The characteristic patterns of the values of both isotopes indicates that the signatures of different water masses are associated with various frontal systems and/or water masses across the transect. The signature of the Polar Front at around latitude 50°S shows the specific response of the isotopic values (δ18O and δ13C) of G. bulloides. Such a response beyond 50°S latitude is ascribable to the general decrease in the ambient temperature, resulting in a continuous increase in δ18O values, while δ13C values decrease as a result of reduced photosynthesis in regions approaching higher latitudes owing to low light penetration. To further corroborate our results, those of many such transects from geographically distinct regions need to be studied for isotopic variations in the calcareous shells of planktonic foraminiferal species. The results have the potential to be used as a proxy to assess the movement of frontal systems in southern high latitude regions.

Possible Source of Intermediate Ions over Marine Environment

Measurements of small, intermediate and large ions made onboard ORV Sagarkanya over the Arabian Sea in May-June 2003 during Arabian Sea Monsoon Experiment (ARMEX) are reported here. The daily averaged values of small-, intermediate-, and large-ion concentrations measured for 36 days during this cruise have been used for analysis. The analysis shows a weak positive correlation of 0.14 between intermediate- and large-ion concentrations, which indicates that the sources of these two types of ions are different over ocean surface. The negative correlation is observed between small- and intermediate-ion concentration for entire period of cruise. In addition, it is seen that the intermediate-ion concentration shows a very good (r = 0.58) and significant positive correlation with sea surface pressure. Based on good negative correlation between small- and intermediate-ion concentrations and good positive correlation between intermediate-ion concentration and sea surface pressure, it has been proposed that attachment of small ions to the ultrafine particles transported from upper troposphere to marine boundary layer is the main source of intermediate ions over ocean surface. This study supports the idea that the main source of ultrafine particles over marine boundary layer (MBL) is entrainment of aerosol particles from the free troposphere.

Modulation of SST, SSS over northern Bay of Bengal on ISO time scale

[1] High resolution observations of atmospheric and oceanic variables are carried out at northern Bay of Bengal from 22nd July to 6th August 2009 on-board ORV Sagar kanya under the Continental Tropical Convergence Zone research/observational programme. Freshening of surface layer by more than 4 psu within 24 hours is observed, which is followed by warming in the surface layer temperature. The heat and salt budget analysis primarily indicates dominant role of advection processes on the evolution of temperature and salinity. The amount of rainfall received at observation site could not explain the observed freshening, thus an extensive analysis using wavelet coherence is done to find out the source of advected fresh water to the observed location. It is found that surface salinity in the northern Bay of Bengal (at 15° N) varies coherently with the rainfall over Ganga-Brahmaputra catchment area on intraseasonal time scale and with lag of about 60 days. Based on above observations, this study hypothesize that the intraseasonal rainfall variation modulates the amount of river discharge, which in turn modulates the salinity over northern Bay of Bengal on intraseasonal time scale. Since surface warming always follows the surface freshening, the time delay between the rainfall over catchment area and freshening at northern Bay of Bengal may explain the post monsoon warming. Importance of atmosphere-ocean coupling in driving the dynamics of the northern bay of Bengal has been clearly brought out in this study.

Chemistry of rainwater and aerosols over Bay of Bengal during CTCZ program

For the first time, simultaneous study on physical and chemical characteristics of PM10, PM2.5, and rainwater chemistry was attempted over the Bay of Bengal in monsoon season of 2009. The aerosols and rainwater samples were collected onboard ship ‘SK-261, ORV Sagar Kanya’ during Oceanographic Observations in the Northern Bay of Bengal under the Continental Tropical Convergence Zone (CTCZ) program conducted during 16 July to 19 Aug 2009. Aerosol samples collected by PM10 and PM2.5 were analyzed for various water soluble (Na+, K+, Ca2+, Mg2+, NH4+, Cl−, SO42− and NO3− and acid soluble (Fe2+, Al3+, Zn2+, Mn3+ and Ni2+) ionic constituents. The pH of rainwater varied from 5.10 to 7.04. Chloride ions contributed most to the total ion concentration in aerosol and rainwater, followed by Na+. Significant contributions of SO42−, NO3− and NH4+ found in PM2.5, PM10 and high concentrations of TSP and non sea-salt SO42− over the mid-ocean is attributed to the long range transport of anthropogenic pollution from the Indian continent. The scavenging ratio was maximum for coarse particles such as Ca2+ and minimum for fine particles like NH4+.

