Impact of sea breeze on wind-seas off Goa, west coast of India

After withdrawal of the Indian Summer Monsoon and until onset of the next monsoon, i.e., roughly during November–May, winds in the coastal regions of India are dominated by sea breeze. It has an impact on the daily cycle of the sea state near the coast. The impact is quite significant when large scale winds are weak. During one such event, 1–15 April 1997, a Datawell directional waverider buoy was deployed in 23 m water depth off Goa, west coast of India. Twenty-minute averaged spectra, collected once every three hours, show that the spectrum of sea-breeze-related ‘wind-seas’ peaked at 0.23 ±0.05 Hz. These wind-seas were well separated from swells of frequencies less than 0.15 Hz. The TMA spectrum (Bouwset al 1985) matched the observed seas spectra very well when the sea-breeze was active and the fetch corresponding to equilibrium spectrum was found to be 77±43 km during such occasions. We emphasize on the diurnal cycle of sea-breeze-related sea off the coast of Goa and write an equation for the energy of the seas as a function of the local wind     

Monthly-mean wind stress along the coast of the north Indian Ocean

Monthly-mean wind stress and its longshore and offshore components have been computed using the bulk aerodynamic method for each of a string of 36 two-degree-latitude by two-degree-longitude squares along the coast of the north Indian Ocean. The data source for the computation is the sixty-year mean resultant winds of Hastenrath and Lamb. The main features exhibited by the components, taking the longshore components as positive (negative) when the Ekman transport is away from (towards) the coast, are: (1) Along the coasts of Somalia and Arabia, the magnitude of the wind stress is among the highest in the north Indian Ocean, and its direction is generally parallel to the coastline. This results in a longshore component which is large (as high as 2·5 dyne/cm²) and positive during the southwest monsoon, and weaker (less than 0·6 dyne/cm²) and negative during the northeast monsoon. (2) Though weak (less than 0·2 dyne/cm²) during the northeast monsoon, the monthly-mean longshore component along the west coast of India remains positive throughout the year. The magnitude of the offshore component during the southwest monsoon is much larger than that of the longshore component. (3) The behaviour of the wind stress components along the east coast of India is similar to that along the Somalia-Arabia coast, but the magnitudes are much smaller.     

Coupling Between the Coastal Ocean and Yaquina Bay,Oregon: Importance of Oceanic Inputs Relative to Other Nitrogen Sources

Understanding of the role of oceanic input in nutrient loadings is important for understanding nutrient and phytoplankton dynamics in estuaries adjacent to coastal upwelling regions as well as determining the natural background conditions. We examined the nitrogen sources to Yaquina Estuary (Oregon, USA) as well as the relationships between physical forcing and gross oceanic input of nutrients and phytoplankton. The ocean is the dominant source of dissolved inorganic nitrogen (DIN) and phosphate to the lower portion of Yaquina Bay during the dry season (May through October). During this time interval, high levels of dissolved inorganic nitrogen (primarily in the form of nitrate) and phosphate entering the estuary lag upwelling favorable winds by 2 days. The nitrate and phosphate levels entering the bay associated with coastal upwelling are correlated with the wind stress integrated over times scales of 4–6 days. In addition, there is a significant import of chlorophyll a to the bay from the coastal ocean region, particularly during July and August. Variations in flood-tide chlorophyll a lag upwelling favorable winds by 6 days, suggesting that it takes this amount of time for phytoplankton to utilize the recently upwelled nitrogen and be transported across the shelf into the estuary. Variations in water properties determined by ocean conditions propagate approximately 11–13 km into the estuary. Comparison of nitrogen sources to Yaquina Bay shows that the ocean is the dominant source during the dry season (May to October) and the river is the dominant source during the wet season with watershed nitrogen inputs primarily associated with nitrogen fixation on forest lands.     

