摘要:

Estimates of the near‐surface Upwelling in the tropical Atlantic ocean (15°S to 15°N), made by Broecker et al. (1978) from a radiocarbon box model and by Wunsch (1984) from a quasi‐geostrophic inversion, differ considerably (in the former case the range was estimated as about 17–20 Sv, while the latter obtained an absolute maximum of about 10 Sv). To understand these two apparently contradictory results, Δ14C constraints were added to the inverse model with realistic estimates of the observational errors in all fields (including the radiocarbon). It is found that the radiocarbon constraints are consistent with the quasi‐geostrophic model and with an upwelling estimated as 7–10 Sv. The near‐surface circulation in the tropical Atlantic is dominated by a northward flux of warm water across the equator necessary to feed the heat losses at high northern latitudes. The Δ14C balance is maintained by the advection of southern hemisphere waters of comparatively low radiocarbon concentration into the tropical zone by upwelling from below of very low radiocarbon water and by its removal to the north, also by advection. The major uncertainty in the radiocarbon calculations derives from the sparcity of sampling so that very great extrapolations are required to estimate the zonal mean Δ14C concentrations and the time histories between the prebomb

ISSN:0148-0227    

DOI:10.1029/JC089iC05p07971

年代:1984

数据来源: WILEY

摘要:

An objective method of estimating regional averages of coherent sea level (SL) change is developed. The technique is applied to a large set of SL data representative of most of the world's continental margins. The results show highly coherent SL changes over many of the regions studied. The method is then applied to the regional averages themselves to develop an overall estimate of the coherent pattern of SL variations existing in the historical SL data set. The pattern is characterized by a coherent rise of SL in all regions except Alaska, Scandinavia (both areas of notorious crustal uplift), and Southeast Asia, where SL appears to be falling. The analysis suggests little or no change in SL prior to the early 1900's. The period since that time has seen an increase in SL that is optimally fit by a linear trend of 23 cm/century. The study results suggest that it is not possible to uniquely determine either a global rate of change of SL or even the average rate of change associated with the existing (inadequate) data set. Indeed, different analysis methods, by themselves, can cause 50% variations in the estimates of SL trend in the existing data set. A signal/noise analysis suggests it should be easy to detect small, future changes in the SL trends estimated for the period 1930–1980. However, detection of theoretically predicted low‐frequency signals (e.g., caused by CO2warming) will be difficult in view of the huge, low‐frequency, natural variability associated with glacial/tectonic proc

ISSN:0148-0227    

DOI:10.1029/JC089iC05p07980

年代:1984

数据来源: WILEY

摘要:

Oscillatory turbulent flow produced near a rough seabed by linear surface waves is analyzed. An analogy to the wall region of steady turbulent flow is shown to be valid very near the bed and provides the basis for a model in which the eddy viscosity is the product of a vertical length scale and the first two Fourier components of a shear velocity based on the instantaneous, local bed shear stress. With this time‐varying eddy viscosity model, an approximate closed‐form solution for the boundary layer flow is obtained. Comparison with available laboratory measurements indicates that the eddy viscosity does in fact vary with time and that the theoretical approach taken here is physically sound. The effect of a time‐varying viscosity on quantities of practical interest is found to be potentially impo

ISSN:0148-0227    

DOI:10.1029/JC089iC05p07989

年代:1984

数据来源: WILEY

摘要:

The solution for the turbulent near‐bottom boundary layer produced by a progressive wave train is advanced to second order in wave steepness. As in the first‐order analysis (part 1) the effective viscosity is assumed to be the product of a vertical length scale and the first few Fourier components of a shear velocity based on the instantaneous, local bed shear stress. An analytical solution for the second‐order flow field is obtained, with attention directed primarily toward the second‐order, wave‐induced steady current or mass transport. The mass transport is found to depend critically on temporal variation of the effective viscosity. The most dramatic result of the analysis is a predicted reversal of the mass transport produced by relatively long waves. This result, for which supporting experimental evidence is presented, has not been predicted previously and cannot be obtained by a time‐invariant eddy viscosity model. Implications of the present results for related problems ar

ISSN:0148-0227    

DOI:10.1029/JC089iC05p07999

年代:1984

数据来源: WILEY

摘要:

A two‐layer model with a rigid lid and a flat bottom on anfplane is employed to study a flow field driven by a density flux through the sea surface or the coast; for example, freshwater discharged from a river. The negative (positive) density flux into the upper layer is represented by the water beneath (over) the interface changing from the lower (upper) layer to the upper (lower) layer, that is, entrainment of the lower (upper) layer by the upper (lower) layer. The subinertial flow pattern constrained by the coast has two components: one is a baroclinic eddy nearby that matches the entrainment region, and the other is a forced, internal Kelvin wave along the appropriate half of the coas

ISSN:0148-0227    

DOI:10.1029/JC089iC05p08008

年代:1984

数据来源: WILEY

摘要:

Heatingqat the surface of a turbulent and initially unstratified ocean may form a thermocline if strong enough. It is proposed here that the heating is strong enough if turbulence occurs infrequently enough past some depth not to transport heat downward. Turbulence is taken not to occur if the gradient Richardson number Ri = N2/(uz2+ Vz2) rises above a critical value of 0.25. The probability that this occurs becomes very small when and where the population Richardson number is greater than about 1.33. This traps the heat above that layer, increasingand forming a thermocline. Let stirring actionKbe generated at a depthDbelow heating, and scale the heating asB=gαq/ρcp. Then the population Richardson number criterion translates: ifH=BD4/K3

