Impact of stratification on internal waves and differential wearing of thermal inversions on the east coast of India

The surface layers of the Bay of Bengal along the east coast of India exhibit intricate stratification owing to the differential distribution of freshwaters. The winter (January–February) cooling of the salinity-induced stable layers results in the development of thermal inversions that deteriorate toward the end of the season. The study focuses on the behavior of the thermal inversions in the light of the variable stratification and the monsoon imposed reversing coastal current. To address the associated processes, a three-dimensional Princeton Ocean Model is applied for the east coast of India, and numerical experiments carried out to study the means by which the thermal inversions tend to perish with the passage of winter. The model domain with variable curvilinear grid uses input fields that comprise realistic bathymetry and initial temperature/salinity conforming to winter/specified stratification. The surface forcing comprises wind stress and diurnal pattern air–sea heat fluxes. The body forcing is derived from the periodic tidal elevations at the open boundaries. It has been found that the thermal inversions tend to sustain as the equator-ward flowing East India Coastal Current (EICC) traps the cool low saline waters between Paradip and Kakinada. The current off Paradip is weak and variable and is not a part of EICC. Consequently, in the absence of replenishment of cool and freshsurface waters, the temperature/salinity gradients get eroded steadily. No thermal inversions are noticed south of Kakinada because of relatively weak current with diminished vertical salinity gradient. As the nature of stratification encountered in the bay is highly variable due to diverse reasons, the behavior of internal waves under different stratification scenarios is also addressed. Numerical experiments indicate that the energy/amplitude of the internal waves are comparable in the surface layers for any stratification, where as it is certain orders exalted in the deeper waters of the strong stratification scenario. Further, it is found that the energies and pattern of the temperature oscillations conform to the nature of mixed tide at the corresponding latitude. The underneath stratification is found to be more responsible for the generation of internal waves compared to the local stratification. This implies that the body forcing emanating from below is the cardinal contributor for the generation of internal waves. The numerical experiment with a flat and uniform bottom showing weak manifestation of internal waves endorses the same. This connotes that the continental slopes are an effective generator of the internal waves and the energy flux conversion of the barotropic tide to internal waves seems to be heavily dependent on the shoaling bottom.     

Modeling the Effects of Tidal Energy Extraction on Estuarine Hydrodynamics in a Stratified Estuary

A 3-D coastal ocean model with a tidal turbine module was used in this paper to study the effects of tidal energy extraction on temperature and salinity stratification and density-driven two-layer estuarine circulation. Numerical experiments with various turbine array configurations were carried out to investigate the changes in tidally averaged temperature, salinity, and velocity profiles in an idealized stratified estuary that connects to coastal water through a narrow tidal channel. The model was driven by tides, river inflow, and sea surface heat flux. To represent the realistic size of commercial tidal farms, model simulations were conducted based on a small percentage (less than 10 %) of the total number of turbines that would generate the maximum extractable energy in the system. Model results show that extraction of tidal in-stream energy will increase the vertical mixing and decrease the stratification in the estuary. Installation of in-stream tidal farm will cause a phase lag in tidal wave, which leads to large differences in tidal currents between baseline and tidal farm conditions. Extraction of tidal energy in an estuarine system has stronger impact on the tidally averaged salinity, temperature, and velocity in the surface layer than the bottom layer even though the turbine hub height is close to the bottom. Finally, model results also indicate that extraction of tidal energy weakens the two-layer estuarine circulation, especially during neap tides when tidal mixing is weakest and energy extraction is smallest.     

Numerical simulation of upwelling off Visakhapatnam on east coast of India during pre-monsoon months

A three-dimensional numerical model of the type described by Johns and coworkers (1992), hereafter referred to as model (J), is applied to study the response of a coastal ocean to pure wind-stress forcing. Conservation equations are applied for mass, momentum, temperature, salinity and turbulence energy. Experiments are performed to investigate the evolution of the thermal structure and upwelling processes along the east coast of India during the pre-monsoon season. A comparison between the computed results and the limited observations on the thermal structure and alongshore currents over the inner-shelf off Visakhapatnam is presented.     