Deep-Sea Benthic Foraminiferal Distribution in South West Indian Ocean: Implications to Paleoecology

Five grab samples from the southwestern part of the Indian ocean were collected by ORV Sagar Kanya during the third expedition to the southern Indian ocean in June 2009. The sediment samples have been analyzed and recorded 36 benthic foraminiferal species belonging to 21 genera and 3 suborders. All the species were taxonomically identified, SEM photographed and illustrated. Deep sea-benthic foraminiferal species at different locations of South of West India Ocean (3150-4125 m water depth) is examined in terms of number of species (n) and diversity (d). The observed depth ranges of benthic foraminifera have been documented to recognize their bathymetric distribution. The valves of these parameters reached their maximum at 3190 m water depth. Productivity continued in the Indo-Pacific Ocean (the biogenic boom) and the Oxygen minimum zone (OMZ) intensified over large parts of Indian Ocean continually. The diversity values show more abrupt trend as depth increases. Species like Epistominella exigua and Pullenia bulloides occur at both 3150 m & 3465 m depths indicating depth persistence. Further, Oridorsalis umbonatus and Melonis sphaeroides occur at both 3150 m & 3465 m depths. Species like Gyroidina sp an indicate of low oxygen environment and Uvigerina hispida-costata indicative of high organic carbon are found to occur at 3150 m & 3740 m respectively. Factor analysis and Pearson correlation matrix was performed on foraminiferal census data of 10 highest ranked species which are present in at least one sample. 3 factors were obtained accounting for 72.81% of the total variance. Thus the study suggests that fluctuations in species diversity at the locations of the present study were related to changes in productivity during the geological past. Further, the faunal data do indicate the early Holocene Indian Ocean was influenced by increased ventilation perhaps by North Atlantic deep water and or circumpolar deep waters.

Latitudinal trends in morphological characteristics of Neogloboquadrina pachyderma (Ehrenberg) along a north–south transect in the south-western Indian Ocean

A total of 25 surficial sediment samples (Peterson grab, gravity and piston cores), collected during the Pilot Expedition to Southern Ocean (PESO) 2004 cruises 199C and 200 onboard the ORV Sagar Kanya along a N–S transect between 9.69 °N and 55.01 °S, and 80 and 40 °E in the Indian Ocean sector of the Southern Ocean (SW Indian Ocean), have been investigated for various morphological features—test size, mean proloculus size and coiling direction (dextral/sinistral forms)—of the planktic indicator species Neogloboquadrina pachyderma (Ehrenberg). The results show that the coiling directions co-vary with temperature and salinity, the abundances of sinistrally coiled forms increasing towards higher latitudes (south of 40 °S), whereas dextrally coiled forms show a reverse trend. Similarly, overall test and proloculus sizes depend largely on the physicochemical properties (salinity, temperature, nutrients, calcium saturation) of the ambient water masses. These observations suggest that, particularly at the boundaries between different water masses, variations in morphological features of N. pachyderma can meaningfully be used to reconstruct paleoceanographic conditions from Indian Ocean sediments.

Evolutionary Features of Marine Atmospheric Boundary Layer (MABL) over the Arabian Sea and the Onset of Monsoon over Kerala during ARMEX-2003

In this study an attempt has been made to examine the evolutionary features of the dynamic and thermodynamic characteristics of the marine atmosphere over the South-East Arabian Sea near 9.22°N, 74.51°E just two to three days prior to the onset of southwest monsoon over Kerala during 2003 and seek the linkages with the large-scale flow in the lower and middle troposphere at that time over the region. The marine meteorological data collected onboard ORV Sagarkanya as part of the experiment ARMEX-2003 for 4–8 June, 2003 are used. The monsoon onset over Kerala occurred on 8 June, 2003. The observed changes in the marine atmospheric boundary layer (MABL) characteristics just two days prior to the onset are discussed. It is observed that the MABL increased in height up to 4 km on 6 June from an initial height 2.8 km on 5 June. The top of the MABL dried up (Relative Humidity RH ~ 30–40%) with weak and variable winds throughout the day on 6 June while the air at 850 hPa is relatively humid (RH ~ 50–80%) but not saturated. A sequential increase in RH is associated with a change in the winds from southwesterly to westerly from 6 June onwards until the onset date. The changes in the lower and middle troposphere flow patterns over the Arabian Sea and Indian region are highlighted.

