Low-frequency variability of currents and temperatures on the Pacific continental shelf off northern Baja California, 1978 to 1979

Currents, coastal winds, coastal sea level, and coastal ocean temperatures were observed at a number of northern Baja California Pacific sites between October 1978 and October 1979. Coastal winds were weakly southward on average and fluctuated north-south throughout the year. Fluctuations were uncorrelated over alongshore separations of 200 km. Coastal winds differed considerably from the large-scale offshore wind estimate (Bakun's Index) both in strength and in variability. At 30°50′N the mean currents were weakly equatorward at 25 m depth and weakly poleward at 42 and 60 m in a water depth of 75 m. The seasonal mean flow was equatorward from October to April and poleward from April to October at the shallower level but poleward all year near bottom. The fluctuations on a time scale of days to weeks were about an order of magnitude stronger than the seasonal variation, were oriented predominantly alongshore, and were quasi-barotropic in nature. Empirical orthogonal analyses showed that almost 90% of the variability could be represented by a single empirical mode. The alongshore fluctuations were significantly correlated with coastal sea level from October to July but during the rest of the year correlation was insignificant, possibly as a result of offshore eddies approaching the shelf.Observations at 25- and 60-m depth at a second mooring 100 km south of the first indicated a mean alongshore divergence from April to July 1979, consistent with long-term mean geostrophic flow patterns for the area. In general, currents were correlated alongshore but a two-week event in May, which manifested itself differently at the two sites, was suggestive of the impingement of an eddy onto the continental shelf. Currents and winds were poorly correlated in general.Temperature variation at sites along 700 km of coast showed a strong seasonal variation with the winter cool period extended by spring upwelling. Days-to-weeks scale fluctuations were similar at the five northernmost sites and correlations alongshore were significant for separations of up to 400 km. No evidence of propagating events was found in coastal temperatures or in currents.     

Nearshore currents on the southern Namaqua shelf of the Benguela upwelling system

Nearshore currents of the southern Namaqua shelf were investigated using data from a mooring situated three and a half kilometres offshore of Lambert's Bay, downstream of the Cape Columbine upwelling cell, on the west coast of South Africa. This area is susceptible to harmful algal blooms (HABs) and wind-forced variations in currents and water column structure are critical in determining the development, transport and dissipation of blooms. Time series of local wind data, and current and temperature profile data are described for three periods, considered to be representative of the latter part of the upwelling season (27 January–22 February), winter conditions (5–29 May) and the early part of the upwelling season (10 November–12 December) in 2005. Differences observed in mean wind strength and direction between data sets are indicative of seasonal changes in synoptic meteorological conditions. These quasi-seasonal variations in wind forcing affect nearshore current flow, leading to mean northward flow in surface waters early in the upwelling season when equatorward, upwelling-favourable winds are persistent. Mean near-surface currents are southward during the latter part of the upwelling season, consistent with more prolonged periods of relaxation from equatorward winds, and under winter conditions when winds were predominantly poleward. Within these seasonal variations in mean near-surface current direction, two scales of current variability were evident within all data sets: strong inertial oscillations were driven by diurnal winds and introduced vertical shear into the water column enhancing mixing across the thermocline, while sub-inertial current variability was driven by north–south wind reversals at periods of 2–5 days. Sub-inertial currents were found to lag wind reversals by approximately 12 h, with a tendency for near-surface currents to flow poleward in the absence of wind forcing. Consistent with similar sites along the Californian and Iberian coasts, the headland at Cape Columbine is considered to influence currents and circulation patterns during periods of relaxation from upwelling-favourable winds, favouring the development of a nearshore poleward current, leading to poleward advection of warm water, the development of stratification, and the creation of potentially favourable conditions for HAB development.     

