Winter Monsoon Period Research Articles

Monsoonal patterns of wave reflection from rubble mound breakwater of Chabahar Bay

This paper presents the results of a one-year field study on the monsoonal reflective response of the rubble mound breakwater (RMB) of Chabahar Bay, located on the northern coast of the Gulf of Oman, Iran. Measurements show that, in general, the correlation between the reflection coefficient and Iribarren number during the winter monsoon period is more remarkable than that of the summer monsoon period. The difference in wave reflection behavior during monsoonal periods is mainly due to the energy proportion of incoming sea and swell waves. Various characteristic wave periods by means of power- and hyperbolic-law prediction functions are explored to enhance the wave reflection prediction, highlighting the significant performance of negative-moment spectral periods Tm−1 and Tm−2 compared with peak and mean spectral periods. Statistical comparison of the performance of Tm−1 and Tm−2 shows that Tm−2 considerably improves the prediction accuracy for moderated energy waves with bimodal sea and swell climates originating from different directions in the winter monsoon and pre-summer monsoon months. However, the prediction improvement is insignificant for unimodal energetic waves observed during the summer monsoon months. Generally, using Tm−2 increases the accuracy of the preexisting equations in predicting the observations of this study.

Winter monsoon brings substantial terrestrial aerosols to Bay of Bengal and adjacent Indian Ocean: Insights from carbon and nitrogen isotopes

It has been currently recognized that air pollutants (e.g., carbon and nitrogen) from South and Southeast Asia have a significant impact on the coastal waters of the northern Indian Ocean. However, the understanding of dynamics and fates of these pollutants in aerosols to the remote ocean through long-distance transportation still needs to be improved. To address this issue, stable isotopic compositions (δ13C, δ15N) of total carbon (TC) and nitrogen (TN) and other geochemical indexes in aerosols are investigated over the Bay of Bengal (BOB) and the adjacent Indian Ocean during the winter monsoon period. The concentrations of TN, TC and aerosol nutrients exhibited a decreasing trend from the northern BOB (N-BOB) to the southern BOB (S-BOB) and Indian Ocean, reflecting the weakening trend of polluted continental influence from the N-BOB to the ocean. The high concentrations of TC (4.28–18.21 μg m−3) in the aerosols of the N-BOB were mainly influenced by the emissions of coal combustion (68 ± 21%), while the high concentrations of TN (0.69–7.31 μg m−3) in the N-BOB were mainly from biofuel/biomass burning (52 ± 31%). In the S-BOB, the lower concentrations of TC (1.42–2.98 μg m−3) and TN (0.38–2.03 μg m−3) mainly originated from the formation of secondary aerosols and the changes in aerosol chemical composition during long-distance transportation from the continent. The primary marine source was responsible for the low TC and TN concentrations over the Indian Ocean. This study quantifies the nitrogen and carbon source shifts from continental anthropogenic activities in the N-BOB to dominant marine and photochemical transformation sources in the remote ocean, quantitatively deepening the understanding of the impact of terrestrial aerosols on the ocean.

Spatiotemporal characterization of PM2.5, O3, and trace gases associated with East Asian continental outflows via drone sounding

