Satellite-Observed Monthly Variability of Sea Surface Chlorophyll-a Concentration: A Case Study in East Malaysia

The Pahang water determined an effect of the sizes of sediment particles on 11 elements distribution during the monsoon season. The elements were of very high concentration in a sediment particle size of 40, 63, and 90 µm. The lower concentration of the elements was associated with a sediment size of 125, 250, and 500 µm. It has been observed in this research study that a concentration of elements increases with a decrease in a size of the sediment particles. The sediment from the Northeast (pre-) monsoon consisted of a finer particle size than the Southwest (post-) monsoon sediments. Overall, the concentration of the elements in the Northeast (pre-) monsoon sediment was higher than the Southwest (post-) monsoon sediment. The Northeast monsoon is highly influenced due to an input of fresh sediment from the rivers into the South China Sea as well as, due to a redistribution in the surface sediment. There were many differences in sediment element content between the two monsoon seasons, but an anthropogenic impact was only found in lead (Pb).

Gulf of Martaban is located at the north of Andaman, and is one of the world most turbid areas. The presence of suspended sediment concentration (SSC) in the water body could reduce the underwater transmittance. This study has been conducted to investigate the variation of SSC over the Gulf of Martaban. Remote sensing reflectance (Rrs) of 667 nm is used as a proxy to represent the sediment SSC variation over the study area. The data for the period of July 2002 to March 2014 acquired from MODIS Aqua 4 km resolution are used in this study. As a result, there is no obvious yearly variation in the SSC cover area. The SSC variation over this study area is found to be seasonal. High homogenous SSC covers area observably during the northeast (NE) monsoon season that occurs from December to January. The sediment cover area could reach the latitude of 15°N that located at the south of the gulf. During southwest (SW) monsoon season that occurs from May to September, low and sparse SSC cover area is observed. As a consequence, the area covered by the SSC is higher during the NE monsoon season as compared to the SW monsoon season. Hence, the SSC cover area during the NE monsoon season is greater than the yearly averaged SSC cover area. Meanwhile the SSC cover area during the rainy SW monsoon season is less than the yearly and NE monsoon season.

This paper discusses the variability in rainfall and trend analysis of annual and seasonal rainfall time series of Shillong and Agartala stations located in the north-east region of India. Commonly used non-parametric statistical methods namely Mann-Kendall and Sen’s slope estimator was used to analyse the seasonal and annual rainfall time series. Statistical analysis showed less variation in annual and south-west monsoon rainfall for both Shillong and Agartala stations. In the total annual rainfall, the share of south-west (SW) monsoon, north-east (NE) monsoon, winter season and summer season rainfall was observed 64.60%, 13.22%, 1.40% and 20.80%, respectively for Shillong station of Meghalaya state. However, the contribution of SW monsoon, NE monsoon, winter season and summer season rainfall in the total annual rainfall was 59.59%, 9.55%, 1.14% and 29.72%, respectively for Agartala station of Tripura state. Non-significant increasing trends of rainfall was observed by 4.54 mm/year, 2.80 mm/year and 2.54 mm/year for annual, SW monsoon, and summer season, whereas, non-significant decreasing trends in rainfall for NE monsoon and winter season was observed with a magnitude of 1.83 mm/year and 1.63 mm/year for Shillong, Meghalaya during 1992 to 2017. Results also revealed that rainfall increased by 1.07 mm/year and 0.18 mm/year in SW monsoon and winter season whereas, rainfall decreased by 7.64 mm/year, 2.58 mm/year and 1.29 mm/year during annual, NE monsoon and summer season non-significantly during 1995 to 2019 in case of Agartala. The findings of present study will be useful for water management and crop planning in hill agriculture of Meghalaya and Tripura state of India.