The effects of river discharge and seasonal winds on the shelf off southeastern South America

The Río de la Plata waters form a low salinity tongue that affects the circulation, stratification and the distributions of nutrients and biological species over a wide extent of the adjacent continental shelf. The plume of coastal waters presents a seasonal meridional displacement reaching lower latitudes (28°S) during austral winter and 32°S during summer. Historical data suggests that the wind causes the alongshore shift, with southwesterly (SW) winds forcing the plume to lower latitudes in winter while summer dominant northeasterly (NE) winds force its southward retreat. To establish the connection between wind and outflow variations on the distribution of the coastal waters, we conducted two quasi-synoptic surveys in the region of Plata influence on the continental shelf and slope of southeastern South America, between Mar del Plata, Argentina and the northern coast of Santa Catarina, Brazil. We observed that: (A) SW winds dominating in winter force the northward spreading of the plume to low latitudes even during low river discharge periods; (B) NE winds displace the plume southward and spread the low salinity waters offshore over the entire width of the continental shelf east of the Plata estuary. The southward retreat of the plume in summer leads to a volume decrease of low salinity waters over the shelf. This volume is compensated by an increase of Tropical waters, which dominate the northern shelf. The subsurface transition between Subantarctic and Subtropical Shelf Waters, the Subtropical Shelf Front, and the subsurface water mass distribution, however, present minor seasonal variations. Along shore winds also influence the dynamics and water mass variations along the continental shelf area. In areas under the influence of river discharge, Subtropical Shelf Waters are kept away from the coastal region. When low salinity waters retreat southward, NE winds induce a coastal upwelling system near Santa Marta Cape. In summer, solar radiation promotes the establishment of a strong thermocline that increases buoyancy and further enhances the offshore displacement of low salinity waters under the action of NE winds.     

Net ecosystem metabolism of a coastal embayment fertilised by upwelling and continental runoff

O₂, N, P and Si net ecosystem metabolism of the Ría de Ares-Betanzos (NW Iberian upwelling system) was estimated during two 3-wk periods of contrasting summer downwelling and autumn upwelling conditions by means of a transient 2-D kinematic box model. The subtidal circulation was positive in both situations, although it was depressed during downwelling and enhanced during upwelling. Concurrently, the ría was fertilised mainly by shelf bottom waters, which introduced from 69% (under downwelling) to almost 100% (under upwelling) of the limiting N nutrients. The ría was an efficient nutrient trap: about 70% of the N nutrients that entered the embayment were retained under downwelling conditions (average flushing time, 9 days) and about 50% under upwelling conditions (average flushing time 3 days). Although the trapping efficiency was lower, the net ecosystem production (NEP) was much higher under upwelling (from 1.0±0.3 to 1.5±0.4 g C m⁻² d⁻¹), than under downwelling favourable winds (from 0.2±0.1 to 0.3±0.1 g C m⁻²). The stoichiometry of NEP suggests that P and N compounds recycled faster than C compounds, specially in the inner segment of the ría. The net degree of silification was twice in the inner than in the outer segment of the ría.     

Surface circulation downstream of the Point Arena upwelling center

Measurements from recently installed 5 MHz high-frequency radar (CODAR) stations south of Point Arena, California, are used to describe surface current patterns during the upwelling season (June-August 2007). The systems provide hourly current maps on a 5-km grid, covering a region from approximately 10 to 150 km offshore (the continental shelf into the deep ocean). These HF-radar observations provide an unprecedented view of circulation in this “coastal transition zone”, between the wind-driven circulation over the shelf and the California Current circulation offshore. Circulation patterns include: (1) bifurcation of the coastal upwelling jet downstream of Point Arena into an along-shelf (down-coast) branch and an offshore branch, and (2) a large-scale anticyclonic meander that often develops into an eddy-like recirculation south of the bifurcation. The “recirculation” feature extends well offshore, with surface currents 50-100 km from the coast consistently opposing the wind stress. The spatial and temporal evolution of the surface current features during upwelling events affects surface transport from Point Arena to areas in the south, increasing the travel time of a substantial fraction of newly upwelled water from a few days to roughly two weeks. Thus, surface currents even far offshore influence coastal transport of nutrients, phytoplankton and larvae on ecologically relevant timescales, with resultant connectivity patterns very different than implied by a simple examination of the mean flow.     