East Asian continental outflows with PM2.5, O3, and other species may determine the baseline conditions and affect the air quality in downwind areas via long-range transport (LRT). To gain insight into the impact and spatiotemporal characteristics of airborne pollutants in East Asian continental outflows, a versatile multicopter drone sounding platform was used to simultaneously observe PM2.5, O3, CO2, and meteorological variables (temperature, specific humidity, pressure, and wind vector) above the northern tip of Taiwan, Cape Fuiguei, which often encounters continental outflows during winter monsoon periods. By coordinating hourly high-spatial-resolution profiles provided by drone soundings, WRF/CMAQ model air quality predictions, HYSPLIT-simulated backward trajectories, and MERRA-2 reanalysis data, we analyzed two prominent phenomena of airborne pollutants in continental outflows to better understand their physical/chemical characteristics. First, we found that pollutants were well mixed within a sounding height of 500 m when continental outflows passed through and completely enveloped Cape Fuiguei. Eddies induced by significant fluctuations in wind speeds coupled with minimal temperature inversion and LRT facilitated vertical mixing, possibly resulting in high homogeneity of pollutants within the outflow layer. Second, the drone soundings indicated exceptionally high O3 concentrations (70–100 ppbv) but relatively low concentrations of PM2.5 (10–20 μg/m3), CO2 (420–425 ppmv), and VOCs in some air masses. The low levels of PM2.5, CO2, and VOCs ruled out photochemistry as the cause of the formation of high-level O3. Further coordination of spatiotemporal data with air mass trajectories and O3 cross sections provided by MERRA-2 suggested that the high O3 concentrations could be attributed to stratospheric intrusion and advection via continental outflows. High-level O3 concentrations persisted in the lower troposphere, even reaching the surface, suggesting that stratospheric intrusion O3 may be involved in the rising trend in O3 concentrations in parts of East Asia in recent years in addition to surface photochemical factors.

Global Warming Weakens the Ocean Front and Phytoplankton Blooms in the Luzon Strait Over the Past 40 years

AbstractThe Luzon Strait is a channel where warmer Kuroshio water from the Pacific Ocean intrudes into the South China Sea (SCS). Under climate change impact, the temperature in marginal seas, including the SCS, rises faster than in open oceans. We speculated that the variation of frontal intensity and its eco‐environmental impact in the Luzon Strait may be different from coastal waters, whose frontal intensity is increasing, thus stimulating phytoplankton growth. To confirm this speculation, 40‐year satellite, and multiple data sources were analyzed in the Luzon Strait. The results showed that strong frontal intensity (front coverage of over 60%) and higher Chlorophyll a content occurred simultaneously in the Luzon Strait during the winter monsoon period. Phytoplankton blooms were enhanced during El Niño years because the stronger Kuroshio intrusion generated stronger fronts and intensified local upwelling in the Luzon Strait. On an interannual scale, the frontal intensity and phytoplankton growth exhibited a significantly decreasing trend in the Luzon Strait over the past 40 years, since the faster warming in the SCS reduced the temperature difference between the Pacific Ocean and the SCS. Warming and weakening fronts reduced the mixed layer depth to the oligotrophic layer, thus limiting the phytoplankton growth. This study confirmed that faster temperature rises in marginal seas reduced the frontal intensity and phytoplankton growth in the strait between oceans and marginal seas.

In situ biomass burning enhanced the contribution of biogenic sources to sulfate aerosol in subtropical cities

Sulfurous gases released by biogenic sources play a key role in the global sulfur cycle. However, the contribution of biogenic sources to sulfate aerosol in the urban atmosphere has received little attention. Emission sources and formation process of sulfate in Guangzhou, a subtropical mega-city in China, were clarified using multiple methods, including isotope tracers and chemical markers. The δ18O of sulfate suggested that secondary sulfate was the dominant component (84 %) of sulfate aerosol, which mainly formed by transition metal ion (TMI) catalyzed oxidation (31 %) and OH radical oxidation (30 %). The factors driving secondary sulfate formation were revealed using a tree boosting model, which suggested that NH3, temperature, and oxidants were the most important factors. The δ34S of sulfate indicated that biogenic sources accounted for annual average of 26.0 % of the sulfate, which increased to 30.4 % in winter monsoon period. Rice straw burning enhanced sulfate formation by promoting the release of reduced sulfur from soil, which is rapidly converted into sulfate under a subtropical urban atmosphere with high concentration of NH3 and oxidants. This study revealed the important influence of rice straw burning on biogenic sulfur emission during the rice harvest, thereby providing insight into the sulfur cycle and regional air pollution.

