摘要:

We have composited historical hydrographic observations for the world ocean for two pentads, 1955–1959 and 1970–1974, and have objectively analyzed these data for the purpose of studying the temporal variability of temperature and salinity in the North Atlantic Ocean. The subtropical gyre (500–1300 m) of the North Atlantic was colder and fresher during 1970–1974 compared to 1955–1959. The relatively large changes in temperature and salinity on constant‐depth surfaces in the subtropical gyre were due to upward displacements of isopycnal surfaces. At intermediate depths the eastern portion of the subarctic gyre was also colder and fresher during the 1970–1974 pentad. In contrast, the western portion of the subarctic gyre exhibited higher temperatures and salinities during 1970–1974 compared to 1955–1959. Characteristic differences in temperature and salinity for both gyres were 0.5°C and 0.025‰, respectively. Analysis of changes in potential vorticity indicates that in the subtropical gyre potential vorticity increased in all density layers above the 26.5 surface, suggesting that convection in this region may have been shallower in 1970–1974 compared to 1955–1959. Below the 26.5 surface, potential vorticity decreased through the 27.3 surface, indicating that deeper than normal convection may have formed the water masses in these layers. At 250‐m depth, relatively large changes in temperature and salinity occurred along the line of maximum climatological mixed layer depth that extends northeast to southwest acros

ISSN:0148-0227    

DOI:10.1029/JC094iC05p06091

年代:1989

数据来源: WILEY

摘要:

The geostrophic adjustment of a stratified coastal current in the presence of a submarine canyon is considered with a mathematical model in which the vertical structure of the fluid is handled with a “level” technique that represents vertical gradients by finite differences. Two situations are investigated in detail: a two‐level system where one level represents the shelf and one the canyon, and a three‐level system with two levels over the shelf and one in the canyon. There are four important length scales in the adjustment process: the initial width of the coastal current, the width of the canyon, and the internal and external radii of deformation. For each vertical mode, the shorter of the radius of deformation for that mode and the width of the coastal current determines the distance over which the perturbing influence of the canyon decays. For typical shelf situations, the external mode decays with the width of the coastal current, while the internal modes decay with the internal radius of deformation. The width of the canyon determines the strength of the cross‐canyon flow and thus, the strength of the canyon's effect on the overlying coastal current, with the interaction becoming smaller as the canyon width becomes smaller than the current width or the radius of deformation. As the cross‐canyon flow becomes small, the importance of the geostrophic balance decreases and the internal density gradients become more important in balancing pressure gradients. Therefore even in the case of flow over a narrow canyon, the isopycnals at the top of the canyon are distorted and there will be some residual circulation on the shelf that is forced by the presence of

ISSN:0148-0227    

DOI:10.1029/JC094iC05p06133

年代:1989

数据来源: WILEY

摘要:

This study examines oceanic surface heat fluxes in the Norwegian, Greenland, and Barents seas using the gridded Navy Fleet Numerical Oceanography Central surface analysis and the First GARP Global Experiment (FGGE) IIc cloudiness data bases. Monthly and annual means of net and turbulent heat fluxes are computed for the FGGE year 1979. The FGGE IIb data base consisting of individual observations provides particularly good data coverage in this region for a comparison with the gridded Navy winds and air temperatures. The standard errors of estimate between the Navy and FGGE IIb winds and air temperatures are 3.6 m/s and 2.5°C, respectively. The computations for the latent and sensible heat fluxes are based on bulk formulas with the same constant heat exchange coefficient of 1.5×10−3. The results show extremely strong wintertime heat fluxes in the northern Greenland Sea and especially in the Barents Sea in contrast to previous studies. The highest monthly average net fluxes are well over 400 W m−2in these two regions in January and February 1979. The estimated uncertainty for the higher heat fluxes based on the comparison between the FGGE IIb and the Navy data sets is about

