摘要:

The motions of eight Autonomous Lagrangian Circulation Explorer (ALACE) floats released near 750 m depth in Drake Passage and followed through the South Atlantic are described and compared with emulations made by advecting model floats through 12 monthly snapshots of velocity from the fine resolution Antarctic model (FRAM). Both ALACEs and FRAM reproduce the major features of the general circulation as follows: strong intermediate depth flow in Drake Passage, bifurcation of the Antarctic Circumpolar Current (ACC) passing over the Falkland Plateau, a strong Falkland Current, its confluence with the Brazil Current, and moderate zonal flow across the South Atlantic. FRAM versus ALACE comparisons are made in both the Eulerian frame and using observed and modeled trajectories. In Drake Passage, where float velocities agree with earlier observations, FRAM velocities are about twice too big. Both FRAM and ALACE velocities are consistent with anO(100 Sv) Falkland Current. In the central South Atlantic the few available float observations indicate the ACC and South Atlantic Current (SAC) to be more localized than in the model. Eddy kinetic energy is much stronger in the observations than in FRAM. Float dispersion in both the model and observations is due primarily to mean shear. Initial RMS particle separation of 100 km grows to nearly 1000 km after 1 year, but most of this is associated with floats that take different paths of the general circulation. The observations indicate that eddy effects are particularly important near the Falkland‐Brazil Current confluence in allowing Antarctic Intermediate Water to transfer from the ACC to the SAC, from which they may enter the subtropical gyr

ISSN:0148-0227    

DOI:10.1029/95JC02538

年代:1996

数据来源: WILEY

摘要:

Using the new ocean tracer CFC‐113 and other hydrographic and chemical data, we have identified and “aged” a Brazil eddy transported across the South Atlantic to the Cape Basin. This is the first observation of such an eddy so far east. It was observed during two RRSDiscoverycruises in January and May 1993, and, using TOPEX‐POSEIDON altimeter data, we have been able to track it between the two locations. TS and nutrient characteristics show that the eddy could not have an eastern Atlantic origin. Comparison with eddies found in the Brazil/Falklands Confluence suggests that this is the most likely source. From the CFC‐113:CFC‐11 ratio it appears that the eddy is in excess of 4 years old, and from the two sightings and the altimeter data we have estimated a transport rate for the eddy and given credence to the CFC‐113:CFC‐11 and CFC‐113:CFC‐12 ageing techniqu

ISSN:0148-0227    

DOI:10.1029/95JC02751

年代:1996

数据来源: WILEY

摘要:

A subset of the Benguela Sources and Transports (BEST) 1992–1993 data is analyzed to study the magnitude and variability of the large scale transports in the area. The data consist of inverted echo sounder series and conductivity‐temperature‐depth stations. The mean 16 month transport values for the upper 1000 m indicate that, of the 13‐Sv northward transport within the Benguela Current, 50% is derived from the central Atlantic (which from geometry may be chiefly South Atlantic water), 25% comes from the Indian Ocean (which may be chiefly Agulhas water), and the remaining 25% may be a blend of Agulhas and tropical Atlantic water. A simple schematic of the transport pattern with a somewhat restricted corridor for Agulhas eddies translation is envisioned. To the west of the eddy corridor flows the South Atlantic source for the Benguela Current; to the east is the Agulhas (Indian Ocean) source. The corridor is breached by South Atlantic and Indian water as the transient eddy field stirs these water

ISSN:0148-0227    

DOI:10.1029/95JC03221

年代:1996

数据来源: WILEY

摘要:

