Wind-generated waves in Hurricane Juan

We present numerical simulations of the ocean surface waves generated by hurricane Juan in 2003 as it reached its mature stage (travelling from deep waters off Bermuda to Nova Scotia and making landfall near Halifax) using SWAN (v.40.31) nested within WAVEWATCH-III (v.2.22; denoted WW3) wave models, implemented on multiple-nested domains. As for all storm-wave simulations, spectral wave development is highly dependent on accurate simulations of storm winds during its life cycle. Due to Juan’s rapid translation speed (accelerating from 2.28 m s⁻¹ on 27 September, 1200 UTC to 20 m s⁻¹ on 29 September, 1200 UTC), an interpolation method is developed to blend observed hurricane winds with numerical weather prediction (NWP) model winds accurately. Wave model results are compared to in situ surface buoys and ADCP wave data along Juan’s track. At landfall, Juan’s maximum waves are mainly swell-dominated and peak waves lag the occurrence of the maximum winds. We explore the influence of surface waves on the wind and show that the accuracy of the wave simulation is enhanced by introducing swell and Stokes drift feedback mechanisms to modify the winds, and by limiting the peak drag coefficient under high wind conditions, in accordance with recent theoretical and experimental results.     

Eddies and Tropical Instability Waves in the eastern tropical Pacific: A review

Mesoscale eddies and tropical instability waves in the eastern tropical Pacific, first revealed by satellite infrared imagery, play an important role in the dynamics and biology of the region, and in the transfer of mass, energy, heat, and biological constituents from the shelf to the deep ocean and across the equatorial currents.From boreal late autumn to early spring, four to 18 cyclonic or anticyclonic eddies are formed off the coastal region between southern Mexico and Panama. The anticyclonic gyres, which tend to be larger and last longer than the cyclonic ones, are the best studied: they typically are 180–500 km in diameter, depress the pycnocline from 60 to 145 m at the eddy center, have swirl speeds in excess of 1 m s⁻¹, migrate west at velocities ranging from 11 to 19 cm s⁻¹ (with a slight southward component), and maintain a height signature of up to 30 cm. The primary generating agents for these eddies are the strong, intermittent wind jets that blow across the isthmus of Tehuantepec in Mexico, the lake district in Nicaragua and Costa Rica, and the Panama canal. Other proposed eddy-generating mechanisms are the conservation of vorticity as the North Equatorial Counter Current (NECC) turns north on reaching America, and the instability of coastally trapped waves/currents.Tropical Instability Waves (TIWs) are perturbations in the SST fronts on either side of the equatorial cold tongue. They produce SST variations on the order of 1–2 °C, have periods of 20–40 days, wavelengths of 1000–2000 km, phase speeds of around 0.5 m s⁻¹ and propagate westward both north and south of the Equator. The Tropical Instability Vortices (TIVs) are a train of westward-propagating anticyclonic eddies associated with the TIWs. They exhibit eddy currents exceeding 1.3 m s⁻¹, a westward phase propagation speed between 30 and 40 km d⁻¹, a signature above the pycnocline, and eastward energy propagation. Like the TIWs, they result from the latitudinal barotropically unstable shear between the South Equatorial Current (SEC) and the NECC with a potential secondary source of energy from baroclinic instability of the vertical shear with the Equatorial Undercurrent (EUC).This review of mesoscale processes is part of a comprehensive review of the oceanography of the eastern tropical Pacific Ocean.     

Surface boundary-layer variability off Northern California, USA, during upwelling

