Arabian Sea Monsoon Experiment Research Articles

Empirical Estimation of Hourly Incoming Shortwave Radiation over the North Indian Ocean

Hourly estimation of incoming shortwave radiation formulae have been developed and given by many scientists. The empirical formulae have to be updated with the latest realistic data. Using Lumb procedure, author attempted to give new regression coefficients for incoming shortwave radiation with in situ observations made during Arabian Sea Monsoon Experiment (ARMEX) cruises over North Indian Ocean for different cloud amounts (from clear sky to overcast conditions). ARMEX field work program has been conducted in June-August 2002 and March-June, 2003 on board R/V “Sagar Kanya”. The shortwave radiation data has been collected using the Albedometer, which is covering entire cloud amounts. Empirical formulae have been developed using sine of solar altitude (sin (h)) with respect to different could amounts. Newly developed Empirical Formulae are better and more accurate as is evidenced by standard deviation. However newly developed empirical formulae are compared with other formulae given by other scientists. The empirical formulae have been compared with available observed data from Bay of Bengal Monsoon Experiment (BOBMEX) and NCEP re-analyzed data. With BOBMEX data is in agreement, however with NCEP showing lower values.

Carbon dioxide and water vapour characteristics on the west coast of Arabian Sea during Indian summer monsoon

Carbon dioxide, water vapour, air temperature and wind measurements at 10 Hz sampling rate were carried out over the coast of Arabian Sea, Goa (15°21′N, 73°51′E) in India. These observations were collected, in association with the surface layer turbulent parameters for the Arabian Sea Monsoon Experiment (ARMEX). In the summer monsoon period, concentration of CO2 was in the range of 550–790 mg m − 3 whereas the water vapour was in the range of 17.5–24.5 g m − 3. The Fast Fourier Transform (FFT) analysis has been performed on these observations to investigate the spectral behaviour of CO2 and water vapour. The relation between CO2 and water vapour on various atmospheric scales has been proposed. CO2 and water vapour observations confirmed the existence of periodicities of large (11, 8 days), meso (5 days) and micrometeorological (20 min) scales.

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.

Natural reduction of CO2 observed in the pre-monsoon period at the coastal station, Goa

Measurements of carbon dioxide (CO2) concentration were made at a coastal land station, Goa, on the west coast of India from March to June 2003 as part of the ARMEX (ARabian sea Monsoon Experiment) campaign. The observations show a systematic reduction (~120 mg m−3) of CO2 concentration during the pre-monsoon months, March–May, during which no significant change in anthropogenic emissions takes place. CO2 shoots up from 520 to 635 mg m−3 in June with the onset of the South West monsoon. Back trajectories show that the source of air mass gradually shifts from the coastal land mass to the open southern Arabian Sea during the pre-monsoon period. The observed reduction in CO2 is explained in terms of earlier measurements in the Arabian Sea indicating maximum chlorophyll a (Sarupria and Bhargava in J Mar Sci 27:292–297, 1998) and minimum partial pressure of CO2 (Sarma in J Geophys Res 108:3225, 2003) in the sea waters off the west coast of India during the pre-monsoon period, cleaner marine air mass advection from the open sea, and negligible local vertical CO2 flux.

Occurrence of extremely low cold point tropopause temperature during summer monsoon season: ARMEX campaign and CHAMP and COSMIC satellite observations

[1] Extreme low cold point tropopause (CPT) temperatures (T ≤ 191 K) are often observed during the monsoon season over the Bay of Bengal (BOB) and adjoining areas. This paper reports frequent occurrences of extreme low CPT temperature over the Arabian Sea (AS) and adjoining areas using radiosonde observations during the Arabian Sea Monsoon Experiment (ARMEX) from 24 June to 15 August 2002. Day-to-day variations in temperature at CPT and at the 100 hPa level observed during the ARMEX campaign show modulation by the wave activity with a period of ∼15 days, and it is observed to be closely associated with the Tropical Easterly Jet (TEJ). Characteristics of wave modulating the temperature at the CPT and at the 100 hPa level are brought out and discussed. Spatial and temporal distribution of low CPT temperature over a wide scale is examined using CHAMP and COSMIC satellite temperature data. These observations show occurrences of low CPT temperatures during the early period of the monsoon season over BOB, AS, and adjoining areas, which often extend to Africa's Horn region. An enhanced low CPT temperature occurrence during the early part of the monsoon appears to be due to the modulation of outgoing long wave-radiation (OLR), CPT temperature, and height by intraseasonal oscillation. Modulation of CPT by intraseasonal oscillation suggests that this oscillation could contribute to dehydration of the lower stratosphere. In addition, a close association is noted between the seasonal variations of the latitude of low CPT temperature and low OLR, which is similar to the anticipated seasonal movement of the Intertropical Convergence Zone (ITCZ).

