Phase Of Monsoon Research Articles

Indian Ocean Dipole : Assessing its impacts on the Indian Summer Monsoon Rainfall (ISMR) across North East India

The Indian Ocean Dipole (IOD), a climatic anomaly, results in sustained sea surface temperature (SST) variations between tropical western and eastern Indian Ocean temperatures. In this study, we studied the variations to inculcate the teleconnections between IOD and Indian summer monsoon rainfall (ISMR) distribution across the country for the period 1960-2020 for all the three phases of ISMR. We analyzed rainfall, SST and low-level wind circulation anomalies for the above mentioned time horizon. Positive IOD events noticeably resulted in increase in summer monsoon rainfall distribution across the country respectively while its negative counterpart led to decrease in rainfall except for the commencement phase of ISMR. The variations in SST, wind circulation and moisture movement processes across the Indian Ocean characterize significant changes in rainfall during the positive and negative phases of IOD especially during the recent decades (1991-2020). The recent time horizon also witnesses enhanced low-level equatorial jets (LEJ) across the equatorial Indian Ocean and the Arabian Sea during the positive IOD events as compared to the prior decades (1960-1990). The effect of moisture convergence zone is also analyzed which results in above rainfall conditions across northeastern and central India. Conversely, negative IOD events were found to subdue any such moisture movement mechanisms. Furthermore, and additional investigation to analyze the effect of IOD on the retreating/withdrawal monsoon across northeast India has been done and it has been observed that a stronger positive IOD is detrimental to the seasonal rainfall (May- September) over North East India (-0.7 one month lag correlation). Furthermore, the DMI index of April-May presented a clear indication of monsoon activity over the area during the withdrawal or retreating phase of the summer monsoon, i.e., during September.

Indian Ocean Dipole : Assessing its impacts on the Indian Summer Monsoon Rainfall (ISMR) across North East India

The Indian Ocean Dipole (IOD), a climatic anomaly, results in sustained sea surface temperature (SST) variations between tropical western and eastern Indian Ocean temperatures. In this study, we studied the variations to inculcate the teleconnections between IOD and Indian summer monsoon rainfall (ISMR) distribution across the country for the period 1960-2020 for all the three phases of ISMR. We analyzed rainfall, SST and low-level wind circulation anomalies for the above mentioned time horizon. Positive IOD events noticeably resulted in increase in summer monsoon rainfall distribution across the country respectively while its negative counterpart led to decrease in rainfall except for the commencement phase of ISMR. The variations in SST, wind circulation and moisture movement processes across the Indian Ocean characterize significant changes in rainfall during the positive and negative phases of IOD especially during the recent decades (1991-2020). The recent time horizon also witnesses enhanced low-level equatorial jets (LEJ) across the equatorial Indian Ocean and the Arabian Sea during the positive IOD events as compared to the prior decades (1960-1990). The effect of moisture convergence zone is also analyzed which results in above rainfall conditions across northeastern and central India. Conversely, negative IOD events were found to subdue any such moisture movement mechanisms. Furthermore, and additional investigation to analyze the effect of IOD on the retreating/withdrawal monsoon across northeast India has been done and it has been observed that a stronger positive IOD is detrimental to the seasonal rainfall (May- September) over North East India (-0.7 one month lag correlation). Furthermore, the DMI index of April-May presented a clear indication of monsoon activity over the area during the withdrawal or retreating phase of the summer monsoon, i.e., during September.

Extended-range forecast of monsoon at smaller spatial domains over India for application in agriculture

The performance of the operational extended-range forecast (ERF) issued by IMD is evaluated for the southwest monsoon 2020. The normal onset of monsoon over Kerala (the southern tip of India) with subsequent rapid progress northward in covering the entire country is very well captured in the ERF with 2 to 3 weeks lead time. The ERF also captured very well the transitions from normal to weaker phase of monsoon in July and the active phase of monsoon during entire August with a lead time of about 3 weeks. The active monsoon condition in the second half of September associated with delayed withdrawal from northwest India was also reasonably well captured in the ERF. Quantitatively, the ERF shows significant skill up to 3 weeks on all India levels. On smaller spatial domains for 36 meteorological subdivisions (met-subdivisions) over India, the performance of category forecasts is evaluated in terms of above normal, normal and below normal. The spatial distribution of the met-subdivision level forecast skill of predicting above normal, normal and below normal categories for the 36 subdivisions during the entire monsoon season of 2020 in terms of correct (forecast and observed category matching) to partially correct (forecast and observed category out by one category) combined categories is found to be 89%, 83%, 80% and 78% for week 1 to week 4 forecasts respectively. The wrong forecasts (forecast and observed category out by two categories) are found to be between 11% in week 1 and 22% in week 4 forecast. Thus, the met-subdivision level forecast shows useful skill and is being used operationally for agrometeorological advisory services of IMD.

