Longshore Wind Stress Research Articles

Tropical Atmospheric Intraseasonal Oscillations Leading to Sea Level Extremes in Coastal Indonesia during Recent Decades

Abstract Coastal flooding induced by sea surface high extreme (HEX) events is an increasing risk to human society and infrastructure as both urban growth in coastal areas and anthropogenic sea level rise continue, especially for island nations like Indonesia. This paper investigates the role of atmospheric intraseasonal oscillations (ISOs), which are dominated by the Madden–Julian oscillation (MJO), in forcing HEXs on the coasts of Indonesia bordering the Indian Ocean. We use satellite altimetry data from 1993 to 2021 and tide gauge observations to detect HEXs, and modeling experiments using both the Regional Ocean Modeling System and a Bayesian dynamic linear model to understand the forcing and processes. We find that HEXs exhibit strong seasonality, with most events occurring during boreal winter (December–February) and spring (March–May) that are dominated by seasonal-to-decadal and intraseasonal variability respectively. In 32% of the 56 HEX events detected, the amplitude of ISO-induced sea level anomalies (SLAs) exceeds that of seasonal-to-decadal SLAs. Surface wind stress associated with atmospheric ISOs is the major forcing for intraseasonal SLAs, and both the remote westerly wind stress from the Indian Ocean equator and northwesterly longshore wind stress at the Indonesian coasts play important roles in driving the HEXs. The MJO is the dominant cause of ISO-dominated HEXs and its impact shows strong seasonal differences. Spring MJOs are associated with stronger convective anomalies over the eastern Indian Ocean equator that drive stronger zonal winds across the equatorial basin that lead to more HEX events compared to winter MJOs when the convection is shifted southward.

The Longshore Transport Enigma and Analysis of a 10-Year Record of Wind-Driven Nearshore Currents

Burnette, C. and Dally, W.R., 2018. The longshore transport enigma and analysis of a 10-year record of wind-driven nearshore currents.Previous analysis of a 10-year record of nearshore directional wave spectra collected with an Acoustic Doppler Current Profiler (ADCP) installed outside the surf zone in Melbourne Beach, Florida, unexpectedly revealed that the long-term average wave-induced radiation stress (Sxy) was nearly balanced between northerly and southerly forcing. More than 4 years of wind data collected at the site with a directional anemometer also showed a nearly balanced net longshore wind stress. However, shoreline offsets at nearby jettied inlets clearly indicate a predominant north-to-south net sediment transport. This enigma was investigated by analyzing the nearshore currents measured by the ADCP, and examining their correlation with the wind and incident waves. Significant correlation was found between the depth-averaged wind-driven longshore current and the incident wave conditions; e.g., the average energy-based significant wave height ___mo is typically larger (0.95 m) when the current is directed to the south than when the current is directed to the north (0.73 m). Guided by the classic Coastal Engineering Research Center (CERC) formula for longshore sediment transport, it is found that is significantly more correlated with southerly directed longshore currents (r = 0.47) than northerly currents (r = 0.21). Also, if a “storm” is defined as whenever Hmo exceeds 1.75 m, 40% of this time, the mean wave direction is out of the northeast quadrant, 33% is from the southeast, and 27% approaches shore-normal. Additionally, during storms, a stronger correlation between Sxy is found with southerly directed wind-driven currents (r = 0.51) than with northerly directed currents (r = 0.32). These findings indicate that net longshore sediment transport may in fact be heavily influenced by the correlation of the local wind with the waves, a feature not included in traditional longshore sediment transport formulas.