Oceanographical patterns during a summer upwelling–downwelling event in the Northern Galician Rias: Comparison with the whole Ria system (NW of Iberian Peninsula)

Summer upwelling and downwelling processes were characterized in the Northern Galician Rias during July and August 2008 by means of sampling carried out onboard R/V Mytilus (CSIC) and R/V Lura (IEO). Thermohaline variables, dissolved oxygen, nutrients, chlorophyll, phytoplankton, ciliates and zooplankton abundances were measured at sections located in the Rias of Viveiro, Barqueiro and Ortigueira and their adjacent shelves. Ekman transport was calculated from QuikSCAT satellite, upwelling intensity estimated with upwelling index from the average daily geostrophic winds, and SST maps obtained from NASA GHRSST satellite. Ekman transport and SST behaviour showed two different patterns: (i) offshore and upwelling favourable conditions on 13–22nd of July; (ii) onshore and downwelling favourable conditions from 23rd July to 19th August. During upwelling, TS diagram showed an intrusion of Eastern North Atlantic Central Water affecting the continental shelf but not the rias. Nutrient salt concentrations increased with depth, reaching their maximum values near the mouth of Ortigueira Ria. During downwelling, coastal water increased its temperature (18.5–19.8 °C) and was retained inside rias; nutrients were nearly depleted, except for the innermost ria (estuarine zone) due to fluvial nutrient inputs. In this inner area, the maximum of chlorophyll-a (Barqueiro Ria) was observed. Low phytoplankton abundances were measured in both cases, even though a short increase in the plankton biomass was observed inside rias during upwelling, while under downwelling a small red tide of Lingulodinium polyedrum was detected. During the upwelling period Northern Rias tend to be mesotrophic systems as revealed by nutrient concentrations, chlorophyll levels and plankton abundances. On the contrary, in similar situations, the Western Rias behaves as eutrophics.     

Disentangling the upwelling mechanisms of the South Brazil Bight

This article presents a suite of long-term numerical simulations that investigate the dynamical mechanisms controlling the circulation in the South Brazil Bight (SBB). The overarching goal of these simulations is to quantify the relative contributions of local wind forcing and the Brazil Current (BC) to the upwelling of nutrient-rich slope water onto the shelf. The model results indicate that the water mass structure of the SBB is controlled by the synergy between wind-driven, inner-shelf upwelling and geostrophic, shelf-break upwelling. The later extends yearlong but the former peaks during the austral summer and decreases towards the winter. The interaction between the poleward flow of the BC and the bottom topography greatly influences the shelf circulation, particularly in the bottom boundary layer. Changes of the SBB coastline direction and shelf width modulate the along-shore pressure gradient and the magnitude of the shelf-break upwelling and downwelling. Thus, although the summer upwelling winds extend over large part of the SBB surface temperatures are warmer in the south because of the cooling effect of the shelf-break upwelling in the northern region. At difference with previous studies of shelf-break dynamics the shelf-break upwelling in our model is not controlled by the uplifting associated with the presence of instabilities of the boundary current or nonlinear accelerations under a variable shelf width. The proposed mechanism is relatively simple. As the boundary current flows along the continental slope, changes in the coastline orientation and along-shore bottom topography modify the along-shore pressure gradient which through geostrophy leads to inshore bottom flow and hence shelf-break upwelling. Such a mechanism can provide insight into upwellings on other western boundary current regions where similar topographic variations exist.     

Generation and evolution of a topographically linked,mesoscale eddy under steady and variable wind-forcing

Numerical simulations with the Regional Ocean Modeling System (ROMS) are used to study the initial spin-up and the evolution of a mesoscale, topographically linked eddy under steady and variable wind conditions. The development of a pool of dense water on the southern Vancouver Island shelf allows cyclonic eddies formed by coastal upwelling off Cape Flattery to spread westward, ultimately contributing to the shelf-wide circulation known as the Juan de Fuca Eddy. This dense water arises through upwelling of water present in the underlying canyon system and tidal mixing over several shallow banks to the north. Tidal mixing is critical to the separation of the eddy from the coast. Although steady upwelling winds with a seasonal mean magnitude (combined with estuarine flow and tides) produce an eddy, only fluctuating winds with timescales and magnitudes typical of the region result in an eddy with a westward extent similar to seasonal observations. With each period of upwelling-favorable winds, newly upwelled water from the coast is entrained into the eddy which grows in size and moves westward. Wind events also significantly affect the baroclinic structure of the eddy. Specifically, during typical summer wind reversals, model surface drifters continue to move cyclonically within the eddy for several days after each downwelling wind event. Under upwelling-favorable wind conditions, model drifters exit the eddy to the southeast as the eddy and coastal upwelling fronts merge into a continuous southeastward shelf break jet.     

