Showing papers on "Wind stress published in 1991"

Quasi-linear Theory of Wind-Wave Generation Applied to Wave Forecasting

Abstract: The effect of wind-generated gravity waves on the airflow is discussed using quasi-linear theory of wind-wave generation. In this theory, both the effects of the waves and the effect of air turbulence on the mean wave profile are taken into account. The main result of this theory is that for young wind sea most of the stress in the boundary layer is determined by momentum transfer from wind to waves, therefore, resulting in a strong interaction between wind and waves. For old wind sea there is, however, hardly any coupling. As a consequence, a sensitive dependence of the aerodynamic drag on wave age is found, explaining the scatter in plots of the experimentally observed drag as a function of the wind speed at 10-m height. Also, the growth rate of waves by wind is found to depend on wave age. All this suggests that a proper description of the physics of the momentum transfer at the air–sea interface can only be given by coupling an atmospheric (boundary-layer) model with an ocean-wave prediction ...

Light limitation of phytoplankton biomass and macronutrient utilization in the Southern Ocean

Abstract: The Antarctic Circumpolar Current (ACC) is unique in that it has continually high concentrations of major plant nutrients but low phytoplankton biomass. This enigmatic phenomenon is the focus of significant speculation that trace nutrients, including Fe, may limit phytoplankton crop size. Global climatologies indicate that the ACC is a region with low surface temperatures, weak density stratification, little summertime surface solar irradiance, and strong wind stress. These physical phenomena act to limit growth rates of the phytoplankton community. Using a photo-physiological description of phytoplankton growth in a simple one-dimensional ecosystem model forced by observations or climatologies of mixing depth and surface irradiance, the authors make an evaluation of the potential for massive, nutrient-exhausting, phytoplankton blooms forming in the ACC. The ACC has persistent mixed layers in excess of 50 m. Literature values and model optimization indicate that the minimal aggregate specific loss rate and typical physical conditions of stratification and surface irradiance, the model predicts that phytoplankton in the ACC would not utilize >10% of the available macronutrients. Without a mechanism for increasing the strength of stratification, the authors predict that massive Fe additions to the Southern Ocean would fail to significantly mitigate the atmospheric CO{sub 2} derived from fossilmore » fuel.« less

Predictability of a coupled ocean-atmosphere model

Abstract: A study is presented to determine the limits on the predictability of the coupled ocean-atmosphere system. Following the classical methods developed for atmospheric predictability studies, the model used is one of the simplest that realistically reproduces many of the important features of the observed interannual variability of sea surface temperature in the tropical Pacific Ocean when forced by observed wind stresses. As no reasonable analysis is available for all the fields, initial conditions for these prediction experiments were taken from a model control run in which the ocean model was forced by the observed surface winds. The atmospheric component of the coupled model is not capable of accurately simulating the large-scale components of the observed wind stress.

On ENSO Physics

Abstract: Two extended integrations of general circulation models (GCMs) are examined to determine the physical processes operating during an ENSO cycle. The first integration is from the Hamburg version of the ECMWF T21 atmospheric model forced with observed global sea surface temperatures (SST) over the period 1970–85. The second integration is from a Max Planck Institut model of the tropical Pacific forced by observed wind stress for the same period. Both integrations produce key observed features of the tropical ocean-atmosphere system during the 1970–85 period. The atmospheric model results show an eastward propagation of information from the western to eastern Pacific along the equator, although this signal is somewhat weaker than observed. The Laplacian of SST largely drives the surface wind field convergence and hence determines the position of large scale precipitation-condensation heating. This statement is valid only in the near-equatorial zone. Air-sea heat exchange is important in the planetary boundary layer in forcing the wind field convergence but not so important to the main troposphere, which is heated largely by condensation heating. The monopole response seen in the atmosphere above about 500 mb is due to a combination of factors, the most important being adiabatic heating associated with subsidence and tropic-wide variations in precipitation. The models show the role of air-sea heat exchange in the ocean heat balance in the wave guide is one of dissipation/damping. Total air-sea heat exchange is well represented by a simple Newtonian cooling parameterization in the near-equatorial region. In the wave guide, advection dominates the oceanic heat balance with meridional advection being numerically the most important in all regions except right on the equator. The meridional term is largely explained by local Ekman dynamics that generally overwhelm other processes in the regions of significant wind stress. The principal element in this advection term is the anomalous meridional current acting on the climatological mean meridional SST gradient. The eastward motion of the anomalies seen in both models is driven primarily by the ocean. The wind stress associated with the SST anomalies forces an equatorial convergence of heat and mass in the ocean. Outside the region of significant wind forcing, the mass source leads to a convergent geostrophic flow, which drives the meridional heat flux and causes warming to the east of the main wind anomaly. West of the main anomaly the wind and geostrophic divergence cause advective cooling. The result is that the main SST anomaly appears to move eastward. Since the direct SST forcing drives the anomalous wind, surface wind convergence, and associated precipitation, these fields are seen also to move eastward.

