Central Bay Research Articles

Seasonal and spatial variations of mesozooplankton energy transfer efficiency determined using remotely sensed SST and Chl-a in the Bay of Bengal

Mesozooplankton (MSP) are the major group of secondary producers in the ocean. They play an important role in both the carbon flux and energy transfer from primary producers to higher trophic levels. The present work explores the significance of how MSP biomass relates to the energy transfer between primary and secondary trophic levels by applying the MSP biomass model developed and validated by Mahesh et al. (2018) in the Bay of Bengal (BoB). The remote sensing products, mean sea surface temperatures (SST) and chlorophyll-a (Chl-a), were acquired from the Aqua (EOS PM) satellite’s Moderate Resolution Imaging Spectroradiometer (MODIS) sensor for the pre-monsoon period in 2013 through to summer 2015. These data were incorporated into the model to estimate MSP biomass which ranged from 0.12 to 33.52 mg C m−3 in the summer and post-monsoon periods in 2014, respectively, and was statistically significant between the seasons (F=19. 43; P=0. 008). The MSP productivity rate varied from 0.07 (summer 2014) to 15.35 mg C m−3 d−1(post-monsoon 2015). The rate was low (1 mg C m−3 d−1) across the central bay during the pre-monsoon period, whilst it was slightly higher (1-3 mg C m−3 d−1) along the neritic waters of the Palk Bay and Kakinada coastlines. The mean energy transfer efficiency from primary producers to MSP production was between 8.81 and 62.36 % during the post-monsoon and summer periods, respectively. This varied significantly between seasons (F=16.61; P=0.005) and was higher in the central bay than coastal waters. The complex interactions between water temperature, primary producer abundance, MSP grazing potential, seasonal variability, and the influential role of monsoons were the major controlling factors governing MSP biomass across the BoB.

Wind and wave climate characteristics and extreme parameters in the Bay of Bengal

Numerical hindcast of typhoon events in the Bay of Bengal from 1981 to 2015 is performed using the Jelesnianski typhoon model. Additionally, with the NCEP wind field data (1981–2015) as the background field, the wind fields over the Bay of Bengal in the 35-year period are reconstructed. With the reconstructed wind field as the driving wind field, wave data for the Bay of Bengal for the 35-year period are simulated by the SWAN wave model. The comparisons showed that the simulated results are in good agreement with the measured data. Based on the hindcast data, the average temporal and spatial distribution characteristics of wind and wave in the Bay of Bengal, including annual, seasonal and monthly average characteristics, are analyzed. Wind and wave in the Bay of Bengal are found to exhibit similar temporal and spatial distribution characteristics. Both wind speeds (Ws) and significant wave height (Hs) are higher in the central bay than in the southern bay and the northern coastal regions. In addition, Ws and Hs are the highest in summer, followed by those in spring and autumn; Ws and Hs are the lowest in winter. Based on annual extreme wind and wave data, extreme wind and wave parameters in the Bay of Bengal are calculated. The results show that the maximum Ws and Hs of 50-year return period in the Bay of Bengal exceed 50 m/s and 16 m, respectively.

Seabed Resuspension in the Chesapeake Bay: Implications for Biogeochemical Cycling and Hypoxia

Sediment processes, including resuspension and transport, affect water quality in estuaries by altering light attenuation, primary productivity, and organic matter remineralization, which then influence oxygen and nitrogen dynamics. The relative importance of these processes on oxygen and nitrogen dynamics varies in space and time due to multiple factors and is difficult to measure, however, motivating a modeling approach to quantify how sediment resuspension and transport affect estuarine biogeochemistry. Results from a coupled hydrodynamic–sediment transport–biogeochemical model of the Chesapeake Bay for the summers of 2002 and 2003 showed that resuspension increased light attenuation, especially in the northernmost portion of the Bay, shifting primary production downstream. Resuspension also increased remineralization in the central Bay, which experienced larger organic matter concentrations due to the downstream shift in primary productivity and estuarine circulation. As a result, oxygen decreased and ammonium increased throughout the Bay in the bottom portion of the water column, due to reduced photosynthesis in the northernmost portion of the Bay and increased remineralization in the central Bay. Averaged over the channel, resuspension decreased oxygen by ~ 25% and increased ammonium by ~ 50% for the bottom water column. Changes due to resuspension were of the same order of magnitude as, and generally exceeded, short-term variations within individual summers, as well as interannual variability between 2002 and 2003, which were wet and dry years, respectively. Our results quantify the degree to which sediment resuspension and transport affect biogeochemistry, and provide insight into how coastal systems may respond to management efforts and environmental changes.