Dynamical structure of the sea off the east coast of Peninsular Malaysia

The regional ocean modelling system (ROMS), at a resolution of 9 km, is used to investigate the dynamics of the sea off the east coast of Peninsular Malaysia (ECPM). The model is configured with two, one-way nested domains. Two simulations are performed in order to understand the dynamical structure of the sea off ECPM. The first simulation is a control run, using climatological monthly mean wind stress, surface freshwater flux, heat and observational oceanic inflow and outflow at open boundaries. The second simulation is an experiment aimed at presenting the seasonally averaged effect of isolated forcing in the absence of wind stress. This procedure allows understanding of the upwelling mechanism in the absence of wind stress forcing within the region. The model simulated the oceanographic features in the region reasonably well, in particular the circulation and temperature patterns conformed to those of Simple Ocean Data Assimilation and observations. Results show the possibility of upwelling in the summer monsoon along the sea off ECPM. This is due to the strong long-shore wind stress, which coincided with lower sea surface height and high baroclinic instability. The strong positive horizontal transport in accordance with the positive vertical transport within 104.5° E and 105.5° E, at latitude 3° N suggests net offshore transport and the occurrence of upwelling. Moreover, results demonstrate that summer upwelling rate at corresponding latitude induced by the long-shore wind stress is 3 × 10−5 m/s larger than the vertical velocity of 0.5 × 10−5 m/s induced by the wind stress curl. This reflects the importance of the long-shore wind stress for inducing the coastal upwelling. High concentrations of phytoplankton biomass through the proxy of chlorophyll-a concentration and the observed upward motion of denser water at the sea surface are due to the upwelled nutrient-rich water.

Wind-Driven Coastal Sea Level Variability in the Northeast Pacific

Abstract The rate of coastal sea level change in the northeast Pacific (NEP) has decreased in recent decades. The relative contributions to the decreased rate from remote equatorial wind stress, local longshore wind stress, and local windstress curl are examined. Regressions of sea level onto wind stress time series and comparisons between NEP and Fremantle sea levels suggest that the decreased rate in the NEP is primarily due to oceanic adjustment to strengthened trade winds along the equatorial and coastal waveguides. When taking care to account for correlations between the various wind stress time series, the roles of longshore wind stress and local windstress curl are found to be of minor importance in comparison to equatorial forcing. The predictability of decadal sea level change rates along the NEP coastline is therefore largely determined by tropical variability. In addition, the importance of accounting for regional, wind-driven sea level variations when attempting to calculate accelerations in the long-term rate of sea level rise is demonstrated.

On the Upwelling off the Southern Tip and along the West Coast of India

Abstract The processes of upwelling off the southern tip (the Kanyakumari coast) and the west coast of India are highly localised features with different forcing mechanisms, and they cannot be treated as a uniform wind-driven upwelling system. Off the Kanyakumari coast, upwelling is due to the south-west monsoon winds that are tangential to the coast. Along the west coast, the area between 8° N and 9° N represents the shadow zone to the influence of the remote forcing on the upwelling; the latter is forced by the longshore wind stress. Moderate to relatively intense upwelling occurs along the Kollam to Mangalore coast (9° N to 13° N) due to the combined action of the longshore wind stress, the coastally trapped Kelvin waves, and the offshore propagating Rossby waves. North of this area (13° N to 15° N), upwelling is weak due to weak wind stress and is closely confined to the coastal belt. This results from the suppressive effect of the southwards-flowing Arabian Sea high saline water on the process of upw...

Steady, frictionally modified wind and current forced shelf circulation: application to Vancouver Island

This paper examines the steady barotropic frictionally controlled circulation over a linear shelf which has an abrupt change in width. The model allows the net longshore transport to be specified and includes forcing due to: (1) flow through a gap in the coastline located along the discontinuity; and (2) longshore wind stress. Rapid longshore variations in the flow occur in the neighbourhood of the shelf width discontinuity and therefore both the cross-shelf and long-shore components of bottom friction must be retained. These give rise to a vorticity equation of elliptic character, in contrast to the parabolic “arrested topographic wave equation”. A “shadow zone” is produced on the wide shelf, adjacent to the shelf width discontinuity, in response to a uniform longshore wind stress. Within the shadow zone the fluid is almost stagnant. As friction increases, greater cross-isobath flow occurs, which reduces the area of the shadow zone. A combination of flow through the strait and longshore wind stress can generate complicated flow patterns in which eddies are ubiquitous. A combination of prescribed non-zero net poleward longshore transport and a uniform longshore wind stress generate a nearshore flow which is equatorward-poleward when the wind is directed equatorward-poleward. Further, the axis of the poleward longshore current migrates towards the shelf edge in response to an equatorward longshore wind stress of increasing magnitude. Seasonal variations in the nearshore flow off Vancouver Island, described by Hickey [(1979) Progress in Oceanography, 8, 191–279] and Freeland et al. [(1984) Atmosphere-Ocean, 28, 288–302] can be qualitatively reproduced by this simple method.