The monsoonal current regimes of the tropical Indian Ocean: Observed surface flow fields and their geostrophic and wind-driven components

Abstract: The annual cycle of current regimes in the tropical Indian Ocean (to 30°S and 120°E) is studied on the basis of long-term observations of the surface wind field, ship drift measurements of surface currents, and subsurface temperature and salinity casts to 400 dbar depth. For the major current domains, the Ekman surface current is computed from wind observations under assumption of a drag coefficient and of an eddy viscosity coefficient; the geostrophic surface flow is calculated from the field of the geopotential anomaly of the ocean surface relative to 400 dbar, constructed from subsurface temperature and salinity casts; the observed current is compiled from ship drift measurements; the Ekman volume transport is estimated using the assumed drag coefficient; and the geostrophic volume transport is obtained from the field of mass transport function between zero and 400 m relative to 400 dbar, constructed from subsurface temperature and salinity casts. The monsoonal reversals of wind stress forcing, which are most dramatic in the northern Indian Ocean and the equatorial zone and more moderate in the southern part of the basin, have diverse consequences for the various current systems. Under the steady southeast trades the westward directed South Equatorial Current (SEC) is basically geostrophic, with an additional strong Ekman contribution in boreal summer. The eastward directed South Equatorial Countercurrent (SCC) is mainly geostrophic, but in boreal summer it is virtually eliminated by the then westward directed Ekman component. The westward/eastward flowing Northeast Monsoon Current/Southwest Monsoon Current (NEM and SWM) of boreal winter/summer are Ekman flows. The Eastward Equatorial Jets (EEJ) during the monsoon transitions have larger geostrophic than Ekman components, while in the northward East African Coastal Current (EAC) and the northeastward Somali Current (SCN) of boreal summer the Ekman prevails over the geostrophic flow. In the latter three current systems the Ekman flow creates mass distributions that in turn entail sympathetic geostrophic currents. The prevailing geostrophic annual mean transports are of order 5 sverdrups (Sv) for the SCC, NEM, and SWM and about 13 Sv for the SEC. A transport performance comparable to that of the perennial SEC is attained by the EEJ during the monsoon transitions.

Variational Estimation of the Wind Stress Drag Coefficient and the Oceanic Eddy Viscosity Profile

Abstract: A variational optimal control technique is used to assimilate both meteorological and oceanographic observations into an oceanic Ekman layer model. An identical twin experiment is discussed first in which the “observations” are created by the dynamic model. The field measurements from the LOTUS-3 (Long-Term Upper Ocean Study-3) buoy are then analysed. By fitting the model results to the data, the unknown boundary condition (the wind stress drag coefficient) and the unknown vertical eddy viscosity distribution are deduced simultaneously from the data, and an optimal estimate of the current field is obtained. Though the model is simple, the results show that the variational assimilation technique is capable of extracting from the available observations a reasonable wind stress drag coefficient and vertical eddy viscosity distribution.