Provenance of terrigenous sediments in the central Bay of Bengal and its relationship to climate changes since 25\u2009ka

Lithogenic grain-size, geochemical elements of core sediments from the central Bay of Bengal (BoB) were analyzed to identify sediment provenance and explore the regional “source-sink” processes since 25 ka. Based on the geochemical compositions of core BoB-88, relative contributions of three end-member sources (Himalayan, Myanmar, Indian Peninsula) were calculated and support the general understanding that Himalayan sources were dominant since the last glacial period, which could reach 70% on average in our study. However, sediments from the Indian Peninsula and Myanmar also contributed nonnegligible materials to the central BoB since 25 ka, especially the former shows an obvious increase since 7.5 ka, which was neglected in the previous studies. To trace the sedimentary processes of different provenance terrigenous materials and reveal their response to climate changes, four typical stages with obvious provenance percent variation, including the Last Glacial Maximum period (LGM), Heinrich 1 (H1), Early Holocene Climate Optimum (EHCO), and mid-late Holocene period, were highlighted and discussed. Sea-level change played a dominant role in the glacial-interglacial scale by controlling the transition of deposition center between the shelf/subaquatic delta and the Bengal Fan, especially for sediment from the Himalayan source, exerting influence on contributions from different provenance end-members between the Holocene and the last glacial period. The opposite directions of the southwest and northeast monsoon current in the BoB have a significant influence on the transfer of the Indian Peninsula and Myanmar materials to the studied core, which led to an increase of up to 11% than the average of the Indian Peninsula and a decrease of up to 9% than the average of the Myanmar source during the Holocene. The balance between sea level (limited transport to the deep sea) and ISM (improved erosion production and output to the BoB) during the mid-late Holocene period maintained the stability of Himalayan source percent. Due to the control of the ISM variations, the Himalayan source showed a decrease of up to 20% and 21% than average during the LGM and H1 period, respectively, and an increase of up to 14% than average during the EHCO. The Myanmar source exhibited fluctuations around the average value during the LGM period, but an obvious increase of up to 21% during the H1 period and a decrease up to 8% during the EHCO, while the Indian Peninsula source percent showed little variations during the LGM and H1, but a decrease of up to 6% during the EHCO. Obvious 1/4 processional and millennial periodic changes (5.0 kyr and 1.1 kyr) of contribution proportions from three end-members provide additional evidences for the climate control on the regional source-sink processes and proved the controlled mechanisms of the orbital parameters and possible influence from the solar activity on millennial scales.

Strengthening of reinforced concrete frames with engineered cementitious composite panels

Infill walls in reinforced concrete frames are susceptible to failure owing to their brittle nature. Their interaction with boundary columns during earthquakes may also cause shear damage in the columns. It is crucial to employ effective seismic strengthening strategies in order to mitigate the seismic risk induced by the infill walls. In this study, a new strengthening technique, conducted by bonding engineered cementitious composite (ECC) precast panels onto infill walls, was investigated. Three reinforced concrete frames, termed bare frame, infilled frame and strengthened frame, were tested by using the pseudo-dynamic testing method. The test specimens were three-storey, three-bay, half-scaled frames with the central bay having hollow clay brick infill walls. The specimens were subjected to three consecutive ground motions scaled to represent earthquakes with return periods of 144, 475 and 2475 years. The experimental results were compared to observe the influence of the infill walls on the reinforced concrete frames, and to demonstrate the performance of the proposed strengthening technique. It was found that application of ECC panels significantly increased the frame strength and stiffness while allowing the system to have a displacement ductility of about four.

