Head Bay Of Bengal Research Articles

Tidal propagation and its non-linear characteristics in the Head Bay of Bengal

The Head Bay of Bengal is highly vulnerable to flooding events caused due to monsoons, cyclones and sea-level rise, owing to its funnel-like shape, high tidal range, presence of numerous river drainage systems, low-lying topography, and shallow bathymetry. Tides dominate the hydrodynamic behaviour and coastal processes in this region and its propagation is quite distinct. The present study uses ADCIRC hydrodynamic model customized for the Head Bay of Bengal, discretized using unstructured finite elements and validated against limited available observations in this data sparse region. The water-level elevations derived from ADCIRC simulation was used to understand the pattern of non-linear tidal propagation with respect to complex coastal geomorphology prevalent in this region. The study finds a marginal amplification of diurnal tide, nearly double amplification of semi-diurnal components, and existence of a degenerate amphidromic point near Meghna delta consistent with previous studies. The spatial and temporal variability of tidal spectral components were examined by applying the techniques of wavelet, harmonic, and time-series analysis at various locations. Maximum amplification of tides occurs at the head of the bay, along a zone enclosing the mouth of tidal inlets; and for regions northward, the tides decay with progressively increasing phase lags. The study signifies dominance of a forced fortnightly tide and tidal asymmetry leading to flood-dominance in the rivers Hooghly, Meghna, and Tetulia. The non-linear properties of tides have been elucidated, and their origin and spatio-temporal variability in these riverine systems were further investigated. Shallow depth and sharp depth gradients were discerned to be the important conditions for the origin of non-linear components. It has been deduced that non-linear tides are generated in regions where propagating tides are accumulated, and amplified in regions where they are funneled. A study of tidal energetics show that maximum rate of tidal energy dissipation is deep inside the rivers at head of the bay. The spatial variability analysis of tidal form factor shows an overall semi-diurnal dominance in the Head Bay.

A Transformed Coordinate Model to Predict Tide and Surge Along the Head Bay of Bengal- Application to Cyclone of 1991 and 1970

The head Bay of Bengal region is one of the most vulnerable regions for extreme water levels associated with severe tropical cyclones. The shallow nature of the head Bay, presence of a large number of deltas formed by major rivers and high tidal range are responsible for storm surge flooding in the region. Specifically, the rise and fall of tidal phases influence the height, duration, and arrival time of peak surge along the coast. The objective of the present study is to evaluate the tide-surge interaction during the super cyclone of 1991 and 1970. A transformed coordinate model is developed to estimate the possible water levels along the coast of Bangladesh.GANIT J. Bangladesh Math. Soc.Vol. 35 (2015) 7-25

The Catastrophe over Jammu and Kashmir in September 2014: A Meteorological Observational Analysis

An observational analysis of the catastrophic rainstorm during 4-6 September 2014 over Jammu and Kashmir (J&K) presented in this study shows that the event was unprecedented in terms of the 24, 48 and 72 h accumulated rainfall. The 24 h accumulated rainfall exceeded the previously determined one-day severe rainstorm limits of 20 cm for a number of stations on 5 and 6 September 2014. Weekly cumulative rainfall (4-10 September 2014) exceeded the average rainfall of the entire monsoon season (June-September) in 6 out 18 districts of the state. A number of stations recorded all-time highest 24, 48 and 72 h accumulated rainfall during the week. Analysis of short-duration intensity of rainfall shows that the heaviest rain-rate (35 mm/h) was recorded over Kawa (Udhampur district). The rain-rate remained less than 20 mm/h at other stations. As Kawa is on the windward side of the Pir Panjal Range, orography seems to have played a significant role. The analysis of synoptic conditions leading to unprecedented rainfall shows that the rains were caused by the interaction of the westwardmoving monsoon low pressure area across central and northwest India and a eastward-moving deep trough in the mid-tropospheric westerlies. The additional low pressure areas that formed over Saurashtra and Kutch on 3 September 2014 and over head Bay of Bengal on 5 September 2014, ensured the vigour of the event was maintained through strong wind and moisture flux in J&K. NWP models could capture heavy rains over J&K only in day 1 forecast.