Numerical Simulation of the Exchange Process within a 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.

Wind-forced Motion on the Northern Great Barrier Reef

Abstract Observations of current, bottom pressure, and wind from the shelf of the Northern Great Barrier Reefare interpreted using a two-layer, frictional, step-shelf model forced by propagating wind stress. Distinguishing features of the region are the many reefs on the shallow shelf, the well-mixed nature of the shelf waters, and the steep continental slope. The instrument army was deployed from April to October 1982 and comprised 14 current meters, nine pressure gauges, and two weather stations. Low-pass [<0,6 cycles per day (cpd)] filtered velocity and pressure data are analyzed separately using a time domain empirical orthogonal function (EOF) analysis. The first EOFs of both velocity and pressure data are found to account for most of the observed variance on the shelf. Time series of these two EOFs are both highly coherent, with the local longshore wind stress at “weatherband” frequencies(<0.3cpd);however, the frequency transfer functions of the wind to the current and of the wind to the pressure ar...

Observations of the Vertical Structure of the Keweenaw Current, Lake Superior

The vertical structure of the Keweenaw Current, on the south shore of Lake Superior, is studied using data from an autonomously profiling current meter moored 2 km offshore (depth = 100 m). Vertical profiles of temperature, current speed, and current direction show current direction strongly aligned east-west with the topographic contours, with currents below 50 m depth most frequently flowing eastward at speeds of 6-12 cm/s, and those above 50 m depth flowing sometimes eastward and sometimes westward at speeds of 10-20 cm/s. Principal component analysis on profiles of eastward velocity reveals two vertical modes which have approximately the barotropic and first baroclinic modal form. The dominant “barotropic” mode contains 90% and 82%, respectively, of the eastward velocity and temperature variances. Strong occurrences of this mode (full-water-column eastward flows with speeds up to 37 cm/s) are related to strong increases in the temporal integral of longshore wind stress, consistent with coastal jet theory. The “baroclinic” mode, containing 6% of the eastward velocity variance, is dominant only during periods of weak longshore wind stress. Its 4-day period is comparable with a typical interval between atmospheric frontal disturbances.

Resonant forcing of coastally trapped waves in a continuously stratified ocean

In a previous paper (Grimshaw, 1987) the resonant forcing of coastally trapped waves was discussed in the barotropic case. In order to extend that theory to more realistic situations, we have considered the analogous theory whereby a longshore current interacts with a longshore topographic feature, or the forcing is due to longshore wind stress, for the case of the continuously stratified ocean. As in the previous theory, near resonance, when a long-wave phase speed is close to zero (in the reference frame of the forcing), the wave motion is governed by a forced evolution equation of the KdV-type. The behaviour of the wave field is characterized by three parameters representing the bandwidth for resonance, the forcing amplitude and the dissipation. We have evaluated these parameters in various practical cases, and found that the bandwidths, which scale with α1/2 when the forcing has dimensionless amplitude α, can often be quite broad. Typically the second, third, or higher, modes may be resonant. Concurrently, the dissipation is also usually significant, leading to a steady state balance between the forcing, dissipation and nonlinear terms.