Vertical propagation of baroclinic Kelvin waves along the west coast of India

A linear, continuously stratified ocean model is used to investigate vertical propagation of remotely forced, baroclinic Kelvin waves along the Indian west coast. The extent of vertical propagation over the length of the coast is found to be an increasing function of the forcing frequency. Simulations show that, over the length of the Indian west coast, vertical propagation is limited at annual and semi-annual periods, but significant at periods shorter than about 120 days. This has two major consequences. First, the depth of subsurface currents associated with these frequencies varies substantially along the coast. Second, baroclinic Kelvin waves generated in the Bay of Bengal at periods shorter than about 120 days have negligible influence on surface currents along the north Indian west coast.     

A study on reflection pattern of swells from the shoreline of peninsular India

Information on reflected surface gravity waves from the shoreline is required for understanding the coastal hydrodynamics. We have quantified the reflected swells (frequency band 0.045–0.12 Hz) from the west and east coast of India based on the spectral wave data derived from the directional waverider buoys. Reflection coefficient, ratio of the reflected and incident spectral energy, was used to quantify the reflected waves. Influence of the seasons, cyclone, relative depth, land/sea breeze, tides and tidal current on the reflected waves were examined. For the locations off the west coast of India, seasons have large impact on the reflection coefficient and were relatively less during the monsoon season due to the increase in incident wave energy. Locations off the east coast of India show almost the same reflection coefficient throughout the year and have no significant seasonal variations. The reflection coefficient off Puducherry was higher than that for other locations due to the low incident wave energy. The reflection coefficient was low during the cyclone period, but the reflected energy during cyclone was higher than that during the normal condition due to the high incident wave energy. High-energy reflected waves show large variation with tide due to the trapping and dissipation of reflected wave by bottom friction and this effect cause low reflection in deep water location than shallow water location. The reflection coefficient decreases with increase in relative depth off west coast of India.     

渤海层化结构及潮汐锋面季节变化的数值研究

采用三维斜压海流模式(POM)模拟了渤海海温的季节变化,以海表与海底温差ΔT作为判别依据,发现3月份前整个渤海的表、底温差小于0.5℃,说明渤海处于充分混合状态;进入4月份以后,莱州湾、渤海中部以及渤海海峡的局部水域出现超过2℃的表、底温差,意味着垂向层化结构开始形成;层化区域面积随着海表热通量增大的趋势可一直持续到8月,9月后由于日照量逐渐减小,季节性温越层逐渐消失,11月以后渤海又恢复到充分混合的状态.     

Role of westerlies and thermohaline characteristics on sea-ice extent in the Indian Ocean Sector of Antarctica

Satellite-derived sea-ice extent in the Indian Ocean Sector during the period November 1978 to December 2006 was studied in relation to the atmospheric forcing and oceanic thermohaline structure. The study revealed that sea-ice extent increased when the ocean exhibited higher stability. Low sea-ice extent was observed during 1985 to 1993, when the zonal winds and latent flux was relatively weak and when the ocean exhibited strong vertical mixing facilitated by low stability thereby, deepening the mixed layer to ∼250 m. This was reflected in the ocean surface layer temperature, which was relatively warm (−0.3°C). Winds increased during 1996 to 2000, but due to higher oceanic stability mixed layer depth shallowed (< 200 m) leading to reduced vertical mixing of deep warmer layers with the surface water, leading to an enhancement in the sea-ice extent.     

Impact of a Tropical Cyclone on Coastal Upwelling Processes

A three-dimensional numerical model is described to study theresponse of a coastal ocean excited by a tropical cyclone in the Bay of Bengal. The numericalexperiments have been carried out using the model to understand the dynamics and thermodynamics ofthe ocean due to different cyclonic systems approaching in different directions. In the firstexperiment, the model is used to simulate the vertical thermal structure of the ocean as a response ofpassage of the less intensified 1997 cyclone, which was skirting the east coast of India before crossingthe Bangladesh coast. The simulations are compared with the buoy data available during the storm period.In the next experiment, it is considered an idealized cyclone with hurricane winds movingnormal to the east coast of India crossing between Visakhapatnam and Kakinada to evolve thermalstructure and currents of the ocean. A net decrease of the SST of 6–7 °C is simulated whenthe severe cyclonic storm moved over the coastal ocean.     

