End Of Monsoon Research Articles

Geochemical tracing of synoptic scale modern dust transport over the Northeast Arabian Sea during the southwest monsoon

During the Southwest monsoon (SWM), aeolian dust is mainly supplied via wet deposition over the northeast Arabian Sea (NEAS). To understand their impact on the biogeochemistry of the Arabian Sea, it is important to identify their sources and characteristics. In this context, wet deposit particulate (WDP) samples were collected at a coastal station (Goa; 15.4° N, 73.8° E) in the NEAS during the SWM for three years. These samples were used to characterize and identify mineral dust sources using mineralogical, elemental, and isotopic (Sr and Nd) signatures. The WDP samples were classified as Beginning of Monsoon (BM, June samples), Mid Monsoon (MM, July–August samples) and End of Monsoon (EM, September samples). Clay mineralogical composition indicate high palygorskite content during BM, which subsequently found to decrease in MM, and almost negligible in EM. However, smectite is highest during MM, with moderate presence of palygorskite during this period. The considerable variation in the relative percentages of clay minerals suggests significant temporal variability in dust sources which is further corroborated by the radiogenic isotopic composition. A strong seasonality in the isotopic composition is observed with 87Sr/86Sr ratio being relatively less radiogenic during MM than the BM and highly radiogenic at the EM. Whereas ƐNd values show an opposite trend to 87Sr/86Sr ratios throughout the monsoon, with more radiogenic ƐNd in the MM, and less radiogenic at the EM. End member mixing plot indicate dominant contribution of dust from the Arabian Peninsula (ARB) and Northeast African (NEA) sources during BM and MM, while a shift towards the Thar desert and Southwest Asian (SWA) sources at the EM. Trace elements associated with different sources were quantified and suggest high Fe concentration is associated with NEA dust sources, despite ARB being major supplier of aeolian dust to the Arabian Sea.

Investigating hydrometeorology of the Western Himalayas: Insights from stable isotopes of water and meteorological parameters

The Himalayas govern the hydrometeorology of the entire Indian subcontinent and feed 19 major rivers on which a large population is dependent. Despite its vast socio-economic relevance, there exist knowledge gaps in the detailed understanding of Himalayan hydrometeorology. The present study attempts to understand subtle hydrometeorological processes concerning precipitation in Western Himalaya (WH), which is further complex due to several mountain ranges in addition to the Great Himalaya and varied land cover. In this backdrop, oxygen and hydrogen isotopic analyses of daily precipitation samples collected from Jammu was done. Major processes and features identified are: (1) Raindrop re-evaporation is dominant, indicated by lower δ18O – δD regression slope (6.6 ± 0.2). (2) Maximum raindrop re-evaporation occurs during Western Disturbances (WD), evident from the low d-excess (<10‰) and high δ18O (>0‰) with low rainfall. (3) Contrary to expectations, negligible moisture is derived from the Mediterranean region (MR) through WD. In contrast, maximum moisture (~97%) is derived from local terrestrial sources, estimated through backward wind trajectories and confirmed by vertical wind velocity (ω) and OLR. (4) The Arabian Sea (AS) moisture contributes more (~11%) compared to the Bay of Bengal (BOB) moisture (~8%) during the monsoon. (5) Isotopically, the most depleted precipitation during July-Aug is associated with continental recycling from the Indo-Gangetic plains and not with the BOB moisture. (6) Isotopic enrichment in precipitation towards end of monsoon is attributed to raindrop re-evaporation, lesser rainout fraction and moisture derived from enriched local sources. (7) Annually, 87% of the moisture for precipitation is derived through continental recycling, and only 13% of the moisture is derived through marine sources (11% from AS and 2% from BOB). Moisture from MR (<0.1%) is negligible.