ISSN:0148-0227    

DOI:10.1029/JC089iC05p08017

年代:1984

数据来源: WILEY

摘要:

From July 1980 to February 1981, ten current meters on three moorings were deployed in the 200‐ to 700‐m‐depth range over the continental slope in the northwestern Gulf of Mexico. The currents were characterized by energetic fluctuations with time scales of a week to several months. Westward drifting Loop Current anticyclones provided the principal driving mechanism for the fluctuations. Longshore current speeds at the 200‐m depth occasionally exceeded 70 cm/s and were persistently>50 cm/s during a 2‐month period in the fall. Except during a hurricane, the currents were only marginally coupled with the winds measured at Brownsville, Texas. Tidal motions accounted for<1% of the current

ISSN:0148-0227    

DOI:10.1029/JC089iC05p08022

年代:1984

数据来源: WILEY

摘要:

Tide gauge data from Key West to Norfolk were used to identify a monthly signal in sea level that is uncorrelated with local shelf‐trapped processes. Time series (1955–1975) of local winds, sea level slope, and river runoff were used in a regression model of sea level to separate a local response and a residual signal. The monthly means of the residual contribution were investigated for their relationship to the seasonal fluctuations of the Gulf Stream. In the Florida Channel, lower sea level is found to correspond to increased flow of the Florida Current. During July and August a marked fall in residual sea level, unrelated to the coastal winds, is found from Key West to Charleston, suggesting that the transport of the Gulf Stream increases both in and north of the Florida Channel during this time. Measured long‐term monthly surface currents at Diamond Shoals, Cape Hatteras, which markedly increase in summer to high velocities, tend to substantiate this claim. An additional wintertime low in residual sea level occurs north of the Florida Channel. The wintertime low does not result from steric heating within the upper 100–150 m of water, nor does it, in contrast to the summertime low, appear to coincide with increased northward surface flow. Monthly mean Sverdrup transport was computed across the North Atlantic between 15°N and 35°N. When compared at the same latitude, residual sea level and Sverdrup transport, both interpreted as indices of Gulf Stream transport, generally disagree in phase during summer. However, north of the Florida Channel they are consistent during winter, assuming that lower sea level at this time reflects increased flow in t

ISSN:0148-0227    

DOI:10.1029/JC089iC05p08033

年代:1984

数据来源: WILEY

摘要:

Wind, waves, currents, and water levels were measured near Lake Erie's southeastern shore during fall 1981. Data were recorded continuously at a wave research tower 6 km offshore at three Waverider buoys deployed around the tower and at three current meter moorings along a transect between the tower and the shore. During four storm episodes, wave and current data were also recorded from a dense array of instruments along a transect across the surf zone. A statistical analysis of the deep water data was performed, and histograms of meteorological parameters, wave parameters, and water level slope were plotted as a function of wind direction. The atmospheric boundary layer was usually unstable; on the average, the water was 2–3°C warmer than the air. This was true for all wind directions. At the tower the highest waves were associated with the greatest fetch distances to the WSW and WNW directions. The longest period waves were also associated with these wind directions. The mean water level slope was opposite the mean wind direction. This discrepancy is most likely due to small leveling errors in the water level gages. The variation of currents with wind direction was also examined. The currents are strongest when the winds are from the west, the quadrant with the strongest wind speeds. However, there are strong currents (5–10 cm/s) for all wind directions. The deep water currents are basically shore parallel, with the onshore/offshore component averaging 39% of the root mean square longshore component. Analysis of the four storm episodes reveals the influence of the offshore currents on the flow regime within the surf zone. Quantitative estimates of the longshore momentum balance outside the surf zone show wind stress and pressure gradient to be the dominant dynamic terms. Inside the surf zone, the radiation stress, wind stress, and bottom stress terms all have comparable effects and are usually larger than the acceleration and pressure gradient terms. In between, there is evidence of longshore flow reversals in the breaker zone and the effect of seiche‐induced offshore currents on the near‐shore flo

ISSN:0148-0227    

DOI:10.1029/JC089iC05p08043

年代:1984

数据来源: WILEY

摘要:

An analysis has been made to aid our understanding of various phenomena associated with turbulent plane jets that issue from river outlets and discharge into quiescent bays. An integrated form of the hydrodynamic equations of flow continuity and momentum balance, coupled with advective‐diffusion mass transport equations, has been formulated into a two‐dimensional spatial domain. Closed‐form analytical solutions are obtained with the aid of similarity functions for the velocity and sediment concentration profiles. A numerical procedure for estimating the areal and volume deposition of the delta sediments is presented. The Atchafalaya River Delta, a newly developing delta in the south central Louisiana Coast, is used to illustrate analytical techniques. The results of analytical predictions are compared with the results predicted by other statistical and generic approaches; an agreement is found. A sensitivity analysis is performed to assess the relative importance of various parameters in the river‐bay‐del

ISSN:0148-0227    

DOI:10.1029/JC089iC05p08054

年代:1984

数据来源: WILEY