Sensible heat fluxes during the active and break phases of the southwest monsoon over the Indian region

Based on the theory given by Saltzman and Ashe (1976), sensible heat fluxes are calculated for the active and break phases of the southwest monsoon over the Indian region. The conclusion drawn is that the sensible heat flux is generally larger during the break monsoon situation when compared with that for the active monsoon situation. The synoptic heat flux is negligible when compared with mean and diurnal heat fluxes over the Indian region even during the monsoon season.

On the Maximum Cloud Zone and the ITCZ over Indian, Longitudes during the Southwest Monsoon

An investigation is presented of the daily variation of the maximum cloud zone (MCZ) and the 7W mb trough in the Northern Hemisphere over the Indian longitudes 70–90°E during April–October for 1973–77. It is found that during June–September there are two favorable locations for a MCZ over these longitudes–on a majority of days the MCZ is present in the monsoon zone north of 15°N, and often a secondary MCZ occurs in the equatorial region (0–10°N). The monsoon MCZ gets established by northward movement of the MCZ occurring over the equatorial Indian ocean in April and May. The secondary MCZ appears intermittently, and is characterized by long spells of persistence only when the monsoon MCZ is absent. In each of the seasons studied, the MCZ temporarily disappeared from the mean summer monsoon location (15–28°N) about four weeks after it was established near the beginning of July. It is reestablished by the northward movement of the secondary MCZ, which becomes active during the absence of the monsoon MCZ, in a manner strikingly similar to that observed in the spring to summer transition. A break in monsoon conditions prevails just prior to the temporary disappearance of the monsoon MCZ. Thus we conclude that the monsoon MCZ cannot survive for longer than a month without reestablishment by the secondary MCZ. Possible underlying mechanisms are also discussed.

Orographic effects on the southwest monsoon: A review

An overview of the problem of orographic effects on the southwest monsoon using the contributions of all the available analytical and numerical models is attempted. A quasi-geostrophic model is applied to deduce the effect of the topographic complex on the Indian peninsula. This model suggests that the southward bending of the low-level isobars on the peninsula can be ascribed to the topographically-induced southward velocity. This southward velocity triggers a Rossby wave to the east of the peninsula which is manifested as a trough on the southern Bay of Bengal.

Hydrographic observations and model simulation of the Bay of Bengal freshwater plume

Hydrographic observations were taken along two coastal sections and one open ocean section in the Bay of Bengal during the 1999 southwest monsoon, as a part of the Bay of Bengal Monsoon Experiment (BOBMEX). The coastal section in the northwestern Bay of Bengal, which was occupied twice, captured a freshwater plume in its two stages: first when the plume was restricted to the coastal region although separated from the coast, and then when the plume spread offshore. Below the freshwater layer there were indications of an undercurrent. The coastal section in the southern Bay of Bengal was marked by intense coastal upwelling in a 50 km wide band. In regions under the influence of the freshwater plume, the mixed layer was considerably thinner and occasionally led to the formation of a temperature inversion. The mixed layer and isothermal layer were of similar depth for most of the profiles within and outside the freshwater plume and temperature below the mixed layer decreased rapidly till the top of seasonal thermocline. There was no barrier layer even in regions well under the influence of the freshwater plume. The freshwater plume in the open Bay of Bengal does not advect to the south of 16 degrees N during the southwest monsoon. A model of the Indian Ocean, forced by heat, momentum and freshwater fluxes for the year 1999, reproduces the freshwater plume in the Bay of Bengal reasonably well. Model currents as well as the surface circulation calculated as the sum of geostrophic and Ekman drift show a southeastward North Bay Monsoon Current (NBMC) across the Bay, which forms the southern arm of a cyclonic gyre. The NBMC separates the very low salinity waters of the northern Bay from the higher salinities in the south and thus plays an important role in the regulation of near surface stratification. (c) 2007 Elsevier Ltd. All rights reserved.

Hydrographic observations and model simulation of the Bay of Bengal freshwater plume