Mesoscale simulation of a flood producing rainstorm over Rangamati, Bangladesh using MM5 model

Mesoscale simulation of a flood producing rainstorm of 21 June 2004 over Rangamati, Bangladesh has been carried out using the MM5 Model. The model performance was evaluated by examining the different predicted and model derived parameters. The MM5 model suggests that the rainstorm over Rangamati was resulted by the large scale weather circulation associated with active conditions of southwest summer monsoon over the head Bay of Bengal which generated favourable conditions for developing the severe mesoscale convections. The strong confluence of southwesterly low level flow transporting large amount of moisture (which vertically extended up to 350 hPa) from the Bay of Bengal towards southeast Bangladesh and its neighbourhoods appears to be one of the striking features. An elongated vorticity maximum of the order of 30-40×10 -5 s -1 was observed across the Rangamati. A well-defined area of low level convergence (-25×10 -5 s -1 ) is associated with strong divergence (25×10 -5 s -1 ) in the high level (200 hPa) has provided favourable conditions for strong convection. The model simulated realistic pattern of rainfall over Rangamati as compared with TRMM observations. Considering the quantitative comparison of the domain average rainfall with TRMM observations, it is found that the model over predicts by 12% as compared with TRMM observed rainfall.

Numerical Study on Storm Surge and Associated Coastal Inundation for 2009 AILA Cyclone in the Head Bay of Bengal

Storm surge from tropical cyclones leads to extreme coastal flooding along low gradient shorelines, barriers, and deltaic system. Typically low-lying deltaic environment comprises of soft sediments and are densely populated, and these regions are quite dynamic and changes in coastal geomorphology are quite rapid from impact of each cyclone. The horizontal extent of coastal flooding depends on factors such as cyclone intensity, wind speed, and onshore topography. One can expect the worst possible scenario of coastal flooding when the time of storm surge coincides with astronomical high tides. Flooding potential from storm surge is quite high for coastal regions in West Bengal, Bangladesh and Myanmar. High tidal amplitude (≈ 4-5 m) along with complex coastline geometry comprising of several inlets, tidal creeks, and river drainage systems cause differential flooding along the onshore regions. The risk level is quite high as flooding from surge can penetrate the river system and inundate low-lying areas along riverbanks. The current study investigates the scenario of coastal flooding associated with the 2009 Aila cyclone for the West Bengal and Bangladesh regions. The numerical Advanced Circulation (ADCIRC) model estimated the storm surge and associated onshore inundation. The computed maximum storm surge was around 4 m along the Sunderban region. The estimated average inland inundation was about 350 m along the coastal belts and major riverbanks. The inundation occurred along 500km coastal stretch, with inland penetration distance reaching a maximum of about 600 m at certain low-lying areas in West Bengal and Bangladesh. This study emphasizes the need for inundation mapping in vulnerable coastal areas, having practical significance in coastal disaster management

Seasonal Prediction of Distinct Climate Anomalies in Summer 2010 over the Tropical Indian Ocean and South Asia

The characteristics and predictability of climate anomalies over the tropical Indian Ocean (TIO) and South Asian region during the boreal summer (June-July-August) of 2010 are investigated on the basis of atmospheric regional model simulations and five forecasts obtained from Asia-Pacific Economic Cooperation Climate Center coupled models. The robust features of summer 2010 are the basin-wide TIO warming and enhanced (suppressed) rainfall over the north Indian Ocean and maritime continent (head Bay of Bengal and parts of monsoon trough region). Our regional atmospheric model experiments corroborate that rainfall over South Asia was mostly determined by the TIO sea surface temperature (SST) warming during summer 2010. Most of the coupled models and their multi-model ensemble (MME) used in this study successfully predict the robust features over the TIO and/or South Asian region with 01 May 2010 initial condition. The positive rainfall anomalies over the west coast of India, southern Peninsular India, and central Bay of Bengal are qualitatively well predicted by the MME. Suppressed rainfall over the northeast Bay of Bengal associated with the northwestward extension of the northwest Pacific ridge is also reasonably predicted by the MME. On the other hand, the MME has a moderate skill in predicting positive rainfall anomalies over the convective zone of southeast TIO due to weak local SST warming. Further, the coupled models and their MME fail to predict the anomalous positive rainfall in northern Pakistan because of their inability in predicting mid-latitude circulation anomalies. This study reveals that the predictive skill of rainfall and circulation anomalies during summer 2010 over the TIO and South Asia is largely attributable to the Indian Ocean basin-wide warming during the decay phase of El Niño. These results indicate that the accurate simulation of the TIO SST by coupled models is critical in determining the 2010 South Asian summer monsoon rainfall.