Hydrography and circulation in the northwestern Bay of Bengal during the retreat of southwest monsoon

The distribution of temperature and salinity in the upper 500 m of the northwestern Bay of Bengal, adjoining the east coast of India, during the retreat of southwest monsoon (September) of 1983 is presented. This study reveals coastal upwelling (limited to the upper 40 m) induced by the local winds. Waters of higher surface salinity near the coast characterize the upwelling. The freshwater influx near the head of the Bay diluted the surface salinity to as low as 26.0 × 10⁻³. The surface circulation was weak and led to a net transport of 2.0 × 10⁶m³.s⁻¹ directed towards northeast.     

Climate Forcing and Salinity Variability in Chesapeake Bay,USA

Salinity is a critical factor in understanding and predicting physical and biogeochemical processes in the coastal ocean where it varies considerably in time and space. In this paper, we introduce a Chesapeake Bay community implementation of the Regional Ocean Modeling System (ChesROMS) and use it to investigate the interannual variability of salinity in Chesapeake Bay. The ChesROMS implementation was evaluated by quantitatively comparing the model solutions with the observed variations in the Bay for a 15-year period (1991 to 2005). Temperature fields were most consistently well predicted, with a correlation of 0.99 and a root mean square error (RMSE) of 1.5°C for the period, with modeled salinity following closely with a correlation of 0.94 and RMSE of 2.5. Variability of salinity anomalies from climatology based on modeled salinity was examined using empirical orthogonal function analysis, which indicates the salinity distribution in the Bay is principally driven by river forcing. Wind forcing and tidal mixing were also important factors in determining the salinity stratification in the water column, especially during low flow conditions. The fairly strong correlation between river discharge anomaly in this region and the Pacific Decadal Oscillation suggests that the long-term salinity variability in the Bay is affected by large-scale climate patterns. The detailed analyses of the role and importance of different forcing, including river runoff, atmospheric fluxes, and open ocean boundary conditions, are discussed in the context of the observed and modeled interannual variability.     

Effect of a broad shallow sill on tidal circulation and salt transport in the entrance to a coastal plain estuary (Mira—Vila Nova de Milfontes, Portugal)

We describe the tidal circulation and salinity regime of a coastal plain estuary that connects to the ocean through a flood tide delta. The delta acts as a sill, and we examine the mechanisms through which the sill affects exchange of estuarine water with the ocean. Given enough buoyancy, the dynamics of tidal intrusion fronts across the sill and selective withdrawal (aspiration) in the deeper channel landward appear to control the exchange of seawater with estuarine water. Comparison of currents on the sill and stratification in the channel reveals aspiration depths smaller than channel depth during neap tide. During neap tide and strong vertical stratification, seawater plunges beneath the less dense estuarine water somewhere on the sill. Turbulence in the intruding bottom layer on the sill promotes entrainment of fluid from the surface layer, and the seawater along the sill bottom is diluted with estuarine water. During ebb flow, salt is effectively trapped landward of the sill in a stagnant zone between the aspiration depth and the bottom where it can be advected farther upstream by flood currents. During spring tide, the plunge point moves landward and off the sill, stratification is weakened in the deep channel, and aspiration during ebb extends to the bottom. This prevents the formation of stagnant water near the bottom, and the estuary is flooded with high salinity water far inland. The neapspring cycle of tidal intrusion fronts on flood coupled with aspiration during ebb interacts with the sill to play an important role in the transport and retention of salt within the estuary.     

