SW Monsoon Season Research Articles

Statistical analysis of climate change over Hanumangarh district

Abstract Precipitation is the most critical element of the hydrological cycle that might influence the frequency of floods or drought. Therefore, for drought and flood forecasting, knowing the precipitation pattern and its trend is mandatory. The research aims to estimate the monthly, seasonal, and annual precipitation trend using statistical (Mann Kendall test & Sen's Slope estimator) and graphical (Innovative Trend Analysis method) for the Hanumangarh district of N-E Rajasthan. The mean monthly precipitation data were gathered from the India-WRIS from 1901 to 2022 (122 years). Seasonal and annual precipitation trend variations were examined by statistical methods and compared with the graphical method. The study results conclude that precipitation trends showed variability for the SW monsoon season from the graphical to the statistical method. For the SW monsoon season, the decreasing trend was seen using statistical methods. In contrast, the graphical method revealed an increasing trend. During the annual precipitation, increasing trends were seen using statistical and graphical methods. It is found that the ITA method is more efficient in finding trends because it does not assume any assumptions like that of the MK test. This study will help water resources engineers and local people with sustainable management, planning, and development of water resources in the Hanumangarh district.

Assessment of climate change in west godavari district of andhra pradesh, India using water balance approach

The present research work sought to analyze climatic changes in the West Godavari district of Andhra Pradesh State by utilizing historical climate data spanning 119 years, from 1901 to 2019. During the first five decades of the twentieth century, the average annual temperature in the research region was 27.18°C, whereas the next five decades were 0.46°C warmer than the first. The twenty-first century's first two decades were even warmer, with an average temperature of 28.2°C, 0.56°C higher. During the SW-monsoon season, the maximum average annual rainfall received was 642.6 mm (about 60% of total rainfall). According to the Thornthwaite water balancing approach, the district has a mega thermal type of climate (A′) based on thermal regime classification and a semi-arid (D) kind of climate based on moisture regime classification. Studies on water balance have been shown to be crucial for comprehending the climate change scenario in any place in the context of strategic plans for managing natural resources and fostering economic growth. Keywords: Climate, Temperature, Rainfall, Water Balance, West Godavari, Andhra Pradesh

Calculation and analysis of cloud attenuation and other cloud parameters in India for earth-space links

• The cloud attenuation is higher during the SW monsoon season followed by NE monsoon, pre-monsoon and winter seasons at the places considered. • The maximum cloud attenuation of 1.67 dB/km is measured in Hyderabad by WVP-NTU cloud detection model at 24 GHz. • The WVP-NTU cloud detection model is more suitable for the five places considered as compared to the SU model. An effort is made to examine the cloud parameters over few stations in India for one year from October 2016 to September 2017. This study includes the investigation of cloud base height, cloud top height, zero degree isotherm height, number of cloud layers, cloud thickness, number of days cloud detected, cloud liquid water content and cloud attenuation at 6 GHz, 12.5 GHz and 24 GHz. Salonen Uppala (SU) model and Water Vapor Pressure-Nanyang Technological University (WVP-NTU) model are used for cloud detection using the radiosonde data at Bangalore, Hyderabad, Mumbai, Thiruvananthapuram and Chennai. The cloud parameters found using both the models are compared. WVP-NTU model shows enhanced values for most of the cloud parameters over SU model. Cloud base height calculated using these models are compared with METAR data and found that Cloud base height from WVP-NTU model matches well with METAR data. Cloud liquid water content and cloud attenuation values are calculated using the ITU-R model. The cumulative distributions (CDF) of cloud attenuation for one year are compared for the places considered. The seasonal CDFs of cloud attenuation using the WVP model are compared with the rain statistics. In this study, the maximum cloud attenuation of 1.67 dB/km is measured in Hyderabad by WVP-NTU combined ITU-R model at 24 GHz.