A five-element mooring array is used to study surface boundary-layer transport over the Northern California shelf from May to August 2001. In this region, upwelling favorable winds increase in strength offshore, leading to a strong positive wind stress curl. We examine the cross-shelf variation in surface Ekman transport calculated from the wind stress and the actual surface boundary-layer transport estimated from oceanic observations. The two quantities are highly correlated with a regression slope near one. Both the Ekman transport and surface boundary layer transport imply curl-driven upwelling rates of about 3×10⁻⁴ m s⁻¹ between the 40 and 90 m isobaths (1.5 and 11.0 km from the coast, respectively) and curl-driven upwelling rates about 1.5×10⁻⁴m s⁻¹ between the 90 and 130 m isobaths (11.0 and 28.4 km from the coast, respectively). Thus curl-driven upwelling extends to at least 25 km from the coast. In contrast, upwelling driven by the adjustment to the coastal boundary condition occurs primarily inshore of the 40-m isobath. The upwelling rates implied by the differentiating the 40-m transport observations with the coastal boundary condition are up to 8×10⁻⁴ m s⁻¹. The estimated upwelling rates and the temperature–nitrate relationship imply curl-driven vertical nitrate flux divergences are about half of those driven by coastal boundary upwelling.     

Evaluation of Vector Winds Observed by NSCAT in the Seas around Japan

In order to validate wind vectors derived from the NASA Scatterometer (NSCAT), two NSCAT wind products of different spatial resolutions are compared with observations by buoys and research vessels in the seas around Japan. In general, the NSCAT winds agree well with the wind data from the buoys and vessels. It is shown that the root-mean-square (rms) difference between NSCAT-derived wind speeds and the buoy observations is 1.7 ms^–1, which satisfies the mission requirement of accuracy, 2 ms^–1. However, the rms difference of wind directions is slightly larger than the mission requirement, 20°. This result does not agree with those of previous studies on validation of the NSCAT-derived wind vectors using buoy observations, and is considered to be due to differences in the buoy observation systems. It is also shown that there are no significant systematic trends of the NSCAT wind speed and direction depending on the wind speed and incidence angle. Comparison with ship winds shows that the NSCAT wind speeds are lower than those observed by the research vessels by about 0.7 ms^–1 and this bias is twice as large for data observed by moving ships than by stationary ships. This result suggests that the ship winds may be influenced by errors caused by ship's motion, such as pitching and rolling.     

Measurement of skin friction distribution along the surface of wind waves

Measurements of local values of the skin friction have been made at many points along the surface of representative wind wave crests in a wind wave tunnel, by use of the distortion of hydrogen-bubble lines. The results obtained at 2.85-m fetch under 6.2 m s^–1 mean wind speed show that the intensity of the skin friction varies greatly along the surface of wind waves as a function of the phase angle. It increases rather continuously at the windward surface toward the crest, attains a value of about 12 dyn cm^–2 near the crest, decreases suddenly just past the crest, and the value at the lee surface is substantially zero Values of the skin friction thus determined along the representative wind waves give an average value of 3.6 dyn cm^–2, rather exceeding the overall stress value of 3.0 dyn cm^–2, which has been estimated from the wind profile. The results are interpreted as that the skin friction bears most of the shearing stress of wind, and that it exerts most intensively around the representative wave crests at their windward faces.     

Analysis of three years of boundary layer observations over the Gulf of Mexico and its shores

Boundary layer observations were made over the Gulf of Mexico over a 3-year period in order to develop and test methods for estimating surface fluxes and boundary layer wind fields. In addition to routinely available buoy and CMAN surface data, six 915 MHz radar wind profilers (RWPs) and RASS profilers were mounted on oil platforms and on the shore. Estimates of surface momentum, sensible heat, and latent heat fluxes have been made from the surface observations using the COARE software. Simulations by the National Weather Service's Eta meteorological model are compared with the observations of surface fluxes and wind profiles. The boundary layer is found to be unstable over 90% of the time, and latent heat fluxes are about five to ten times larger than sensible heat fluxes, as usually found over tropical oceans. Eta model simulations of surface fluxes are within about ±50% of COARE estimates of the fluxes based on surface observations. Most of the time, COARE-derived fluxes at 11 sites are within a factor of two of each other at any given hour. In multi-day case studies, COARE calculations are found to agree with Eta model simulations of these fluxes and parameters within a factor of two most of the time. Eta model simulations of wind speeds in the boundary layer tend to exceed the RWP observations by 1–2 m s⁻¹ near shore and by 2–6 m s⁻¹ at distances of 100–200 km offshore.     

