Showing papers on "Wind shear published in 1993"

The western boundary current of the seasonal subtropical gyre in the Bay of Bengal

Abstract: Hydrographic data collected during March–April 1991 show the presence of a poleward current along the western boundary of the Bay of Bengal north of about 10° N carrying warmer waters of southern origin. The inshore side of the current was marked by cooler, more saline waters brought to the surface due to the presence of the current which transported approximately 10 × 106 m3/s. The hydrography is suggestive of many of the features that have been associated with the western boundary currents of the subtropical gyres of the world oceans: a recirculation zone, waves, eddies, etc. These features, however, were not satisfactorily resolved in the data. Using available climatologies of monthly mean ship drifts, seasonal hydrography, and monthly mean wind stress, we propose that the poleward current is the western boundary current of a seasonal anticyclonic subtropical gyre which forms in the Bay during January, is best developed during March-April, and decays by June. The gyre and the western boundary current are unique because of their seasonal character. The pattern of circulation leading to formation and decay of the gyre is reproduced reasonably well in the computation of the monthly mean barotropic transport induced by the curl of wind stress, which has a well-defined annual cycle due to the monsoons and which is conducive to the formation of an anticyclonic gyre only during the months of January-May. The pattern of circulation due to baroclinic transport induced by the wind stress curl, however, is not known at present, and this makes it difficult to conclude unequivocally that the wind stress curl over the bay is the sole mechanism to force the gyre.

Subtidal circulation over the northern California shelf

Abstract: The circulation over the shelf and upper slope off northern California, between 38°N and 42°N, was observed using moored current and temperature recorders deployed as part of the Northern California Coastal Circulation Study (NCCCS), from March 1987 through October 1989. The results of this study provide a larger-scale view of the wind-driven circulation than that described through the 1981–1982 Coastal Ocean Dynamics Experiment (CODE), particularly with regard to alongshore and temporal variations. From an improved description of the frequency structure of wind and current, a very low frequency (VLF) signal is identified in the observed currents at frequencies below those typical of wind forcing. The majority of this VLF variance appears to be accounted for by persistent flow events associated with the presence of mesoscale circulation in the adjacent ocean. The longer duration of the NCCCS also allows an improved description of the seasonality of flow regimes off northern California. Three oceanic seasons are identified: an upwelling season (April-July), a relaxation season (August-November), and a storm season (December-March). Alongshore variations in the strength of upwelling, in the strength of the alongshore flow, both near-surface and undercurrent, and in water temperature not only are a function of latitude, as is the wind, but they also correspond to location relative to promontories, notably Cape Mendocino. Immediately south of Cape Mendocino, the near-surface flow exhibits an equatorward minimum and a temperature minimum, whereas the undercurrent exhibits a poleward maximum. Conversely, at the moorings immediately north of the cape, temperatures are a maximum and the undercurrent exhibits a minimum. The maximum in near-surface temperature relates to a minimum in upwelling; no significant correlation was found between local wind and current immediately north of Cape Mendocino. This upwelling minimum and the upwelling maximum south of the cape were also observed as persistent sea surface temperature patterns in satellite imagery.

Turbulence structure and mass transfer across a sheared air–water interface in wind-driven turbulence

Abstract: The mass transfer mechanism across a sheared air–water interface without bubble entrainment due to wave breaking was experimentally investigated in terms of the turbulence structure of the organized motions in the interfacial region in a wind-wave tank. The transfer velocity of the carbon dioxide (CO2) on the water side was measured through reaeration experiments of CO2, and the fluid velocities in the air and water flows were measured using both a hot-wire anemometer and a laser-Doppler velocimeter. The results show that the mass transfer across a sheared air–water interface is more intensively promoted in wind shear, compared to an unsheared interface. However, the effect of the wind shear on the mass transfer tends to saturate in the high-shear region in the present wind-wave tank, where the increasing rate of mass transfer velocity with the wind shear decreases rapidly. The effect of the wind shear on the mass transfer can be well explained on the basis of the turbulence structure near the air–water interface. That is, surface-renewal eddies are induced on the water side through the high wind shear on the air–water interface by the strong organized motion generated in the air flow above the interface, and the renewal eddies control the mass transfer across a sheared interface. The mass transfer velocity is correlated with the frequency of the appearance of the surface-renewal eddies, as it is in open-channel flows with unsheared interfaces, and it increases approximately in proportion to the root of the surface-renewal frequency. The surface-renewal frequency increases with increasing the wind shear, but for high shear the rate of increase slows. This results in the saturated effect of the wind shear on the mass transfer in the high-shear region in the present wind-wave tank. The mass transfer velocity can be well estimated by the surface-renewal eddy-cell model based on the concept of the time fraction when the surface renewal occurs.