Sources of Salinity Variation in a Coastal Lagoon in a Karst Landscape

Bahia de la Ascension (BA) is a shallow, mangrove-fringed coastal bay connected to the Caribbean through two inlets, outlined by the Mesoamerican Barrier Reef System. This work represents an initial investigation of the relative contribution of hydrometeorological and hydrodynamic forcing on salinity variation in this lagoon. Our objective is to assess the sensitivity of the salinity in BA to fluctuations in freshwater inflow and coastal oceanography. Two field trips were undertaken during rainy and dry seasons in 2007. Surface salinity was mapped across the system and CTD deployments carried out within BA and in the sea end-member to characterize temperature, conductivity, and water level. Also, cross-sectional CTD profiles were implemented to examine vertical stratification. The water balance indicated that 16 % of rainfall over the drainage basin (DB) becomes groundwater discharge plus surface runoff into BA during dry season, while 68 % of the precipitation input to the DB is supplied through groundwater–surface runoff to the bay during rainfalls. This combined inflow showed larger fluctuations than direct rainfall and, thus, has a greater potential to alter the seasonal salinity variations within BA. The tidal signal is selectively attenuated within BA, as diurnal frequencies are more readily filtered out than semidiurnal frequencies. Mesohaline conditions in the southwest bay are associated with freshwater sources, while saline water masses in the inlet are influenced by prevalent SE winds in the region and tidal phase, establishing a strong horizontal SW-NE estuarine salinity gradient.     

Oceanography of the U.S. Pacific Northwest Coastal Ocean and estuaries with application to coastal ecology

Ocean processes are generally large scale on the U.S. Pacific Northwest coast; this is true of both seasonal variations and event-scale upwelling-downwelling fluctuations., which are highly energetic. Coastal upwelling supplies most of the macronutrients available for production, although the intensity of upwelling-favorable wind forcing increases southward while primary production and chlorophyll are higher in the north, off the Washington coast. This discrepancy could be related to several mesoscale features: the wider, more gently sloping shelf to the north, the existence of numerous submarine canyons to the north, the availability of Columbia River plume water and sediment north of the river mouth, and the existence of a semi-permanent eddy offshore of the Strait of Juan de Fuca. We suggest that these features have important effects on the magnitude and timing of macronutrient or micronutrient delivery to the plankton. These features are potentially important as well to transport pathways and residence times of planktonic larvae and to the development of harmful algal blooms. The coastal plain estuaries, with the exception of the Columbia River, are relatively small, with large tidal forcing and highly seasonal direct river inputs that are low to negligible during the growing season. Primary production in these estuaries is likely controlled not by river-driven stratification but by coastal upwelling and exchange with the ocean. Both baroclinic mechanisms (the gravitational circulation) and barotropic ones (lateral stirring by tide and, possibly, wind) contribute to this exchange. Because estuarine hydrography and ecology are so dominated by ocean signals, the coastal estuaries, like the coastal ocean, are largely synchronous on seasonal and event time scales, though, intrusions of the Columbia River plume can cause strong asymmetries between Washington and Oregon estuaries especially during spring downwelling conditions. Water property correlation increases between spring and summer as wind forcing becomes more spatially coherent along the coast. Estuarine habitat is structure not only, by large scale forcing but also by fine scale processes in the extensive intertidal zone, such as by solar heating or differential advection by tidal, curents.     

Modelling December 2004 Indian Ocean tsunami: a coastal study

December 2004 tsunami in the Indian Ocean region has been simulated using MIKE-21 HD model. The vertical displacement of the seabed is incorporated into the numerical simulation by using time-varying bathymetry data. In the open ocean, sea surface height from altimeter observation has been used to validate the model results. To the west of the rupture zone, the crest is observed to precede the trough of the tsunami waves while to the east, trough preceded the crest. The model performance along the coastal region has been validated using de-tided sea levels from tide gauge measurements at Tuticorin, Chennai, Vishakapattanam, and Paradip ports along the east coast of India. Unique coastal characteristics of the tsunami waves, wave height, and wave celerity are reasonably simulated by the numerical model. Spectral analysis of tide gauge observations and corresponding model results has been done, and the distribution of frequency peaks from the analysis of gauge observations and the model results is observed to have a reasonable comparison. Low-frequency waves, contributed from the coastally trapped edge waves, are found to dominate both the tide gauge observations and the model results. The subsequent increase in the tsunami wave height observed at Chennai, Vishakapattanam, and Paradip has been explained on the basis of coastally trapped edge waves. From the validation studies using altimeter data and tide gauge data, it is observed that the model can be used effectively to simulate the tsunami wave height in the offshore as well as in the coastal region with satisfying performance.     

