Peak Southwest Monsoon Research Articles

Assessment of satellite-based Net Primary Productivity models in different biogeochemical provinces over the northern Indian Ocean

ABSTRACT The biological productivity in the oceanic ecosystem plays a vital role in the global carbon cycle and energy flow in the oceanic food chain. Therefore, proper monitoring of phytoplankton primary production in the ocean is essential to understand the role and responses of aquatic ecosystems to global climate change. Its quantification is also important for higher trophic-level studies and sustainable ecosystem management. The advent of satellite technology has provided an opportunity to retrieve Net Primary Productivity (NPP) at high spatial and temporal resolution. However, for efficient utilization of satellite NPP it is imperative to assess the models for their accuracy and variability. The present study described NPP estimated from MODIS-Aqua satellite data (2003–2021) using five models (VGPM, Eppley-VGPM, CbPM, CAFE, and SABPM) and their variability in the four biogeochemical provinces (ARAB, MONS, INDW and INDE) in the northern Indian Ocean. The temporal variability indicated all models exhibit seasonal variability in ARAB and INDW with two prominent peaks during peak southwest monsoon (June-August) and winter monsoon (December-January). However, there was significant difference in the magnitude. The seasonal peak was primarily due to increase in NPP due to summer upwelling and winter mixing. The spatially averaged diversity index indicated maximum variability in INDW (74.6–77.1%), followed by INDE (77.2–78.0%), ARAB (77.2–78.4%), and MONS (78.6–78.9%). The satellite retrieved NPP by various models showed maximum diversity in the region above 15°N and west of 60°E in AS and 20°N and east 90°E in the BoB indicating a significant influence of discharge from perennial rivers resulting in poor performance of the models in these regions. The variability of other models was then assessed with respect CbPM as the model was known to perform better. The Eppley-VGPM showed a maximum (55%) and SABPM showed a minimum (11%) Mean Absolute Percentage Difference with respect to CbPM. The long-term spatio-temporal and quantifiable variability between the NPP models, and biogeochemical provinces, indicated that the study provided useful information on the selection and utilization of an appropriate model for future sustainability and climate change studies.

Comparative account on phytoplankton functional community dynamics in the Alappuzha mud bank region (southwest coast of India) by HPLC-CHEMTAX and microscopy approaches

Phytoplankton community composition and its temporal dynamics were evaluated in the Alappuzha near shore waters (<15 m depth), including mud bank region, located along the southwest coast of India, using HPLC-derived marker pigments and microscope-based species composition data. The study has shown a characteristic phytoplankton succession pattern, wherein an exceptional increase in chlorophyll a (Chl a, >10 mg m−3), observed in the whole study region during the peak southwest monsoon (SWM - July) period, apparently unveiled the biological manifestation of nutrient enrichments facilitated by the coastal upwelling phenomenon. A conspicuous increase in fucoxanthin (a biomarker for diatoms) perceived across the study region clearly indicated the preponderance of diatoms (mainly Thalassiosira sp., Skeletonema costatum, Trieres mobiliensis, Nitzschia sp. etc.), particularly during the peak SWM period. The CHEMTAX calculations have shown the significant contribution of Chl a (av. 76% of total Chl a) from diatoms, regardless of seasons, compared to other taxonomic groups. Furthermore, the combined results of HPLC-CHEMTAX analyses have demonstrated the seasonal occurrence of various small-sized phytoplankton functional groups (PFGs), specifically cyanobacteria, cryptophytes and chlorophytes, along with the predominant large-sized PFGs, i.e. diatoms (mostly) and dinoflagellates. The relative increase in zeaxanthin prevalent during the warm periods (Pre-SWM and Post SWM) signified the presence of cyanobacteria. By contrast, during the SWM period, a conspicuous increase in Chl b and alloxanthin recorded in the whole study region indicated the abundance of green algae and cryptophytes, respectively. Likewise, a nominal increase in peridinin (a marker for dinoflagellates) prevalent during the onset of SWM was apparently due to the proliferation of dinoflagellates, Tripos furca and Prorocentrum micans (microscopically confirmed). Even though the overall CHEMTAX results clearly complemented the microscopy derived phytoplankton composition data, especially the abundance of large sized PFGs, (i.e. diatoms and dinoflagellates), this particular analysis could enable the portrayal of seasonal dynamics of small-sized PFGs (mainly cyanobacteria and cryptophytes) as well, as this information is highly significant as far as the study region is concerned.

