Southwest Monsoon Season Research Articles

Evidence for brown carbon absorption over the Bay of Bengal during the southwest monsoon season: a possible oceanic source.

The near UV-visible light-absorbing organic carbon (OC) of ambient aerosols, referred to here as brown carbon (BrC), significantly influences the atmospheric radiative forcing on both regional and global scales. Here, we documented BrC absorption in the aqueous and methanol extracts of marine aerosols collected over the Bay of Bengal (BoB: September-October 2017) and a city, Visakhapatnam (May-June 2018), in southern India during the southwest monsoon (i.e., a transition period with weak continental impact). The absorption spectra of BrC over the BoB showed several peaks around 300-400 nm and differ from those observed over Visakhapatnam. The absorption coefficient of BrC over the BoB, unlike Visakhapatnam data, does not seem to covary with other chemical proxies of biomass burning (non-sea-salt or nss-K+) and coal combustion (nss-SO42-) in the continental outflows, suggesting a different source of BrC over the BoB. Besides, we observed higher proportions of water-insoluble organic carbon (WIOC/OC: 0.89 ± 0.02) and significant enrichment of Mg2+ over Na+ (i.e., relative to seawater) in BoB aerosols. This result and the backward air mass trajectories both hinted their major source of OC from marine-derived organic matter. In contrast, the absorption spectra of BrC over Visakhapatnam are like those from biomass burning emissions in the Indo-Gangetic Plain. This observation is further supported by the satellite-based fire counts and backward air mass trajectories. Therefore, our study underscores the BrC aerosols from the oceanic sources and southern India, hitherto unknown, and can improve our understanding of the regional climate effects of carbonaceous aerosols if included in models.

Changes in aerosol size distributions over the Indian Ocean during different meteorological conditions

Aerosol emissions in South Asia are large. The emitted aerosols can travel significant distances and, during the Asian southwest monsoon especially, are prone to modification through cloud processing and wet scavenging while being transported. The scale of emissions and transport means that the global climate impact of these aerosols are sensitive to modification en route, but the process-level understanding is still largely lacking. In this study, we analyse long-term aerosol data measured at an observatory established in Hanimaadhoo, Republic of Maldives, to investigate the long-term properties of aerosols over the Indian Ocean as well as to understand the effect of precipitation on the aerosol particle size distribution during long-range transport. The observatory location is ideal because it is a receptor site with little local influence, and, depending on the season, receives either polluted air masses coming from the Indian subcontinent or clean marine air masses from the Indian Ocean. We analysed the sub-micron particle number size distribution measured during the years 2004–2008, and 2014–2017, and this is the first inter-seasonal long-term study of the sub-micron aerosol features in the region. The aerosol origin and its relative exposure to wet scavenging during long-range transport were analysed using back-trajectory analysis from HYSPLIT. By comparing aerosol measurements to precipitation along its transport, this study shows that there is a substantial change in particle number size distributions and concentrations depending on the amount of rainfall during transport. During the southwest monsoon season, the aerosol size distribution was notably bi-modal and total particle concentrations clearly reduced in comparison with the prevailing aerosol size distribution during the northeast monsoon season. Precipitation during transport usually corresponded with a greater reduction in accumulation mode concentrations than for smaller sizes, and the shape of the median size distribution showed a clear dependence on the trajectory origin and route taken.

Concentration factor of metals in zooplankton and their seasonality in Kalpakkam coast, southwest Bay of Bengal