Hydrodynamic conditions associated with the formation,maintenance and dissipation of a phytoplankton thin layer in a coastal upwelling system

During May–June 2005, a 17-d cruise was carried out in Ría de Pontevedra (Galician Rías Baixas) to study the physical–biological interactions that may lead to subsurface aggregations of phytoplankton organisms in thin layers (TLs). Physical processes governed the initiation and development, maintenance, and decline of a diatom (toxin producing Pseudo-nitzschia spp. and Chaetoceros socialis) TL during an upwelling relaxation-upwelling–downwelling sequence. Differences in shear profiles appeared to lead to the formation of a TL during upwelling events. These results reveal that the coupling between maximum values of shear and buoyancy frequency can shape a subsurface chlorophyll maximum (SCM) into a TL. The effect of shear upon phytoplankton patches, which has been predicted on the basis of theoretical studies, has been corroborated in this study in which the vertical distribution of an observed TL was controlled by physical processes.     

The coupled Hudson River estuarine-plume response to variable wind and river forcings

Observations of the Hudson River plume were taken in the spring of 2006 in conjunction with the Lagrangian Transport and Transformation Experiment using mooring arrays, shipboard observations, and satellite data. During this time period, the plume was subjected to a variety of wind, buoyant, and shelf forcings, which yield vastly different responses in plume structure including a downstream recirculating eddy. During weak and downwelling winds, the plume formed a narrow buoyant coastal current that propagated downstream near the internal wave speed. Freshwater transport during periods when the downwelling wind was closely aligned with the coast was near the river discharge values. During periods with a cross-shore component to the wind, freshwater transport in the coastal current estimated by the mooring array is less than the river discharge due to a widening of the plume that leads to the internal Rossby radius scaling for the plume width to be invalid. The offshore detachment of plume and formation of a downstream eddy that is observed surprisingly persisted for 2 weeks under a variety of wind forcing conditions. Comparison between mooring, shipboard, and satellite data reveal the downstream eddy is steady in time. Shipboard transects yield a freshwater content equal to the previous 3 days of river discharge. The feature itself was formed due to a large discharge following a strong onshore wind. The plume was then further modified by a brief upwelling wind and currents influenced by the Hudson Shelf Valley. The duration of the detachment and downstream eddy can be explained using a Wedderburn number which is largely consistent with the wind strength index described by Whitney and Garvine (J Geophys Res 110:C03014 1997).     

Numerical study on the summer upwelling system in the northern continental shelf of the South China Sea

A three-dimensional baroclinic nonlinear numerical model is employed to investigate the summer upwelling in the northern continental shelf of the South China Sea (NCSCS) and the mechanisms of the local winds inducing the coastal upwelling, associated with the QuikSCAT wind data. First, the persistent signals of the summer upwelling are illustrated by the climatological the Advanced Very High-Resolution Radiometer (AVHRR) Sea Surface Temperature (SST) image over 1985–2006 and field observations in 2006 summer. Then, after the successful simulation of the summer upwelling in the NCSCS, four numerical experiments are conducted to explore the different effects of local winds, including the wind stress and wind stress curl, on the coastal upwelling in two typical strong summer upwelling regions of the NCSCS. The modeled results indicate that the summer upwelling is a seasonal common phenomenon during June–September in the NCSCS with the spatial extent of a basin-scale. Typical continental shelf upwelling characteristics are clearly shown in the coastal surface and subsurface water, such as low temperature, high salinity and high potential density in the east of the Hainan Island, the east of the Leizhou Peninsula and the southeast of the Zhanjiang Bay (noted as the Qiongdong-QD), and the inshore areas from the Shantou Coast to the Nanri Islands of the Fujian Coast (noted as the Yuedong-YD). The analysis of the QuikSCAT wind data and modeled upwelling index suggests that the local winds play significant roles in causing the coastal upwelling, but the alongshore wind stress and wind stress curl have different contributions to the upwelling in the Qiongdong (QDU) and the coastal upwelling in the Yuedong (YDU), respectively. Furthermore, model results from the numerical experiments show that in the YD the stable alongshore wind stress is a very important dynamic factor to induce the coastal upwelling but the wind stress curl has little contribution and even unfavorable to the YDU. However, in the QD the coastal upwelling is strongly linked to the local wind stress curl. It is also found that not only the offshore Ekman transport driven by the alongshore wind stress, the wind stress curl-induced Ekman pumping also plays a crucial effect on the QDU. Generally, the wind stress curl even has more contributions to the QDU than the alongshore wind stress.     

