Northeast Monsoon Period Research Articles

Surface Boundary-Layer Characteristics Over a Tropical Inland Station: Seasonal Features

Some characteristic features of the atmospheric surfacelayer over an inland tropical station, Lucknow (27°N, 81°E)in India, are investigated for different seasons using micrometeorological data collected at the 10-m levelof an instrumented tower. Seasonal monsoonal variations in wind circulation (north-easterliesand south-westerlies) dominate the flow over this part of the region. This study utilizesthe wind, temperature and humidity data obtained for a period of one year from February 1996 to January 1997. The diurnal variation of mean parameterssuch as temperature, relative humidity, and wind speed are studied for different seasonsnamely pre-, south-west, post- and north-east monsoon periods. Turbulent statistics are computedusing the eddy correlation technique, and are studied under the framework of Monin–Obukhov similarity theory with results compared with otherexperimental studies reported in the literature. It is found that the turbulence statistics arenearly independent of season and the near-neutral values of the normalized standard deviations ofthe three wind components, temperature and specific humidityare found to be 1.00 ± 0.04 (vertical), 2.63 ± 0.36(longitudinal), 2.19 ± 0.06 (lateral), 6.56 ± 0.33and 6.45 ± 0.33 respectively.

Diurnal and seasonal variabilities in surface ozone at a high altitude site Mt Abu (24.6°N, 72.7°E, 1680m asl) in India

Abstract Measurements of surface ozone, CO, CH4 and oxides of nitrogen have been made at a high altitude site Mt Abu (24.6°N, 72.7°E, 1680 m asl), India for the period 1993–2000. Diurnal patterns in ozone do not show daytime in situ photochemical buildup at this site throughout the year. On the contrary, lower ozone mixing ratios are observed during the day. Interestingly, diurnal pattern changes significantly after the northeast monsoon period (winter) and shows a unique double hump in spring at this site. Seasonal variation in ozone shows a pronounced maximum (monthly average about 46 ppbv) in the late autumn and winter, unlike many other global sites. Local pollutants are major contributors to the ozone levels during this period, while regional transport plays a role in spring. Lowest ozone mixing ratio (monthly average about 25 ppbv) is observed during southwest monsoon period (summer), when clean marine air from the Arabian Sea and Indian Ocean reaches to this site. The unique meteorology over this region seems to play an important role in seasonal as well as in diurnal variations in ozone. Background and continental ozone levels are estimated to be 33.4±13.3 and 48.1±9 ppbv, respectively, over this region of India. A correlation study between ozone and CO indicates possibility of incomplete photochemical processes over Asia. Annual average mixing ratios of CO, CH4 and oxides of nitrogen are observed to be about 131.4±35.8 ppbv, 1.63±0.04 ppmv and 1.5±1.4 ppbv, respectively.

Coexistence and reproductive isolation of the sympatric echinoids Diadema savignyi Michelin and Diadema setosum (Leske) on Kenyan coral reefs

The sympatric echinoids Diadema savignyi and D. setosum coexist in shallow reef lagoons throughout East Africa. The reproductive strategies of these echinoids were studied to investigate reproductive isolation as a possible mechanism for maintaining the coexistence of these closely related species. The annual reproductive cycle and lunar periodicity were determined by gonad index measurements, histological examination of gametogenesis, and induction of spawning with injections of KCl. The peak reproductive period of D. savignyi coincided with the north-east monsoon period (when light and temperatures are high) as gonad indices were high (>8%) beginning in February and peaked at 9.7% in May. Gonad indices subsequently rapidly decreased (by 26%) in June at the beginning of the cooler south-east monsoon period. However, the presence of sperm and ova in most months of the year indicates continuous gametogenesis with reduced reproductive effort during the cooler months. The annual cycle for D. setosum showed less of a seasonal trend as gonad indices remained above 7% throughout much of the year but tended to be highest when temperatures were lower. This is the first confirmation of continuous reproduction in these two species at the equator. The reproductive patterns of both species remained consistent over 2 years of sampling. Both species exhibited a synchronized lunar spawning periodicity during the 3 months sampled, with D. setosum spawning on lunar days 8–10 and D. savignyi spawning after the full moon (lunar days 17–18). Whereas spawning in D. savignyi was very tightly synchronized, 20% of D. setosum individuals still spawned after the peak spawning period. The coexistence of these closely related species appears to be maintained by temporal reproductive isolation during the lunar spawning period reinforced by seasonal differences in reproductive effort.