Abstract Concentrations of 12 different trace elements (Cd, Ce, Co, Cr, Cu, Fe, La, Mn, Ni, Pb, U and Zn) in dissolved form and in tissue of mesozooplankton isolated from Klapakkam coast were analyzed, to understand their bioaccumulation potential. Dissolved metal concentrations in water was found to be in the order Fe > Zn > Cu > Pb > U > Cr > Mn > Ni > La > Cd > Ce > Co, whereas, concentration in mesozooplankton tissues followed the trend of Fe > Zn > Mn > Cr > Ni > Pb > Cu > Ce > La > Co > U > Cd. Summer season was found to have high metal concentrations in the water and zooplankton tissue than the southwest and north east monsoon seasons. The bioconcentration factor (BCF) always remained high (BCF ≥ 1) indicating active accumulation of metals by mesozooplankton. The sequential trend of BCF was Mn > Ce > Fe > Cr > Ni > Co > La > Cu > Pb > Zn > Cd > U. Concentrations of rare earth elements viz. Ce and La, were significantly high in the study area in the dissolved form (range (mean ± SD): Ce: 0.2–3.8 μg L−1 (0. 7 ± 0. 8); La: 0.3–6.7 μg L−1 (1.8 ± 2.0)) and zooplankton tissue (Ce: 5.1–44.2 μg g−1 (21.55 ± 10.96); La: 2.5–20.9 μg g−1 (10.1 ± 5.4)) as well. The zooplankton community was dominated by copepods and there emerged a strong positive correlation between the soluble and accumulated metals in mesozooplankton in case of Zn and Cr. This study has provided a novel insight into metal distribution in zooplankton often act as the major food for marine fishes, and play an important role in the transfer of toxic metals in the marine food chain.

Synergy between INSAT-3D infra-red and GPM microwave radiometer for precipitation studies

The Indian National Satellite System (INSAT) based Multispectral Rainfall Algorithm (IMSRA) operationally provides precipitation estimates using measurements provided by the Kalpana satellite from 2008 onwards and the INSAT-3D from 2014 onwards over Indian land and oceanic regions for dissemination to the users. Here, an attempt is made to estimate rainfall over the Indian monsoon region by the synergistic use of geostationary INSAT-3D IMSRA derived rainfall estimates and rain data from the Global Precipitation Measurement (GPM) Microwave Imager (GMI) using a suitable objective analysis method. The objective criteria of a successive correction method benefit from high spatial and temporal resolutions of the geostationary satellite and infrequent but more accurate microwave-based rainfall estimates, particularly during heavy rain conditions. The method is applied to various case studies for the southwest monsoon season of 2015 at both instantaneous and daily scale. A comparison is made with ground data and also with independent global concurrent rainfall products. The merged rainfall estimates shows noticeable improvement over the Infrared (IR)-based rainfall estimates over various parts of the Indian land and oceanic regions on a daily scale. Overall, results reveal that the GMI type radiometer in low inclination orbit like the Megha-Tropiques and Tropical Rainfall Measurement Mission (TRMM) could be more useful for synergistic use with INSAT-3D. It has the potential to predict more accurately near real-time tropical rainfall, particularly over the Indian monsoon region, which makes it a valuable source of information for flood forecasting, agriculture and water resource management, numerical weather prediction, and numerous other applications in the hydro-meteorological sciences.

Long-term chlorophyll-a dynamics in tropical coastal waters of the western Bay of Bengal.

The long-term distribution of in situ optically active substances (OAS), accuracy assessment of satellite retrieved chlorophyll-a (chl-a) and its long-term trend has been carried out at a coastal site of the north-western Bay of Bengal. The temporal distribution of chl-a, total suspended matter (TSM) and absorption due to coloured dissolved organic matter at 440nm (aCDOM440) discerned a common peak during southwest monsoon season (August-October). Chl-a also showed a prominent peak during pre-southwest monsoon period (March-April). The spatial variability of TSM and aCDOM440 was maximum during southwest monsoon, whereas in the case of chl-a, it was during pre-southwest monsoon. The accuracy assessment of chl-a retrieved from Moderate Resolution Imaging Spectroradiometer-Aqua (MODISA), Ocean Colour Monitor-2 (OCM-2) and Visible Infrared Imager Radiometer Suite (VIIRS) showed overestimation in nearshore waters. The error in satellite measurement of chl-a was within the range of 33 to 51%. The chl-a retrieved from MODISA was most accurate as indicated by statistical analysis. The long-term trend in satellite chl-a clearly indicated bi-modal distribution with a primary peak during pre-southwest monsoon attributed to recurrent phytoplankton bloom that was mostly confined to nearshore waters. Whereas, the secondary peak in chl-a, during the end of southwest monsoon, spreads far offshore.