Fronts, water masses and heat content variability in the Western Indian sector of the Southern Ocean during austral summer 2004

High density CTD and XBT sections were covered from 35° to 56°S along 45°E and 57°30′E to investigate the morphology of the main fronts in the southwest Indian Ocean, as a part of the Indian pilot expedition to the Southern Ocean on board ORV Sagar Kanya. Northern branch of the Subtropical Front (NSTF) was observed at ∼ 35°30′S along 45°E. Along 57°30′E, the signature of the Agulhas Return Front (ARF) + Subtropical Front (STF) was identified with a rapid decrease in surface temperature between 43°30′ and 45°S and it is located with a southward shift compared to that at 45°E. The Subantarctic Front (SAF) was distinguished as two fronts as northern SAF (SAF1) and southern SAF (SAF2) along both the meridional sections. Polar Front1 (PF1) was identified between 49° and 50°S whereas Polar Front2 (PF2) was identified between 52° and 54°S along 45°E. This study reveals a southward shift of the oceanic fronts (ARF + STF) from west to east, with a maximum southward displacement of > 2° latitude at 57°30′E. A novel finding of this study is that along 45°E, SAF1 merged with ARF and SSTF and SAF2 ∼ 4° latitude southwards from the merged fronts whereas along 57°30′E, SAF1 was not identified as a merged front with ARF and STF as opposed to earlier studies [Belkin, I.M., Gordon, A.L., 1996. Southern Ocean fronts from the Greenwich Meridian to Tasmania. Journal of Geophysical Research 101, 3675–3696]. The thermocline region was absent south of PF. An enhancement in the mixed layer thickness from 42° to 52°S occurred in association with the strengthening of the wind forcing. Major water masses like Subtropical Surface Water, Subantarctic Surface Water, Mode Water, Antarctic Intermediate Water, Circumpolar Deep Water and Antarctic Bottom Water were identified along 45°E. Upper-ocean heat-content computation revealed a remarkable drop of 989 × 10 7 J m −2 at ∼ 42°S and 1405 × 10 7 J m −2 at ∼ 44°S along 45° and 57°30′E, respectively. We believe that this sudden drop in heat content affects the meridional heat transfer which is crucial to the regional climatic variability.

Amino sugars in suspended particulate matter from the Bay of Bengal during the summer monsoon of 2001

Amino sugars (AS) are important constituents of organic matter. However, very little is known about their cycling in marine waters. In this research, we assessed the distribution and cycling of these compounds in waters of the Bay of Bengal. For this purpose, samples of suspended particu late matter (SPM) were collected from 8 depths (surface to 1000 m) at 6 locations during the 166th cruise of the ORV Sagar Kanya in the Bay of Bengal in July/August 2001. The SPM samples were analysed for particulate organic carbon (POC), particulate nitrogen (PN) and AS concentrations and composition. The AS varied between 0.4 and 17.5 nmol/l. Concentrations were high in the surface waters and generally decreased with increasing depth. AS concentration decreased from the south to north. AS accounted for 0.01 to 0.71% and 0.05 to 2.37% of POC and PN, respectively. Rapid decrease in AS-C% and AS-N% with depth indicates that these compounds were preferentially degraded relative to bulk POC and PN. The composition of AS suggests that glucosamine (GLU-N) and galactosamine (GAL-N) were present in the surface SPM samples, and their abundance decreased from surface downwards. Relatively, low values of GLU-N/GAL-N ratio indicate that the organic matter was mostly derived from the detritus of micro-organisms. Our data suggest that chitin, a polymer of the glucosamine produced by many marine organisms was not the major source of AS in the Bay. Rapid cycling of these compounds indicates their importance in the cycling of nitrogen in marine waters

4D-variational assimilation over the Bay of Bengal using the Adjoint of a Variable Resolution Global Ocean Model

The adjoint technique has been widely used over the last two decades in applications involving the dynamics of the Atmosphere and Ocean. This present study applies this technique in the assimilation of oceanic parameters such as temperature and net-surface heat flux, in a variable resolution Ocean General Circulation Model (OGCM). The National Center for Environmental Prediction (NCEP) 1999 July–August global data (sea-surface temperature and derived surface heat flux) and vertical temperature profiles collected from ORV Sagar Kanya and INS Sagardhwani during the Bay of Bengal Monsoon Experiment (BOBMEX 1999) are used for assimilation over the Bay of Bengal. The net-surface heat flux and temperature obtained after assimilation show good agreement with their respective seasonal patterns over the whole Indian Ocean, though there are some differences in the numerical values of heat fluxes. The vertical profiles of temperature after assimilation over the Bay of Bengal show strong resemblance with the corresponding ship observations, emphasizing the importance of the adjoint technique in ocean data assimilation.

