摘要:

A recently obtained series of bottom pressure measurements from various positions in the Drake Passage (DP) has been used to estimate the transport variability of the Antarctic Circumpolar Current (ACC) under the assumption that the variability is predominantly barotropic and thus proportional to the north‐minus‐south pressure difference. The standard deviations in derived transport range from 5.3 Sv in 1993 to 8.9 Sv in 1990 (10‐day filter; 1 Sv = 106m3s−1). All values are less than the 10 Sv (10‐day filter) obtained during the International Southern Ocean Studies (ISOS) program at DP between the mid 1970s and early 1980s [Whitworth and Peterson, 1985]. Although some of this discrepancy is due to differences in procedure in dealing with gaps in the data, our data suggest that the ACC transport can be persistently less variable over several years than was previously thought. In particular, the ISOS data set contains two large‐amplitude changes in pressure difference over timescales less than 2 weeks, while no similar events were observed in the more recent data. Although the more recent pressure gauges were deployed at different positions to those used in ISOS, their reliability as indicators of ACC transport changes has been established by examination of simultaneous pressure measurement

ISSN:0148-0227    

DOI:10.1029/96JC02003

年代:1996

数据来源: WILEY

摘要:

The dynamic response to a westerly wind burst which occurred during the Coupled Ocean Atmosphere Response Experiment in the warm pool of the equatorial Pacific Ocean is described using velocity, hydrography, and microstructure measurements. Turbulent fluxes distributed momentum input from the wind over a near‐surface layer of variable thickness. Coriolis and pressure gradient terms combined to induce a wavelike response whose frequency was close to the local inertial frequency. Wind stress variations on near‐inertial timescales interfered both constructively and destructively with the wave response, exerting considerable influence on the observed curre

ISSN:0148-0227    

DOI:10.1029/96JC02005

年代:1996

数据来源: WILEY

摘要:

A westerly wind burst observed in the warm pool of the western equatorial Pacific Ocean cooled the ocean's surface layer by about 0.8°C. Turbulent entrainment at the base of this layer caused cooling but also heating due to the reversal of the vertical temperature gradient during rain events. Consequently, the cumulative effect of turbulent entrainment was minimized. Following the wind burst, a sustained eastward surface current contributed to high current shear and turbulent dissipation rates at the top of the thermocline. As a result, most of the heat transfer into the thermocline occurred after the wind burst had ended. The cruise‐averaged turbulent flux into the thermocline was 17 ± 10 W m−2, which suggests that the annual mean is only a few watts per square meter. The restratification of the upper ocean in the aftermath of the wind burst is accounted for partly (but not wholly) by local turbulent entrainment. Despite heavy precipitation, upper ocean salinity generally increased during the cruise. Advection appears to have been the dominant factor governing local salinity ch

ISSN:0148-0227    

DOI:10.1029/96JC02006

年代:1996

数据来源: WILEY

摘要:

A new, multibasin reduced model of the global thermohaline circulation has been developed that builds upon the single‐basin Atlantic model recently described inSakai and Peltier[1995]. The model comprises individual, two‐dimensional, Atlantic, Indian, and Pacific components which are linked via a circumpolar basin representative of the Southern Ocean. It also includes a complete seasonal cycle for sea surface temperature, a sea ice component, and an acceptably accurate representation of the influence of both wind stress and bottom topography. The circulation in the individual basins is described using a stretched coordinate system in order to allow the use of reduced vertical resolution where high resolution is unnecessary. Values for the control parameters of the new multibasin model are established in the same manner as was employed in analysis of the single‐basin model, and the basic conclusion of that study, that there exists a natural oscillatory mode of behavior of the deep circulation on a timescale ranging from a century to several millennia, is confirmed. The precise timescale of the internal variability is determined by the detailed nature of the hydrological cycle which is herein constrained far more realistically than was possible for the single basin model. The simulations performed with the model deliver a rather realistic facsimile of the millennium timescale Dansgaard‐Oeschger oscillations [Dansgaard et al., 1984] that were such a prominent feature of North Atlantic glacial climate according to climate proxy data from the Greenland Ice Core Project and Greenland Ice Sheet Project ice cores from Summit, Greenland, when boundary conditions are fixed to those appropriate to full glacial conditions. We also describe a sequence of paleoceanographic experiments that have been designed to explore the sensitivity of the deep circulation to the impact of the specific surface freshwater anomalies that are known to have developed during the last deglaciation event of the current ice age. The simulation of the response of the thermohaline circulation to such anomalies provides strong additional support for the notion that modulation of the strength of the overturning flow in the Atlantic basin played a very important role in the Younger‐Dryas climate

