摘要:

Using a robust diagnostic model, the steady circulation in the world ocean is determined from climatological annual mean fields of hydrographic data and wind stress. The upper circulation reflects the surface geopotential anomaly of the input hydrographic data. In addition, the “warm water route” is prominent: the water passing through the Indonesian seas flows westward across the Indian Ocean to return to the North Atlantic. In the deep Atlantic Ocean, the North Atlantic Deep Water is transported southward by the dominant deep western boundary current in the depth range from about 1500 m to the bottom, whereas the Weddell Sea Deep Water does not extend northward from the Argentine Basin. An important result is a westward current appearing near 25°N in the middepth Atlantic Ocean. Such a current has been inferred from tracer distributions but hardly detected with objective methods. The velocity maximum at middepth suggests that the current is not a lower part of a wind gyre but is a baroclinic flow caused by a thermohaline effect. In the Indian Ocean a swift middepth current appears along the western boundary, but it may be questionable because the resulting heat transport does not agree with earlier estimates. The deep water flowing into the southern basins from the Antarctic Circumpolar Current does not flow further northward even in the Madagascar Basin. The cause of these unsuccessful results is probably that the present model does not take into account large seasonal variations in the Indian O

ISSN:0148-0227    

DOI:10.1029/92JC00867

年代:1992

数据来源: WILEY

摘要:

A time‐averaged description of the formation and spreading of North Pacific subtropical mode water (NPSTMW) is presented. The formation of NPSTMW is studied using TRANSPAC expendable bathythermograph data. The data are transformed into a stream coordinate system and averaged. The coordinate system uses the position of the Kuroshio front as its origin. Maps are presented of the temperature structure and mixed layer distribution in winter relative to the Kuroshio front. The formation of NPSTMW occurs in two separate stages, cooling and thickening. As water flows at the surface in the Kuroshio in winter, the water is cooled by the intense ocean‐atmosphere heat flux taking place in the area. When water leaves the Kuroshio to the south, the thermocline underneath becomes deeper, convection is able to penetrate deep into the water column, and the heat flux acts more to thicken the surface mixed layer rather than reduce the temperature. The area south of the Kuroshio, where relatively homogeneous waters are found, is the source region for NPSTMW. At the end of the winter the thick mixed layers are assumed to be covered over by a seasonal thermocline and become NPSTMW. The Levitus (1982) data are used to study the way in which NPSTMW is spread away from its source region by the subtropical circulation. The distribution of potential vorticity (PV) on four NPSTMW isopycnals is studied, with low PV used as a tracer for NPSTMW. The PV reveals a pattern of “differential spreading,” whereby NPSTMW formed in the western part of the source region is advected to the west after its formation. NPSTMW formed farther east is advected more to the south or east after formation. Differential spreading allows NPSTMW to be found in a large part of the northwestern subtropical gyre, well away from its source region. The distribution of NPSTMW described by Masuzawa (1969), with NPSTMW getting colder from west to east, is explained as a combination of the distribution of NPSTMW at its source and the way it is spread after it is

ISSN:0148-0227    

DOI:10.1029/92JC01001

年代:1992

数据来源: WILEY

摘要:

The historical data base is used to study property changes in both the Western Mediterranean Deep Water (WMDW) and the Levantine Intermediate Water (LIW). Changes in WMDW properties during the past century have been described previously, although on a more limited data base. We are not aware of any previous study of changes in LIW properties. In the extensive data base we used, increases appear in both WMDW temperature and salinity, from 1909 to the present, which substantiate previously reported observations. In addition, we find that the density of WMDW seems to have increased as well, which disagrees with previous suggestions that it has remained constant. We observe that the WMDW temperature increase displays a distinct acceleration starting about 1955 and that a similar, although less conspicuous, acceleration occurs in the WMDW salinity increase. From our study of historical data on LIW properties, the LIW salinity also appears to have increased since 1909. We argue that the warming trend in WMDW may well be a response to the salinity increase, which seems to be imported from the eastern Mediterranean by LIW, and as such our observations endorse a recently published hypothesis. The increase in LIW salinity, in turn, is attributed to changes in the eastern Mediterranean freshwater budget, resulting from damming of major rivers that drain either directly or indirectly into the eastern Mediterranean. Finally, we demonstrate that the basin has not yet reached a new steady state after this freshwater disturbance and that the response time of the system seems to be of the order of 100 years.