Geoacoustic and physical properties of carbonate sediments of the Lower Florida Keys

 Near-surface sediment geoacoustic and physical properties were measured from a variety of unconsolidated carbonate sediments in the Lower Florida Keys. Surficial values of compressional and shear speed correlate with sediment physical properties and near-surface acoustic reflectivity. Highest speeds (shear 125–150 m s^-1; compressional 1670–1725 m s^-1) are from sandy sediments near Rebecca Shoal and lowest speeds (shear 40–65 m s^-1; compressional 1520–1570 m s^-1) are found in soft, silty sediments which collect in sediment ponds in the Southeast Channel of the Dry Tortugas. High compressional wave attenuation is attributed to scattering of acoustic waves from heterogeneity caused by accumulation of abundant shell material and other impedance discontinuities rather than high intrinsic attenuation. Compared to siliciclastic sediments, carbonate sediment shear wave speed is high for comparable values of sediment physical properties. Sediment fabric, rather than changes due to the effects of biogeochemical processes, is responsible for these differences.     

Discussion on the critical wind speed for wind-wave generation on the basis of shear-flow instability theory

Observed critical wind speeds for the generation of wind waves are compared with those derived from a shear-flow instability theory. The theory predicts that the critical wind speed depends on the fetch and, for the case of infinite fetch, it is 93 cm s^–1 at 30 cm above the mean water surface, which agrees well with observations at sufficiently large fetch. For water containing soap, the much larger critical wind speeds which are observed cannot be explained by the reduction of surface tension alone. A qualitative discussion suggests that the elasticity of surface films of soap can effectively increase the critical wind speed.     

One-dimensional modelling of convective CO2 exchange in the Tropical Atlantic

Diurnal changes in seawater temperature affect the amount of air–sea gas exchange taking place through changes in solubility and buoyancy-driven nocturnal convection, which enhances the gas transfer velocity. We use a combination of in situ and satellite derived radiometric measurements and a modified version of the General Ocean Turbulence Model (GOTM), which includes the National Oceanic and Atmospheric Administration Coupled-Ocean Atmospheric Response Experiment (NOAA-COARE) air–sea gas transfer parameterization, to investigate heat and carbon dioxide exchange over the diurnal cycle in the Tropical Atlantic. A new term based on a water-side convective velocity scale (w_*w) is included, to improve parameterization of convectively driven gas transfer. Meteorological data from the PIRATA mooring located at 10°S10°W in the Tropical Atlantic are used, in conjunction with cloud cover estimates from Meteosat-7, to calculate fluxes of longwave, latent and sensible heat along with a heat budget and temperature profiles during February 2002. Twin model experiments, representing idealistic and realistic conditions, reveal that over daily time scales the additional contribution to gas exchange from convective overturning is important. Increases in transfer velocity of up to 20% are observed during times of strong insolation and low wind speeds (<6 m s⁻¹); the greatest enhancement from w_*w to the CO₂ flux occurs when diurnal warming is large. Hence, air–sea fluxes of CO₂ calculated using simple parameterizations underestimate the contribution from convective processes. The results support the need for parameterizations of gas transfer that are based on more than wind speed alone and include information about the heat budget.     

Comparison of wind-stress algorithms and their influence on wind-stress curl using buoy measurements over the shelf off Bodega Bay, California

