摘要:

Current meter data have been analyzed from seven moorings on the continental slope along the central California coast, from Point Piedras Blancas to Point Reyes. The goal was to examine the subtidal variability in the 100 m to 1000 m depth range, particularly with regard to alongshore propagating events and interactions with eddies and meanders of the California Current offshore. The 2‐year time series available off Point Sur were first analyzed in conjunction with the local and remote surface wind stress and coastal synthetic subsurface pressure, and then correlated with shorter coincident current records moored at similar depths to the north and south. The poleward flowing California Undercurrent was the most prominent feature at all the moorings except at one site located well into the Monterey Submarine Canyon. The strongest poleward flows over the slope occurred in 3‐ to 4‐month bursts, not phase locked with the seasons, with vector speeds exceeding 40 cm s−1. South of the canyon, an approximately monthly signal was identified which propagated poleward, upward, and offshore. The behavior of this signal was consistent with that of an internal coastal Kelvin wave generated at the surface by remote wind stress to the south and was likely not of equatorial origin. The wave was apparently scattered by the abrupt topography of the canyon, since its energy persisted to the north of the canyon but with unstable phase. At least three eddy‐meander interaction events were observed. These warm, deep (>1000 m), anticyclonic features reversed the flow over the slope to equatorward when they moved onshore and interrupted the flow of the undercurrent. One event forced anomalously strong (>15 cm s−1) onshore flows off Monterey Bay and offshore flows off Point Sur. While quantitative transport estimates could not be made with this sparse data set, it seems apparent that such events play a significant role in the exchange of water properties between the shelf and the

ISSN:0148-0227    

DOI:10.1029/97JC00436

年代:1997

数据来源: WILEY

摘要:

The data from two current meter moorings off Point Sur, California, were analyzed to examine the energy transfers over the continental slope in the California Current system (CCS). The method used was to calculate terms in the heat equation at intermediate depths between instruments, using the thermal wind relation to estimate the horizontal temperature gradients from the vertical shear. Time series ofu,v, andTwere collected for 17 months (May 1989 to October 1990) at site P2 on the 800‐m isobath and for 12 months (May 1990 to May 1991) at site P3, 25 km farther offshore on the 1800‐m isobath. Instrument depths were 100, 350, and 500 m at both moorings plus 1000 m at site P3. Mean values of the energy conversion terms were computed at sites P2 and P3 at 225 m depth and at site P2 at 425 m depth. The local change of eddy potential energy (EPE) was approximately balanced (∂EPE/∂t≈ 0) for each case but the dynamics were quite different. At P2 225, the sources of EPE were advection by the mean flow and baroclinic instability, which converted mean potential energy (MPE) to EPE. The losses came from eddy advection out of the region and the vertical eddy heat flux (VEHF), which converted EPE to eddy kinetic energy (EKE). At P2 425, baroclinic instability provided the major source of EPE. This, plus a small positive contribution from the VEHF, was balanced by advection of EPE out of the region by both the mean flow and the eddies. At P3 225, mean advection and baroclinic instability were negligible, and the downward eddy heat flux (VEHF) was balanced by advection of heat out of the region by the eddies. Time series of terms in the EPE equation were used to examine high energy transfer events at both moorings. Events at P2 involved both horizontal and vertical processes and had longer timescales (several days to weeks) than those at P3, which had much shorter timescales (2–5 days) and were dominated by vertical processes. Compared to the Gulf Stream, the mean baroclinic instability at P2 (both depths) was smaller by at least a factor of 10. A term‐by‐term comparison between the mean values off Point Sur and the Kuroshio Extension showed different processes, with the eddies gaining strength at the expense of the mean flow off Point Sur and the opposite occurring in the Kur

ISSN:0148-0227    

DOI:10.1029/97JC02329

年代:1997

数据来源: WILEY

摘要:

The seasonal circulation associated with the East Australian Current is examined using a set of steric heights derived from the historical hydrology and expendable bathythermograph data collected in the region. The data are separated into a network of regional bins allowing for known oceanographic and topographic features and a two‐harmonic best fit to the seasonal cycle is obtained in each bin. Maps of the annual‐frequency component of the surface and depth‐integrated steric heights (handP) show the development and progression of the EAC flow regime through a complete seasonal cycle. The EAC has a strong seasonal cycle from 25°S to 45°S, with strongest southward flow in austral summer. The seasonal cycle in surface flow over the continental shelf is documented by two independent methods, geostrophically, using cross shelf sea level gradients derived from coastal tide gauge data and steric heights at the continental shelf edge, and directly from merchant ship observations. The two estimates are in good agreement. The seasonal cycle in the EAC is more pronounced than in other midlatitude western boundary currents for which data are available. At 28°S, the strength of the total Tasman Sea transport (southward flow) varies between a minimum transport of 7 Sv in winter (July) to a maximum of 16 Sv in summer. The semiannual frequency components ofhandPis important near 30°S near the EAC outflow, but not elsewhere. The seasonal cycle of the EAC is not due to strong seasonal variations in Tasman Sea wind stress curl east of the region of interest. Seasonally reversing zonal flows occur offshore north of 25°S, which are apparently locally forced by reversing wind stress curls; but if these flows were fed from the south by the EAC current system, the EAC would have to be weaker in summer, not stronger. The Leeuwin Current Extension along Australia's west and south coasts may pass up the east coast of Australia, providing an important contribution to the enhanced southward flow of the EAC in summer. The vigorous anticyclonic eddies of the EAC also show a marked seasonal cycle, and this is probably an important part of the mechanism for the strong seasonal cycle of the EAC south of 25°S. The location of the strongest anticyclonic eddy in the EAC moves steadily southward throughout the summer season, and the phase of the coastal EAC appears also to move southward, contrary to the expectations of linear theory and to the hypothesis that the Leeuwin Current Extension is the major cause of the sea

ISSN:0148-0227    

DOI:10.1029/97JC00227

年代:1997

数据来源: WILEY

摘要:

Seasonal and interannual variability of the sea surface temperature field in the Adriatic Sea is analyzed from the low‐resolution advanced very high resolution radiometer data. The spatial resolution of 18 km allowed analysis of only basin and subbasin scale features. Average monthly and seasonal sea surface temperature fields for the entire studied period (1984–1992) are discussed. The analysis shows the absence of any permanent sea surface thermal features in the Adriatic Sea. The south Adriatic sea surface temperature minimum presumably associated to the cyclonic gyre, previously considered as one of the permanent features, appears to be recurrent, being prominent only in late autumn and early winter, i.e., in the preconditioning and a deepwater formation phases. The major Ionian water inflow is documented in autumn while the thermal signature of the western surface outflow of Adriatic water appears most prominent in winter. The variability of the basin‐wide thermal pattern in the Adriatic reveals four distinct seasons, which is different from both the eastern and western Mediterranean, where only two major patterns are recognized. A prominent interannual signal occurs in a northward extension of the warm water plume along the eastern coast, which in some years reaches the northernmost corner of the Adriatic, while in other situations it remains trapped in the south Adriatic cyclonic gyre. The surface thermal signature of the south Adriatic gyre also varies on an interannual timescale, and it was weak or completely absent during the period 1984–1986 while it was rather prominent in the period 1987–1992. A constant trend of sea surface temperature decrease in the center of the south Adriatic gyre and in the northernmost corner of the Adriatic was evidenced over the studi

ISSN:0148-0227    

DOI:10.1029/97JC01720

年代:1997

数据来源: WILEY

摘要:

Utilizing recently developed deep‐sea mooring technology, we present the first interannual time series measurements of the ocean temperature, salinity, and velocity fields in the Bering Sea basin. These were made during spring and summer of 1992 and 1993 and late winter to summer of 1994, in 2195 m of water. Results show a weak background flow of ∼5 cm/s with semidiurnal tides superimposed. Anticyclonic (clockwise) eddies pass by sporadically, inducing currents up to ∼80 cm/s and depressing the isopycnals and isotherms up to ∼200 m. The thermal structure in the upper 400 m shows a deep temperature maximum from warm Alaskan Stream inflows and a minimum above due to winter cooling. A sea surface cooling event was observed in the late winter of 1994, with cold water penetrating to ∼120‐m depth. Spring warming begins in late April to early May of each year, marked by the arrival of the 3.75°C isotherm. The local wind plays little role in forcing ocean currents at the site except at the inerti

ISSN:0148-0227    

DOI:10.1029/97JC00881

年代:1997

数据来源: WILEY

摘要:

Phytoplankton is known to be a key element in the production and eventual oceanic efflux of dimethyl sulfide (DMS) to the atmosphere. We hypothesized that the alternation ofPhaeocystis pouchetiiand diatoms, the two major algal components of the spring bloom, would modulate the input of particulate organic sulfur (POS), dimethylsulfoniopropionate (DMSP), and DMS into the mixed layer of the marginal ice zone. A bloom of diatoms is expected to present similar pathways but to have very different rates of POS/DMSP/DMS production and POS/DMSP sinking and no or low DMS flux to the atmosphere as contrasted to the cycling occurring during theP. pouchetiiphase of the bloom. Our initial hypothesis cannot be accepted based on our observations in the Barents Sea during the spring of 1993. The contribution of diatoms to the water column budgets of DMSP and DMS was significant and cannot be overlooked. We suggest that the physiological stage of the bloom is perhaps more important to biogeochemical cycling than its phytoplankton species composition in controlling DMSP and DMS fluxes in Arctic waters. Loss of paniculate DMSP in the mixed layer was mainly by release into the dissolved pool and by sedimentation father than by grazing, except in ice‐free waters. Cycling of DMS in the mixed layer was predominantly biological in ice‐free waters, while in Polar Front waters, ventilation was proportionally more important due to depressed microbiol

ISSN:0148-0227    

DOI:10.1029/96JC03870

年代:1997

数据来源: WILEY

摘要:

The model ofMeylan and Squire[1996], which treats solitary ice floes as floating, flexible circular disks, is incorporated into the equation of transport for the propagation of waves through a scattering medium, assumed to represent open ice pack in a marginal ice zone. The time‐independent form of the equation is then solved for homogeneous ice conditions allowing for dissipation due to scattering, together with extra absorption from interactions between floes, losses in the water column, and losses arising from the inelastic character of the sea ice including local brash. The spatial evolution of wave spectra as they progress through the pack is investigated with the aim of explaining the field data ofWadhams et al. [1986]. Specifically, the change toward directional isotropy experienced by waves as they travel into the ice interior is of interest. In accord with observations, directional spread is found to widen with penetration until eventually becoming isotropic, the process being sensitive to wave period. The effect of absorption on the solution is investigate

ISSN:0148-0227    

DOI:10.1029/97JC01453

年代:1997

数据来源: WILEY

摘要:

We present acoustic Doppler current profiler (ADCP) survey results resolving three‐dimensional current structure in thermocline stratification atop Cobb Seamount (130.8°W, 46.8°N). Mean flow includes clockwise circulation near the bottom and an east‐northeastward background current. Clockwise flow is not strong enough to isolate fluid from the background current as in a Taylor cap unless this occurs in the deepest 50 m, outside ADCP coverage. Diurnal currents (subinertial, 0.69ƒ) are amplified within a few kilometers radially and about 100 m vertically of the seamount by up to 5.3 times ambient K1tidal currents. Their diagnostics include clockwise propagation around the seamount with first azimuthal wavenumber, clockwise rotation in time, narrow current ellipses oriented nearly along bathymetric contours with positively correlated radial and azimuthal components, counterclockwise turning with depth, and downward propagation. They are compared to the stratified seamount‐trapped wave of first azimuthal wavenumber and third‐highest subinertial resonant frequency (0.70ƒ) calculated numerically using measured bathymetric and stratification profiles. Inviscid free wave currents exhibit symmetric patterns not consistent with the measurements; in a forced damped wave maintained by ambient tidal currents against parameterized friction of 2 day timescale, as appropriate to dissipation timescales estimated from microstructure measurements, symmetries are broken such that each observed current diagnostic is matched. When the wave is linearized about a steady, baroclinic, clockwise current representing the measured mean flow, its frequency and structure are very weak