Hydrographic observations were taken along two coastal sections and one open ocean section in the Bay of Bengal during the 1999 southwest monsoon, as a part of the Bay of Bengal Monsoon Experiment (BOBMEX). The coastal section in the northwestern Bay of Bengal, which was occupied twice, captured a freshwater plume in its two stages: first when the plume was restricted to the coastal region although separated from the coast, and then when the plume spread offshore. Below the freshwater layer there were indications of an undercurrent. The coastal section in the southern Bay of Bengal was marked by intense coastal upwelling in a 50 km wide band. In regions under the influence of the freshwater plume, the mixed layer was considerably thinner and occasionally led to the formation of a temperature inversion. The mixed layer and isothermal layer were of similar depth for most of the profiles within and outside the freshwater plume and temperature below the mixed layer decreased rapidly till the top of seasonal thermocline. There was no barrier layer even in regions well under the influence of the freshwater plume. The freshwater plume in the open Bay of Bengal does not advect to the south of 16 degrees N during the southwest monsoon. A model of the Indian Ocean, forced by heat, momentum and freshwater fluxes for the year 1999, reproduces the freshwater plume in the Bay of Bengal reasonably well. Model currents as well as the surface circulation calculated as the sum of geostrophic and Ekman drift show a southeastward North Bay Monsoon Current (NBMC) across the Bay, which forms the southern arm of a cyclonic gyre. The NBMC separates the very low salinity waters of the northern Bay from the higher salinities in the south and thus plays an important role in the regulation of near surface stratification. (c) 2007 Elsevier Ltd. All rights reserved.

Boundary-layer heights over the monsoon trough region during active and break phases

The thermodynamic structure and the heights of the boundary layer over the monsoon trough region of the Indian southwest monsoon are presented for the active and break phases of the monsoon. Results indicate significant and consistent variation in boundary-layer heights between the active and break phases.

Turbulent heat flux variation over the Monsoon-Trough region during MONTBLEX-90

Tower data collected during the Monsoon-Trough Boundary Layer Experiment (MONTBLEX-90) have been analysed to understand the observed structure of the surface layer over an arid region (Jodhpur) and a moist region (Kharagpur) during active and weak phases of the 1990 southwest monsoon. Turbulent heat and momentum fluxes are estimated by the eddy correlation method using sonic data. The turbulent momentum flux at both Jodhpur and Kharagpur was larger when the winds were stronger, reaching a maximum of the order of 0.5 N m(-2) on 5 and 6 August when a low pressure system was located over the region. The heat flux at Jodhpur is high during weak monsoon days, the maximum being 450 W m(-2), whereas during active days the flux never exceeds 200 W m(-2). At Kharagpur, the flux does not vary significantly between active and weak monsoon days, the maximum in either phase being 160 W m(-2) At Jodhpur, there is significant contrast in the near-surface air temperature, being higher during weak monsoon days as compared to active days. Cloud cover did not vary significantly in both the regions. The turbulent heat flux variation at both the sites appears to be correlated mainly with soil mixture, and less sensitive to cloud cover.

Genesis of the monsoon trough boundary layer experiment (MONTBLEX)

This paper sets out the motivation for carrying out an observational experiment on the atmospheric boundary layer along the monsoon trough, in the light of earlier studies of the atmospheric boundary layer in India and elsewhere, and the significant role that the trough has been shown to play as a key semi-permanent feature of the southwest monsoon. The scientific objectives of the experiment are set out, and its planning and execution are touched upon. Some of the gains resulting from the experiment are mentioned, and lessons for the future about the conduct of such programmes are drawn.

The June 2010 Nicobar Earthquake: Fault Reactivation on the Subducting Oceanic Plate

The similar to 1300-km-long rupture zone of the 2004 Andaman-Sumatra megathrust earthquake continues to generate a mix of thrust, normal, and strike-slip faulting events. The 12 June 2010 M(w) 7.5 event on the subducting plate is the most recent large earthquake on the Nicobar segment. The left-lateral faulting mechanism of this event is unusual for the outer-rise region, considering the stress transfer processes that follow great underthrusting earthquakes. Another earthquake (M(w) 7.2) with a similar mechanism occurred very close to this event on 24 July 2005. These earthquakes and most of their aftershocks on the subducting plate were generated by left-lateral strike-slip faulting on north-northeast-south-southwest oriented near-vertical faults, in response to north-northwest-south-southeast directed compression. Pre-2004 earthquake faulting mechanisms on the subducting oceanic plate are consistent with this pattern. Post-2004, left-lateral faulting on the subducting oceanic plate clusters between 5 degrees N and 9 degrees N, where the 90 degrees E ridge impinges the trench axis. Our study observes that the subducting plate off the Sumatra and Nicobar segments behaves similarly to a chip of the India-Australia plate, deforming in response to a generally northwest-southeast oriented compression, an aspect that must be factored into the plate deformation models.