On the Generation of African Squall Lines

Abstract: Squall lines (SLs) form an important component of the meteorology of northern Africa, and in particular, contribute substantially to rainfall totals. Their generation requires the existence of a potentially unstable low-level supply of moisture overlain by dry desert air and vertical wind shear beneath the midlevel African easterly jet. The instability may be released (and an SL initiated) by factors such as surface heating, topography, African waves, or surface evaporation. The relative importance of each of these factors and the means by which they impact on SL generation is reviewed. This is followed by a detailed analysis of one month of satellite imagery and surface data for August 1985 over a portion of central northern Africa. The novelty of our study lies in the temporal resolution of the satellite imagery, which with 21 images per day allows the identification of a large number of short-lived SLs (4-h duration or less). On the southern fringes of the Sahara these are likely to contribute...

Adaptation of the near-surface wind to the development of sand transport

Abstract: A model of wind-blown sand transport is described with particular emphasis on the feedback between the grain cloud and the near-surface wind. The results from this model are used to develop Owen's (1964) hypothesis that ‘the grain layer behaves, so far as the flow outside it is concerned, as increased aerodynamic roughness whose height is proportional to the thickness of the layer’. The hypothesis is developed to show the influence this dynamic roughness has on the turbulent boundary layer above the saltation layer. Two processes are identified which influence the path of the system towards equilibrium. The first is the feedback between the near-surface wind and the grain cloud in which the quantity of sand transported is limited by the carrying capacity of the wind. The second is due to the temporal development of an internal boundary layer in response to the additional roughness imposed on the flow above the grain layer by the grain cloud. A similarity is noted between the temporal response of a turbulent boundary layer to sand transport and the spatial response of a turbulent boundary layer downstream of a step increase in surface roughness. Finally it is noted that the work may have important implications for transport rate prediction in unsteady winds.

Equatorial variability and resonance in a wind-driven Indian Ocean model

Abstract: A numerical isopycnal ocean model has been designed and applied to model the Indian Ocean north of 25°S. Vertical normal modes are used in the open boundary conditions and for selections of initial layer depths. A 21-year integration with a reduced Hellerman-Rosenstein monthly averaged wind stress has been made with 3.5-layer and 1.5-layer versions of the model. Both solutions reproduce the main features of the observed wind-driven seasonal circulation in the Indian Ocean above the main thermocline. The transient semiannual equatorial surface jets are more intense, more coherent, and in better phase agreement with observations when three layers are active. The associated undercurrents below the main thermocline are also included in the 3.5-layer model solution. Second baroclinic-mode, reflecting, equatorial Kelvin and Rossby waves combine to give a semiannual, resonant basin mode. Experiments with an equatorial band of semiannual zonal winds suggest a very strong response of the Indian Ocean to wind forcing with this period. Further, the amplitudes of the 28–30 day oscillations in the western equatorial model region are found to be strongly damped with depth; they have upward phase propagation and downward energy propagation.

A review of aeolian transport processes in cold environments

Abstract: Aeolian environments in Canada experience seasonally cold temperatures. The portion of the annual wind transport occurring between late autumn and early spring usually is perceived as trivial because low temperature transport is difficult to measure reliably, particularly in remote northern areas, and because warm climate based semi-empirical wind erosion models are intractable for temperatures below 0°C. Very little is known about the processes contributing to the phenomenal aeolian transport associated with the Pleistocene Epoch, but supply limiting factors were likely as important then as they are in contemporary high latitude environments, although the wind and solar radiation regimes of this glacial period are not exactly replicated. Field and simulation work on the boundary layer and surface controls, which include wind shear velocity, air density, sediment texture, pore water, snow, ice, and vegetation, suggests a complex system of interactions. Frozen and wet surfaces, traditionally viewed as stab...