Comparison of transit time models for exploring seasonal variation of preferential flow in a Moso bamboo watershed

Understanding seasonal variations of the hydrological fluxes and preferential flow is crucial in exploring contaminant fate and transport processes in a watershed of interest. Here, performance of different StorAge Selection (SAS)-based rainfall-runoff models was compared for a Moso bamboo watershed under the East Asian monsoon climate. Most of these models are composed of a two-parallel-reservoir flow module and a companioned SAS module. The results show that the temporal variation in streamflow is well captured by a two-parallel-reservoir flow module. The tracer simulations from different models show that two-uniform-reservoir (SSU) model with no SAS function has the lowest NSE value (0.17), the model including an SAS in one of the two reservoir outflows (SSD(U) and SSD(L)) improves performance (NSE greater than 0.5). The performance in simulating streamflow δ18O is further improved when SAS functions are applied for outflows from both reservoirs (SSD (U, L), NSE = 0.76). Additional inclusion an SAS function for ET fluxes in the model (SSD (U, ET, L)) results in a minor improvement from SSD (U, L). The results of SSD (U, ET, L) have achieved the best transport performance (NSE = 0.77), with implication for preferential flow in the watershed. The preferential flow for deep drainage and lower reservoir outflow shows seasonal variation, with more young water in the East Asian summer monsoon period than that in the East Asian winter monsoon period. The upper reservoir storage and deep drainage rates are the main factors determining the temporal variation of preferential flow for deep drainage and lower reservoir outflow, respectively.

Characteristic Variation of Particulate Matter-Bound Polycyclic Aromatic Hydrocarbons (PAHs) during Asian Dust Events, Based on Observations at a Japanese Background Site, Wajima, from 2010 to 2021

Asian dust (AD) events and total suspended particles (TSPs) were observed at the Kanazawa University Wajima Air Monitoring Station (KUWAMS), a Japanese background site, during the East Asian winter monsoon periods (from November to May of the following year) from 2010 to 2021. Nine kinds of polycyclic aromatic hydrocarbons (PAHs) were determined in each TSP sample. In this study, a total of 54 AD events were observed. According to the different pathways of long-range transportation, AD events were divided into AD-high events (transported at higher altitudes, approximately 4000 m) and AD-low events (transported at lower altitudes, approximately 2500 m). The TSP concentrations increased sharply in the AD events and were higher in the AD-high events (39.8 ± 19.5 μg/m3) than in the AD-low events (23.5 ± 10.5 μg/m3). AD did not have a significant effect on the ΣPAHs characteristic variation, as ΣPAHs (ΣPAHs = fluoranthene, pyrene, benz[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, benzo[ghi]perylene, and indeno [1,2,3-cd]pyrene) concentrations in non-AD periods, AD-high events, and AD-low events were 543 ± 374, 404 ± 221, and 436 ± 265 pg/m3, respectively. The PAH compositions were also consistent. As a result, the TSP concentration was affected by the input air mass transported at higher altitudes from the desert region while the PAH concentration was impacted by the air mass at lower altitudes, which carried the PAHs emitted from fossil fuels and biomass combustion in northeastern China. Moreover, the health risks of PAHs were calculated with the inhalation lifetime cancer risk, which ranged from 10−6 to 10−5 ng/m3, indicating a potential carcinogenic risk at the KUWAMS during the East Asian winter monsoon periods.

Impact of initial and lateral open boundary conditions in a Regional Indian Ocean Model on Bay of Bengal circulation