Characteristics of raindrop size distribution (DSD) are studied during the southwest (SW) and northeast (NE) monsoon seasons using 4 1/2 years of DSD measurements made at Gadanki (13.5°N, 79.2°E) by an impact‐type disdrometer. The observed DSD is found to be distinctly different in the NE monsoon from that of the SW monsoon. The stratified DSD (based on rain rate) shows more small drops and fewer bigger drops in the NE monsoon than in the SW monsoon, particularly in the low rain rate regime. This feature is not an anomalous one, rather observed consistently in all the years. The diurnal variation of DSD (in terms of mass‐weighted mean diameter, Dm) plots in both monsoons show large values of Dm in the evening hours. Several possible reasons, from instrumental problems to geophysical mechanisms, for the observed DSD differences are examined. The wind field and the range of Dm values indicate that the DSDs in the SW and NE monsoons are continental and oceanic in nature, respectively. Further, high temperature and intense convective activity (vigorous updraughts) in the SW monsoon modify the DSD through evaporation, drop sorting, and collision‐coalescence processes. Copyright © 2009 Royal Meteorological Society

Microbial community dynamics and taxon-specific phytoplankton production in the Arabian Sea during the 1995 monsoon seasons

The coupling of temporal and spatial variations of chlorophyll a concentration and the East Asian monsoons in the southern Taiwan Strait

The analysis of chlorophyll, sea surface temperature (SST), wind speed and nitrate data have been carried out in the monthly, seasonal and inter-annual scales during 1999–2004. The monthly averaged chlorophyll concentration indicates high chlorophyll concentration (0.50–2.0 mg/m3) in the southern peninsula around the tip of India. The movement of chlorophyll from the Arabian Sea and Gulf of Mannar region towards the east via Sri Lankan region has been observed during the southwest monsoon (SWM) season. The algal bloom has been observed both during southwest and northeast monsoon (NEM) period. The SST has been observed to be high (29–31 °C) during the spring inter monsoon (SIM) during March–May and low during SWM and NEM (~27 °C). Wind speed has been observed to be very high (8–12 m/s) during the SWM and NEM periods. The relationship between in situ nitrate and temperature has been established with R2 value 0.912 with 1537 data points. The nitrate concentration has been observed to be high (0.20–0.50 µmol/l) during SWM due to the upwelling process. Relationship has been established between chlorophyll, SST, wind speed and nitrate covering the seasonal averaged data over five-year period. The increase in wind speed may be causing upwelling and mixing phenomenon, bringing up the nutrient rich bottom water to surface and mixes up the water column and hence the decrease in SST. So, the enhancement in productivity/phytoplankton chlorophyll biomass has been observed. The interrelationship of the parameters in the Arabian Sea, Indian Ocean and the Bay of Bengal subsets has been derived. The chlorophyll in the Indian Ocean has been observed to be primarily dependent on nitrate (R2 = 0.39) and SST (R2 = 0.38) and to a lesser extent wind speed (R2 = 0.11). But, the Arabian Sea and Bay of Bengal chlorophyll has been observed to be more dependent on SST (R2 = 0.43 and 0.52) followed by nitrate (R2 = 0.30 and 0.27) and wind speed (R2 = 0.18 and 0.11), obtained from regression analysis.

Seasonal variation in hydrographic conditions in Taiwan Strait is strongly influenced by the monsoonal system. During northern winter, the China Coastal Current, pushed by the north-east (NE) monsoon, moves southwards into Taiwan Strait and during northern summer, the South China Sea Surface Current, driven by the south-west (SW) monsoon, invades the strait until the NE monsoon again prevails. As the SW monsoon wanes (in northern autumn), the Kuroshio Branch Current enters from the southern part of the strait, but stagnates in the middle because of interference by the China Coastal Current. As the NW monsoon wanes (in northern spring), the stagnation ceases and the SW monsoon begins. We characterised zooplankton (including copepods and ichthyoplankton) communities during a period when the SW monsoon was prevalent (in August), at the onset of the NE monsoon (in November) and as the NE monsoon waned (in March). Multivariate analyses of zooplankton composition and species abundances demonstrated that the structures of communities are closely related to oceanic variables (such as temperature, salinity and upwelling), which, in turn, are heavily influenced by the monsoons. The zooplankton faunas in Taiwan Strait are a mixture of local species and intruding species, the latter introduced from along the China coast during northern winter and from the South China Sea during northern summer. Our findings are fundamental to practical ecosystem management and an effective long-term monitoring programme.