ISSN:0148-0227    

DOI:10.1029/96JC00539

年代:1996

数据来源: WILEY

摘要:

The flow which constitutes the conveyor belt in the Hamburg large‐scale geostrophic ocean general circulation model has been investigated with the help of a particle tracking method. In the region of North Atlantic Deep Water formation a thousand trajectories were calculated backward in time to the point where they upwell from the deep ocean. Both the three‐dimensional velocity field and convective overturning have been used for this calculation. Together, the trajectories form a representative picture of the upper branch of the conveyor belt in the model. In the Atlantic Ocean the path and strength (17 Sv) of the conveyor belt in the model are found to be consistent with observations. All trajectories enter the South Atlantic via Drake Passage, as the model does not simulate any Agulhas leakage. Large changes in water masses occur in the South Atlantic midlatitudes and subtropical North Atlantic. Along its path in the Atlantic the water in the conveyor belt is transformed from Antarctic Intermediate Water to Central North Atlantic Water. On the average the timescale on which the water mass characteristics are approximately conserved is only a few years compared to the timescale of 70 years for the conveyor belt to cross the Atlantic. The ventilation of thermocline waters in the South Atlantic midlatitudes is overestimated in the model due to too much convective deepening of the winter mixed layer. As a result the fraction of the conveyor belt water flowing in the surface layer is also overestimated, along with integrated effects of atmospheric forcing. The abnormally strong water mass transformation in the South Atlantic might be related to the absence of Agulhas leakage in the mo

ISSN:0148-0227    

DOI:10.1029/96JC02162

年代:1996

数据来源: WILEY

摘要:

Coarse resolution general circulation ocean models show a tendency to accumulate an excess of bomb14C on the eastern margin of the subtropical gyres, while observations suggest an excess is found in the west. Here simulations of the bomb14C transient in the North Atlantic Ocean are made using coarse resolution (1.0° × 1.2°) and “eddy‐permitting” (1/3° × 2/5°) tracer transport models. The former employs large horizontal diffusivities to parameterize eddy transfer. Both models employ the same air‐sea exchange parameterization, with a specified tropospheric history of Δ14C. The coarse resolution model accumulates14C evenly over the whole subtropical gyre, while the eddy‐permitting model has a pronounced maximum column inventory in the western part, consisent with the observed patterns. The prescence of explicit eddies enhances the ventilation of density surfaces in the recirculation zone on the western margin of the gyre, suggesting that the zonal gradient hinted at in the observations is intimately tied to properties of geostrophic eddy dynamics which are not captured parametrically in the coarse

ISSN:0148-0227    

DOI:10.1029/96JC01698

年代:1996

数据来源: WILEY

摘要:

The representation of dynamic ocean topography (ζ) through spherical harmonic (SH) and orthonormal (ON) expansions was studied using TOPEX altimeter data, three potential coefficient models used to define geoid undulations, and three estimates of ζ from oceanographic data and global circulation models (GCMs). The ON expansions are desirable when one wishes to study the spectral characteristics of a function in a defined domain such as the ocean. The potential coefficient models tested were JGM‐2, JGM‐3, and GRIM4_C4b. Each model was augmented with the OSU91A potential coefficients from degree 71 to 360. The ζ models were those ofLevitus[1982] and values implied by the POCM_4B (Semtner/Chervin) model and a Los Alamos National Laboratory Model POP(96) (Malone, Smith, Dukowicz). The latter two models were defined over a 2‐year time period. Values of ζ were computed from 2 years of TOPEX data using the three potential coefficient models. The ON expansions of ζ from the TOPEX data were then compared to the estimates from the oceanographic data. The differences, to ON degree 14, with the POCM_4B model and the TOPEX results were ±14.0 cm (JGM‐2), ±12.4 cm (JGM‐3), and ±14.4 cm (GRIM4_C4b). A comparison with the other ζ estimates using TOPEX/JGM‐3 gives differences of ±14.3 cm (Levitus) and ±13.3 cm (POP (96)). The comparisons were made only to degree 14 because (1) the correlation between the ζ coefficients from TOPEX data and POCM_4B fell off beyond degree 14 and (2) the geoid undulation accuracy, in the ocean region, was equal to the ζ signal near degree 14. These results suggest ζ estimates made above degree 14 may be contaminated by geoid undulation errors. Also suggested from the comparisons was that the TOPEX/JGM‐3 estimates of ζ were more reliable than those from oceanographic data to degree 8 (2500‐km resolution). The ζ estimates from the POCM_4B and POP(96) models, 2‐year averages, agreed well north of 40°S. Below this the differences could reach 40 cm in the Antarctic Circumpolar Current (60°S, 215°). The differences between the TOPEX/JGM‐3 and POCM_4B ζ estimates exceeded 20 cm in a number of places (e.g., (20°N, 140°), (5°S, 130°), (60°S, 220°), (45°N, 320°)). The largest differences (−62 cm) occurred in the Banda Sea. The ζ representations were used to calculate upper ocean geostrophic velocities in the east/west and north/south directions. Excluding a 10° band on either side of the equator, the difference (TOPEX versus POCM_4B) was ±2.5 cm/s with the magnitude of the total velocity being 4.8 cm/s. The difference was consistent with the error estimates of the velocities implied by the errors in the JGM‐3 coefficients to degree 14. The ζ estimates were also determined from four recent mean sea surface grids and the results compared to the POCM_4B model through the ON representation. The MSS grids used were the OSUMSS95, the UTCSRMSS95, the GFZ/D‐PAF MSS95A, and the CNES/GRGS MSS95. The best agreement, to degree 14, was found with the OSUMSS95 (±11.1 cm) and the CSRMSS95 (±11.5 cm). The comparisons were poorer (±15 cm) when a mean sea surface was used where no mean inverted barometer correction had been applied to the gridded data. Although substantial progress has been made in the past 10 years in the determination of the Earth's gravitational potential, the accuracy limitations of geoid undulation determination still hinder the comparison and assimilation of altimeter data and oceanographic data. The need for a dedicated gravity satellite mission, to yie

ISSN:0148-0227    

DOI:10.1029/96JC01987

年代:1996

数据来源: WILEY

摘要:

The well‐known fact that tropical sea level can be usefully simulated by linear wind driven models recommends it as a realistic test problem for data assimilation schemes. Here we report on an assimilation of monthly data for the period 1975–1992 from 34 tropical Pacific tide gauges into such a model using a Kalman filter. We present an approach to the Kalman filter that uses a reduced state space representation for the required error covariance matrices. This reduction makes the calculation highly feasible. We argue that a more complete representation will be of no value in typical oceanographic practice, that in principle it is unlikely to be helpful, and that it may even be harmful if the data coverage is sparse, the usual case in oceanography. This is in part a consequence of ignorance of the correct error statistics for the data and model, but only in part. The reduced state space is obtained from a truncated set of multivariate empirical orthogonal functions (EOFs) derived from a long model run without assimilation. The reduced state space filter is compared with a full grid point Kalman filter using the same dynamical model for the period 1979–1985, assimilating eight tide gauge stations and using an additional seven for verification [Miller et al., 1995]. Results are not inferior to the full grid point filter, even when the reduced filter retains only nine EOFs. Five sets of reduced space filter assimilations are run with all tide gauge data for the period 1975–1992. In each set a different number of EOFs is retained: 5, 9, 17, 32, and 93, accounting for 60, 70, 80, 90, and 99% of the model variance, respectively. Each set consists of 34 runs, in each of which one station is withheld for verification. Comparing each set to the nonassimilation run, the average rms error at the withheld stations decreases by more than 1 cm. The improvement is generally larger for the stations at lowest latitudes. Increasing the number of EOFs increases agreement with data at locations where data are assimilated; the added structures allow better fits locally. In contrast, results at withheld stations are almost insensitive to the number of EOFs retained. We also compare the Kalman filter theoretical error estimates with the actual errors of the assimilations. Features agree on average, but not in detail, a reminder of the fact that the quality of theoretical estimates is limited by the quality of error models they assume. We briefly discuss the implications of our work for future studies, including the application of the method to full ocean general circulation models and coupled