Characteristics of gravity waves in the mesosphere observed with the middle and upper atmosphere radar: 1. Momentum flux

Abstract: We observed wind motions from 60- to 90-km altitudes with the MU radar (35°N, 136°E) in the four observation periods: October 1986, June 1987, July 1990, and January/February 1991. Mean wind profiles were fairly consistent with results of Kyoto meteor radar observations (35°N, 136°E) collected in 1983–1985 at 80- to 110-km altitudes, and zonal mean winds generally agreed well with CIRA 1986 except for the profiles below 70 km in January/February 1991, although discrepancy of amplitudes sometimes ranged up to 10 to 20 m/s. Characteristics of frequency spectra of radial wind velocities were basically similar among results determined in four observation periods. That is, oblique spectra had a logarithmic slope of about −5/3 in the frequency range lower than 4–5 × 10−4 s−1. Gravity waves with periods longer than 20–50 min to 5 hours (the lowest limit of the spectral analysis) were found to carry a large part of the zonal momentum flux, while a dominant frequency component was not detected for the meridional flux. Zonal and meridional drag of mean winds induced by the gravity wave with periods from 5 min to 8–10 hours were +51 m/s/d and −4 m/s/d in June at 75 km and −4.0 m/s/d and +7.4 m/s/d in October at 70 km, respectively. The major part of the drag force was also found in the wave component with periods larger than 30 min. The day-to-day variation in the zonal momentum flux showed a good correlation with the vertical shear of the zonal mean wind, which suggests effects of the gravity wave activities on mean wind fields, or the effect of shear variation on the gravity wave activity.

A simulation of the Cerro Hudson SO2 cloud

Abstract: An isentropic trajectory model is used to simulate the evolution of the southern hemisphere SO2 cloud associated with the eruption of Cerro Hudson. By matching the parcel trajectories with total ozone mapping spectrometer SO2 retrievals, the principal stratospheric injection region is determined to be between 11 and 16 km in altitude. This region is characterized by weak wind shears and is located just poleward of the subtropical jet in the outer fringe of the stratospheric polar vortex. The lack of wind shear in the injection region explains the slow zonal dispersal of the SO2 cloud which was still clearly observed 19 days after the eruption. The trajectory model simulation of the SO2 cloud shows good agreement with observations for 7 days after the eruption. Using the potential vorticity and potential temperature estimates of the initial eruption cloud, the cloud position relative to the polar night jet is shown to be nearly fixed up to September 2, 1991, which was as long as the cloud was observed. This result suggests that the lower stratospheric polar and midlatitude regions are nearly isolated from each other during the late August period.

Directional attributes of the ocean surface wind stress vector

Abstract: In a recent paper by Geernaert (1988) it was suggested that the stress and wind vectors are rarely aligned and that the deviation of the stress vector direction from the wind vector is related to the surface heat flux. It was argued that since cold air advection over the open ocean generally corresponds to upward heat flux, the consequent rotation of the winds to the right with height during cold air advection could be responsible for observations of the stress vector to be to the right of the wind flow for unstable stratifications. Conversely, observations of the stress vector to the left of the flow for stable stratifications were suggested to be associated with the turning of the wind vector to the left of the flow for warm air advection, or downward heat flux. In that study only 30% of the statistical variance of the angle between the wind and wind stress vectors could be explained by air-sea heat flux, and it was suggested that the orbital velocity spectrum of surface waves (i.e., data unavailable in that study) could be important for explaining at least part of the remaining variance. In this study we report on measurements of the wind stress vector and wave directional spectrum collected during the Synthetic Aperture Radar X-Band Ocean Nonlinearities experiment. Conducted on the Chesapeake Light Tower during 1988, the experiment yielded data reported herein that provide convincing evidence that the stress vector direction, during light winds and near-zero air-sea heat flux, is governed in large part by the direction of long waves on the ocean surface. Further, the measurements suggest that the stress vector has a direction which is a blend between the wind direction and the swell direction.