Frontal circulation induced by up-front and coastal downwelling winds

Two-dimensional (cross-shelf and depth) circulation by downwelling wind in the presence of a prograding front (with isopycnals that slope in the same direction as the topographic slope) over a continental shelf is studied using high-resolution numerical experiments. The physical process of interest is the cross-shelf circulation produced by northeasterly monsoon winds acting on the Kuroshio front over the East China Sea outer shelf and shelfbreak where upwelling is often observed. However, a general problem is posed and solved by idealized numerical and analytical models. It is shown that upwelling is produced shoreward of the front. The upwelling is maintained by (1) a surface bulge of negative vorticity at the head of the front; (2) bottom offshore convergence beneath the front; and (3) in the case of a surface front that is thin relative to water depth, also by upwelling due to the vorticity sheet under the front. The near-coast downwelling produces intense mixing due to both upright and slant-wise convection in regions of positive potential vorticity. The analytical model shows that the size and on-shore propagating speed of the bulge are determined by the wind and its shape is governed by a nonlinear advection–dispersion equation which yields unchanging wave-form solutions. Successive bulges can detach from the front under a steady wind. Vertical circulation cells develop under the propagating bulges despite a stable stratification. These cells can have important consequences to vertical exchanges of tracers and water masses.     

Numerical simulation and analysis of the mean coastal circulation off California

A two-dimensional numerical model is applied to a coastal ocean wherein alongshore elevation and density gradients, normally calculated by a three-dimensional model, are instead supplied by climatologically averaged data for the California Current System between 25 and 40°N. Surface wind stress is also obtained from climatological data. Both surface and bottom boundary layers are resolved in the model calculations; a second moment turbulence closure submodel supplies vertical diffusivities. Near steady state solutions are possible when surface buoyancy flux is imposed at the surface.Model results are as follows: Southward wind stress produces a broad equatorward current with an embedded coastal jet in accordance with previous studies. Positive wind stress curl reduces the jet current and produces a poleward undercurrent which then surfaces as the curl is increased. The jet currents are reduced and poleward flow increases as bottom steepness increases; to a lesser extent, inclusion of the beta effect has a similar effect. The existence of near bottom, poleward or equatorward flow is explained rather simply in terms of the bottom stress resulting from the alongshore balance of surface wind stress and vertically integrated pressure gradient, the latter involving the alongshore surface elevation and density gradient. A further finding is that the upwelling circulation associated with wind stress is confined to the top 200 to 300 m of the ocean along the California coast.     

Coherence of river and ocean conditions along the US West Coast during storms

The majority of water and sediment discharge from the small, mountainous watersheds of the US West Coast occurs during and immediately following winter storms. The physical conditions (waves, currents, and winds) within and acting upon the proximal coastal ocean during these winter storms strongly influence dispersal patterns. We examined this river–ocean temporal coherence for four coastal river–shelf systems of the US West Coast (Umpqua, Eel, Salinas, and Santa Clara) to evaluate whether specific ocean conditions occur during floods that may influence coastal dispersal of sediment. Eleven years of corresponding river discharge, wind, and wave data were obtained for each river–shelf system from USGS and NOAA historical records, and each record was evaluated for seasonal and event-based patterns. Because near-bed shear stresses due to waves influence sediment resuspension and transport, we used spectral wave data to compute and evaluate wave-generated bottom-orbital velocities. The highest values of wave energy and discharge for all four systems were consistently observed between October 15 and March 15, and there were strong latitudinal patterns observed in these data with lower discharge and wave energies in the southernmost systems. During floods we observed patterns of river–ocean coherence that differed from the overall seasonal patterns. For example, downwelling winds generally prevailed during floods in the northern two systems (Umpqua and Eel), whereas winds in the southern systems (Salinas and Santa Clara) were generally downwelling before peak discharge and upwelling after peak discharge. Winds not associated with floods were generally upwelling on all four river–shelf systems. Although there are seasonal variations in river–ocean coherence, waves generally led floods in the three northern systems, while they lagged floods in the Santa Clara. Combined, these observations suggest that there are consistent river–ocean coherence patterns along the US West Coast during winter storms and that these patterns vary substantially with latitude. These results should assist with future evaluations of flood plume formation and sediment fate along this coast.     