Boron Isotopic Analysis of Representative Atmospheric Aerosols Derived From Long-Range Transported/Local Emission on an Islet Offshore NE Taiwan

To identify pollutant origins and their potential sources from either long-range transported (LRT) or local emissions, the Boron (B) concentrations and the B isotopic compositions (δ11B) in the water-soluble fraction of aerosols were determined. These aerosols were collected from an islet offshore in northeastern Taiwan, Peng Chia Yu (PCY), a non-residential islet, from January 1998 to March 2000. This islet may be influenced by various pollutants or chemical transport during monsoon periods, but suffers minimal human perturbation locally. The B in the specimens falls to 0.3–1.63 ng m−3 during the SW monsoon seasons when compared to 0.46–2.56 ng m−3 in the NE monsoon. However, the δ11B results show no clear variations in both monsoon seasons (10.7–24.3‰), regardless of differences in air mass origin. A two end-member mixing scenario is proposed to explain our observations using the obtained δ11B and 1/[B] results. The ocean endmember is characterized by high 1/[B] and high δ11B; while other endmember is from continental endmember or anthropogenic contributions that are characterized by of 1/[B] and lowδ11B (δ11B < 10‰). Based on these chemical and isotopic results, we found aerosol emissions in northern Taiwan are characterized as low B with nearly constant δ11B, when compared with long-range transported continental endmembers derived from nearby regions. This study provides preliminary B and δ11B levels in aerosols derived from LRT/local emissions and discusses potential monsoonal effects on aerosols offshore of NE Taiwan.

Sediment Characterization and Monsoonal Impact on Beach Morphology at the Three Beaches along the Karachi Coast, Pakistan

Sediment characteristics plays important role in the determination and functioning of coastal ecosystems. The present study is an attempt to evaluate the variability in sediment characteristics of the three beaches: Clifton, Sandspit and Buleji, at the Karachi coast. The samples were collected during pre-monsoon and south-west monsoon seasons to evaluate the seasonal differences. The sediments of the three sites showed variations in moisture content, organic matter and grain size. The highest mean moisture (27.17%) and organic contents (3.5 %) were recorded from the sediments of Clifton. The maximum fraction of sediments (> 80 %) consist of fine to very fine sand at all studied sites. The sediments of Clifton were high in very fine sand fraction and low in coarse sand fraction as compared to Sandspit and Buleji. The sediments of all the three studied sites are moderately sorted, negatively coarse-skewed and showed leptokurtic distribution. The studied beaches of Clifton, Sandspit and Buleji shows dissimilarity in sediment characteristics as indicated through Cluster and PCA analysis. The monsoonal influence on sediment characteristics was also observed at all the three studied beaches. The strong wave action during SW monsoon season results in the deposition of the coarser sediments at the high tide level thus increasing the steepness of the beaches.

Vertical distributions and columnar properties of the aerosols during different seasons over Kattankulathur (12.82oN, 80.04oE): A semi-urban tropical coastal station

Seasonal characteristics of the aerosols over Kattankulathur (12.83° N, 80.04° E) is studied using Micro Pulse Lidar observations during 2016–2018 and Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observation, Moderate Resolution Imaging Spectroradiometer and Ozone Monitoring Instrument datasets during 2006–2018. The vertical distribution of the aerosols is characterized as the steep decrease with altitude nearly exponentially up to 2–3 km during the northeast (NE) monsoon and winter seasons mainly dominated by clear marine and polluted continental aerosols. While it gradually decreases with altitude up to 4–5 km during pre-monsoon and southwest monsoon (SW) seasons mainly dominated by dust aerosols. The mean aerosol loadings near the surface are ~0.38 km−1 during NE monsoon and winter, ~0.30 km−1 pre-monsoon, and 0.22 km−1 during SW monsoon seasons. Such difference in the seasonal distribution of aerosol loading is mainly attributed to the difference in surface insolation, convection, long-range transport, and the atmospheric boundary layer (ABL) structure and dynamics. The ABL contributes 61%, 46%, 38%, and 63% of the overall aerosol optical depth (AOD) during the winter, pre-monsoon, SW monsoon, and NE monsoon seasons, respectively. The AOD shows a substantial seasonal variation with two local maxima during May and October and minima during January and July. The maximum and minimum AOD is found to be ~0.59 ± 0.16 and 0.4 ± 0.05, respectively. The Angstrom exponent (AE) (fine mode fraction (FMF)) shows a marked seasonal variation with a maximum value of 0.84 (0.60), indicating the dominance of finer particles during winter and minimum value of 0.54 (0.27), indicating dominance coarse particles during SW monsoon. The AE and UV aerosol index values suggest that Kattankulathur is mainly characterized by large-sized absorbing aerosols (30%) such as dust mixture and sea salt particles. Small-sized non-absorbing aerosols such as biogenic aerosols are rarely (<1%) observed.

