Showing papers on "Wind stress published in 2003"

Reduced drag coefficient for high wind speeds in tropical cyclones

Abstract: The transfer of momentum between the atmosphere and the ocean is described in terms of the variation of wind speed with height and a drag coefficient that increases with sea surface roughness and wind speed. But direct measurements have only been available for weak winds; momentum transfer under extreme wind conditions has therefore been extrapolated from these field measurements. Global Positioning System sondes have been used since 1997 to measure the profiles of the strong winds in the marine boundary layer associated with tropical cyclones. Here we present an analysis of these data, which show a logarithmic increase in mean wind speed with height in the lowest 200 m, maximum wind speed at 500 m and a gradual weakening up to a height of 3 km. By determining surface stress, roughness length and neutral stability drag coefficient, we find that surface momentum flux levels off as the wind speeds increase above hurricane force. This behaviour is contrary to surface flux parameterizations that are currently used in a variety of modelling applications, including hurricane risk assessment and prediction of storm motion, intensity, waves and storm surges.

Oceanography of the U.S. Pacific Northwest coastal ocean and estuaries with application to coastal ecology

Abstract: Ocean processes are generally large scale on the U.S. Pacific Northwest coast; this is true of both seasonal variations and event-scale upwelling-downwelling fluctuations., which are highly energetic. Coastal upwelling supplies most of the macronutrients available for production, although the intensity of upwelling-favorable wind forcing increases southward while primary production and chlorophyll are higher in the north, off the Washington coast. This discrepancy could be related to several mesoscale features: the wider, more gently sloping shelf to the north, the existence of numerous submarine canyons to the north, the availability of Columbia River plume water and sediment north of the river mouth, and the existence of a semi-permanent eddy offshore of the Strait of Juan de Fuca. We suggest that these features have important effects on the magnitude and timing of macronutrient or micronutrient delivery to the plankton. These features are potentially important as well to transport pathways and residence times of planktonic larvae and to the development of harmful algal blooms. The coastal plain estuaries, with the exception of the Columbia River, are relatively small, with large tidal forcing and highly seasonal direct river inputs that are low to negligible during the growing season. Primary production in these estuaries is likely controlled not by river-driven stratification but by coastal upwelling and exchange with the ocean. Both baroclinic mechanisms (the gravitational circulation) and barotropic ones (lateral stirring by tide and, possibly, wind) contribute to this exchange. Because estuarine hydrography and ecology are so dominated by ocean signals, the coastal estuaries, like the coastal ocean, are largely synchronous on seasonal and event time scales, though, intrusions of the Columbia River plume can cause strong asymmetries between Washington and Oregon estuaries especially during spring downwelling conditions. Water property correlation increases between spring and summer as wind forcing becomes more spatially coherent along the coast. Estuarine habitat is structure not only, by large scale forcing but also by fine scale processes in the extensive intertidal zone, such as by solar heating or differential advection by tidal, curents.

On the wave age dependence of wind stress over pure wind seas

Abstract: Data from five recent field campaigns are selected for pure wind sea, deep water, and fully rough flow conditions. The combined data set includes a wide range of wave ages, with high variability in both friction velocity and wave phase speed. These data, which are expected to follow Monin-Obukhov similarity scaling, are used to investigate the influence of wave age on wind stress. The relationship between the dimensionless roughness and inverse wave age is found to be zo/σ = 13.4 (u*/cp)3.4, where zo is the surface roughness length, σ is the standard deviation of the surface elevation, u* is the friction velocity, and cp is the wave phase speed at the peak of the spectrum. This relationship, which represents a significant dependence of roughness on wave age, was obtained using a procedure that minimizes the effects of spurious correlation in u*. It is also shown to be consistent with the wave age relationship derived using an alternate form of the dimensionless roughness, namely, the Charnock parameter zog/u*2, where g is the gravitational constant.

Gas transfer velocities measured at low wind speed over a lake

Abstract: The relationship between gas transfer velocity and wind speed was evaluated at low wind speeds by quantifying the rate of evasion of the deliberate tracer, SF 6, from a small oligotrophic lake. Several possible relationships between gas transfer velocity and low wind speed were evaluated by using 1-min-averaged wind speeds as a measure of the instantaneous wind speed values. Gas transfer velocities in this data set can be estimated virtually equally well by assuming any of three widely used relationships between k600 and winds referenced to 10-m height, U10: (1) a bilinear dependence with a break in the slope at ; 3.7 ms 21 , which resulted in the best fit; (2) a power dependence; and (3) a constant transfer velocity for U10 ,; 3.7 ms 21 , with a linear dependence on wind speed at higher wind speeds. The lack of a unique relationship between transfer velocity and wind speed at low wind speeds suggests that other processes, such as convective cooling, contribute significantly to gas exchange when the wind speeds are low. All three proposed relationships clearly show a strong dependence on wind for winds .3.7 m s 21 which, coupled with the typical variability in instantaneous wind speeds observed in the field, leads to average transfer velocity estimates that are higher than those predicted for steady wind trends. The transfer velocities predicted by the bilinear steady wind relationship for U10 ,; 3.7 ms 21 are virtually identical to the theoretical predictions for transfer across a smooth surface.

