Longshore Wind Research Articles

Wind-generated, high-frequency edge waves

The effect of longshore wind stress on high-frequency edge waves, over a convex, exponential, semi- infinite sheet, is determined analytically. Comparison is made with physical data measured at Port Kembla Harbour, on the eastern coast of Australia. Good agreement is obtained for the amplitudes of the waves, as well as for their periods.

Stratification in a small lagoon in the Great Barrier Reef

Temperatures were measured in a small lagoon in the windward reef flat of Davies Reef in the central Great Barrier Reef and examined on three time scales to gain three perspectives on thermal stratification and the trapping of bottom water. Profiling for stratification and dye revealed layering where bottom water was trapped and released by the successive capping and uncapping of the lagoon by a diurnal thermocline. A 1-month monitoring array revealed a solar synchronization, with the temperature of reef-flat water exceeding temperatures of lagoon water by up to 1 5�C within 1 h of midday, and lagoon stratification lagging this by 1 h. There was also a lunar synchronization with mixing proceeding during nocturnal rising tides. Lagoon surface and bottom temperatures were also monitored for 11 months. The amplitude of the diurnal stratification showed no coherence either with the amplitude of the tide (marked spring-neap tides) or with scalar wind stress. The low frequency amplitude of the diurnal oscillation was coherent with the longshore wind vector at periods near 3 6 days and in a band approximately from 10 to 40 days. Daily stratification increased when winds were poleward and decreased when winds were equatonvard. Events of flushing were separated on average by 9 h, but the most frequently observed separation was 5 h and only 10% of separations exceeded 18 h during the 11 months.

Wind-Induced Sea-Surface Slopes on the West Florida Shelf

Abstract Tidal and meteorological records at stations in the eastern Gulf of Mexico have been studied. The sea-level response is a maximum for winds along the coast and varies symmetrically with angle. The coherence is maximum at periods of 4–10 days. The horizontal coherence of sea level is high out to 500 km for 4–10 day periods. The horizontal coherence for wind (measured at coastal stations) is high out to at least 500 km. The amplitude of the response of sea level to winds is larger by a factor of 4 here, on a broad shelf, than on the Oregon coast, which is more narrow by about the same ratio. A response of ∼16 cm is induced by ∼4 m s−1 wind. This response, or transfer function, is uniform over the spectral range (4–100 days). The sea-level response to the longshore wind stress is not linear, but to the power 0.8 ± 0.1, and is attributed to the relatively low tidal currents in this region. The large horizontal coherences of wind and sea level imply broad longshore flows extending 500 km or more along...

The General Circulation in a Dissipative Ocean Basin with Longshore Wind Stresses

Solutions of the Stommel equation are presented which take the form of free waves in the interior of the ocean basin, driven by convergences and divergences in coastal transports brought about by the variation of the longshore wind stress around the coast. These waves have been termed “coastal waves” and result from the beta-effect in the presence of a uniform frictional process, such as the loss of momentum by the ocean to the atmosphere. The coastal waves which typically transport 5 Sv extend significantly into the interior of the ocean from all boundaries except the western boundary, and also drive a westward nonlinear current, appear to be an important feature in the general circulation. A good example of a quasi-steady wavefield induced by intermediate-scale coastline geography occurs in the Flinders Current off the south coast of Australia. The western boundary current, of course, compensates for imbalances in interior transport. Its structure results from forcing, both by this transport and the longshore wind stress on the western coast itself, which produces no net transport outside of the boundary layer.