Response of phytoplankton assemblages to variations in environmental parameters in a subtropical bay (Chabahar Bay, Iran): Harmful algal blooms and coastal hypoxia

The present work is a comprehensive study on the spatiotemporal variation of environmental parameters in relation to phytoplankton assemblages over a one-year period (March 2017–March 2018) at a semi-enclosed subtropical bay and adjacent coastal waters in the north of Gulf of Oman. The physical and biological parameters were mostly controlled by the southwest monsoon which usually lasts 4 months (June–September) in the region. The outstanding phenomena which highly affected the physicochemical parameters and biological activity were thermal stratification (established in the May–October period), and rising concentration of nutrients due to monsoon-driven upwelling in the late monsoon and post-monsoon periods. As a consequence, the hypoxic condition was established and the pH was also decreased to values smaller than 8.0. It seems that the nitrate concentration limits phytoplankton growth during most times of the year. However, the results showed that the phytoplankton population in the fall inter-monsoon period may also be controlled by silicate availability as its concentration declined to values as small as 0.8 μM in January. Chlorophyll a concentration showed a strong peak in January and a weaker peak in October. The former is due to the intense blooms of Noctiluca scintillans and the later was coincided with high nutrient concentrations and high phytoplankton density and diversity. Diatoms (93%) were dominant during the course of sampling and dinoflagellates (4.5%) were second in dominance. Exceptions were observed in March 2017 when the outbreak of dinoflagellates led to extensive water discoloration and in April 2017 during the massive bloom of Mesodinium rubrum. The most abundant genera were diatoms Pseudo-nitzschia (34.8%), Nitzschia (23.9%), Hemiaulus (20%), and Leptocylindrus (6.1%). While dinoflagellates blooms mostly occurred during high nutrients concentrations in late SW monsoon and inter-monsoon seasons; the bloom of potentially harmful diatoms occurred during the SW monsoon season.

Turbulence characteristics of surface boundary layer over the Kalpakkam tropical coastal station, India

In this study turbulent fluxes and their intensity features are studied in different seasons at the tropical Indian coastal station, Kalpakkam. Measurements from Ultrasonic anemometer at 10 m agl over 30-day period of four seasons (winter 1–30 January; summer/spring 1–30 April; SW monsoon 1–30 July; NE monsoon 1–30 October) in 2013 and 2014 are used for this work. Various surface layer parameters viz, friction velocity (u*), Obukhov length (L), momentum flux (M), turbulent heat flux (H), turbulence kinetic energy (TKE) are computed using eddy correlation method. Results indicate that the study region is highly turbulent in summer followed by NE monsoon, winter and SW monsoon seasons. Derived parameters indicate that shear is the main contributing mechanism for TKE generation during SW monsoon and both shear and buoyancy contributed for the generation of TKE in other seasons. Site specific turbulent intensity relationships were developed by analyzing second order moments of 3D wind components as a function of stability parameter (z/L). The turbulent components of wind followed 1/3 power law in the unstable regime and − 1 power law in the stable regime. Comparisons with previous studies indicate that the turbulent intensity for horizontal winds at the coastal station is relatively less especially in the unstable conditions. The derived relationships are found to be unique and vary seasonally and suggest their application for improved modeling of atmospheric dispersion in the study domain. Rate of dissipation of TKE (ϕe) for stable and unstable conditions at the observation site is different from the earlier proposed relationships in the literature. Thus, a new relationship is proposed for the better fit of the data at this site.

