摘要:

Four years of TOPEX/Poseidon data (October 1992 to October 1996) are assimilated into a high‐resolution primitive equation model [Gent and Cane, 1989] of the tropical Pacific Ocean. The assimilation method used is a degraded version of the Singular Evolutive Extended Kaiman filter approach [Pham et al., 1998] in which the reduced order basis is fixed. The along‐track TOPEX/Poseidon sea surface height residuals are introduced in the model every 3 days. Initialization of the model runs appears to be crucial. This is particularly true for the 1992–1996 period, which is characterized by several El Niño and La Niña events and exhibits quite anomalous climatological behavior. An in situ database dedicated to this period was built and used to initialize the model properly. The simulation results with and without assimilation were compared with expendable bathythermograph and Tropical Atmosphere‐Ocean in situ observations. The impact of sea surface height assimilation is observed both on the surface and subsurface temperature and velocity fields. The deficiencies of the free model in terms of means and variability structures are significantly reduced. More importantly, the evolution of the model fields over time is considerably improved by the assimilation. These supposedly more realistic assimilated four‐dimensional fields are used to investigate the mechanism responsible for the 1992–1996 warm pool replenishment and depletion. This approach provides insights into the combined role of zonal and meridional transports in balancing volume changes in

ISSN:0148-0227    

DOI:10.1029/2000JC900011

年代:2000

数据来源: WILEY

摘要:

A modeling study of the bomb14C transient is presented for the Pacific Ocean. A primitive equation ocean circulation model has been configured for a high‐resolution domain that accounts for the Indonesian Throughflow (ITF). Four separate runs were performed: (1) seasonal forcing with 20 Sv of ITF transport, (2) seasonal forcing with 10 Sv of ITF transport, (3) seasonal forcing with no ITF transport, and (4) interannual forcing with 15 Sv of ITF transport. This study has two main objectives. First, it is intended to describe the time evolution of the bomb14C transient. This serves as a tool with which one can identify the physical processes controlling the evolving bomb14C distribution in the Pacific thermocline and thus provides an interpretive framework for the database of Δ14C measurements in the Pacific. Second, transient tracers are applied to the physical oceanographic problem of intergyre exchange. This is of importance in furthering our understanding of the potential role of the upper Pacific Ocean in climate variability. We use bomb14C as a dye tracer of intergyre exchange between the subtropical gyres and the equatorial upwelling regions of the equatorial Pacific. Observations show that while the atmospheric Δ14C signal peaked in the early to mid‐1960s, the Δ14C levels in the surface water waters of the subtropical gyres peaked near 1970, and the Δ14C of surface waters in the equatorial Pacific continued to rise through the 1980s. It is shown that the model exhibits skill in representing the large‐scale observed features observed for the bomb14C transient in the Pacific Ocean. The model successfully captures the basin‐scale inventories of bomb14C in the tropics as well as in the extratropics of the North Pacific. For the equatorial Pacific this is attributed to the model's high meridional resolution. The discrepancies in the three‐dimensional distribution of bomb14C between the model and data are discussed within the context of the dynamical controls on the Δ14C distribution of bomb14C

ISSN:0148-0227    

DOI:10.1029/1999JC900228

年代:2000

数据来源: WILEY

摘要:

Seven years of conductivity‐temperature‐depth data enable direct computations of volume transport and a description of the temporal changes of water properties in the western subarctic Pacific. The cooling of the western subarctic surface layer is shown through the observation of the thermohaline transition, which occured in the western subarctic during 1990–1997. Its main consequences were changes of the Oyashio path and restratification of the western subarctic water. The example presented here uses data obtained during the Canada/Russia International North Pacific Ocean Climate Study, as well as the Russian project Subarctic to show that a warming and ventilation of the midlayers of the western subarctic (200–1000 dbar) occurred because of the influx of warm subtropical waters to the Oyashio and Sea of Okhotsk and to the weak coastal Oyashio before the transition. This influx appears to have reversed in the early 1990s; now subarctic intermediate waters displace the remnants of subtropical waters that penetrated to the Sea of Okhotsk through the Soya Strait and create strong stratification in the Oyashio area. This transition appears to have begun during the reinforcement in the coastal

ISSN:0148-0227    

DOI:10.1029/1999JC900330

年代:2000

数据来源: WILEY

摘要:

The proportion of18O in a sample of seawater is an excellent tracer of its past history as, away from the surface, it is conservative and also nondynamical. The range of values in source waters is also large, and the accuracy achievable in modern measurement high. Here we bring together for the first time a global data set of over 6000 individual measurements from the past 40 years. The properties of this data set are described. Noteworthy features include the hitherto unnoticed, but distinctive, contribution of North Pacific Upper Water to the δ18O:salinity relationship, and different origins of the deeper waters of the Atlantic and Pacific Oceans. We also make a plea here to the international community to contribute unpublished data to the archive for the use of all

ISSN:0148-0227    

DOI:10.1029/2000JC900005

年代:2000

数据来源: WILEY

摘要:

We propose a new approach to estimate the vertical variability of the salinity field. The method is based on combined vertical modes of temperatureTand salinitySand reconstructs salinity profiles via a weighted least squares procedure. The major advantages of this new approach over using a climatologicalT–Srelationship are that seasonal‐to‐interannual variability is better taken into account and that the method combines in a consistent way different sources of information such asTandSprofiles and sea surface height. The present results estimate the salinity along 165°E in the western Pacific Ocean for the 1993–1998 period and emphasize the importance of the salinity on sea level variability. The reverse problem of estimating the salinity variability along the water column from the satellite‐derived sea level and sea surface fields is also investigated. Finally, comparison with in situ salinity observations demonstrates the possibility of extracting useful information about the salinity variability from the TOPEX/Poseidon alti

ISSN:0148-0227    

DOI:10.1029/1999JC900279

年代:2000

数据来源: WILEY

摘要:

As a result of its remarkably high salinity and despite its small volume input, remnants of Red Sea Water (RSW) have been identified in the Agulhas Current, over 6000 km distant of their source. This provided the motivation to investigate the long‐term mean spreading of RSW throughout the Indian Ocean, using a comprehensive set of observations, taken from the National Oceanographic Data Center archives and from the World Ocean Circulation Experiment Hydrographic Program for the Indian Ocean. After emerging from the Gulf of Aden into the Arabian Sea, RSW spreads predominantly southwestward along the African continental slope, as indicated by strongly inclined isohalines across the Arabian Sea. There is some monsoon variability, so that during the winter monsoon there is more RSW present in the Gulf of Aden and an intensification of southward spreading along the western boundary, between 12° and 5°N. Elsewhere the intermediate depth salinity field of the Indian Ocean appears relatively stationary. Between 5° and 10°S, in the region of the South Equatorial Current, isohalines of the RSW layer become quasi‐zonal across the width of the Indian Ocean, only dipping southward toward the western boundary west of 50°E. South of here, there is a strong tongue of RSW spreading southward through the Mozambique Channel and into the Agulhas Current. These conclusions concur with previous localized investigations of intermediate water properties. Using a simple mixing model, the percentage of RSW throughout the Indian Ocean was quantified. It was found that the flux of salt into the Gulf of Aden from the Red Sea is similar to that estimated to cross 32°S in the Agulhas Current. This result implies that all the RSW which is mixed into the interior of the Indian Ocean may eventually be exported at the western boundary. Furthermore, it implies that RSW is the dominant component of the salt budget for the intermediate layer and that input from the Indonesian Seas and via diapycnic processes

ISSN:0148-0227    

DOI:10.1029/1999JC900306

年代:2000

数据来源: WILEY

摘要:

Current measurements using a shipboard acoustic Doppler current profiler (ADCP) and two satellite‐tracked buoys, together with conductivity‐temperature‐depth (CTD) casts, were carried out over the continental shelf in the southern East China Sea in July 1995. Two transects for ADCP measurements were defined and divided into segments of ∼56 km. In order to remove diurnal and semidiurnal tidal flows from observed flows, four round‐trip ADCP surveys were conducted along each segment over diurnal tidal periods of ∼24 hours and 50 min. The outflow through the Taiwan Strait, i.e., the Taiwan Current, with velocities of 35–40 cm s−1, was clearly observed to flow northeastward along the coast of China. The volume transport was estimated as at least 1 Sv. The Taiwan Current probably increased width and decreased velocity in the central East China Sea. Bifurcation into two branches of the Taiwan Current shown by previous studies was not observed. Another current entering onto the continental shelf northeast of Taiwan, i.e., the Kuroshio Branch, was found with velocities>15 cm s−1in the surface, and it helped to make an anticyclonic eddy. The main portion of the Kuroshio Branch flowed northeastward along the 100 m isobath, with a volume transport of ∼0.3 Sv. The Taiwan Current was clearly separated from the Kuroshio Branch by the area of very weak or southw