Monsoon-phase regulates the decoupling of auto- and heterotrophic respiration by mediating soil nutrient availability and root biomass in tropical grassland

Soil respiration is considered as one of the most sensitive parameters regulated by the interplay of different soil biophysical parameters, ground cover and climatic conditions. As per the global predictions, monsoon variability in South Asian countries can affect various soil processes. However, studies exploring the interactive role of vegetation and precipitation conditions on soil respiration and its components (viz., auto- and heterotrophic) are lacking. A mini-trenching method was performed to explore the alterations in auto- and heterotrophic components of soil respiration as well as their regulatory variables in grassland vegetation under different summer monsoonal phases. Three vegetation densities (viz. low, medium and high) based on herbaceous cover, and three monsoon phases (viz. onset, active and end) of the tropical summer monsoon were considered. The findings revealed that heterotrophic respiration was the major contributor to the total soil respiration under medium- and high-density vegetation with percent contribution of 62–80% and 53–88% respectively. Contrastingly, autotrophic respiration was the major contributor (54–56%) to soil respiration in low-density vegetation during active- and monsoon-end phases. Amongst the three monsoon phases, variation in soil respiration components was most pronounced during the active-monsoon phase. Structural Equation Modelling results revealed microbial biomass and root biomass as key regulators of auto- and heterotrophic respiration. Remarkably, a partitioning of root biomass parallel to the soil respiration was observed during monsoon-end phase where coarse root biomass and fine root biomass emerged as regulators of auto- and heterotrophic respiration, respectively. Moreover, for monsoon-onset and -end phases, a completely different set of variables involved in regulation of soil respiration components were identified. Our results revealed that the monsoon-phase mediates the root and nutrient dynamics which led to the differential decoupling of the components of soil respiration. Thus, inclusion of (below)-ground cover and monsoon-phase wise variation in soil biophysical parameters in the global biogeochemical models would be more helpful for better prediction of carbon balance in different ecosystems.

Assessment of Lead-Lag and Spatial Changes in simulating different epochs of the Indian summer monsoon using RegCM4

The present study examines the efficacy of the regional climate model RegCM4 in depicting the onset, active, and break phases of the Indian summer monsoon (ISM). The model is driven by National Centers for Environmental Prediction (NCEP)/NCAR Reanalysis 1 project (NNRP1) data of 28 years (1981–2008) for each monsoon separately. High-resolution gridded rainfall analysis dataset of India Meteorological Department (IMD) and the monsoon epochal report from National Climate Center (NCC) are used for verification. Besides assessment of model performance using qualitative methods, several standard statistical techniques are used to understand the capability of the model in representing monsoonal epochs.A relationship between the SST changes over the Niño-3.4 region and the ISM over all India and the monsoon core region (MCR) is established where a strong dependency in the rainfall variation over the MCR with the SST variation is found than all India rainfall. The performance of the model is satisfactory in portraying the spatial and temporal intraseasonal variability (ISV) of monsoon over MCR and all India regions. It is found that the model performance is higher in simulating the onset and break phases (dry days) than the active phases. The limited skill of the RegCM4 model in representing the active phase is due to the model simulated low temperature and weak pressure gradient over the MCR which prevents the convection and given rise to small and weak active phases over the region. The mechanism from the pre-onset to onset of monsoon, and the monsoon advance between the onset and the monsoon ISV is also well captured by the RegCM4. Weak tropospheric temperature gradient over the Indian subcontinent during the pre-onset period causes delay onset over the Indian subcontinent and the subsequent high tropospheric temperature gradient during the mid of June causes a faster progression of the active/break phases of monsoon. At the same time, simulation of the monsoon phase by the model is heavily regulated by the changes of SST over the Niño-3.4 region. The signal of ENSO variability, forces to the model through the ICBC, modulates the capability of the regional model in identifying the interannual and ISV of monsoon over the MCR region.