Prevalence and seasonal trends of malaria at general hospital, Palanpur Banaskantha, Western part of India

Aim: Malaria is one of the known endemic disease in India. Malaria is transmitted by infected female anopheles mosquito vector transmitting mainly two Plasmodial species named P. falciparum and P. vivax. Prevalence of malaria is affected by atmospheric and socioeconomic condition of geographical area. Aim of the study is to know the prevalence of malaria and the seasonal trend of malaria at general hospital Palanpur, Banaskantha in western part of India. Material and Methods: This study was carried out at Central Laboratory, General Hospital, Palanpur, Banaskantha over period of July 2014 to June 2018. 41660 samples from fever patients were collected. We used Microscopic examination for the diagnosis of malaria by preparing thick and thin smears from these 41660 samples and stained using field stain. Results: We had collected total 41660 samples of fever patients. Out of 41660 samples, 688 samples were detected positive for malaria. So prevalence of malaria in our hospital was 1.65%. Prevalence of P. Vivax, P. Falciparum and mixed (infection of P. Falciparum and P. Vivax at a same time) were found 77.90%, 21.80% and 0.30% respectively. Malaria was seen relatively higher in males and affecting 15-30 years of age group more. Most cases of malaria were found in months of September and October near the end of monsoon and post monsoon season. Conclusion: From our study we concluded that there was high prevalence of P. vivax than P. Falciparum infection. Maximum numbers of cases were after major rainfall, reported in month of September and October. Morbidity and mortality of malaria must be reduced by strengthening Malaria Surveillance, risk factor assessment during pre and postmonsoon period. Premonsoon preventive actions need strengthening and strict monitoring of all these actions are needed. Keywords: Prevalence, Plasmodium vivax, Plasmodium falciparum, Malaria, Thin and Thick smear, Seasonal trends.

Trend analysis of river flow and groundwater level for Shipra river basin in India

The declining baseflow and drying up of river during the lean period are important issues in major rivers systems in India. In recent times, more focus has been given to formulate an action plan for river rejuvenation. The present study aims at the assessment of declining flow regime in Shipra river in the Madhya Pradesh state of India. Long-term daily runoff data recorded at the Ujjain gauge discharge site on Shipra river was analyzed to identify trends in dependable flows using a non-parametric Mann–Kendall test. The declining trend for dependable river flow at various probability levels was observed at Ujjain during the beginning of monsoon (month of July) and at the end of monsoon (months of September and October). However, a rising trend was observed during the mid-monsoon season (month of August). The trend analysis of pre-monsoon and post-monsoon groundwater level data indicated falling groundwater level trend in the entire Shipra basin. Significant declining trends in groundwater levels were observed in the middle reach area of the basin where important cities like Indore and Ujjain are located. Wigley and Jones’s index was used to judge the rising or declining flow regime of Shipra river. Average annual rainfall, as well as river runoff, has decreased substantially during the last two decades. The trend analysis of flow and groundwater level data of the Shipra river basin and information derived using Wigley and Jones index will be useful to formulate strategies for the river rejuvenation planning.

Simultaneous infection of Hepatitis A and Hepatitis E viruses amongst acute viral hepatitis patients: A hospital-based study from Uttarakhand.

Background:Enterically transmitted viral agents like Hepatitis A virus (HAV) and Hepatitis E virus (HEV) are common causes of viral hepatitis in developing countries. Double infections by both agents, as their routes of entry are similar, are common. Overall this is a major health issue for our country. This study was carried out to learn about the seroprevalence of HAV & HEV (and double infections if any) infections in acute viral hepatitis (AVH) cases attending our hospital.Materials and Methods:This is a retrospective cross-sectional study of a 2-years duration carried out in the serology lab of Dept. of Microbiology, AIIMS Rishikesh. Continuously collected samples totaling 617 cases, presenting with Acute Viral Hepatitis was included in the study. Cases with suggestive history were tested for IgM anti-HAV and IgM anti-HEVrespectively. Commercially available ELISA kits were put into use. Standard Statistical Package (SPSS 23) was put to use for statistical analysis.Results:HAV & HEV seroprevalence in AVH cases were found to be 14.7% (91/617) and 28.04% (173/617), respectively. Dual infection of HAV and HEV was found in 5.9% (32/617) of study subjects. The prevalence of HAV and HEV among males were14.2% and 34.26%, respectively while that in female were 15.36% and 21.16%, respectively. Infection was predominantly found in young adults. Distinct seasonal variation was observed, period towards the end of monsoon, and beginning of winter recorded more cases. Both year, most of the positive cases are seen in the months of August and September.Conclusion:The infection rate of HEV is higher than HAV amongst AVH cases. This and relatively high co-infection rate (5.9%) is significant in terms of the need for regular screening of HEV in pregnant women as well as the urgent need to improve hygiene amongst the population. This data will help in future vaccine strategies and sanitation programs in this part of the country.

