摘要:

Between September 1991 and March 1994 the oceanic region off the east coast of Australia at 30°S was the subject of an intense observational program Part of the World Ocean Circulation Experiment, the objective was to improve our understanding of the dynamics of the East Australian Current (EAC) and to measure its volume and energy flux. The two main components were the Pacific Current Meter Array 3 (PCM3) with six moorings spanning a total distance of 120 km, and repeat, high‐resolution hydrographic surveys. The long‐term average fromPCM3 shows the EAC as a narrow, swift, and highly variable current centered at 40 km from the coast with strong shear in the upper 1000 m and mean southward velocities of 0.6 m s−1in the core. The measurements also revealed a thick countercurrent underneath the poleward flow. The mean volume transport for the entire section covered by the array (between the coast and 154.4°E) was 22.1±4.6 Sv toward the south with an rms variability of 30 Sv. Combining the data from the PCM3 array with the hydrographic sections generated nine detailed snapshots of the absolute geostrophic current field. The snapshots were used to evaluate the reliability of the PCM3 data for determining volume transports. The rms difference between transports derived from direct current observations and from hydrographie data was 3.8 Sv, indicating that the PCM3 array can be used to describe the current variability despite the sparse spatial distribution of velocity measurements. Transport variability was dominated by periods between 90 and 140 days, which are attributed to eddy shedding in the region. Occasional intense northward flow events were observed where total transports reached up to 50 Sv northward. On those occasions the southward boundary current changed from being surface intensified and mainly baroclinic to northward and more barotropic, related to the northward migration past the array of the EAC coastal separati

ISSN:0148-0227    

DOI:10.1029/1999JC000121

年代:2000

数据来源: WILEY

摘要:

On the basis of hydrographic observations taken in the vicinity of Denmark Strait, a primitive equation model is used to investigate physical mechanisms that control the exchange through the strait. The dense water transport is topographically controlled and predictions byWhitehead[1998] andKillworth and McDonald[1993] are consistent with numerical model results. The distribution of temperature and thickness of the modeled plume is in good agreement with the high‐resolution hydrographic dat

ISSN:0148-0227    

DOI:10.1029/2000JC900111

年代:2000

数据来源: WILEY

摘要:

Aquatic vegetation controls the mean and turbulent flow structure in channels and coastal regions and thus impacts the fate and transport of sediment and contaminants. Experiments in an open‐channel flume with model vegetation were used to better understand how vegetation impacts flow. In particular, this study describes the transition between submerged and emergent regimes based on three aspects of canopy flow: mean momentum, turbulence, and exchange dynamics. The observations suggest that flow within an aquatic canopy may be divided into two regions. In the upper canopy, called the “vertical exchange zone”, vertical turbulent exchange with the overlying water is dynamically significant to the momentum balance and turbulence; and turbulence produced by mean shear at the top of the canopy is important. The lower canopy is called the “longitudinal exchange zone” because it communicates with surrounding water predominantly through longitudinal advection. In this region turbulence is generated locally by the canopy elements, and the momentum budget is a simple balance of vegetative drag and pressure gradient. In emergent canopies, only a longitudinal exchange zone is present. When the canopy becomes submerged, a vertical exchange zone appears at the top of the canopy and deepens into the canopy as the depth of submergence

ISSN:0148-0227    

DOI:10.1029/2000JC900145

年代:2000

数据来源: WILEY

摘要:

Hydrographic and velocity observations of the North Brazil Current (NBC) retroflection region during the 1990–1991 Western Tropical Atlantic Experiment (WESTRAX) are examined with the intent of extracting dynamical information about the NBC eddy shedding. A comparison is performed between the depth structure of empirical orthogonal functions and the dynamical normal modes of the NBC retroflection region on a beta plane centered on 5°N. The barotropic and first two baroclinic modes account for about 75% of the vertical structure of the NBC flow. Thus, a three‐layer model of the NBC region is suitable for dynamical studies of the NBC retroflection. In terms of flow structure the upper layer represents the retroflection of the surface layers of the NBC to feed the North Equatorial Countercurrent. The middle layer represents the separating subthermocline waters of the NBC which feed the North Equatorial Undercurrent. The third layer represents a weak meandering flow that may be thought of as the Deep Western Boundary Current signature in the three‐layer ocean. In terms of PV, a well‐defined front separates the NBC waters from the North Equatorial Current in the upper layer. Both upper and middle layers present closed PV contours associated with the eddy in the process of pinching off from the retroflecting NBC. The NBC separation region, although equatorial, complies reasonably well with the basic quasigeostrophic (QG) assumptions. Therefore QG methods are applied to investigate the NBC meander growth. By isolating the effect of PV anomalies in each of the layers on each of the layers, baroclinic growth is verified to occur during the NBC eddy

