摘要:

Imagery of ocean surface waves obtained from synthetic aperture radar (SAR) provides a unique two‐dimensional fine‐resolution view of the ocean wave field which has been long thought to be of considerable value for studying air‐sea inter‐action, wave generation and propagation, and wave‐current refraction. Wave spectra from SAR have also been thought to be extremely useful for updating and verifying global wave forecast models. Such promise prompted a SAR to be placed on Seasat, an oceanographic satellite which operated in a rather glorious mode for only a little over 3 months in 1978. This imagery has been well studied and indeed has demonstrated the feasibility of acquiring useful directional wave spectra from space [e.g.,Vesecky and Stewa

ISSN:0148-0227    

DOI:10.1029/JC093iC12p15365

年代:1988

数据来源: WILEY

摘要:

During the Shuttle Imaging Radar‐B (SIR‐B) mission in October 1984, synthetic aperture radar (SAR) images were taken off the coast of Japan. Wave spectra derived from SIR‐B imagery were compared with simultaneous wave buoy observations. Waves at the time of flight were nearly perpendicular to the SAR flight direction and were composed of two major systems: a swell of about 200‐m wavelength and wind waves with 70‐m wavelength. Both the swell and wind wave peaks were identifiable in the SAR image spectrum after stationary response correction. Peak frequencies derived from SIR‐B images and from the wave gauge agreed satisfactorily. However, the relative magnitudes of the peaks are not in accordance with the surface measurements. Possible reasons for the disagreement ar

ISSN:0148-0227    

DOI:10.1029/JC093iC12p15367

年代:1988

数据来源: WILEY

摘要:

During October 1984, coincident shuttle imaging radar‐B synthetic aperture radar (SAR) imagery and wave measurements from airborne instrumentation were acquired. The two‐dimensional wave spectrum was measured by both a radar ocean wave spectrometer and a surface contour radar aboard the aircraft. In this paper we compare two‐dimensional SAR image intensity variance spectra with these independent measures of ocean wave spectra to verify previously proposed models of the relationship between such SAR image spectra and ocean wave spectra. The results illustrate both the functional relationship between SAR image spectra and ocean wave spectra and the limitations imposed on the imaging of short‐wavelength, azimuth‐travel

ISSN:0148-0227    

DOI:10.1029/JC093iC12p15374

年代:1988

数据来源: WILEY

摘要:

SIR‐B synthetic aperture radar ocean‐wave spectra for a 200‐km pass crossing the Agulhas current off the coast of Africa are analyzed. A significant enhancement of one spectral peak along the northern edge of the current is attributed both to amplification of the waves by refraction‐dominated wave‐current interaction and to transient specular backscatter contributions spatially correlated to

ISSN:0148-0227    

DOI:10.1029/JC093iC12p15389

年代:1988

数据来源: WILEY

摘要:

Synthetic aperture radar (SAR) image intensity spectra measured during the Shuttle Imaging Radar‐B (SIR‐B) mission on October 6 and 8, 1984, over the North Sea are compared with ocean wave spectra obtained from a wave prediction model. The relationship between both spectra is discussed in the light of a previously proposed SAR imaging theory of ocean waves. Most of the measured SAR image spectra of October 6 show double peaks, while the hindcast ocean wave spectra have only single peaks. It is shown that the double peaks are generated by the SAR imaging mechanism. They occur only when velocity bunching is sufficiently strong near the range direction and when enough spectral energy density is present in this spectral region. Double peaks are generated when the SAR modulation transfer function (MTF) consisting of the sum of the complex real aperture radar (RAR) MTF and the velocity bunching MTF has a strong minimum near the range direction by which the wave spectrum is cut into two, in general unequal, pieces. The azimuth angle at which this minimum occurs depends strongly on the phase of the RAR MTF. In general, the two peaks are not located symmetrically with respect to the range direction, nor do they have equal heights. We interpret the reasonably good agreement between simulated and measured SAR image spectra during the SIR‐B mission over the North Sea as further experimental evidence in support of the validity of the previously proposed SAR imaging theory of ocean

ISSN:0148-0227    

DOI:10.1029/JC093iC12p15403

年代:1988

数据来源: WILEY

摘要:

The time‐averaged flow of a three‐layer eddy‐resolving quasi‐geostrophic ocean model is considered in the light of two analytical models of wind‐driven gyres, in which the vertical structure of the gyre is set by assuming that potential vorticity is uniform beneath layers exposed to forcing. The first, due to Young and Rhines (1982), supposes that the depth‐integrated meridional transport is set by the imposed wind‐stress curl. In contrast, in the baroclinic Fofonoff gyres of Marshall and Nurser (1986, 1988), inertial aspects of the flow are emphasized without imposing a Sverdrup constraint. The mean fields from the model are seen to lie between the two extremes represented by these analytical solutions. In the interior of the gyre away from inertial boundary currents and jets, the Sverdrup constraint is obeyed, and the mean flows resemble the Young and Rhines solution. However, the essential character of the overspun recirculation, including the weakly depth‐dependent nature of the currents in its return flow, seem to be well captured in the inertial limit considered by Mars