Aflatoxin exposure in children living in Mirpur, Dhaka: data from MAL-ED companion study

Dietary exposure to aflatoxin is implicated in growth faltering of children. Despite the high burden of childhood stunting in urban Bangladesh, there are no data on long-term exposure to aflatoxin. This study aimed to explore aflatoxin exposure levels in a group of children followed longitudinally. The current study used data and biospecimens collected during 2010–2014 as part of the MAL-ED birth cohort study in an urban slum of Mirpur, Dhaka where children were followed from birth to 36 months. AFB1-lysine adduct concentrations were determined by isotope dilution mass spectrometry from plasma samples collected at 7, 15, 24, and 36 months of age. The limit of detection was 0.5 pg of AFB1-lys/mg albumin. In 744 plasma samples, the geometric mean of AFB1-lysine/mg albumin was 1.07 pg (range 0.04–123.5 pg/mg albumin). The proportion of children with detectable aflatoxin exposure was 10.1, 20.9, 17.9, and 61.7% for 7, 15, 24, and 36 months, respectively. Reduction in breastfeeding prevalence (80% at 24 months vs. 38% in 36 months) corresponded with the high-level detection of AFB1-lysine at the age of 36 months. AFB1-lysine concentrations were the highest at the end of monsoon. This study reveals that 62% of children in slum settlement were exposed to aflatoxin by the end of the third year of life. High aflatoxin exposure was detected at the end of rainy season and with the introduction of family food. These findings suggest interventions to ameliorate the problem of chronic aflatoxin exposure including childhood stunting.

Usability of estimated soil moisture derived from gridded rainfall in agricultural operations

Soil moisture information has not been used extensively under PAN India mode operationally in the country due to sparse network of soil moisture measuring stations. Effort has been made to estimate the soil moisture using gridded rainfall data using soil water balance (SWB) model and subsequently to compare quantitatively with insitu soil moisture measured by gravimetric method. It is observed from the quantitative validation with ground observations that during monsoon season the accuracy with the observed was found to be fairly accurate from the early part of the July to the end of monsoon when the soil was almost recharged with the rainfall. One of the limiting factors is that during dry period and in the low rainfall areas accuracy is relatively low. Thus, the soil moisture information generated using gridded rainfall data can be used from the second month (July) of monsoon season to the end of the monsoon season on PAN India mode for irrigation scheduling and determination of sowing dates and for contingency crop planning etc.

Seroprevalence of Hepatitis A Virus (HAV) and Hepatitis E Virus (HEV) Co-infection in the Patients Presenting with Acute Viral Hepatitis Attending a Tertiary Care Hospital in North India