A study of wind stress and heat flux over the open ocean by the inertial-dissipation method

Abstract: A bow-mounted propeller anemometer and fast-response temperature sensors were operated during several cruises of CSS Dawson. Spectra of wind speed and temperature fluctuations were measured over the open ocean for a wind speed range of 6 to 21 m s−1 and a sea-air temperature difference range of ±6°C. Wind stress on the sea surface and sensible heat fluxes were determined by the inertial-dissipation method over a wide range of wind speeds for both stable and unstable atmospheric conditions. Neutral drag and sensible beat flux coefficients as functions of the wind speed at a 10-m reference height are in excellent agreement with the only existing eddy fluxes measured over the ocean from a stable platform and also with open sea inertial-dissipation measurements from a ship.

Mean circulations of boundary-layer rolls in lake-effect snow storms

Abstract: An observational study of the roll-average flow fields of boundary-layer rolls is presented. Data used for this purpose were collected by dual-Doppler radar and aircraft measurements taken over southern Lake Michigan during the 1983/1984 field operations of Project Lake Snow. The roll circulations agreed well with findings of past observational, numerical and theoretical studies, with cross-roll components roughly 10% of the convective internal boundary layer (CIBL)-mean wind speeds and weaker vertical components. Along-roll winds were systematically stronger in the rollupdraft regions than in the roll-downdraft regions, probably due to distortion of the along-roll wind profile by the rolls. Comparison of observed wind profiles to those required by roll formation mechanisms found by past numerical and theoretical studies suggested that the observed rolls were formed by the along-roll wind shear (Asai, 1970) or wind shear curvature (Kuettner, 1971) in the lowest 0.2Z i, whereZ i is the height of the top of the CIBL.

Reduction of horizontal wind speed in a boundary layer with obstacles

Abstract: The reduction of horizontal wind speed at hub height in an infinite cluster of wind turbines is computed from a balance between a loss of horizontal momentum due to the drag and replenishment from above by turbulent fluxes. This reduction is derived without assumptions concerning the vertical wind profile above or below hub height, only some basic assumptions on turbulent exchange have been made. Two applications of the result are presented, one considering wind turbines and one pressure drag on orographic obstacles in the atmospheric boundary layer. Both applications are basically governed by the same kind of momentum balance.

Derivation of satellite wind model functions using operational surface wind analyses: An altimeter example

Abstract: One year of global surface wind products from multiple operational numerical weather prediction (NWP) forecast/analysis systems are used as comparison data to derive an empirical wind speed model function for the Geosat altimeter. The resulting model function is nearly identical to the modified Chelton-Wentz model for wind speeds from 4.5 to 15 m/s. Highly skewed distributions at low wind speeds are consistent with specular reflections and antenna mispointing errors hypothesized by others on the basis of extremely limited data. Mesoscale variability in the wind field and synoptic scale errors in the NWP products are shown to account for about 30 percent of the observed scatter of sigma(0) at each wind speed. The remaining scatter is largest at low winds, and decreases to a nearly constant value of about 12 percent at speeds greater than 7 m/s. Model function uncertainty expressed more traditionally in units of wind speed is examined for historical model functions as well as the present NWP-based model. The historical models have significant biases at high wind speeds owing to the lack of comparison in situ data used in their construction. The present study demonstrates that modern operational NWP surface wind products are sufficiently accurate to allow development of fully empirical model functions and associated error analyses.

Modeling seasonal variability in the wind-driven upper-layer circulation in the Indo-Pacific region

Abstract: A basin-scale fine-resolution primitive equation reduced-gravity model forced by the climatological monthly wind of Hellerman and Rosenstein is used to study seasonal variability of the wind-driven upper-layer circulation in the Indo–Pacific region. The model domain is limited to the tropical Pacific Ocean and the eastern Indian Ocean. The use of the open boundary conditions allows the model to account for a net transport of mass from the Pacific to the Indian Ocean. An active eddy field is modeled east of Mindanao in the strong shear zone formed between the North Equatorial Current and the North Equatorial Countercurrent. This eddy field gives rise to the energetic shorter time scale variations in the western Pacific and appears to be responsible for considerable variability in surface currents noted previously. A distinct current bifurcation at the southern coast of New Britain in the Solomon Sea is modeled. The western branch of the resulting flow exits through Vitiaz Strait, while the eastern...