A simulation of the Last Glacial Maximum climate using the NCAR-CCSM

Abstract: The National Center for Atmospheric Research-Community Climate System Model (NCAR-CCSM) is used in a coupled atmosphere–ocean–sea-ice simulation of the Last Glacial Maximum (LGM, around 21,000 years ago) climate. In the tropics, the simulation shows a moderate cooling of 3 °C over land and 2 °C in the ocean in zonal average. This cooling is about 1 °C cooler than the CLIMAP sea surface temperatures (SSTs) but consistent with recent estimates of both land and sea surface temperature changes. Subtropical waters are cooled by 2–2.5 °C, also in agreement with recent estimates. The simulated oceanic thermohaline circulation at the LGM is not only shallower but also weaker than the modern with a migration of deep-water formation site in the North Atlantic as suggested by the paleoceanographic evidences. The simulated northward flow of Antarctic Bottom Water (AABW) is enhanced. These deep circulation changes are attributable to the increased surface density flux in the Southern Ocean caused by sea-ice expansion at the LGM. Both the Gulf Stream and the Kuroshio are intensified due to the overall increase of wind stress over the subtropical oceans. The intensified zonal wind stress and southward shift of its maximum in the Southern Ocean effectively enhances the transport of the Antarctic Circumpolar Current (ACC) by more than 50%. Simulated SSTs are lowered by up to 8 °C in the midlatitudes. Simulated conditions in the North Atlantic are warmer and with less sea-ice than indicated by CLIMAP again, in agreement with more recent estimates. The increased meridional SST gradient at the LGM results in an enhanced Hadley Circulation and increased midlatitude storm track precipitation. The increased baroclinic storm activity also intensifies the meridional atmospheric heat transport. A sensitivity experiment shows that about half of the simulated tropical cooling at the LGM originates from reduced atmospheric concentrations of greenhouse gases.

Observations of SST-Induced Perturbations of the Wind Stress Field over the Southern Ocean on Seasonal Timescales

Abstract: The surface wind stress response to sea surface temperature (SST) over the latitude range 308‐608 Si n the Southern Ocean is described from the National Aeronautics and Space Administration’s QuikSCAT scatterometer observations of wind stress and Reynolds analyses of SST during the 2-yr period August 1999 to July 2001. While ocean‐atmosphere coupling at midlatitudes has previously been documented from several case studies, this is the first study to quantify this relation over the entire Southern Ocean. The spatial structures of the surface wind perturbations with wavelengths shorter than 108 latitude by 308 longitude are closely related to persistent spatial variations of the SST field on the same scales. The wind stress curl and divergence are shown to be linearly related, respectively, to the crosswind and downwind components of the SST gradient. The curl response has a magnitude only about half that of the divergence response. This observed coupling is consistent with the hypothesis that SST modification of marine atmospheric boundary layer (MABL) stability affects vertical turbulent mixing of momentum, inducing perturbations in the surface winds. The nonequivalence between the responses of the curl and divergence to the crosswind and downwind SST gradients suggests that secondary circulations in the MABL may also play an important role by producing significant perturbations in the surface wind field near SST fronts that are distinct from the vertical turbulent transfer of momentum. The importance of the wind stress curl in driving Ekman vertical velocity in the open ocean implies that the coupling between winds and SST may have important feedback effects on upper ocean processes near SST fronts.

Ekman transport and pumping in the California Current based on the U.S. Navy's high‐resolution atmospheric model (COAMPS)

Abstract: [1] Upwelling in the California Current system due to Ekman transport and pumping was estimated using a high-resolution (9-km grid) atmospheric model reanalysis. Model winds, verified with satellite-measured winds at 13 locations, had weekly averaged wind components within 1.7 m s−1 RMS, and wind gradients within 1.6 m s−1 per 100 km RMS. Model wind hind-casts from May 1999 to September 2000 revealed narrow bands (about 10 by 50 km) of strong wind stress and wind stress curl parallel to the coast and adjacent to major coastal promontories. These bands, which are sub-grid scale in operational models, were capable of generating local upwelling greater than 10 m d−1 and downwelling greater than 5 m d−1. Peak summer estimates for the California Current system indicate vertical transport due to Ekman pumping was roughly 1.0 × 10−6 m3 s−1, and Ekman transport from alongshore wind stress was about 0.5 × 10−6 m3 s−1. These estimates suggest that Ekman pumping from wind stress curl is as important as Ekman transport from alongshore winds in the California Current system.