Tropical cyclones over the Arabian Sea during the monsoon onset phase

AbstractGenerally, monsoon conditions are unfavourable for cyclogenesis in the Arabian Sea due to the strong vertical wind shear present over the basin which prevents the vertical development of cyclones. However, there are instances where tropical cyclones form over the basin during the onset phase of the Indian summer monsoon (ISM). The current study examines the evolution of the synoptic conditions in the years during 1979–2019 in which a cyclone forms during the onset phase of the ISM. The effect of various atmospheric and oceanic factors, the modulation of wind circulation over the Indian subcontinent and the north Indian Ocean region due to the possible influence of tropical and extratropical weather systems has been taken into consideration. Our analysis indicates that a weak mean monsoonal wind circulation caused by the southward intrusion of subtropical westerlies (at 200 hPa) is conducive for cyclogenesis over the Arabian Sea during the monsoon onset phase. Such a weak onset phase of the monsoon, coupled with the seasonal warm SSTs, may lead to cyclone formation in the Arabian Sea.

Response of coastal phytoplankton pigment composition to tropical cyclone Fani

Phytoplankton pigment composition was evaluated during the pre-cyclone phase (PRCP) and post-cyclone phase (POCP) of tropical cyclone Fani in the coastal waters of the northwestern Bay of Bengal. The chromatographic analysis revealed higher pigment diversity and an increase in individual pigment concentration during POCP. Chlorophyll–a (chl–a) was the dominant pigment during PRCP and POCP, followed by fucoxanthin. However, chl–a and fucoxanthin concentrations increased 18- and 14-folds, respectively, during the POCP, signifying Bacillariophyta bloom. Complementing microscopy confirmed the dominance of the toxic Bacillariophyta species Pseudo-nitzschia pungens (reaching 5.47 × 104 cells l−1) during the POCP. The cyclone-induced nutrient recharge of the ambient medium could have promoted phytoplankton growth, causing the reappearance of diatom bloom during the later phase of the pre-southwest monsoon. Small-sized Prymnesiophyta and Cryptophyta were not detected microscopically; however, they were identified by chromatographic analysis through pigment markers during POCP.

A 145 kyr record of upstream changes in Indian monsoon circulation and its link to southern high-latitude climate

The influence of northern high latitude climate variability on the South Asian Summer Monsoon has been extensively studied using both instrumental and proxy based climate data. In comparison, only a few studies have attempted to explore the southern high latitude association of the South Asian Summer Monsoon. Here we present a 145 kyr long new data (oxygen and carbon isotopic abundance of two depth-stratified species of foraminifera viz. Globigerinoides ruber and Globorotalia menardii) from a sediment core on sub-millennial to millennial scale resolution from the southwestern tropical Indian Ocean - a region swept by the southeasterly wind during boreal summer (referred to as ‘summer’ hereafter), which transforms to the southwest monsoon wind after crossing the equator. The available records of past summer monsoon variability are predominantly based on reconstruction of downstream hydrology, which is identifiable with the thermodynamics of the system. The present study is a reconstruction of the dynamics of the summer monsoon circulation from an upstream region which is ideally suited for exploring its link with the southern high-latitude climate. We identified several occurrences of interchanging intense and weak monsoon circulation events during the 145 kyr period. We further find that warm (cold) conditions in Antarctica show a near one-to-one coupling with weak (strong) monsoon phases, suggesting a strong mechanistic link between them during the period.

Influence of Indian Summer Monsoon on Tropopause, Trace Gases and Aerosols in Asian Summer Monsoon Anticyclone Observed by COSMIC, MLS and CALIPSO