Internal waves and mixing in the upper equatorial Pacific Ocean

Abstract: Microstructure measurements in the equatorial Pacific at 140°W in late 1984 show a pronounced diurnal variation in both high-frequency internal wave energy and kinetic energy dissipation rate. Observations indicated that after sunset, internal waves (presumably generated by convective overturns in the mixed layer) propagate downward and increase turbulence levels in the pycnocline. It is proposed that large mixed layer eddies in the South Equatorial Current interact with the large shear caused by the Equatorial Undercurrent to generate a westward going anisotropic wave field. The momentum transport in the radiated wave field results in a drag force on the equatorial mean flow field. The observed mean wind stress at 140°W during Tropic Heat I (which is twice as large as the annual mean wind) is closer to the estimated radiation stress (∼−10−4 m2 s−2) at the base of the mixed layer (≈30 m) than to the estimated turbulent stress (∼−10−5 m2 s−2). A wave dissipation model based on the observed turbulent kinetic energy dissipation rate is introduced in order to estimate the wave momentum flux divergence in the stratified region above the undercurrent core. The model predicts that most of the downward wave momentum flux penetrates through the undercurrent core. It is hypothesized that when the wind stress is strong, the equatorial Pacific Ocean responds by generating a westward traveling internal wave field which transports much of the surface wind stress below the actively mixing surface layer.

A Comparison of Measured and Wind-derived Ekman Transport at 11°N in the Atlantic Ocean

Abstract: A comparison of measured and wind-derived ageostrophic transport is presented from a zonal transect spanning the Atlantic Ocean along 11°N. The transport per unit depth shows a striking surface maximum that decays to nearly zero at a depth of approximately 100 m. We identify this flow in the upper 100 m as the Ekman transport. The sustained values of wind stress and the penetration depth of the Ekman transport reported here are considerably greater than in previous observations, which were made in conditions of light winds. The transport of 12.0 ± 5.5 × 106 m3 s−1, calculated from the difference of geostrophic shear and shear measured by an acoustic Doppler current profiler, is in good agreement with that estimated from the shipboard winds, 8.8 ± 1.9 × 106 m3 s−1, and from climatology, 13.5 ± 0.3 × 106 m3 s−1. Qualitatively, the horizontal distribution of the wind-driven flow was best predicted by the shipboard winds. The cumulative transport increased linearly over the western three-fourths of t...

Mean circulation and variability on the eastern Canadian continental shelf

Abstract: The circulation and variability on eastern Canadian continental shelves is reviewed with emphasis on horizontal and vertical structure in the water column. Mean and seasonal currents, sheared by stratification and friction, generally follow bathymetry and are driven primarily by surface wind stress and buoyancy fluxes. In certain near-resonant embayments, nonlinear interactions of the tide also make significant contributions to the mean and low-frequency currents. Direct forcing of low-frequency shelf circulation by offshore currents and pressure fields is of lesser importance according to recent observations and models. At higher frequencies, the wintertime response to atmospheric forcing occurs in three important frequency bands: surface wave (periods of 2–20 s), inertial (17–22 h) and synoptic or subtidal (2–10 days). Most of the subtidal energy in the subsurface pressure (SSP) field is attributable to direct local forcing by alongshore wind and to remote forcing by coastal-trapped waves (CTW). The subtidal current variability is less coherent with these sources primarily because of small scale circulations created by vortex stretching, topographic steering and scattering by the rugged shelf bathymetry. Bottom friction and scattering control the ratio of locally and remotely forced subtidal energy as a function of position on the shelf. Intermittent inertial currents in stratified shelf waters, often associated with mesoscale atmospheric forcing, exhibit a 180° phase change across the pycnocline and are inhibited near the coast. Surface waves generated by storm winds are strongly dissipated in shallow water, where they enhance bottom stresses that: (1) balance the surface wind stress; and (2) may contribute to the damping of inertial waves. Specific examples of these phenomena are drawn from observations on the Scotian Shelf and in the Gulf of Maine.