The Mean and Seasonal Circulation off Southwest Nova Scotia

Long-term measurements off southwest Nova Scotia reveal the following features of the mean circulation: (a) a westward longshore coastal current (4–10 cm s−1, (b) an anticyclonic gyre around Brown Bank (5–15 cm s−1, and (c) an upwelling circulation off Cape Sable (1–2 cm s−1, at bottom). The gyre circulation appears permanent but the coastal current and upwelling exhibit annual variations of the same order as the means. There is a distinct annual signal in the longshore transport at Cape Sable (maximum westward in winter), whereas the mean transport (0.14 × 106 m4 s−1) is consistent with both geostrophic estimates and budget requirements in the Gulf of Marine. Strong seasonal cycles are also found in the salinity and density fields at Cape Sable which appear to be controlled both by buoyancy input from the coastal current and local mixing effects. A linear diagnostic model indicates that the primary dynamical balance for the circulation is between a longshore pressure gradient and longshore mean density and stratification gradients which have summer maxima. Lesser contributions arise from longshore wind, offshore density gradient and centrifugal upwelling. Tidal rectification, deduced from coherent modulations of the semidiurnal tidal streams and low-frequency currents, supports the Browns Bank gyre circulation and drives both westward and offshore components of the near-bottom flow off Cape Sable. Thus the “centrifugal upwelling” hypothesis fails and the main driving force for Cape Sable upwelling appears to be the longshore density variations maintained by tidal mixing.

On Wind and Ocean-Velocity Correlations in a Coastal-Upwelling System

This note studies the response of a simple linear baroclinic coastal-upwelling model to fluctuating longshore winds. Correlations between wind stress and velocities are computed explicitly. It is shown that these correlations depend primarily upon the wind-stress spectrum and that, for a realistic spectrum, the wind stress leads the longshore velocity by approximately one day. The computed correlations agree remarkably well with observations and dismiss the belief that time lags ought to be directly related to the local inertial period, i.e., some fraction of the local pendulum day.

Thermal waves on the Queensland shelf

Thermocline waves are found on the continental slope and shelf of Queensland. Slope waves of period near 90 days accompany upwelling and downwelling. Shelf thermal waves are created by onshore surges of shelf- break water, which is 1-4.5�C cooler than lagoon water, depending on the season. The bottom surges are investigated by means of the cross-spectra of temperatures with longshore winds, with atmospheric pressure and with mean sea level. Longshore winds are found to be the driving mechanism and significant coherence is found near periods of 4 and 8 days (summer and winter, respectively) and in a band with periods of 10-70 days, perhaps peaking near 30 days. Correlation and cross-spectral analyses of temperature at sites 100 km apart suggest that the short period thermal waves popagate equatorward wheras the long period waves, which contain most of the thermal covariance, form a standing mode. Cross-spectral analysis of the root mean square amplitude of diurnal thermal variations with the longer-period thermal waves suggests that localized tidal upwelling brings bottom water over reefs.

Upwelling by internal tides and kelvin waves at the continental shelf break on the Great Barrier Reef

A time series of 50 days duration was obtained of sea levels and winds and of temperature and currents at six depths from 27 to 104 m at 18�19'S.,147�21'E. on the continental shelf break between the Great Barrier Reef and the Coral Sea. The sea-level signal had a predominantly mixed solar and lunar semidiurnal tidal period. The currents consisted of a semidiurnal tidal component oriented primarily cross-shelf, except near the sea floor, superimposed on a low-frequency, predominantly longshore, southward component, coherent with depth, in geostrophic balance, and modulated by the longshore wind component Large fluctuations in temperature were observed, consisting of a low-frequency component, possibly generated by internal Kelvin waves, and iiucruarions of predominantiy solar semidiurnai iidai period. The latter fiiictuations are interpreted as evidence of internal tides of amplitude up to 110 m that may be generated by the interaction of the longshore currents with topographic irregularities in the shelf. It is suggested that, during any long-term studies of water properties near the shelf break, some additional monitoring of short-term temporal variations should be carried out to avoid data aliasing by internal tides. The bottom boundary layer appears to be very active in vertical mixing. Internal tides may be very important in introducing other water components, e.g. nutrients, to the outer Great Barrier Reef.

Continental shelf waves and their influence on the circulation around the Great Barrier Reef

Winds and atmospheric pressure, sea level and water currents were measured at several locations over the continental shelf, both east and west of the Great Barrier Reef, between 14.5�s. and 20�S., from June to November 1980. The dominant wind direction changed from westward over the Coral Sea to north- westward (roughly parallel to the shore) over the shelf. A strong non-tidal low-frequency signal in all sea- level and longshore current data was found, highly coherent from site to site and strongly correlated with the longshore wind component over the shelf, though not with the atmospheric pressure. A model of wind- driven barotropic shelf waves is used to explain a number of observations, such as the invariance of temporal fluctuations of longshore current with distance from shore, and the northward longshore propagation of oceanic disturbances at a speed equal to twice that of the first-mode barotropic free shelf wave, a speed one order of magnitude smaller than that of the wind system. The low-frequency current fluctuations resulted in large water displacements, up and down the coast. Low-frequency cross-shelf currents were much weaker and less coherent. Two upwelling mechanisms are internal tides and internal Kelvin waves coupled to the barotropic shelf waves.

