摘要:

The effects of eddies on the mixed layer and on the transfer of properties between the mixed layer and thermocline are studied in a high‐resolution model of an unstable front. A spring scenario is considered, with a mixed layer maintained against surface warming by wind mixing. Tilting of the buoyancy gradients by ageostrophic flow is found to shallow the mixed layer over the front, giving a mixed layer as shallow as 10 m in some places, compared with the equilibrium Monin‐Obukhov depth of ∼30 m away from the front. Variations of mixed layer depth with time and lateral advection lead to a subduction field that is substantially different from the vertical velocity field. Vertical velocities associated with the growing instabilities push tongues of a ventilation tracer, initially only present within the surface layer, down to depths of up to 100 m. After 24 days a quarter of this ventilation tracer has been deposited in the thermocline. Conversely, the vertical motions pull nutrients from the thermocline up into the mixed layer, giving enhanced surface values along the front. In the basin average the geostrophic eddies give vertical eddy diffusivities for a ventilation tracer of O(3 ‐ 10×10

ISSN:0148-0227    

DOI:10.1029/2000JC900018

年代:2000

数据来源: WILEY

摘要:

A new method is presented for combining historical hydrography with the acoustic travel time measurements from an inverted echo sounder (IES) to obtain time series of full water column profiles of temperatureTand specific volume anomaly. Hydrography is used to demonstrate that profiles constructed from the IES measurement account for more than 90% of theTand δ variance through the main thermocline range. Horizontal gradients between neighboring pairs of instruments, when combined with measurements from bottom pressure sensors and deep current meters, provide accurate estimates of absolute geostrophic velocity profiles. A section of IESs and current meters across the North Atlantic Current (NAC) along World Ocean Circulation Experiment line ACM6 near 42.5°N is analyzed using these methods, and the Eulerian mean temperature and absolute velocity sections are calculated. The mean transport for the combined NAC and northward flow within the inshore portion of the Mann Eddy is 146±13 (× 106m3s−1). The temporal standard deviation of the northward transport is 41×106m3s−1, arising not only from changes in the transport of the NAC but also from lateral shifts of the currents and eddies captured by the fixed span of moorings. Assumption of a level of no motion at the bottom and at 2000 dbar underestimates the true absolute transport by 33 or 60%, respectively. Historical measurements indicate that 50–60×106m3s−1circulates within the Mann Eddy adjacent to the North Atlantic Current, thus 86–96×106m3s−1appears to be throughput at the ACM6 line. Because the throughput observed on the ACM6 line considerably exceeds the combined historical estimates of northward and eastward outflow from the Newfoundland Basin of ≈ 30×106m3s−1, these measurements suggest a substantial (≈ 50 × 106m3s−1) recirculation or southward flow within the Newfoundland Basin in addition to the historical recirc

ISSN:0148-0227    

DOI:10.1029/2000JC900097

年代:2000

数据来源: WILEY

摘要:

Satellite‐tracked drifting buoys, deployed in the Canary Basin as part of the Subduction Experiment between July 1991 and October 1993 and the French Semaphore Experiment during October 1993, were used to obtain a description of surface currents and temperature in the Canary Basin. The study focuses on surface water convergence, eddy energy production, and heat transport. The Azores Current associated with the subtropical convergence zone is clearly visible at 34°N, and bifurcates around 22°W, with the major branch of the current circling the Madeira plateau and joining the Canary Current along the continental slope. Eddy kinetic energy maxima are found along the Azores Current. The mean current revealed a region of maximum convergence north of the Azores Current around longitude 29°W occurring with a negative heating anomaly and positive work done by the Reynolds stress. The southward meridional temperature fluxes in the Ekman layer (0–50 m) between 37°W and the African and European coast are estimated between −0.076±0.022×l015W, produced by mean southward volume transport in our study area. The residual between local surface heat fluxes and horizontal convergence of heat implies a vertical heat convergence process associated with mesoscale temperature and

ISSN:0148-0227    

DOI:10.1029/2000JC900096

年代:2000

数据来源: WILEY

摘要:

All Agulhas rings that were spawned at the Agulhas retroflection between 1993 and 1996 (a total of 21 rings) have been monitored using TOPEX/Poseidon satellite altimetry and followed as they moved through the southeastern Atlantic Ocean, decayed, interacted with bottom topography and each other, or dissipated completely. Rings preferentially crossed the Walvis Ridge at its deepest parts. After having crossed this ridge they have lower translational speeds, and their decay rate decreases markedly. Half the decay of long‐lived rings takes place in the first 5 months of their lifetimes. In addition to the strong decay of rings in the Cape Basin, about one third of the observed rings do not seem to leave this region at all but totally disintegrate here. The interaction of rings with bottom topography, in particular with the Vema Seamount, is shown frequently to cause splitting of rings. This will enhance mixing of the rings' Indian Ocean water into that of the southern Atlantic. This localized mixing may well provide a considerable source of warm and salty Indian Ocean water into the Atlantic overturning circulatio

ISSN:0148-0227    

DOI:10.1029/1999JC000046

年代:2000

数据来源: WILEY

摘要:

This paper examines the characteristics of planetary wave signatures that have been found in the Along Track Scanning Radiometer averaged sea surface temperature (ASST) record for 1991–1996. Longitude‐time plots for every latitude between 5° and 50°, north and south, reveal westward propagating wave‐like patterns at many locations, whose speed decreases with latitude like baroclinic Rossby waves. A two‐dimensional Radon transform method is used to measure the wave speed and its variation with location and time, which broadly matches the Rossby wave speeds predicted by the most recent theory and those measured by TOPEX altimetry, although there are some discrepancies. At low latitudes the thermally detected speeds are slower than expected, a possible consequence of sampling limitations. Wave signatures are clearest between 25° and 40°S, where the meridional temperature gradient is strongest. Here observed speeds are 20–30% greater than theoretical predictions. Planetary wave speed varies considerably with longitude. In general, it increases toward the west of ocean basins, and distinct differences between ocean basins are evident. The propagation characteristics of the waves appear to change abruptly at locations consistent with latitudinal variations in seafloor bathymetry, particularly midocean ridges. In addition, eastward propagating signatures are found in the Southern Ocean. The results demonstrate the value of the ASST data set as a tool for studying basin‐scale wave processes as a complement to the use of altimetry. By observing the thermal signature of Rossby waves the method has the potential to clarify their influence on air‐sea interaction processes and to contribute to climate

ISSN:0148-0227    

DOI:10.1029/2000JC900067

年代:2000

数据来源: WILEY

摘要:

The influences of the Kuroshio Current (KUC), Taiwan Warm Current (TWC), and surface wind stress on the Yellow and East China Seas (YES) are examined using tracers in a Princeton Ocean Model. Two experiments are performed: one with wind stress and one without. Seasonal variations in the inflow and outflow of the TWC and Tsushima Current are specified to examine the effects of the current transports. Two separate tracers are inserted into each model experiment to track the pattern of KUC and TWC waters into the YES. The two main areas of KUC and TWC water movement into the Yellow Sea are the southern entrance to the Yellow Sea trough and the Yangtze Relict River valley. Results indicate that KUC and TWC waters advect into the Yellow Sea regardless of wind stress. However, in the Yellow Sea during winter the wind stress increases KUC and TWC concentration at 20 m. The wind stress also produces short time period events that horizontally advect water masses and spatially smooth seasonally averaged water mass concentrations. The bottom Ekman layer appears to be one of the mechanisms driving the northward bottom flow across the East China Sea shelf. The bottom friction layer is stronger in summer when the TWC velocity is high. The bottom friction layer draws KUC water across the bottom of the continental shelf into the Yangtze Relict River valley and generates upwelling along the Chinese coast.

ISSN:0148-0227    

DOI:10.1029/2000JC900093

年代:2000

数据来源: WILEY

摘要:

The spatial distribution of the energy flux, energy dissipation, current magnitude, and surface elevation amplitude of the major tidal constituents over the northwest European shelf are examined in detail. These distributions are obtained from the harmonic analysis of a 6 month three‐dimensional simulation of 28 tidal constituents. The model currents are validated by a comparison of computed tidal harmonics against those derived from a harmonic analysis of up to 278 current time series. A similar comparison is performed for tidal elevations based on 257 tide gauges. Calculations show that for the most significant tidal constituents (e.g.,M2,S2andK2) there is a major energy flux in the deep water along the shelf edge off the northwest of Scotland, with some energy leaking onto the shelf and into the North Sea. A second source of energy is across the shelf edge at the southern end of the Celtic Sea, with this energy flux propagating into the Irish Sea and southern North Sea, where the majority of the energy is dissipation in the shallow regions. Significantly different distributions are found for the diurnal and shallow water constituents, and the spatial distributions of energy flux and dissipation of the various constituents are considered. The accuracy of separating tidal current harmonics using model data of less than a synodic period is examined with reference to theS2andK2tide. Calculations suggest that the accuracy of computed currents comparable to those obtained from observations can be obtained from an analysis of a 60 day period compared with the synodic period forS2andK2of 182 days, a significant saving in computer tim