Emerging role of Indian ocean on Indian northeast monsoon

This study examines the emerging role of Indian Ocean sea surface temperature (SST) on the inter-annual variability (IAV) of Indian north-east monsoon rainfall (NEMR). The IAV of NEMR is associated with the warm SST anomaly over east Bay-of-Bengal (BoB) (88.5oE–98.5oE; 8.5oN–15.5oN) and cool SST anomaly over east equatorial Indian Ocean (80.5oE–103.5oE; 6.5oS–3.5oN). The gradient of SST between these boxes (i.e. northern box minus southern box) shows strong and robust association with the Indian NEMR variability in the recent decades. For establishing the teleconnections, SST, mean sea level pressure, North Indian Ocean tropical storm track, and circulation data have been used. The study reveals that during the positive SST gradient years, the inter-tropical convergence zone (ITCZ) shifts northwards over the East Indian Ocean. The tropical depressions, storms and cyclones formed in the North Indian Ocean moves more zonally and strike the southern peninsular India and hence excess NEMR. While, during the negative SST gradient years, the ITCZ shifts southwards over the Indian Ocean. The tropical depressions, storms and cyclones formed in the North Indian Ocean moves more northwestward direction and after crossing 15oN latitude re-curve to north-east direction towards head BoB and misses southern peninsular India and hence, deficient NEMR.

Impact of Northwest Pacific anticyclone on the Indian summer monsoon region

Influence of northwest (NW) Pacific anticyclone on the Indian summer monsoon (ISM), particularly over the head Bay of Bengal and monsoon trough region, is investigated. Strong NW Pacific anticyclone during summer induces negative precipitation anomalies over the head Bay of Bengal and Gangetic Plain region. Westward extension of moisture divergence and dry moisture transport from NW Pacific associated with anticyclone (ridge) and local Hadley cell-induced subsidence are responsible for these negative precipitation anomalies. The impact is maximum when the anticyclone and Indian Ocean basin warming co-occur. This contributes significantly to year-to-year variability of ISM.

Do cold, low salinity waters pass through the Indo-Sri Lanka Channel during winter?

During winter, along the east coast of India, the near-surface flow is characterized by the southward-flowing East India Coastal Current (EICC) which bends around Sri Lanka and enters into the south-eastern Arabian Sea (AS). This current carries cooler, low-salinity waters from the head Bay of Bengal (BoB) into the south-eastern AS. But due to a lack of any direct in situ measurements, it is not clear whether any part of this current that flows through the Indo-Sri Lanka Channel (ISLC) is significant. An attempt is made in this study to look for any observational evidence for the southward flow of cooler, low salinity waters through the ISLC during winter. In the absence of direct in situ measurements on the observed currents in the non-navigable shallow ISLC, the observed high resolution, advanced very high resolution radiometer (AVHRR) sea surface temperature (SST), and sea-viewing wide field-of-view sensor (SeaWiFS) chlorophyll-a and historic sea surface salinity (SSS) data are utilized as tracers to track any southward water flow through the Pamban Pass and Adam's Bridge in the ISLC. The analysis suggests that both the non-navigable shallow Pamban Pass and the Adam's Bridge in the ISLC act as barriers and limit the southward flow of cooler, low salinity waters into the Gulf of Mannar in the south during winter.

