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Estuaries are known to be strong source for atmospheric CO2, however, little information is available from Indian estuaries. In order to quantify CO2 emissions from the Indian estuaries, samples were collected at 27 estuaries all along the Indian coast during discharge (wet) period. The emissions of CO2 to the atmosphere from Indian estuaries were 4–5 times higher during wet than dry period. The pCO2 ranged between ∼300 and 18492 μatm which are within the range of world estuaries. The mean pCO2 and particulate organic carbon (POC) showed positive relation with rate of discharge suggesting availability of high quantities of organic matter that led to enhanced microbial decomposition. The annual CO2 fluxes from the Indian estuaries, together with dry period data available in the literature, amounts to 1.92 TgC which is >10 times less than that from the European estuaries. The low CO2 fluxes from the Indian estuaries are attributed to low flushing rates and less human settlements along the banks of the Indian estuaries. Key Points Indian estuaries emits ~2TgC annually to atmosphere An order of magnitude high fluxes during discharge period Intensive microbial decomposition of organic matter is the major source

In this study, an attempt has been made to examine the relationship between summer monsoon rainfall (June–September) and the total number of depressions, cyclones and severe cyclones (TNDC) over Bay of Bengal during the post-monsoon (October–December) season. The seasonal rainfall of the subdivisions (located in south India) (referred as rainfall index – RI), is positively and significantly correlated ( r =0.59; significant at >99% level) with the TNDC during the period, 1984–2013. By using the first differences (current season minus previous season), the correlations are enhanced and a remarkably high correlation of 0.87 is observed between TNDC and RI for the recent period, 1993–2013. The average seasonal genesis potential parameter (GPP) showed a very high correlation of 0.84 with the TNDC. A very high correlation of 0.83 is observed between GPP and RI for the period, 1993–2013. The relative vorticity and mid-tropospheric relative humidity are found to be the dominant terms in GPP. The GPP was 3.5 times higher in above (below) normal RI in which TNDC was 4 (2). It is inferred that RI is playing a key role in TNDC by modulating the environmental conditions (low level vorticity and relative humidity) over Bay of Bengal during post-monsoon season which could be seen from the very high correlation of 0.87 (which explains 76% variability in TNDC). For the first time, we show that RI is a precursor for the TNDC over Bay of Bengal during post-monsoon season. Strong westerlies after the SW monsoon season transport moisture over the subdivisions towards Bay of Bengal due to cyclonic circulation. This circulation favours upward motion and hence transport moisture vertically to mid-troposphere which causes convective instability and this in turn favour more number of TNDC, under above-normal RI year.

Time-series observations were conducted off Visakhapatnam, central west coast of Bay of Bengal, from October 2007 to April 2009 to examine the influence of physical and atmospheric processes on water column nutrients biogeochemistry. The thermal structure displayed inversions of 0.5 to 1.0° C during winter and were weaker in summer. The water column was vertically stratified during the entire study period and was stronger during October–November 2007 and August–December 2008 compared to other study periods. High concentrations of chlorophyll-a and nutrients were associated with the extreme atmospheric events. The strong relationship of nutrients with salinity indicates that physical processes, such as circulation, mixing and river discharge, have a significant control on phytoplankton blooms in the coastal Bay of Bengal. Phosphate seems to be a controlling nutrient during winter whereas availability of light and suspended matter limits production in summer. Formation of low oxygen conditions were observed in the bottom waters due to enhanced primary production by extreme atmospheric events; however, re-oxygenation of bottom waters through sinking of oxygen-rich surface waters by a warm core (anticyclonic) eddy led to its near recovery. This study reveals that atmospheric and physical processes have significant impacts on the water column biogeochemistry in the coastal Bay of Bengal.

In this study, an attempt has been made to develop a simple multiple regression model to forecast the total number of depressions and cyclones (TNDC) over Bay of Bengal during summer monsoon (June–September) season using the data for the period, 1995–2016. Four potential predictors (zonal wind speed at 850 hPa in May and April SST in the North Australia–Indonesia region, 05°S–15°S; 120°E–160°E; March NINO 3.4 SST and geopotential height at 200 hPa in the region, 0°N–10°N; 80°E–100°E) have been identified to forecast TNDC. A remarkably high multiple correlation coefficient of 0.92 has been observed with the TNDC which explains 85% variability. The methodology has been tested for the recent 5 years (2012–2016) and found a good agreement between the observed and forecast values of TNDC except in 2015 in which the observed and predicted TNDC were 2 and 0, respectively. It is interesting to see high and significant correlations between the above predictors and the genesis potential parameter (GPP) during summer monsoon season. This GPP depends on the relative vorticity at 850 hPa, mid troposphere relative humidity, thermal instability between 850 and 500 hPa, and vertical wind shear between 200 and 850 hPa. It is inferred that the above predictors are influencing the environmental conditions over Bay of Bengal which, in turn, influencing the genesis of cyclones during summer monsoon season. The impact of ENSO (El-Nino-Southern Oscillation) and La-Nina in TNDC is examined and found that the vertical wind shear and relative vorticity are high and the GPP was almost double in ENSO compared with that in La-Nina which favoured high (low) TNDC under ENSO (La-Nina).