LONG WAVES OVER THE GREAT BARRIER REEF

Low-frequency forcing of water currents over the continental shelf °f Australia is quite strong and should be taken into account when the flow for durations greater than 1 day is important. In the case of the Queensland coast, the longshore wind generates barotropic continental shelf waves, raising or lowering the mean sea level by as much as 30 cm and generating longshore currents over the continental shelf, even very close to the coast, that are often larger than the tidal currents. These wind-driven currents can reverse sign, flowing alternately northward and southward, although the longshore wind stress, though fluctuating, does not change direction. To reproduce such phenomena in an analytical or computer model of wind-driven currents, it is necessary to extend the offshore boundaries of the model offshore from the continental shelf break.

Upwelling as a Source of Nutrients for the Great Barrier Reef Ecosystems: A Solution to Darwin's Question?

The Great Barrier Reef shelf ecosystem is examined for nutrient enrichment from within the seasonal thermocline of the adjacent Coral Sea using moored current and temperature recorders and chemical data from a year of hydrology cruises at 3 to 5 wk intervals. The East Australian Current is found to pulsate in strength over the continental slope with a period near 90 d and to pump cold, saline, nutrient rich water u p the slope to the shelf break. The nutrients are then pumped inshore in a bottom Ekman layer forced by periodic reversals in the longshore wind component. The period of this cycle is 12 to 25 d in summer (30 d year round average) and the bottom surges have an alternating onshoreoffshore speed up to 10 cm S-'. Upwelling intrusions tend to be confined near the bottom and phytoplankton development quickly takes place inshore of the shelf break. There are return surface flows which preserve the mass budget and carry silicate rich Lagoon water offshore while nitrogen rich shelf break water is carried onshore. Upwelling intrusions penetrate across the entire zone of reefs, but rarely into the Lagoon. Nutrition is del~vered out of the shelf thermocline to the living coral of reefs by localised upwelling induced by the reefs. Bottom chlorophyll concentrations average 0.4 mg m-3 a t the inner reefs and 0.8 mg m-3 near the shelf break while surface concentrations average 0.3 mg m-3; annual top and bottom variances are respectively 0.5 and 1.9 mg m-3 and there is no apparent seasonal cycle. The estimated onshore nitrogen flux in a 10 m thick bottom layer gives an annual nitrogen input of 20 yg at 1-' throughout the water column in a 50 km zone of reefs, an enormous value for tropical waters.

Low-Frequency Circulation at the Edge of the Scotian Shelf

Abstract Records of current, temperature and salinity from a two-year mooring program at the shelf break off Nova Scotia are examined to determine the low-frequency oceanic responses to the driving surface wind field and fluctuating offshore currents associated with the Gulf Stream. The seasonal mean and subtidal variance (at periods of 2–10 days) of the cross-shelf currents reflect the strong annual cycle in the wind field measured at Sable Island. The mean vertical shear suggests a simple Ekman response to winter increases in the longshore wind component, but this model fails quantitatively because 1) the inferred surface-layer (20 m) transport is much too large and 2) the deep (150 m) “return” flow shows no annual signal. The excessive offshore near-surface transport in winter must be reconciled with the relatively stationary position of the shelf/slope-water boundary (SSB) by invoking intense cross-frontal mixing and/or a seasonal mean alongshore pressure gradient. The seasonal mean longshore currents...

The Role of Wind-Generated Mixing in Coastal Upwelling

Abstract A simple parameterization of mixing processes originally developed by Kraus and Turner (1967) is included in a two-dimensional, two-layer theory of wind-driven coastal upwelling. Mixed-layer deepening is a competition between entrainment due to wind stirring, stabilization due to surface beating, and upwelling vertical velocity. For longshore winds favorable to upwelling, the temperature of the surface water mass being pushed offshore by the wind is determined by that of the incoming subsurface water mass, by the surface heating, and by the mixing dynamics. An upwelling steady state is possible with surface heating. In this state, the vertical upwelling velocity is exactly matched by the entrainment velocity, and the resulting cold water flux into the mixed layer is balanced by surface heating; surface temperature increases linearly with distance offshore. Time-dependent numerical solutions exhibit a, tendency toward this steady-state solution. Winds favorable to downwelling cause detrainment of ...