The main objectives of this study were to compare three wind-stress algorithms of varying intricacy and estimate the extent to which each method altered computed wind-stress curl. The algorithms included (1) a simple bulk formula for neutral conditions that is dependent only on wind velocity components; (2) a formula that in addition to dependence on wind components includes a simplified effect of thermal stability through differences in air and sea temperatures; and (3) an algorithm that includes full treatment of dynamics and atmospheric stability. Data for the analysis were from a field program that used a special buoy network off Bodega Bay during 28 June–4 August 2001.A diamond-shaped setup of five closely separated buoys in Bodega Bay allowed for one of the first attempts to compute wind-stress curl over the ocean using buoy measurements. Based on an analysis of the available dataset, the marine layer over Bodega Bay is characterized by positive wind-stress curl with a median value around 0.2 Pa (100 km)⁻¹ and maximum values reaching 2.5 Pa (100 km)⁻¹. Positive wind-stress curl was observed for all wind speed conditions, whereas negative wind-stress curl episodes were associated mostly with low-wind conditions.Comparison of wind-stress curl computed using the three algorithms showed that differences among them can be significant. The first and third algorithms indicated similar stress curl (difference around 10%), but the differences between these two and the second algorithm were much higher (approximately 40%). The reason for the difference is the stability correction, which in the third algorithm strongly decreases with an increase in wind speeds, but stays at a similar level for all wind speeds in the second algorithm. Consequently, for higher wind speeds the variability of wind stress calculated using the second algorithm is greater than for the other two algorithms, causing significant differences in computed wind-stress curl (root mean-square error equal to 0.19 Pa (100 km)⁻¹).Despite the apparent biases in computed wind stress and wind-stress curl among the algorithms, all of them show a significant trend of decreasing sea-surface temperature (SST) with increasing wind-stress curl. The bootstrapping analysis has revealed that both the along-shore wind stress and wind-stress curl have noticeable correlation with the changes in the sea-surface temperature as an indirect indication of the upwelling. An additional analysis, based on the low-pass filtered data, showed also significant agreement between the measured divergence in the cross-shore surface transport and the wind-stress curl computed for all three algorithms.     

Continental shelf waves off the fukushima coast Part I: Observations

Current records obtained in the inshore region along the Fukushima coast are analyzed. The existence of periodical current fluctuations whose period is about 100 hours and whose amplitude is as large as 15–25cm s^–1 is recognized. Auto-spectral analyses are made also for sea level, atmospheric pressure and wind records. Each spectrum has significant peaks at the similar period to the current spectrum. The wind spectrum has a broad peak compared with the current. The periodical current fluctuations propagate southward with speed of 3–5 km h^–1. These propagation speeds seem to correspond to those of the second-and third-mode shelf waves.     

Surface currents derived from satellite-tracked buoys off Namibia

To study the flow field off Namibia (20–30°S, 10–15°E), 48 satellite-tracked buoys were deployed and tracked in six bimonthly batches between July 1994 to September 1995. In situ supporting wind information was collected from a weather buoy moored off Lüderitz, from coastal stations and from voluntary observing ships. Buoy drift tracks were compared with surface topography data from the TOPEX/POSEIDON satellite and satellite infrared images. Most of the buoys drifted in a northwesterly direction, the buoys deployed in the south generally moving faster and diverging more from the coast than the northern buoys. The overall maximum daily drift velocity was 72 cm s^-1, but typical speeds were 10–30 cm s^-1. In the proximity of the coast some buoys experienced transient southward sets associated with the effect of coastal trapped waves, while tracks north of 23°S showed inertial oscillations.     

A Recent Full-Depth Survey of the Alaskan Stream

We examine results from a cruise in May 1997. CTD casts to near the bottom were made south of the Aleutian Islands, across Amchitka Pass, and north of the islands. We computed a westward geostrophic speed of 123 cm s^–1 at 173.5°W in the Alaskan Stream. The computed volume transport there, referred to the bottom, was 25×10⁶m³s^–1. On other similar sections, transports were 8–15 × 10⁶ m³s^–1. Various complex variations in geopotential height along the Stream apparently altered the cross-stream gradients, and hence the transports. Rotational tendencies were also present. Northward inflow through Amchitka Pass was quite strong (6 × 10⁶ m³s^–1). Data north of the islands supported the existence of a zero-velocity reference level of variable depth.     