Introduction: Hepatitis A virus (HAV) and Hepatitis E virus (HEV) are both transmitted enterically, resulting in acute viral hepatitis (AVH) in developing countries like India. HAV has a world-wide distribution and affects infants and young children in developing countries, and its epidemics are not very common. HEV is restricted to tropical countries and affects older children and young adults, and its epidemics are common. Co-infection with both viruses may lead to serious complications. Aim: This study was done to determine prevalence of HAV and HEV in patients presenting with AVH and the co-infection of HAV and HEV in these patients. Materials and Methods: A cross-sectional study of one year’s duration was conducted in the Department of Microbiology, Dr. Ram Manohar Lohia Hospital, New Delhi. 1230 patients presenting with AVH were considered in the study. Serum samples were analyzed for IgM anti-HAV and IgM anti-HEV for the detection of HAV and HEV infection, respectively, using commercially available ELISA kits. Results: The seroprevalence of HAV- and HEV-positive patients was 15.5% and 27.2%, respectively. The seroprevalence of both HAV and HEV in patients with acute viral hepatitis was 5.1%. The prevalence of HAV and HEV among males (14.6% and 29.8%) was higher than in females (16.6% and 23.4%). These infections were predominantly seen during end of monsoons and beginning of winter. Conclusion: The prevalence of HEV is much higher than that of HAV; co-infection rate of 5.1% mandates the screening for HEV which will be of immense importance in pregnant women and improving levels of personal hygiene among lower socio-economic population.

Modified temperature index model for estimating the melt water discharge from debris-covered Lirung Glacier, Nepal

Abstract. In the Nepalese Himalayas, the complex topography, occurrence of debris covered glaciers, and limited data availability creates substantial difficulties for modelling glacier melt. The proper recognition of melt processes governs the accurate estimation of melt water from glacier dominated systems, even in the presence of debris-covered glaciers. This paper presents a glacier melt model developed for the Lirung sub-basin of Langtang valley, which has both a clean glacier area, 5.86 km2, and a debris-covered glacier area, 1.13 km2. We use a temperature index approach to estimate sub-daily melt water discharge for a two week period at the end of monsoon, and the melt factor is varied according to the aspect and distributed to each grid processed from the digital elevation model. The model uses easily available data and simple extrapolation techniques capable of generating melt with limited data. The result obtained from this method provides accurate estimate with an R2 value of 0.89, bias of 0.9% and Nash-Sutcliffe efficiency of 0.86, and suitable in Himalaya where data availability is major issue.

Prevalence of Hepatitis A virus (HAV) and Hepatitis E virus (HEV) in the patients presenting with acute viral hepatitis

Background: Hepatitis A virus (HAV) and Hepatitis E virus (HEV) are both enterically transmitted, resulting in acute viral hepatitis (AVH) in developing countries. They pose major health problems in our country. This study was done to determine prevalence of HAV and HEV in patients presenting with AVH and the co-infection of HAV and HEV in these patients. Materials and Methods: A cross-sectional study of 2-years duration was conducted in the Department of Microbiology, KMC, Mangalore. A non-random sampling of 958 patients presenting with AVH was considered in the study. On the basis of history, serum samples were analysed for IgM anti-HAV and IgM anti-HEV for the detection of HAV and HEV, respectively using commercially available ELISA kits. Data collected was analysed by using Statistical Package for the Social Sciences (SPSS) version 11.5. Results: The seroprevalence of HAV- and HEV-positive patients were 19.31% and 10.54%, respectively. The seroprevalence of both HAV and HEV in patients with acute viral hepatitis was 11.5%. The prevalence of HAV and HEV among males (68% and 31%) was higher than in females (31% and 20%) and was predominantly seen among young adults. These infections were predominantly seen during end of monsoons and beginning of winter. Conclusion: Though the prevalence of HAV is much higher than that of HEV, co-infection rate of 11.5% mandates the screening for HEV which will be of immense importance in pregnant women and improving levels of personal hygiene among higher socio-economic population. These data will be essential for planning of future vaccination strategies and for better sanitation programme in this part of the country.