The South Asia Seasonal Climate Outlook Forum (SASCOF) issues seasonal tercile precipitation forecasts to provide advance warning of anomalously dry or wet monsoon seasons in South Asia. To increase objectivity of the SASCOF seasonal outlook, the World Meteorological Organisation recommends using a multi-model ensemble combining the most skilful dynamical seasonal models for the region. We assess the skill of 12 dynamical models at forecasting seasonal precipitation totals for 1993–2016 for the southwest (June–July–August–September) and northeast (October–November–December) monsoon seasons at regional and national levels for Afghanistan, Bangladesh, Nepal, and Pakistan, using identical forecast periods, hindcast initialisation months and domain used at the SASCOF. All models demonstrate positive skill when regionally-averaged, especially for the southwest monsoon season, noting considerable spatial differences. Models exhibit highest skill where correlation between observed precipitation and El Niño Southern Oscillation (ENSO) is highest, e.g., central/north India and Nepal during the southwest monsoon, and Afghanistan and north Pakistan during the northeast monsoon. Model skill is especially low in northwest India and northeast of South Asia during the southwest monsoon, e.g., Bangladesh (despite high precipitation totals) coinciding with a weak ENSO teleconnection. The Indian Ocean Dipole teleconnection is less pronounced in the southwest monsoon season, whereas the spatial pattern for the northeast monsoon closely resembles that of ENSO. Due to high variability in model skill, we recommend basing the SASCOF forecast on a multi-model ensemble of all models but discounting poorly performing models at the national level.

In the present study, impact of precipitation disparity on groundwater level fluctuation was carried out in Vellore district, Tamil Nadu, India, using geospatial techniques. There are five rain gauge stations in the study area in which three rain gauge stations, namely Alangayam, Jolarpettai and Pernampet, receive more precipitation when compared with the average annual precipitation of Tamil Nadu state (920 mm). The other two stations, namely Madanur and Natrampalli, receive less than 920 mm of precipitation annually. The overall average annual precipitation of the study area is 913.6 mm. More than 100 mm precipitation is received in all the five rain gauge stations during southwest (SW) and northeast (NE) monsoon seasons. The maximum monthly precipitation is usually recorded during the month of November and the minimum precipitation is recorded during June. The post-monsoon precipitation is around 10.8 mm, which is almost negligible in the study area. The contribution of precipitation by various seasons is in the following sequence: Southwest monsoon > Northeast monsoon > Pre-monsoon > Post-monsoon. The spatial disparity study indicates that the intensity of average annual, pre-monsoon and post-monsoon precipitations increase towards west in the study area. The intensity of precipitation is more in the northern part during SW monsoon season, whereas the intensity is more in the southern part during NE monsoon season. The spatial disparity analysis of groundwater fluctuation shows that the depth of groundwater (below ground level) increases towards west during all the monsoon seasons. The minimum, mean and maximum depths of occurrence of groundwater in this region are, respectively, 1.6, 9.6 and 21.15 m. Declining trend in the regional groundwater level is observed from December to June because of less precipitation during non-monsoon season. However, the monsoon (both SW and NE monsoon) precipitation recharges the groundwater from June to December to reach the maximum in the month of December.

In the present study, impact of precipitation disparity on groundwater level fluctuation was carried out in Vellore district, Tamil Nadu, India, using geospatial techniques. There are five rain gauge stations in the study area in which three rain gauge stations, namely Alangayam, Jolarpettai and Pernampet, receive more precipitation when compared with the average annual precipitation of Tamil Nadu state (920 mm). The other two stations, namely Madanur and Natrampalli, receive less than 920 mm of precipitation annually. The overall average annual precipitation of the study area is 913.6 mm. More than 100 mm precipitation is received in all the five rain gauge stations during southwest (SW) and northeast (NE) monsoon seasons. The maximum monthly precipitation is usually recorded during the month of November and the minimum precipitation is recorded during June. The post-monsoon precipitation is around 10.8 mm, which is almost negligible in the study area. The contribution of precipitation by various seasons is in the following sequence: Southwest monsoon > Northeast monsoon > Pre-monsoon > Post-monsoon. The spatial disparity study indicates that the intensity of average annual, pre-monsoon and post-monsoon precipitations increase towards west in the study area. The intensity of precipitation is more in the northern part during SW monsoon season, whereas the intensity is more in the southern part during NE monsoon season. The spatial disparity analysis of groundwater fluctuation shows that the depth of groundwater (below ground level) increases towards west during all the monsoon seasons. The minimum, mean and maximum depths of occurrence of groundwater in this region are, respectively, 1.6, 9.6 and 21.15 m. Declining trend in the regional groundwater level is observed from December to June because of less precipitation during non-monsoon season. However, the monsoon (both SW and NE monsoon) precipitation recharges the groundwater from June to December to reach the maximum in the month of December.