Export pathways for river discharged fresh water in the northern Gulf of Mexico

Abstract: [1] A numerical simulation of the Gulf of Mexico (GoM) using the Navy Coastal Ocean Model (NCOM) is used to identify the pathways by which fresh water discharged by major rivers in the northern Gulf is exported away from the region. The NCOM, a new primitive equation ocean model with a hybrid sigma/geopotential level vertical coordinate, is described along with its application to the GoM region. Trajectories from surface drifters are analyzed to show evidence of the seasonally shifting alongshore and cross-shelf transport in the region. The model results are used to determine the preferred locations and times of year for cross-shelf and along-shelf export of low-salinity water from the northern GoM. The annual cycle of local wind stress plays an important role in shifting the export pathway of the fresh water discharged from the major rivers (primarily the Mississippi River) toward the east in the spring/summer, where it can be transported offshore by the currents associated with deep ocean mesoscale eddies, and toward the west in the fall/winter, where it is transported southward along the Mexican coastline as a coastally trapped current.

Modeling circulation in lakes: Spatial and temporal variations

Abstract: The influence of spatial and temporal variations in wind forcing on the circulation in lakes is investigated using field data and the three-dimensional Estuary and Lake Computer Model (ELCOM) applied to Lake Kinneret. Lake Kinneret field data from six thermistor chains and eight wind anemometers deployed during July 2001 are presented. Internal wave motions are well reproduced by the numerical model when forced with a spatially uniform wind taken from a station near the lake center; however, simulated seiche amplitudes are too large (especially vertical mode 2) and lead observations by 3‐10 h (for a 24-h period wave) at different locations around the lake. Consideration of the spatial variation of the wind field improves simulated wave amplitude, and phase error at all stations is reduced to less than 1.5 h. This improvement is attributable to a better representation of the horizontally averaged wind stress and can be reproduced with a spatially uniform wind that has the same horizontally averaged wind stress as the spatially varying wind field. However, a spatially varying wind field is essential for simulating mean surface circulation, which is shown to be predominantly directly forced by the surface-layer‐averaged wind stress moment.

Cross-shelf exchange in a model of the Ross Sea circulation and biogeochemistry

Abstract: Transport of warm, nutrient-rich Circumpolar Deep Water (CDW) onto Antarctic continental shelves and coastal seas has important effects on physical and biological processes. The present study investigates the locations of this transport and its dynamics in the Ross Sea with a high-resolution three-dimensional numerical model. The model circulation is forced by daily wind stress along with heat and salt fluxes calculated from atmospheric climatologies by bulk formulae. All surface fluxes are modified by an imposed climatological ice cover. Waters under the Ross Ice Shelf are not included explicitly, but their effect on temperature and salinity is imposed in a buffer zone at the southern end of the model domain. A simple nutrient uptake is calculated based on the climatological chlorophyll distribution and Monod uptake kinetics. Model circulation is strongly affected by bottom topography, due to weak stratification, and agrees with schematics of the general flow and long-term current measurements except near the southern boundary. The sea-surface temperature is similar to satellite estimates except that the warmest simulated temperatures are slightly higher than observations. There is a significant correlation between the curvature of the shelf break and the transport across the shelf break. A momentum term balance shows that momentum advection helps to force flow across the shelf break in specific locations due to the curvature of the bathymetry (that is, the isobaths curve in front of the flow). For the model to create a strong intrusion of CDW onto the shelf, it appears two mechanisms are necessary. First, CDW is driven onto the shelf at least partially due to momentum advection and the curvature of the shelf break; then, the general circulation on the shelf takes the CDW into the interior.

The large-scale time-mean ocean circulation in the Nordic Seas and Arctic Ocean estimated from simplified dynamics

Abstract: A simplified diagnostic model of the time-mean, large-scale ocean circulation in the Nordic Seas and Arctic Ocean is presented. Divergences in the surface Ekman layer are extracted from observed climatological wind stress fields. Similarly, divergences caused by the meridional thermal wind transport (relative to the bottom) are calculated from an observed climatological density field. These known quantities are then used to force the model's bottom geostrophic velocities. Both scaling arguments and direct observations show that for long time scales the bottom currents are closely aligned with contours of flH, (where f is the Coriolis parameter and H is the depth of the seabed). Due to the weak planetary vorticity gradient at high latitudes, the f/H field is dominated by topography and is characterized by multiple regions of closed isolines. The only frictional effect included in the model is bottom stress. By then integrating the depth-integrated vorticity equation over the area spanned by a closed f/H contour, and assuming that the same contour is a streamline of the bottom geostrophic flow, we derive an analytical expression for the bottom geostrophic velocity on this f/H contour. For the few contours that are not closed, current measurements are used as boundary conditions. Model results are compared with near-bottom current measurements in both the Nordic Seas and the Arctic Ocean. In addition comparison is made with observations from surface drifters in the Nordic Seas by adding the observed thermal wind shear to the modeled bottom flow. The agreement is surprisingly good, suggesting that the simple model is capturing some of the most important processes responsible for the large-scale circulation field. Features like the subgyre recirculations in the Nordic Seas, the gyres in the Canadian and Eurasian Basins, the East Greenland Current, the Norwegian Atlantic Current and the Arctic Circumpolar Boundary Current are all well reproduced by the model. The simplicity of the model makes it well suited as a dynamical framework for interpreting the large-scale circulation pattern in the Nordic Seas and Arctic Ocean.