Relationship between summer monsoon rainfall and cyclogenesis over Bay of Bengal during post-monsoon (October–December) season

In this study, an attempt has been made to examine the relationship between summer monsoon rainfall (June–September) and the total number of depressions, cyclones and severe cyclones (TNDC) over Bay of Bengal during the post-monsoon (October–December) season. The seasonal rainfall of the subdivisions (located in south India) (referred as rainfall index – RI), is positively and significantly correlated (r=0.59; significant at >99% level) with the TNDC during the period, 1984–2013. By using the first differences (current season minus previous season), the correlations are enhanced and a remarkably high correlation of 0.87 is observed between TNDC and RI for the recent period, 1993–2013. The average seasonal genesis potential parameter (GPP) showed a very high correlation of 0.84 with the TNDC. A very high correlation of 0.83 is observed between GPP and RI for the period, 1993–2013. The relative vorticity and mid-tropospheric relative humidity are found to be the dominant terms in GPP. The GPP was 3.5 times higher in above (below) normal RI in which TNDC was 4 (2). It is inferred that RI is playing a key role in TNDC by modulating the environmental conditions (low level vorticity and relative humidity) over Bay of Bengal during post-monsoon season which could be seen from the very high correlation of 0.87 (which explains 76% variability in TNDC). For the first time, we show that RI is a precursor for the TNDC over Bay of Bengal during post-monsoon season. Strong westerlies after the SW monsoon season transport moisture over the subdivisions towards Bay of Bengal due to cyclonic circulation. This circulation favours upward motion and hence transport moisture vertically to mid-troposphere which causes convective instability and this in turn favour more number of TNDC, under above-normal RI year.

Features of nocturnal low level jet (NLLJ) observed over a tropical Indian station using high resolution Doppler wind lidar

High resolution Doppler wind lidar measurements made during the period 01 April 2012 to 31 March 2014 over Pune (18°32′N, 73°51′E, 559m Above Mean Sea Level), India have been used to study Nocturnal Low Level Jet (NLLJ) occurrence and its characteristics. Vertical profiles of horizontal wind in the altitude range from 100m to 3000m (at every 50m interval) and averaged over 5min have been used to study time–height variations during local nighttime. On several occasions during nighttime the wind profiles showed a narrow region of strong wind speed below 1000m altitude from surface, suggesting the presence of the low level jet. Analysis of the data indicates that NLLJ occurs more frequently (~66%) during pre-monsoon season (March–May) and on only 14% of the nocturnal period during SW monsoon season (June-September). Mean jet core heights during pre-monsoon, monsoon, post-monsoon (October–November), and winter (December–February) seasons are found to be 687m, 691m, 593m, and 586m respectively. Seasonal mean jet core speeds during pre-monsoon and monsoon are higher than those during winter. There are some occasions during monsoon season when hourly mean jet speeds during nighttime are as high as 15–20ms−1. Horizontal wind directions in the NLLJ during different seasons are consistent with the seasonal mean flow over the tropical Indian region. Most frequently occurring jet core height is in the height range 600–700m with almost 65% of the cases having jet core heights<700m and maximum frequency of occurrence of jet speed is in the range 9–11ms−1. Large east–west temperature gradients, inertial oscillations, stability in the lower atmosphere seem to be some of the factors that play significant role in the formation and sustenance of NLLJ over the location during different seasons.

Shoreline Change Analysis of Mangalore Coast and Morphometric Analysis of Netravathi-Gurupur and Mulky-pavanje Spits