Estimates of Horizontal Divergence and Vertical Velocity in the Equatorial Pacific

Abstract: Horizontal divergence and vertical velocity in the surface mixed layer of the equatorial Pacific between 90° and 150°W are estimated from current measurements obtained from trajectories of freely drifting buoys during 1979–1990. The 12-year averaged horizontal divergence is predominantly meridional and has a maximum magnitude of 3–4 (× 10−6 s−1) in a 20-km-wide latitude band cantered on the equator. Using the equation of continuity, this divergence corresponds to an upwelling velocity of 1.5–2 (× 10−4 m s−1) at 50-m depth. The seasonal variations of equatorial divergence are in good agreement with the local zonal wind stress.

Wind-forced Variations in Sea Surface Height in the Northeast Pacific Ocean

Abstract: Sea surface height (SSH) anomalies from the Geosat altimeter for the northeast Pacific Ocean were analysed to determine their annual and interannual fluctuations over a 2.5-year period. The interannual anomalies suggested large-scale changes in the intensity of the California and Alaska currents, with a weak California Current for the first year (1986–1987), which strengthened during the second year, partly by a diversion of flow from the Alaskan gyre into the California Current and partly by a decrease in SSH along the coast. In the California Current between about 36° and 46°N, the annual fluctuations in SSH showed westward phase propagation. These observations were modeled using a linearized potential vorticity equation with one active layer, forced by realistic wind stress curl, which resembled a standing wave. The annual fluctuations in SSH were produced primarily by Ekman pumping, because Rossby waves are coastally trapped poleward of about 37°N. The predicted response had excellent phase a...

Effect of wind changes during the Last Glacial Maximum on the circulation in the Southern Ocean

Abstract: Present-day surface wind stress climatology is manipulated to simulate wind conditions during the last glacial maximum. These estimated wind fields force a one-layer, wind-driven numerical model of the southern ocean to determine if a change in the strength of the surface wind stress can shift the location of the Antarctic Polar Front, which is part of the Antarctic Circumpolar Current. A change in the forcing by a factor of 0.5–2.0 results in a change in the speed of the flow by an identical factor with no change in position. However, if the present-day wind climatology is shifted meridionally, there is a change in both strength of the circulation and spatial pattern. A shift of the wind stress of more than 5° of latitude is required to produce a shift in the location of the polar front.

Single-Doppler Radar Observations of a Mini-Supercell Tornadic Thunderstorm

Abstract: During the early evening hours of 19 May 1989, the CHILL 10-cm Doppler weather radar observed most of the lifetime of an unusually small tornadic thunderstorm. Throughout the event, the parent thunderstorm echo top remained below 6.7 km MSL The low-altitude echo diameter, as defined by the 25-dBZ contour, was only 15 km. Despite its small size, both visual and radar observations indicated that this storm contained many of the organizational features often noted in large, “classical” southern Great Plains supercells. The synoptic setting in which this storm occurred was atypical for supercell development in that both the thermodynamic instability and vertical wind shear magnitudes were limited. This documentation of a tornadic storm that developed in a nonthreatening environment mid that presented a small, seemingly inconsequential radar appearance demonstrates some of the challenges that will be faced by automated Doppler radar-based severe weather detection algorithms.

Tracking mesoscale ocean features in the Caribbean Sea using Geosat altimetry

Abstract: The authors use Geosat Exact Repeat Mission altimetry data to track mesoscale ocean features in the Caribbean Sea. Because of the topography of the basin extensive mesoscale features exist, and have been studied and modeled. Models indicate the presence of eddy flow patterns in parts of the basin. The meteorology of the region is dominated by the Intertropical Convergence Zone, which shifts with the seasons. It has been shown that the seasonal wind stress curl positively correlates with total volume transport through the Caribbean Sea. Satellite altimetry provides an accepted means of studying mesoscale features of ocean dynamics which vary with time. The data reveals the appearance of two anticyclonic features, one each year, which appear and drift westward at speed considerably less that surface flow speeds. It also indicates the presence of a cyclonic feature in the southwest corner of the Columbian basin which varies with time. Marine geoid corrections unfortunately remove any features which are not time dependent from the data.