Coastal upwelling activity on the Pacific shelf of the Baja California Peninsula

Abstract: High primary productivity on the Pacific coast of the Baja California Peninsula is usually related to coastal upwelling activity that injects nutrients into the euphotic zone in response to prevailing longshore winds (from the northwest to north). The upwelling process has maximum intensity from April to June, with the coastal upwelling index varying from 50 to 300 m3/s per 100 m of coastline. Along the entire coast of the peninsula, the upwelling intensity changes in accordance with local wind conditions and bottom topography. Spatial variability can also be modulated by the influence of mesoscale meanders of the California Current. We have identified the seasonal and synoptic variability of upwelling signatures on the Baja California shelf, using averaged monthly and weekly sea surface temperature (SST) distributions obtained from remote sensing imagery from the Advanced Very High Resolution Radiometer in the period from 1996 to 2001. Analysis of SST distribution and direct experimental data on temperature and nutrient concentration shows that the areas with the coldest SST anomalies were closely related to the bottom slope, shelf width, and coastline orientation relating to wind direction. We also assume that the nutrient transport into the coastal lagoons may be forced by the coupling of coastal upwelling and tidal pumping of surface waters into the lagoon system.

Structure and dynamics of the Indian-Ocean cross-equatorial cell

Abstract: The cross-equatorial cell (CEC) in the Indian Ocean is a shallow (z≳−500 m) meridional overturning circulation, consisting of northward flow of southern-hemisphere thermocline water, upwelling in the northern hemisphere, and a return flow of surface water. In this study, several types of ocean models, varying in complexity from a 1 1 2 -layer analytic model to a state-of-the-art general circulation model (GCM), are used to investigate CEC structure and its dynamics. Pathways are illustrated by tracking model drifters from the northern-hemisphere upwelling regions, both forwards in time to follow the surface pathways and backwards in time to follow the subsurface flows. In the subsurface branch, cross-equatorial flow occurs via a western-boundary current, where strong horizontal mixing can alter the sign of its potential vorticity. In contrast, surface pathways cross the equator in the interior ocean at almost all longitudes. Sources of CEC water are flow into the basin in the southeastern ocean, subtropical subduction, and the Indonesian Throughflow. The models differ in which source is most prominent, a consequence of their different parameterizations of vertical-mixing processes and basin boundary conditions. The surface, cross-equatorial branch is driven by the annual-mean component of the zonal wind stress τx. It is predominantly antisymmetric about the equator with westerlies (easterlies) north (south) of the equator, and so is roughly proportional to latitude y. The resulting negative wind curl drives a southward Sverdrup flow across the equator. For a τx that is exactly proportional to y, the Ekman pumping velocity is identically zero; as a consequence, no geostrophic currents are generated by the wind, and the Sverdrup transport is equal to the Ekman drift. In GCM solutions, the southward, cross-equatorial flow occurs just below the surface (z

An optical technique for the measurement of longshore currents

Abstract: [1] We present an optical method (optical current meter) to measure the longshore component of nearshore surface currents by measuring the alongshore drift of persistent sea foam in the surf zone. The method uses short time series of video data collected from an alongshore array of pixels. These space-time data are first Fourier transformed to a frequency-wave number spectrum and, finally, to a velocity spectrum. A model of the velocity spectrum is fit to the observed spectrum to estimate the foam drift velocity. Confidence intervals and other measures of the input and output data quality are calculated. Field test comparisons were made against an in situ bidirectional electromagnetic current meter on the basis of 1 month of video data from the 1997 Sandy Duck field experiment. The root mean square error between the two approaches was 0.10 m/s. Linear regression analysis showed the gain between the two instruments to not be statistically different from one. Differences between the surface and interior measurements were compared to forcing mechanisms that may cause surface velocity shear. Velocity offsets and alongshore wind stress were well correlated for cases when waves and wind were not aligned to within ±45°, when wind- and wave-forced currents are reasonably separable. Calculated wind-dependent surface current shear, modeled as a surface boundary layer, correlated well with the observed velocity offsets for observations of nonalignment between wind and waves. This technique can be applied to study large-scale coastal behavior.

Wind Stress Vector over Ocean Waves

Abstract: Previous investigations of the wind stress in the marine surface layer have primarily focused on determining the stress magnitude (momentum flux) and other scalar variables (e.g., friction velocity, drag coefficient, roughness length). However, the stress vector is often aligned with a direction different from that of the mean wind flow. In this paper, the focus is on the study of the stress vector direction relative to the mean wind and surface-wave directions. Results based on measurements made during three field campaigns onboard the R/P Floating Instrument Platform (FLIP) in the Pacific are discussed. In general, the wind stress is a vector sum of the 1) pure shear stress (turbulent and viscous) aligned with the mean wind shear, 2) wind-wave-induced stress aligned with the direction of the pure wind-sea waves, and 3) swell-induced stress aligned with the swell direction. The direction of the wind-wave-induced stress and the swell-induced stress components may coincide with, or be opposite to,...