This study uses three simulations (Exp-1, Exp-2, and Exp-3) from a nested basin-scale, regional, eddy-permitting Indian Ocean model forced with CORE-II inter-annual forcing, to study the impact of initial and lateral open boundary conditions on Bay of Bengal (BoB) circulation. It also evaluates the state of art reanalysis products from GODAS, SODA3 and ORAS5. Sensitivity studies using more realistic initial and lateral boundary conditions suggest that accurate initial conditions are essential for realistic simulation of the mean and the variability of temperature and salinity in the upper ocean. We also show that accurate lateral boundary conditions are also essential for realistic basin-scale simulations for the upper ocean circulation in the BoB. The model captured well the observed seasonal and inter-annual variability of salinity in the BoB. The comparison of surface currents from the regional simulations and reanalysis products with OSCAR analyses shows Exp-3 (with improved initial and lateral boundary​ conditions) to be the closest among all three. The summer monsoon current is also most realistic in Exp-3, while the reanalysis products are unable to reproduce the BoB Coastal Current structure and magnitude. This study also reports the first evidence of a coastally trapped, narrow boundary current at thermocline depth along the eastern boundary of BoB. The presence of this boundary current is noticed in all three regional solutions while most prominent in Exp-3 and nearly absent in the reanalysis products. The salt transport across 8°N shows large seasonal and inter-annual variations during 2003–2009 in the best simulation (Exp-3), with standard deviation ranging between 77–150 psu-m3/s up to 1000 m depth. Apart from summer and winter monsoon periods, which show the intrusion of salt into BoB from the equatorial Indian Ocean in the upper layers, we found significant salt transport at deeper depths (300–1500 m) as well. The integrated inter-annual variations of the seasonal salt flux at different depths across 8 °N show presence of strong seasonal cycle in the upper layer (0–200 m). However, the seasonality is most prominent in the upper 50 m. The intra-seasonal variation in the deeper layers are more prominent as compared to the upper layer.

Seasonal Features and Potential Mechanisms of Submesoscale Processes in the Southern Bay of Bengal during 2011/12

Abstract This study investigates the seasonal features and generation mechanisms of submesoscale processes (SMPs) in the southern Bay of Bengal (BoB) during 2011/12, based on the output of a high-resolution model, LLC4320 (latitude–longitude–polar cap). The results show that the southern BoB exhibits the most energetic SMPs, with significant seasonal variations. The SMPs are more active during the summer and winter monsoon periods. During the monsoon periods, the sharpening horizontal buoyancy gradients associated with strong straining effects favor the frontogenesis and mixed layer instability (MLI), which are responsible for the SMPs generation. The symmetric instability (SI) scale is about 3–10 km in the southern BoB, which can be partially resolved by LLC4320. The SI is more active during summer and winter, with a proportion of 40%–80% during the study period when the necessary conditions for SI are satisfied. Energetics analysis suggests that the energy source of SMPs is mainly from the local large-scale and mesoscale processes. Baroclinic instability at submesoscales plays a significant role, further confirming the importance of frontogenesis and MLI. Barotropic instability also has considerable contribution to the submesoscale kinetic energy, especially during summer. Significance Statement Submesoscale processes (SMPs) are ubiquitous in the Bay of Bengal (BoB). Affected by the seasonally reversing monsoon, abundant rainfall and runoff, and equatorial remote forcing, the upper circulation in the BoB is complex, featuring active mesoscale eddies and rich submesoscale phenomena, making the BoB a “natural test ground” for submesoscale studies. It is found in this work that characteristics of SMPs in the BoB are quite different from other regions. In the southern bay, SMPs are most active during the summer and winter monsoons due to the frontogenesis, enhanced mixed layer instability (MLI), and symmetric instability. These findings could deepen our understanding on multiscale dynamic processes and energy cascade in the BoB and have implications for the study of marine ecology and biogeochemical processes.

Seasonal variation of transit time distribution and associated hydrological processes in a Moso bamboo watershed under the East Asian monsoon climate

Understanding seasonal variation of the transit time distribution of stream discharge and evapotranspiration is crucial in exploring the flow pathways and the rainfall-storage-streamflow processes in a watershed of interest . Here, both time-variant and time-invariant StorAge Selection (SAS) functions were adopted to estimate the transit time distribution in a Moso bamboo watershed under the East Asian monsoon climate. Daily stable isotopic compositions (δ18O, δ2H) of precipitation and streamflow at the watershed outlet from 1 October 2017 to 31 December 2020 were measured to calibrate the model parameters. The time-variant SAS method considering watershed wetness performs slightly better than the time-invariant SAS method, implying adding a watershed wetness indicator in the time-variant model results in only a minor improvement. Young water fraction and median age as two streamflow age measures were derived from the time-variant SAS model. The median age decreases abruptly within several days at three periods (one is in January 2020, and the other two periods are in Meiyu season) with larger uncertainty, while the estimated young water fraction varies gradually with much smaller uncertainty. The young water fraction derived from the time-variant SAS method appears to be more reliable than that estimated by the tracer cycle damping method. For stream discharge, the storage preferentially releases younger water to discharge under wetter conditions in the East Asian summer monsoon (EASM; June-September) period, while this phenomenon does not occur in the East Asian winter monsoon (EAWM; October-May) period. The larger hourly rainfall intensity for the whole EASM period (mean intensity 2.08 mm/h) likely enhances direct contribution of event water to streamflow than that in the EAWM period (mean intensity 0.75 mm/h). The evapotranspiration is estimated to extract older water than stream discharge in both the EAWM period and the EASM period. That is likely related to the occurrence of ecohydrological separation, and root water uptake below the soil-bedrock interface.

