Titli was the cataclysmic storm to afflict Indian coast during 2018. Tropical cyclones may inflate nutrients on the surface layer of the ocean by entrainment and mixing, as a result, the upper oligotrophic and euphotic layers may be recharged with rich nutrients resulting in triggering phytoplankton blooms and significant increase in primary production. The biological response using the chlorophyll concentration from Oceansat-2 OCM and the Net Primary Productivity from MODIS-A to this cyclone and its relation with the cyclone intensity has been studied. Role of physical oceanography parameters has been studied using multiple satellite-derived parameters like SST, TCHP, SLA, Ekman pumping velocity, wind stress and wind vectors and eddies during the passage of cyclone Titli. The results showed an increase in the surface chlorophyll concentration from 0.08 (before) to 0.32 mg m$^{-3}$ (after), whereas NPP enhanced from 457 to 1313 mgC m$^{-2}$ day$^{-1}$ with an SST drop of ${\sim}$2.58$^{\circ}$C (30.25$^{\circ}$–27.67$^{\circ}$C). The study confirms that the physical parameters are the key components that influence the biological response in addition to the cyclone intensity.

Analyses of the marine geophysical data collected in four oceanographic cruises during 1974–2008 inside the Gulf of Kachchh, northwest India was done, and geology and crustal structure were interpreted for the first time. Seabed topography deduced from satellite-derived bathymetry and bathymetric investigations has revealed the tide-dominated Gulf (> 6 m high) has ${\sim}$ 50 m maximum depth along the axial part. The satellite-derived free-air gravity anomalies and their horizontal gradients together with earlier published results helped in noting major crust features like faults. Analyses of single and multi-channel seismic reflection data had enabled identifying reflection strata pattern and their continuity, wave-field and acoustic characters. 2-D model studies of ocean-bottom seismic refraction data (following ray-tracing technique), satellite-derived free-air gravity anomalies, magneto-telluric data along a profile in middle of the Gulf and magnetic anomalies near Gulf mouth have led to determine various physical properties: velocity, density, resistivity and the magnetic of the subsurface layers. They have been utilized in interpreting three-layered crust structure of the Gulf. Further, presence of ${\sim}$ 3.0 km thick low,  ${\sim}$ 3.7 km/s velocity and 2.2 gm/cc density layer overlain by about 2.1 km thick high, 4.6 km/s velocity, and 2.55 gm/cc density layer noted. Considering the physical properties in correlation with mainland geology and crustal structure from onshore Suthri-1 and inner shelf GK-01-A1 well logs and coast parallel Mandvi–Mundra deep seismic study results, low and high-velocity layers have been noted as low velocity Mesozoic sedimentary rock formations overlain by high velocity Deccan volcanic rock formations. The volcanic rock thickness increases to the east are noted. The Tertiary sediments above them are ${\sim}$ 1.2 km thick. The major crustal structures and geology identified contribute to understand regional geology, structure and tectonic framework of the Kachchh region and in particular, of the study area. The fractured Deccan volcanic rocks and their intervening intra-trappean sediments are possibly considered as potential hydrocarbon-bearing formations (?) in view of the underlying thick Mesozoic sediments.

$\bf{Highlights}$

$\bullet$ Seismic reflection, OBS, and magneto-telluric data revealed three layered crustal structure of the Gulf

$\bullet$ We have identified fractured Deccan volcanic rocks and their intervening intra-trappeans sediments inside the Gulf

$\bullet$ Presence of low velocity Mesozoic sedimentary rock formations overlain by high velocity Deccan volcanic rock formations

Tropical Cyclone Ockhi was an intense cyclone, with a peculiar and long track, in the Arabian Sea in 2017. It caused severe damage to coastal infrastructure and death of 282 people. Indian National Centre for Ocean Information Services (INCOIS) issued the Joint INCOIS-IMD (India Meteorological Department) bulletins on the Ocean State Forecasts (OSF) and alerts/warnings during Ockhi. Validation of the OSF from INCOIS using buoys reveals that the forecasts were in good agreement with the observations [average correlation 0.9, RMSE ${\le}$0.8 m (for larger waves), and scatter index ${\le}$25%]. Climatological analysis of Genesis Potential Index (GPI) suggests that the southeast Arabian Sea, where the TC-Ockhi was intensified, had all the favourable conditions for intensification during November/December. Moreover, it was found that four days before the genesis of Ockhi, the environmental vorticity and relative humidity were more favourable for the cyclogenesis compared to vertical wind shear and potential intensity. The intensification rate was rapid as experienced by earlier cyclones in this region. Also, the cyclone track closely matched the background tropospheric winds. The present study suggests that the forecasters should look into the background dynamic and thermodynamic conditions extensively in addition to multi-model guidance to better predict the genesis, intensity and track of the cyclones.

$\bf{Higlights}$

$\bullet$ In the Arabian Sea, during the TC-Ockhi, the forecasts of wave parameters from the model forced with bias-corrected ECMWF winds resulted in very good agreement with observations.

$\bullet$ Climatologically, TC-Ockhi region has large potential for the genesis and intensification of TC due to an enhanced low-level cyclonic vorticity and the reduction in vertical wind shear.

$\bullet$ During the TC-Ockhi period, low-level vorticity and mid-tropospheric relative humidity were the dominant contributing factors, which lead to an enhanced GPI in the Arabian Sea.

$\bullet$ TC-Ockhi also had rapid intensification in a similar fashion the earlier cyclones in this region behaved.

$\bullet$ There is no abnormality also in the TC-Ockhi track, as the TC-Ockhi track matches well with the background tropospheric flow.

The present study highlights the wave–tide interactions in a tide-dominant coast Versova, along the west coast of India. Versova is a macro tidal area and is home to fisheries. Model simulations are carried out to investigate wave–tide interactions with and without incorporating water level (WL) variations in the model setup. The simulation results are compared with the observed data at Versova. Model comparison with wave observation shows that the simulated significant wave height (Hs) reproduced the observed wave heights with an accuracy of scatter index = 8% and correlation = 0.94 with the inclusion of WL variations. The incorporation of WL variations created the energy modulations in the low-frequency part of the wave spectra, raising the periodical modulations in wave height. This low-frequency wave energy modulation is absent in the without WL simulations, resulting in underestimation of energy density which causes underestimation of Hs by ${\sim}$1 m. Hence this study strongly suggests that water level variations must be incorporated into the wave model to accurately represent wave modulations which are significant during monsoon and extreme events in the tide-dominant coastal areas.