Tillage and Irrigation Requirements of Sorghum (&amp;lt;i&amp;gt;Sorghum bicolor&amp;lt;/i&amp;gt; L.) at Hamelmalo, Anseba Region of Eritrea

Most Eritrean farmers do not adopt soil conservation measures and till even sloppy fields 2 - 4 times for planting sorghum (Sorghum bicolor L.) with a view to facilitate rainwater intake. Field experiments were conducted at Hamelmalo to optimize tillage and irrigation requirements of sorghum in loamy sand. Tillage treatments were conventional tillage (4 times) on existing slopes (CTf), conventional tillage on managed plots (terraced) with residue (CTm + R) and without residue (CTm &#45 R), reduced tillage (single tillage 4 days after heavy rainfall) on managed plots with residue (RTm + R) and without residue (RTm &#45 R) and no tillage (direct planting) on managed plots with residue (NTm + R) and without residue (NTm &#45 R) randomized in four replications. Tillage in CTm and CTf was same. Experiment was repeated in year II along with a new experiment in split plot design with same tillage treatments in main plots and 4 irrigation treatments in subplots in 3 replications. Irrigation treatments were rainfed (I0), 70 mm irrigation at 50% depletion of soil moisture in CTm &#45 R from 1 m profile after end of monsoon (I1), 70 mm irrigation 7 days after irrigation in I1 (I2), and 70 mm irrigation 7 days after irrigation in I2 (I3). Bulk density increased and infiltration rates decreased by harvesting due to tillage but changes were lower in residue plots of NT and RT than CT. Optimum soil moisture for emergence of sorghum was within 0.145 ± 0.002 m3 m-3 at which soil strength was well below critical level for root growth. Soil strength in tilled layer due to intermittent wetting and drying following planting exceeded 2000 k Pa when dried below 0.143 m3 m-3 moisture. Soil profile in CTf did not recharge by rainfall even by end of the rainy season, whereas it was fully wetted in level and terraced plots. Conservation measures resulted 80 - 150 mm of residual moisture per 2 m of soil profile at sorghum harvesting. Residual moisture was relatively more in residue and irrigated plots than in nonresidue and CTf plots. Soil bunding and levelling alone raised sorghum yields in RT + R to 2887 kg ha-1 under rainfed and 3980 kg ha-1 under 70 mm irrigation 21 days after last rainfall of the season (I1). Corresponding yields in CTf were 501 kg ha-1 under rainfed and 1161 kg ha-1 under irrigation. Single preplanting tillage 4 days after heavy rainfall (RT) was as good as 2 - 4 tillage (CT) practiced by farmers. Sorghum yields in Hamelmalo could be about 2752 kg ha-1 by water use of 344 mm and 4009 kg ha-1 by 432 mm. Water use in CTf was lowest (208 mm) under rainfed.

Forecasts of Rainfall (Departures from Normal) over India by Dynamical Model

Intra-seasonal lack(excess) of rainfall that lead to situations of meteorological drought (flood) in one or other part of country every year apart from inter-annual variability of rainfall(Krishnamurthy, 2000). India received 88% of long period average (normal) rainfall 886.9mm during southwest monsoon season(monsoon) in 2014. Northwest(NW), central, northeast (NE) and south peninsula of India received 79%, 90%, 88% and 93% of normal rain respectively. Twelve meteorological subdivisions i.e. about 30% area of country received deficient category(below 19% of normal) by the end of monsoon. Forecasts of Global Forecasting System(GFS/ T574L64) at ESSO-NCMRWF have been studied for all the thirty six meteorological subdivisions(subdivisions) of India during monsoon(June to September) 2014. Predicted values of weekly percentage departure of rainfall from normal are found in good agreement with observed kind of departures(-ve or less than normal) on more than about 80% of the cases at sub divisional spatial scales. Model forecasts for monsoon season were consistently found reasonably good for both the recent years 2009 and 2014 when India received 22% and 12% less rainfall than normal respectively. Scores o f models forecasted weekly rainfall over meteorological subdivisions for monsoon 2014 are found improved compared to those for monsoon in 2009.