ISSN:0148-0227    

DOI:10.1029/JC094iC05p06145

年代:1989

数据来源: WILEY

摘要:

The Azores Current, south of the Azores Archipelago, is part of the subtropical North Atlantic gyre. Using an international hydrographic data set, we analyze mean and seasonal geostrophic transport fields in the upper 800 m of the ocean in order to determine the origin of the Azores Current in the western basin and seasonal changes in the related flow. Geostrophic currents are obtained by using the method applied by Stramma (1984) in the eastern basin. The Azores Current is found to originate in the area of the Southwest Newfoundland Rise (Figure 10). In winter an almost uniform current connects this region of origin with the Azores Current, while a branching into two current bands is observed in summer, with the southern band forming a marked cyclonic loop. Within the upper 800 m, all of the transport in the northern band and about 70% of the transport in the southern band recirculates in the eastern basin. Additionally, expendable bathythermograph data from the Azores Current region indicate an increase of eddy potential energy from winter to summer.

ISSN:0148-0227    

DOI:10.1029/JC094iC05p06159

年代:1989

数据来源: WILEY

摘要:

The geographical and temporal distributions of worldwide ship drift velocities were plotted in order to see where and when the observations were made and to identify what appear to be major gaps in the data. Curiously, large areas of the North Atlantic, North Pacific, and South Pacific were found to be devoid of observations in some months during the years 1920–1934 when the number of yearly observations is large. An estimated 700,000 observations are missing. These would significantly enhance the usefulness of the data set if they could be found and added to it, especially in the Pacific where the data density is lo

ISSN:0148-0227    

DOI:10.1029/JC094iC05p06169

年代:1989

数据来源: WILEY

摘要:

A numerical model can be fit to data by minimizing a positive quadratic function of the differences between the data and their model counterparts. The rate at which algorithms for computing the best fit to data converge depends on the size of the condition number and the distribution of eigenvalues of the Hessian matrix, which contains the second derivatives of this quadratic function. The inverse of the Hessian can be identified as the covariance matrix that establishes the accuracy to which the model state is determined by the data; the reciprocals of the Hessian's eigenvalues represent the variances of linear combinations of variables determined by its eigenvectors. The aspect of the model state that are most difficult to compute are those about which the data provide the least information. A unified formalism is presented in which the model may be treated as providing either strong or weak constraints, and methods for computing and inverting the Hessian matrix are discussed. Examples are given of the uncertainties resulting from fitting an oceanographic model to several different sets of hypothetical data.

ISSN:0148-0227    

DOI:10.1029/JC094iC05p06177

年代:1989

数据来源: WILEY

摘要:

Various aspects of near‐bottom ocean variability, including the alongshore temperature and current, the cross‐slope temperature, the onshore‐offshore flow, and the area‐averaged relative vorticity on the western slope of the Florida Strait near 27°N have been measured with a three‐point acoustic reciprocal transmission. Three pairs of reciprocal travel time data were extracted from acoustic pulse responses of a 1983 (August) triangular transmission experiment. Temperatures and currents were estimated from the sum and difference, respectively, of reciprocal travel times, and an area‐averaged vorticity was estimated from circulation following Stokes' theorem. The estimations agree well with the concurrent direct measurements at two nearby moorings. Low‐frequency fluctuations of temperatures, currents, and vorticity show a strong weekly period, clearly and consistently describing a near‐bottom onshore‐offshore water movement on the western slope in association with the Gu

ISSN:0148-0227    

DOI:10.1029/JC094iC05p06197

年代:1989

数据来源: WILEY

摘要:

A turbulent entrainment model of a hydrothermal plume was modified to include the effects of vertical structure in the background temperature and salinity fields. Owing to the stably stratified salinity field in the Pacific, relatively warm, salty water spreads at an equilibrium level, where the plume density equals the ambient density. In contrast, the unstable salinity profile in the Atlantic gives the plume added buoyancy, and it continues to rise to an equilibrium level where it is relatively cool and fresh. These attributes result from the mixing between the plume and the surrounding seawater at each level below the spreading layer. The equilibrium temperature anomaly is not directly proportional to the source strength but depends on the mixing and background salinity gradient. The net entrainment of surrounding seawater is greater in the Atlantic plume model than in the Pacific plume model for the same buoyancy flux. Recent data from the TAG hydrothermal field at 26°N on the Mid‐Atlantic Ridge and data from a vent field on the Endeavor segment of the Juan de Fuca Ridge are compared to model results. The model predicts the penetration height and θ‐Scharacteristics of the Atlantic plume, and it explains some of the differences between the observed Atlantic and Pacific hydrothermal p

ISSN:0148-0227    

DOI:10.1029/JC094iC05p06213

年代:1989

数据来源: WILEY

摘要:

A synoptic analysis of the dynamic topography of the sea surface in a (531 km)2domain including the sites of the Polymode Synoptic Dynamics Experiment and Local Dynamics Experiment is presented. This study is based on all available GEOS 3 and Seasat altimeter passes between May and August 1978 referenced to an altimetric mean sea surface. The altimeter synoptic time series are compared to in situ estimates calculated from Polymode expendable bathythermographs, conductivity‐temperature‐depth probes, deep floats, and current meters. Wherever the altimeter coverage is good, both analyses show remarkable agreement. The following results are obtained: (1) the altimeter/in situ correlation in 0.56 on average and never falls below 0.38, (2) the beginning of operations of Seasat in the first days of July 1978 has a dramatic effect on all indicators, (3) the variance for the altimeter estimate is no more than 9% higher on average than the variance of the in situ estimate, and (4) the daily mean sea level is measured to within 2.4 cm on average. A first guess of the four‐dimensional stream function was calculated using the altimeter estimate at the surface and an empirical extension scheme. The resulting field was specified as initial and boundary values of the Harvard quasi‐geostrophic model. It is shown that the assimilation makes the altimeter estimate converge toward the in situ estimate especially at the subthermocline levels where spatial correlations of 0.60 are achieved after 8 days. A simultaneous assimilation of the altimeter stream function and 1400‐m Polymode current meter stream function was also performed, with even bette

ISSN:0148-0227    

DOI:10.1029/JC094iC05p06221

年代:1989

数据来源: WILEY

摘要:

Mesoscale mapping of the ocean sound speed field in a 1000×1000 km area by means of ocean acoustic tomography is greatly enhanced by augmenting a few acoustic moorings with a movable ship‐based receiver. Computer simulations based on realistic noise levels in the measured acoustic travel times give 5% (1%) residual variance in ΔC(x;y,z) for four (six) acoustic source moorings in an ocean perturbed in the gravest baroclinic mode. For comparison, objective mapping based on traditional vertical profiles requires 3 times the steaming distance to yield equivalent residual error. Detailed results depend on many parameters: the assumed mesoscale spectrum and vertical mode structure, the number of observed multipaths, the mooring configuration, the number of ship stations and the travel time signal level (due to mesoscale eddies) and noise level (due to internal waves and position‐keeping errors). These parameters have critical values, below which there is distinct deterioration and beyond which there is little gain. We believe that the critical values can be attained in practice so the ultimate limit on mesoscale mapping is imposed by the internal wave‐induced travel time error. This assumes that position keeping of the submerged acoustic sources and receiver by a combination of satellite navigation and high‐frequency acoustics can be achieved with ±10‐m accuracy. The present study assumes a stationary ocean; a second paper will deal with reciprocal transmissions yielding currents and hence the barotropic mode. This is required in a dynamic ocean model for estimating ΔC(x,y,z;t). All this is preparatory to a tomography experiment in the Greenland Sea

ISSN:0148-0227    

DOI:10.1029/JC094iC05p06232

年代:1989

数据来源: WILEY