Inter-annual Changes of Water Temperature in the Southern South China Sea's Continental Shelf: The Influence of ENSO on Malaysian Waters

An extensive amount of data from the World Ocean Database (WOD) that covers the southern South China Sea (SSCS) were analysed to study the long-term trends and variations of the El-Nino Southern Oscillation (ENSO) event. The WOD data were selected from 1951 to 2010, which included the area within 0–10 °N and 100–117 °E. El-Nino and La-Nina anomaly results showed that the sea surface temperature anomalies (SSTa) trend of SSCS was higher during offset-El-Nino years and lower during offset-La-Nina years. Besides, the inter-annual vertical water temperature profile revealed that the deepening of the warmer water temperature occurred in February and August of the offset-El-Nino years. Next, analysis of the potential area for coastal upwelling showed that the uplifting of cold water (26 °C) at the east coast of Peninsular Malaysia (ECPM) occurred during the peak southwest monsoon, whereas at the northwest of Sabah waters (NWSW), it occurred during the offset northeast monsoon. El-Nino suppressed the coastal upwelling at the ECPM due to the intense mixing of warm water during the offset-El-Nino years. Meanwhile, El-Nino enhanced the coastal upwelling in the NWSW due to the establishment of a positive wind stress curl anomaly at the northwest Borneo coast.

Influence of warm core eddy on the vertical distribution of autotrophic pico- and nanoplankton in the Bay of Bengal

ABSTRACT The distribution and ecology of autotrophic pico- and nanoplankton were studied in the warm core eddy-affected northern Bay of Bengal (BoB) during the peak south-west monsoon of 2012. Small phototrophs showed diverse response patterns to the contrasting environmental setting in the northern BoB during the south-west monsoon. The convergence of surface waters in the eddy-influenced region was evidenced in terms of a deeper mixed layer and lower concentrations of . Synechococcus and picoeukaryotes were found to be more abundant in coastal waters as compared with the warm core eddy-influenced and offshore waters. Prochlorococcus was more abundant in the eddy-influenced and the offshore regions possibly as a consequence of their ability to survive in low-nutrient/oligotrophic waters. The abundance of Synechococcus increased noticeably in the coastal and subsurface waters due to their affinity towards higher concentrations of nutrients. The present data therefore support the view of a community shift induced by warm core water eddies and show their importance in promoting primary production by Prochlorococcus in the mixed layers of the BoB.

Seasonal production of nitrous oxide in a tropical estuary, off western India

Estuaries are highly productive ecosystems, and estuarine emission of N2O contributes significantly to the global nitrogen (N) budget. Presented here are the spatio-temporal distribution and water–air flux of N2O from a monsoonal, tropical estuary, the Mandovi located off Goa, during three consecutive years of 2011, 2012 and 2013. Despite substantial input from various sources, the estuary was moderately enriched with nitrogenous nutrients (≤30μM). However, the N transformations do not seem to contribute towards considerable emissions of N2O. Nonetheless, the seasonal upwelling along the west coast of India influences the water column chemistry at the estuarine mouth, as the upwelled water intrudes ∼13 km inside the estuary, leading to significant changes in terms of N2O cycling. A high build-up of N2O in the bottom waters was observed annually during peak southwest monsoon (SWM). The highest concentrations of N2O ∼164 nM (∼2459 %, August 2011), 106 nM (∼1566 %, September 2012), and 156 nM (∼2320 %, August 2013) were recorded at the estuarine mouth that was associated with a coastally-derived upwelled water mass. However, the average concentrations of N2O during pre- and post-monsoon were 10.88 ± 2.07 nM and 11.07 ± 2.85 nM, respectively, for the three years.The water–air flux of N2O was highest during SWM and was lowest during pre- and post-monsoons. The average flux by the estuary varied from ∼3.98 - 48.46μmol m−2d−1. The average annual emission from the Mandovi estuary was estimated to be ∼5.5 × 106 g N2O y−1, which seems to be a smaller fraction when compared to coastal emissions. Our study suggests that while the Mandovi estuary remains in saturation with respect to N2O, it acts as a modest source of N2O to the atmosphere, especially during SWM.