Controls on seasonal variations of silicate weathering and CO2 consumption in two river catchments on the NE Tibetan Plateau

Water samples from the Buha and Shaliu Rivers, located on the semi-arid northeastern Tibetan Plateau, were collected weekly over a one year period. The major ionic compositions of water samples were measured and the daily contents of suspended particulate material (SPM) were monitored in both rivers in order to investigate the influence of lithology, climate and physical erosion on seasonal silicate weathering. In the Shaliu River, weathering of trace amounts of calcite contributes more than 50% of the Ca2+ and HCO3- to the river water. Through high-resolution variations of Ca2+ concentrations and elemental ratios, the signal of carbonate precipitation is captured at the end of monsoon in this river. The measured physical erosion rate is only 8.7–16.0mm/kyr in this semi-arid region, which is 2–3 orders of magnitude lower than that in the Himalaya and nearby regions.In contrast with several orders of magnitude in seasonal variations of silicate weathering rates in both catchments, the distinct lithology between the catchments only leads to a 15 times difference of annual net CO2 consumption. The correlation analysis shows that seasonal silicate weathering is strongly dependent on water discharge in the semi-arid area. The most important observation is that, unrecognized by the previous studies, both physical erosion rate and air temperature exhibit two distinct trends with silicate weathering rates (and net CO2 consumption) during the years. The two trends might suggest that temperature plays a more important role on the CO2 consumption rate before the mid-monsoon under a condition of low water discharge than that after the monsoon with a high water discharge. During the period before the mid-monsoon, the relationship between temperature and silicate weathering rate exhibits higher activation energy than after the mid-monsoon, suggesting a greater dissolution of uneasily weatherable minerals from groundwater, frozen soil, and/or dust input. The relationship between erosion and weathering during the period before the mid-monsoon indicates a faster increase of silicate weathering rate, because freezing erosion produces large amount of high surface area minerals.

Short- and long-term temporal variations in salinity and the oxygen, carbon and hydrogen isotopic compositions of the Hooghly Estuary water, India

The Hooghly River estuary provides a unique experimental site to understand the effect of monsoonal river discharge on freshwater and seawater mixing. Water samples collected bi-weekly for a duration of 17months were analyzed for salinity, δ18O, δ13CDIC, as well as δD to investigate the differential mixing of freshwater and seawater. The differences in salinity and δ18O of samples collected during low and high tides on the same day are strongly correlated suggesting a well mixed water column at our sampling site. Low salinity and depleted δ18O during monsoon is consistent with increased river discharge as well as high rainfall. We identified different slopes in a δ18O versus salinity plot for the estuary water samples collected during monsoon and non-monsoon seasons. This is driven by composition of the freshwater source which is dominated by rainwater during monsoon and rivers during non-monsoon months. Selected δD analyses of samples indicate that groundwater contributes significantly to the Hooghly Estuary during low rainfall times of the year. δ13CDIC measured in the water recorded low values towards the end of monsoon indicating low productivity (i.e. increased organic respiration) while progressively increasing δ13CDIC values from October till January as well as during some of the pre-monsoon months can be explained by increasing productivity. Very low δ13CDIC (~−20‰) suggests involvement of carbon derived from anaerobic oxidation of organics and/or methane with potential contribution from increased anthropogenic water supply. An estimate of seawater incursion into the Hooghly Estuary at different times of the year is obtained by using salinity data in a two-component mixing model. Presence of seawater was found maximum (31–37%) during February till July and lowest (less than or equal to 6%) from September till November. We notice a temporal offset between Ganges River discharge farther upstream at Farakka and salinity variation at the Hooghly Estuary. We believe that this time lag is a result of the physical distance between Farakka and Kakdweep (our sampling location) and put constraints on the travel time of river water during early monsoon.