Bimodal variation of SST and related physical processes over the North Indian Ocean: special emphasis on satellite observations

Using sea surface temperature (SST) and wind speed retrieved by the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI), for the period of 1998–2003, we have studied the annual cycle of SST and confirmed the bimodal distribution of SST over the north Indian Ocean. Detailed analysis of SST revealed that the summer monsoon cooling (winter cooling) over the eastern Arabian Sea (Bay of Bengal) is more prominent than winter cooling (summer monsoon cooling). A sudden drop in surface short wave radiation by 57 W m−2 (74 W m−2) and rise in kinetic energy per unit mass by 24 J kg−1 (26 J kg−1) over the eastern Arabian Sea (Bay of Bengal) is observed in summer monsoon cooling period. The subsurface profiles of temperature and density for the spring warming and summer monsoon cooling phases are studied using the Arabian Sea Monsoon Experiment (ARMEX) data. These data indicate a shallow mixed layer during the spring warming and a deeper mixed layer during the summer monsoon cooling. Deepening of the mixed layer by 30 to 40 m with corresponding cooling of 2°C is found from warming to summer monsoon cooling in the eastern Arabian Sea. The depth of the 28°C isotherm in the eastern Arabian Sea during the spring warming is 80 m and during summer monsoon cooling it is about 60 m, while over the Bay of Bengal the 28°C isotherm is very shallow (35 m), even during the summer monsoon cooling. The time series of the isothermal layer depth and mixed layer depth during the warming phase revealed that the formation of the barrier layer in the spring warming phase and the absence of such layers during the summer cooling over the Arabian Sea. However, the barrier layer does exist over the Bay of Bengal with significant magnitude (20–25 m). The drop in the heat content with in first 50 m of the ocean from warming to the cooling phase is about 2.15 × 108 J m−2 over the Arabian Sea.

Validation of MODIS derived aerosol optical depth and an investigation on aerosol transport over the South East Arabian Sea during ARMEX-II

Abstract. The influence of wind and humidity on aerosol optical depth (AOD) over the Arabian sea is being investigated using MODIS (Moderate Resolution Imaging Spectroradiometer) Level 3 (Collection-5) and NCEP (National Centres for Environmental Prediction) reanalysis data for the second phase of the Arabian Sea Monsoon Experiment (ARMEX-II) over the South East Arabian Sea (SEAS) in the pre-monsoon period (14 March–10 April 2003). In order to qualify MODIS data for this study, MODIS aerosol parameters were first compared with ship borne Microtops measurements. This showed correlations 0.96–0.97 in the case of spectral AODs and a correlation 0.72 for the angstrom exponents. The daily AOD data from MODIS and winds from NCEP reveal that the ship observed episodic enhancement and decay of AOD at the TSL (Time Series Location) during 23 March–6 April 2003 was caused by the southward drift of an aerosol pocket driven by an intensification and reduction of surface pressure in the North Western Arabian Sea with a low altitude convergence prevailing over SEAS. The AOD increase coincided with a decrease in the Angstrom exponent and the fine mode fraction suggesting the pocket being dominated by coarse mode particles. A partial correlation analysis reveals that the lower altitude wind convergence is the most influential atmospheric variable in modulating AOD over the ARMEX-II domain during the TSL period. However, surface winds at a distant zone in the north/north west upwind direction also had a moderate influence, though with a lag of two days. But this effect was minor since the winds were not strong enough to produce marine aerosols matching with the high AODs over the ARMEX-II domain. These findings and the similarity between MODIS column mass concentration and the ship borne QCM (Quartz Crystal Microbalance) measured coarse mode mass concentration, suggest that the aerosol pocket was mostly composed of coarse mode mineral dust in the lower atmospheric altitudes transported from the Arabian deserts.

Warm pool thermodynamics from the Arabian Sea Monsoon Experiment (ARMEX)