Time-dependent stratification in the Gauthami-Godavari estuary

The time-dependent salinity stratification in an environment of highly variable turbulence of Gauthami-Godavari estuary is described. From time series observations at three stations over two tidal cycles each in the winter (December 1989) and the dry (April 1990) seasons in the Gauthami-Godavari estuary, the development of stratification is correlated with periods of substantially reduced velocity shear. Stratification is observed to be greatest around the low water slacks and least around the high water slacks. The formation of stratification relaxes viscous constraints and a buoyancy circulation rapidly develops. The breakdown of stratification drastically modifies the circulation and largely removes the vertical shear associated with the density driven flow. Destratification occurs around the high water slacks in the lower reaches fairly close to the mouth of the estuary. The variations in the fields of mass will strongly affect the response of the velocity field.     

Research Progress of Long-term Ocean Temperature Changes in the Southern Ocean

In the recent decades， a large amount of anthropogenic heat has been absorbed and stored in the Southern Ocean. Results from observations and climate models' simulations both show that the Southern Ocean displays large warming in the upper and subsurface ocean that maximizes at 45°~40°S. However， the underlying mechanisms and evolution processes of the Southern Ocean temperature changes remain unclear， leaving the Southern Ocean to be a hotspot of climate change studies in the recent years. The present study summarized the current progress in the observations and numerical modeling of long-term temperature changes in the Southern Ocean. The effects of changes in wind， surface heat flux， sea-ice and other factors on the ocean temperature changes were presented， along with the introduction to the role of oceanic mean circulation and eddies. The present study further proposed that a deepening of the understanding in the Southern Ocean temperature change may be achieved by investigating the fast and slow responses of the Southern Ocean to external radiative forcing， which are respectively associated with the fast adjustments of the ocean mixed-layer and the slow evolution of the deep ocean. Specifically， the striking and fast mixed-layer ocean warming north of 50°S is tightly related to the surface heat absorption over upwelling regions and wind-driven meridional heat transport， resulting in enhanced warming around 45°S. While in the slow response of the Southern Ocean temperature， the enhanced ocean warming shifts southward and downward， mainly associating with the heat transfer from oceanic eddies. The Southern Ocean temperature has pronounced climatic effects on many aspects， such as global energy balance， sea-level rise， ocean stratification changes， regional surface warming and atmospheric circulation changes. However， large model biases/deficiencies in simulating the present-day climatology and essential ocean dynamic processes last in generations of climate models， which are the main challenge in advancing our understanding in the mechanisms for the Southern Ocean climate changes. Therefore， to achieve reliable future projections of the Southern Ocean climate， substantial efforts will be needed to improve the model performances and physical understanding in the relative role of various processes in ocean temperature changes at different time scales.     

Seasonal Stratification Characteristics of Vertical Profiles of Water Body in Lake Lugu