The physical structure of cold filaments near Point Arena, California, during June 1987

Abstract: Shipboard conductivity-temperature-depth, acoustic Doppler current profiler, and continuous temperature and salinity observations were made in the coastal transition zone off Point Arena, California (39°N, 124°W), during June 15–28, 1987, to describe the hydrographic structure and velocity fields associated with the cold filaments found there. An adaptive sampling plan was used to measure the properties of these filaments, guided in real time by satellite Advanced Very High Resolution Radiometer sea surface temperature imagery and feedback from the in situ sensors. The primary feature observed was a large, cool (12.0°–13.5°C), salty (32.7–33.0 psu) filament which extended over 200 km offshore from Point Arena and exceeded 500 m depth. This feature was bounded in the offshore direction by a continuous equatorward meander, with offshore velocities (60–87 cm s−1) on the northern edge of the cool filament and onshore velocities (69–92 cm s−1) along the southern edge, and persisted for at least 3 weeks. A second feature was advected into the study area from the north by an anticyclonic eddy offshore and later merged with the Point Arena filament. Smaller (30 km wide by 50–100 km long by 50–100 m deep) very cold (10.0°–12.0°C) high salinity (>33.0 practical salinity units (psu)) features were observed within the Point Arena filament, but persisted for only 6–10 days. The net volume transport of the larger feature was offshore at ∼3 × 106 m3 s−1 and suggests it was fed by an inflow to the region from the north. The smaller features were correlated with bursts of equatorward wind stress on an event by event basis but not with times of large-scale wind stress convergence. Surface drifters deployed during the experiment closely followed the surface dynamic topography. Some of the drifters followed the path of the offshore meander, while others moved south inshore.

Phytoplankton pigment patterns and wind forcing off central California

Abstract: Mesoscale variability in phytoplankton pigment distributions of central California during the spring-summer upwelling season are studied via a 4-yr time series of high-resolution coastal zone color scanner imagery. Empirical orthogonal functions are used to decompose the time series of spatial images into its dominant modes of variability. The coupling between wind forcing of the upper ocean and phytoplankton distribution on mesoscales is investigated. Wind forcing, in particular the curl of the wind stress, was found to play an important role in the distribution of phytoplankton pigment in the California Current. The spring transition varies in timing and intensity from year to year but appears to be a recurrent feature associated with the rapid onset of the upwelling-favorable winds. Although the underlying dynamics may be dominated by processes other than forcing by wind stress curl, it appears that curl may force the variability of the filaments and hence the pigment patterns.

Frequency downshift in narrowbanded surface waves under the influence of wind

Abstract: It is well known that the spectral peak of wind-induced gravity waves on the sea surface tends to shift to lower frequencies as the fetch increases. In past theories the nonlinear dynamics subsequent to Benjamin–Feir instability has been found to initiate the downshift in narrow-banded waves in the absence of wind. However, these weakly nonlinear theories all predict the downshift to be only the first phase of an almost cyclic process. Limited by the length of a wave tank, existing experiments are usually made with relatively steep waves which often break. Although there is a theory on how breaking adds dissipation to stop the reversal of the initial trend of downshift, the details of breaking must be crudely characterized by semi-empirical hypotheses. Since the direct role of wind itself must be relevant to the entire development of wind-wave spectrum, we examine here the effect of wind on the nonlinear evolution of unstable sidebands in narrow-banded waves. We assume that the waves do not break and consider the case where the nonlinear effects that initiate the downshift, energy input by wind and damping by internal dissipation all occur on the same timescale. This means that not only must the waves be mild but the wind stress intensity must also lie within a certain narrow range. With these limitations we couple the air flow above the waves with Dysthe's extension of the cubic Schrodinger equation, and examine the initial as well as the long-time evolution of a mechanically generated wavetrain. For a variety of wind intensities, downshift is indeed found to be enhanced and rendered long lasting.