ISSN:0148-0227    

DOI:10.1029/JC093iC12p15427

年代:1988

数据来源: WILEY

摘要:

A case study evaluating the predictive capability of an upper layer circulation model of the northwest Indian Ocean is presented. The model is a nonlinear, reduced gravity model incorporating realistic boundary geometry and is forced by observed winds. Model results for the fall of 1985 are compared with and evaluated against U.S. Navy bathythermograph and NOAA satellite data collected during August–November 1985. An assessment is made of the model's ability to simulate correctly the circulation structure. Ship wind observations are converted to wind stress for model forcing by a procedure developed by Legler and Navon (1988). The model is only moderately successful in reproducing the structure of the large, rather homogeneous pool of water located off the Arabian Peninsula in September. However, the model behaves remarkably well in the dynamically active region around Socotra. Major fronts and eddies frequently observed in the region during the transition period between the southwest and the northeast monsoon appear in the 1985 model results and compare well, both temporally and spatially, with the observational data. Thus given accurate wind information, the model appears highly effective in dynamically active regions and demonstrates potential as a useful prognostic tool for evaluation of the Arabian Sea when real time winds become availabl

ISSN:0148-0227    

DOI:10.1029/JC093iC12p15437

年代:1988

数据来源: WILEY

摘要:

Deep fluctuations in current along the equator in the Central Pacific are dominated by coherent structures which correspond closely to narrow‐band (in wave number) propagating equatorial waves. Currents were measured roughly at 1500 and 3000 m depths at 5 moorings between 144 and 148°W from January 1981 to March 1983 as part of the Pacific Equatorial Ocean Dynamics (PEQUOD) program. In each frequency band resolved, a single complex empirical orthogonal function accounts for half to three quarters of the observed variance in either zonal or meridional current. Dispersion for equatorial first meridional Rossby and Rossby gravity waves is consistent with the observed vertical‐zonal coherence structure. The observations indicate that energy flux is westward and downward in long first meridional mode Rossby waves at periods 45 days and longer and eastward and downward in short first meridional mode Rossby waves and Rossby‐gravity waves at periods 30 days and shorter. A local minimum in energy flux occurs at periods corresponding to a maximum in upper ocean meridional current energy contributed by tropical instability waves. Total vertical flux (energy transport per unit zonal distance) across the 9‐ to 90‐day period range i

ISSN:0148-0227    

DOI:10.1029/JC093iC12p15455

年代:1988

数据来源: WILEY

摘要:

Surface layer coefficients for wind profiles, wind stress, and heat flux in typical open sea conditions are briefly reviewed. Businger‐Dyer flux‐gradient relationships and a Charnock wind stress formula fit the empirical data and are dimensionally consistent. These have been solved by an iterative method, and the results are presented in a tabular form suitable for climatological calculations from marine wind and temperature d

ISSN:0148-0227    

DOI:10.1029/JC093iC12p15467

年代:1988

数据来源: WILEY

摘要:

The southern section of the Agulhas western boundary current system exhibits unique characteristics as regards ocean/atmosphere heat flux processes. The Agulhas Retroflection region's high heat flux core from 37°S to 41°S, 16°E to 22°E does not demonstrate a distinct annual cycle of turbulent heat fluxes (latent and sensible) as is characteristic of its northern hemisphere counterparts. Rather, a weak semiannual heat flux cycle is found with maximum average losses during winter and summer (200 and 211 W/m2) and minimum losses during spring and autumn (185 and 162 W/m2). Upstream where the Agulhas Current is closer to land, winter heat losses exceed those of summer, but the differences are small. This behavior contrasts with that encountered at the poleward ends of northern hemisphere western boundary currents where winter heat fluxes are several times those of summer. The main reason for this difference is persistent westerly and southwesterly wind flow over the Agulhas Retroflection region throughout the year which ensures that cold, unsaturated maritime air repeatedly forces loss of heat from the ocean's surface. Spatial heat flux gradients associated with the Agulhas‐Subtropical Convergence surface temperature front are more pronounced in summer than in winter, indicating that cyclogenesis locally may be less seasonally dependent than in the northern hemisphere situation. Average oceanic cooling rates in the core region of the Retroflection, based on net heat flux calculations and a mixed surface layer of 75 m, range from 1.35°C/month during winter to 0.25°C/month during summer. Interannual variability in ocean/atmosphere heat fluxes within the Agulhas Retroflection region often exceeds the variability illustrated by the annual cycle. West of the Agulhas Retroflection core region, interannual sea surface temperature (SST) anomalies are more influential in the generation of heat flux anomalies by virtue of their large temporal variability. This high SST variability is primarily attributed to interannual changes in flux of Agulhas Current water into the southeast Atlantic Ocean. Oceanic heat loss within this warm water zone is an important modifying influence to both ocean and atmosphere, thus meriting further r

ISSN:0148-0227    

DOI:10.1029/JC093iC12p15473

年代:1988

数据来源: WILEY