Various developmental projects have come up in the coastal zone of Dakshina Kannada and Udupi districts in the last few decades. A number of beaches are subjected to erosion and lots of coastal properties are destroyed specially during SW monsoon season along the Mangalore coast in recent years. In order to protect the beaches and the properties, a number of seawalls - each ranging in length between 100 m and 2500 m - are built since 1980s. A very few of them built scientifically, but most of them are just dumping of granite blocks directly on the shoreline. In the present study, an attempt is made to understand long-term (1967-2013) shoreline changes as well as erosion / deposition patterns due to natural (including sea-level rise) and anthropogenic activities along this coast. The study area extends from Yermal in the north to Talpady in the south covering a total length of 45.63km. Topomap of 1967 and multi-temporal satellite images have been analyzed using Remote Sensing and GIS techniques to demarcate shoreline positions and to assess the impact of anthropogenic and hydrological factors on coastal morphology of this region. An integrative approach of Remote Sensing and GIS techniques as well as seasonal field studies clearly illustrates the reasons for shoreline and beach morphological changes. Construction of seawalls has resulted in shifting of erosion sites from one place to another adjacent place, whereas, breakwaters have been acting as barriers for littoral drift. Overall, areas subjected to erosion are higher compared to those subjected to accretion in the study area. Applications of Remote Sensing and GIS have led to understanding shoreline and beach morphological changes. These have also provided data analysis tools and methods to evaluate the geospatial patterns in short-term and long-term changes. The results of this study will be useful for shoreline, coastal zone management and implementation of CRZ act.

Current climate and climate change over India as simulated by the Canadian Regional Climate Model

The performance of the fifth generation of the Canadian Regional Climate Model (CRCM5) in reproducing the main climatic characteristics over India during the southwest (SW)-, post- and pre-monsoon seasons are presented in this article. To assess the performance of CRCM5, European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) and Interim re-analysis (ERA-Interim) driven CRCM5 simulation is compared against independent observations and reanalysis data for the 1971–2000 period. Projected changes for two future periods, 2041–2070 and 2071–2100, with respect to the 1971–2000 current period are assessed based on two transient climate change simulations of CRCM5 spanning the 1950–2100 period. These two simulations are driven by the Canadian Earth System Model version 2 (CanESM2) and the Max Planck Institute for Meteorology’s Earth System Low Resolution Model (MPI-ESM-LR), respectively. The boundary forcing errors associated with errors in the driving global climate models are also studied by comparing the 1971–2000 period of the CanESM2 and MPI-ESM-LR driven simulations with that of the CRCM5 simulation driven by ERA-40/ERA-Interim. Results show that CRCM5 driven by ERA-40/ERA-Interim is in general able to capture well the temporal and spatial patterns of 2 m-temperature, precipitation, wind, sea level pressure, total runoff and soil moisture over India in comparison with available reanalysis and observations. However, some noticeable differences between the model and observational data were found during the SW-monsoon season within the domain of integration. CRCM5 driven by ERA-40/ERA-Interim is 1–2 °C colder than CRU observations and generates more precipitation over the Western Ghats and central regions of India, and not enough in the northern and north-eastern parts of India and along the Konkan west coast in comparison with the observed precipitation. The monsoon onset seems to be relatively well captured over the southwestern coast of India, while the monsoon withdrawal occurs too late in comparison with observations. Boundary forcing errors are generally of the same magnitude or larger for the SW-monsoon season and smaller for the other seasons when compared to performance errors. For the two future periods analyzed, both CanESM2 and MPI-ESM-LR driven CRCM5 simulations imply a general warming over India in the twentyfirst century, with maximum increases projected for the pre-monsoon season. However, conflicting climate change signal is noted for precipitation for both future periods with respect to the 1971–2000 current period, with CanESM2 driven simulation suggesting an increase in precipitation in future climate, which is consistent with the projected increase in evapotranspiration, while MPI-ESM-LR driven simulation suggests a decrease in precipitation and evapotranspiration. The conflicting climate change signal in precipitation is also reflected in those variables that are tightly linked with precipitation, such as total runoff, highlighting the need for multi-model ensembles to better represent the uncertainties related to future projections.