Estuaries have been under sampled to establish them as sources or sinks of the atmospheric carbon dioxide. Such poor coverage is well known for tropical, particularly monsoon driven, estuaries. In an attempt to study the variability in CO2 in a tropical monsoon estuary we made systematic time‐series observations in the Gautami Godavari estuarine system in the east coast of India. Our 18 month‐long extensive monitoring in the tropical Godavari estuarine system revealed pH >7.8 during dry period that decreased by 1.5 ± 0.01 during peak discharge period. The decrease in pH was associated with high nutrients and bacterial activities suggesting significant organic carbon decomposition. High bacterial respiration (20.6 ± 7.2 μMC l−1 d−1) in the estuary resulted in very high pCO2 of ∼30,000 μatm during peak discharge period, which otherwise were <500 μatm during dry period. Such high pCO2 levels were unknown to occur in any aquatic region. Several major and minor estuaries flow into the northern Indian Ocean from the Indian subcontinent and the monsoon associated processes make these systems chimney for emitting CO2 to atmosphere unrealized hitherto. Key Points Tropical estuaries are significant source of CO2 Monsoonal river discharges bring acidic waters to estuary High temporal variability in pCO2 levels in the estuary

A super cyclone with a central pressure of 912 hPa which was the lowest sofar in any tropical cyclone in Bay of Bengal, hit Paradeep coast on 29th October, 1999. The system was stationary for 30 hours after landfall and a record rainfall of 530 mm was observed at Paradeep on 30th October. TMI (Tropical Rainfall Measuring Mission (TRMM) Microwave Imager) SST images for the period, 25th to 31st October have been examined to see the impact of this super cyclone on SST field. A record decrease of 6 degree C in SST has been observed at 17.4 degree N; 88.9 degree E on 29th October, for the first time, due to the passage this super cyclone.

An attempt has been made to examine the relationship between UOHC and Uh for the severe cyclones during 1990-2006. Uh has been computed from 6 h track positions of each storm (www.imd.gov.in; T3.0 onwards) and the corresponding UOHC (it was given as 'tropical cyclone heat potential' prior to the storm) data were taken from the NOAA website (http://iridl.ldeo. columbia.edu/SOURCES/.NOAA/.NCEP/ .EMC/.CFS/.DAILY/.BelowSeaLevel/)6. Data for 13 severe cyclones during premonsoon (April and May) and postmonsoon (October and November) seasons were used in the computations. Details of the period, lowest central pressure (mb) of the storm, maximum sustained wind speed (kn) and the intensity (T number) of the storms are shown in Table 1. Low (less than 2 m/s) and high (more than 9 m/s) values of Uh were avoided in the analysis. Due to nonavailability of real-time datasets, we were forced to use the pre-storm UOHC data from the NOAA website.

A very severe cyclonic storm “Aila” hit West Bengal on 26 May 2009. The storm intensified when it encountered with a warm core (SST = 31°C) anti-cyclonic eddy (ACE4) in the north Bay of Bengal. The storm intensity increased by 43% due to this eddy, which is comparable with that (34%) obtained from a best fit line (derived from several numerical experiments over north-west Pacific Ocean). The shallow mixed layer of the large-scale ocean and deep mixed layer inside the eddy appear to be crucial parameters besides translation speed of the storm (Uh), ambient relative humidity and thermal stratification below mixed layer, in the storm intensification. From the eddy size and Uh, the eddy feedback factor is found to be about 0.4 (i.e. 40%), which is close to the above. Since there exists an inverse relationship between Uh and UOHC (upper ocean heat content), slow (fast) moving storms require high (low) UOHC. The warm ACE4 with a high UOHC of 149 kj/cm 2 (300% higher than the climatological value) and deep warm layer (D26 = 126 m) opposes the cooling induced by the storm and helps for the intensification of the storm through the supply of large enthalpy (latent + sensible) flux.

In order to examine the influence of forcing (river flow and tides) and anthropogenic activities (dredging and dam regulation) on stratification, a study was conducted over a period of 19 months (June 2008–December 2009) in the Gautami–Godavari estuary (G–GE) during spring and neap tide periods covering entire spectrum of discharge over a distance of 36 km from the mouth. The bathymetry of the estuary was recently changed due to dredging of ∼20 km of the estuary from the mouth for transportation of barges. This significantly changed the mean depth and salinity of the estuary from its earlier state. The variations in the distribution of salinity in the Godavari estuary are driven by river discharge during wet period (June–November) and tides during dry period (December–May). The weak stratification was observed during high discharge (July–August) and no discharge (January–June) periods associated with dominant fresh water and marine water respectively. The strong stratification was developed associated with decrease in discharge during moderate discharge period (October–December). Relatively stronger stratification was noticed during neap than spring tides. The 15 psu isohaline was observed to have migrated ∼2–3 km more towards upper estuary during spring than neap tide suggesting more salt enters during former than latter period. Total salt content was inversely correlated with river discharge and higher salt of about 400×10 6 m 3 psu was observed during spring than neap tide. Flushing times varied between less than a day and more than a month during peak and no discharge periods respectively with lower times during spring than neap tide. The flushing times are controlled by river discharge during high discharge period, tides during dry period and both (river discharge and tides) under moderate discharge period. This study suggests that modification of discharge, either natural due to weak monsoon, or artificial such as dam constructions and re-routing the river flow, may have significant impact on the stratification and biogeochemistry of the Godavari estuary.

In the absence of earlier studies, an attempt is made to identify the vulnerable areas of the Indian coast for the damages due to Tsunami based on an earlier study reported in the context of sea level rise due to greenhouse effect. It is inferred that 12-18 degree N stretch is safe on the west coast when compared with the other regions. 10-12 degree N; 14-16 degree N and 20 degree N on east coast and 9-10 degree N; 21-24 degree N on the west coast are most vulnerable for the damages due to tsunami and storm surges. This information is useful to evacuate people during Tsunami and storm surges.