This study focuses on describing the patterns and trends of five selected rainfall indices in Peninsular Malaysia, based on daily rainfall data from 1975 to 2004. Five rainfall indices based on two main seasons, the northeast and southwest monsoons, were analyzed: total rainfall, frequency of wet days, rainfall intensity, frequency of wet days (extreme frequency), and rainfall intensity (extreme intensity) exceeding the long-term mean 95th percentile. The findings indicated that the eastern areas of the Peninsula were strongly influenced by the northeast monsoon, while the southwest monsoon had the greatest impact on the western part of the Peninsula, particularly the northwest. In studying the trends of these rainfall indices, the non-parametric Mann–Kendall test was used. The serial correlation and cross-correlation structure of the data were accounted for in determining the significance of the Mann–Kendall test results. It was found that there were differences in trend patterns over the Peninsula during both seasons, with a decrease in total rainfall and a significant decrease in frequency of wet days leading to a significant increase in rainfall intensity over the Peninsula, except in eastern areas, during the southwest monsoon. In contrast, a trend of significantly increasing total rainfall and an increase in frequencies of extreme rainfall events during the northeast monsoon caused a significantly increasing trend in rainfall intensity over the Peninsula to be observed. However, no significant trend was observed with respect to extreme intensity during both monsoons over the Peninsula. The findings of this study suggest that rainfall patterns in Peninsular Malaysia are very much affected by the northeast monsoon, based on the larger magnitude of changes observed in the rainfall indices.

The phytoplankton variability and ocean productivity have been studied with the retrieval of chlorophyll concentration (CC) from various satellite ocean colour missions. The phytoplankton distribution is related to ocean processes and physical parameters such as temperature, wind speed, currents, etc., at regional and global scales. In the current study, the phytoplankton productivity has been observed in the southeast Arabian Sea during southwest (SW, June–September) and northeast (NE, October–December) monsoon seasons using merged chlorophyll product datasets during the years 2017 and 2018. There has been an observation of high chlorophyll concentration (~5.0 mg m−3) in the southeast Arabian Sea (southern tip of India) during SW monsoon and less in NE monsoons (~1.0 mg m−3). The spread in CC has been observed maximum during July–September. High anomaly of CC was observed along north-eastern Arabian Sea during northeast monsoon months of 2017 due to the very severe cyclonic storm Ockhi. Similarly, during August 2018, high chlorophyll concentration was observed (18.8 mg m−3) in south-eastern tip as a result of heavy rainfall and discharge of nutrient rich riverine water. The sea surface temperature (Modis-Aqua SST) has been observed to be cooler (26–27°C) in the SE Arabian Sea coastal waters during July–August as compared to warmer (28–30°C) surrounding waters. During the SW monsoon (June–August), the wind speed has been observed to be of high magnitude (~10 m/s), which was not evident during September. The alongshore (north-westerly) wind has been observed during July–September in the near coastal water that causes coastal to offshore moving Ekman mass transport (EMT). Similarly, the alongshore (north-easterly) wind has been observed during December in the southern tip of India and off Sri Lankan west coast. The strong zonal EMT (–1500 to –500 kg/m/s) has been observed in the southeast Arabian Sea during July–September 2017 and 2018 which is weak to moderate (> –500 kg/m/s) in NE monsoon months. Statistical analysis has been made between SST, wind speed and CC which shows significant correlation coefficient (r > 0.6, p < 0.001). Hence, the present study confers that the alongshore wind and high wind speed regions are favourable to upwelling events and have been responsible for cooling of SST resulting in high productivity zones in the southeast Arabian Sea, which is important from the point of view of marine fishery and ecosystem assessment.

Monsoon-related changes in surface hydrography and productivity in the Bay of Bengal over the last 45 kyr BP

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