Seasonal and spatial dynamics of mesozooplankton community in a subtropical embayment

Mesozooplankton abundance and community composition are influenced by hydrography and biological interactions. In subtropical systems influenced by seasonal monsoons, seasonal cycle of plankton communities could be disrupted and restructured by short but intense weather events. Our survey of a shallow subtropical embayment to the northern part of the South China Sea demonstrated temporal variations in zooplankton biomass and composition between monsoon periods: not only were the wet (summer monsoon) and dry (winter monsoon) periods distinctive from each other, but also the transition phase between the two. The meroplankton communities were distinct between the monsoon periods with fish, decapods and echinoderm peaked during the wet season and cirripedia bloomed in dry season. This pattern can be attributed to both changes in growth conditions and influx of individuals through monsoon-driven water currents. The community shift during the transitional phase, defined by the rapid changes in salinity and temperature, corresponded to the reported reproductive cycles of marine organisms, including commercially important fishes, crustaceans, and echinoderms. The interannual variability observed during the wet period reflected changes in rainfall between consecutive years, highlighting the importance of having a long time series with which to establish a baseline information to better manage nursery habitats like the one surveyed, particularly in light of increasing coastal development. Subtropical coastal zooplankton communities are highly dynamic with multiple pulses and interruptions driven by weather events.

Columnar aerosol optical depth, water vapor and ozone over a semi-arid urban location of western India: Potential sources and direct radiative effects

Continuous measurements of Aerosol Optical Depth (AOD), columnar water vapor (WV) and Ozone (O3) concentrations were made over a semi-arid urban location Rajkot (22.29°N, 70.74°E, and 142 m AMSL) in the western India. The annual mean values of AOD, WV and O3 were 0.37 ± 0.15, 1.44 ± 0.6 gm cm−2 and 243 ± 18 DU during 2016–2019. AOD showed large seasonal variability mostly dominated by abundant coarse mode aerosols (Angstrom Exponent, AE < 0.65 and AOD ∼ 0.37) during pre-monsoon and monsoon, mainly caused by wind-blown and transported dust from nearby Thar and Kutch desert. Mixture of fine and coarse mode aerosols were abundant (AE = 1.13 ± 0.26) during post-monsoon influenced by both anthropogenic and natural sources. Columnar WV were also higher in pre-monsoon and monsoon seasons. The O3 concentrations is higher during pre-monsoon (247 DU) and lower during monsoon (230 DU). The vertical distribution of aerosol extinction coefficient derived from satellite (CALIPSO) based observations revealed the dominance of dust aerosols (∼74 %) in pre-monsoon; smoke and polluted dust in winter and post monsoon period. The trajectory clusters and concentration weighted trajectory maps also indicated the combined effect of anthropogenic (especially from the northern part of India) and natural aerosol sources during the post-monsoon period. Associated with the higher anthropogenic components during post-monsoon season, aerosol direct radiative forcing depicted surface dimming as high as − 48.7 ± 12.6 Wm−2, resulting in atmospheric forcing of 42.6 ± 12.5 Wm−2.