Before the onset of the south Asian summer monsoon, sea surface temperature (SST) of the north Indian Ocean warms to 30–32°C. Climatological mean mixed layer depth in spring (March–May) is 10–20 m, and net surface heat flux (Qnet) is 80–100 W m−2 into the ocean. Previous work suggests that observed spring SST warming is small mainly because of (1) penetrative flux of solar radiation through the base of the mixed layer (Qpen) and (2) advective cooling by upper ocean currents. We estimate the role of these two processes in SST evolution from a two‐week Arabian Sea Monsoon Experiment process experiment in April–May 2005 in the southeastern Arabian Sea. The upper ocean is stratified by salinity and temperature, and mixed layer depth is shallow (6 to 12 m). Current speed at 2 m depth is high even under light winds. Currents within the mixed layer are quite distinct from those at 25 m. On subseasonal scales, SST warming is followed by rapid cooling, although the ocean gains heat at the surface: Qnet is about 105 W m−2 in the warming phase and 25 W m−2 in the cooling phase; penetrative loss Qpen is 80 W m−2 and 70 W m−2. In the warming phase, SST rises mainly because of heat absorbed within the mixed layer, i.e., Qnet minus Qpen; Qpen reduces the rate of SST warming by a factor of 3. In the second phase, SST cools rapidly because (1) Qpen is larger than Qnet and (2) advective cooling is ∼85 W m−2. A calculation using time‐averaged heat fluxes and mixed layer depth suggests that diurnal variability of fluxes and upper ocean stratification tends to warm SST on subseasonal timescale. Buoy and satellite data suggest that a typical premonsoon intraseasonal cooling event occurs under clear skies when the ocean is gaining heat through the surface. In this respect, premonsoon SST cooling in the north Indian Ocean is different from that due to the Madden‐Julian oscillation or monsoon intraseasonal oscillation.

Seasonal changes in aerosol characteristics over Arabian Sea and their consequence on aerosol short-wave radiative forcing: Results from ARMEX field campaign

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.

Simulation of intense organized convective precipitation observed during the Arabian Sea Monsoon Experiment (ARMEX)

We examine a deep precipitating system that formed over the west coast of India during 26–28 June 2002 producing heavy rainfall of 2–61 cm day−1. The system developed into a well‐marked low pressure area due to interaction between an eastward moving westerly trough and a westward moving monsoon low. We used the PSU/NCAR Mesoscale Model (MM5) to make 10‐day interactively nested simulations at 90, 30, and 10 km grid‐resolutions. We used observations from a special data set collected during an Arabian Sea Monsoon Experiment (ARMEX) conducted June–August, 2002. Nudging the observations produced a balanced atmosphere which, in turn, matched the location of vortex with cloud clusters observed from satellite. The simulated rainfall corresponded well with observations. We then use the simulated fields as forcing and boundary conditions for MM5 run in cloud‐system resolving mode at 2 km grid‐resolution to detail cloud‐clusters embedded within the monsoon disturbance. We examined the sensitivity to different physical parameterizations and also the effect of continuous nudging four dimensional data assimilation (FDDA) on the rainfall forecasts. While the simulation of the convective event improved with certain combinations of physical parameterizations, the rainfall was not forecasted at the correct location, no matter which parameterization was used, unless continuous FDDA was performed in all domains throughout the integrations. Finally, cloud‐cluster properties of the cloud‐system resolving simulations were compared with observations.

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.

Variation of coastal atmospheric boundary layer characteristics with convective activity along the west coast of India during the Arabian Sea Monsoon Experiment (ARMEX) 2002

This paper investigates the characteristic features of the coastal atmospheric boundary layer (CABL) along the west coast of India during the south-west monsoon (SWM) 2002. Extensive surface and upper-air findings were obtained during the same period from the Arabian Sea Monsoon Experiment (ARMEX; 15th June to 15th August 2002) 2002. The operational general circulation model (GCM) of the National Centre for Medium Range Weather Forecasting (NCMRWF) was used in this study to see the spatial variation of the CABL during two specific convective episodes that led to heavy rainfall along the west coast of India. The impact of a non-local closure (NLC) scheme employed in the NCMRWF GCM was carried out in simulating the CABL. The same episodes were also simulated using a similar parameterization scheme employed in the high resolution mesoscale modelling system (MM5). The diurnal variation of CABL is better represented from MM5 simulation. Comparing the MM5 simulation with that of the coarser grid NCMRWF GCM, we observed that the NCMRWF GCM underestimates the values of both latent heat flux (LHF) and the coastal atmospheric boundary layer height (CABLH). Results from MM5 therefore indicate that the best way to move forward in addressing the short-comings of coarse grid-scale GCMs is to provide a parameterization of the diurnal effects associated with convection processes.

Study of aerosol transport through precipitation chemistry over Arabian Sea during winter and summer monsoons

Precipitation samples over the Arabian Sea collected during Arabian Sea Monsoon Experiment (ARMEX) in 2002–2003 were examined for major water soluble components and acidity of aerosols during the period of winter and summer monsoon seasons. The pH of rain water was alkaline during summer monsoon and acidic during winter monsoon. Summer monsoon precipitation showed dominance of sea-salt components (∼90%) and significant amounts of non-sea salt (nss) Ca 2+ and SO 4 2−. Winter monsoon precipitation samples showed higher concentration of NO 3 − and NH 4 + compared to that of summer monsoon, indicating more influence of anthropogenic sources. The rain water data is interpreted in terms of long-range transport and background pollution. In summer monsoon, air masses passing over the north African and Gulf continents which may be carrying nss components are advected towards the observational location. Also, prevailing strong southwesterly winds at surface level produced sea-salt aerosols which led to high sea-salt contribution in precipitation. While in winter monsoon, it was observed that, air masses coming from Asian region towards observational location carry more pollutants like NO 3 −and nss SO 4 2− that acidify the precipitation.