In flight doppler weather radar wind shear detection system

Abstract: An airborne doppler radar wind shear detection system has a volumetric scanning pattern for providing atmospheric measurement data for individual resolution cells that are formed into a 3-D grid of atmospheric data samples. Volumetric feature extraction modules identify and group resolution cells having particular features into air masses of interest. A spatial feature association and filtering module combines the air masses of interest into a 3-D representation of atmospheric conditions and filters out ground clutter. A contextual matching and temporal tracking module compares the 3-D representation to known wind shear models and compares successive 3-D representations to one another to aid in identifying hazardous wind shear conditions in the aircraft flight path.

Hodograph Curvature and Updraft Intensity in Numerically Modeled Supercells

Abstract: A set of numerical simulations of supercell thunderstorms has been carried out with a range of low-level curvatures in the environmental hodograph and midlevel shears. They cover a range of hodograph “shape,” as measured by the integrated helicity of the lowest 3 km of the hodograph. The peak updraft occurs in the first hour of the storms and tends to be greater for larger values of environmental helicity. There is also a slight tendency for greater updraft intensity with lesser values of midlevel shear. Significantly, air in the core of the updrafts at midlevels (∼5 km) is not the most unstable air at the level. The most buoyant air rises in a region with a downward-directed pressure gradient force, which slows its ascent. Conversely, pressure gradient forces at lower levels (2–3 km) accelerate less buoyant air upward into the core of the midlevel updrafts. The pressure gradient force is larger in the cases with more curvature in the environmental wind than the low-curvature environments. This i...

Formation mechanisms for low-altitude Es and their relationship with neutral Fe Layers: Results from the METAL Campaign

Abstract: The METAL campaign was a multi-instrument campaign conducted in September-October 1991 that was designed to investigate the relationship between neutral and ionized metallic layers in the high-latitude lower ionosphere. Measurements included electron density profiles and electric fields from the EISCAT UHF incoherent scatter radar, ionosonde measurements of Es layers, neutral Fe profiles from lidar and rocket observations of winds (by chaff release), and measurements of plasma density and ion composition (by mass spectrometer). Es layers were observed on eight occasions. The measurements are here compared with simulations of Es formation by tidal wind shear and electric fields. The main features of the Es layers are found to be consistent with formation by tidal wind shear alone in one case and by the combined action of wind shear and electric fields in the remaining cases. On three occasions, layers were seen to descend rapidly, in one case to below 90-km altitude, and the rate of descent was found to be consistent with the expected action of the observed electric fields. Sporadic neutral Fe layers were observed on seven occasions, including four cases in close correlation with Es. However, the neutral layers were generally not at exactly the same altitude as the Es, nor did they follow the same altitude changes. The results indicate that both wind shears and electric fields are important for the formation of thin layers and the vertical transport of metal ions in the high-latitude lower thermosphere. They show that neutral Fes layers, although they may be formed in close association with Es layers, can persist and migrate independently of Es layers. This latter observation suggests that partial reionization of neutral layers by intense auroral particle precipitation may explain some observations of multiple Es layers.

The Coupling of Gravity Waves and Turbulence at White Sands, New Mexico, from VHF Radar Observations

Abstract: Doppler spectra taken with the VHF Doppler radar at White Sands Missile Range are used to describe the winds and turbulence for 10 days in March–April 1991. The large power aperture product of this radar provides excellent data coverage in 150-m layers over the entire height range used, about 5–20 km. The results show that gravity-wave activity and small-scale turbulence are significantly enhanced at all levels during times when wind speeds in the troposphere, near 5.6 km (about 500 hPa), are strong. Largest enhancements are found in the lower stratosphere, near 16–18 km, where the mean log C2N is increased by over 10 dB during times of strong winds at low levels. Mean winds, wind shears, and static stability in the lower stratosphere were found to be nearly the same, regardless of wind speeds at low levels. The authors conclude that the enhanced turbulence is due to an effect not described by the local background wind and static stability, and suggest that this effect is upward-propagating gravi...