Study on the rainfall climatology of Regional Research Station, Paiyur for crop planning

Regional Research Station, Paiyur is located in Krishnagiri taluk of Dharmapuri District and more than 80 per cent of the area is under dryland farming. The daily rainfall data for 28 years were analysed for its variability and probability. The mean annual rainfall was 918 mm with 32 per cent variability. The contribution from Winter, Summer, Southwest monsoon, Northeast monsoon periods to total rainfall was 1.3, 14.6, 49.8 and 34.3 per cent. Rains during May, July, August, September and October are more dependable. Each standard week from 29th to 46th receive a rainfall of more than 20 mm, indicating the crop growing period from July third week to November 2nd week

Activity of cloud-to-ground lightning observed in Sri Lanka and in surrounding area of the Indian Ocean

Birthday UNIVERSITY OF COLOMBO Birthday 11.6568 Normal 0 false false false MicrosoftInternetExplorer4 st1\:*{behavior:url(#ieooui) } /* Style Definitions */ table.MsoNormalTable {mso-style-name:Table Normal; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-parent:; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin:0cm; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:Times New Roman; mso-ansi-language:#0400; mso-fareast-language:#0400; mso-bidi-language:#0400;} Activity of cloud-to-ground lightning flashes observed during the monsoon thunderstorms over Sri Lanka with a lightning locating system consisting of two direction finders (DF) is presented. The Northeast monsoon produced over 884 cloud-to-ground flashes with a peak lightning rate of 96 flashes per hour whereas Southwest monsoon produced 3,294 flashes with a peak rate of 104 flashes per hour. A relative flash density maximum of 198 flashes per 28×28 km 2 was observed over Ratnapura area (6.68N, 80.40E). The percentage of positive flashes and average peak lightning current values for negative flashes are found to be 6.4% and 36 kA respectively for the Northeast monsoon period and 1.7% and 40 kA respectively for the Southwest monsoon period for the flashes that struck within 10-250 km range. No significant difference in average peak lightning current values were observed for the negative and positive flashes in both monsoons. doi:10.4038/sljp.v2i0.177 Sri Lankan Journal of Physics, Vol.2 (2001) 13-19

Zooplankton: pre-southwest and northeast monsoons of 1993 to 1994, from the North Arabian Sea

Zooplankton samples from the North Arabian Sea Environment and Ecosystem Research (NASEER) cruises were analyzed to determine the basic taxonomic composition, biomass (standing stock) and the total and copepod numeric abundance; these characteristics are discussed with reference to the different monsoon periods. Cruises carried out during March 1993 and May 1994, categorized as pre-southwest monsoon periods, and a cruise in December 1994, categorized as a northeast monsoon period, are discussed in detail. The biomass of January 1992 versus August 1992 and August 1992 versus March 1993 differed significantly (F = 6.44, P≤ 0.05). Ranges of highest and lowest biomass from each cruise are also given. Distinct “high” and “low” production areas of statistically significant difference (F = 12.67, P≤ 0.05) were observed. The “high” and “low” production areas were mobile and followed the surface wind circulation patterns (wind reversal pattern) during the northeast and southwest monsoon periods. Overall zooplankton showed a patchy distribution. The overall zooplankton abundance and total copepod counts differed significantly between the Cruises 3 versus 4 and 4 versus 5 (F = 15.67, P≤ 0.05 and F = 34.39, P≤ 0.05, respectively). There was no significant difference (P≥ 0.05) in biomass, between eutrophic and oligotrophic stations, suggesting no difference between near shore and offshore waters. Thirty-eight taxonomic groups were identified from the samples, with copepods being the most dominant group, followed by chaetognaths and siphonophores. Copepods constitute an average of 52.50 to 74.93% of the total zooplankton count and reach maxima of 92.14% of the total zooplankton count at the outset of the southwest monsoon (March 1993) and 91.39% at the outset of the active northeast monsoon (December 1994).