Sea surface temperature (SST) from weekly global advanced very high resolution radiometer data for the period of 1982–1992 and estimates of the surface forcing due to wind stress and net heat flux are used to investigate a global monthly climatology of large‐scale oceanic frontal zones (OFZs), the variability of SST gradient in several frontal zones, and the meridional frontogenesis in the North Pacific. Subpolar frontal zones are identifiable in the SST gradient field throughout the whole year while subtropical frontal zones appear as seasonal phenomena in the northern hemisphere. The magnitude and position of subtropical fronts were found to vary synchronously in both hemispheres, with maximum intensity in November to March. During this period, subtropical fronts in the northern hemisphere become clearly identifiable, separate features, while in the southern hemisphere their southern boundaries tend to merge with the subpolar front. In the northern hemisphere, the northward shift of the subtropical frontal zones during late boreal spring results in a merger of the subpolar and subtropical zones by August. In the southern hemisphere, subtropical fronts separate from the subpolar front during this period and become clearly distinguishable, though less intense. Singular spectrum analysis of the 11‐year time series of SST gradient in several OFZs revealed a seasonal signal as well as interannual variations that, in the equatorial zone, are related to El Niño events. The amplitude of the annual cycle is comparable in the subtropical and subpolar North Pacific, but the subtropics leads the subpolar zone by about 4 months. Combined forcing due to wind stress and net surface heat flux was found to be sufficient to describe most of the observed frontogenesis variability in North Pacific midlatitudes on seasonal timescales, but not on shorter timescales. In the subtropical zone, the Ekman convergence alone was not sufficient to provide the observed frontogenesis/frontolysis on seasonal time scales. In fact, when the maximum frontogenesis associated with Ekman convergence occurs in late spring/early summer, this is counteracted by the frontolytic effect of Ekman pumping and the dominating frontolytic effect of surface heat forcing. In summer, Ekman convergence is more important in the subpolar area where it is enhanced by the smaller frontogenetic effect of both Ekman pumping and surface heat forcing. Surface forcing due to wind stress and heat flux considered in this study does not explain the high‐amplitude intraseasonal frontogenesis variations, especially in the subpola

ISSN:0148-0227    

DOI:10.1029/95JC02992

年代:1996

数据来源: WILEY

摘要:

The hypothesis that fronts are sites of active subduction is examined using density, temperature, salinity, and horizontal velocity data from a trio of surveys of the Azores Front done in May 1991 and March 1992. These surveys were made using a SeaSoar equipped with a conductivity‐temperature‐depth profiler and a shipboard acoustic Doppler current profiler. The potential density and potential voracity indicate that dense water from the north side of the front may be sliding down beneath the surface outcrop. This apparently subducting isopycnal has a great deal of temperature and salinity variability. Horizontal velocity is nearly parallel to isopycnals, indicating that the time rate of change and vertical advection must be small. The thermal wind balance is observed to be valid, especially in the region of the largest horizontal density gradients. Shear at the base of the mixed layer is likely due to near‐inertial motions. The potential vorticity is dominated by the planetary vorticity, except at the front, where vertical shears (the tilting term) become large. The tilting term acts to reduce the magnitude of the potential vorticity at the front, in agreement with simple theoretical models. The magnitude of the tilting term is similar to the total vorticity in the seasonal thermo

ISSN:0148-0227    

DOI:10.1029/95JC02867

年代:1996

数据来源: WILEY

摘要:

A set of numerical simulations is used to investigate the Pacific Ocean circulation north of 20°S, with emphasis on the Kuroshio/Oyashio current system. The primitive equation models used for these simulations have a free surface and realistic geometry that includes the deep marginal seas, such as the Sea of Japan. Most of the simulations have 1/8° resolution for each variable but range from 1/2°, 1.5‐layer reduced gravity to 1/16°, six layer with realistic bottom topography. These are used to investigate the dynamics of the Kuroshio/Oyashio current system and to identify the processes that contribute most to the realism of the simulations. This is done by model‐data comparisons, by using the modularity of layered ocean models to include/exclude certain dynamical processes, by varying the model geometry and bottom topography, and by varying model parameters, such as horizontal grid resolution, layer structure, and eddy viscosity. In comparison with observational data, the simulations show that the barotropic mode, at least one internal mode, nonlinearity, high “horizontal” resolution (1/8° or finer), the regional bottom topography, and the wind forcing are critical for realistic simulations. The first four are important for baroclinic instability (eddy‐mean energetics actually show mixed barotropic‐baroclinic instability), the wind curl pattern for the formation and basic placement of the current system, and the bottom topography for the distribution of the instability and for influences on the pathways of the mean flow. Both the Hellerman and Rosenstein (1983) (HR) monthly wind stress climatology and 1000‐mbar winds from the European Centre for Medium‐Range Weather Forecasts (ECMWF) have been used to drive the model. East of about 150°E, they give a mean latitude for the Kuroshio Extension that differs by about 3°, approximately 34°N for HR, 37°N for ECMWF, and 35°N observed. The subarctic front is the northern boundary of the subtropical gyre. It is associated with the annual and April–September mean zero wind stress curl lines (which are similar), while the Kuroshio Extension is associated with wintertime zero wind stress curl. This means that part of the flow from the Kuroshio must pass north of the Kuroshio Extension and connect with the Oyashio and subarctic front. Realistic routes for this connection are flow through the Sea of Japan, a nonlinear route separated from the east coast of Japan, and bifurcation of the Kuroshio at the Shatsky Rise. In addition, the six‐layer simulations show a 3‐Sv meridional overturning cell with southward surface flow and northward return flow centered near 400 m depth. Baroclinic instability plays a critical role in coupling the shallow and abyssal layer circulations and in allowing the bottom topography to strongly influence the shallow circulation. By this means, the Izu Ridge and Trench and seamounts upstream and downstream of these have profound influence on (1) the mean path of the Kuroshio and its mean meanders south and east of Japan and (2) on separating the northward flow connecting the Kuroshio and the Oyashio/subarctic front from the east coast of Japan. Without the topographic influence, the models show an unrealistic northward current along the east coast of Japan. In essence, the topography regulates the location and strength of the baroclinic instability. The baroclinic instability gives eddy‐driven deep mean flows that follow thef/hcontours (wherefis the Coriolis parameter andhis the depth of the water column) of the bottom topography. These abyssal currents then strongly influence the pathway for subtropical gyre flow north of the Kuroshio Extension and steer the mean meanders in the Kuroshio south and east of Japan. This is corroborated by current meter data from the Kuroshio Extension Regional Experiment (World Ocean Circulation Experiment line PCM 7). The meander path south of Japan depends on the occurrence of baroclinic instability west of the Izu Ridge; otherwise, a straight path occurs. The pathway shows little sensitivity to the Tokara Strait transport over the range simulated (36–72 Sv in yearly means). However, interannual increases in wind forcing or Tokara Strait transport give rise to a predominant meander path, while decreases yield a predominant straight path. Resolution of 1/8° in an ocean model is comparable to the 2.5° resolution used in atmospheric forecast models in the early 1980s based on the first internal mode Rossby radius of deformation. Model comparisons at 1/8° and 1/16° resolution and comparisons with current meter data and Geosat altimeter data show that 1/16° resolution is needed for adequate eastward penetration of the high eddy kinetic energ