Spatial heterogeneities in aerosol properties over Bay of Bengal inferred from ship-borne and MODIS observations during ICARB-W cruise campaign: Implications to radiative forcing

Comprehensive investigations during the last decade have clearly established that aerosols have a significant impact on the climate. This paper reports the results of the spatial variations in aerosol optical depth (AOD) and fine mode fraction (FMF) characteristics as a function of latitude and longitude over the Bay of Bengal (BoB) and the Northern Indian Ocean (NIO) during ICARB-W cruise period of 27th December 2008–30th January 2009 from onboard Sunphotometer and MODIS (Terra, Aqua) satellite measurements. Very high AOD 500 (0.7–0.8) occurred over the north head BoB adjacent to the northeastern Indian coast and the lowest AOD 500 (0.1–0.2) occurred in central BoB far away from the coasts, and in a small area in the northeastern part close to Myanmar coast as well as over NIO. The highest values (as high as 1.2) of Ångström exponent, α occurring over northeast BoB (regions close to Bangladesh and Myanmar) indicate relative abundance of accumulation mode particles and very low values of α (below 0.7) over central part of BoB as well as southern BoB/NIO suggesting dominance of coarse-mode sea spray aerosols. Terra/Aqua MODIS AOD 550 and cruise measured AOD 500 using Sunphotometer showed good agreement ( R 2 = 0.92) over the BoB. The total mass concentrations over BoB during cruise period were remarkably high, with a mean value of 28.4 ± 5.7 μg m −3 . Aerosol FMF was higher than 0.7 over the BoB, while FMF over NIO was about 0.5. NCEP reanalysis data on winds at 850 hPa, along with 5-days airmass back trajectories via HYSPLIT model, suggested transport of continental aerosols from the central and northern India over the BoB by the strong westerly/northwesterly winds. Regionally averaged clear sky aerosol (net) forcing over BoB during the winter is −28.9 W m −2 at the surface and −10.4 W m −2 at the top of the atmosphere whereas, the ARF values estimated over NIO at TOA, surface and in the atmosphere are −6.4, −18.3 and +11.9 W m −2 , respectively. ► The spatial variation of columnar measurements of AOD 500 observed high values over northern/northwestern parts of the BoB in which AODs were as high as ∼0.7; while the low values of AOD 500 (<0.2) were observed in the northeastern regions of BoB (Myanmar/Bangladesh coast) and southern BoB which opens to the vast Indian Ocean. ► α showed highest values of ∼1.5, even though generally high values prevailed over the eastern as well as northern coastal regions of India and β followed the pattern similar to that of AOD with a mean value of 0.17 ± 0.09 over BoB. ► Total mass concentration showed low values over the central BoB and equatorial Indian Ocean with a mean value of 23.6 ± 4.3 μg m −3 , high values over northern BoB (mean 45.4 ± 10.2 μg m −3 ) and moderate mass concentrations with a mean value of 32.3 ± 7.1 μg m −3 , persisted along the east coast of India with higher values very near to the ports. ► FMF is found to be higher than 0.7 over BoB indicates the dominance of fine mode particles which were produced mainly due to manmade sources in the surrounding regions and got transported across the oceanic region. ► The diurnally averaged aerosol forcing over entire BoB at the surface is −28.9 W m −2 and at the TOA is −10.4 W m −2 . The difference between the TOA and the surface forcing is +18.5 W m −2 which is absorbed in the atmosphere. Similarly, the ARF values estimated over NIO at TOA, surface and in the atmosphere are −6.4, −18.3 and +11.9 W m −2 , respectively.

Study of Dynamical Structure of an Unusual Monsoon Depression formed over the Bay of Bengal during August 2006

Monsoon depressions, the main rain-producing systems over the Indian region along and near their tracks, are found to intensify the monsoon circulation by organizing low-level convergence. The normal track of the monsoon depressions is along the position of the monsoon trough at the surface, i.e., northwestward from the Head Bay of Bengal. Most of the monsoon depressions dissipate within one or two days after landfall. An unusual monsoon depression formed in the Bay of Bengal during the 1st week of August 2006 causing heavy to very heavy rainfall over Madhya Pradesh, Maharashtra and Gujarat States of India. The track of this depression was anomalously southward from the mean track of the August depressions. It maintained its intensity during its longer travel. This paper addresses some of the dynamical characteristics of the depression in relation to its southward/westward track and longer travel. It is observed that horizontal advection of absolute vorticity above 550 hPa (below 600 hPa) along west (east) of the depression and maximum divergence of absolute vorticity below 400 hPa dominated for the westward movement of the depression. Increased moisture supply from the Arabian Sea (after the landfall of the depression) helped to maintain the intensity of the system throughout its long travel. The energy conversion terms revealed the strengthening of the zonal flow at higher levels prior to the formation of the depression.