Influence of hypoxia on phosphorus cycling in Alappuzha mud banks, southwest coast of India

The southwest coast of India is known to experience two concurrent processes during the southwest monsoon, namely coastal upwelling and mud banks at selected stretches such as that off Alappuzha. The mud bank sediments were found to contain very high concentrations of phosphorus (ave. 2255 ± 278 μg/g), and the present study measured changes in concentration of elements such as carbon, phosphorus, iron, and manganese within the Alappuzha mud banks, to understand the influence of upwelling on the cycling of these elements. The results showed that coastal upwelling led to hypoxic conditions within the mud bank region during the peak southwest monsoon, which induced significant changes in the phosphorus fractions in the sediments. There was an enrichment of iron, caused by the efflux of reactive Fe from the subsurface sediment layers and depletion of manganese, due to reductive dissolution of Mn oxides and subsequent release of Mn. The low molar ratios of total organic carbon to total nitrogen (9 ± 1) and total organic carbon to organic phosphorus (118 ± 57) generally indicated that the organic matter was relatively fresh and derived from marine plankton. Sedimentary phosphorus also exhibited wide fluctuations in its geochemical forms during hypoxia. Five geochemical forms of P such as exchangeable P (Ex-P), Fe and Mn bound P (Fe-P), P from Al oxides, non-reducible Fe bound P and organic P (Non Red-P), calcium bound P (Ca-P) and refractory organic P (ROP) were extracted, adopting sequential extraction procedure. The percentage contribution of various P forms were 2%–11% Ex-P, 4%–25% Fe-P, 4%–10% Non Red-P, 60%–78% Ca-P and 4%–12% ROP. The fractionation study revealed that precipitation of apatite and vivianite are the significant pathways of phosphorus enrichment in mud bank sediments. The study indicates that the enrichment of labile P fractions could be mediating the enhancement in biological production in the Alappuzha mud banks during the southwest monsoon.

Response of phytoplankton to heavy cloud cover and turbidity in the northern Bay of Bengal

An interesting physiological response of phytoplankton to large fluctuations in underwater photosynthetically active radiation (PAR) levels in the northern Bay of Bengal has been presented here. This study is primarily based on a 12-day time series observation in the northern Bay of Bengal during the peak Southwest Monsoon (July 2012), when the study region was recurrently exposed to alternating cloudy and sunny sky conditions. On overcast days, the PAR available underwater at the time series location (TSL) drastically decreased, with the noontime PAR at the surface water (2 m) usually being ~600 µmol m−2 s−1 on sunny days and declining to ~50 µmol m−2 s−1 on heavily overcast days. Closely linked with the sunny and cloudy days at TSL, chlorophyll a concentration in the water column showed noticeable features; it increased in the upper water column (surface-40 m) and decreased in the lower water column (41–80 m) on cloudy days, while the reverse was the case on sunny days. Based on in-situ and laboratory experimental data, it was observed that these temporal changes in the vertical distribution of chlorophyll a in the northern Bay of Bengal were due to the short-term physiological acclimation of phytoplankton to large changes in underwater PAR.

Ecological indications of copepods to oxygen-deficient near-shore waters

Dissolved oxygen deficiency and its impacts on marine organisms is an important research topic world over. In this study, we utilized the advantage of a shallow seasonal hypoxic zone occurring along the Southwest Coast of India. Also, we adopted the standard live staining technique to assess the ecological responses of copepods to oxygen-deficient waters. Based on 18 weekly/biweekly sampling sessions in the nearshore-waters along the Southwest coast of India, spanning over the Pre-Southwest Monsoon (April) to Late-Southwest Monsoon (September) of 2014, this study showed a noticeable decrease in the total abundance of copepods in the hypoxic subsurface waters associated with coastal upwelling. This feature was noticed particularly during the Peak-Southwest Monsoon (July–August), which was contrary to the usual temporal abundance pattern of copepods in the aerated surface waters. During the Southwest Monsoon, the percentage of dead copepods was noticeably high in the study area, especially in the subsurface waters, due to the physiological stress from hypoxic waters. Indicator Value (IndVal) analysis of copepods demarcated 8 indicator species (IndVal > 40) under different environmental conditions. During the Pre-Southwest Monsoon, the indicator species were Acartia erythraea, A. danae, Centropages orsini and C. furcatus, whereas Oithona similis was the indicator during the Peak-Southwest Monsoon. Pseudodiaptomus serricaudatus and Centropages tenuiremis were the indicators during the Late-Southwest Monsoon, whereas Temora turbinata was an indicator during both the Early- and Late- Southwest Monsoon periods. The overall pattern in the distribution of copepods evidenced the dominance of calanoids during the Pre-Southwest Monsoon and that of cyclopoids (O. similis, Oithona sp.,) during the Peak-Southwest Monsoon. Multivariate analysis showed that the observed temporal shift in the composition and increase in the number of dead individuals of copepods in the study region were mainly governed by the coastal upwelling-associated hydrographical settings.