The influence of meteorological variation on the upwelling system off eastern Hainan during summer 2007–2008

The influence of meteorological variation, i.e., typhoon and precipitation events, on the coastal upwelling off the eastern Hainan Island was studied based on observations taken during two upwelling seasons. The observations were made in August 2007 and July 2008, respectively. We found that, in principle, similar structure of sea surface temperature and bottom temperature prevailed in both observational periods, providing evidence that upwelling events occur frequently during the summer monsoon along the eastern Hainan shelf. Based on a simple momentum balance theory, we studied the balances between momentum fluxes, wind stress, and bottom stress. The results showed that the Burger number is S ≈ 1, indicating that the cross-shelf momentum flux divergence was balanced by the wind stress and the onshore return flow occurred in the interior of the water column. Hence, a conceptual model of the upwelling structure was built for further understanding of upwelling events. In addition, it was also observed that variations in the strength of upwelling are controlled by storm events, i.e., strong northerly winds change the structure of the thermocline on the shelf significantly. The strong mixing caused by wind reduces the strength of the thermocline, in particular in coastal seas. Based on our conceptual model, a frontal zone between mixed coastal water and offshore water develops which destabilizing the water column and hence decreases the upwelling strength. Freshwaters from the two main rivers in the Wenchang Bay are confined to the coastal area less than 20–30 m deep, as confirmed by our water mass analysis. Freshwater discharge stabilized the water column, inhibiting the upwelling as shown by the potential energy calculation. Consequently, estuarine water only inhibits the upwelling in the near coastal area. Therefore, it can be concluded that estuarine water does not have a significant impact on upwelling strength on the shelf.     

Monsoon-induced upwelling off the Vietnamese coast

During the southwest monsoon from July 8 to 28, 2003, an interdisciplinary cruise took place in the central area of Vietnamese upwelling with “MV Nghien Cuu Bien” in the South China Sea. Physical observations in the upwelling area are analyzed with respect to local/regional wind forcing and far field forcing. Nutrients and phytoplankton measurements are discussed with respect to exchange processes between different water masses. The wind-induced coastal upwelling by local wind forcing is much weaker than in the previous years due to weaker-than-normal winds. This can be attributed to the far field forcing of the 2002/2003 El Niño event which modulates the upwelling intensity. The atmospheric conditions reflect the typical situation after an El Niño event which weakens the wind-induced coastal upwelling, reduces the latent heat flux, and results in higher-than-normal sea-surface temperatures. The general circulation pattern during SW monsoon is driven by the spatial asymmetry in the monsoon forcing. The flow pattern is characterized by an upwelling-induced northward undercurrent and a recently detected southward countercurrent. The resulting stretching deformation of this flow pattern forms an offshore jet between ～12°N and 12.5°N and causes a local enhancement of the upwelling intensity. The upwelling due to stretching deformation is a peculiarity, which makes the Vietnamese upwelling area different to other upwelling areas. A budget of the upwelling components is presented: the strongest contribution in 2003 to the Vietnamese upwelling is the dynamical upwelling due to the clockwise rotation of the northward undercurrent. The internal radius of deformation separates the upwelling area from the offshore area as well as different water masses. Mekong River and the Gulf of Thailand waters which are offshore show nutrient depletion. Therefore, high chlorophyll maxima cannot be explained by nutrient supply from river runoff. The dynamical upwelling brings in nutrient-rich Maximum Salinity Water into the euphotic zone. This causes a subsurface chlorophyll maximum between 20 and 40 m water depth along the northward undercurrent. Deflection from the Redfield ratio in the C:N ratio and negative excess nitrogen identifies the region as nitrogen-limited which may favor cyanobacteria blooms. The consequence is a unique feature in new production: in the upwelling area, new production is based on upwelled nitrate, whereas offshore in the nutrient-depleted Mekong and Gulf of Thailand water, new production is based in addition on nitrogen fixation.     