The existence of the Asian Summer Monsoon Anticyclone (ASMA) during the summer in the northern hemisphere, upper troposphere and lower stratosphere (UTLS) region plays a significant role in confining the trace gases and aerosols for a long duration, thus affecting regional and global climate. Though several studies have been carried out, our understanding of the trace gases and aerosols variability in the ASMA is limited during different phases of the Indian monsoon. This work quantifies the role of Indian Summer Monsoon (ISM) activity on the tropopause, trace gases (Water Vapor (WV), Ozone (O3), Carbon Monoxide (CO)) and aerosols (Attenuated Scattering Ratio (ASR)) obtained from the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC), Microwave Limb Sounder (MLS), Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite observations, respectively, during the period 2006–2016. Enhancement in the tropopause altitude, WV, CO, ASR and low tropopause temperatures, O3 in the ASMA region is clearly noticed during peak monsoon months (July and August) with large inter-annual variability. Further, a significant increase in the WV and CO, and decrease in O3 during the active phase of the ISM, strong monsoon years and strong La Niña years in the ASMA is noticed. An enhancement in the ASR values during the strong monsoon years and strong La Niña years is also observed. In addition, our results showed that the presence of deep convection spreading from India land regions to the Bay of Bengal with strong updrafts can transport the trace gases and aerosols to the upper troposphere during active spells, strong monsoon years and La Niña years when compared to their counterparts. Observations show that the ASMA is very sensitive to active spells, strong monsoon years and La Niña years compared to break spells, weak monsoon years and El Niño years. It is concluded that the dynamics play a significant role in constraining several trace gases and aerosols in the ASMA and suggested considering the activity of the summer monsoon while dealing with them at sub-seasonal scales.

Downstream and In Situ Genesis of Monsoon Low‐Pressure Systems in Climate Models

AbstractThe monsoon low‐pressure systems (LPSs) are a major contributor to the rainfall over India. The genesis of LPS in climate models is not well understood. Here, we track the LPS activity in 11 coupled climate models using an automated tracking algorithm and classify their genesis mechanism broadly into two categories—in situ and downstream. We find that the in situ genesis mechanism dominates in all models, with an average of 56% systems categorized under this category, while 63% of the observed LPS had in situ genesis. The average downstream genesis in the models is 32%, closer to the observed 30%. About 12% and 7% of the LPS genesis could not be attributed to either of the categories in the models and observations, respectively, due to the presence of both types of genesis mechanisms. Although the bulk statistics of the in situ and downstream LPS genesis across the models in boreal summer is comparable to that of observations, substantial inter‐model variability is observed. Also, we find significant differences in the temporal distribution of downstream LPS genesis in models. Although the models realistically capture the percentage of downstream LPS for the whole monsoon season, they tend to simulate a higher number of genesis in the early phase of monsoon as opposed to the observed peak in August and September, which is linked to a stronger Rossby wave activity in the models in June.

Quantitative estimations of iron oxide minerals in the Late Pleistocene paleosol of the Yangtze River Delta: Implications for the chemical weathering, sedimentary environment, and burial conditions

Iron oxide minerals are widely present in soil and known to be closely associated with soil-forming environments. In this study, a detailed quantitative analysis of the iron oxide minerals in the Late Pleistocene hard clay of the Yangtze River Delta (YRD) was conducted using new integrated methods of magnetism, selective chemical-dissolution, and diffuse reflectance spectroscopy. A variety of established chemical weathering indexes and optically stimulated luminescence (OSL) chronology were also applied in the investigations of the hard clay in the study area. These methods allowed for detailed examinations of the chemical weathering, sedimentary environmental conditions, and the post-depositional processes of the hard clay deposits in the YRD. This study’s A-CN-K diagram revealed that the hard clay had entered an intermediate weathering stage. In addition, various chemical weathering indexes demonstrated that the hard clay had undergone three stages of chemical weathering from bottom to top as follows: 1) A relatively high stage of chemical weathering; 2) A low chemical weathering stage with a slightly increasing trend; and 3) A relatively high chemical weathering stage with a weakly decreasing trend. It was found that when χ, χfd, Fed/Fet, Gt, and Hm/Gt were not reliable indicators of chemical weathering for the hard clay, Fed could be a useful proxy. In the current study, based on the OSL ages and several paleoclimatic proxies, it was suggested that the climate conditions in the YRD had transitioned from a relatively weak to a strong East Asian summer monsoon phase during the periods between the marine oxygen isotope stages (MIS) 4 and MIS 3. Furthermore, the low values of χ and χfd in the hard clay indicated an intense dissolution of fine-grained strong magnetic minerals. Meanwhile, the relatively high Feo/Fed ratios in the superficial horizon displayed the predominance of poorly crystalline forms of iron oxides. In summary, the analysis results implied that the hard clay had undergone post-burial processes of ferrolysis, as well as dissolved organic matter translocation from the overlying Holocene sediment. The aforementioned post-burial processes for the hard clay had occurred under reduction–oxidation cycles, which were mainly driven by the rise of the groundwater levels with the Holocene transgression.