ISSN:0148-0227    

DOI:10.1029/95JC01674

年代:1996

数据来源: WILEY

摘要:

In November–December 1992, we measured microstructure, the hydrographic field, and the velocity field in the western equatorial Pacific near 1°43′S and 156°E as the first part of the Tropical Ocean ‐ Global Atmosphere ‐ Coupled Ocean Atmosphere Response Experiment. During our stay, we observed two westerly wind bursts with maximum speeds of about 10 m s−1, rainfalls of about 20 mm h−1, and nearly 2 weeks of calm and dry weather. During wind bursts, mixing in the upper 1 MPa was dramatic and rapid, and the major turbulence in the thermocline was produced by near‐inertial shears following the wind bursts, during which dissipation rates and Richardson numbers were comparable to those in the central Pacific. By the end of the October–November westerly burst, the averaged eddy diffusivity of momentum in the mixed layer was about 10−2m2s−1. Below the mixed layer, a layer about 0.50–0.75 MPa thick with high shear and strain along with a high dissipation rate persisted even after the wind burst. During calm days, mixing was primarily driven by nighttime convection and was confined to the upper 0.1 or 0.2 MPa. In general, no diurnal deep cycle in dissipation rate was found below the mixed layer, unlike in the central equatorial Pacific. Entrainment below the mixed layer occurred when both wind work and convection were large. On average, the entrainment heat flux at the base of the mixed layer was about 6 (±2) W m−2. Salinity gradients were important to density between 0.3 and 0.6 MPa. However, the salinity stratification did not control entrainment mixing. During calm and dry weather, the mixed layer heat content was governed by the vertical divergence of the radiative flux, that is, the net surface heat flux minus the radiative flux out the bottom of the layer. Both the entrainment flux and the advective flux (which was estimated as the residual term) were smaller than uncertainties in the surface heat flux. During the October–November wind burst, however, the advective term was one of the most dominant terms in the heat budget. Although the entrainment heat flux was small over short timescales, it may be important in long‐term budgets because the long‐term net sur

ISSN:0148-0227    

DOI:10.1029/95JC02553

年代:1996

数据来源: WILEY

摘要:

We here present conductivity‐temperature‐depth and chemical (CO2, PO4, O2, alkalinity, andpH) observations across an anticyclonic (fresh‐core) ring in the Oyashio Current near Bussol Strait, Kuril Islands. These measurements suggest that the core of the ring is a region of enhanced vertical mixing, with attendant effects on both spatial and temporal patterns of CO2exchange. In particular, the difference between atmospheric and oceanicpCO2at the air‐sea boundary shows a pronounced seasonal reversal in sign. In winter,pCO2is everywhere higher in the ocean than in the atmosphere; however, because mixing within the core of the ring brings additional amounts of deep, CO2‐rich water upward the highest values ofpCO2occur above the ring. In summer,pCO2outside the ring boundary is also higher in the ocean than in the atmosphere; however, because enhanced mixing within the ring now promotes high values of nutrient flux, primary productivity, and CO2uptake,pCO2values there are lower than in the atmosphere. These data suggest the importance of including mesoscale physical processes in the interpretation of oceanic CO2exchange and demonstrate the usefulness of biogeochemical properties in studies of ring dynamics and