Noctiluca and copepods grazing on the phytoplankton community in a nutrient-enriched coastal environment along the southwest coast of India

The relative grazing impact of Noctiluca scintillans (hereafter referred only Noctiluca) and copepods (Acrocalanus gracilis, Paracalanus parvus, Acartia danae and Oithona similis) on the phytoplankton community in an upwelling-mudbank environment along the southwest coast India is presented here. This study was carried out during the Pre-Southwest Monsoon (April-May) to the Late Southwest Monsoon (August) period in 2014. During the sampling period, large hydrographical transformation was evident in the study area (off Alappuzha, Southwest coast of India); warmer Pre-Southwest Monsoon water column condition got transformed into cooler and nitrate-rich hypoxic waters during the Southwest Monsoon (June-August) due to intense coastal upwelling. Copepods were present in the study area throughout the sampling period with a noticeable increase in their abundance during the Southwest Monsoon. On the other hand, the first appearance of Noctiluca in the sampling location was during the Early Southwest Monsoon (mid-June) and thereafter their abundance increased towards the Peak Southwest Monsoon. The grazing experiments carried out as per the food removal method showed noticeable differences in the feeding preferences of Noctiluca and copepods, especially on the different size fractions of phytoplankton. Noctiluca showed the highest positive electivity for the phytoplankton micro-fraction (av. 0.49±0.04), followed by nano-fraction (av. 0.17±0.04) and a negative electivity for the pico-fraction (av. -0.66±0.06). In total ingestion of Noctiluca, micro-fraction contribution (83.7%) was significantly higher compared to the nano- (15.7%) and pico-fractions (0.58%). On the other hand, copepods showed the highest positive electivity for the phytoplankton nano-fraction (av. 0.38±0.04) followed by micro- (av. -0.17±0.05) and pico-fractions (av. -0.35±0.05). Similarly, in total ingestion of copepods, nano-fraction (69.7%) was the highest followed by micro- (28.9%) and pico-fractions (1.37%). The grazing pressure of Noctiluca on the total phytoplankton was found to be 27.7% of the standing stock and 45.6% of the production, whereas in the case of copepods, it was 9.95% of the standing stock and 16.6% of the production. The study showed that the grazing pressure of Noctiluca on the total phytoplankton as well as larger phytoplankton fraction was 2.8- and 8-folds higher than that of the copepods. This suggests the leading role of Noctiluca as an effective grazer of larger phytoplankton along the southwest west coast of India, especially during the Peak/Late Southwest Monsoon.

Copepod grazing and their impact on phytoplankton standing stock and production in a tropical coastal water during the different seasons