Fire-induced progressive collapse mechanisms of steel frames with partial infill walls

This paper presents a numerical investigation on progressive collapse mechanisms of steel frames with partial infill walls under fire scenarios using ABAQUS platform. The load redistributions and collapse mechanisms of steel frames subjected to different fire scenarios (edge bay and central bay fires) are studied. The influences of strength, opening percentage and layout of partial infill walls are also analyzed. The results of the numerical analysis indicate that the presence of partial infill walls causes combined tensile force and bending moment at the ends of surrounding beams. The strut action of partial infill walls and the overturning moment imposed by the frame in the collapse bay are identified as load redistribution mechanisms of the steel frame. The steel frame using higher strength steel, more number and lower opening percentage of partial infill walls collapses at higher temperatures. Furthermore, the analyses identify three deformation modes of beams depended on the layout of partial infill walls. Finally, a preliminary design method based on equivalent compressive strut model is proposed with good accuracy, in order to prevent the steel frame with partial infill walls from fire-induced collapse in engineering practice.

Late Quaternary deep and surface water mass evolution in the northeastern Indian Ocean inferred from carbon and oxygen isotopes of benthic and planktonic foraminifera

New planktonic and benthic foraminiferal stable isotope records from core YDY05 (northeastern Indian Ocean) provide new insights into paleoceanographic changes in the northeastern Indian Ocean since the last glacial period. The distinct δ18O decrease was observed since the beginning of the deglaciation to the mid-Holocene (∼8–6 kyr BP), possibly reflecting a reduction in surface salinity in the central Bay of Bengal (BoB) water, which probably resulted from strengthened precipitation, concurrent enhanced river discharge and rising sea-level, related to the intensification of Indian Summer Monsoon (ISM). Variations in benthic δ13C and δ13CPlanktonic-Benthic in our core site reflect significant variations in source water characteristics over the LGM-Holocene. The large δ13CPlanktonic-Benthic offset during the glacial period suggests a more sluggish deep water circulation, and lower δ13CPlanktonic-Benthic from the deglaciation to the Holocene suggests an enhanced deep water circulation in the central BoB. The drastic depletion in benthic δ13C during the glacial period suggests a significant reduction of North Atlantic Deep Water (NADW) intrusion and a progressive influx of Antarctic Bottom Water (AABW) and 12C-rich Circumpolar Deep Water (CDW) into the central BoB. In contrast, since the deglaciation, the central BoB experienced a drastically increased intrusion of better ventilated and 13C-rich NADW. The differences in benthic δ18O between the LGM section and the Holocene exceeds the ice volume effect by ∼0.5‰, providing further evidence that the deep water mass of the central BoB was influenced by the less dense NADW, instead of the AABW, since the last deglaciation.

ЦЕРКОВЬ МЕДХАНЕ АЛЕМ В АДИ КЕШО И СЛОЖНОСТИ ДАТИРОВКИ СКАЛЬНЫХ ПАМЯТНИКОВ РЕГИОНА ТЫГРАЙ (ЭФИОПИЯ)

ЦЕРКОВЬ МЕДХАНЕ АЛЕМ В АДИ КЕШО И СЛОЖНОСТИ ДАТИРОВКИ СКАЛЬНЫХ ПАМЯТНИКОВ РЕГИОНА ТЫГРАЙ (ЭФИОПИЯ)

Monitoring Intraseasonal Oscillations in the Indian Ocean Using Satellite Observations

AbstractIntraseasonal oscillations (ISOs) in the Indian Ocean play a significant role in determining the active (wet) and break (dry) cycles of the southwest monsoon rainfall. In this study, we use satellite‐derived precipitation, sea level anomalies (SLAs), sea surface salinity (SSS), sea surface temperature, and surface winds to monitor the 30‐90‐day, 10‐20‐day, and 3‐7‐day ISOs, and how they influence local dynamics. The main focus of this work, however, is showing the importance of using SLA and SSS to monitor ISOs. Mixed Rossby gravity waves were found to induce convection associated with the southern cell of the 10‐20‐day mode, with surface winds from the northern cell modulating coastal Kelvin waves in the Bay of Bengal. The 10‐20‐day SSS response is instead more closely related to wind‐induced upwelling in the central Bay of Bengal and river runoff in the northern Bay. The 3‐7‐day mode was found to have a weak oceanic signal, as the monsoon trough is mainly positioned over land, though SSS captured the structure of the signal most clearly. This study highlights the need for high spatial resolution SLA in order to adequately capture 3‐7‐day oscillations in the monsoon trough.