The large-bodied copepods off Masirah Island, Oman: An investigation of Southwest Monsoon onset and die-off

Abstract Masirah Island, situated off the southern coast of Oman in the Arabian Sea, is surrounded by upwelled water during the Southwest Monsoon (SWM) and was used as a base for time-series studies of the copepod community for a decade beginning in 2006, covering the Spring Intermonsoon (SIM), April–May, and including June, and the SWM period, August–October. Subeucalanus pileatus and Calanoides natalis were the two most abundant copepod species (33.9% and 32.7% respectively of the total abundances). These two species shifted their dominance during the two main seasonal periods. During the SWM, C. natalis was the dominant species occurring mainly at sea surface temperatures (SST) between 18 and 27 °C while S. pileatus dominated during the SIM occurring at warmer SSTs, frequently between 25 and 28 °C. Results from generalized linear models indicated that the season and the SST were the best predicted variables affecting the abundances of C. natalis and S. pileatus. Calanoides carinatus was also affected by wind speed and wind direction, meaning that strong SWM winds and low temperatures (seasonal upwelling) favor the occurrence of this species. Upwelling favorable winds lasted longer in 2015/2016 than in 2009/2010; peak winds shifted from June in 2009–2011 to July in 2012–2016. The SST was higher in 2016 than in the previous five years. Time-series of C. natalis life stages showed three large peaks of early copepodites from June through September 2016 during the SWM season, in contrast to only one peak observed at the beginning of the SWM season from previous years. This result suggests that C. natalis was able to capitalize on the longer SWM in 2016 by continuing to reproduce throughout September. Diapausing C. natalis reached the surface in mid-to-late June as stage CV, ruling out any reproduction by this species at its overwintering depth. Unfavorable SST at the end of the SWM corresponded to a downward migration by C. natalis in 2015. Overall, the physical forcing of this upwelling region appears to be changing and C. natalis seems to be adapting to these changing conditions by continuing to reproduce at the surface as long as SST is favorable.

Health risks associated with fluoride intake from rural drinking water supply and inverse mass balance modeling to decipher hydrogeochemical processes in Vattamalaikarai River basin, South India

Preliminary investigation reveals that fluorosis is reported due to the continuous intake of fluoride-rich groundwater in Vattamalikarai River basin, Tamil Nadu, India. A detailed study was attempted for evaluating the health risks associated with the intake of fluoride-rich groundwater supplied to the rural community. Groundwater samples were collected from 60 and 58 dug and tube wells during winter and southwest (SW) monsoon seasonsrespectively. The samples were analyzed for the determination of fluoride and other chemical parameters to examine the fitness for drinking water. Spatio-temporal variation maps reveal that fluoride concentration is high during SW monsoon season when compared with the winter season in this region. The fluoride bearing minerals present in hornblende-biotite gneiss and charnockite rock formations leached into the groundwater during rock-water interaction. To understand the subsurface hydrogeochemical reactions, inverse mass balance model was developed using NETPATH code. The model output indicates that calcite dilution, silicate (hornblende and biotite) weathering, ion exchange (Ca/Na and Mg/Na) and illite precipitation are the dominant processes controlling the groundwater chemistry along the flow paths. Non-carcinogenic risks to children and adults (women and men) were evaluated by working out intake exposure of groundwater. Hazard quotient (HQ) based on fluoride intake was calculated for children and adults. It varied from 0.08 to 2.21 with an average of 1.07 for adults. For children, it varied from 0.01 to 2.99 with the mean of 1.44. About 78%, 69% and 61% of the samples fall under the risk category for children, women and men during winter season. However, more number of samples possessed health risks (83% of samples for children, 73% of samples for women and 64% of samples for men) during SW monsoon season.

Assessing the Possibility of Appearing RIP Current at Quy Nhon Beach, Binh Dinh Province

RIP current is a strong, popular water flow at beaches in Vietnam as well as in the world, with the direction of flow from the shore to the sea. This is a dangerous flow, each year there are unfortunate accidents for swimmers. This study used the measured data source implemented by the Center for Environmental Fluid Dynamics in 2018 to build the model MIKE 21 Coupled for simulating the hydrodynamic field and the rip current at Quy Nhon beach area. Through calculation scenarios under different conditions show that, the area near the amusement park (An Duong Vuong road) and the southern-most area of Quy Nhon beach are likely to appear rip current in the northeast wind season. In the southwest monsoon season, the intensity of the rip current is strong, appearing near the Victory Monument area. These areas are often crowded by swimmers with local people and tourists, so pay attention to avoid accidents that may happen.