Biological Characteristics of Central Indian Basin Waters During the Southern Summer

Phytoplankton biomass, taxonomy, primary productivity, and photosynthetically available radiation (PAR) were studied as part of baseline data collection for prospective nodule mining in the Central Indian Basin during the ORV Sagar Kanya cruise SK-120 in January 1997. The phytoplankton cell counts and chlorophyll a estimates showed low biomass level, suggesting low rates of primary productivity in the region studied. The average chlorophyll a value was 0.775 mg m−3 at surface and 17.75 mg m−2 in the water column. Similarly, average primary productivity at surface was 3.72 mg C m−3 d−1 and was 51.23 mg C m−2 d−1 in column. The chlorophyll a maxima at 50 to 80 m was the characteristic feature of the euphotic zones of the area. Average phytoplankton counts at the surface were low (3960 cells/l), compared to those at 25 m (6421 cells/l) and 75 m (5187 cells/l). At most of the stations mesozooplankton biomass was maximum in the top 50 m water column, indicating the importance of grazing in the euphotic zone. Appreciable quantities of mesozooplankton were observed below the euphotic zone, where settlement of chlorophyll a occurs. The low iron concentration in the water and its relationship with the water column productivity were correlated. The results show that waters in the CIB have low productivity in the surface as well as subsurface layers. This is expected to change in this case of a mining discharge in to these layers, possibly locally affecting the existing marine ecosystems. The final impact of such mining activity may remain negligible in the deep sea environment.

Measurements of the ion concentrations and conductivity over the Arabian Sea during the ARMEX

Measurements of the small‐, intermediate‐, and large‐ion concentrations and the atmospheric electric conductivity of both polarities have been made over the Arabian Sea on four cruises of ORV Sagarkanya during the Arabian Sea Monsoon Experiment (ARMEX) during the monsoon and premonsoon seasons of 2002 and 2003. Seasonally averaged values of the total as well as polar conductivity are much higher during the monsoon than premonsoon season. Surprisingly, however, the concentration of small ions is less and those of large and intermediate ions are more during the monsoon than premonsoon season. The diurnal variations observed during the premonsoon season show that the nighttime small ion concentrations are about an order of magnitude higher than their daytime values. On the contrary, the daytime concentrations of the intermediate and large ions are much higher than those of their nighttime values. No such diurnal variations in ion concentrations are observed in monsoon season. Also examined are the variations in ion concentrations of different categories with distance from the coastline in different seasons and the ion concentration changes associated with the precipitation of various types that occurred over ORV Sagarkanya. It is sufficient to invoke the ion‐aerosol attachment process to explain our premonsoon observations. However, the generation of highly charged large ions by the bubble‐breaking process caused by the wave breaking due to strong southwesterly surface winds (10–12 m s−1) over the Arabian Sea is postulated to explain the monsoon season observations.

Intermediate ion formation in the ship's exhaust

Measurements of the small‐, intermediate‐ and large‐ion concentrations and atmospheric electric conductivity have been made onboard ORV Sagarkanya off the west coast of India during the Arabian Sea Monsoon Experiment (ARMEX) ‐ 2003. On a few days during the pre‐monsoon season, the intermediate ion concentration is observed to periodically increase by 20–60% of its ambient value with a periodicity of ∼2 hours. An examination of the ion‐concentration variations with respect to cruise pattern and the prevailing wind direction reveals that the observed increases in intermediate ion concentration are caused by the ship's exhaust polluting the air over the measurements site. Formation of intermediate ions in the engine's exhaust has been associated with the generation of chemiions within the engine combustors which provide centers for the rapid growth of molecular clusters and the formation of electrically charged sulphuric acid/water aerosols.

Effect of the onset of southwest monsoon on the atmospheric electric conductivity over the Arabian Sea

Measurements of the atmospheric electric conductivity and small, intermediate, and large ion concentrations made at four locations over the Arabian Sea onboard ORV Sagarkanya in the premonsoon and monsoon seasons during the Arabian Sea Monsoon Experiment in 2002–2003 are reported. Observations show a threefold increase in the daily average value of conductivity over a period of ∼10 days preceding the onset of southwest monsoon. The enhanced values of conductivity are accompanied with a higher concentration of large ions. Although ion concentrations in all categories exhibit large fluctuations, the average concentration levels of small ions decrease and of large ions increase during the monsoon period. It is proposed that the enhancement in conductivity associated with the onset of monsoon is due to the highly charged large ions produced by the bubble bursting process at the air‐sea interface caused by the breaking waves during the high‐wind‐speed periods over the Arabian Sea. The correlation coefficient between the large ion concentration and wind speed measured at Sagarkanya is found to increase from 0.41, 0.16, and 0.23 in the premonsoon and two monsoon periods to 0.56 during the monsoon onset period. The enhanced value and the large variability of large ion concentrations observed during the monsoon season are associated with the spatial and temporal heterogeneity in not only their production but also their dissipation by the cloud scavenging processes over the Arabian Sea.