Seasonal and Interannual Variability of Current, Temperature and Salinity off Southwest Nova Scotia

Strong annual signals are found in temperature, salinity, and current measurements at two sites off Cape Sable, Nova Scotia. Differences in means and annual cycles indicate that the along-isobath current is not controlled by wind stress at these time scales. The phase of the primary peak of freshwater runoff into the Gulf of St. Lawrence (RIVSUM) is consistent with the advection of a salinity minimum to Cape Sable at an average speed of 6 km∙d−1, but the secondary peak is not observed and may be masked by the seasonal inflow of slope water into the Gulf of Maine via Northeast Channel. A diffusive model, forced by air–sea interaction, provides a reasonable fit to the annual temperature cycles with vertical diffusivities of 15–50 × 10−4 m2∙s−1. Correlations between residual longshore wind stress and along-isobath current, deep salinity, and temperature are consistent with wind-driven upwelling. Furthermore, warm-core rings in the slope water sometimes enhance onshore heat and salt fluxes to create positive temperature and salinity anomalies off Cape Sable, especially in late summer. February ocean temperature anomalies to 50 m are related to winter-average cross-shore wind (and related heat flux residuals), but do not persist on seasonal time scales.

Large-Scale, Low-Frequency Response on the Continental Shelf Due to Localized Atmospheric Forcing Systems

Abstract The first-order wave equation model of Gill and Schumann, which describes the flow induced on the continental shelf by the longshore wind stress, evaluated at the coast, is extended in three ways. First, we relax the nondivergent approximation and hence are able to estimate the response in the Kelvin wave mode. Second, we include the full wind stress forcing, and allow for an offshore variation in the wind stress. We also include a representation of pressure forcing. Thus we are able to describe the response due to localized forcing systems. Third, we consider the coupling of the deep-ocean region to the shelf region across a continental slope region, modeled as a discontinuity in depth at the shelf break. The results are applied to the response on the shelf due to a localized forcing system, either traveling longshore parallel to the coast, or traveling offshore normal to the coast.

Long shelf waves generated by a coastal flux

Solutions are presented for the shelf response of barotropic and stratified oceans to an oscillatory coastal flux through a strait. The flux is modeled as the return interior flow due to an Ekman transport that arises from a fictitious square‐wave longshore wind stress. The width 2 d, of the square‐wave stress is taken to be that of the strait and the response due to the coastal flux determined from longwave wind forced solutions that exist in the literature. The net longshore energy flux for the nth strait forced shelf wave mode, wave number kn, is shown to be proportional to [bnsin(knd)( knd)]2, where is the coastal flux spectrum and bn the modal “wind” coupling coefficients. Results compare favorably with existing barotropic theories and estimates of net longshore energy flux obtained for the Australian Coastal Experiment region are shown to be in reasonable agreement with observations at periods of 9 days, when forcing by Bass Strait is assumed greatest.

Momentum and heat balances in steady coastal currents

The momentum balance for coastal currents forced by a longshore wind stress and a longshore density gradient is examined for coastlines of various orientations. We isolate the coastal circulation by requiring that the longshore wind stress and density gradient become uniform in the adjacent deep ocean in which the longshore bottom pressure gradient is assumed to be zero, and derive a vorticity equation for the longshore current for a prescribed cross-shelf variation of wind stress and density structure. Solutions of this equation are obtained for a uniform longshore wind stress along a coastline with an exponential shelf in two situations: (A) For a uniform cross-shelf density on the inner shelf and a zero longshore bottom pressure gradient on the outer shelf, the division between the two regions occurring approximately at the depth of the main thermocline, and (B) for a thermohaline structure which is independent of the cross-shelf co-ordinate such that the coastal current is barotropic. It is shown that on a zonal coast the coastal transports due to the wind stress forcing are of O(σ τ Wl ϱ 0β ) , and due to the density gradient forcing of O(− σ ϱ 0 gh T 2/ β∂δ 0/ ∂l) where τ Wl and − ϱ 0 ∂δ 0/ ∂l are respectively, the longshore wind stress and surface density gradient, ϱ 0 is a reference density for seawater, g is the acceleration of gravity, h T is the scale depth for the main thermocline, β= df dy where f=2Ω sinϕ in which Ω is the angular speed of rotation of Earth, and ϕ is the latitude, and σ= W T W is a shelf parameter in which W T is the exponential shelf scale depth and W is an eddy scale. On eastern boundaries the transports are less [especially in (A)] and are identically zero on western boundaries on which the transport is determined by external (non-local) processes. The longshore velocity structure on the western boundary depends strongly on σ and incorporates a system of currents and counter-currents. The heat balance for the coastal circulation is investigated by considering the solution of the heat conservation equation for the surface temperature with the cross-shelf boundary conditions of zero heat flux through the coast and into deep ocean. It is found that the sea surface temperature anomaly is zero for wind stress forcing and positive for density gradient forcing (at constant salinity). Mixed forcing however may lead to upwelling which is characterized by a negative sea surface anomaly. Upwelling occurs on non-western boundaries on which there is a longshore wind stress of sufficient intensity or on western boundaries where there is an external transport of sufficient magnitude to generate a net transport up the longshore surface density gradient. This occurs on eastern coasts where there is poleward advection of warm water and sufficiently strong equatorward meridional winds, and on western coasts in a polar gyre where there is a strong equatorward external transport and a poleward temperature gradient. The Ekman circulation maintains the upwelling anomalies.