Chemical composition of rain water and influence of airmass trajectories at a rural site in an ecological sensitive area of Western Ghats (India)

Samples of rain water were collected during monsoon season (June to September) of 2006 and 2007 at Hudegadde, a rural site located in an ecological sensitive area of Western Ghats. The collected samples were analyzed for pH, conductivity and major ions. At this site, rainwater pH varied from 4.20 to 7.39 with 5.65 as volume weighed mean. The observed mean was slightly lower than the average pH reported at most of the Indian continental sites. Monthly variation showed that average pH of rain water was the lowest during September (end of monsoon) and the highest during July (peak of monsoon). Overall, marine sources had dominating influence at this site. However, significant influence of anthropogenic and crustal sources from local as well as inter-continental regions was also noticed. As compared to NO 3 − , higher concentration of SO 4 2− was noticed which might be due to contribution from industrial activities responsible for SO2 emission. At this site, influence of five types of airmass trajectories was noticed i.e. i) C.I.O. (Central part of Indian Ocean)-when air masses blown from Maldives and nearby region of central Indian ocean. These airmasses had higher concentrations of nss Ca2+ which did not show any adverse impact on the pH; ii) N.W.I.O.(North-West Indian Ocean)-when airmasses travelled from oceanic region close to north-east Africa. These airmassses had higher concentrations of nss sulphate and nitrate and gave rise to acid rain; iii) S.W.I.O. (South -West Indian Ocean)- when airmasses came from southern part of Indian ocean (close to Mauritius). During these airmasses, rain water samples had almost equal ratio of nss SO 4 2− and nss Ca2+ similar to N.W.I.O but very low NO 3 − ; iv) Gulf-when airmasses were observed coming from Gulf region. Although these airmasses contributed only 2% of the total number of samples but carried high amount of nss SO 4 2− which gave rise to acid rain. The second lowest pH was observed during these airmasses which might be due to very high nssSO 4 2− /nssCa2+ ratios; v) N.W.I.O. + S.W.I.C. (North-West Indian Ocean+South-West Indian Continental)- when airmasses originated from north-west Indian Ocean travelling towards south continental part of India and then arriving to the site. During these airmasses, samples showed typical influence of urban activities having high concentrations of nss SO 4 2− and NO 3 − leading to the lowest pH of rain water.

Eddy-Mediated Regime Transitions in the Seasonal Cycle of a Hadley Circulation and Implications for Monsoon Dynamics

Abstract In a simulation of seasonal cycles with an idealized general circulation model without a hydrologic cycle and with zonally symmetric boundary conditions, the Hadley cells undergo transitions between two regimes distinguishable according to whether large-scale eddy momentum fluxes strongly or weakly influence the strength of a cell. The center of the summer and equinox Hadley cell lies in a latitude zone of upper-level westerlies and significant eddy momentum flux divergence; the influence of eddy momentum fluxes on the strength of the cell is strong. The center of the cross-equatorial winter Hadley cell lies in a latitude zone of upper-level easterlies and is shielded from the energy-containing midlatitude eddies; the influence of eddy momentum fluxes on the strength of the cell is weak. Mediated by feedbacks between eddy fluxes, mean zonal winds at upper levels, and the mean meridional circulation, the dominant balance in the zonal momentum equation at the center of a Hadley cell shifts at the transitions between the regimes, from eddies dominating the momentum flux divergence in the summer and equinox cell to the mean meridional circulation dominating in the winter cell. At the transitions, a feedback involving changes in the strength of the lower-level temperature advection and in the latitude of the boundary between the winter and summer cell is responsible for changes in the strength of the cross-equatorial winter cell. The transitions resemble the onset and end of monsoons, for example, in the shift in the dominant zonal momentum balance, rapid shifts in the latitudes of maximum meridional mass flux and of maximum convergence at lower levels, rapid changes in strength of the upward mass flux, and changes in direction and strength of the zonal wind at upper and lower levels. In the monsoonal regime, the maximum upward mass flux occurs in an off-equatorial convergence zone located where the balance of the meridional geopotential gradient in the planetary boundary layer shifts from nonlinear frictional to geostrophic. Similar dynamic mechanisms as at the regime transitions in the simulation—mechanisms that can act irrespective of land–sea contrasts and other inhomogeneities in lower boundary conditions—may be implicated in large-scale monsoon dynamics in Earth’s atmosphere.