Response of a high-resolution ocean circulation model to winds from different sources in simulating summer monsoon freshening in the North Bay of Bengal: A case study

A very high-resolution model for the Bay of Bengal is configured to simulate submesoscale flow features, which are otherwise absent in coarse resolution models. In this manuscript, we present the response of this model to simulate summer monsoon freshening in the northern Bay of Bengal when forced with two different winds, one based on weather forecast model output from National Centre for medium range weather forecast (NCMRWF) and the other based on satellite-derived SCATSAT ocean surface winds for 2017 as a case study. Our results reveal that the spatio-temporal evolution of lateral freshwater dispersal in the northern Bay of Bengal is sensitive to wind vorticity and eddy formation/evolution. A reasonably realistic decrease of 2–3 psu in the sea surface salinity at the mooring location of 18oN, 90oE towards the end of September 2017 is simulated in the model when forced with the SCATSAT winds, whereas significantly weaker freshening occurs at the mooring location when the model is forced with the NCMRWF winds. A filament of strong positive vorticity connecting two cyclonic eddies in the north central Bay in the NCMRWF wind forced simulations prevents the direct southward advection of freshwater, resulting in higher salinity at the mooring location. A similar mechanism that would prevent southward advection of fresh water is absent in the SCATSAT forced model run. Westward moving Rossby waves in the SCATSAT forced model run are found to play an important role in the evolution of freshening events at various locations in the northern Bay of Bengal. Lagrangian Coherent Structures (LCS) derived from the model currents suggest that the background conditions were favourable to advect freshwater from Ganges-Brahmaputra towards the mooring location in the northern Bay of Bengal in the SCATSAT forced simulations.

Numerical modelling of tidally generated internal wave radiation from the Andaman Sea into the Bay of Bengal

Semi-diurnal internal waves generated by tides in a high resolution numerical model that includes the Andaman Sea archipelago are found to propagate into the central Bay of Bengal and reach the coasts of India and Sri Lanka. The waves are also present in subsurface temperature records from RAMA moorings, and their propagation speed across the Bay of Bengal agrees well with satellite remote sensing from MODIS imagery in spite of the hydrostatic nature of the model. The internal waves are simulated by a fully coupled ocean-atmosphere prediction system, exchanging surface fluxes between the air and sea at high frequency and at high resolution. For the ocean, a hydrostatic model including diurnal and semi-diurnal tides provides a 2 km resolution representation of the entire Bay of Bengal. In the ocean model simulations, the semi-diurnal internal waves interact with the mesoscale circulation and surface waves and modify the flow and the stratification. By comparing coupled ocean-wave model runs with tides and without tides, but forced by identical surface fluxes from the atmosphere, it is demonstrated that the inclusion of diurnal and semi-diurnal tides act to cool the core of the thermocline while increasing the temperature above and below it along the pathway of the internal waves, a result that likely is due to vertical mixing by the waves.

Performance of CMIP5 wind speed from global climate models for the Bay of Bengal region

AbstractRealistic estimates of surface wind speed (WS) are an essential prerequisite for process‐based air–sea interaction studies and numerical modelling needs. In this context, historical and projected WS estimates obtained from global and regional climate models warrant proper assessment and necessary bias corrections before it can be optimally used for rigorous analysis and research needs. Adequate evaluation and bias correction of WS estimates are therefore crucial to understand the extremes. For example, they have direct implications on extreme wind‐wave characteristics that can influence the coastal zones. The present study performed a detailed evaluation of WS obtained from the Coupled Model Intercomparison Project Fifth Phase (CMIP5) products to assess their projections for the Bay of Bengal region. A suite of global climate models (GCMs) is employed to generate the CMIP5 projections under four Representative Concentrative Pathways (RCPs) of 2.6, 4.5, 6.0, and 8.5 and based on differential CO2 emission scenarios. The present study also used the Research Moored Array for African–Asian–Australian Monsoon Analysis and Prediction (RAMA) buoys located in the central Bay of Bengal in order to validate and skill assess the CMIP5 wind products under varying RCPs. Besides, an intercomparison exercise that was performed between RAMA buoys data and merged satellite altimeter data from the French Research Institute for Exploitation of the Sea/Laboratory of Oceanography from Space (IFREMER/CERSAT) provided the necessary confidence to ascertain the quality of CMIP5 WS products. The study signifies that a moderate positive correlation was noticed in the WS comparison between CMIP5 GCM products and the RAMA buoys (maximum correlation of .64), and the correlation factor varied between the suite of models used in CMIP5 experiments. This exercise would provide detailed know‐how on the performance of various GCMs and also provide a basis to select the best performing GCMs for the Bay of Bengal region. Analysis of the upper 10% (90th percentile) showed a maximum underestimation/overestimation of 2.5 and 1.5 m·s−1, respectively, for WS comparison between CMIP5 and RAMA buoys data. Probability density of WS data fitted to Weibull distribution showed an increase of moderate WS (6–8 m·s−1) during the period of study. Although the CMIP5 GCMs are not able to represent the contemporary WS climatology satisfactorily, the models such as HadGEM2‐ES, HadGEM2‐CC, CanESM2, and ACCESS‐1.3 showed the best performance concerning near‐surface WS for the Bay of Bengal region.