Large scale features and assessment of spatial scale correspondence between TMPA and IMD rainfall datasets over Indian landmass

Daily rainfall datasets of 10 years (1998-2007) of Tropical Rainfall Measuring Mission (TRMM) Multi-satellite Precipitation Analysis (TMPA) version 6 and India Meteorological Department (IMD) gridded rain gauge have been compared over the Indian landmass, both in large and small spatial scales. On the larger spatial scale, the pattern correlation between the two datasets on daily scales during individual years of the study period is ranging from 0.4 to 0.7. The correlation improved significantly (similar to 0.9) when the study was confined to specific wet and dry spells each of about 5-8 days. Wavelet analysis of intraseasonal oscillations (ISO) of the southwest monsoon rainfall show the percentage contribution of the major two modes (30-50 days and 10-20 days), to be ranging respectively between similar to 30-40% and 5-10% for the various years. Analysis of inter-annual variability shows the satellite data to be underestimating seasonal rainfall by similar to 110 mm during southwest monsoon and overestimating by similar to 150 mm during northeast monsoon season. At high spatio-temporal scales, viz., 1 degrees x1 degrees grid, TMPA data do not correspond to ground truth. We have proposed here a new analysis procedure to assess the minimum spatial scale at which the two datasets are compatible with each other. This has been done by studying the contribution to total seasonal rainfall from different rainfall rate windows (at 1 mm intervals) on different spatial scales (at daily time scale). The compatibility spatial scale is seen to be beyond 5 degrees x5 degrees average spatial scale over the Indian landmass. This will help to decide the usability of TMPA products, if averaged at appropriate spatial scales, for specific process studies, e.g., cloud scale, meso scale or synoptic scale.

Phytoplankton size structure in the southern Bay of Bengal modified by the Summer Monsoon Current and associated eddies: Implications on the vertical biogenic flux

The present study combines field and satellite observations to investigate how hydrographical transformations influence phytoplankton size structure in the southern Bay of Bengal during the peak Southwest Monsoon/Summer Monsoon (July-August). The intrusion of the Summer Monsoon Current (SMC) into the Bay of Bengal and associated changes in sea surface chemistry, traceable eastward up to 90 degrees E along 8 degrees N, seems to influence biology of the region significantly. Both in situ and satellite (MODIS) data revealed low surface chlorophyll except in the area influenced by the SMC During the study period, two well-developed cydonic eddies (north) and an anti-cyclonic eddy (south), closely linked to the main eastward flow of the SMC, were sampled. Considering the capping effect of the low-saline surface water that is characteristic of the Bay of Bengal, the impact of the cyclonic eddy, estimated in terms of enhanced nutrients and chlorophyll, was mostly restricted to the subsurface waters (below 20 m depth). Conversely, the anti-cyclonic eddy aided by the SMC was characterized by considerably higher nutrient concentration and chlorophyll in the upper water column (upper 60 m), which was contrary to the general characteristic of such eddies. Albeit smaller phytoplankton predominated the southern Bay of Bengal (60-95% of the total chlorophyll), the contribution of large phytoplankton was double in the regions influenced by the SMC and associated eddies. Multivariate analysis revealed the extent to which SMC-associated eddies spatially influence phytoplankton community structure. The study presents the first direct quantification of the size structure of phytoplankton from the southern Bay of Bengal and demonstrates that the SMC-associated hydrographical ramifications significantly increase the phytoplankton biomass contributed by larger phytoplankton and thereby influence the vertical opal and organic carbon flux in the region. (C) 2014 Elsevier B.V. All rights reserved.

Diurnal and seasonal variation of mixing ratio and delta C-13 of air CO2 observed at an urban station Bangalore, India

We present here observations on diurnal and seasonal variation of mixing ratio and delta C-13 of air CO2, from an urban station-Bangalore (BLR), India, monitored between October 2008 and December 2011. On a diurnal scale, higher mixing ratio with depleted delta C-13 of air CO2 was found for the samples collected during early morning compared to the samples collected during late afternoon. On a seasonal scale, mixing ratio was found to be higher for dry summer months (April-May) and lower for southwest monsoon months (June-July). The maximum enrichment in delta C-13 of air CO2 (-8.04 +/- 0.02aEuro degrees) was seen in October, then delta C-13 started depleting and maximum depletion (-9.31 +/- 0.07aEuro degrees) was observed during dry summer months. Immediately after that an increasing trend in delta C-13 was monitored coincidental with the advancement of southwest monsoon months and maximum enrichment was seen again in October. Although a similar pattern in seasonal variation was observed for the three consecutive years, the dry summer months of 2011 captured distinctly lower amplitude in both the mixing ratio and delta C-13 of air CO2 compared to the dry summer months of 2009 and 2010. This was explained with reduced biomass burning and increased productivity associated with prominent La Nina condition. While compared with the observations from the nearest coastal and open ocean stations-Cabo de Rama (CRI) and Seychelles (SEY), BLR being located within an urban region captured higher amplitude of seasonal variation. The average delta C-13 value of the end member source CO2 was identified based on both diurnal and seasonal scale variation. The delta C-13 value of source CO2 (-24.9 +/- 3aEuro degrees) determined based on diurnal variation was found to differ drastically from the source value (-14.6 +/- 0.7aEuro degrees) identified based on seasonal scale variation. The source CO2 identified based on diurnal variation incorporated both early morning and late afternoon sample; whereas, the source CO2 identified based on seasonal variation included only afternoon samples. Thus, it is evident from the study that sampling timing is one of the important factors while characterizing the composition of end member source CO2 for a particular station. The difference in delta C-13 value of source CO2 obtained based on both diurnal and seasonal variation might be due to possible contribution from cement industry along with fossil fuel / biomass burning as predominant sources for the station along with differential meteorological conditions prevailed.