Circulation and water masses of the Arabian Sea

The dynamics and thermodynamics of the surface layer of the Arabian Sea, north of about 10N, are dominated by the monsoon-related annual cycle of air-sea fluxes of momentum and heat. The currents in open-sea regime of this layer can be largely accounted for by Ekman drift and the thermal field is dominated by local heat fluxes. The geostrophic currents in open-sea subsurface regime also show a seasonal cycle and there is some evidence that signatures of this cycle appear as deep as 1000 m. The forcing due to Ekman suction is an important mechanism for the geostrophic currents in the central and western parts of the Sea. Recent studies suggest that the eastern part is strongly influenced by the Rossby waves radiated by the Kelvin waves propagating along the west coast of India. The circulation in the coastal region off Oman is driven mainly by local winds and there is no remotely driven western boundary current. Local wind-driving is also important to the coastal circulation off western India during the southwest monsoon but not during the northeast monsoon when a strong (approximately 7 × 10⁶m³/sec) current moves poleward against weak winds. This current is driven by a pressure gradient which forms along this coast during the northeast monsoon due to either thermohaline-forcing or due to the arrival of Kelvin waves from the Bay of Bengal. The present speculation about flow of bottom water (deeper than about 3500 m) in the Arabian Sea is that it moves northward and upwells into the layer of North Indian Deep Water (approximately 1500–3500m). It is further speculated that the flow in this layer consists of a poleward western boundary current and a weak equatorward flow in the interior. It is not known if there is an annual cycle associated with the deep and the bottom water circulation.     

Computer model of wind, waves, and longshore currents during a coastal storm

A mathematical model has been developed to forecast or hindcast wind, waves, and longshore currents during the passage of a coastal storm. Storm intensity is a function of the barometric pressure gradient which is modeled by rotating an inverted normal curve around the center of an ellipse. The length and orientation of the major and minor axes of the ellipse control the size and shape of the storm. The path of the storm is determined by a sequence of storm positions for the hindcast mode, and by interpolated positions assuming constant speed and direction for the forecast mode. The site location, shoreline orientation, and nearshore bottom slope provide input data for the shore position. The geostrophic wind speed and direction at the shore site are computed from the latitude and barometric pressure gradient. The geostrophic wind is converted into surface wind speed and direction by applying corrections for frictional effects over land and sea. The surface wind speed and direction, effective fetch, and wind duration are used to compute wave period, breaker height, and breaker angle at the shore site. The longshore current velocity is computed as a function of wave period, breaker height and angle, and nearshore slope. The model was tested by comparing observed data for several coastal locations with predicted values for wind speed, wave period and height, and longshore current velocity. Forecasts were made for actual storms and for hypothetical circular and elliptical storms.     

Physical control of primary productivity on a seasonal scale in central and eastern Arabian Sea

Usingin situ data collected during 1992–1997, under the Indian programme of Joint Global Ocean Flux Study (JGOFS), we show that the biological productivity of the Arabian Sea is tightly coupled to the physical forcing mediated through nutrient availability. The Arabian Sea becomes productive in summer not only along the coastal regions of Somalia, Arabia and southern parts of the west coast of India due to coastal upwelling but also in the open waters of the central region. The open waters in the north are fertilized by a combination of divergence driven by cyclonic wind stress curl to the north of the Findlater Jet and lateral advection of nutrient-rich upwelled waters from Arabia. Productivity in the southern part of the central Arabian Sea, on the other hand, is driven by advection from the Somalia upwelling. Surface cooling and convection resulting from reduced solar radiation and increased evaporation make the northern region productive in winter. During both spring and fall inter-monsoons, this sea remains warm and stratified with low production as surface waters are oligotrophic. Inter-annual variability in physical forcing during winter resulted in one-and-a-half times higher production in 1997 than in 1995.     

Mesoscale modeling study of the oceanographic conditions off the southwest coast of India

A high resolution model, using the Miami Isopycnic Coordinate Ocean Model (MICOM), has been implemented for the first time to study the seasonal circulation and coastal upwelling off the southwest Indian coast during 1974. This model is part of a model and data assimilation system capable of describing the ocean circulation and variability in the Indian Ocean and its predictability in response to the monsoon system. Along the southwest coast of India the dominant coastal current is the reversing West Indian Coastal Current which is well simulated and described, in addition to the weaker undercurrent of the opposite direction. Upwelling of cold water, 4‡C lower than offshore temperatures appear in April. The upwelling intensifies with the southwest monsoon and is simulated in accordance within situ observations. Upwelling appears to be strongest off Cochin and Quilon, and the upwelling of cold water is seen together with a decrease in salinity in the model simulation.     

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.     

Seasonal variation of the salinity in the Zuari estuary,Goa, India

The annual salt budget of the Zuari is examined. The characteristics of the estuary differ markedly from the low run off season during November–May to the heavy run off period of the southwest monsoon from June to October. During November–May the estuary is vertically mixed and the two processes controlling the transport of salt are run off induced advective transport out of the estuary, and tidally induced diffusive transport into the estuary. The magnitude of the latter is about 20% larger, leading to a salinity rise in the estuary. The diffusion coefficient has been estimated to be 233 ± 101 m²/sec. With the onset of the southwest monsoon, the run off increases dramatically, and the estuary loses about 75% of its salt during the first two months of the season. About 2/3 of this loss is recovered in the next two months when the run off decreases. Because the estuary is partially stratified during June–October, gravitational circulation is expected to play a role in addition to tidal diffusion and run off. The magnitude of its contribution has, however, not yet been determined.     

