We present a critical analysis of the observations and interpretation of VLF subionospheric measurements related to the main Nepal Gorkha earthquake which occurred on 25 April 2015 (Mw7.8) and its major aftershock on 12 May 2015 (Mw7.3). The VLF narrowband signal used is from North West Cape (NWC) (19.8 kHz) VLF transmitter located in Australia and recorded at Allahabad (latitude 25.41°N, longitude 81.93°E). Allahabad is located very close (~360 km) to these earthquake epicenters. Two widely used analysis, viz., (1) terminator time and (2) nighttime fluctuation techniques, are applied to extract seismic related effects in the NWC narrowband VLF data. The terminator time analysis yields statistically significant shifts of ~45 and ~26 min, respectively, in evening terminator time in the NWC VLF amplitude signal, 1 day before both the earthquakes. The nighttime fluctuation method shows a consistent, statistically significant, increase in three parameters 1 day before the earthquake. The observed terminator time and nighttime fluctuation shifts were associated with these earthquakes only after scrutinizing possible contributions from other potential sources such as solar activity; other earthquakes on the signal path; and meteorological disturbances such as lightning activity, wind speed, and temperature along the transmitter‐receiver great circle path. The VLF subionospheric signal analysis results unambiguously point toward the presence of seismically excited atmospheric gravity waves during these major earthquakes and their important role in providing the coupling between the seismic source region and overlying ionosphere.

During the Bay of Bengal (BoB) Boundary Layer Experiment (BoBBLE) in the southern BoB, time series of microstructure measurements were obtained at 8 • N, 89 • E from 4-14 July, 2016. These observations captured events of barrier layer (BL) erosion and re-formation. Initially, a three-layer structure was observed: a fresh surface mixed layer (ML) of thickness 10-20 m; a BL below of 30-40 m thickness with similar temperature but higher salinity; a high salinity core layer, associated with Summer Monsoon Current. Each of 93 reason for this is the lack of direct turbulence and mixing observations, particularly in 94 the BoB. In the BoB, measurements of vertical mixing have been made in the north (Lu-95 cas et al. 2016; Mahadevan et al. 2016) and near Sri Lanka (Jinadasa et al. 2016). Here 96 we present micro-structure measurements that captured the erosion of the barrier layer 97 and its re-formation during a 10-day time series in the southern BoB during the summer 98 monsoon of 2016. The data have been used to understand the characteristics of mixing 99 5in the barrier layer, and the mechanism of barrier layer formation and erosion. Our data 100 suggest that the advection of high salinity surface waters by the SMC to the southern 101 BoB has an important role in the barrier layer erosion. 102The paper is organized as follows: The measurements and methodologies are de-103 scribed in Section 2. Observations of barrier layer formation and erosion are presented 104 in Section 3. Formation mechanisms of the barrier layer and its turbulent characteristics 105 are addressed in Section 4. Section 5 details the mechanism of barrier layer erosion. 106 A 1D model analysis is presented in Section 6. The summary and conclusions of the 107 present study are given in Section 7. 108 2. Methods and field campaign 109

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This paper reports the results of aerosol optical characteristics over a tropical urban station at Pune, India, during drought and normal monsoon years, based on ground-based and satellite observations for the period 2008-2010. Ground-based data from MICROTOPS-II and AERONET, and satellite data products from Moderate Resolution Imaging Spectroradiometer (MODIS) and Ozone Monitoring Instrument (OMI) sensors, were utilized in the study. Detailed analysis of this data revealed that the maximum values of aerosol optical depth (AOD) and minimum values of precipitable water content (PWC) were observed during a drought year (2009) compared to normal monsoon years (2008 and 2010). In order to characterize aerosols further, the Ångström parameters α and β were evaluated. Using the least squares method, α is calculated in the spectral interval of 380-1020 nm, along with the coefficients a 1 and a 2 of the second-order polynomial fitted to the plotted logarithm of AOD versus the logarithm of wavelength. Meteorological parameters, such as temperature and relative humidity, have been measured during the course of the study, as well as the variations in monthly rainfall over the experimental site. The high surface temperatures and low rainfall amounts during the pre-monsoon and monsoon seasons of 2009 lead to an increase in AOD as compared to those in 2008 and 2010. The ground-based observations from MICROTOPS-II reveal good correlation with satellite observations of the MODIS and OMI sensors, and also correlate with the AERONET observations, corroborating the results. Discrimination of aerosol types from radiometric measurements and the tropospheric circulation obtained from the NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) reanalysis were also discussed, and found to be consistent.

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The Kochi Backwater (KB) is the second largest wetland system in India. It is connected to the sea at Fort Kochi and Munambam (30 km north of Kochi). As the tide is forced through two openings, its propagation in the backwater system is very complicated, particularly in the northern arm of the estuary.Using synchronous water level (WL) and current measurements in the KB from a network of stations during 2007-08, it was convenient to demarcate the northern KB into two distinct regions according to the tidal forcing from the north (Pallippuram) and south (Vallarpadam). This demarcation is useful for computing the propagation speeds of the dominant tidal constituents in the northern branch of the KB with dual opening for opposing tides. WL variations indicated that M2 tide (Principal lunar semidiurnal constituent) dominated in the sea level variance, followed by the K1 constituent (Luni-solar declinational diurnal constituent). The M2 tidal influence was strongest near the mouth and decayed in the upstream direction. The propagation speed of the M2 tide in the southern estuary was ~ 3.14 m/s.The ratio of the total annual runoff to the estuarine volume is ~ 42 that indicate the estuary will be flushed 42 times in a year. KB can be classified as a monsoonal estuary where the river discharge exhibits large seasonal variation.

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.

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