Soil heat flux and day time surface energy balance closure at astronomical observatory, Thiruvananthapuram, south Kerala

Soil heat flux is an important input component of surface energy balance. Estimates of soil heat flux were made in the year 2008 using soil temperature data at Astronomical Observatory, Thiruvananthapuram, south Kerala. Hourly values of soil heat flux from 00 to 24 LST are presented for selected days typical of the winter, pre-monsoon, SW monsoon and NE monsoon seasons. The diurnal variation is characterized by a cross-over from negative to positive values at 0700 h, occurrence of maximum around noon and return to negative values in the late evening. The energy storage term for the soil layer 0–0.05 m is calculated and the ground heat flux G ∗ is estimated in all seasons. Daytime surface energy balance at the surface on wet and dry seasons is investigated. The average Bowen’s ratio during the wet and dry seasons were 0.541 and 0.515, respectively indicating that considerable evaporation takes place at the surface. The separate energy balance components were examined and the mean surface energy balance closure was found to be 0.742 and 0.795 for wet and dry seasons, respectively. When a new method that accounts for both soil thermal conduction and soil thermal convection was adopted to calculate the surface heat flux, the energy balance closure was found to be improved. Thus on the land surface under study, the soil vertical water movement is significant.

Oxygen minimum zone of the open Arabian Sea: variability of oxygen and nitrite from daily to decadal timescales

Abstract. The oxygen minimum zone (OMZ) of the Arabian Sea is the thickest of the three oceanic OMZ. It is of global biogeochemical significance because of denitrification in the upper part leading to N2 and N2O production. The residence time of OMZ water is believed to be less than a decade. The upper few hundred meters of this zone are nearly anoxic but non-sulfidic and still support animal (metazoan) pelagic life, possibly as a result of episodic injections of O2 by physical processes. We report on discrete measurements of dissolved O2 and NO2–, temperature and salinity made between 1959 and 2004 well below the tops of the sharp pycnocline and oxycline near 150, 200, 300, 400, and 500 m depth. We assemble nearly all O2 determinations (originally there were 849 values, 695 of which came from the OMZ) by the visual endpoint detection of the iodometric Winkler procedure, which in our data base yields about 0.04 mL L−1 (~ 2 μM) O2 above the endpoint from modern automated titration methods. We acknowledge that much lower (nanomolar) O2 values have been measured recently with the STOX (Switchable Trace amount OXygen) sensor in the eastern tropical South Pacific, and that similar conditions may also prevail in the Arabian Sea OMZ. In spite of the error in O2 measurements at vanishingly low levels, we argue that the temporal trends of the historic data should still hold. We find 632 values acceptable (480 from 150 stations in the OMZ). The data are grouped in zonally paired boxes of 1° lat. and 2° long. centered at 8, 10, 12, 15, 18, 20, and 21° N along 65 and 67° E. The latitudes of 8–12° N, outside the OMZ, are treated in passing. The principal results are as follows: (1) an O2 climatology for the upper OMZ reveals a marked seasonality at 200 to 500 m depth with O2 levels during the northeast monsoon and spring intermonsoon seasons elevated over those during the southwest monsoon season (median difference, 0.08 mL L−1 [~ 3.5 μM]). The medians of the slopes of the seasonal regressions of O2 on year for each of the NE and SW monsoon seasons are −0.0043 and −0.0019 mL L−1 a−1, respectively (−0.19 and −0.08 μM a−1; n = 10 and 12, differing at p = 0.01); (2) four decades of statistically significant decreases of O2 between 15 and 20° N but an opposing trend toward an increase near 21° N are observed. The mechanisms of the balance that more or less annually maintain the O2 levels are still uncertain. At least between 300 and 500 m, the replenishment is inferred to be due to isopycnal re-supply of O2, while at 200 (or 250?) m it is diapycnal, most likely by eddies. Similarly, recent models show large vertical advection of O2 well below the pycnoclines and oxyclines. The NO2– distribution, taken as an indicator of active NO3– reduction, does not show a trend in the redox environment for a quarter of a century at a GEOSECS station near 20° N. In the entire OMZ, the regression slopes on year within seasons for the rather variable NO2– do not present a clear pattern but by other measures tended to an increase of NO2–. Vertical net hauls collect resident animal (metazoan) pelagic life in the NO2– maximum of the OMZ at O2 levels well below the lower limit of the Winkler titration; the extremely low O2 content is inferred from the presence of NO2– believed to be produced through microbial NO3– reduction. Instead of the difficult measurement by the STOX sensor, the relation between the very low O2 inferred from presence of NO2– and mesozooplankton should be studied with 100 to 150 L bottles rather than nets. The spatial (within drift stations) and temporal (daily) variability in hydrography and chemistry is large also below the principal pycnocline. The seasonal change of hydrography is considerable even at 500 m depth. Future O2 or nutrient budgets for the OMZ must not be based on single cruises or sections obtained during one season only. Steady state cannot be assumed any longer for the intermediate layers of the central Arabian Sea.