A Similarity Hypothesis for Air–Sea Exchange at Extreme Wind Speeds

Abstract: Hurricane intensity is sensitive to fluxes of enthalpy and momentum between the ocean and atmosphere in the high wind core of the storm. It has come to be recognized that much of this exchange is likely mediated by sea spray. A number of representations of spray-mediated exchange have appeared in recent years, but when these are applied in numerical simulations of hurricanes, storm intensity proves sensitive to the details of these representations. Here it is proposed that in the limit of very high wind speed, the air–sea transition layer becomes self-similar, permitting deductions about air–sea exchange based on scaling laws. In particular, it is hypothesized that exchange coefficients based on the gradient wind speed should become independent of wind speed in the high wind limit. A mechanistic argument suggests that the enthalpy exchange coefficient should depend on temperature. These propositions are tested in a hurricane intensity prediction model and can, in principle, be tested in the field.

Local and remote forcing of currents and temperature in the central Southern California Bight

Abstract: [1] A comprehensive study of the central Southern California Bight shows that subtidal currents are dominated by relatively long time scales (10–25 days), large alongshore scales, and significant offshore and upward phase propagation. A one-dimensional model shows that observed fluctuating, poleward propagating (speeds 140–260 cm s−1) alongshore pressure gradient disturbances account for a much larger fraction of the alongshore velocity variance than local wind stress (at least 40% in both seasons) and have the longer periods of the dominant currents. With the addition of local wind stress, about half the velocity variance can be accounted for, and overall, about 5% more variance in spring than in summer. Results are consistent with generation of disturbances by remote wind stress several hundred kilometers equatorward of the bight and alongcoast propagation as low-mode coastally trapped waves. The large-scale remote forcing is also responsible for much of the velocity variance on the adjacent shelf, the semienclosed Santa Monica Bay. A nonlinear, three-dimensional model shows that water is pushed into the bay initially as part of a throughflow, later becoming an eddy that gradually fills the bay, producing counterflow on its shoreward side. On the shelf, local alongshore wind stress accounts for only 25% of the velocity variance in spring and none in summer. The large-scale disturbances also produce significant temperature fluctuations throughout the region, via lateral advection of the mean alongshore temperature gradient. Local wind-driven coastal upwelling is responsible for temperature fluctuations on the inner shelf during several 2–4 day events in spring, but only very near the coastal wall.

The Role of Katabatic Winds on the Antarctic Surface Wind Regime

Abstract: Antarctica is known for its strong and persistent surface winds that are directed along topographic pathways. Surface winds are especially strong during the winter period. The high directional constancy of the wind and the close relationship of the wind direction to the underlying terrain can be interpreted as evidence of katabatic wind activity. Observations show that the directional constancy of the Antarctic surface wind displays little seasonal variation. Summertime winds cannot be expected to contain a significant katabatic component, owing to enhanced solar heating of the ice slopes. Observations also show that the coastal environs are subjected to wide variation in atmospheric pressure associated with frequent cyclone activity. The robust unidirectional nature of the Antarctic surface wind throughout the year implies that significant topographic influences other than those from katabatic forcing must be acting. Idealized numerical simulations have been performed to illustrate the potential...

The impact of the wind stress curl in the North Atlantic on the Atlantic inflow to the Norwegian Sea toward the Arctic

Abstract: [1] The Norwegian Atlantic Current (NwAC) through the Norwegian Sea serves as a conduit of warm and saline Atlantic water from the North Atlantic to the Arctic Ocean, an important factor for climate and ecology. In this study, we concentrate on how the North Atlantic wind stress curl (NAWSC) affects interannual variability on the major branch of the NwAC—the Norwegian Atlantic Slope Current (NwASC). Based on wind stress data from the NCEP reanalysis and estimated volume transport of the NwASC from current records during 1995–2003 in the Svinoy section (62°N), our analysis shows that the volume transport in the NwASC exhibits a maximum correlation of 0.88 with the zonally integrated NAWSC at 55°N 15 months earlier. Our findings reveal the NAWSC to be a major forcing for interannual variability of the NwASC in the range of 3.0- to 5.3 Sv (Sv = 106 m3 s−1). The 15 month time lag appears to be in accordance with a forced baroclinic Rossby wave in response to the local Ekman pumping changing the baroclinicity and strength of the North Atlantic Current (NAC). After being converted to a nearly barotropic shelf edge current along the Irish-Scottish shelf through interaction with bottom topography, it appears as a barotropic response downstream in the Svinoy section. This result suggests the possibility of predicting conditions influenced by the NwASC more than a year in advance, using the NAWSC as a proxy.