ISSN:0148-0227    

DOI:10.1029/2000JC900078

年代:2000

数据来源: WILEY

摘要:

A recent numerical investigation of “self‐organization” [Werner and Fink, 1993] suggests that the feedback process between currents and sediment response can result in “self‐organized” patterns and can be used to predict beach cusp formation and spacing. A similar model based on self‐organization is tested here in order to understand the processes occurring during beach cusp formation and development, to evaluate the sensitivity toward the parameters used, and to examine how the model might relate to field observations. Results obtained confirm the validity of the self‐organization approach and its capacity to predict beach cusp spacing, with values in fair agreement with the available field measurements, with most of the input parameters primarily affecting the rate of the process rather than the final spacing. However, changes in the random seed and runs for large numbers of swash cycles reveal a dynamical system with significant unpredictable behavior. Cusp spacing tends to change with time, and cusp regularity shows large long‐term variations. Cusps are found to be accretionary in the swash zone, and in agreement with most observations, mean flows are horn divergent over developed topography. Simulations over nonplanar slopes characterized by the presence of preexisting nonrhythmic or cuspate features have been performed. Results indicate that preexisting large‐amplitude cusps are destroyed if their spacing is substantially different from that expected under self‐organization and that the final spacing is consistent with that predicted by the model for an equivalent plane beach. These findings support the hypothesis that self‐organization is a robust mechanism f

ISSN:0148-0227    

DOI:10.1029/2000JC900095

年代:2000

数据来源: WILEY

摘要:

Seasonal mean circulation in the Santa Barbara Channel (SBC) consists of poleward flow on the northern shelf and equatorward flow on the southern shelf. While the equatorward flow is in the direction of the persistent, upwelling‐favorable wind, the inshore poleward flow is against the wind. The channel circulation also has a strong seasonal signal that the cyclonic circulation strengthens considerably in spring and summer. A three‐dimensional model is used to simulate the observed currents over an 8‐month period from January to August 1994. The model assimilates the moored temperature and salinity data obtained from a comprehensive circulation study in the SBC‐Santa Maria Basin (SMB). The model also incorporates surface wind forcing; the mesoscale wind stress pattern is derived from a dense network of weather buoys. The model simulation reproduces well the surface mean currents and their seasonal (monthly) variability. The transient (less than seasonal) model velocity response consists of two fundamental modes, a wind‐forced mode and an alongshore pressure gradient‐forced mode. The wind‐driven circulation is concentrated west of the channel in the SMB. The alongshore pressure gradient‐forced motion is concentrated in the channel along the northern coast. Analysis of the evolution of inshore poleward current suggests that the spring transition in this region may coincide with the end of the wint

ISSN:0148-0227    

DOI:10.1029/1999JC000159

年代:2000

数据来源: WILEY

摘要:

A new remote sensing technique based on video image processing has been developed for the estimation of nearshore bathymetry. The shoreward propagation of waves is measured using pixel intensity time series collected at a cross‐shore array of locations using remotely operated video cameras. The incident band is identified, and the cross‐spectral matrix is calculated for this band. The cross‐shore component of wavenumber is found as the gradient in phase of the first complex empirical orthogonal function of this matrix. Water depth is then inferred from linear wave theory's dispersion relationship. Full bathymetry maps may be measured by collecting data in a large array composed of both cross‐shore and longshore lines. Data are collected hourly throughout the day, and a stable, daily estimate of bathymetry is calculated from the median of the hourly estimates. The technique was tested using 30 days of hourly data collected at the SandyDuck experiment in Duck, North Carolina, in October 1997. Errors calculated as the difference between estimated depth and ground truth data show a mean bias of −35 cm (rms error = 91 cm). Expressed as a fraction of the true water depth, the mean percent error was 13% (rms error = 34%). Excluding the region of known wave nonlinearities over the bar crest, the accuracy of the technique improved, and the mean (rms) error was −20 cm (75 cm). Additionally, under low‐amplitude swells (wave heightH≤ 1 m), the performance of the technique across the entire profile improved to 6% (29%) of the true water depth with a mean (rms) error of

ISSN:0148-0227    

DOI:10.1029/1999JC000124

年代:2000

数据来源: WILEY