Amino acid composition and &amp;lt;i&amp;gt;δ&amp;lt;/i&amp;gt;&amp;lt;sup&amp;gt;15&amp;lt;/sup&amp;gt;N of suspended matter in the Arabian Sea: implications for organic matter sources and degradation

Abstract. Sedimentation in the ocean is fed by large aggregates produced in the surface mixed layer that sink rapidly through the water column. These particles sampled by sediment traps have often been proposed to interact by disaggregation and scavenging with a pool of fine suspended matter with very slow sinking velocities and thus a long residence time. We investigated the amino acid (AA) composition and stable nitrogen isotopic ratios of suspended matter (SPM) sampled during the late SW monsoon season in the Arabian Sea and compared them to those of sinking particles to understand organic matter degradation/modification during passage through the water column. We found that AA composition of mixed layer suspended matter corresponds more to fresh plankton and their aggregates, whereas AA composition of SPM in the sub-thermocline water column deviated progressively from mixed layer composition. We conclude that suspended matter in deep waters and in the mixed layers of oligotrophic stations is dominated by fine material that has a long residence time and organic matter that is resistant to degradation. SPM in areas of high primary productivity is essentially derived from fresh plankton and thus has a strong imprint of the subsurface nitrate source, whereas SPM at oligotrophic stations and at subthermocline depths appears to exchange amino acids and nitrogen isotopes with the dissolved organic carbon (DOC) pool influencing also the δ15N values.

Monsoon and cyclone induced wave climate over the near shore waters off Puduchery, south western Bay of Bengal

Seasonal and annual variations in wave characteristics are studied based on measurements during 2009–2011 using wave rider buoy. Presence of swells generated by the south Indian Ocean cyclones are found over the south western Bay of Bengal during pre-monsoon season. Maximum significant wave height is measured 6m during Thane cyclone in December, 2011. Long period waves are observed mostly during the summer (SW) monsoon and are negligible in the winter (NE) monsoon period. Short period waves dominate (63%) the wave climate during the NE monsoon. Wave spectra during the SW monsoon are multi peaked whereas during the post monsoon season single peaked spectra are found. Single peaked spectra observed during SW monsoon of 2011 coincides with the presence of positive Indian Ocean Dipole. Waves during the pre monsoon and SW monsoon season are influenced by sea breeze. Analysis indicates that directional width is minimum for waves from the NE since they are wind waves and maximum for waves from SE since they are swells. Study indicates that wave climate of the south western Bay of Bengal is in contrast to that in eastern Arabian Sea during the SW monsoon.

Recent Studies on Wind Seas and Swells in the Indian Ocean: A Review

Winds and waves data measured at a few locations along the west coast of India as well as wave model results were analysed to study the wave characteristics in the deep as well as nearshore regions during different seasons. During fair weather season, we can observe distinct and systematic diurnal variation in wave height and wave period, especially, simultaneous increase in wave height and decrease in wave period with increase in local wind speed. This is due to superimposition of wind seas generated by the sea breeze system with the pre-existing swells. The swells observed in the Arabian Sea are from SW direction during SW monsoon season and from SW/SSW and NW directions during both pre-monsoon and post-monsoon seasons. The NW swells which occur during Shamal events show distinct characteristics such as an increase in wave height, decrease in swell period and a common propagation direction (northwest) for wind seas and swells. Also the swells generated at 400 S, propagate in the north Indian Ocean, and enter the Bay of Bengal without affecting the Arabian Sea. Fine resolution winds (say, MM5 or WRF) are necessary to understand the effect of land-sea breeze system on wind-sea generation in the coastal regions. Here, we review the recent work on wind seas and swells in the Indian Ocean and coastal regions that we have carried out using measured wave data and model results.