Spatial and temporal characteristics of rainfall over a forested river basin in NW Borneo

The spatial and temporal patterns of rainfall over the Baram River Basin (BR) in Sarawak (Malaysian Borneo) were characterised through cluster analysis and multivariate statistics for the 25 year period from 1990 to 2014. Baram River Basin recorded an average annual rainfall of 3654 mm with high spatial and temporal variations. Ward’s method based analytical hieratical clustering (AHC) identified three homogeneous clusters (HC’s) of rain gauging stations (HC-I, HC-II, and HC-III). Rain gauges in HC-I are located in high rainfall domain, HC-II gauges are in moderate rainfall domain and HC-III gauges are in low rainfall domains. The moderate elevation regions in the Baram basin recorded the highest amounts of rainfall compared to lower and upper elevated regions. Multivariate statistics show variation in the distribution of monthly and annual rainfall within and between individual HC’s. Mean annual rainfall recorded at individual HC’s varies by around 1000 mm. Though the mean monthly rainfall distribution in HC’s varies by more than 70 mm, all clusters show a common pattern of high and low rainfall seasons related to monsoon characteristics of the region. The northeast monsoon (NEM) is the wettest period with highest mean rainfall falling in the months of November and December at all stations. In comparison, the southwest monsoon (SWM) is the driest season with the lowest mean rainfall falling in July. It was also noted that the inter-monsoon periods are actually wetter than the SWM season. Overall, rainfall in the Baram shows high inter-annual variability with dominant low rainfall domain in the lower (coastal) and upper (plateau) reach with highly localised high rainfall domain surrounded by fluctuating moderate rainfall areas in the middle reaches of the river basin. The localised nature of high rainfall domain in the central (south and southwest) part of the basin suggests that the surrounding elevated mountains and hills in that region influence the high rainfall recorded. The findings of the present research will aid in formulating water resource management plans, large scale plantation and agricultural practices and any other hydrological development projects in the region.

Microbial Composition and Variability of Natural Marine Planktonic and Biofouling Communities From the Bay of Bengal.

The Bay of Bengal (BoB) is the largest bay in the world and presents a unique marine environment that is subjected to severe weather, a distinct hydrographic regime and a large anthropogenic footprint. Despite these features and the BoB’s overall economic significance, this ecosystem and its microbiome remain among the most underexplored in the world. In this study, amplicon-based microbial profiling was used to assess the bacterial, archaeal, and micro-eukaryotic content of unperturbed planktonic and biofilm/biofouling communities within the BoB. Planktonic microbial communities were collected during the Southwest monsoon season from surface (2 m), subsurface (75 m), and deep-sea (1000 m) waters from six south-central BoB locations and were compared to concomitant mature biofouling communities from photic-zone subsurface moorings (∼75 m). The results demonstrated vertical stratification of all planktonic communities with geographic variations disappearing in the deep-sea environment. Planktonic microbial diversity was found to be driven by different members of the community, with the most dominant phylotypes driving the diversity of the photic zone and rarer species playing a more influential role within the deep-sea. Geographic variability was not observed in the co-located biofouling microbiomes, but community composition and variability was found to be driven by depth and the presence of macro-fouling and photosynthetic organisms. Overall, these results provide much needed baselines for longitudinal assessments that can be used to monitor the health and evolution of this dynamic and critically important marine environment.

Modeling of Raindrop Size Distribution Observed Using Micro Rain Radar Over Darjeeling (27.05oN, 88.26oE): An Eastern Himalayan Region

Micro rain radar (MRR-2) provides the measurement of the drop size distribution DSD with altitude and rain rates. In this paper, we show a new form of gamma drop size distribution (DSD) model. MRR-2 is based on the principle of Radar spectrum measurement of continuous-wave of frequency modulation. DSD measurements for the rain rates ~ 0–25 mm/h and the height from 150 to 2000 m during the south-west monsoon season for the period 2009 and 2010 are used in this study. From the variance of DSD ( $$\sigma^{2}$$ ) and mass-weighted mean drop diameter as well as after the mathematical calculation, two parameters ( $$\varLambda$$ and µ) are obtained for the use of lower-order moments. Both the moments are utilized for the best suitable of gamma DSD. Measured DSD for the different rain rates and different altitudes below the melting layer height (~ 2–3 km) compared among the exponential, lognormal and gamma DSD model over Darjeeling in the Eastern Himalaya. The correlation between best fit measured DSD and gamma distribution indicate that they correlate well for both lower and higher order moments for the heights 150 m and 1050 m. Whereas they correlated only for higher order moments for the 450 m, 1500 m and 1950 m. Thus, it is found that the observed pattern of DSD agrees well to the gamma DSD model.