ISSN:0148-0227    

DOI:10.1029/1999JC900309

年代:2000

数据来源: WILEY

摘要:

Near‐surface drifters were deployed in the California Current System during the period 1993–1994 in order to study mean and eddy currents. The deployment scheme allowed a 2 year period where data density was high enough to make meaningful estimates of eddy statistics and of seasonal mean currents and variances on a 5° grid. Eddy variability, which tended to be isotropic, was higher near the coast (standard deviations around 0.2 m s−1) and decreased offshore (down to ∼0.1 m s−1). Seasonal mean surface flow west of California was southward during all seasons: if a Davidson Current existed during 1993–1995, it was probably either short‐lived or only found closer to the coast than our study area, i.e., within 100–150 km of shore. In some cases, drifter populations within a discrete feature were high enough to allow some detailed resolution of its behavior. We note a tendency for individual eddies (which can last for up to 3–5 months) to drift westward at rates of O(5 km day−1). Not all eddies in the region are surface‐intensified. There is clear evidence for at least the occasional presence of a thin, meandering southward jet in the region that can be 1000 km or more long. This jet can be identified with that found in the Coastal Transition Zone program in association with cool filaments. The jet appears to become broader and slower south of Point Conception. We hypothesize (on the basis of others' results as well as ours) that the eddies that are strongest at the surface result primarily from instabilities on a southward alongshore mean current and that eddies that are maximal at depth are spawned by the

ISSN:0148-0227    

DOI:10.1029/1999JC900327

年代:2000

数据来源: WILEY

摘要:

This article discusses the results of an experimental survey on the performance of a select group of open boundary conditions (OBCs) for three‐dimensional, primitive equation models. The focus is on passive conditions, where the model response at the open boundaries is dictated by the interior dynamics. The performance of the OBCs is tested in a series of numerical experiments conducted in a rotating channel which includes variable bottom topography and density stratification. The experiments are selected to enhance nonlinear phenomena that comprises advection and the propagation of dispersive waves. The first two experiments study coastal upwelling with constant and time‐varying wind forcing. The third experiment analyses the barotropic and baroclinic response of a coastal ocean due to the passage of a traveling storm. The open boundary condition with the best overall performance is the flow relaxation scheme on sea surface elevation and transport, a radiation condition for internal velocities, and a combined scheme of flow advection and relaxation for the temperature field. A modified gravity wave radiation scheme provides reasonable responses when combined with radiation conditions for internal velocities and an advection equation for temperature if the forcing is not changing direction rapidly at the open boundary. Schemes using the method of characteristics or traditional wave advection schemes fail when combined with radiation conditions for the baroclinic m

ISSN:0148-0227    

DOI:10.1029/1999JC900317

年代:2000

数据来源: WILEY

摘要:

Here we report on two field experiments from Lake Alpnach (surface area: 4.8 km2; maximum depth: 34 m) that were designed to study the process of boundary mixing and to estimate its efficiency, the ratio between the turbulent kinetic energy converted into potential energy and dissipated into heat, for diapycnal tracer transport. Lake Alpnach is particularly suited for this purpose because it is known from earlier experiments that (1) its currents follow a regular oscillatory pattern, associated with basin‐wide standing internal waves (seiches, period ∼0.5 days and 1 day, respectively), and (2) diapycnal tracer transport is mainly caused by boundary mixing. During one of the experiments reported here, strong seiches were excited regularly and damped on a timescale of the order of 3 days; during the other experiment seiching motion was comparably weak. If seiching is excited regularly, we find a persistent well‐mixed bottom layer of 4–5 m height at the deepest part of the lake. In the absence of regular seiching the layer disappears within 10–20 days. On the sloping bottom boundaries the well‐mixed layers (1) are of much more transient nature, (2) exhibit different thermal structure, and (3) decrease in thickness toward shallower depth inversely proportionally to the stability at the same depth in the lake interior. Their reduced thickness is possibly the result of repeatedly occurring intrusions of boundary mixed water masses that are observed to extend horizontally ∼100–200 m into the lake interior. As a consequence of repeated generation of intrusions, mixing on the sloping boundaries is expected to be considerably more efficient compared to mixing over flat bottom boundaries. The observed mixing efficiency, the ratio of the rate of change of potential energy below depthzcaused by turbulence to the energy loss by bottom friction below depthz, increases indeed from 0.01±0.01 in the deepest well‐mixed layers to 0.15±0.04 in t

ISSN:0148-0227    

DOI:10.1029/1999JC900303

年代:2000

数据来源: WILEY