Flood generation during the SW monsoon season in northern Thailand

Abstract This paper analyses the annual floods for the city of Chiang Mai in northern Thailand using a relatively long dataset (1921–2009) to study the mechanisms behind flood generation in this region. Four floods of different magnitudes were chosen for the empirical analysis. Daily rainfall, flow and baseflow data are analysed together with information about tropical storm occurrence–frequency and El Ninõ Southern Oscillation (ENSO) events to identify the occurrence and causes of floods. We found that floods are caused by a variety of factors but the most extreme floods are often caused by a combination of wet catchment conditions and heavy rainfall due to monsoonal effects or tropical storms. Tropical storms play a variable role in that their occurrence in the earlier part of the monsoon season may only wet up the catchment and have no part to play in the peak-flow generation. However, heavy rainfall due to tropical storms during the later part of the monsoon season may result in extreme floods. Typhoon Damrey (2005) is such an example as a 100 year flood event occurred after the passage of this typhoon across northern Thailand. Flood forecasting and management therefore need more high-resolution information about rainfall patterns and tropical storm activity.

Hydrodynamic Modelling of Chennai Coast from a Coastal Zone Management Perspective

Kankara, R.S.; Mohan, R., and Venkatachalapathy, R., 2013. Hydrodynamic modelling of Chennai coast from a coastal zone management perspective. A thorough knowledge of hydrodynamic conditions of coastal waters is a primary requirement for understanding and resolving environmental management issues. The present study is aimed at characterizing the hydrodynamic regime of Chennai coastal waters through field investigations and numerical modelling. To this end, time series oceanographic data on tide, current, and wind were collected for 15 days each during SW monsoon and NE monsoon seasons. The data analysis revealed that tides were semidiurnal in nature and that the ratio of diurnal (O1, K1) and semidiurnal (M2, S2) constituents was 0.19 and 0.24 for the two respective seasons. The average current was 17 cm/s N during SW monsoon and 10 cm/s S during NE monsoon. A two-dimensional hydrodynamic model was also set up and validated with the field data successfully. The model results of water level and current were compared with field observations for both seasons and found to be satisfactory. The correlation was 0.89 and 0.99 for surface elevation and 0.56 and 0.73 for current speed. The relative mean absolute error (RMAE) and index of agreement (IoAd) were also computed for both seasons. The RMAE and IoAd analysis revealed that model validation was found to be “excellent” for water level and “very good” for current.

Utilisation of ‘Aerostat’ Doppler Weather Radar in nowcasting of convective phenomena

‘Aerostat’ system is a part of the air defence radar network, adopted by the Indian Air Force. Many meteorological instruments have been integrated with this system, including Doppler Weather Radar (DWR). The ground-based DWR has a maximum range of 300 NM, however, it generally uses 150 NM range on scan mode. The scan mode images are provided at half an hour interval, which are being utilised very effectively for nowcasting of thunderstorms at various IAF bases. In the present study, utilisation of DWR images for nowcasting of thunderstorms / dust storms is discussed over NW India with the help of a few case studies during pre-monsoon and SW monsoon seasons of 2008. Further, products generated through operational meso-scale NWP model runs have been studied in order to obtain indications / guidance for expected convective activity over the area at least 24-36 hours in advance. Thus, short-range weather forecasts through NWP models can be used as an advance indication for careful monitoring of DWR images in near real time. It has been found that the DWR is a very good tool to track the movement of significant weather echoes around the airfields, which can be very helpful in issuing appropriate warnings / advisories with sufficient lead time. Meso-scale NWP models are capable of generating reliable indications for expected convective activity at least 24-36 hours in advance. The integration of both the inputs can increase the accuracy and reliability of location and time specific prediction of convective activity.&#x0D;