Nutrient biogeochemistry in an upwelling-influenced estuary of the Pacific northwest (Tillamook Bay,Oregon, USA)

Tillamook Bay, Oregon, is a drowned river estuary that receives freshwater input from 5 rivers and exchanges ocean water through a single channel. Similar to other western United States estuaries, the bay exhibits a strong seasonal change in river discharge in which there is a pronounced winter maximum and summer minimum in precipitation and runoff. The behavior of major inorganic nutrients (phosphorus, nitrogen, and silica) within the watershed is examined over seasonal cycles and under a range of river discharge conditions for October 1997–December 1999. Monthly and seasonal sampling stations include transects extending from the mouth of each river to the mouth of the estuary as well as 6–10 sites upstream along each of the 5 major rivers. Few studies have examined nutrient cycling in Pacific Northwest estuaries. This study evaluates the distributions of inorganic nutrients to understand the net processes occurring within this estuary. Based upon this approach, we hypothesize that nutrient behavior in the Tillamook Bay estuary can be explained by two dominant factors: freshwater flushing time and biological uptake and regeneration. Superimposed on these two processes is seasonal variability in nutrient concentrations of coastal waters via upwelling. Freshwater flushing time determines the amount of time for the uptake of nutrients by phytoplankton, for exchange with suspended particles, and for interaction with the sediments. Seasonal coastal upwelling controls the timing and extent of oceanic delivery of nutrients to the estuary. We suggest that benthic regeneration of nutrients is also an important process within the estuary occurring seasonally according to the flushing characteristics of the estuary. Silicic acid, nitrate, and NH₄ ⁺ supply to the bay appears to be dominated by riverine input. PO₄ ⁻³ supply is dominated by river input during periods of high river flow (winter months) with oceanic input via upwelling and tidal exchange important during other times (spring, summer, and fall months). Departures from conservative mixing indicate that internal estuarine sources of dissolved inorganic phosphorus and nitrogen are also significant over an annual cycle.     

Mineral ballast and particle settling rates in the coastal upwelling system off NW Africa and the South Atlantic

The ocean off NW Africa is the second most important coastal upwelling system with a total annual primary production of 0.33 Gt of carbon per year (Carr in Deep Sea Res II 49:59–80, 2002). Deep ocean organic carbon fluxes measured by sediment traps are also fairly high despite low biogenic opal fluxes. Due to a low supply of dissolved silicate from subsurface waters, the ocean off NW Africa is characterized by predominantly carbonate-secreting primary producers, i.e. coccolithophorids. These algae which are key primary producers since millions of years are found in organic- and chlorophyll-rich zooplankton fecal pellets, which sink rapidly through the water column within a few days. Particle flux studies in the Mauretanian upwelling area (Cape Blanc) confirm the hypothesis of Armstrong et al. (Deep Sea Res II 49:219–236, 2002) who proposed that ballast availability, e.g. of carbonate particles, is essential to predict deep ocean organic carbon fluxes. The role of dust as ballast mineral for organic carbon, however, must be also taken into consideration in the coastal settings off NW Africa. There, high settling rates of larger particles approach 400 m day⁻¹, which may be due to a particular composition of mineral ballast. An assessment of particle settling rates from opal-production systems in the Southern Ocean of the Atlantic Sector, in contrast, provides lower values, consistent with the assumptions of Francois et al. (Global Biogeochem Cycles 16(4):1087, 2002). Satellite chlorophyll distributions, particle distributions and fluxes in the water column off NW Africa as well as modelling studies suggest a significant lateral flux component and export of particles from coastal shelf waters into the open ocean. These transport processes have implications for paleo-reconstructions from sediment cores retrieved at continental margin settings.     