Sea Level Changes Along Bangladesh Coast in Relation to the Southern Oscillation Phenomenon

Interannual variations of sea level along the Bangladesh coast are quite pronounced and often dominate the long-term sea level trends that are taking place. The El Niño/Southern Oscillation (ENSO) induced variation is an important component of interannual mode of variations. The present article deals with the relationship between the sea level variations along the Bangladesh coast and the Southern Oscillation phenomenon. The mean tide level data of monsoon season (June to September) pertaining to Hiron Point (in Sundarbans) and Char Changa (on the mouth of Meghna River) have been analyzed and correlated to the Southern Oscillation Index (SOI). The annual variation of mean tide level in the coastal areas of Bangladesh reveals that the tide level reaches its peak during the monsoon season. The maximum tide level during the calendar year is recorded in August. Thus, it is not surprising that the inundation of the coastal belt of Bangladesh due to the floods is most common during the summer monsoon season, especially from July to September. Therefore, the sea level variations during the monsoon are of paramount importance to Bangladesh. The results of the present study show that both at Hiron Point and Char Changa there is a substantial difference between the mean tide level during the El Niño and La Niña monsoons. The mean tide level at Hiron Point is higher by about 5 cm during August of La Niña years as compared to that during the El Niño years. The difference at Char Changa, which is located at the mouth of Meghna River, is much higher. This is probably due to the increased fresh water discharge into the Meghna River during La Niña years. Thus at the time of crossing of a monsoon depression, the chances of widespread inundation are higher during a La Nin~a year as compared to that during an El Niño year. The Correlation Coefficients (CCs) between Mean Tide Levels (MTLs) at Hiron Point and Char Changa and the SOI during September (at the end of monsoon) are +0.33 and +0.39 respectively. These CCs are statistically significant at 90% and 95% levels, respectively. These results may find applications in the preparedness programs for combating sea level associated disasters in Bangladesh.

Influx and efflux of moisture in a desert soil during a 1 year period

Spatial and temporal variation of moisture flux in soil over a 1 year period was studied in the Thar desert of India. Tritium was injected at a depth of 0.8 m at 18 sites distributed over an area of 50 km2 and at 20 adjacent points at a single site over a sand dune, before the 1994 monsoon. Vertical soil profiles were collected from the 18 sites after the end of monsoon and at the end of the year and from the multiple‐injections site at different time intervals throughout the year. Moisture flux values, computed from tritium and moisture distribution in soil profiles, show that the moisture influx at a site depends upon the grain size of the top 20 cm layer and that about 68% of the total influx of water below the injection depth is lost during the summer months. The data for the multiple‐injections site show that the tritium spike was displaced to a depth of 4 m by the end of the monsoon, and it remained stationary during the postmonsoön period of 8 months, although about 64% of the soil moisture was lost. The spiked layer showed enrichment in the specific activity of tritium during summer months. These observations suggest that vapor diffusion is the dominant process for moisture loss from the Thar desert soil. The net input (natural recharge) to groundwater due to 1994 precipitation was 62 mm.

Soil moisture movement and groundwater recharge by tritium tracer tagging technique

The paper deals with the study of rate of soil moisture movement and recharge to groundwater system at Indian Agricultural Research Institute (IARI) farm for the years 1973, 1974 and 1975. The utility of artificially injected tritium, experimental techniques adopted and a quantitative determination of recharge and moisture movement rates are discussed. Corrections for upward movement of moisture after the end of monsoon have been used to calculate the average rates of water movement below the injection level and groundwater recharge. Average rates of soil moisture movement and recharge are 9.4 mm/d and 27.4% (193 mm) of monsoon precipitation for three years respectively. Under favourable conditions the diffusion coefficients for soil water system are calculated assuming Gaussian distribution of tritium concentrations in the soil profiles. The average value of diffusion coefficient is 2.5 × 10−5 cm2/s for the sandy loam soils. The variation in recharge is mainly due to inherent variability in soil physical properties and vegetative cover. The technique is not found reliable for higher water table conditions on account of lateral flow.