ISSN:0148-0227    

DOI:10.1029/2000JC900129

年代:2000

数据来源: WILEY

摘要:

Lowered acoustic Doppler current profiles (LADCP) from an early March 1995 cruise across the Agulhas Current show a swift, narrow undercurrent flowing northeast along the continental slope. Neither this Agulhas Undercurrent nor the adjacent deep extension of the Agulhas Current are evident from measurements of water properties alone, and their absence from the conventional referencing of geostrophic current estimates biases net southward transport estimates high by several sverdrups. Here we refine the original transport calculation by removing barotropic tides and by estimating instrumental and sampling errors. Two additional LADCP sections, from cruises in late March and June 1995, also show the undercurrent and the deep extension of the Agulhas. Differences in the current structure are evident. The Agulhas Current extends throughout the water column in March, but extends only to 2300 m depth in June. Additionally, the current extends further offshore in March. Of the three available LADCP sections, only those from early March and June have sufficient sampling to calculate the net southward transport of the Agulhas Current and Undercurrent. The two estimates, 78±3 and 76±2 Sv, are nearly identical. Consideration of water properties on density surfaces shows that although the undercurrent carries intermediate water with Red Sea Water influence northward, the bulk of this water mass is flowing southward, away from its source, in the Agulhas Curren

ISSN:0148-0227    

DOI:10.1029/1999JC000201

年代:2000

数据来源: WILEY

摘要:

An overview is given of the top‐to‐bottom flow, thermohaline, oxygen, and nutrient structures between the Bering Sea and the Fiji islands, based on high‐resolution field measurements along 179°E. The zonal flow is dominated by a banded mesoscale structure, reflecting the highly sheared structure and instabilities of the major ocean currents crossing the date line. The major midlatitude currents are 100–200 km wide, have maximum speeds of ∼40 cm s−1, and extend to ∼2000 m, when measured by the 5 cm s−1isotach. El Niño conditions in 1993 caused a merger of the North Equatorial Countercurrent with the Equatorial Undercurrent, leading to a 600 km wide band of equatorial eastward flow (40–60 cm s−1) in the upper 300 m. Beneath, a 400 km wide and 1.2 km deep westward jet (10–30 cm s−1) is found. Between 1500 and 3500 m the equatorial flow is predominately to the southeast, and between 3500 and 5000 m it is toward the northwest. Consonant with banded shear flow and flow‐topography interactions, the physical and chemical property distributions along 179°E are complicated, reflecting the effects of differential property advection, mixing, and water mass transformation. Well‐defined property fronts occur in shear zones between currents of different origin. The thermohaline, oxygen, and nutrient tongues associated with North and South Pacific intermediate and deep waters have considerable internal structure, indicative of mesoscale processes and contorted flow paths. The abyssal flow near the date line indicates northward movement of lower circumpolar water to the Hess rise, whence it turns westward and north, west of the Emperor Seamounts; properties east of this seamount chain are very uniform. The deep and abyssal waters of the isolated Aleutian Basin differ substantially from those in the adjacent Pacific, by being warmer (Δ = 0.1°C), less saline (Δ = 0.01), poorer in oxygen (Δ = 50 μmol kg−1), and much richer in silicate (Δ = 80 μmol kg−1). An isolated basin within the Wake‐Necker Ridge also shows some unusual bo

ISSN:0148-0227    

DOI:10.1029/1999JC000030

年代:2000

数据来源: WILEY

摘要:

Results are presented from an eddy‐resolving primitive equation ocean model to test the hypothesis that geostrophic eddies can be parameterized through a variety of downgradient closures. Time‐mean eddy fluxes of temperature, isopycnic thickness, and quasi‐geostrophic potential vorticity are diagnosed on level surfaces from 5 years of model data and are spatially correlated with the corresponding time‐mean gradients. We find no discernible correlation between the absolute eddy fluxes and mean gradients. However, after decomposing the eddy fluxes into “rotational” and “divergent” components we find a positive correlation between the divergent eddy fluxes and the mean gradients, although the overall correlation coefficients remain small, typically O(0.2). The correlation between the eddy temperature fluxes and mean temperature gradients is positive only over the upper 2 km and is negative at depth, suggesting that the abyssal temperature fluxes are upgradient. In contrast, the correlations between eddy fluxes and mean gradients of isopycnic thickness and potential vorticity are positive over the most of the fluid column. The overall correlations are similar for both thickness and potential vorticity closures. We further decompose the divergent component of the eddy fluxes into components directed perpendicular and parallel to mean contours. We find that both the perpendicular and the parallel components are simil

ISSN:0148-0227    

DOI:10.1029/1999JC000041

年代:2000

数据来源: WILEY

摘要:

A high‐resolution, nonlinear, barotropic, finite element tidal model is developed for removing tidal elevations from satellite altimeter observations in the northeast Pacific Ocean. Surface elevations and currents for the constitutentsM2,S2,N2,K2,K1,O1,P1, andQ1are computed using boundary forcing from the Oregon State University global tidal model, TPXO.3, and the effects of the tidal potential, Earth tide, and ocean self‐attraction and loading. Amplitudes and phases computed from the harmonic analyses of altimeter time series at crossover locations along the tracks of the TOPEX/Poseidon satellite are assimilated into the model, and the corrected harmonics are compared with pelagic and coastal tide and pressure gauge measurements, and with the TPXO.3 and FES95.2.1 global models. Diurnal tidal currents along the British Columbia and Alaska continental shelves are shown to be strongly influenced by the presence of continental shelf waves. The surface manifestation of these waves is clearly evident inK1coamplitude plots.K1andM2energy fluxes are calculated along these shelves and the potential generation of internal tides is discussed. Barotropic tidal dissipation is estimated in several key subregions, and theM2total for the Gulf of Alaska is found to be approximately half that of previous estima

ISSN:0148-0227    

DOI:10.1029/1999JC000122

年代:2000

数据来源: WILEY

摘要:

Long time series of sea level from tide gauges along the north side of the Hawaiian Ridge and shorter series of dynamic heights inferred from inverted echo sounders moored just north of the main Hawaiian Islands are examined for evidence of internal tides at theM2frequency. We find that the amplitudes and phases of theM2tidal components have low‐frequency variability, which is consistent with a superposition of an internal tide with the larger barotropic tide. Further, the low‐frequency variability is correlated with low‐frequency changes in the depth of the pycnocline, which suggests a simple physical mechanism to account for the low‐frequency modulations in the internal tidal amplitude. These modulations are coherent for long distances along the Hawaiian Ridge, indicating a coherent generation of the internal tide that is consistent with acoustic observations in the North Pacific and with recent analyses of sea surface heights from satellite al

ISSN:0148-0227    

DOI:10.1029/2000JC900140

年代:2000

数据来源: WILEY

摘要:

Many subsurface passages connecting the deep parts of the oceans have a topography such that the location of the most pronounced horizontal constriction does not coincide with that of the sill. When applying rotating hydraulic theory to describe the deep water flow through channels of this type, a problematic situation emerges since the standard analysis assumes a well‐defined controlling section. The present study demonstrates how the associated critical flow problem arising from the hydraulic model may be resolved on the basis of functional‐theoretical analysis. The methodology is applied to the deep water flow into the Baltic proper through the Bornholm Channel, where multiple critical cross sections were found. These, as well as the associated critical transport predictions, are discussed, and it is found that only one of them serves de facto as a control with unidirectional f

ISSN:0148-0227    

DOI:10.1029/2000JC900136

年代:2000

数据来源: WILEY