Low frequency fluctuations off the Natal coast

Conditions on the continental shelf off the Natal coast are described in terms of the effect on fluctuations in currents, temperature, and (to a limited extent) sea level. The results of measurements made at four mooring points along the coast are then analyzed for fluctuations in the 2‐ to 20‐day period range. The longshore wind component is shown to play a major role in the circulation dynamics; however, the effect of topographic variations and the associated flow of the Agulhas Current also serve to produce markedly differing regimes. Thus in the northern region it is found that the currents are almost entirely wind dominated, while further south, nonlocal signals propagating into the regions produce a different form of response. The measurements are not comprehensive enough to allow more detailed investigations as to the nature of these signals.

On the theory of deep flow in the Hudson Shelf Valley

The time averaged near‐bottom current in the New York Bight for a selected 3‐day period is idealized as a steady state response to the mean wind at the John F. Kennedy International Airport and the mean inflow condition across the Long Island shelf. The idealization invokes potential vorticity conservation in a homogeneous ocean subject to a linear bottom friction and results in shelf currents whose longshore components are in general agreement with the averages of the observed. In the Hudson Shelf Valley, qualitative agreement with the observation is achieved in the onshore component which is directed nearly along the valley axis. Nearshore, within 40 km or so of the coast, the response is, to a substantial degree, wind driven. Nearshore upvalley mean flow is due partly to shore‐parallel wind stresses that are directed away from the valley and is due partly to topographic deflection of an alongshore current that is driven in part by the inflow condition and in part by the alongshore component of the mean wind stress. Because of the important role of bottom friction, an alongshore current driven by longshore winds undergoes sharper (and more realistic) deflection across the valley than does an alongshore current driven by the inflow condition.

A numerical model of circulation in a continental shelf-silled fjord coupled system

A numerical model of continental shelf-silled fjord coupled systems was constructed motivated by the great range of oxic and anoxic fjord deep water conditions, and bottom water renewal processes found in silled estuaries. The model is a cross-sectional, time dependent, non-linear, finite-difference model that includes friction, stratification and topography but assumes no longshore propagation or advection. While the model is of a general nature, this study considers a specific case of a high latitude subarctic fjord opening on to the Gulf of Alaska shelf. The experiments simulating fjord circulation focus on bottom water renewal, include modeling the inflow of denser shelf water over the sill into the fjord basin by both static (initial conditions) and dynamic processes (freshwater inflow and long-fjord winds inside the fjord, and longshore winds on the adjacent shelf). The results of the modeling experiments suggest that Coriolis force should be considered in modeling narrow fjord basins. Winds within the fjord are important in the surface circulation but in the statified case, have little effect on deep circulation. The effect of freshwater inflow to Resurrection Bay is shown to be minimal. Meteorological forcing of hydrographic conditions on the adjacent shelf (coastal upwelling and downwelling) greatly affects fjord wide circulation, especially bottom water circulation and renewal. The results suggest that deep-water renewal is nearly continuous year around although the driving forces (e.g. wind driven coastal upwelling and downwelling) and associated results (e.g. variations in hydrography) are strongly seasonal. Comparison of hydrographic, meteorological and current data from the Resurrection Bay-continental shelf system with the model results show agreement.

A Model of Mesoscale Air-Sea Interaction in a Sea Breeze-Coastal Upwelling Regime

Abstract A time-dependent, two-dimensional numerical model is constructed by coupling a four-layer atmosphere to a two-layer ocean through fluxes of heat and momentum. Idealized experiments are performed to investigate the oceanic response to sea breeze forcing, changes induced in the sea breeze by coastal upwelling, and air-sea feedback during periods of active coastal upwelling. This problem is motivated by the fact that the time scale of the coastal upwelling response is short compared to most oceanic response times and is comparable to the sea breeze time scale. When forced with a longshore wind stress, the model ocean reproduces several features commonly observed in coastal upwelling regimes, including an equatorward surface jet, a poleward undercurrent, and a region of low sea surface temperatures near the coast. For the cases considered here, the sea breeze contributes significantly to the mean longshore wind stress and, consequently, plays a role in driving the coastal upwelling circulation. It al...