Diurnal evolution of orographic precipitating clouds over the southernmost part of the Western Ghats of India during summer and winter monsoons

AbstractIn the present communication, diurnal evolution of vertical structure of precipitating clouds over the southernmost part (7°–11°N) of the Western Ghats (WG) of Indian region during summer and winter monsoon periods is discussed using 17 years of Tropical Rainfall Measuring Mission (TRMM) precipitation radar measurements. This part of the WG experiences both summer and winter monsoons and provides a unique opportunity to study the orographical processes involved in the formation of precipitating clouds at diurnal scales during two contrasting background environments. During summer monsoon, the lower tropospheric winds are dominated by the presence of strong low‐level westerly jets, which interact with steep slopes of the WG whereas during winter monsoon, winds are very weak easterlies and encounter the WG from eastern side. The composite longitudinal distribution of vertical structure of precipitating clouds in terms of frequency of occurrence of radar reflectivities ≥23 dBZ is constructed at four local time intervals. The results are discussed in terms of bands of high frequency of occurrence of precipitating clouds and their longitudinal position with respect to the WG. A band of high frequency of occurrence of precipitation clouds is observed to be moving eastwards and their potential formation mechanism is attributed to the forced lifting over the upslope regions and to the upstream blocking over the offshore during the summer monsoon. During winter monsoon, a two band structure is observed one over the southern BOB, which is attributed to the persistent low‐pressure system observed over this region and another over the upslope of the WG from eastern side. Thus the present study for the first time brings out the diurnal evolution of the vertical structure of precipitating clouds during summer and winter monsoon over the southernmost part of the WG and discuss the potential physical processes involved in their formation.

Quantifying the contribution of evaporation from Lake Taihu to precipitation with an isotope-based method

ABSTRACT Moisture recycling plays a crucial role in regional hydrological budgets. The isotopic composition of precipitation has long been considered as a good tracer to investigate moisture recycling. This study quantifies the moisture recycling fractions (f r) in the Lake Taihu region using spatial variations of deuterium excess in precipitation (d P) and surface water vapour flux (d E). Results show that d P at a site downwind of the lake was higher than that at an upwind site, indicating the influence of lake moisture recycling. Spatial variations in d P after sub-cloud evaporation corrections were 2.3, 1.4 and 3.2 ‰, and d E values were 27.4, 32.3 and 31.4 ‰ for the first winter monsoon, the summer monsoon and the second winter monsoon, respectively. Moisture recycling fractions were 0.48 ± 0.13, 0.07 ± 0.03 and 0.38 ± 0.05 for the three monsoon periods, respectively. Both using the lake parameterization kinetic fractionation factors or neglecting sub-cloud evaporation would decrease f r, and the former has a larger influence on the f r calculation. The larger f r in the winter monsoon periods was mainly caused by lower specific humidity of airmasses but comparable moisture uptake along their trajectories compared to the summer monsoon period.

Improvement in Estimation of Phytoplankton Size Class in Arabian Sea Using Remote Sensing Measurements

Phytoplankton is the principal component of the ocean ecosystem, whose biomass (represented as total chlorophyll-a (Chl-a) concentration) and size structure are the key ecological indicators. With an aim to retrieve phytoplankton size class (PSC) in Arabian Sea (a region with limited measurements), this study focuses on modification of two existing algorithms, one for measuring Chl-a and the other for size fraction. PSC is estimated using a regionally tuned three-component model with Chl-a concentration taken as input. Parameters of the model are modified for Arabian Sea (AS) using <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in-situ</i> data to estimate three size classes namely micro-, nano- and pico-plankton (model is referred as AS-PSC-2021). Accuracy of PSC retrieval depends on Chl-a concentration, which is estimated from spectral remote sensing reflectance ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R_rs</i> (λ)) using Quasi Analytical Algorithm (QAA) along with non-linear optimization. This hybrid approach for Chl-a estimation is named “QAA-optimized model”. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">In-situ</i> measurements of Chl-a concentration, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R_rs</i> (λ) and PSC fractions collected during April to May-2017 inter-monsoon period) from Arabian Sea are used along with the previous measurements from winter-monsoon periods of the year 2009, 2010 and 2011 for model development and validation. The PSCs were estimated using pigment concentrations obtained from High Performance Liquid Chromatography (HPLC). The Chl-a concentration obtained from QAA-optimized model was verified with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in-situ</i> data (N=44) and found to perform well (slope=0.95, bias=0.04, RMS=0.13 and R <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> =0.90). Also, the regionally tuned PSC model (AS-PSC-2021) showed a significant improvement in retrieving the size fraction when compared to previous PSC algorithm proposed for the region. The AS-PSC-2021 model was able to reproduce the variability in all the three size classes showing the dominance of pico- and micro-plankton at lower and higher Chl-a concentration, respectively, but more importantly the dominance of nano-plankton at mid Chl-a range. It is important to have a regionally tuned algorithm for key ecological parameters such as Chl-a and PSCs to assess the ecosystem in a perspective of ocean warming and climate change.