High‐frequency response of wind‐driven currents measured by drifting buoys and altimetry over the world ocean

Abstract: [1] We investigate the wind-driven current response over the world ocean using SVP drifting buoys, ERS-1 and ERS-2, and TOPEX altimetry data and ECMWF wind stress fields. Wind-driven ageostrophic currents are estimated from drifting buoys by removing altimetry deduced geostrophic currents from observed drifting velocities. Removing the geostrophic signal enhances the coherence with wind stress at periods longer than 10 days. For superinertial periods lower than 20 days, a two-parameter (i.e., angle and amplitude) model is fitted to the isolated ageostrophic signal to study the upper ocean response to surface wind stress in that frequency band. The model, which can explain up to 30% of the variance in some areas, indicates that the currents spiral to the right (left) of the wind with depth in the Northern (Southern) Hemisphere, in agreement with the Ekman theory. The absolute value of the angle parameter is found to exhibit significant spatial and seasonal variability. The vertical eddy viscosity and the Ekman layer depth as deduced from Ekman theory are compared to previous results. Their seasonal variability is also studied. Model results are then used to compute the mean and standard deviation of ageostrophic currents in 5° boxes over the world ocean. The equatorial divergence and the subpolar and subtropical convergence zones are well reproduced.

Evolution of stratification over the New England shelf during the Coastal Mixing and Optics study, August 1996-June 1997

Abstract: [1] To investigate the processes influencing the evolution of stratification over continental shelves a moored array was deployed on the New England shelf from August 1996 to June 1997. Temperature, salinity, and current observations spanning the water column were obtained at four midshelf sites, along with meteorological measurements at a central site to estimate the wind stress and the surface heat and freshwater fluxes. Four processes contributed to the seasonal evolution of the stratification. (1) The breakdown of the seasonal thermocline in fall was primarily due to wind forcing, not surface cooling, and occurred in four discrete steps associated with westward, along-coast wind stress events. Eastward wind stress events of similar magnitude did not reduce the stratification. (2) The water at midshelf remained stratified throughout most of the winter due to saltier shelf-slope front water displaced onshore by anomalously strong and persistent eastward alongcoast wind stresses. (3) The gradual redevelopment of the thermocline, beginning in April, was primarily a one-dimensional response to increasing surface heat flux. (4) Stratification in early April and throughout May was substantially enhanced by low-salinity water associated with river runoff from southern New England that was driven eastward and offshore by upwelling-favorable (eastward) wind stresses.

Air‐sea interaction and circulation changes in the northeast Atlantic

Abstract: The upper ocean of the Rockall Trough exhibits coherent interannual variations in temperature and salinity over the past 26 years, with highs in the mid-1980s and late 1990s and lows in the late 1970s and early 1990s, and with ranges of ±0.5°C and ±0.05 in salinity. The origins of the interannual changes are discussed, covering three potential influencing factors: the propagation of anomalies developed upstream of the basin, the effect of local air-sea interaction, and the result of changes of regional circulation bringing different water masses into the region. The changes in heat and freshwater content of the upper ocean are directly compared to observed variations in air-sea heat and freshwater fluxes over the period of the time series. It is shown that the role of the atmosphere in locally altering the oceanic properties, particularly salinity, is relatively small and insufficient to explain the changes. Two recent hydrographic surveys are analyzed to ascertain how the distribution of water masses to the south of the basin may influence the properties of the northern Rockall Trough upper ocean, and the results are reviewed in the context of historical analyses. It is found that the critical factor in determining the properties is the varying amount of relatively cool and fresh North Atlantic Current water mixing with the dominant water mass, the warm saline Eastern North Atlantic Central Water at the entrance to the basin. Variations of inflowing water masses are caused by east-west changes in the location of the subpolar front, and the relationship of these changes in regional circulation to the North Atlantic wind stress field is discussed.

Modeling study of turbulent mixing over the continental shelf: Comparison of turbulent closure schemes

Abstract: limitation is imposed. During upwelling-favorable winds, the majority of turbulent mixing occurs in the top and the bottom boundary layers and in the vicinity of the vertically and horizontally sheared coastal jet. Turbulent mixing in the coastal jet is primarily driven by shear-production. The near-surface flow on the inner shelf becomes convectively unstable as wind stress forces the upwelled water to flow offshore in the surface layer. During downwelling-favorable winds, the strongest mixing occurs in the vicinity of the downwelling front. The largest turbulent kinetic energy and dissipation are found near the bottom of the front. Turbulence in the bottom boundary layer offshore of the front is concentrated between recirculation cells which are generated as a result of symmetric instabilities in the boundary layer flow. INDEX TERMS: 4219 Oceanography: General: Continental shelf processes; 4568 Oceanography: Physical: Turbulence, diffusion, and mixing processes; 4255 Oceanography: General: Numerical modeling; 4279 Oceanography: General: Upwelling and convergences;

Impact of flow distortion corrections on turbulent fluxes estimated by the inertial dissipation method during the FETCH experiment on R/V L'Atalante