The interseasonal features of precipitation microphysics over Thiruvananthapuram and Kolkata - the two tropical stations of Indian sub-continent

A comparative study of precipitation microphysics along with the possible causative mechanisms for the inter-seasonal variation of raindrop size distribution over Thiruvananthapuram and Kolkata – the two tropical stations of Indian sub-continent has been highlighted in this paper. In addition to the contrasting geographical and climatic variations, the importance of the two locations lies in the fact that the rainfall over Thiruvananthapuram represents the Arabian Sea branch of south-west monsoon while that over Kolkata represents the Bay of Bengal branch. The study reveals a distinct domination of raindrops of diameter 2 mm and above for the pre-monsoon months (March–May) over Kolkata with respect to Thiruvananthapuram, while for the monsoon months (June–September), no such appreciable differences are noticed. Correspondingly, the continental and maritime clouds also show their contrasting impacts in the rainfall microphysics during these inter-seasonal phases for the two locations. It has been observed that rainfall evolving from the continental clouds during the pre-monsoon period over Kolkata produces distinct diurnal variation of rainfall occurrences, which are also associated with the presence of raindrop of diameter 3 mm and above. Correspondingly, such variations are absent for the maritime clouds which dominate the monsoon rainfall for both the locations. Thus it can be said that the nature of clouds along with the surface temperature plays an important role in the varying microphysics of precipitation during the inter-seasonal phases of monsoon over these two cities of India.

Variabilities in the community structure of phytoplankton in the upwelled and non-upwelled waters of southeastern Arabian Sea during the early summer monsoon

The community structure of microphytoplankton was assessed along the southeastern Arabian Sea during the early phase of the summer monsoon. The study records an intense coastal upwelling along the southernmost region (off Thiruvananthapuram), which decreased further north. High chlorophyll-a (10.8 mg m-3) and nutrient concentration was recorded in the coastal waters of Thiruvananthapuram. Even though off Mangalore (12° N) and off Goa (15° N) where upwelling was confined to narrow coastal zone, also showed high chlorophyll-a concentration, 3.98 mg m-3 and 6.31 mg m-3, respectively. The upwelled waters were dominated by centric diatoms (Thalassiosira sp.) and the non-upwelled waters (12° N and 15° N) were dominated by dinoflagellates. Microphytoplankton cell density was the highest along off Thiruvananthapuram (4.8×104 cells L-1), with maximum cell density along the coastal waters (1.4×104 cells L-1). Phytoplankton community of upwelled and non-upwelled waters showed significant variations with 60 % similarity between phytoplankton communities of upwelled waters.

Spatio‐temporal variability in the stratiform/convective rainfall contribution to the summer monsoon rainfall in India

AbstractSpatio‐temporal variability of contributions of stratiform and convective rainfall to Indian monsoon (June–September) rainfall have been investigated using hourly rainfall data of well distributed 126 stations in India for the period 1969–2015. The criteria used for identifying stratiform rainfall are hourly rainfall ≤5 mm and spatial homogeneity. The study showed that Indian monsoon rainfall exhibits two distinctive features, viz. stratiform component dominating over peninsular (southern) India and convective component dominating over northern India. The diurnal variation shows domination of convective activity in the afternoon hours over the northern parts of India. Intra‐seasonal variability in the stratiform rainfall is the lowest over the West Coast stations through‐out the season. While it shows low values during the onset (June) and withdrawal (September) phases of monsoon and higher values during July–August in Peninsular India. Inter‐annual variations in the convective rainfall are larger than that of stratiform rainfall all over the country. Trend analysis indicates that both stratiform and convective rainfall are decreasing over the central parts of the country and increasing significantly along West Coast and western parts of the country. During excess monsoon year, convective rainfall activity is more than in deficit monsoon year, in Central India. The results brought out in the study will be useful as a proxy for understanding the spatial and temporal variability of the latent heating fields over India in the monsoon season and validation of model simulations of clouds and rainfall types.