Seasonal Transport Variation in the Western Subtropical North Atlantic: Experiments with an Eddy-resolving Model

Abstract: A high-resolution model of the wind-driven and thermohaline circulation in the North and equatorial Atlantic Ocean is used to study the structure and variability of the boundary current system at 26°N, including the Florida Current, the Antilles Current, and the Deep Western Boundary Current (DWBC). The model was developed by Bryan and Holland as a Community Modeling Effort of the World Ocean Circulation Experiment. Subsequent experiments have been performed at IfM Kiel, with different friction coefficients, and different climatologies of monthly mean wind stress: Hellerman–Rosenstein (HR) and Isemer–Hasse (IH). The southward volume transports in the upper 1000 m of the interior Atlantic, at 26°N, are 25.0 Sv (Sv ≡ 106m3s−1) for HR, and 34.9 Sv for IH forcing, in good agreement with the transport from the integrated Sverdrup balance at this latitude (23.9 Sv for HR, 35.6 Sv for IH). The return flow of this wind-driven transport, plus the southward transport of the DWBC (6–8 Sv), is partitioned between the Florida Current and Antilles Current. With HR forcing, the transport through the Straits of Florida is 23.2 Sv; this increases to 29.1 Sv when the wind stresses of IH are used. The annual variation of the simulated Florida Current is very similar to previous, coarse-resolution models when using the same wind-stress climatology (HR); the annual range (3.4 Sv) obtained with HR forcing is strongly enhanced (6.3 Sv) with IH forcing. The meridional heat transport at 26°N, zonally integrated across the basin, is in phase with the Florida Current; its annual range increases from 0.44 PW (HR) to 0.80 PW (IH). The annual signal east of the Bahamas is masked by strong transport fluctuations on a time scale of O(100 days), caused by an instability of the Antilles Current. By averaging over several model years, an annual cycle is extracted, which is in phase with the wind stress curl over the western part of the basin.

On the Annual Cycle of Equatorial Upwelling in the Central Atlantic Ocean

Abstract: The annual cycle of the upper ocean's vertical velocity component (w) on the equator at 28°W is examined by integrating the continuity equation using current meter data from the Seasonal Response of the Equatorial Atlantic Experiment. The annual cycle consists in part of an intense, but brief (∼1 month), upwelling season beginning with the onset of strong easterly wind stress in boreal spring. This upwelling is followed by weaker downwelling during the summer despite the persistence of strong easterly wind stress. The record-length averaged w profile shows that maximum upwelling (0.6 × 10−3 cm s−1) is located slightly above the core of the Equatorial Undercurrent and downwelling is located below the base of the thermocline. The standard deviations are about tenfold the magnitude of the means. Independent evidence supporting these results are that 1) sea surface temperature (SST) is related to w during the springtime changes in easterly wind stress with the observed and computed isotherm displacem...

Gulf Stream water on the shelf and upper slope north of Cape Hatteras

Abstract: Although intrusions of water from the Gulf Stream have often been observed over the Carolina shelf, there has been no published report of Gulf Stream water near the continental margin north of Cape Hatteras. By examining sea surface temperature distributions and hydrographic data collected over an 11-yr period, we have found that water discharged from the Gulf Stream often appears over the shelf and upper slope north of Cape Hatteras. For example, in the band between 36 and 38°N, roughly 80–300 km north of the tip of Cape Hatteras, Gulf Stream water was detected 13–27% of the time at sites on the upper slope and 3–9% of the time at locations on the shelf. In most instances, sea surface temperature distributions suggest that the Gulf Stream water which appeared near the continental margin was not part of the Gulf Stream's main current but was fluid which had been expelled from the Gulf Stream. The Gulf Stream water found on the shelf did not appear to significantly influence the local circulation or alongshelf density structure. Its subtidal flow was largely driven by the alongshelf wind stress; and in most instances its vertical density profile nearly matched that of adjacent shelf water situated alongshore. In contrast, the discharged Gulf Stream water observed over the upper slope was significantly less dense than abutting fluid of equivalent depth located along the slope to the northeast. The circulation associated with this density contrast often conveyed Middle Atlantic Bight shelf water seaward of the continental margin. The resulting rate of shelf water export is not well resolved by the available data. Some rough estimates put it at 0.1 Sv, comparable with the estimated mean alongshore transport of shelf water over the Middle Atlantic Bight shelf.