Streamflow Variability in Mahaweli River Basin of Sri Lanka during 1990–2014 and Its Possible Mechanisms

This study investigates the variation of seasonal streamflow and streamflow extremes in five catchments of the Mahaweli River Basin (MRB) Sri Lanka from 1990 to 2014, and the relationship between streamflow and seasonal rainfall in each catchment is then examined. Furthermore, the influence of Indian Ocean Dipole (IOD) and El Nino and Southern Oscillation (ENSO) on the seasonal rainfall and streamflow in the upper (UMRB) and lower reaches (LMRB) of MRB are explored. It’s found that the rainfall amount in southwest monsoon (SWM) season contributes 29.7% out of annual total rainfall in the UMRB, while the LMRB records 41% of the total rainfall during the northeast monsoon (NEM) season. The maximum streamflow of upper (lower) Mahaweli catchments is observed in the SWM (NEM) season. Catchments in the UMRB (LMRB) recorded strong interannual variability of seasonal overall flow (Q50), Maximum 10-day, and 30-day flows during the SWM (NEM) season. It’s further revealed that the catchment streamflow in the UMRB is closely correlated with the SWM rainfall in the interannual time scale, while streamflow of catchments in the LMRB is closely associated with the NEM rainfall. The effects of ENSO and IOD on streamflow are consistent with their impacts on rainfall for all catchments in MRB, with strong seasonal dependent. These suggested that the sea surface temperature anomalies in the both Indian Ocean and tropical Pacific Ocean are important factors affecting the streamflow variability in the MRB, especially during the SWM season.

Observed and model‐simulated thermodynamic processes associated with urban heavy rainfall events over Bangalore, India

AbstractA total of 32 rainfall events spanning the period from 2012 to 2014 over the urban Indian city of Bangalore were simulated using the Weather Research and Forecast (WRF) model. Model simulations were carried out with a four‐nested domain initialized with Global Forecast System (GFS) data and the forecast was generated on an hourly basis. The forecasted rainfall at hobli level (Bangalore has 34 hobli divisions with an area of each hobli of the order of about 10 km2) was evaluated in terms of its intensity and pattern of spatial distribution by comparing it with corresponding rain‐gauge observations. Also, the rainfall forecast skill of the model was evaluated statistically by computing root mean square error, bias and mean absolute error. Thermodynamic variables such as equivalent potential temperature, convective available potential energy (CAPE), convective inhibition (CIN), K index (KI), lifted index (LI) and total totals index (TTI) were also derived from the simulated model parameters for all the events and then verified against corresponding observations. The results showed that the WRF model could simulate the rainfall events and associated thermodynamic features qualitatively; however, there were few hoblis where the relative errors in the forecast were > 100%. The forecast errors were relatively lower for cases during the southwest monsoon season compared with other seasons. It was found that the model underestimated thermodynamic indices such as CAPE, dew point depression and the simulated LI were positive; these were indicative of the model's limitation in simulating intense convection and a possible reason for underpredicted rainfall simulations.

Studies on the variability of upwelling system in the South Central Vietnam waters under the impact of ENSO events and its impact on hydrographic conditions of the Ninh Thuan-Binh Thuan waters

In this paper, we have exploited and analyzed data from HYCOM + NCODA Global 1/12.5o Reanalysis database in the period of 20 years from 1993 to 2012 and from 2013–8/2016 for the data assimilation of the HYCOM + NCODA Global 1/12° Reanalysis - for 22 depth layers in the East Sea area. The research results show that there are changes in the upwelling phenomenon and hydrological structure in the Ninh Thuan - Binh Thuan waters under the influence of ENSO phenomenon as follows: La Niña can degrade and eliminate the upwelling phenomenon. In the waters of Ninh Thuan - Binh Thuan when upwelling phenomenon occurs, it often manifests itself in the rapid decline of seawater temperature and rapid increase in seawater salinity in the depth layer of 50–100 m. The research results also show that in the East Sea region in 2010, during the Southwest monsoon season (July 2010), the phenomenon of downwelling occurred quite strongly in the middle of the Hoang Sa archipelago and the west of the Philippines and expanded toward southeast part of Hainan island. The cause of this special dynamic phenomenon may be due to the influence of the wind regime (average wind stress in July 2010).