On the generation of the seasonal coastal upwelling in the Gulf of Guinea

Seasonal upwelling in the coastal waters of the Gulf of Guinea is unusual because classical coastal upwelling theory does not even qualitatively hold there; there is no significant relationship between the upwelling and the local longshore wind stress. In this paper an attempt is made to qualitatively explain the Gulf of Guinea seasonal upwelling using coastal trapped forced wave dynamics. Two independent sets of data were used to compare theory with observation. Both indicate that provided dissipation of the travelling wind‐forced wave is taken into account, the theory agrees qualitatively with observation. The implication is that the upwelling is generated by coastal wind forcing several hundred kilometers east of the upwelling region. The Gulf of Guinea coastal upwelling therefore does not seem to be related to upwelling generated along the equator or to deep ocean basin forcing—it is not an ‘anti El Niño,’ as has been postulated elsewhere.

Low‐frequency wind‐induced sea level oscillations along the east coast of North America

Atmospherically adjusted coastal sea level observations (ASL) and wind measurements at stations along the east coast of North America from Cape Hatteras, North Carolina, to Eddy Point, Nova Scotia, for a 6‐month winter period were used to study the spatial structure of ASL and wind stress on the continental shelf and to determine the low‐frequency response of ASL to wind forcing. ASL and wind stress were coherent over distances of 1300 km for motions with periods between 60 and 600 hours. Northward movement of storm systems and of the associated ASL response was observed for stations north of Cape May, New Jersey. South of Cape May the phase of ASL suggested southward movement of pressure disturbances. Longshore wind accounted for 55% of ASL energy and led ASL by 8–12 hours. Cross‐shore winds were generally not coherent with ASL. In the Middle Atlantic Bight (MAB), sea level response was 17 cm for a 1 dyn cm−2 longshore wind and symmetric in the longshore wind stress directions. In contrast, from Nantucket north, sea level response to longshore wind stress was weaker (9 cm for a 1 dyn cm−2 wind to the northeast). The response for wind to the northeast was twice as large as the response for wind stress toward the southwest. If coastal winds were representative of winds over the shelf, the observations imply that the wind‐driven longshelf geostrophic currents were larger for unit wind stress in the MAB than in the Georges Bank‐Scotian Shelf region. The regional change in ASL response may be due to the large semidiurnal tidal currents which occur in the Georges Bank region or to the rough topography on the Scotian Shelf, which effectively increases bottom friction. The regional change in ASL response suggests a divergence in longshore flow and an onshore flow on the shelf south of Nantucket for winds to the southwest. A low‐frequency longshelf ASL slope was observed in the MAB that was incoherent with longshelf wind stress.

Coastal Upwelling/Downwelling Cycles in Southern Lake Superior

Extensive current meter, hydrographic and wind data were collected in the region of the Keweenaw Current, a strong coastal jet, during the late spring and early summer regime of Lake Superior. These data were compared with surface wind and pressure patterns over the region with the following conclusions: 1) Longshore winds associated with migrating low-pressure systems generate onshore Ekman transport and coastal downwelling. The coastal downwelling results in an intensification of the onshore internal pressure gradient, and consequently the coastal jet. Speeds of over 75 cm s−1 are both observed and calculated, suggesting the Keweenaw Current, at these times, is in geostrophic equilibrium. 2) Longshore winds associated with migrating high-pressure systems generate offshore Ekman transport and coastal upwelling. Upwelling velocities of 13-50 m day−1, an offshore surface transport layer of 10 m, a near-shore deceleration of the Keweenaw Current and current direction fluctuations of near- and sub-inertial period are found associated with the upwelling. The period of passage of highs (upwelling) and lows (downwelling) in July is about 4–6 days. This gives the Keweenaw Current the appearance of a pulsating coastal jet of period 4–6 days and amplitude about 60 cm s−1.