A model for the natural and anthropogenic aerosols over the tropical Indian Ocean derived from Indian Ocean Experiment data

The physical, chemical and radiative properties of aerosols are investigated over the tropical Indian Ocean during the first field phase (FFP) of the international Indian Ocean Experiment. The FFP was conducted during February 20 to March 31, 1998. The results shown here are from the Kaashidhoo Climate Observatory (KCO), a new surface observatory established on the tiny island of Kaashidhoo (4.965°N, 73.466°E) in the Republic of Maldives. From simultaneous measurements of aerosol physical, chemical, and radiative properties and the vertical structure from lidar, we have developed an aerosol model which, in conjunction with a Monte Carlo radiative transfer model, successfully explains (within a few percent) the observed solar radiative fluxes at the surface and at the top of the atmosphere. This agreement demonstrates the fundamental importance of measuring aerosol physical and chemical properties for modeling radiative fluxes. KCO, during the northeast monsoon period considered here, is downwind of the Indian subcontinent and undergoes variations in the aerosol visible optical depth τν from ∼0.1 to 0.4, with a monthly mean of ∼0.2. Lidar data suggest that the aerosol is confined largely to the first 3 kms. Sulfate and ammonium contribute ∼29% to τν; sea‐salt and nitrate contributes ∼17%; mineral dust contributes ∼15%; and the inferred soot, organics, and fly ash contribute 11%, 20%, and 8% respectively. We estimate that anthropogenic sources may contribute as much as 65% to the observed τν. We consider both an externally and an internally mixed aerosol model with very little difference between the two in the computed radiative forcing. The observed scattering coefficients are in the upper range of those reported for other oceanic regions, the single‐scattering albedos are as low as 0.9, and the Angstrom wavelength exponents of ∼1.2 indicate the abundance of submicron aerosols. In summary, the data and the model confirm the large impact of anthropogenic sources. The surface global fluxes (for overhead Sun) decrease by as much as 50 to 80 W m−2 owing to the presence of the aerosols, and the top of the atmosphere fluxes increase by as much as 15 W m−2, thus indicating that anthropogenic aerosols are having a large impact on the tropical Indian Ocean.

Bacterivory in the northwestern Indian Ocean during the intermonsoon – northeast monsoon period

Bacterial grazing loss rates were studied by radioactive labeling of natural bacteria with L-(4,5- 3H) leucine and from the rate of disappearance of bacterial cells in the northwestern Indian Ocean. Bacterivory was measured in a mixed sample that had been combined from various depths across the euphotic zone. Experiments were performed on 26 occasions at 19 stations in the Gulf of Oman and the Arabian Sea during the intermonsoon–northeast monsoon period (November–December 1994). Combined uptake of radiolabeled bacteria (ULB) in 1–8 and 8–100 μm size fractions was somewhat lower than loss of label (LBL) measured in the bacterial fraction (0.2–1.0 μm), suggesting loss of radioactivity from the grazers due to metabolism. The less sensitive rate of disappearance of bacterial cells (LBC) was on average 51% higher than LBL estimates. Results from ULB and LBL measurements revealed that bacterivory was higher in the Gulf of Oman (average loss rate 4.1% h -1) than in the Arabian Sea where rates were slightly higher inshore (1.7% h -1) than in the central gyre. Heterotrophic nanoflagellates in the 1–8 μm size fractions were identified as the primary bacterivores. Microzooplankton (8–100 μm) accounted for 33% of total bacterivory in the Gulf of Oman but only 16% in the central Arabian Sea. Time-course experiments conducted at two stations indicated that diel changes in bacterivory may be substantial in the northwestern Indian Ocean.

PERFORMANCE EVALUATION OF SOLAR SUBMERSIBLE PUMP

A SP-4-8 Gundfos eight stage 50 mm submersible pump was erected and its performance evaluated at 30 m head in Coimbatore district. The module (Model BHPV- 1230A) had an output of 30 W±10 per cent at solar intensity of 100mWcm². Data on solar intensity, PC module output and pump output were observed for a period of one year. The results revealed that the discharge was directly proportional to solar intensity. The efficiency of PV system in converting solar energy into direct electricity varied from 3.72 to 5.15 per cent up to 50 mWem solar radiation intensity, whereas it was 5.2 to 7.04 per cent beyond 50 mWem. The pump efficiency varied from 42 to 54 per cent, but maintained around 43 per cent. The average pump discharge was 1700/h during the two monsoon periods and 2200/h in summer and winter. The predicted water flow based on sunshine hours varied from 19.000 21,700/day in winter and 17,300 20.0001 day day in summer. The same during south west and north east monsoon periods were 7600 - 12,000 and 11,500 13.500/day respectively.