Simulating the transient evolution and abrupt change of Northern Africa atmosphere–ocean–terrestrial ecosystem in the Holocene

We present the first synchronously coupled transient simulation of the evolution of the northern Africa climate-ecosystem for the last 6500 years in a global general circulation ocean–atmosphere–terrestrial ecosystem model. The model simulated the major abrupt vegetation collapse in the southern Sahara at about 5 ka, consistent with the proxy records. Local precipitation, however, shows a much more gradual decline with time, implying a lack of strong positive vegetation feedback on annual rainfall during the collapse. The vegetation change in northern Africa is driven by local precipitation decline and strong precipitation variability. In contrast, the change of precipitation is dominated by internal climate variability and a gradual monsoonal climate response to orbital forcing. In addition, some minor vegetation changes are also simulated in different regions across northern AfricaThe model also simulated a gradual annual mean surface cooling in the subtropical North Atlantic towards the latest Holocene, as well as a reduced seasonal cycle of SST. The SST response is caused largely by the insolation forcing, while the annual mean cooling is also reinforced by the increased coastal upwelling near the east boundary. The increased upwelling results from a southward retreat of the North Africa monsoon system, and, in turn, an increased northeasterly trade wind. The simulated changes of SST and upwelling are also largely consistent with marine proxy records, albeit with a weaker magnitude in the model.The mismatch between the collapse of vegetation and gradual transition of rainfall suggests that the vegetation collapse is not caused by a strong positive vegetation feedback. Instead, it is suggested that the Mid-Holocene collapse of North African vegetation is caused mainly by a nonlinear response of the vegetation to a precipitation threshold in the presence of strong climate variability. The implication to the modeling and observations is also discussed.     

Temporal variability in SO<Subscript>2</Subscript> exposures at Hawai’i Volcanoes National Park,USA

Sulfur dioxide measured at the Visitor’s Center of Hawai’i Volcanoes National Park, USA exhibits seasonal and diurnal patterns and is sensitive to rainfall, but the relationship between wind direction and SO₂ is too inconsistent to support deterministic predictions of hourly SO₂ based on hourly wind direction. Although SO₂ at the Visitor Center has usually been below regulatory levels, high SO₂ levels and adverse health effects remain a concern. This investigation identified patterns in hourly SO₂ based on wind direction, time of day, month, and rainfall occurrence using 4 years of hourly data. Empirical probabilities were investigated using a Bayesian approach. Winds from the volcanic vents were rare, but when they did occur SO₂ was elevated about half of the time. Conversely, half of the hours with elevated SO₂ occurred when volcanic vents were not directly upwind. Episodes of elevated SO₂ tended to occur during the months of November–March and between 8:00 a.m. and 5:00 p.m. Rainfall was associated with a marked reduction in SO₂ (29–81% depending on wind direction). Individuals that wish to avoid exposure to volcanic fumes can take these patterns into account.     

Abrupt change in Vietnam coastal upwelling as a response to global warming

Coastal upwelling is of great social and economic significance to the human community. The Vietnam coastal upwelling is a fundamental part of upwelling systems in the South China Sea, western Pacific Ocean. However, its sedimentary response to global warming has been largely unknown. Here we reconstruct a 3200-year sedimentary record based on high-resolution geochemical analyses of a marine core from off the Vietnamese coast. The results reveal an abrupt sedimentary change since approximately 1950 ce. The abrupt change is explained by concurrent rapid intensification of the coastal upwelling. This can be ascribed to significant global change since the beginning of the 20th century. Global warming could enhance wind speed during the summer and strengthen the coastal upwelling. The forced upwelling results in an increase in nutrient availability and marine primary production; however, ongoing global warming may also increase the risk of more frequent algal blooms in this area.     

Wind waves and sediment transport regime off the south-central Kerala coast,India

Wind waves in the innershelf of the south-central Kerala coast, south-west India were measured at four locations during different seasons. Simultaneously, numerical models were developed to simulate the wave and sediment transport regime of the innershelf. Strong monsoonal influence is seen in the wave characteristics with greater amplitudes, lower periods and switch-over from SW to SWW–W direction. The net annual longshore sediment transport is southerly in the innershelf and northerly in the surf zone. These counter-directional transports are linked by seasonally reversing the cross-shore transports. In the locations where the transports in the longshore and cross-shore directions are balanced, stable beaches prevail. Erosion/accretion tendency prevails in locations where these transports are not balanced. The southern and northern parts of the coast where onshore transports are predominant could be accreting zones. The erosion/accretion pattern deduced from the sediment transport model corresponds well with the long-term erosion/accretion trend for this coast.     