ISSN:0148-0227    

DOI:10.1029/97JC01451

年代:1997

数据来源: WILEY

摘要:

To interpret measurements at Cobb Seamount, the observational signatures in stratified seamount‐trapped waves, with emphasis on forcing and frictional influences, are demonstrated for bathymetry and stratification representing a typical midlatitude seamount. Inviscid waves propagate azimuthally with standing wave radial and vertical structure. This includes current ellipses parallel or perpendicular to isobaths with uncorrelated radial and azimuthal velocities, nearly horizontal depth regions within which currents rotate in time clockwise or counterclockwise, and velocity turning with depth alternately clockwise and counterclockwise in successive quarter periods. Waves are explained conceptually in terms of “stratified slope‐Kelvin waves” (plane, propagating, ƒ plane topographic Rossby waves in stratification over a planar sloping bottom). An inviscid stratified seamount‐trapped wave is a cross‐isobath (radial and vertical) mode, of equal amplitude stratified slope‐Kelvin waves propagating upslope and downslope, that resonates in the azimuthally reentrant waveguide of the sloping seamount sides. Cross‐isobath structure is controlled by (1) ray refraction by slope angle variations, (2) Airy function behavior near turning points where bottom slope decreases to the ratio between wave and buoyancy frequencies, and (3) geometric compression by cylindrical geometry. In a stationary forced damped wave maintained by ambient tidal currents against dissipation concentrated near the summit where currents are strongest, cross‐isobath standing wave structure is broken with rays carrying energy upslope dominating; currents propagate phase downward and outward in addition to azimuthally, have ellipses oriented across isobaths with positively correlated radial and azimuthal velocities, and are dominated by clockwise rotation in time and counterclockwise

ISSN:0148-0227    

DOI:10.1029/97JC01452

年代:1997

数据来源: WILEY

摘要:

Effects of radially and vertically sheared mean azimuthal flow on trapped waves at a seamount are shown to include changes to resonance frequency and spatial structure and the formation of two types of critical surface. In waves calculated using bathymetry, stratification strength, and frequency (0.66ƒ) appropriate to amplified diurnal currents observed at Cobb Seamount (46.8°N), mean flow based on measurements there does not cause critical surfaces. Even though clockwise mean flow vorticity is a significant fraction of ƒ, effects on the wave are limited to weakly reduced spatial scales and an increase in resonance frequency much weaker than estimates of the Doppler shift. The Doppler shift is opposed by variations in mean horizontal shear that cause a downslope potential vorticity gradient opposite that of the bottom slope supporting the wave; mean vertical shear is unimportant. In the general case, mean flow can cause (1) a stratified seamount‐trapped wave critical surface where the intrinsic frequency vanishes, causing both horizontal and vertical scales of the wave to diminish and (2) an internal wave critical surface where the intrinsic frequency rises to the effective Coriolis frequency, or low‐frequency bound for superinertial internal waves. An internal wave critical surface bounds a “superinertial cap” within which subinertial currents are effectively superinertial. Mean flow at Fieberling Guyot (32.4°N) reduces the effective Coriolis frequency by 0.43ƒ and causes a superinertial cap having an internal wave critical surface for diurnal currents (0.93ƒ) that coincides with measured turbulence maxima extending several kilometers away laterally above

ISSN:0148-0227    

DOI:10.1029/97JC01221

年代:1997

数据来源: WILEY