The grazing rate of copepods on the total and size-fractionated phytoplankton biomass in a coastal environment (off Kochi, southwest coast of India) were measured during pre-monsoon (PRM), peak southwest monsoon (PKSWM), late southwest monsoon (LSWM) and post-southwest monsoon (PSWM). The phytoplankton standing stock (chlorophyll a-Chl. a) and growth rate (GR) were less during the PRM (Chl. a 0.58mgm-3; GR 0.23±0.02) and PSWM (Chl. a 0.89mgm-3; GR 0.30±0.05) compared to PKSWM (Chl. a 6.67mgm-3; GR 0.43±0.02) and LSWM (Chl. a 4.09mgm-3; GR 0.40±0.04). The microplankton contribution to the total Chl. a was significant during the PKSWM (41.83%) and LSWM (45.72%). Copepod density was lesser during the PRM (1354Nom-3) and PSWM (1606Nom-3) than during PKSWM and LSWM (4571 and 3432Nom-3, respectively). Seasonal changes in phytoplankton biomass, phytoplankton size structure, and copepod community were closely related to the hydrographical transformations in the study domain. Dominant calanoid copepods in the study region ingested 8.4 to 14.2% of their daily ration from phytoplankton during the PRM and PSWM, which increased to >50% during the PKSWM and LSWM. The cyclopoid Oithona similis was abundant during the PKSWM, ingesting only 21% of their daily ration from phytoplankton. Temporal variation in the phytoplankton biomass and copepod species composition caused differences in community level top-down control. The copepod community ingestion on phytoplankton was high during the LSWM (18,583μgCm-3d-1), followed by PKSWM (9050μgCm-3d-1), PSWM (1813μgCm-3d-1), and PRM (946μgCm-3d-1). During the low Chl. a period (PRM and PSWM), dominant calanoid copepods showed a positive selectivity for the micro- and nano-phytoplankton size fractions, whereas during the high Chl. a period (PKSWM and LSWM), they showed a positive selection for nano-phytoplankton fractions. Irrespective of the seasons, dominant calanoid copepods showed a negative selection of pico-phytoplankton fraction. The cyclopoid O. similis and Poecilostomatoid Corycaeus danae showed a positive selection of nano- and pico-phytoplankton fractions rather than micro-fraction. The grazing pressure of copepod community ingestion on micro-fraction was less (0.56% of the phytoplankton biomass and 1.06% of the phytoplankton production) during the PKSWM. This study provides, for the first time, clear findings on the seasonal variation in the top-down control of phytoplankton by copepods in a tropical coastal water ecosystem and discusses its implications on phytoplankton blooming, plankton food web, and biogeochemistry.

Wave Hindcasting Using WAM and WAVEWATCH III: A Comparison Study Utilizing Oceansat-2 (OSCAT) Winds

In this study, the state-of-the-art third generation wave models WAM and WAVEWATCH-III (WWIII) have been used to predict waves for the North Indian Ocean over 1°× 1° (lat × long) grid resolutions utilizing six-hourly processed winds which was possible with the launch of OCEANSAT-2 (OSCAT) by ISRO, on 23 September 2009. The study demonstrates the application of interpolated OSCAT winds to hindcast waves across the globe using two deep-water models WAM and WAVEWATCH III (WWIII) and to compare the skill of the models using error analysis. In this context, the wave models WAM and WWIII were forced using OSCAT winds for the year 2011 over the global domain as well as for the North Indian Ocean (regional domain) using appropriate boundary conditions/inputs. The output from the models such as significant wave height (Hs) and mean wave period (Tc) were validated with an NDBC buoy measurement during January to December 2011 in the North Atlantic Ocean and for few selected buoys in the Bay of Bengal for the month of July 2011 (peak southwest monsoon). The comparisons of OSCAT winds with NDBC wind measurements reveal that the overall trend and dominant directions are consistent with the observational data. The validation of significant wave parameters of the selected buoy in the North Atlantic ocean revealed very high correlation (R>0.9) with percentage error <20% for Hs and Tc. The comparisons between the observed and predicted wave parameters in the Bay of Bengal showed that percentage error ranged between 9 to 24% for Hs and within 10% for Tc. It is noted that WAM describes the variability of wave heights realistically with smaller error estimates and better correlation coefficients than WWIII. Such validation studies are proven to be useful in quantifying the performance of these wave models which are being utilized or yet to be further explored for routine operational applications as well as for long-term wave hindcasting in the strategic areas of importance in deep as well as shallow waters by nesting with suitable near-shore models in the littoral zone.