The nearshore current along a high-rainfall, trade-wind coast-Nicaragua

Runoff from abundant rainfall on the watersheds along the east coast of Nicaragua results in a well-defined nearshore current, extending 20–40 km out from the coast. Important terms in the controlling dynamical balance are the Coriolis forces, and eddy frictional forces. Calculations of the diabathic surface slope 0(10 −5) show a smooth setup of 4–5 cm from the outer edge of the current to the shoreline. A longshore surface slope of 0(10 −8) appears to be set up by the longshore wind stress, and further computations allow an estimate of ∼ 6 gr cm −1 s −1 for the dynamic eddy viscosity. An analytical expression including diabathic surface slope and density gradient, parabathic surface slope, wind stress, and quadratic bottom friction reproduces the salient observed features of the nearshore current. These include the pronounced maximum in the parabathic (along-shelf) current about 10 km off the coast, a complex diabathic velocity structure, and a shelf countercurrent just seaward of the outer edge. Further calculations suggest that the dominant driving arises from the freshwater-induced density gradients, accounting for upward of 80% of the flow velocity. As suggested by Royer (1982), the prevailing trade wind exerts an onshore wind stress that serves the important role of maintaining the integrity of the density gradients via the convergence of a surface Ekman layer toward the coast.

The Large-Scale Environment of the Poleward-Flowing Leeuwin Current, Western Australia: Longshore Steric Height Gradients, Wind Stresses and Geostrophic Flow

Abstract Steric heights have been estimated using T–S relationships for all available XBTs within 10–40°S, 105–130°E to 1980, and merged with steric heights from hydrology data. These data were subdivided into bins following the Western Australian coast, and a two-harmonic best fit to the seasonal cycle obtained in each bin. Seasonal departures from the annual mean of surface steric height are combined with similar departures of pressure-corrected coastal tide gage data to obtain maps of seasonal departure in geostrophic flow, both on and off the continental shelf. These are added to annual mean patterns to give total seasonal flows, for comparisons with earlier work. Wind stress vectors, wind stress curls and longshore wind stress components are also obtained, using data from a marine climate altas. The results give an improved picture of the broad-scale environment surrounding the Leeuwin Current (a narrow, rapid poleward current along the Western Australian continental shelf edge) and of the wind stres...

Gulf Stream and wind‐induced current variability on the Georgia continental shelf, winter 1978

Low‐frequency longshore current fluctuations on the continental shelf off Georgia and their relationships with local atmospheric forcing and Gulf Stream displacement were examined for a 3‐month period from January to April 1978. (Acoustic travel time and bottom pressure measurements at a station on the continental slope were used to determine the depth of the main thermocline, as an indicator of Gulf Stream displacement.) On the middle shelf, current variability was dominated by local wind forcing at periods longer than 2 days, with very little Gulf Stream influence. Longshore wind stress was the main driving force at periods longer than 4 days, while cross‐shore wind contributed at shorter periods. In contrast, on the outer shelf, current fluctuations in the upper layer were highly coherent with Gulf Stream displacement at a 12‐day period, and marginally coherent with longshore wind at a 6‐day period. Linear regression analysis showed that Gulf Stream and local wind forcing accounted for most of the fluctuations in the upper layer over the shelf break for time scales greater than 5 days and at around 2 days. A low multiple coherence window at 2.8–5 days was probably due to Gulf Stream frontal eddies. In the lower layer over the shelf break the current fluctuations had a character intermediate between the upper layer currents (Gulf Stream dominated) and mid‐shelf currents (wind dominated).