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.

Intense rainfall events over the west coast of India

The west coast of the Indian peninsula receives very heavy rainfall during the summer Monsoon (June–September) season with average rainfall over some parts exceeding 250 cm. Heavy rainfall events with rainfall more than 15 cm $day^{-1}$ at one or more stations along the west coast of India occur frequently and cause considerable damage. A special observational programme, Arabian Sea Monsoon Experiment,as carried out during the monsoon season of 2002 to study these events. The spatial and temporal distributions of intense rainfall events, presented here, were used for the planning of this observational campaign. The present study using daily rainfall data for summer monsoon season of 37 years (1951–1987) shows that the probability of getting intense rainfall is the maximum between $14^\circ N-16^\circ N$ and near $19^\circ N$. The probability of occurrence of these intense rainfall events is high from mid June to mid August, with a dip in early July. It has been believed for a long time that offshore troughs and vortices are responsible for these intense rainfall events. However, analysis of the characteristics of cloud systems associated with the intense rainfall events during 1985–1988 using very high resolution brightness temperature data from INSAT-IB satellite shows that the cloud systems during these events are characterized by large spatial scales and high cloud tops. Further study using daily satellite derived outgoing longwave radiation (OLR) data over a longer period (1975–1998) shows that, most of these events (about 62%) are associated with systems organized on synoptic and larger scales. We find that most of the offshore convective systems responsible for intense rainfall along the west coast of India are linked to the atmospheric conditions over equatorial Indian Ocean.

Moist convective instability over the Arabian Sea during the Asian summer monsoon, 2002

AbstractThe moist convective instability over the Arabian Sea during the Arabian Sea Monsoon Experiment 2002 is studied using high resolution radiosonde data over the Arabian Sea off the west coast of India and inland at Goa (15°21′N,73°51′E). The vertical structure of the lower troposphere is discussed in terms of the profiles of conserved variables and the time series of convective available potential energy (CAPE), mixed‐layer depth, lifting condensation level and level of free convection. Analysis shows that the Arabian Sea within 200 km of the west coast of India is characteristically a low‐CAPE region irrespective of whether it is a good or bad monsoon season. Little or no variability in the mean (0–500 Pa) lapse rate during July and August indicates the quasi‐equilibrium state of the tropical atmosphere during the summer monsoon with an approximate balance between the convective and large‐scale forces. Large‐scale forcing for the ascent of surface air seems to be of prime importance for triggering deep convection and rainfall over the East Arabian Sea. Case studies of precipitating convection over inland at Goa during the monsoon indicate a moistening and warming of the lower troposphere following the convection. Copyright © 2006 Royal Meteorological Society.

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.

Hydrography and water masses in the southeastern Arabian Sea during March–June 2003

This paper describes the hydrographic observations in the southeastern Arabian Sea (SEAS) during two cruises carried out in March–June 2003 as part of the Arabian Sea Monsoon Experiment. The surface hydrography during March–April was dominated by the intrusion of low-salinity waters from the south; during May–June, the low-salinity waters were beginning to be replaced by the highsalinity waters from the north. There was considerable mixing at the bottom of the surface mixed layer, leading to interleaving of low-salinity and high-salinity layers. The flow paths constructed following the spatial patterns of salinity along the sections mimic those inferred from numerical models. Time-series measurements showed the presence of Persian Gulf and Red Sea Waters in the SEAS to be intermittent during both cruises: they appeared and disappeared during both the fortnight-long time series.

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.

Observed variability of sea surface salinity and thermal inversions in the Lakshadweep Sea during contrast monsoons

[1] The sea surface salinity (SSS) of the Lakshadweep Sea (LS) shows large seasonal variability due to horizontal advection of low (high) salinity waters from south (north) during winter (summer) monsoon. The measurements made in the LS during the Arabian Sea Monsoon Experiment (ARMEX), have shown a dramatic drop of 3.4 PSU in the SSS during the winter following an active (2003) monsoon that is double that of the drop of 1.7 PSU observed during the winter following a weak (2002) monsoon. The resultant near-surface thermal inversions also have shown large differences in the life cycle and depth of occurrence between these two winters.