Thermospheric response to ion heating in the dayside cusp

Recent observations from the CHAMP satellite indicate that neutral density enhancements are common in the northern dayside cusp. The neutral density in this region can be nearly a factor of 2 larger than in adjacent regions of the thermosphere on the poleward and equatorward sides of the cusp. The presence of density enhancements implies that the neutral atmosphere is being heated in the cusp region causing upwelling. A high-resolution model of the global thermosphere is used to study the thermospheric response to heating in the northern dayside cusp. It is found that heating in the cusp results in the creation of a neutral fountain. Specifically, upward drift of the thermosphere within the cusp region is followed at higher altitudes by poleward and equatorward movement out of the cusp region and the gradual subsidence of the neutral gas. Density enhancements of the magnitude observed by the CHAMP satellite occur in the model results for sufficiently strong heating in the cusp. Neutral temperature enhancements also occur and are strongest near the poleward and equatorward boundaries of the cusp region.     

Modeling seasonal circulation, upwelling and tidal mixing in the Arafura and Timor Seas

The extensive shallow tropical seas off northern Australia, encompassing the Arafura and Timor Seas, have been identified as one of the most pristine marine environments on the planet. However, the remoteness and the absence of major industrial development that has contributed to this status have the additional consequence that relatively little is known about these systems. This study is the first to model oceanographic conditions across the tidally dominated Arafura and Timor Seas, and their seasonal variability. The results are based on a high-resolution (0.05°) ocean circulation model forced by realistic winds, waves and tides. The main focus of the study is on physical processes that influence the distributions of sediments and primary productivity across the system. Regions of high bottom stress and tidal mixing have been identified, including a large offshore area around Van Diemen Rise (Timor Sea). Lagrangian particle tracks have revealed a seasonal overturning cell that stretches across the Gulf of Carpentaria (Arafura Sea) with upwelling and downwelling on either side of the Gulf. The presence of coastal upwelling and downwelling is shown to provide a dynamically consistent explanation for the persistent turbid boundary layer observed around the shallow coastal waters of the Gulf.     

A numerical study of the effect of the marginal sea on coastal upwelling in a non-linear inertial model

Inertia theory and the finite element method are used to investigate the effect of marginal seas on coastal upwelling. In contrast to much previous research on wind-driven upwelling, this paper does not consider localized wind effects, but focuses instead on temperature stratification, the slope of the continental shelf, and the background flow field. Finite element method, which is both faster and more robust than finite difference method in solving problems with complex boundary conditions, was developed to solve the partial differential equations that govern coastal upwelling. Our results demonstrate that the environment of the marginal sea plays an important role in coastal upwelling. First, the background flow at the outer boundary is the main driving force of upwelling. As the background flow strengthens, the overall velocity of cross-shelf flow increases and the horizontal scale of the upwelling front widens, and this is accompanied by the movement of the upwelling front further offshore. Second, temperature stratification determines the direction of cross-shelf flows, with strong stratification favoring a narrow and intense upwelling zone. Third, the slope of the continental shelf plays an important role in controlling the intensity of upwelling and the height that upwelling may reach: the steeper the slope, the lower height of the upwelling. An additional phenomenon that should be noted is upwelling separation, which occurs even without a local wind force in the nonlinear model.     

The Guajira upwelling system

The coastal upwelled waters of the Guajira coast, the most northerly peninsula of South America, were studied on the basis of historical data bases, remotely sensed data, and three oceanographic cruises. The Guajira Peninsula is the locus of particularly strong upwelling because it protrudes into the Caribbean Low-Level Wind Jet and its west coast parallels the direction of the strongest winds. The year-round upwelling varies with the wind forcing: strongest in December–March and July, and weakest in the October–November rainy season. The east–west temperature, salinity and density front that delimits the upwelling lies over the shelf edge in the east of the peninsula but separates from the south-westward trending topography to the west. A coastal westward surface jet geostrophically adjusted to the upwelling flows along the front, and an eastward sub-surface counterflow is trapped against the Guajira continental slope. The undercurrent shoals toward the western limit of the upwelling, Santa Marta, beyond which point it extends to the surface. Some of the westward jet re-circulates inshore with the counterflow but part continues directly west to form an upwelling filament. Much of the mesoscale variation is associated with upwelling filaments, which expel cooler, chlorophyll-rich coastal upwelling waters westward and northward into the Caribbean Sea. Freshwater plumes from the Magdalena and Orinoco rivers influence the area strongly, and outflow from Lake Maracaibo interacts directly with upwelled waters off Guajira. Another important factor is the Aeolian input of dust from the Guajira desert by episodes of offshore winds.