ISSN:0148-0227    

DOI:10.1029/92JC00767

年代:1992

数据来源: WILEY

摘要:

Leads in pack ice have long been considered important to the thermodynamics of the polar regions. A winter lead affects the ocean around it because it is a density source. As the surface freezes, salt is rejected and forms more dense water which sinks under the lead. This sets up a circulation with freshwater flowing in from the sides near the surface and dense water flowing away from the lead at the base of the mixed layer. If the mixed layer is fully turbulent, this pattern may not occur; rather, the salt rejected at the surface may simply mix into the surface boundary layer. In either event the instability produced at the surface of leads is the primary source of unstable buoyancy flux and, as such, exerts a strong influence on the mixed layer. Here as many as possible of the disparate and almost anecdotal observations of lead oceanography are assembled and combined with theoretical arguments to predict the form and scale of oceanographic disturbances caused by winter leads. The experimental data suggest the velocity disturbances associated with lead convection are about 1–5 cm s−1. These appear as jets near the surface and the base of the mixed layer when ice velocities across the lead are less than about 5 cm s−1. The salinity disturbances are about 0.01 to 0.05 psu. Scaling arguments suggest that the geostrophic currents set up by the lead density disturbances are also of the order of 1–5 cm s−1. The disturbances are most obvious when freezing is rapid and ice velocity is low because the salinity and velocity disturbances in the upper ocean are not smeared out by turbulence. In this vein, lead convection may be characterized at one extreme as free convection in which the density disturbance forces the circulation. At the other extreme, lead convection may be characterized as forced convection in which the density disturbance is mixed rapidly by boundary layer turbulence. The lead numberLo, which is the ratio of the pressure term to the turbulence term in the momentum equation, and the turbulent lead numberLot, which is the ratio of buoyant production to shear production in the turbulent kinetic energy equation, define the boundary between the free and forced regimes. ForLoandLotless than one, both the large‐scale circulation and the turbulence are forced by surface stress. ForLoandLotgreater than one, both the large‐scale circulation and the turbulence are forced by the buoyancy flux. The magnitudes of velocity and salinity disturbances from a model developed elsewhere, suitable to free convection, agree with what few observations we have. The results of a forced convection model, developed here, suggest salinity disturbances of the order of 0.01–0.02 practical salinity units, with the maximum occurring at the surface of the lead and decreasing substantially below 5–10 m. This unstable gradient is a unique characteristic of lead convection. Though the salinity disturbances may be small when ice velocities are large, the buoyancy flux in leads has a major effect on the boundary

ISSN:0148-0227    

DOI:10.1029/92JC00684

年代:1992

数据来源: WILEY

摘要:

A three‐dimensional hydrodynamic model is developed to study the circulation in the Arabian Gulf. The model contains realistic basin geometries and bathymetries of the Arabian Gulf and a good portion of the Gulf of Oman and is driven by monthly climatological winds, evaporation, and net ocean heat gain in both gulfs and the Shatt‐al‐Arab discharge. It is found that the cyclonic circulation in the southern portion of the gulf is primarily driven by the evaporation‐induced freshening from the Strait of Hormuz. In the northwestern corner of the gulf, the Shatt‐al‐Arab discharge maintains the cyclonic circulation, which would otherwise be anticyclonic. The northwestward intrusion of fresher water along the Iranian coast is weakened by northwesterly winds in winter but strengthens and extends almost to the head of the gulf in summer owing to the warming of the Gulf of Oman waters, the development of the summer thermocline in the Arabian Gulf, and diminishing winds. The southward coastal current along the Arabian coast is most prominent between the head of the gulf and Qatar. The model also predicts a strong southward coastal jet east of Qatar which is primarily wind driven. No similar coastal jet can be developed in the Gulf of Salwa, we

ISSN:0148-0227    

DOI:10.1029/92JC00841

年代:1992

数据来源: WILEY

摘要:

A free surface tropical ocean model is presented for explicit prediction of the actual topography of sea surface and other variables, for further intercomparative studies among differently formulated oceanic general circulation models (OGCMs), and data assimilation of the observed sea level. Based on an OGCM without the rigid‐lid approximation developed at the Institute of Atmospheric Physics (Beijing, China), the dynamic formulation and numerics of the model have some unique features such as explicit treatment of sea level elevation as a prognostic variable, calculation of the departures of thermodynamic variables in time integration by subtracting the standard stratification, introduction of the σ vertical coordinate, total available energy consideration, and a barotropic‐baroclinic mode‐splitting algorithm for computational efficiency. The model covers the tropical Pacific Ocean between 30°N and 30°S with horizontal grid spacing of 1° in latitude and 2° in longitude. There are 14 vertical layers with 8 layers in the top 200 m. Prior to further applications of the model, simulations of the annual mean and seasonal cycle of the tropical Pacific Ocean have been carried out to examine performance of this free surface OGCM forced by climatological atmospheric fluxes. It is shown that with the appropriate treatments of the governing equations and numerical methods, the free surface baroclinic OGCM can be computed at least as efficiently as a rigid‐lid model. By eliminating the rigid‐lid approximation, keeping explicitly the available surface energy and energy conversion related with divergence of vertically integrated flow, and by subtracting the standard stratification, the model would be more accurate, reasonable, and effective in representing the energy cycle in the real ocean. The misrepresentation of energy conversion caused by the rigid‐lid approximation is shown and estimated on the basis of the simulated data. The simulated results show that the model is able to reproduce not only the observed current system and thermal structure and their variations but also the actual sea level in the tropical Pacific, which shares many observed sea surface features. The preliminary comparison with other existing simulations shows some noticeable differences in the simulated circulation in spite of using the same wind stress, which need to be inve