Modeling interannual variability in the Indian Ocean using momentum fluxes from the operational weather analyses of the United Kingdom Meteorological Office and European Centre for Medium Range Weather Forecasts

Abstract: A 16-level general circulation model of the Indian Ocean is used to reproduce interannual variability in ocean circulation from 1987 to 1990 using wind stresses from the United Kingdom Meteorological Office (UKMO) operational forecast model analyses. The results of this integration are compared with those obtained using the Hellerman and Rosenstein [1983] (HR) climatological forcing. The differences in the two wind products are large, as are the resulting surface and subsurface currents. Model results are compared with observations at 55°E and 73°E on the equator. The undercurrent using UKMO stresses is more strongly eastward than that using HR, and in the four years studied, the model always simulates an undercurrent in February-March at Gan (73°E), contrasting with the inconstancy of its appearance in the years sampled by the observations. Interannual differences in the surface currents can be large, as much as 100 cm s−1 in May on the equator. The years 1987 and 1988 are worthy of special consideration, since 1987 was a drought year over India and 1988 had above average rainfall. Therefore, the period 1987–1988 is repeated using stresses from the European Centre for Medium Range Weather Forecasts (ECMWF). There are large differences between the ECMWF and the UKMO stresses and, consequently, in the model currents, where differences can be as much as 80 cm s−1 over a large area of the equatorial zone and in the Somali current region. At undercurrent depth, the response at Gan (73°E) is very similar in the first half of the year in all three integrations, but differs significantly in the second half with the UKMO and ECMWF stresses inducing a semiannual response, whereas HR leads to an annual cycle. Although interannual differences are large on a month to month basis, they are not long lasting. Interannual variations in model sea surface temperatures (SST) resulting from variations in the surface stress are poorly related to observed SST anomalies. There are no direct measurements of currents in the 1987–1990 period with which to validate the model simulations. There are, however, satellite measurements of the distortion of the sea surface and so comparison is also made between the simulations of dynamic topography from the model and GEOSAT altimetry data. While there is broad-scale similarity in pattern between the three model simulations and the GEOSAT observations, detailed agreement between the fields is poor. The model results are also compared to the dynamic topography derived from Levitus data.

Estimating the influence of weather on ship performance

Abstract: From extensive and detailed calculations of the added resistance due to wind and waves for a wide range of ships having Series 60 forms, relatively simple methods are provided for estimating added resistance, speed loss at constant power and power increase at constant speed, in head weather Oblique sea added resistance is obtained by modifying head sea results by a direction reduction factor derived from Series 60 model tests in oblique waves oblique wind added resistance is calculated from an approximate formula interpreting van Verlekom's method, leading to a direction-reduction factor applied to head wind resistance Separate calculations can be made for wind and sea each coming from its own direction, with its own strength

The Local and Nonlocal Response of Conception Bay to Wind Forcing

Abstract: In this paper the response of Conception Bay to wind forcing is discussed. Current meter and thermistor chain observations are analysed and compared with output from a reduced-gravity numerical model. The model incorporates realistic coastal geometry and is driven by wind stress calculated from observed winds. Moorings were deployed in the bay during 1989 and 1990. In 1990 the moorings were placed within the coastal waveguide around the head of the bay and show that southwesterly winds generate an upwelling event on the western side that moves around the head of the bay and is suggestive of Kelvin wave propagation. Data analysis shows that the thermocline response is strongly coherent between each mooring at periods of 2–10 days, and winds measured at a nearby station are found to be strongly coherent with the observed temperature fluctuations. Two versions of the reduced-gravity model are applied—one models Conception Bay alone and ignores “upstream” influences and another includes neighboring T...

Wind-Forced variability of the deep eastern north Pacific: Observations of seafloor pressure and abyssal currents

Abstract: Data from an array of bottom pressure gauges and a string of current meters in the vicinity of 47 deg N, 139 deg W, are used to examine the deep-ocean variability forced by ocean surface wind stress curl from August 1987 to June 1988. Bottom geostrophic currents are computed from the pressure gauge array, and these correspond well to the long-period directly measured currents at 3000 m. The supratidal-period bottom pressure variations are coherent at 95% confidence with the wind stress curl in period bands of 3-4 days and 15-60 days but removed in distances of 400 and 700 km to the northwest and the southeast, respectively. A linear, two-layer hydrodynamic model is used to examine the theoretical forcing produced by random-phased surface wind fields for the conditions of the eastern north Pacific and the 15- to 60-day-period observed response is reproduced credibly. To model 3- to 15-day variations, more realistic models are required.