For the purpose of exploring seasonal stratification characteristics of water hydrochemistry, the seasonal dynamics and vertical thermal stratification of water temperature in Lake Lugu, the vertical profiles of water temperature (Temp), Electrical Conductivity (EC), Dissolved Oxygen (DO), pH and Chlorophyll-a (Chl-a) of Lake Lugu were monitored in January, April, July and October 2015, respectively. The results indicated that water body of Lake Lugu appeared thermal stratification in spring, summer and autumn, however, in winter, the water temperature in vertical direction was homogeneous. The thermocline was located between 10 and 25 m, nevertheless, it moved down to range from 20 to 30 m in autumn. In addition, water temperature in hypolimnion was maintained almost as a constant and consistent with annual temperature, indicating water body was stable all along. The results showed that the thermal stratification had some influences on vertical distributions of DO, EC, pH and Chl-a. The significant stratification of DO, EC and pH was found, especially in summer, DO and pH values in thermocline peaked due to greatly stable thermal stratification and temperature increase. In hypolimnion, DO concentration and pH value were very small. Moreover, Chl-a concentration was higher in the surface and lower in the bottom water, implying that human should be highly alter to prevent the emergence of a large area of algae in Lake Lugu. EC took on decreasing variation, besides, lower in the thermocline. While,Lugu Lake water salinity was lower and substantially constant (~ 0.10‰), without considering the effects of salinity, both in vertical sections and in epilimnion, thermocline and hypolimnion, there all existed a simple linear function of the relationship between EC andwater temperature, showing that Lugu Lake was affected by natural climate and keeps natural state.     

Ocean tidal loading affecting precise geodetic observations on Greenland: Error account of surface deformations by tidal gravity measurements

Air-borne and satellite based altimetry are used to monitor the Greenland ice-cap. Since these measurements are related to fiducial sites at the coast, the robustness of the height differences depends on the stability of these reference points. To benefit from the accuracy of these methods on the centimeter level, station corrections regarding the Earth tides and the ocean tidal loading have to be applied. Models for global corrections esp. for the body tides are available and sufficient, but local corrections regarding the effect of the adjacent shelf area still have to be inferred from additional observations. Near the coast, ocean tidal loading causes additional vertical deformations in the order of 1 to 10 cm. Therefore, tidal gravity measurements were carried out at four fiducial sites around Greenland in order to provide corrections for the kinematic part of the coordinates of these sites. Starting in 1993 four stations were occupied on Greenland for a one year record each.The results show the expected strong tidal anomalies due to ocean tidal loading. The loading computations confirmed these observations, but it turned out that with global models only about 50 % of the observed effect can be explained. This means that at these stations a vertical deformation of up to ± 3.5 cm is not corrected applying these global models.     

Impacts of a Wave Farm on Waves,Currents and Coastal Morphology in South West England

This study investigates impacts of a wave farm on waves, currents and coastal morphology adjacent to the wave farm, which is located in the Southwest of England (the Wave Hub). In this study, we focus on the interaction between waves and tides due to the presence of the wave farm and its effects on wave radiation stresses, bottom shear stresses and consequently on the sediment transport and the coast adjacent to the wave farm, using an integrated numerical modelling system. The modelling system consists of the near-shore wave model SWAN, the ocean circulation model ROMS and a sediment transport model for morphological evolution. The results show that tidal elevation and tidal currents can have a significant effect on waves and that tidal forcing and waves have a significant effect on bottom shear stresses. Waves can impact on the processes related to the bottom boundary layer and mixing intensity in the water column. The wave farm has an impact on the gradients of radiation shear stresses and bottom shear stresses that modify current speeds and wave heights, which in turn impact on the near-shore sediment transport and the resulting morphological changes. Bed load transport rates show a decrease when the wave farm is present, even during storm conditions. The results highlight the importance of the interactions between waves and tides when modelling coastal morphology with presence of wave energy devices.     

Mudbank regime off the Kerala coast during monsoon and non-monsoon seasons

Field experiments conducted in the nearshore ocean to understand the dynamics of mudbank off Kerala, south-west coast of India, are highlighted. Real time monitoring of the nearshore ocean off Purakkad, Kerala was accomplished using pressure transducers for nearshore surface wave measurements, and current sensors for nearshore velocity measurements. Comprehensive information on the spatial structure of mudbank was obtained from aerial surveys. Extensive data collected on surface waves and currents in the nearshore ocean, indicate that the infra-gravity (IG) waves (leaky modes and trapped edge wave modes), and far infra-gravity (FIG) waves coupled with strong shoreline reflections and undertow play an important role in the dynamics associated with the mudbanks off Kerala during the monsoon season. During the non-monsoon season evidence for progressive edge waves in the infragravity frequency band, an energetic gravity wave band and a strong undertow with weak reflections was observed.