Monthly Mean Wind Stress and Sverdrup Transports in the North Atlantic: A Comparison of the Hellerman–Rosenstein and Isemer–Hasse Climatologies

Abstract: The monthly mean wind stress climatology of Hellerman and Rosenstein (HR) is compared with the climatology of Isemer and Hasse (IH), which represents a version of the Bunker atlas (BU) for the North Atlantic based on revised parameterizations. The drag coefficients adopted by IH are 21% smaller than the values of BU and HR, and the calculation of wind speed from marine estimates of Beaufort force (Bft) is based on a revised Beaufort equivalent scale similar to the scientific scale recommended by WMO. The latter choice significantly increases wind speed below Bft 8, and effectively counteracts the reduction of the drag coefficients. Comparing the IH stresses with HR reveals substantially enhanced magnitudes in the trade wind region throughout the year. At 15°N the mean easterly stress increases from about 0.9 (HR) to about 1.2 dyn cm−1 (IH). Annual mean differences are smaller in the region of the westerlies. In winter, the effect due to the reduced drag coefficient dominates and leads to smaller stress values in IH; during summer season the revision of the Beaufort equivalents is more effective and leads to increased stresses. Implications of the different wind stress climatologies for forcing the large-scale ocean circulation are discussed by means of the Sverdrup transport streamfunction (ψs): Throughout the subtropical gyre a significant intensification of ψs takes place with IH. At 27°N, differences of more than 10 Sv (1 Sv ≡ 106 m3 s−1) are found near the western boundary. Differences in the seasonality of ψs are more pronounced in near-equatorial regions where IH increase the amplitude of the annual cycle by about 50%. An eddy-resolving model of the North Atlantic circulation is used to examine the effect of the different wind stresses on the seasonal cycle of the Florida Current. The transport predicted by the numerical model is in much better agreement with observations when the circulation is forced by IH than by HR, regarding both the annual mean (29.1 Sv vs 23.2 Sv) and the seasonal range (6.3 Sv vs 3.4 Sv).

A Description of the Annual Cycle in Sea Surface Temperature and Upper Ocean Heat in the Equatorial Atlantic

Abstract: Temperature and velocity time series, obtained by surface moorings during the Seasonal Response of the Equatorial Atlantic Experiment, are used to investigate the role of ocean dynamics upon the annual cycle of equatorial sea surface temperature (SST) and upper ocean heat. The annual cycle in SST is explained by different mechanisms, each operant at different phases of the cycle. The boreal springtime decrease in SST results from upwelling in response to the seasonal intensification of easterly wind stress. This upwelling causes the seasonal formation of the cold tongue along the equator in the central and eastern portions of the basin. An early summer increase in SST is attributed to the meridional convergence of Reynolds' heat flux associated with surface current instability-generated waves. After the instability waves abate, SST and mixed layer depth remain relatively steady from late summer through fall when the advective terms are small and cancelling, suggesting that surface heating is then...

Recent Climate Changes in the Northern Hemisphere

Abstract: The consistency of analyzed changes in surface wind stress, sea level pressures and surface temperatures between 1980–86 and previous periods indicates the reality of statistically significant and substantial climate changes in the Northern Hemisphere, especially over the North Pacific, on decadal time scales. Cooling in North Pacific sea surface temperatures and warming along the west coast of North America and Alaska are ascribed mainly to the changes in thermal advection associated with a deeper and more extensive Aleutian Low.

Effects of Diabatic Cooling in a Shear Flow with a Critical Level

Abstract: The response of a two-dimensional, stably stratified shear flow to diabatic cooling, which represents the evaporative cooling of falling precipitation in the subcloud layer, is examined using both a linear analytical theory and a nonlinear numerical model. The ambient wind is allowed to reverse its direction at a certain height and the cooling is specified from the surface to a height below the wind reversal level. From a scale analysis of the governing equations a nonlinearity factor of the thermally induced finite-amplitude wave, gQ0l(cpT0U02N), is found. From a scale analysis of the linear system, it is shown that the wind shear can modify the condition in which the upstream propagation of the density current is opposed by the ambient wind. When the shear and the basic wind are of opposite sign, small basic wind is enough to prevent the upstream propagation of the density current. This is because part of the cooling is used to compensate the positive vorticity associated with the positive wind...