Hydrography, nutrients, suspended organic matter, and primary production in a shallow fjord system on the west coast of Norway

Abstract The investigation was carried out at three stations in a coastal area, characteristic for the west Norwegian coast. 21 cruises cover the period February to October 1982. The hydrography (T °C, S %º, O2/H2S), the nutrient concentration (NO3 -, SiO2, PO4 3-), suspended organic biomass (chlorophyll a, phaeophytin, particulate organic carbon, and nitrogen), and in situ 14C-assimilation were measured at fixed depths. The predominant northerly and southerly winds and the resulting on-shore and off-shore transport of water had a significant influence on the hydrography at the outer station in Raunefjorden, somewhat less at the intermediate station in Kviturdvikpollen, but were of minor influence at the permanently stratified inner station in Vågsböpollen. Hydrogen sulphide occurred in the bottom water of both Vagsbopollen and Kviturdvikpollen from June to October. Maximum concentrations were up to 6 times higher than reported in a study 18 years ago in the same localities. The concentrations of nutrients and suspended organic matter were highest in Vågsböpollen and Kviturdvikpollen. Highest suspended concentrations were found at the oxic/anoxic interface. In Raunefjorden the nutrient and suspended organic matter concentrations were strongly influenced by upwelling of nutrient-rich water. The annual primary production at the three stations ranged from 180 to 230 g C/m2 per year. The topography of the fjords, the fresh-water run-off, and changes in the predominantly longshore wind stress are essential for an understanding of the phytoplankton dynamics in the area.

Wind-driven circulation on the northern Great Barrier Reef continental shelf in summer

Observations of wind, atmospheric pressure, sea levels and current are presented for the northern (9°14′S) Great Barrier Reef continental shelf for November 1981 to May 1982. Strong low-frequency non-tidal oceanic fluctuations were observed, resulting in alternating northward and southward transport and a weak ‘mean’ circulation, and are the result of long ‘arrested’ topographic waves driven by a quasi-steady longshore wind stress and damped by turbulent bottom friction. The coefficient of friction for low-frequency currents is found to be proportional to the tidal velocities. The coefficient is also found to be much larger than that in the central (15–19°S) region of the Great Barrier Reef continental shelf, and this difference is attributed to the greatly enhanced energy dissipation by secondary circulation around coral reefs in the reef-studded northern region. Sough of Cape York (10·5°S), the primary effect of the cross-shelf wind is to complement the geostrophic set-up due to the longshore current. Intense tidal currents in the vicinity of Cape York combined with a dissipative western boundary in Torres Strait appear to prevent long wave propagation north of Cape York where a steady-state analytical model is used to show that both cross- and longshore wind components generate the reversing currents observed through Bligh Entrance.

The Fluctuating Longshore Pressure Gradient on the Pacific Northwest Shelf: A Dynamical Analysis

Abstract The majority of papers on Pacific Northwest shelf dynamics have emphasized the relationship between longshore wind stress τ3, and longshore velocity v. However, attempts to illustrate a balance of momentum in the longshore direction have not been encouraging: τ3*sol;H (where H is water depth) has insufficient magnitude to balance the vertically averaged longshore acceleration Vt,, at least during summer. In this paper it is demonstrated that the missing momentum is provided by the longshore pressure gradient force −py,. The pressure gradient was estimated using tide gauge and atmospheric pressure data at stations separated by roughly 400 km. Seasonal and long-term means from Hickey and Pola and, in some cases, nonseasonal monthly anomalies from Enfield and Allen were added to the sum of the tide gauge and atmospheric pressure data to form time series of total subsurface pressure. The pressure data were multiplied by an offshore deny factor to simulate coastal trapping. The analysis was performed ...