Relationship between precipitation isotopic compositions and synoptic atmospheric circulation patterns in the lower reach of the Yangtze River

In this study, the stable isotopic compositions (δ18O, δ2H and d-excess) in daily precipitation from October 2017 to May 2020 in the lower reach of the Yangtze River (LYR) were examined to understand the relationship between stable isotopic compositions in precipitation and synoptic atmospheric circulation patterns (SACPs). Atmospheric circulations over the LYR were classified into seven patterns according to the Kirchhofer method with three, one and three SACPs in the East Asian winter monsoon period (EAWM), the monsoon transition period and the East Asian summer monsoon period (EASM), respectively. Precipitation events between most SACPs are distinguishable for either δ18O or d-excess, except for two SACPs in the EAWM, and two SACPs in the EASM. Local meteorological variables are primary factors determining the variation of isotopic compositions in precipitation for three SACPs in the EAWM. Though the precipitation-producing air masses come from different seas under two different SACPs in the EASM, i.e., the South China Sea for one and the western Pacific for the other, neither δ18O nor d-excess is distinguishable. Nevertheless, the relationships between isotopic composition and meteorological variables appear to be distinguishable between these two SACPs. The SACP dominant in the monsoon transition tends to have higher δ18O (mean value) and δ2H (mean value) in precipitation than all other SACPs. The d-excess in precipitation of this SACP is more likely altered by subcloud evaporation, and the retrieval of moisture source d-excess information should be treated with cautions. Similarly, d-excess in Meiyu rain, classified in one of the SACPs in the EASM period, is very likely altered by moisture recycling along the atmospheric moisture transport pathway, evident from the significant negative relationship between d-excess in precipitation and mean specific humidity along the backward trajectories. Thus d-exess of Meiyu rain is very likely positively deviated from its oceanic moisture source. Overall, the findings can provide valuable insights for understanding the relationship between stable isotopes in precipitation and SACPs in the East Asian monsoon region.

Wave Characteristics in Northeast Coastal Waters of Zhoushan Island Under the Influence of Winter Monsoon

Waves at 12 m water depth in the northeast coastal waters of Zhoushan Island were measured from November 2014 to February 2015, and the characteristics were described. The study showed that the significant wave height varied from 0.08 to 1.35 m with the average of 0.36 m during the winter monsoon period, but the maximum of the maximum wave height is close to the maximum value of some typhoon waves of the same place. The monthly main wave direction varied from ENE to N to NE to ENE. Relatively high correlations exist between some wave parameters and relationships between them were identified. Most of the waves have a bimodal spectrum and the influence of monsoon on waves is intermittent.

Determining the Sources and Transport of Brown Carbon Using Radionuclide Tracers and Modeling