Seasonal dynamics and biological productivity in the Arabian Sea Euphotic Zone as simulated by a three‐dimensional ecosystem model

A nitrogen‐based, seven‐component ecosystem model of Fasham [1993] type has been coupled to a three‐dimensional quasi‐geostrophic ocean general circulation model and applied to a simulation of the seasonal variability of physical and ecosystem variables in the northwestern Indian Ocean. The comparison of the solution with available data (seasonal changes in surface chlorophyll at certain key positions from the climatological coastal zone color scanner (CZCS) database, satellite‐derived annual primary production, and data from cruises) has highlighted many agreements but also some disagreements between the model and observations. In particular, the model produces an entrainment bloom in May‐June in the central Arabian Sea for which there is no evidence in the CZCS data and does not reproduce the observed August‐September bloom in the northern Arabian Sea. In the latter case an analysis showed that zooplankton grazing control prevents the development of the model bloom. A comparison of our solution with the results obtained by McCreary et al. [1996] using a 2.5‐layer physical‐ecosystem model showed that, despite numerous differences between the models, the period and location for most blooms in these solutions coincide closely. The model reproduced the differences in primary productivity between the northeast and southwest monsoon periods and showed that the model phytoplankton in the main upwelling areas were only seriously nutrient limited during the spring intermonsoon period. For large parts of the year the model predicts that phytoplankton production is closely coupled to zooplankton grazing with blooms only occurring when there are rapid changes in phytoplankton growth rate due either to the entrainment of nitrate into the mixed layer or decreased light limitation when the mixed layer is shallowing thereby allowing the phytoplankton to escape from grazing control. The predicted particle flux from the euphotic zone was of the order of 40% of the primary production and lagged the latter quantity by 8‐10 days. This time lag meant that the correlation between daily values of the two quantities was low, and this may partly explain the difficulties of finding good correlations between observations of these two quantities.

The Relationship Between some Large-scale Atmospheric Parameters and Rainfall over Southeast Asia: A Comparison with Features over India

Monthly rainfall data for 135 stations for periods varying from 25 to 125 years are utilised to investigate the rainfall climatology over the southeast Asian monsoon regime. Monthly rainfall patterns for the regions north of equator show that maximum rainfall along the west coasts occurs during the summer monsoon period, while the maximum along the east coasts is observed during the northeast monsoon period. Over the Indonesian region (south of the equator) maximum rainfall is observed west of 125 °E during northern winter and east of 125 °E during northern summer.

Seasonal and long term variations of aerosol content in the atmospheric mixing region at a tropical station on the Arabian sea-coast

Abstract Altitude profiles of aerosol number density in the atmospheric mixing region obtained using a bistatic CW lidar at Trivandrum (8.55 °N, 77 °E), a station situated on the West coast, near the southern tip of the Indian peninsula, during the period 1989 to 1994 showed a systematic seasonal dependence. The number density shows a peak at ~ 300–400 m which is quite prominent during the local summer period and becomes less discernable during the North-East monsoon period. The mixing region aerosol optical depth estimated using these aerosol number density profiles shows a seasonal variation with two maxima; one during the winter to summer transition period and the other during the South-West monsoon period. A long term increasing trend is also observed in the aerosol optical depth from 1989 to 1994. A comparison of the mixing region aerosol optical depth with the total aerosol optical depth obtained using a solar radiometer has shown that this increasing trend is not confined to the mixing region alone but extends to higher altitudes also. This study also shows that under quiescent conditions, ~ 30% on average of the total aerosol optical depth, at 0.5 μm wavelength, is contributed by the aerosols in the altitude region below 1 km.