Numerical simulation of seasonal mesoscale atmospheric flow‐field variables using ARW over the Singapore region: impact of land use land cover

AbstractIn the present study, the effect of land use land cover information on the simulation of seasonal planetary boundary layer (PBL) flow‐field variables over the Singapore region is analysed using the Advanced Research Weather Research and Forecasting (ARW) mesoscale model. The results achieved are compared using the standard ARW Moderate Resolution Imaging Spectroradiometer (ARW‐MODIS) global land cover data and a regional land cover map produced at the Centre for Remote Imaging, Sensing and Processing (ARW‐CRISP), Singapore. The high‐resolution simulations are conducted with triple‐nested domains (horizontal grid spacing 27, 9 and 3 km) and 51 unequally spaced vertical sigma levels. The evolution of PBL variables and rainfall are modelled for four seasons (northeast monsoon season, first inter‐monsoon period, southwest monsoon season and second inter‐monsoon period). For the purpose of numerical simulations, 30 days in each season were considered and the model‐simulated variables were validated with in situ available meteorological observations and rainfall at four stations over Singapore. The simulation results highlight the broad variation of the low‐level flow‐field in different seasons. The ARW mesoscale model simulated diurnal trends in surface meteorological variables and rainfall are in better agreement with observations by using the ARW‐CRISP regional land cover map compared to ARW‐MODIS global land cover data. Stronger winds are found during monsoon seasons compared with inter‐monsoon periods. Overall, the results of the present study indicate that regional level air quality monitoring and prediction capabilities around Singapore can be improved by using up‐to‐date land cover products designed to take into account regional land cover characteristics.

Circulation patterns in a shallow tropical reservoir: Observations and modeling

The diurnal circulation pattern is deduced in Kranji Reservoir, a shallow, elongated tropical reservoir in Singapore via field observations and results from the 3-D Estuary, Lake, and Coastal Ocean Model (ELCOM) model for the month of July 2012 which is during Singapore’s Southwest Monsoon season. Detailed comparisons of the modeled temperature and velocity fields with field measurements during a dry period of July 21–26 2012 without rainfall or inflow events are presented in terms of point comparisons, longitudinal profiles and cross-reservoir transverse velocity profile, thus providing a good representation of the flow field. Both observation and modeling show that the diurnal circulation pattern is driven by wind and by temperature gradients due to upwelling which depend on the time of the day. There is a daytime circulation pattern characterized by one large loop along the main channel, and a nighttime circulation characterized by two loops: one large loop between the reservoir dam and a location K1 (2500 m upstream from the dam where inflow tributary flows merge), and one small loop between K1 and the tributaries. These circulation patterns are consistent with the observed horizontal distribution of chlorophyll a during the Southwest Monsoon months over years 2008–2012. High chlorophyll a concentration occurs near the dam as expected due to downwelling within the large circulation loop and also close to the tributaries due to the nutrient from the catchments being trapped within the smaller loop. These results demonstrate the contributing effect of hydrodynamic flow on the biomass distribution that can occur in shallow tropical systems and which affects the diurnal and seasonal variation of biomass for such systems.

Mesoscale eddy variability and its linkage to deep convection over the Bay of Bengal using satellite altimetric observations

In the Bay of Bengal (BoB), surface circulation is strongly influenced by the Indian Monsoon and notable local eddying that modulates the East India Coastal Current (EICC). In this study, we apply an eddy tracking algorithm to 25 years (1993–2018) of satellite altimetric observations to identify, analyze, and track mesoscale eddies in the BoB from their generation to dissipation. We compare the characteristics and trajectories of these eddies during the southwest (SW) and northeast (NE) monsoon seasons and during the pre- and post-SW monsoon periods to better understand the seasonality of the local eddy field. We find high eddy generation in the eastern BoB associated with instability induced by coastal Kelvin waves and the westward propagating Rossby waves, but we found the most robust eddies in the western BoB around the EICC. Ocean heat content (OHC) in the BoB was compared with warm-core eddies and we find that warm-core eddies drive changes in OHC. We also compared the eddy field during strong and weak SW monsoon forcing and with varying Indian Ocean Dipole (IOD) and El Niño-Southern Oscillation (ENSO) conditions, finding a statistically significant relationship with warm-core eddies and sea surface temperatures (SSTs; R2 = 91%) and outgoing longwave radiation (OLR; R2 = 88%) during periods of strong ENSO and IOD forcing. To understand the impact of these eddies on local atmospheric convection we found the composite surface structure of anticyclonic and cyclonic eddies of warm-core and cold-core SSTs, finding a close relationship between the eddy-composite SSTs and OLR.