AbstractThe isotope tracer technique plays a key role in identifying the sources and atmospheric processes affecting pollution. The sources of brown carbon (BrC) at Guangzhou during 2017–2018 were characterized by positive matrix factorization with radiocarbon isotope constraints and multiple linear regression analysis. The primary emission factors of fossil fuel combustion (FF) and biomass burning (BB) accounted for 34% and 27% of dissolved BrC absorption at λ = 365 nm (Abs365), respectively. The total mean light absorption contributed by secondary sources was 39%. The FF‐origin Abs365 changed insignificantly throughout the year and was dominant in the summer monsoon period, whereas the Abs365 from BB and secondary nitrate formation increased and contributed larger fractions during the winter monsoon period. Transported BrC was estimated using an index of 7Be/(7Be + n210Pb). Higher values were generally accompanied by lower Abs365, whereas lower values were associated with higher Abs365, indicating that BrC absorption of aerosols transported from the upper‐atmosphere is lower than that of aerosols transported near the surface. Based on the positive correlations between 210Pb and Abs365 and non‐fossil dissolved organic carbon in the winter monsoon period, we estimated that the contribution of invasive BrC (include ground and upper‐atmosphere level) to total absorption during the period of elevated BrC was ∼50%. The transported BrC was likely related to BB organic aerosols and secondary nitrate formation processes. This study supports radionuclides as a novel method for characterizing the sources and transport of BrC that can be applied in future atmospheric research.

Characteristics of Stable Isotopes in Precipitation and Their Moisture Sources in the Guanling Region, Guizhou Province

Environmental prediction is one of the crucial means for social sustainable development. Based on the continuous sampling of atmospheric precipitation in Guanling County, Guizhou Province, China, from March 2016 to February 2017, and combining the reanalyzed data of the National Centre for Environmental Prediction/National Centre for Atmospheric Research with the Hybrid Single-Particle Lagrangian Integrated Trajectory model, this paper analyzed the variations of δ2H and δ18O in precipitation at the synoptic scale in the Guanling region. The results showed that the variations of δ2H, δ18O, and d-excess values in precipitation exhibited remarkable seasonal variability. The stable isotopic values in precipitation in the winter half-year were higher than those in the summer half-year. The meteoric water line of the winter half-year was close to the annual meteoric water line. The results showed that there was more than one fundamental source of moisture. It was affected by the winter monsoon period, which is longer than the summer monsoon period, so the local evaporation of water vapor participating in the water circulation had a greater impact. With the increase of precipitation, the intercept and slope of the meteoric water line gradually decreased, which indicated that the secondary evaporation was weak under the effect of stable isotope subcloud cluster. The correlations of precipitation δ18O with temperature T and precipitation P vary with time scales. As the time scale decreased, the correlation between δ18O and the temperature and precipitation improved. When P ≤ 5 mm and 10°C &lt; T ≤ 30°C, the most sensitive changes in stable isotopes were observed. In the study area, the backward trajectory model showed that the moisture in the winter half of the year was mainly from the transportation of the westerlies wind, replenishment, and local reevaporation of near-source ocean water, while the water in the summer half of the year mainly came from the transportation of water from the ocean at low latitudes.

Understanding the microzooplankton mediated food web of the winter–spring Noctiluca bloom in the Northeastern Arabian Sea Ecosystem

The dynamics of Northeastern Arabian Sea (NEAS) is governed by convective mixing in winter and stratification during the inter-monsoon spring seasons, wherein blooms of green Noctiluca scintillans is a recurrent phenomenon. The study investigated the role of microzooplankton (MZP) in mediating the microbial food web in bloom waters of Noctiluca scintillans. The winter monsoon and inter-monsoon spring periods for the years 2009, 2011 and 2012 were considered for the present study as the occurrence and intensity of bloom fluctuates between these seasons. The MZP mediation seems to be through the top-down controls (preferential grazing) on ciliates (CTS) in bloom waters, consequently supporting high mesozooplankton standing stock and inevitably higher stock of copepods. Among the three years of observations, this relation was noticeable for the year 2012, as the intense bloom of Noctiluca (3 ×107 cells L−1) coincided with the highest number of copepods. On the contrary, in the non-bloom waters, a simple classical food web seems to be operational as diatoms dominated the phytoplankton community. Furthermore, the functional role of two CTS species viz. Codonellospsis ecaudata and Steenosemella ventricosa were most influential in structuring the community and microbial food web in the bloom events. The study suggests that during the bloom of Noctiluca scintillans, MZP mediated food web seems to be relatively significant in sustaining the high secondary production in the NEAS.