ISSN:0148-0227    

DOI:10.1029/92JC00911

年代:1992

数据来源: WILEY

摘要:

Decreased consumption of leaded gasoline in the United States over the past two decades has drastically altered the flux and isotopic composition of Pb entering the western North Atlantic from the atmosphere. Here we exploit the resulting temporal changes in the distribution and isotopic composition of oceanic Pb to investigate interactions between dissolved and particulate Pb in the oceanic water column. Measurements of dissolved Pb isotopic composition on samples collected in 1987 in the upper water column near Bermuda demonstrate that surface water206Pb/207Pb decreased from ∼1.203 to ∼1.192 since 1983 and that a measurable change propagated to 300–500 m since the 1984 profile of Shen and Boyle (1988). The first accurate measurements of suspended particulate Pb in an open ocean profile show concentrations of 1–3 pmol/L, equal to 2–4% of total Pb. Vertical profiles of (1) the stable lead isotopic composition and (2) the ratio of total Pb to210Pb in suspended particles closely track contemporaneous depth variations in these ratios for dissolved Pb throughout the upper 2000 m of the Sargasso Sea near Bermuda. Thus suspended particles reach isotopic equilibrium with ambient sea water Pb on a time scale which is shorter than their residence time with respect to vertical removal, in agreement with equilibrium scavenging hypotheses based on interpretations of Th isotope distributions. A simple flux model suggests that the effect of deep ocean scavenging processes on the flux and isotopic composition of Pb sinking on large particles was minor throughout the preanthropogenic and most of the anthropogenic era but has become important as surface inputs decrease to pre‐leaded gasoline levels and may exceed the contribution of surface‐derived Pb flux in th

ISSN:0148-0227    

DOI:10.1029/92JC00759

年代:1992

数据来源: WILEY

摘要:

Profiles of dissolved methane in the upper 300 m of a 190,000 km2area of southern Californian waters were determined during November 1989 and March 1990. Methane in near‐surface waters ranged from 1 to 1416 nmol L−1. The lowest concentrations were at depths>200 m. The highest levels were found along the coast, especially in the region extending from Point Conception to Santa Barbara. Vertical profiles of methane are characterized by maxima in supersaturation within a narrow density band located within the pycnocline; this layer is continuous over a large geographic area. Offshore stations exhibiting elevated levels of methane are spatially coincident with mesoscale eddies of the California Current system. Interpretation of hydrographic and chemical data suggests that CH4‐rich waters are upwelled from relatively shallow areas that are characterized by numerous hydrocarbon seeps. The data are consistent with a nearshore source of methane, presumably submarine seeps. Subsurface maxima in offshore waters are likely maintained by lateral transport of allochthonous CH4along density surfaces from coastal waters and autochthonous microbial produ

ISSN:0148-0227    

DOI:10.1029/92JC00865

年代:1992

数据来源: WILEY

摘要:

The areal ice distribution in a pack ice field is described using a number of classes, characterized by different ice thicknesses. Each class may have a snow layer on top and evolves according to the thermodynamic forcing (same for all classes) and is followed through time. Mechanical forcing leads to ice export and deformation. The ice export is quantified by the parameteredescribing the areal export of ice per unit of time. The deformation is quantified by the parameterrdescribing the area of open water generated by deformation per unit of time. The fraction of summer heat absorbed in leads and utilized for ice melting is quantified by the parametergop. The model is applied to the Arctic Ocean. It is run to a steady state for different combinations ofe,r, andgop, and the response of the thickness distribution to these parameters may thereby be quantified. It is shown that the ice generally gets thinner for increasing export. The response to deformation is dependent ongopsuch that the mean thickness increases withrfor smallgop(i.e., no effect or just a small melting effect of the heat absorbed in leads) but decreases withrfor largegop. The overall effect ofgopis that the mean thickness decreases with increasinggop. Forgopclose to 1 and moderate values ofeandrthe ice sheet melts completely in the summer.

ISSN:0148-0227    

DOI:10.1029/92JC00814

年代:1992

数据来源: WILEY

摘要:

Laboratory experiments and theory were conducted to observe the flow patterns and transport in both buoyancy‐driven and wind‐driven rotating fluids. In “lock‐exchange” experiments, water with one density flows into a second basin after a sliding gate is removed. Water of a second density flows back into the first basin. The size and location of the currents for various values of density difference, rotation rate, and assorted sidewall geometries was recorded. Volume flux of the fluid was also measured and compared with a theory for lock‐exchange flow of a rotating fluid. In a separate group of experiments with a passive upper layer, easterly winds (like those in the Arctic Ocean) drive the upper level water into the Arctic Ocean and therefore oppose the buoyant exchange. Westerly winds would drive the water out of the Arctic Ocean. This indicates that the exchange between the Arctic Ocean and the Greenland‐Norwegian Sea is likely to be driven by buoyancy rather than by driven by wind. Crude estimates of the volumetric and fresh water exchange rate from the lock‐exchange formulas are compared with observed ocean fluxes, and approximate agr

ISSN:0148-0227    

DOI:10.1029/92JC00735

年代:1992

数据来源: WILEY