Ensemble based regional ocean data assimilation system for the Indian Ocean: Implementation and evaluation

A high-resolution ocean circulation model for the Indian Ocean (IO) using Regional Ocean Modeling System (ROMS) is operational at Indian National Centre for Ocean Information Services (INCOIS) which provides ocean state forecasts for the Bay of Bengal (BoB) and the Arabian Sea (AS) to the Indian Ocean rim countries. To provide an improved estimate of ocean state, a variant of Ensemble Kalman Filter (EnKF), viz., the Local Ensemble Transform Kalman Filter (LETKF) has been developed and interfaced with the present basin-wide operational ROMS. This system assimilates in-situ temperature and salinity profiles and satellite track data of sea-surface temperature (SST). The ensemble members of the assimilation system are initialized with different parameters like diffusion and viscosity coefficients and are subjected to an ensemble of atmospheric fluxes. In addition, one half of the ensemble members respond to K profile parameterization mixing scheme while the other half is subjected to Mellor–Yamada mixing scheme. This strategy aids in arresting the filter divergence which has always been a challenging task. The assimilated system simulates the ocean state better than the present operational ROMS. Improvements permeate to deeper ocean depths with better correlation and reduced root-mean-squared deviation (RMSD) with respect to observations particularly in the northern Indian Ocean which is data rich in density. Analysis shows domain averaged RMSD reduction of about 0.2–0.4 °C in sea surface temperature and 2–4 cm in sea level anomaly. The assimilated system also manages to significantly improve the thickness of the temperature inversion layers and the duration of its occurrence in northern Bay of Bengal. The most profound improvements are seen in currents, with an error reduction of 15 cm/s in zonal currents of central Bay of Bengal.

Reconsidering a proportional system of timber-frame structures through ancient mathematics books: a case study on the Muryangsujŏn Hall at Pusŏksa Buddhist Monastery

ABSTRACTThe mathematics references in ancient China, the Zhoubi Suanjing and the Jiuzhang Suanshu, present information on formative ideas of ancient people and their perception of objects. The introduction to the Yingzao Fashi mentions mathematical sources, including the Zhoubi Suanjing. Both of these books focus on the philosophical concept of Tianyuan difang (Heaven is round and Earth is square), as well as inscribed and circumscribed circles. The square root of 2(√2), which can be derived from this part, proves to be an essential criterion for building, seen in Korea, China, and Japan. Using the exemplary Koryŏ building, the Muryangsujŏn Hall at Pusŏksa Buddhist Monastery, this thesis shows that the standard ground plan width of the outermost bay has a √2 ratio to the central bay width. Its cross-section, likewise, proves that √2 times or twice the distance or height (relying on the height of the eave columns) are applied to the distance or height between each column and purlin in the application of arithmetic and geometric concepts. In the future, this work will be a reference for the reconstruction design of ancient buildings prior to the Koryŏ period, analogous to the Muryangsujŏn Hall.