Detailed observation of the wind-exerted surface flow by use of flow visualization methods

Detailed observations were performed of the wind-exerted surface flow, before and after the generation of wind waves. As flow visualization techniques, 6 classes of polystyrene beads of from 0.33 mm to 1.93 mm in diameter, with a specific gravity of 0.99, and also, hydrogen bubble lines, were used. Experiments were carried out at three ranges of the wind speed: 4.0, 6.2 and 8.6ms^–1 in the mean in the wind-wave tunnel section, and the observations were made at 2.85 m in fetch. In the case of 6.2 m s^–1, when the initial surface skin flow attains 0.22 cm in the scale thickness and 16 cm s^–1 in the surface velocity in about 3 second from the onset of the wind, regular waves of about 1.7 cm in wave length appear on the water surface. In one second after that, the downward thrust of the surface flow and the consequent forced convection commences, and the transition of the surface layer to a turbulent state occurs. Ordinary wind waves begin to develop from this state. In developed wind waves the viscous skin flow grows on the windward side of the crests, frequently producing macroscopic skin flows, and these skin flows converge to make a downward thrust at the lee side, and the viscous skin layer disappears there. The velocity of the downward flow has a maximum at the phase of about 30, and the value is of the order of 10 cm s^–1 at 4-mm depth after the orbital velocity of the sinusoidal wave is subtracted. As the process through which the wind stress acts on the water surface, it is considered that the following particular one may be real: the skin friction concentrated at the windward side of the crest produces skin flows, which thrust into the inner region to make the forced convection, carrying the acquired momentum. The viscous shearing stress just before the generation of the surface undurations was about 1/4 of the total shearing stress under the existence of wind waves. It is considered that the increase of the wind stress by wind waves is caused by this mechanism.     

Measurements of the water-following capability of holey-sock and TRISTAR drifters

Since 1985, a number of measurements have been made in deep water to determine the water-following characteristics of mixed layer drifters with both holey-sock and TRISTAR drogues at 15 m depth. The measurements were done by attaching two neutrally buoyant vector measuring current meters (VMCMs) to the top and the bottom of the drogues and deploying the drifters in different wind and upper ocean shear conditions for periods of 2–4 h. The average velocity of the VMCM records was taken to be a quantitative measure of the slip of the drogue through the water, observed to be 0.5-3.5 cm s⁻¹. The most important hydrodynamic design parameter which influenced the slip of the drogue was the ratio of the drag area of the drogue to the sum of the drag areas of the tether and surface floats: the drag area ratio R. The most important environmental parameters which affected the slip were the wind and the measured velocity difference across the vertical extent of the drogue. A model of the vector slip as a function of R, vector wind and velocity difference across the drogue was developed and a least squares fit accounts for 85% of the variance of the slip measurements. These measurements indicated that to reduce the wind produced slip below 1 cm s⁻¹ in 10 m s⁻¹ wind speed, R > 40. Conversely, if the daily average wind is known to 5 m s⁻¹ accuracy, the displacement of the R = 40 drifter can be corrected to an accuracy of 0.5 km day⁻¹.     

Statistics of Wind and Waves off Tsuyazaki, Fukuoka, in the Eastern Tsushima Strait

The variability of the sea surface wind and wind waves in the coastal area of the Eastern Tsushima Strait was investigated based on the hourly data from 1990 to 1997 obtained at a station 2 km off Tsuyazaki, Fukuoka. The annual mean wind speed was 4.84 m s⁻¹, with strong northwesterly monsoon in winter and weak southwesterly wind in summer. Significant wave heights and wave periods showed similar sinusoidal seasonal cycles around their annual means of 0.608 m and 4.77 s, respectively. The seasonal variability relative to the annual mean is maximum for wave heights, medium for wind speeds, and minimum for wave periods. Significant wave heights off Tsuyazaki turned out to be bounded by a criterion, which is proportional to the square of the significant wave period corresponding to a constant steepness, irrespective of the season or the wind speed. For terms shorter than a month, the significant wave height and the wave period were found to have the same spectral form as the inshore wind velocity: white for frequencies less than 0.2 day⁻¹ and proportional to the frequency to the −5/3 power for higher frequencies, where the latter corresponds to the inertial subrange of turbulence. The spectral levels of wave heights and wave periods in that inertial range were also correlated with those of the inshore wind velocity, though the scatter was large. This revised version was published online in August 2006 with corrections to the Cover Date.     