Simulation of spatial variation of plankton communities in the South Central Vietnam sea by ROMS model

This study preliminarily applies the Regional Ocean Modeling System (ROMS) in the two major monsoon seasons (Northeast and Southwest monsoons) for the South Central Vietnam sea (9–14.5oN, 105–112oE), in which the hydrodynamic and ecological modules are coupled. The results show that the plankton only develop in 200 m water on the top, concentrated mainly in the 0–70 m layer and in maximum biomass of 15–40 m layer. In the Northeast monsoon season, the plankton are concentrated mainly in the northern part and open seas of the area, while in the Southwest monsoon season, they are concentrated in the upwelling and adjacent southern areas. These results correctly reflect the basic law of the development of plankton communities in the sea area.

Discernment of near-oceanic precipitating clouds into convective or stratiform based on Z–R model over an Asian monsoon tropical site

Type of precipitating near-oceanic clouds are found out and tracked by inferring it from the near-surface empirical relationships of rainfall rate and radar reflectivity (Z–R), which are established using a micro-rain radar (MRR) and a Joss–Waldvogel disdrometer at a tropical coastal station Thiruvananthapuram (8.5°N, 76.9°E), an Asian monsoon near-oceanic site, almost at the tip of Indian peninsula. It is also found out from the vertical variation of Z–R relation that coalescence is prevalent than breakup, as a result of the collision of rain drops, along with evaporation of smaller drops existed as the rain comes down. The DSD/radar reflectivity/rainfall data from the MRR, disdrometer and manual rain gauge were inter-compared, and found to have good agreement. The MRR radar bright band signature, which is an indication of melting height, is validated using the Wyoming radiosonde data. The whole rain events from March to September 2007 were separated in to bright band (BB) and non-bright band (NBB) cases. The variation of Z with R for all the heights (up to 6000 m with 200 m intervals) is modeled with a function Z = ARb corresponding to both BB and NBB cases, separately for pre-monsoon and southwest monsoon seasons. And, it is realised that there are two distinct fits with different slopes and intercepts existing for Z–R relations corresponding to BB and NBB cases throughout aloft during both the seasons. It is found out that the presence of radar BB or NBB signatures, with corresponding distinct Z–R relations even just at near-surface, which can be used as a proxy for discerning the clouds into stratiform or convective, respectively.

On the extreme rainfall events during the southwest monsoon season in northeast regions of the Indian subcontinent

AbstractHydrological disasters are recurrent over the northeast Indian region (NEI) due to heavy downpours within a short span of time. Therefore, the characteristics of rainfall extremes during the southwest monsoon over the NEI, their evolution and dissipation features, are discussed using rainfall data from the Tropical Rainfall Measuring Mission (TRMM) 3B42 V7 and the Climate Prediction Center for 16 years from 2000 to 2015. Circulation features during the lead–lag of extreme events are also examined using the National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis products. Thirty‐two extreme events such that rainfall of the day exceeds the 90th percentile were identified. During the extreme events, heavy rainfall is located mainly in the NEI without much spread in the aerial domain. The highest number of extreme rain events is found in July, followed by June and August. Trend analysis of the rainfall indicates a significant decrease in low rainfall events and this decreasing trend is compensated by the significant increasing trend in very heavy (>95 percentile) to extremely heavy (>99 percentile) rainfall events. The average life of the extreme rainfall events is found to be about 3 days during which the NEI receives more than 100 mm/day; however, on individual examination, this ranges from 2 to 5 days. A strengthening of the southerly component of lower level wind from the Bay of Bengal and an updraft due to convergence at 850 hPa over the region favours the rainfall peak. The sea level pressure anomaly gradient and vertical velocity show a sudden development of conditions favourable for a heavy downpour over the northeast region.