Zooplankton field heterogeneity formation on the mesoscale: elements of the theory and empirical characteristics

The continuous records of mesozooplankton biomass (with the spatial averaging of pump track every 20 km) were carried out from the surface layer (2–6 m depth) over 5 transects in the western part of the Indian Ocean during the end of the north-east monsoon period. The spatial autocorrelation functions have shown that the average size of zooplankton heterogeneities on the mesoscale is close to 40 km. To model the field of zooplankton biomass b( t, x) the model based on the equation of non-conservative substance diffusion was applied: db bt + u( db dx ) = k( d 2b dx 2 ) + nb + a(t, x) , where t is the time; x the horizontal coordinate; u the velocity of mass transport by a current; k the coefficient of horizontal turbulent diffusion and n the normalised velocity of zooplankton growth. The field a( t, x) characterises the rest of the local fluctuations of zooplankton biomass not associated with the mentioned processes. The techniques of solving the equation based on the spectral theory of the random fields was solved by means of the evaluation of the field autocovariation functions.

FIELD EVALUATION OF IN-SI U MOISTURE CONSERVATION PRACTICE

A field experiment was conducted for two years during north-east monsoon period to study the effect of in-site water harvesting measure on different crops under rainfed condition at the Agricultural College and Research Institute, Killikulam, Tamil Nadu. Among the moisture conservation practices, contour ploughing was significantly superior to contour bunding but was on par with broad based ridging. Economics wise cowpea and castor were found to fetch more profit.

Heavy metals in surface sediment and epibenthic macroinvertebrates from the coastal zone and continental slope of Kenya

In surface sediments, a statistically significant increase was measured in the concentration of copper and cadmium along all transects, and of zinc along only the most southern (Gazi) transect, radiating into the Indian Ocean perpendicular to the Kenyan coast. Mean copper and cadmium increased from 5 to 30 μg g −1 dry wt and from 0.01 to 0.34 μg g −1 dry wt in shallow coastal (±20 m depth) to deep-sea stations (±2000 m depth), respectively. These gradients were found both during the south-east monsoon and north-east monsoon period. The shallow estuarine zone of the Sabaki river mouth showed significantly enhanced levels of total organic carbon and nitrogen and all metals analysed, except cadmium. In crustaceans, the concentration of copper and particularly cadmium was significantly above baseline levels, varying from 45 to 90 μg g −1 dry wt and 1.0 to 8.5 μg g −1 dry wt, respectively. Zinc levels (49–102 μg g −1 dry wt) were at about baseline levels or a little elevated. On the contrary, lead showed very low concentrations, varying from 0.1 to 0.6 μg g −1 dry wt. Other species generally showed the same pattern.

Evidence for eddy formation in the eastern Arabian Sea during the northeast monsoon

The seasonal formation of a large (500–800 km diameter) anticyclonic eddy in the upper 300–400 m of the eastern Arabian Sea during the northeast monsoon period (December‐April) is indicated from hydrographic and satellite altimetry sea level observations, as well as from numerical model experiments. The center of the eddy circulation is approximately 10°N, 70°E, just to the west of the north‐south Laccadive Island chain. In this paper the eddy is called the Laccadive High (LH). In some ways it is a mirrorlike counterpart to the Great Whirl, which develops during the southwest monsoon off the Somali coast (western Arabian Sea). The LH occurs at the same latitude but on the opposite side of the basin during the reversed monsoon. It is different from the Great Whirl, however, in its formation process, its intensity, and its decay. The hydrographic data obtained from surveys all during a single season give sufficiently close station spacing to allow reasonable contouring of the geopotential surfaces and of the properties within and around the LH region with minimum time aliasing. The Geosat altimeter record extends over 4 years, during which the seasonal variability of the LH indicates a dynamic relief of approximately 15–20 cm, which is in good agreement with the hydrographic observations. The altimetry time series also suggests a westward translation of the LH by January with a subsequent dissipation in midbasin. The model used is a wind‐forced three‐layer primitive equation model which depicts a LH in agreement with the timing, position, and amplitude of both the hydrographic and altimetric measurements. The numerical simulation includes a passive tracer located in the western Bay of Bengal; the western advection of the tracer around the south coasts of Sri Lanka and India in December and January is consistent with the appearance of low‐salinity water observed to extend into the Arabian Sea during this period. The modeling studies suggest that both local and remote forcing are important in formation of the LH.