On vertical velocity fluctuations and internal tides in an upwelling region off the west coast of India

Hourly fluctuations of vertical velocity in relation to components of flow and wind and temperature oscillations at a morring site in the shelf waters off the west coast of India are discussed. The vertical velocities were computed from a time series of vertical temperature profiles assuming that horizontal advection of temperature is negligible. The computed values at a depth of 40 m during the 72-h period of observation were of the order of 10⁻¹ to 10⁻²cm s⁻¹, with a mean value of −2·77 × 10⁻² cm s⁻¹ indicating a net upward movement of water. The computed vertical velocity showed fluctuations of about 2–3 h, in addition to weaker signals of about 12 h. Based on the spectral estimates, we speculate that these fluctuations of 2–3 h in the vertical velocity may be caused by the fluctuations in the along-shore wind. The oscillations of isotherms found in the temperaturedepth time series and the spectral estimates of temperature and cross-shore flow component showed a periodicity of about 12 h, which indicated the presence of semi-diurnal internal waves. The fact that these internal wave troughs were associated with the measured onshore flow suggested that the waves were propagating offshore. The computed stability parameters showed little evidence of instability or mixing. It was found that the isotherm troughs in the temperaturedepth time series at about 12-h period coincided with high vertical shear in the cross-shore direction and low values of Brunt Vaisälä frequency.     

Lagrangian measurements in the Kamchatka Current and Oyashio

From 1988 to 1993, 23 satellite-tracked drifting buoys entered the Kamchatka Current. The buoy trajectories showed a well-formed, high-speed current that originated near Shirshov Ridge, and flowed southward through Kamchatka Strait. During some years, the buoys turned eastward at 50°N, while in other years they were transported as far south as Japan (40°N). Only one buoy entered the Sea of Okhotsk. Eddies were evident in many of the buoy trajectories. Greatest maximum daily velocities (>100 cm s^–1) were observed south of Kamchatka Strait, with 50–60 cm s^–1 being more common.     

Evaluation of the relationship between parameters of the radar signal backscattered by the sea surface at grazing angles and the wind wave breaking characteristics

The results of investigations of the statistical characteristics of spikes of radar signals scattered by the sea surface at grazing angles and borizontally polarized radiation are reported. The measurements were conducted at wind speeds ranging from 2.8 to 9 m s^–1 and at various azimuthal directions relative to the general sense of wind wave propagation. The relationship between the theoretically and experimentally derived distribution parameters allows a conclusion about the linear dependence of spike amplitudes on the extent of instable areas. The theoretical model provides a qualitatively authentic azimuthal nature of the variation of the mean radar signal.Translated by V. Puchkin.     

Analysis of temporal and spatial characteristics of waves in the Indian Ocean based on ERA-40 wave reanalysis

Based on the 45-year (09/1957-08/2008) European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis (ERA-40) wave reanalysis dataset, this study analyzes interannual and interdecadal variabilities and intraseasonal oscillations of sea surface wind speed (WS), wind sea wave height (H^w), swell wave height (H^s) and significant wave height (H_s) in the Roaring Forties and tropical waters of the Indian Ocean, to determine swell propagation characteristics. The results show: (1) monthly variabilities of H^s in the Roaring Forties are in good agreement with those in tropical waters of the Indian Ocean; swell plays a dominant role in mixed waves throughout most of the Indian Ocean; and WS, H^w, H^s, and H_s exhibit a significant increasing trend over the 45-year study period. (2) H^s in the Roaring Forties and tropical waters of the Indian Ocean share a common period of 9.8–10.4 years on an interdecadal scale; and WS and H^s in the Roaring Forties and H^s in the tropical waters of the Indian Ocean share a common period of approximately 8 days (weekly oscillation) on an intraseasonal scale. (3) Swell of the Roaring Forties needs approximately 30 h to fully respond to the wind in this region. Approximately 84 h are required for H^s to propagate from the Roaring Forties to the tropical waters of the south Indian Ocean, while it takes approximately 132–138 h for H^s to propagate from the Roaring Forties to the tropical waters of the north Indian Ocean.