Linear systems analysis of momentum on the continental shelf and slope of the central Great Barrier Reef

Abstract: Current meter records from a mooring transect deployed across the continental shelf and slope of the central Great Barrier Reef, Australia during 1985 have been analyzed in a study of the subtidal momentum balance. In the 3–20 day wave band, a single-input linear systems model of the subtidal along-shelf flow, driven by across-shelf pressure gradient (i.e., assuming semi-geostrophic balance), explained over 70% of the variance on the shelf, but only 30% at the shelf break and upper slope. A two-input model driven by along-shelf horizontal pressure gradient and wind stress, and incorporating along-shelf acceleration and bottom stress, explained approximately 60% of the variance on both the shelf and upper slope. The model responses evidently combine local wind-driven circulation and freely-propagating continental shelf waves. Linear resistance coefficients estimated from the two-input model averaged 0.07 cm s−1, but were higher (0.09) within the reef matrix and lower (0.06) near the coast.

The role of the turbulent stress divergence in the equatorial Pacific zonal momentum balance

Abstract: From a comprehensive set of upper ocean measurements made during a moderate E1 Nifio in boreal spring 1987, we reassess the role of turbulence in transporting momentum vertically at the equator. An examination of the terms in the vertically integrated zonal momentum equations indicates that on short time scales the zonal pressure gradient is not balanced by the surface wind stress despite an apparent balance of these terms on longer (seasonal) time scales. The vertical redistribution of zonal momentum is complex. The strength of the wind determines both the magnitude and, likely, the mechanisms of momentum transport between the surface and the core of the undercurrent. During low wind

The fortnightly mean circulation of chesapeake bay

Abstract: Data from several current measurement programmes were used to examine the temporal and spatial variability of Chesapeake Bay circulation at time scales of 15 days and longer. First, year-long current records from two stations were analysed for response to runoff, wind stress and gravitational forcing. A strong spring freshet in April resulted in an annual maximum in the longitudinal density gradient, yet the surface current at mid-bay actually declined at the time of this maximum. The reason for this response lies in a prolonged period of northward wind which effectively reduced the strength of the two-layer gravitational flow. This shows that seasonal changes in the wind field can modulate the gravitational flow and that these annual wind shifts are a significant factor in the circulation of this estuary. Statistical analysis also indicates correlation between surface current and longitudinal wind at time scales of 15 days and longer, implying that meteorological effects cannot be completely filtered out at these frequencies. These data demonstrate that Chesapeake Bay circulation, unlike runoff, does not change by orders of magnitude between seasons, indicating that an examination of spatial patterns of circulation can be made using data collected during different deployment periods. A total of 168 current records from 1977 to 1983, averaged over periods of at least 15 days, were used to examine the spatial structure of the mean bay-wide circulation. Surface velocity, while predominantly down-bay, shows considerable longitudinal structure. This is to be expected considering the different meteorological conditions under which the data was collected. However, bottom observations show remarkably coherent up-bay flow regardless of the measurement period, illustrating the dominant role of topography and the importance of the gravitational mode in bay circulation.

Annual and interannual variability of baroclinic transports across Line P in the northeast Pacific Ocean