Dissolved aluminium in the surface microlayer of the eastern Arabian sea

Measurements of dissolved aluminium (Al) in surface microlayer (SML) samples from the eastern Arabian Sea during the southwest (summer) and northeast (winter) monsoon periods have revealed much higher concentrations (23–657 nmol kg −1) than in the sub-surface layer (SSL; collected at 1 m depth). Higher concentrations observed during the summer monsoon and directional trends in the distribution, with concentration maxima situated away from the influence of fluvial discharge, are best explained by the atmospheric deposition of mineral dust aerosol and its subsequent dissolution in the surface seawater. The low Al concentrations in the SSL (11–296 nmol kg −1) varied in line with those in the SML. However, Al was significantly higher there than in the rest of the water column (up to 3000 m), indicating that much of the dissolved aerosol Al was removed in the upper few metres of the water column. The non-nutrient-type behaviour of dissolved Al did not support a mechanism of Al removal by biological uptake. High Al levels in the SML observed during the summer monsoon were associated with low primary productivity. The considerably lower values observed during the winter monsoon, a period of higher primary productivity in the Arabian Sea, could be due to removal from solution by adsorption onto biogenic particles.

Simulation of barotropic wind-driven circulation in the upper layers of Bay of Bengal and Andaman Sea during the southwest and northeast monsoon seasons using observed winds

A two-dimensional, nonlinear, vertically integrated model was used to simulate depth-mean wind-driven circulation in the upper Ekman layers of the Bay of Bengal and Andaman Sea. The model resolution was one third of a degree in the latitude and longitude directions. Monthly mean wind stress components used to drive the model were obtained from the climatic monthly mean wind data compiled by Hastenrath and Lamb. A steady-state solution was obtained after numerical integration of the model for 15 days. The sensitivity of the model to two types of open boundary conditions, namely, a radiation type and clamped type, was tested. A comparison of simulated results for January with available ship drift data showed that the application of the latter along the open boundary could reproduce all the observed features near the boundary and the interior of the model domain. The model was integrated for 365 days to study the circulation during the southwest and northeast monsoon seasons. The model was successful in simulating the broad features of circulation including gyres and eddies observed during both the seasons, the development of north equatorial current during the northeast monsoon period and eastward moving monsoon drift current up to 90°E during the southwest monsoon season. During the latter season, two anticyclonic gyres were observed in the central and the southern parts of the Bay. A cyclonic type of circulation was prevalent in the central and western parts of the Bay of Bengal during the northeast monsoon months of November and December. The simulated western boundary current along the east coast of India, flows northward and southward during the southwest and northeast monsoon seasons respectively. It is presumed that this western boundary current, simulated during both the seasons, is locally wind-driven.

Effects of Meteorological Factors on Toxic Red Tide Events in Sabah, Malaysia

Abstract. Red tides of the toxic dinoflagellate Pyrodinium bahamense var. compressum have occurred almost annually since 1976 in the coastal waters of the west coast of Sabah, Malaysia. Field obSérvations have indicated that the timings of these outbreaks could be linked to meteorological forcings. All the red tides have occurred during either the northeast or southwest monsoon periods. Matching of red tide outbreaks with total monthly rainfall, monthly‐averaged index of wind‐induced surface water mixing, and monthly‐averaged upwelling index for the period from 1975 to 1989 failed to show any definite pattern. However, more red tide events occurred in months with high mixing index values. The absence of definable patterns could be due to the use of monthly, rather than daily, values in the calculations.

Surveillance for Japanese encephalitis in villages near Madurai, Tamil Nadu, India

A simple dusk index was developed to monitor the density of recognized vectors of Japanese encephalitis virus (JEV) based on hand catches around cattlesheds at dusk and parous rates. When used routinely in combination with sentinel animal studies for surveillance in villages with a high prevalence (46·2%) of neutralizing antibodies against JEV in children under 16 years, there was a peak in vector density and virus activity during the north-east monsoon period, October–December. The reasons for an unusual outbreak of cases of encephalitis during the summer months of 1984 are discussed.