Abstract: [1] The FETCH campaign was for a large part devoted to the measurement and analysis of turbulent fluxes in fetch-limited conditions. Turbulent measurements were performed on board the R/V L'Atalante, on an ASIS spar buoy and on aircraft. On the R/V L'Atalante, turbulent data were obtained from a sonic anemometer and from a microwave refractometer. The main focus of this paper is to present results of momentum and heat fluxes obtained from the R/V L'Atalante, using the inertial-dissipation method and taking into account flow distortion effects. Numerical simulations of airflow distortion caused by the ship structure have been performed to correct the wind measurements on the R/V L'Atalante during the FETCH experiment. These simulations include different configurations of inlet velocities and six relative wind directions. The impact of airflow distortion on turbulent flux parameterizations is presented in detail. The results show a very large dependence on azimuth angle. When the ship is heading into the wind (relative wind direction within ±38° of the bow), the airflow distortion leads to an overestimation of the drag coefficient, associated with a wind speed reduction at the sensor location. For relative wind directions of more than ±38° from the bow, flow distortion causes the wind to accelerate at the sensor location, which leads to an underestimate of the drag coefficient. The vertical displacement of the flow streamlines could not be fully established by numerical simulation, but the results are in qualitative agreement with those inferred from the data by prescribing the consistency of momentum flux as a function of azimuth angle. Both show that the vertical elevation of the flow can be considered as constant (1.21 m from numerical simulations) only within about ±20° from bow axis. Values of vertical displacements up to 5 m are found from the data for high wind speeds and beam-on flows. Our study also shows that the relative contributions of the streamline vertical displacement and the mean wind speed underestimate or overestimate vary significantly with relative wind direction. The relative contribution due to vertical streamline displacement is higher for heat flux than for momentum flux. The consistency of our correction for airflow distortion is assessed by the fact that the correction reduces the standard deviation of the drag coefficient: only if this correction is taken into account, do the curves of the drag coefficient versus wind speed become similar for data corresponding to wind in the bow direction and from the side. When the complete numerical airflow correction is applied to the data set limited to relative wind directions at ±30° from the bow axis, the drag coefficient formula is CD10N × 1000 = 0.56 + 0.063 U10N, for U10N > 6 m s−1. This formula provides CD10N values comparable to the ones found from the ASIS buoy data for wind speeds of about 13 m s−1. They are however smaller by 9% at higher winds (>15 m s−1). This formula is also similar, within a few percent, to the parameterizations of Smith [1980], Anderson [1993], and Yelland et al. [1998]. The exchange coefficient for evaporation is found to be 1.00 × 10−3 on average with a small standard deviation of 0.31 × 10−3. A slight increase of CE10N value with wind speed is, however, observed with a variation of about 20% (0.2 × 10−3) for wind speeds between 6 and 17 m s−1, following CE10N × 1000 = 0.82 + 0.02 U10n, for U10n > 6 m s−1.

Relative effects of wind stress curl, topography, and stratification on large‐scale circulation in Lake Michigan

Abstract: [1] This paper uses the results from two multiseason numerical model simulations of Lake Michigan hydrodynamics to examine the relative effects of wind stress curl, topography, and stratification on large-scale circulation. The multiseason simulations provide a period long enough to encompass the full range of atmospheric and thermal conditions that can occur in the lake. The purpose of this paper is to diagnose the relative importance of various mechanisms responsible for the large-scale circulation patterns by analyzing the vorticity balance in the lake on a monthly timescale. Five different model scenarios are used to isolate the predominant mechanisms: (1) baroclinic lake, spatially variable wind stress; (2) barotropic lake, spatially variable wind stress; (3) baroclinic lake, spatially uniform wind stress; (4) barotropic lake, spatially uniform wind stress; and (5) barotropic lake, linearized equations, spatially uniform wind stress. By comparing the results of these five model scenarios it is shown that the cyclonic wind stress curl in the winter and the effect of baroclinicity in the summer are primarily responsible for the predominantly cyclonic flow in the lake. Topographic effects are also important but are not as significant as wind stress curl and baroclinic effects. Nonlinear effects are much smaller.

Second and first baroclinic Kelvin modes in the equatorial Pacific at intraseasonal timescales

Abstract: [1] TOPEX/Poseidon sea level and time series from the TAO (Tropical Atmosphere Ocean) array of moorings over 1992–1999 are used to investigate the spatiotemporal characteristics of equatorial Kelvin waves in the Pacific at period shorter than 180 days Spectral analyses show high oceanic energy in two separate bands of period, around 70 days and 120 days Signals are coherent in both bands all along the equator Ocean General Circulation Model (OGCM) simulations forced by different wind stress fields are also analyzed and exhibit the same features Observed and modeled sea level, dynamic height and 20°C isotherm depth, band-pass filtered around 70 days, show an eastward propagation at phase speed of 24–29 m s−1, typical of the first baroclinic Kelvin mode The meridional and vertical structures of the equatorial waves at the onset of the 1997 El Nino are also those expected for a first Kelvin mode At 120-day period, phase speeds, meridional and vertical structures are closer to those expected for a second baroclinic mode Both modes are nevertheless present in the two frequency bands A simple Kelvin wave model shows that the wind at 120-day period seems to force the Kelvin waves at the same period However, the predominance of second baroclinic mode at 120-day period cannot be explained solely by the characteristics of the wind OGCM simulations rather suggest the importance of internal oceanic processes such as vertical redistribution of energy from first to second baroclinic mode as the waves propagate eastward Model outputs over 1984–1999 indicate that these equatorial Kelvin waves at 70- and 120-day period may have an effect on El Nino since they seem to be stronger at its onset