Abstract: The more than 25 years (mainly during 1959–1983) of the hydrographiclSTD data along Line P in the northeastern part of the North Pacific Ocean have been examined to depict the annual and interannual variability of baroclinic transports in the upper 1000 m of water. The mean transport across the line, between Station P and the coast, based on well over 100 transects is 5 × 106 m3 s−1 (Sv), poleward. The western and eastern half of the line contribute 3 and 2 Sv, respectively. The fluctuations of transport are greater in the eastern half due presumably to the greater eddy motion on that side. Transports relative to the 1200 and 1500 db surfaces are, respectively, 20 and 40% greater than those relat'lve to the 1000 db surface for each side of the line. There is very little annual cycle of transports. However, the tendency exists for the transports to be slightly greater in winter than in summer. This cycle is more discernible for the transports across the eastern half of the line. The time-series transports are characterized by the presence of a well-defined interannual variability. For the entire and the western half of the line persistent strong or weak transports occur for 3–4 years at a time. Spectral peak at a period of 6–7 years is evident but is not significant, even at the 80% confidence interval. At the annual frequency the observed and computed (based on curl of wind stress) transports are highly correlated for the eastern half of the line, suggesting that the coastal portion of the transports are more responsive to changing wind-stress curl than for the offshore portion. It would appear that at the seasonal scale a significant part of the atmospheric forcing goes to produce transports in the barotropic mode in the offshore waters. There is a small out-of-phase relationship between the Alaska Current and the coastal component of the California Current. While this result is not inconsistent with the bifurcation proposal it is more likely that this is due to the in-phase relationship of the coastal currents along the Pacific coast of Canada-U.S.A. The widespread occurrence of ocean events at the period of 6–7 years in the North Pacific is believed to be associated with atmospheric forcing of similar time scale. Information upon the barotropic component of the circulation is lacking. Such data are needed to understand the coupling between the atmosphere and the ocean in both the annual and interannual scales.

A wind-wave tank study of the azimuthal response of a Ka-band scatterometer

Abstract: The azimuthal response of a 36-GHz scatterometer from wind-generated waves in large wave tanks is presented. The radar was operated at an incidence angle of 30 degrees from nadir and with vertical polarization. Measurements were also made of turbulent stress in air flow above the water surface and the cross-wind and along-wind wave slopes. The friction velocity, u/sub */, ranged from 5 to 100 cm/s. For each wind speed, the azimuthal variation of radar cross section is expressed in terms of the classical three-term Fourier cosine series. As the wind speed increases, the azimuthally averaged radar cross section increases, and the upwind-downwind asymmetry decreases, in general agreement with field observations. However, this Ka-band data set reveals that the cross-wind modulation is not a monotonic function of wind speed, but instead it reaches a peak at u/sub */ of about 25 cm/s. The wave slope data exhibit exhibit a similar feature in the short wave directional development. Measurements from 35- and 95-m fetches indicate that the azimuthal response of the radar is more directly related to wind stress than wind speed. >

The dependence of wind stress on wave height and wind speed

Abstract: Three near-neutral boundary layer data sets were investigated with the aim of finding a dependence of wind stress on both wind speed and significant wave height. The data set most representative of open-ocean wave height, wind speed, and momentum flux conditions, was selected and analyzed by means of the least-squares method to produce a new parameterization for the wind stress as a function of both wind speed and significant wave height. This study shows that the wind stress, and consequently the drag coefficient, decreases with increasing wave height for a fixed wind speed. The study also shows that the curvature of the wind profile decreases with increasing wave height and that the C(DN) = A + BU-bar(10) form for the drag coefficient parameterization is inadequate. A drag coefficient that applies to both smooth and rough flows is proposed. These results are more applicable for open-ocean deep-water conditions and less applicable for sheltered, closed, shallow water sites.

An upper‐ocean general circulation model for the north pacific: Preliminary experiments

Abstract: A model with two active layers, a mixed layer and a pycnocline layer, over a semipassive deep ocean is described. The model is used to simulate a climatological seasonal cycle in the upper North Pacific. The formulation is similar to that in Cherniawsky et al. (1990). The model resolution is 1° latitude by 1.5° longitude, extending from 62°N to the equator. It is driven with monthly wind stress (Hellerman and Rosenstein, 1983) and with Newtonian heat and freshwater fluxes, which were inferred from climatological (Levitus, 1982) sea‐surface monthly temperatures and annual mean salinities. The monthly temperature anomalies (without the annual mean) are multiplied by a prescribed gain factor and advanced in time, compensating for time delay in the response of the mixed layer. No‐slip and no‐flux constraints are applied on north, east, west and land boundaries, while the following open boundary conditions are used at the equator: (a) free‐slip on zonal velocities in the two layers; (b) a prescribed m...