Comprehensive evaluation of multisatellite precipitation estimates over India using gridded rainfall data

This study presents a comprehensive evaluation of five widely used multisatellite precipitation estimates (MPEs) against 1 degrees x 1 degrees gridded rain gauge data set as ground truth over India. One decade observations are used to assess the performance of various MPEs (Climate Prediction Center (CPC)-South Asia data set, CPC Morphing Technique (CMORPH), Precipitation Estimation From Remotely Sensed Information Using Artificial Neural Networks, Tropical Rainfall Measuring Mission's Multisatellite Precipitation Analysis (TMPA-3B42), and Global Precipitation Climatology Project). All MPEs have high detection skills of rain with larger probability of detection (POD) and smaller ``missing'' values. However, the detection sensitivity differs from one product (and also one region) to the other. While the CMORPH has the lowest sensitivity of detecting rain, CPC shows highest sensitivity and often overdetects rain, as evidenced by large POD and false alarm ratio and small missing values. All MPEs show higher rain sensitivity over eastern India than western India. These differential sensitivities are found to alter the biases in rain amount differently. All MPEs show similar spatial patterns of seasonal rain bias and root-mean-square error, but their spatial variability across India is complex and pronounced. The MPEs overestimate the rainfall over the dry regions (northwest and southeast India) and severely underestimate over mountainous regions (west coast and northeast India), whereas the bias is relatively small over the core monsoon zone. Higher occurrence of virga rain due to subcloud evaporation and possible missing of small-scale convective events by gauges over the dry regions are the main reasons for the observed overestimation of rain by MPEs. The decomposed components of total bias show that the major part of overestimation is due to false precipitation. The severe underestimation of rain along the west coast is attributed to the predominant occurrence of shallow rain and underestimation of moderate to heavy rain by MPEs. The decomposed components suggest that the missed precipitation and hit bias are the leading error sources for the total bias along the west coast. All evaluation metrics are found to be nearly equal in two contrasting monsoon seasons (southwest and northeast), indicating that the performance of MPEs does not change with the season, at least over southeast India. Among various MPEs, the performance of TMPA is found to be better than others, as it reproduced most of the spatial variability exhibited by the reference.

Fingerprinting environmental conditions and related stress using stable isotopic composition of rice (Oryza sativa L.) grain organic matter

Sea level rise (SLR) is a primary factor responsible for inundation of low-lying coastal regions across the world, which in turn governs the agricultural productivity. In this study, rice (Oryza sativa L.) cultivated seasonally in the Kuttanad Wetland, a SLR prone region on the southwest coast of India, were analysed for oxygen, hydrogen and carbon isotopic ratios (delta O-18, delta H-2 and delta C-13) to distinguish the seasonal environmental conditions prevalent during rice cultivation. The region receives high rainfall during the wet season which promotes large supply of fresh water to the local water bodies via the rivers. In contrast, during the dry season reduced river discharge favours sea water incursion which adversely affects the rice cultivation. The water for rice cultivation is derived from regional water bodies that are characterised by seasonal salinity variation which co-varies with the delta O-18 and delta H-2 values. Rice cultivated during the wet and the dry season bears the isotopic imprints of this water. We explored the utility of a mechanistic model to quantify the contribution of two prominent factors, namely relative humidity and source water composition in governing the seasonal variation in oxygen isotopic composition of rice grain OM. delta C-13 values of rice grain OM were used to deduce the stress level by estimating the intrinsic water use efficiency (WUEi) of the crop during the two seasons. 1.3 times higher WUE, was exhibited by the same genotype during the dry season. The approach can be extended to other low lying coastal agro-ecosystems to infer the growth conditions of cultivated crops and can further be utilised for retrieving paleo-environmental information from well preserved archaeological plant remains. (c) 2015 Elsevier Ltd. All rights reserved.