Assessment of TRMM-based TMPA-3B42 and GSMaP precipitation products over India for the peak southwest monsoon season

Reliable information of rainfall and its variability at finer spatio-temporal scales over the Indian monsoon region is crucial for various applications in hydrology, meteorology, water resource management, and evaluation of high-resolution numerical model outputs. In this study, the Tropical Rainfall Measuring Mission (TRMM)-based high-resolution multisatellite rainfall products namely, TRMM Multisatellite Precipitation Analysis (TMPA)-3B42 and Global Satellite Mapping of Precipitation (GSMaP) are evaluated against recently released gauge-based India Meteorological Department (IMD) gridded data set. Both the research (delayed mode) and near real-time rainfall products of TMPA-3B42 (V7 and RT) and GSMaP (MVK and NRT) are evaluated over India at a daily scale for the peak southwest monsoon months of July–August. Two study periods (2000–2010 and 2009–2013) are used to accommodate the analyses based on data availability. Comparison of TMPA-3B42 (V7 and RT) and GSMaP-MVK with IMD gauge-based data for 2000–2010 shows that these data sets are able to represent large-scale monsoon rainfall spatial features, but are having region-specific biases. The interannual variability of monsoon rainfall averaged over India is also reasonably captured. However in terms of biases, TMPA-3B42RT overestimates rainfall by about 21% and GSMaP-MVK underestimates by about 22% at all-India scale. In terms of continuous skill metrics, TMPA-3B42V7 is closer to the IMD gauge-based observations possibly due to gauge adjustments. Categorical verification against gauge-based observations shows that the satellite-based data sets have difficulties in rainfall detection primarily over the northern Jammu & Kashmir regions, northeast India, and southern peninsular India. At subregional scale within India, satellite rainfall estimates show rather lower skill score in rainfall identification and larger uncertainty in rainfall estimation over the northeast India. Comparison of near real-time products of TMPA and GSMaP for July–August months of 2009–2013 shows that TMPA-3B42RT overestimates and GSMaP-NRT underestimates rainfall over most parts of India. Even though TMPA-3B42 (V7 and RT) and GSMaP-MVK rainfall data sets show similar diurnal variations over the Indian monsoon region, TMPA-3B42RT (GSMaP-MVK) shows larger (smaller) magnitude in diurnal cycle as compared to TMPA-3B42V7. Results from different skill metrics suggest that both research and near real-time versions of TMPA products can be used with relatively higher confidence as compared to GSMaP over the Indian region during peak monsoon period.

Anomalous current structure in the eastern equatorial Indian Ocean during the south-west monsoon of 1994

Analysis of hydrographic data collected along 80°, 84° and 88°E between 5 °N and 3 °S during peak south-west monsoon (July–August) of 1994 in the eastern equatorial Indian Ocean (EEIO) reveals thermohaline variability and anomalous current structure in the upper ocean. A warm (28.4–28.8°C) and less-saline (33 to 34.8) deep (60-75m) homogeneous layer characterizes the surface mixed layer. The south-west monsoon current, advecting the Arabian Sea High Salinity Water eastwards,appears as an eastward undercurrent (˜50 cm s–1) below 125 m at 80 °E between 1 °and 1.5 °N and in the upper 100 m in the vicinity of the equator with an eastward increase of core velocity between 75 and 100 m. This undercurrent has a transport of 4 Sv at 80 °E. The westward flow in the upper layers north of the equator is anomalous, as the surface winds have an eastward zonal component. The associated westward transport is 44 Sv between 84 °and 88 °E. This equatorial westward flow appears to be influenced by the anomalously strong north-westward coastal current off Sumatra and westward propagating Rossby waves.

Patterns of co-variability between physical and biological parameters in the Arabian Sea

The relationship between physical forcing and biological response observed in the Arabian Sea for the years 1978–1986 were examined. Spatial and temporal patterns of variability in a climatological time-series of three possible physical forcing parameters and CZCS-derived phytoplankton pigment concentration during the annual cycle were quantified using single and joint empirical orthogonal function (EOF) and singular-value decomposition (SVD) analyses. Monthly composites of the NASA regional pigment data were interpolated to fill data voids and binned corresponding to the physical flux data. Nearly all the spatial-temporal analyses consistently partitioned a large portion of the variability using only 1 or 2 dominant modes and indicated a lag in the timing of the peak pigment concentration behind the maxima in physical forcing. In all cases, major modes of variability resembled the Southwest Monsoon pattern, with the Northeast Monsoon contributing very little to the total variance and covariance. The Joint EOF and SVD analyses incorporated subtle features surrounding the peak Southwest Monsoon phenomena. Correlation maps of the joint EOF analysis depicted differences in spatial variability of pigment concentration associated with stress and curl, showing areas of curl-driven upwelling distinct from coastal upwelling, with possible off-shore advection of the curl-induced high pigment waters.