Spatial distribution and spectral characteristics of aerosol single scattering albedo over the Bay of Bengal inferred from shipborne measurements

Aerosol single scattering albedos (SSA) were deduced from extensive and collocated measurements of spectrally resolved aerosol scattering (σsca) and absorption (σabs) coefficients, carried out for the first time over the entire Bay of Bengal (BoB). Notwithstanding the high values of σsca, and σabs, comparatively higher values of SSA were noticed over the head BoB (north of 16°N) and coastal regions while lower values of SSA persisted over the Central BoB indicating the dominance of absorbing aerosols far away from continental source regions. At mid visible wavelength (550 nm) SSA ranged from 0.84 to 0.96 over different parts of the BoB showing large heterogeneity. However, more than 80% of the values lay between 0.9 and 0.95. Spectral variation of SSA was distinctly different over the northern and southern BoB.

Pattern characteristics of Indian monsoon rainfall using principal component analysis (PCA)

In the present study the Principal Component Analysis (PCA) is used to determine the dominant rainfall patterns from rainfall records over India. Pattern characteristics of seasonal monsoon rainfall (June–September) over India for the period 1940 to 1990 are studied for 68 stations. The stations have been chosen on the basis of their correlation with all India seasonal rainfall after taking the ‘ t’ Student distribution test (5% level). The PCA is carried out on the rainfall data to find out the nature of rainfall distribution and percentage of variance is estimated. The first principal component explains 55.50% of the variance and exhibits factor of one positive value throughout the Indian subcontinent. It is characterized by an area of large positive variation between 10°N and 20°N extending through west coast of India. These types of patterns mostly occur due to the monsoon depression in the head Bay of Bengal and mid-tropospheric low over west coast of India. The analysis identifies the spatial and temporal characteristics of possible physical significance. The first eight principal component patterns explain for 96.70% of the total variance.

Dynamics of monsoon trough over northwest and adjoining westcentral Bay of Bengal

&#x0D; Daily variations of surface and thermodynamical parameters in the vicinity of eastern end of the monsoon trough located over northwest and adjoining westcentral Bay of Bengal have been studied. A model sequence for the strengthening/weakening of the monsoon trough over head Bay of Bengal, which is the seat of cyclogenesis during southwest monsoon, has been proposed. Monsoon trough activity over the sea seems to possess a 4 day cycle. The primary forcing for the activation of monsoon trough appears to be the sea surface temperature (SST). The lag between peak SST and minimum surface pressure appears to be 2 days. The lag between peak SST and enhanced rainfall over Orissa seems to be 4 days.

Analysis of moist convective instability over Indian monsoon region and neighbourhood

The nature of deep convection over the Indian monsoon region and neighbourhood during different seasons is investigated by analysing Dry Static Energy (DSE), Moist Static Energy (MSE), Precipitable Water Content (PWC) and Convective Available Potential Energy (CAPE) computed from 13 years (1982-1994) monthly mean data obtained from the National Centre for Environmental Prediction (NCEP) reanalysis.&#x0D; &#x0D; It is seen from this study that the mean atmosphere over the Indian monsoon region is convectively unstable at lower levels during all seasons with highest degree of instability and maximum PWC during the monsoon season compared to other seasons. The results also show that during the monsoon season from June to September the convectively most unstable region is situated over the Head Bay of Bengal (HBOB) region and decreases gradually from West Pacific (WP), Equatorial South Indian Ocean (ESIO) and to Arabian Sea (AS) regions. Similarly the CAPE value is also highest over HBOB region and it is about 362 Joules/kg (24%) more than the CAPE value over WP region. The composite MSE profiles for strong and weak monsoon years indicate higher values of MSE at all levels during the strong monsoon years compared to weak monsoon years. It is also observed that the surface MSE and PWC over Indian monsoon region during June to September show significant positive correlation with All India Summer Monsoon Rainfall (AISMR).