Spatial–Temporal Variation in Sea Surface Temperature from Landsat Time Series Data Using Annual Temperature Cycle

Zhang, K.; Jiang, T., and Huang, J., 2019. Spatial–temporal variation in sea surface temperature from Landsat time series data using annual temperature cycle. In: Jung, H.-S.; Lee, S.; Ryu, J.-H., and Cui, T. (eds.), Advances in Remote Sensing and Geoscience Information Systems of Coastal Environments. Journal of Coastal Research, Special Issue No. 90, pp. 58-65. Coconut Creek (Florida), ISSN 0749-0208.Sea surface temperature (SST) plays an important role in aquatic ecosystems and the biogeochemical cycle. Multi-temporal remote-sensing observations may be desirable alternatives to traditional in situ sensors for SST measurement. However, the frequently used low-to-moderate-resolution remote sensors usually cannot identify subtle SST variations in coastal areas due to the pixel radiance contamination caused by shoreline influences. For alleviating this problem, the SST of Jiaozhou Bay (JZB) between 1986 and 2017 was estimated by means of Landsat thermal infrared data and the single-channel retrieval algorithm. The retrieved results were validated by field-measured water temperature and bootstrap method. Then, the estimated SST was divided into five-year intervals and calculated the SST climatology for each period. With use of the annual temperature cycle fitting model, the time series data not only demonstrated the spatial–temporal variation of the water temperature of JZB but also effectively compensated for the lack of remote sensing data due to adverse weather in summer. The estimation results showed that the SST of JZB increased gently during the past 30 years, especially in coastal areas. The increase of SST near artificial facilities was evident, indicating the influence of urbanization and industrialization in coastal area. A correlation analysis of SST and meteorological factors revealed that air temperature influenced SST variation, especially in the central bay, whereas wind speed and precipitation had nearly no influence.

Combined analysis of sulfur and carbon contents, and foraminifer as paleoenvironmental indicators in tidal flat sediments on Miura Peninsula, Japan

Reconstruction of natural hazards from tidal flat sediments is essential for assessment of coastal hazards, and is based on knowledge of recent sediments. This study therefore examined recent sediments deposited on a narrow, muddy tidal flat on Miura Peninsula, metropolitan Tokyo, Japan, by means of sedimentological analysis, C, N, and S elemental analysis, carbonate content, 137Cs dating, and analysis of benthic foraminifera (species composition and degree of preservation). 137Cs profiles of four sediment cores of ∼20 cm in length showed that the sediments were deposited after the release of 137Cs by the accident at the Fukushima No. 1 Nuclear Power caused by the Tohoku-oki tsunami at March 2011. Results of C/S ratios of mud fractions indicated that the intertidal deposits in the central bay are characterized by low C/S ratios (4.9–11.4) and the bayhead deposits of the upper intertidal and supratidal zones by higher ratios (13.9–24.9). A fluid–mud layer identified in the central bay had the same C/S ratios and carbonate contents as marine sediments, indicating that the fluid-mud deposits probably resulted from resuspension of mud caused by storm-wave action after March 2011. Faunal analysis of benthic foraminifera showed that very low density of foraminifera can be regarded as one of the defining characteristics of fluid-mud deposits.

Sedimentary responses to the sea level and Indian summer monsoon changes in the central Bay of Bengal since 40 ka

We present a sedimentary multi-index comprehensive study on sediment core BoB-56 collected from the central Bay of Bengal (BoB) with an attempt to understand the sedimentary processes and their responses to sea level change and the Indian summer monsoon (ISM) since the last glacial period. We revealed the temporal evolution of terrigenous input and provided a brief discussion of the implications of our sedimentary indexes for ocean productivity. Then, we proposed a simple depositional model in the central BoB since the last glacial period. As indicated by the geochemistry, mineralogy and physical indexes, including linear sedimentary rate (LSR), Ti/Ca, Rb/Sr and quartz/calcite ratios, the terrigenous input was higher during the last glacial period than during the Holocene period, and the sea level is suggested to be responsible for this significant distinction through the depositional center transition in the BoB at the glacial-interglacial scale. Similar terrigenous input patterns with solar radiation and precipitation before 18 ka indicated that the monsoon controlled these situations at precession-related scales. Synchronous changes with records in the north Atlantic Ocean during the last deglaciation and early Holocene suggested terrigenous input responses to the climate changes that also recorded by high latitude sedimentary system at millennial scales. Indicated by the element (CaCO3, Ba and Sr) authigenic accumulation rates, the paleoproductivity in the central BoB was at a roughly equivalent level during the last glacial period and the Holocene period. Different terrestrial nutrient inputs and ISM-related ocean surface stratifications were suggested to be responsible for the level of paleoproductivity. This study provides a brief understanding of the sedimentary response to the climate and emphasizes the different roles of the sea level and ISM in the central BoB.