ISSN:0148-0227    

DOI:10.1029/96JC01684

年代:1996

数据来源: WILEY

摘要:

The spatial covariances of the time‐dependent density and geostrophic velocity fields off southern California are determined from a unique set of repeated hydrographic observations (44 cruises) made by the California Cooperative Oceanic Fisheries Investigations from 1984 to 1994. The covariances and objective analysis are used to combine direct velocity observations, from shipboard acoustic Doppler current profiler (ADCP) measurements made on a recent survey (October 1993), with hydrographic observations. The analysis reduces ageostrophic noise in the ADCP velocities by smoothing and enforcing horizontal nondivergence; additionally, the velocities are mapped over scales that are dynamically consistent with the hydrography. Maximum surface geostrophic flow in the California Current in October 1993 is about 35 cm s−1, 50% larger than that estimated assuming a 500‐m level of no motion. Absolute flow at 500 m isO(10 cm s−1) and indicates that the surface eddy field penetrates through the thermocline. Uncertainty in the geostrophic reference calculated from the ADCP measurements is ofO(4 cm s−1). The velocity residual (objectively analyzed minus raw ADCP estimates) exhibits smaller correlation scales than the geostro

ISSN:0148-0227    

DOI:10.1029/96JC02004

年代:1996

数据来源: WILEY

摘要:

We use a set of mixed‐layer drifting buoy trajectories from the California Current (20°N–40°N) during 1985–1990 to obtain statistically reliable estimates of the mean currents, the mean variance field, and the geographically varying diffusivity, integral timescales, and integral space scales. Typical values for the diffusivity are 1.1–8.7 × 107cm2s−1, while the timescales and space scales are 2.1–7.1 days and 16–59 km, respectively. The variance field displays a strong westward gradient out to 125°W, and diffusivity shows a tendency to decrease toward the southwest part of the domain. Significant anisotropy is found in the variance field near the coastal boundary and at 30°N, 130°W, which is the region where the subarctic and northern subtropical fronts approach the California Current. The antisymmetric component of the diffusivity tensor indicates that cyclonic eddies dominate the mesoscale signature of drifters in this region. We seek simple parameterizations to relate the scales of motion of the random velocity field to the diffusivity by testing least squares fits to κ∞∝u02Tand κ∞∝u0L, whereu02is the velocity variance. We found no cases for which these two hypotheses could be distinguished. For the meridional component the linear regressions are not successful, which suggests that the meridional departure velocities result from a flow regime that is significantly organized by, for example, waves or coherent structures. A subset of the drifters measured temperature along their tracks, and we use the resultant data to produce the first direct estimates of the horizontal eddy heat flux divergence based on Lagrangian estimates. In addition, we separately compute the “eddy diffusivity” parameterization of the eddy heat flux divergence, ∇ · 〈u′θ′〉 = ∇(κ∇Θ), using our diffusivity estimates and a sea surface temperature climatology. The two independent terms agree well, which provides a measure of reassurance about the diffusivity estimates. The eddy heat flux divergence in the California Current is very small (<5 W m−2) and does not appear to be significant in the lo

ISSN:0148-0227    

DOI:10.1029/96JC02008

年代:1996

数据来源: WILEY