Variability of the low-level circulation of the South American Monsoon analysed with complex networks

Understanding the variability of low-level atmospheric circulation regimes is key for understanding the dynamics of monsoon systems. The South American Monsoon is characterized by strong year-long trade winds that are channeled southward into the South American Low-Level Jet after crossing the Amazon basin, which in turn is elementary for the moisture transport to Southern South America. In this study, we utilize streamflow wind networks, a type of climate networks that tracks the local flow of the wind field, together with the analysis of composites of wind, precipitation, and geopotential height fields, to investigate the variability of the South American low-level circulation. The streamflow wind networks are used here as they are able to directly track the wind flow and encode its spatiotemporal characteristics in their topology. We focus on intraseasonal variations in terms of active and break monsoon phases on the one hand, and on the interannual variability in terms of the impacts of the El Niño-Southern Oscillation on the other hand. Our findings highlight the importance of the South American Low-Level Jet, its spatial position and variability. Our study reveals the relation of the active and break regimes to anomalous high- and low-pressure systems over the southern Atlantic that are connected to Rossby wave trains from the southern Pacific, as well as the impact of these regimes on the cross-equatorial low-level flow. In addition, the streamflow networks that we use demonstrate significant shifts of the dominant wind flow pattern during El Niño and La Niña episodes.

Lightning Activity Variability with Prevailing Weather Parameters and Aerosol Loading Over Dry and Wet Regions of India

This paper presents the lightning activity, aerosol optical depth (AOD) and climatic parameters (Bowen ratio, relative humidity, rainfall, maximum surface temperature and maximum updraft speed) over wet (Northeast India—NEI) and dry (Northwest India—NWI) land surfaces in a comparative analysis. The analyses are performed on flash counts and weather data of 17 years (1998–2014) retrieved from the Tropical Rainfall Measuring Mission (TRMM) lightning imaging sensors (LIS) and the Moderate Resolution Imaging Spectroradiometer (MODIS), respectively. The first higher peak for flash counts during pre-monsoon (April–May) and second lower peak during September over wet NEI and dry NWI regions indicate the development of strong electrified storms during pre-monsoon and weakly electrified clouds during the withdrawal phase of the southwest summer monsoon. The monthly means of flash counts, Bowen ratio, maximum surface temperature, AOD and maximum updraft speed are higher by 27, 81, 16, 50 and 16%, respectively, for NWI than those of NEI. The relative humidity and rainfall are higher by 12% and 73% for NEI than those of NWI, respectively. The Pearson’s correlation coefficients of lightning activity with climatic parameters and AOD demonstrate the distinctive orographic lifting, moisture content and vertical wind shear in the upper troposphere in the regional climatic zone of dry NWI in comparison to that of wet NEI.

Convection over the eastern equatorial Indian Ocean and Iranian branch of Asian summer monsoon anticyclone modulates the wet and dry phases of summer monsoon rainfall over Kerala

AbstractThis study examined factors affecting wet and dry phases of monsoon rainfall over Kerala, using modern‐era retrospective analysis for research and applications (MERRA 2) data set. Based on circulation pattern, our study shows that the wet and dry phases of monsoon over Kerala are related to modulation of convective activity over the eastern equatorial Indian Ocean (EEIO) and the Iranian branch (Western flank) of Asian Summer monsoon anticyclone (ASMA). Outgoing longwave radiation (OLR) and cloud top pressure (CTP) show an anomalous dipole (quadrupole) pattern over the Equatorial Indian Ocean (EIO) during the wet (dry) phase of Kerala summer monsoon rainfall (KSMR). We found that KSMR is enhanced during intensification of the Iranian branch of ASMA and suppressed by anomalous deep convection over the EEIO. Further, anomalous lower level cyclonic circulation over the Western North Pacific (WNP) is pulling low‐level jet (LLJ) through Southern Peninsular India, hence playing a vital role in establishing the KSMR wet phase. Contrarily, the dry phase of KSMR is associated with weakening of the Iranian branch of ASMA in the upper level.