Seasonal controls on surface pCO2 in the central and eastern Arabian Sea

The variability in partial pressure of carbon dioxide (pCO₂) and its control by biological and physical processes in the mixed layer (ML) of the central and eastern Arabian Sea during inter-monsoon, northeast monsoon, and southwest monsoon seasons were studied. The ML varied from 80–120 m during NE monsoon, 60–80 m and 20–30 m during SW- and inter-monsoon seasons, respectively, and the variability resulted from different physical processes. Significant seasonal variability was found in pCO₂ levels. During SW monsoon, coastal waters contain two contrasting regimes; (a) pCO₂ levels of 520–685 μatm were observed in the SW coast of India, the highest found so far from this region, driven by intense upwelling and (b) low levels of pCO₂ (266 μatm) were found associated with monsoonal fresh water influx. It varied in ranges of 416–527 μatm and 375–446 μatm during inter- and NE monsoon, respectively, in coastal waters with higher values occurring in the north. The central Arabian Sea pCO₂ levels were 351–433, 379–475 and 385–432 μatm during NE-inter and SW monsoon seasons, respectively. The mixed layer pCO₂ relations with temperature, oxygen, chlorophylla and primary production revealed that the former is largely regulated by physical processes during SW- and NE monsoon whereas both physical and biological processes are important in inter-monsoon. Application of Louanchiet al (1996) model revealed that the mixing effect is the dominant during monsoons, however, the biological effect is equally significant during SW monsoon whereas thermodynamics and fluxes influence during inter-monsoons.     

Climatic and Tidal Forcing of Hydrography and Chlorophyll Concentrations in the Columbia River Estuary

Hydrographic patterns and chlorophyll concentrations in the Columbia River estuary were compared for spring and summer periods during 2004 through 2006. Riverine and oceanic sources of chlorophyll were evaluated at stations along a 27-km along-estuary transect in relation to time series of wind stress, river flow, and tidal stage. Patterns of chlorophyll concentration varied between seasons and years. In spring, the chlorophyll distribution was dominated by high concentrations from freshwater sources. Periods of increased stream flow limited riverine chlorophyll production. In summer, conversely, upwelling winds induced input of high-salinity water from the ocean to the estuary, and this water was often associated with relatively high chlorophyll concentrations. The frequency, duration, and intensity of upwelling events varied both seasonally and interannually, and this variation affected the timing and magnitude of coastally derived material imported to the estuary. The main source of chlorophyll thus varied from riverine in spring to coastal in summer. In both spring and summer seasons and among years, modulation of the spring/neap tidal cycle determined stratification, patterns of mixing, and the fate of (especially freshwater) phytoplankton. Spring tides had higher mixing and neap tides greater stratification, which affected the vertical distribution of chlorophyll. The Columbia River differs from the more tidally dominated coastal estuaries in the Pacific Northwest by its large riverine phytoplankton production and transfer of this biogenic material to the estuary and coastal ocean. However, all Pacific Northwest coastal estuaries investigated to date have exhibited advection of coastally derived chlorophyll during the upwelling season. This constitutes a fundamental difference between Pacific Northwest estuaries and systems not bounded by a coastal upwelling zone.     

Seasonal and diurnal variability of thermal structure in the coastal waters off Visakhapatnam

Seasonal and diurnal variability of thermal structure in the coastal waters off Visakhapatnam has been examined in relation to the flow field and surface winds utilizing the hourly data of temperature and currents taken at a fixed location over a tidal cycle at monthly intervals. The coastal currents in the pre-monsoon period and strong near-surface winter cooling processes affect the thermal structure of the coastal sea. Upwelling which is predominant during March to May with an intermittent relaxing event helps in the development of a strong layered thermal structure while convective mixing due to winter inversions during November to February causes weak thermal gradients in the water column.     

Numerical simulation of the exchange process within shallow bar-built estuary

Great South Bay is the largest of a series of interconnected bar-built estuaries on the south shore of Long Island, New York. The depth-averaged barotropic motions in the bay were simulated by using a finite element two-dimensional numerical model. The barotropic motions were driven with astronomical tides, subtidal coastal sea level fluctuations induced by longshore wind stress over the adjacent continental shelf, and local wind stress over the surface of the bay. There was vigorous exchange at tidal frequencies between the western part of Great South Bay and the surrounding waters, but the tidal exchange was heavily damped in the eastern part of the bay. At subtidal frequencies the volume exchange did not exhibit significant attenuation in the interior of the bay. In the eastern part of Great South Bay, the magnitude of the subtidal volume exchange could exceed that of the tidal exchange. The principal mode of subtidal volume exchange was found to be associated with subtidal sea level fluctuations along the coast, which characteristically caused a filling or emptying of the system from all open boundaries of Great South Bay.