ISSN:0148-0227    

DOI:10.1029/95JC02924

年代:1996

数据来源: WILEY

摘要:

This study compares four surface wind products which may be used to force ocean circulation models of the North Pacific: wind stress derived from the Atlas special sensor microwave imager (SSM/I) based surface wind analyses (July 1987–June 1990), the Goddard Earth Observing System (GEOS) wind assimilation product at 10 m (March 1985–February 1990), and, at the appropriate overlapping times, the European Centre for Medium‐Range Weather Forecasts (ECMWF) wind analyses and the Comprehensive Ocean‐Atmosphere Data Set (COADS). Overall, there is good agreement in spatial and temporal variations between COADS and the model‐based analyses. The COADS product used in this study, based on monthly averaged winds, underestimates the monthly averaged stresses in the higher latitudes where the submonthly variance is high. In the tropics and subtropics where the variance is lower, the COADS stresses are closer to climatology than the weaker model‐based winds. The largest differences between the model‐based analyses occur during winter in the northernmost region of the North Pacific basin where the spatial and temporal variability is largest. The seasonal mean Atlas SSM/I stresses are stronger overall than the other model‐based analyses, with differences reaching as high as 0.09 N m−2in the high latitudes during winter. The Atlas product is also consistently stronger in the curl. Of the four curl products, the Atlas SSM/I and GEOS are closest in pattern and intensity in the tropics in both summer and winter. These also have the closest wavenumber spectral distribution in the high latitudes during winter. Integrations of a nonlinear, baroclinic quasi‐geostrophic ocean model using three of the wind stress curl distributions show differences of 30–45% in the mean integrated volume transport in the Kuroshio Extension. The Atlas SSM/I wind stress curl seems to force a more realistic subtropical gyre (in terms of transport and eddy energy levels) and western boundary current structure. The GEOS curl seems to force a more realistic subpolar gyre circulation. Overall, the assimilation of the SSM/I wind speed data improves the comparison between the model‐based analyses and ship‐based observations and produces a more realisti

ISSN:0148-0227    

DOI:10.1029/95JC03015

年代:1996

数据来源: WILEY

摘要:

The sea surface height (SSH) variations of the North Pacific ocean and the Kuroshio Extension region, in particular, are examined by frequency and wavenumber decompositions of a 1/8°, six‐layer primitive equation Pacific Ocean model and of the Geosat Exact Repeat Mission (ERM) data. Both data sets exhibit peaks in variability at 1 and 2 cycles per year over much of the Kuroshio Extension region. This study is restricted to these two frequencies. Annual variations of equatorial currents in both data sets are similar in both space and time, with the variations in the South Equatorial Current appearing as annual westward propagations. Annual variations in the strength of the Kuroshio Extension are manifested mainly through changes in the strength of the recirculation gyres on the southern side of the current. Annual transport maxima for the Kuroshio Extension occur around late October for both the model and Geosat. Large‐scale variations (length scales greater than 1000 km) of the model and Geosat have comparable amplitudes. The main differences between the model SSH and the Geosat ERM data occur over regions where seasonal steric variations are significant (from 20°N to 30°N). Wavenumber spectra over the Kuroshio Extension region reveal similar dynamics in both data sets. Much of the energy in wavenumber spectra appears as westward propagating SSH anomalies near the theoretical Rossby wave dispersion relations. As the Rossby wave dispersion relation changes with latitude (shifting to shorter wavelengths with higher latitudes), the peaks in the wavenumber decompositions follow. Thus the dynamics are generally consistent with quasi‐geostrophic dynamics in both the model and altimeter data. Wavelengths of propagating SSH anomalies which have spectral peaks near the Rossby dispersion curve are longer in the Geosat and model than wavelengths indicated by theory. In the semiannual frequency below 35°N, westward propagation dominates over eastward propagation in both Geosat and the model. Most differences in the dynamics of the model and Geosat occur at shorter length and timescales, with Geosat showing higher amplitudes at the shorter scales than

ISSN:0148-0227    

DOI:10.1029/95JC02081

年代:1996

数据来源: WILEY