Response of macrozoobenthic communities to summer monsoon upwelling and related hypoxia in the south eastern Arabian Sea shelf

Coastal upwelling in the south eastern Arabian Sea (SEAS) leads to oxygen depletion over the continental shelf during the summer monsoon season (June–September), with latitudinal gradients in intensity. Based on two surveys in the onset (June) and peak (August) phases of the summer monsoon, the present study evaluates the response of macrozoobenthic communities (size >500 μm) to upwelling and consequent hypoxia (dissolved oxygen <0.2 ml/l) in the central sector of the SEAS shelf (10–12°N, 30–200 m). From the onset to the peak monsoon, macrozoobenthic density increased five-fold in the mid-shelf (50 m water depth), and nearly doubled in the outer shelf (100 m water depth) and shelf edge (200 m water depth). This was found to be a direct consequence of recruitment and proliferation of opportunistic polychaetes, particularly the spionid Paraprionospio pinnata, which was the single dominant species (52–78%) at all depths during the peak monsoon. With the establishment of the monsoon, the shelf communities (particularly 50–100 m depth sites) are thus transformed from relatively diverse assemblages to dense, single-species dominated ones. The shelf-edge communities (150–200 m depths), which are impacted with the perennial Arabian Sea oxygen minimum zone, and therefore harbour opportunist-dominated communities year-round. It is postulated that larvae of hypoxia-tolerant taxa are transported from the shelf edge by the process of upwelling onto the shelf. The settlement and survival of these larvae are regulated by the nature of shelf sediments and by the prevailing hypoxia. Thus, substantial recruitment of opportunists occurred in the outer and mid-shelf (50–100 m), but not in the inner shelf (30 m), where sedimentation from river discharge hindered settlement and survival of juveniles.

Active and break phases of the South American summer monsoon: MJO influence and subseasonal prediction

The role of the Madden–Julian Oscillation (MJO) in producing active and break periods of the South American (SA) monsoon and the performance of the ECMWF and NCEP models in predicting these periods at multiweek lead times are assessed. Two monsoon indices, based on precipitation and wind, are proposed to characterize these periods. The models represent well the observed association of active and break monsoon days with large scale convection and circulation anomalies. Although reproducing approximately the distribution of active and break days proportions in each phase of the MJO cycle, models produce a phase shift between observed and simulated distributions because they establish the teleconnection between Central Pacific and South America, as well as its impacts, sooner than in observations. The predictive skill of both rainfall and wind anomalies is limited to about 2 weeks, with the monsoon wind index displaying higher correlation score till week 3. The forecast performance is apparently not affected by initialization on active or break monsoon days. However, it is higher for prediction of lower precipitation in break days than heavier rainfall in active days. Wind is much better predicted than rainfall for active days, which could be used for extreme rainfall events forecast. Although relatively small at shorter lead times, the MJO contribution is the major source of rainfall predictability after week 3. To improve the multiweek prediction of SA monsoon, models need not only to predict correctly the MJO phase, but also to reproduce in the right phase the MJO-related SA rainfall anomalies.

Weather based rules for yellow mosaic disease prediction on soybean in Madhya Pradesh

The yellow mosaic disease (YMD) found to be one of the major constraints in soybean production in Madhya Pradesh. For timely management of this disease and for reducing the calendar based prophylactic pesticides spray, this study was conducted to screen out critical weather limits for peak YMD incidence. Field experiments as well as farmers field survey were conducted during kharif seasons of 2015 to 2019 in four districts viz. Tikamgarh, Chhatarpur, Datia and Jabalpur of Madhya Pradesh to record spacio-temporal variability of whitefly and YMD on soybean. Isolation of DNA from the infected soybean leaf samples was executed as per the standardized protocol of DNA isolation and molecular level identification of mungbean yellow mosaic India virus (MYMIV) was carried out as well. It was noted that the preceding week’s weather conditions significantly influenced the peak YMD incidence during the study periods. It was observed that maximum temperature, sunshine hours, number of whitefly population were positively related with YMD at the significant level; while the rainfall, evening relative humidity were negatively related. A weather based rule was worked out for peak prediction of YMD in soybean during the previous two weeks period when the following conditions were satisfied: Whitefly population per plant was above 12, maximum temperature was above 33°C and passive phase of monsoon was for more than a week. The transmission rate was found to be higher when dry period coincide with maximum temperature above 33°C for greater than or equal to 5days. This weather based rule for peak YMD prediction was also validated.