Forecasting Indian summer monsoon rainfall by outgoing longwave radiation over the Indian Ocean

The satellite derived outgoing longwave radiation (OLR) over the Indian Ocean (30°N–30°S and 40°E–100°E) from 1974 to 1996 has been analysed for the relationship with the Indian summer monsoon total (June–September) rainfall. The OLR of two regions appears to be related to summer monsoon rainfall. One of the regions is located over the Head Bay of Bengal (near 22.5°N and 92.5°E) during May and the other one over the south Indian Ocean (near 30°S and 97.5°E) during April. The average OLR (index) for these two regions shows a strong and stable relationship with the Indian summer monsoon rainfall and they are found to be independent. A multiple linear regression equation is developed to predict the Indian summer monsoon rainfall using these indexes and the empirical relations are verified on independent data. Good results were obtained in forecasting the summer monsoon rainfall for the whole of India. The forecast of summer monsoon rainfall for west-central India and all-India rainfall for July also appears to be encouraging. The indexes, thus, seem to be useful in long-range forecasting of the Indian summer monsoon rainfall. Copyright © 2000 Royal Meteorological Society

Thermodynamic structure of the boundary layer in relation to a monsoon depression over the Bay of Bengal ? A case study

Aerological observations carried out on board ORV Sagarkanya at a stationary location (20° N, 89° E) over the Head Bay of Bengal during August 18–21, 1990 were analysed to study the thermodynamic structure of the marine boundary layer in relation to a monsoon depression which formedin situ with its centre at 20° N, 88° E. The q(mixing ratio) reversal observed at 850 hPa prior to formation of the low pressure area shifted to a higher level (h<700 hPa) with the formtion of the low. Positive buoyancy below 850 hPa prior to the formation of the low indicated conditions favourable for deep convection. When the low pressure area intensified into a depression, negative buoyancy was observed at lower levels.

Diurnal variation of cloudiness during southwest monsoon season using I NSAT-IB radiance data

An attempt has been made to study the diurnal variation of convective clouds. For this study 3 hourly full resolution infrared data of INSAT-IB have been used for the monsoon season (Jun-Sep) of 1987-89. The area of study extends from 35°N to 25°S and 40oE to l00oE, which is subdivided into small areas of 2.5x 2.5 Lat./Long. Mean temperature and the fractional area covered by clouds colder than a given threshold temperature over each sub area are the parameters used for this study. Two threshold temperatures. namely 265°K &amp; 235oK are chosen to represent convective clouds and deep convective clouds respectively. Using the three hourly observations, times of maximum and minimum convective activity are also obtained. Maximum convective activity is observed over head Bay of Bengal at about noon and this maximum migrates westward onto land till midnight and swings back to oceanic area by morning. This eastwest oscillation is less over equatorial regions (open ocean).

Bay depressions during monsoon 1978

Depressions forming in the head Bay of Bengal and moving westnorthwards across central parts of, India exert a profound influence on monsoon activity over the country. Some depressions form in situ in the Bay of Bengal. Some others originate in South China Sea. Or are remnants of typhoons in the. Southwest Pacific, which travel westwards into the Bay from across Thailand and Burma as weak tropical disturbances. The paper presents the synoptic features that favour. Formation, intensification and movement of such depressions during June to September of 1978. These pre-conditions are in conformity with earlier findings.

On development of a monsoon low over northwest India

A case of an unusual formation of a monsoon low pressure area over Rajasthan and neigh-bourhood during its vertical structure and the associated rainfall resemble a typical monsoon depression. Development took place when a mid-tropospheric vortex moved in a southerly direction from the Jammu &amp; Kashmir region and was superposed on the monsoon trough which had moved into Rajasthan due to movement of a Bay depression. The low developed as a downward propagaton of the mid-tropospheric vortex in a manner resembling the formation of a typical monsoon depression over the head Bay of Bengal. The low pressure area caused excessive rainfall over Rajasthan. The low showed an unusual movement initially in an easterly direction giving the impression of being a western disturbance, then in a westerly direction and finally in a southwesterly direction. The movement is examined with reference to the 200 mb flow over the area.