Large-eddy simulations of the wind-induced turbulent Ekman layer

Abstract: At urbulent Ekman layer created by a steady wind near the water surface is investigated using the numerical method of large-eddy simulations. The classical case of a flow unaffected by density stratification and surface waves is revisited to understand the internal structure of the flow and implications of the traditional assumptions of constant effective viscosity and the ‘f -plane’ approximation. A series of numerical experiments reveals that the Ekman solution needs correcting even in this case. The examination of the effective viscosity hypothesis confirms its validity but shows that the viscosity varies strongly with depth. It increases in the subsurface layer of thickness about 1/4 the turbulent length scale and decreases below this level. A Bessel function solution is proposed that corresponds to the approximate effective viscosity profile and matches with the LES results. Strong flow dependence on the latitude and wind direction is detected and explained by the effects of redistribution of turbulent kinetic energy between the velocity components and modification of the vertical transfer of turbulent momentum. In this paper, we consider the classical problem of a turbulent flow generated near the ocean surface by a steady wind stress in the presence of Earth’s rotation. Interest in this flow goes back to Ekman’s landmark work published in 1905. (An interesting historical review of Fridtjof Nansen’s polar expedition and other events preceding Ekman’s paper is given by Walker (1991).) Ekman assumed a balance between the Coriolis force, viscous friction and the pressure gradient, adopted the approximation of constant vertical eddy viscosity Az ,a ndderived a solution now known as the ‘Ekman spiral’. In the case of a steady wind in the x-direction, the steady-state Ekman velocity profile in the open ocean is (for the northern hemisphere) u = V0 cos π + π D z

Upper Ocean Sensitivity to Wind Forcing in the South China Sea

Abstract: The Naval Research Laboratory (NRL) Layered Ocean Model (NLOM) has been used to investigate the sensitivity of the upper South China Sea (SCS) circulation to various atmospheric wind forcing products. A 1/16° 6-layer, thermodynamic Pacific Ocean north of 20°S version of NLOM has been integrated using observed climatological monthly mean winds (Hellerman and Rosenstein, 1983) and climatologies based on two atmospheric prediction models: the European Centre for Medium-Range Weather Forecasts (ECMWF) and the National Centers for Environmental Prediction (NCEP). ECMWF products include the 10 meter winds (at both 1.125° and 2.5° resolution) and surface stresses (1.125°). The NCEP forcing (1.875°) is a surface stress product. Significant differences exist in the wind stress curl patterns and this is reflected in the upper ocean model response, which is compared to observational data. The model experiments suggest the generation of the West Luzon Eddy is controlled by positive wind stress curl. The degree of Kuroshio intrusion into the SCS, however, is not affected by wind stress curl but is governed by the coastline geometry of the island chain within Luzon Strait. The summertime offshore flow from the Vietnamese coast is present in all simulations but the dipole structure on either side of the jet is variable, even among experiments with similar wind stress curl patterns. The ECMWF surface stresses exhibit spurious coastal wind stress curl patterns, especially in locations with significant orographic features. This manifests itself in unrealistic small scale coastal gyres in NLOM. High resolution basin-scale and coastal models might be adversely affected by these stresses.

High resolution simulations on the North Aegean Sea seasonal circulation

Abstract: . The seasonal characteristics of the circulation in the North Aegean Sea are examined with the aid of a climatological type simulation (three-year run with perpetual year forcing) on a fine resolution grid (2.5 km by 2.5 km). The model is based on the Princeton Ocean Model with a parameterisation of plume dynamics that is employed for the input of waters with hydrographic properties that are different than the properties of basin waters, as the Black Sea Water (BSW) outflow through the Dardanelles Strait and riverine sources. The model is nested with a sequence of coarser regional and basin-wide models that provide for the long-term interaction between the study area and the Eastern Mediterranean at large. The results are employed to discuss the response of the North Aegean to the important circulation forcing mechanisms in the region, namely wind stress, heat and salt fluxes, buoyancy due to rivers and the BSW outflow (which is low in salinity and occasionally low in temperature) and the interaction with the Southern Aegean. The high resolution allows for the detailed representation of the complicated topography that presides in the region. This helps produce a rich eddy field and it allows for variability in the pathways of BSW that has implications in the basin hydrography and circulation. Key words. Oceanography: general (continental shelf processes; numerical modeling)