Bay of Bengal‐East Asia‐Pacific Teleconnection in Boreal Summer

AbstractA new teleconnection pattern (the BEAP) across the Bay of Bengal‐East Asia‐Pacific region in boreal summer is revealed in this study using mainly ERA‐Interim reanalysis data from the European Centre for Medium‐Range Weather Forecasts. The BEAP index (BEAPI) is defined as the signed sum of standardized apparent moisture sinks at five centers along the pathway. Correlation analysis of the apparent heat sources and apparent moisture sinks has verified the existence of the BEAP teleconnection. Variations in BEAP can affect precipitation anomalies resulting from the anomalous moisture transport and the antiphase surface temperature variation. Wave flux analysis has verified the Rossby wave propagation route that originates around the central Bay of Bengal and extends across North China to the West Pacific. La Niña‐type sea surface temperature anomalies (SSTAs) appearing simultaneously in the same season can excite a positive BEAP pattern by enhancing convection over the Bay of Bengal, while El Niño‐type SSTAs have the opposite effect. Significant correlation between the BEAPI and the SSTAs can last from early summer to early winter. Numerical experiments confirm the BEAP teleconnection pattern and the associated physical processes.

The relationship between Indian monsoon rainfall and low-pressure systems

Indian summer monsoon precipitation is significantly modulated by synoptic-scale tropical low pressure areas (LPAs), the strongest of which are known as monsoon depressions (MDs). Despite their apparent importance, previous studies attempting to constrain the fraction of monsoon precipitation for which such systems are responsible have yielded an unsatisfyingly wide range of estimates. Here, a variant of the DBSCAN algorithm is implemented to identify nontrivial, coherent rainfall structures in TRMM-3B42 precipitation data. Using theoretical considerations and an idealised model, an effective capture radius is computed to be 200 km, providing upper-bound attribution fractions of 57% (17%) for LPAs (MDs) over the monsoon core zone and 44% (12%) over all India. These results are also placed in the context of simpler attribution techniques. A climatology of these clusters suggests that the central Bay of Bengal (BoB) is the region of strongest synoptic organisation. A k-means clustering technique is used to identify four distinct partitions of LPA (and two of MD) track, and their regional contributions to monsoon precipitation are assessed. Most synoptic rainfall over India is attributable to short-lived LPAs originating at the head of the BoB, though longer-lived systems are required to bring rain to west India and east Pakistan. Secondary contributions from systems originating in the Arabian Sea and south BoB are shown to be important for west Pakistan and Sri Lanka respectively. Finally, a database of precipitating-event types is used to show that small-scale deep convection happens independently of MDs, whereas the density of larger-scale convective and stratiform events are sensitive to their presence—justifying the use of a noise-rejecting algorithm.

Nursery habitat use for larval and juvenile Pacific herring Clupea pallasii in Miyako Bay on the Pacific coast of northern Japan

To clarify nursery habitat use for larval and juvenile Pacific herring Clupea pallasii, we collected eggs, larvae, and juveniles at several sampling sites in inner Miyako Bay from 2012 to 2016. Adult C. pallasii used sea grass beds on the west side of inner Miyako Bay as a spawning area. In contrast, their offspring grew up to approximately 100 mm in total length (TL) on the east side of the inner bay, where they stayed until July. C. pallasii seemed to leave the nursery area on the east side of the bay with the rise in temperature in August. Therefore, C. pallasii shifted habitats with ontogenetic development, from the west side of the inner bay in the egg stage, to the east side of the inner bay in the larval and early juvenile stages, and then to the central bay area in the later juvenile stage. Environmental surveys showed lower salinity near the bottom of the east side than the west side of inner Miyako bay, suggesting that larval and juvenile C. pallasii appeared to use the area with low salinity on the east side of Miyako Bay as a nursery.