Wind relaxation as possible cause of the South China Sea Warm Current     

Shenn-Yu Chao  Ping-Tung Shaw  Joe Wang 《Journal of Oceanography》1995,51(1):111-132

Winter appearance of a northeastward warm current off the southern coast of China against gale force winds is well documented but lacks a plausible explanation. Relaxation of northeasterly winds is envisaged here as a possible cause of the South China Sea Warm Current in winter. A three-dimensional circulation model for the South China Sea is first driven to equilibrium by climatological forcings. Thereafter, wind forcing is relaxed from the 15th day of each month for 9 days. In winterlike months from December to April, the wind relaxation invariably triggers a northeastward current of which the location and alongshore span are comparable to that of the observed warm current. This current is driven by the pressure gradient along the northwestern boundary of the South China Sea, sea level being high to the southwest and low to the northeast. The sea level gradient is built up by the monsoon-driven southwestward coastal current along the northwestern boundary and, after wind relaxes, triggers a return current and a sea level drop that expand southwestward from the southern coast of China to the east coast of Vietnam. The current is initially barotropic, becoming increasingly baroclinic in time as warm waters from the south are advected northeastward. The model also suggests that the sea level gradient is present in most of the months of the year, but is not as dramatic as in winter to trigger fundamental changes in the circulation of the South China Sea.  相似文献   

Numerical Study of the Upper-Layer Circulation in the South China Sea   总被引：7，自引：0，他引：7  

Ching-Sheng Chern  Joe Wang 《Journal of Oceanography》2003,59(1):11-24

Upper-layer circulation in the South China Sea has been investigated using a three-dimensional primitive equation eddy-resolving model. The model domain covers the region from 99° to 122°E and from 3° to 23°N. The model is forced by the monthly averaged European Centre for Medium-Range Weather Forecasts (ECMWF) model winds and the climatological monthly sea surface temperature data from National Oceanographic Data Center (NODC). Inflow and outflow through the Taiwan Strait and the Sunda shelf are prescribed monthly from the Wyrtki estimates. Inflow of the Kuroshio branch current in the Luzon Strait is assumed to have a constant volume transport of 12 Sv (1 Sv = 10⁶ m³/s), and the outflow from the open boundary to the east of Taiwan is adjusted to ensure the net volume transport through all open boundaries is zero at any instant. The model reveals that a cyclonic circulation exists all year round in the northern South China Sea. During the winter time this cyclonic eddy is located off the northwest of Luzon, coinciding with the region of positive wind stress curl in this season. This cyclonic eddy moves northward in spring due to the weakening of the northeast winds. The cyclonic circulation becomes weak and stays in the continental slope region in the northern South China Sea in the summer period. The southwest wind can raise the water level along the west coast of Luzon, but there is no anticyclonic circulation in the northern South China Sea. After the onset of the northeast monsoon winds in fall, the cyclonic eddy moves back to the region off the west coast of Luzon. In the southern South China Sea and off the Vietnam coast, the model predicts a similar flow structure as in the previous related studies. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Sedimentary geology of the Columbia River Estuary     

Christopher R Sherwood  Joe S Creager 《Progress in Oceanography》1990,25(1-4)

A multiyear study of the sedimentary geology of the Columbia River Estuary has provided valuable data regarding sediment distribution, bedform distribution, and suspended sediment distribution on spatial and temporal scales that permit delineation of sedimentary environments and insight into the sedimentary processes that have shaped the estuary. In comparison to other more-intensively studied estuaries in North America, the Columbia River estuary has relatively larger tidal range (maximum semidiurnal range of 3.6m) and large riverflow (6,700m³s⁻¹). Variations in riverflow, sediment supply, and tidal flow occur over a range of time scales, making the study of modern processes, as they relate to long-term effects, particularly challenging.Analyses of more than 2000 bottom-sediment grab samples indicate that the bed material of the estuary varies in a relatively narrow range between 0 and 8 phi (1.0 and 0.0039mm) with an overall mean size of 2.5 phi (0.177mm). Sediment size decreases generally in the downstream direction. Sediments from the upriver channels are coarse (1.5–2.0phi; 0.25–0.35mm) and moderately sorted; sediments in the central estuary show wider range and variation in grain size and sorting (1.75–6.0phi; 0.016–0.3mm). Sediment from the entrance region has a mean size of 2.75phi (0.149mm) and is well sorted. Seasonal changes in sediment size distributions occur and are best delineated by those samples containing more than 10% mud (silt plus clay). Sediments containing a significant fine fraction generally occur only in the peripheral bays and in channels isolated from strong currents. Thin deposits of fine sediments are occasionally found in main channels, and the ephemeral nature of these sediments suggest that they may erode and produce the silty rip-up clasts that appear intermittently in the same regions.The distribution of bedforms of various size and shape has been mapped with side-scan sonar during three seasons and at various tidal stages. The presence of bedforms with wavelengths of 6–8m and alternating slip faces about 40cm high indicates that the deeper portion of the entrance region is dominated by tidally reversing lower flow regime sediment transport. Bedforms in the upper reaches of the estuary are much larger, with heights of up to 3m and wavelengths of up to 100m. These bedforms, and the smaller, superimposed bedforms, imply downstream transport under fluvial conditions. In the central estuary, bedforms in the deep portion of the main channels are oriented upriver while those on the shallow flanks of the channels are oriented seaward. The landward limit of upriver bedform transport varies seasonally in response to riverflow fluctuations.A complex array of sedimentary environments exists in the Columbia River estuary. Each environment is influenced by the relative importance of waves, fluvial currents, and tidal currents, as modified by the presence or absence of estuarine circulation, vegetation, or human activity. The importance of these enviroments to the ecosystem of the estuary is discussed in subsequent papers in this volume.  相似文献   

Transition of Tidal Waves from the East to South China Seas over the Taiwan Strait: Influence of the Abrupt Step in the Topography   总被引：7，自引：0，他引：7  

Sen Jan  Ching-Sheng Chern  Joe Wang 《Journal of Oceanography》2002,58(6):837-850

Observations of tidal waves between the East and South China Seas (ECS and SCS) over the Taiwan Strait (TS) suggest that the diurnal tides simply appear as one southward-propagating wave from the ECS to the SCS through the TS. The semidiurnal tides, however, behave differently in that they appear as a southward-propagating Kelvin wave in the western TS and a nearly standing wave in the eastern TS, and then diminish rapidly over the shallow shoal in the southern TS. A smaller-domain model, with sea-level boundary conditions derived from a larger-domain tidal model, was first used to simulate tides in the TS to an overall percentage of accuracy of about 90%. Subsequent numerical experiments and theoretical analysis revealed that the southward-propagating semidiurnal tides to be impeded and then reflected as they arrive at an abrupt, deepened step in the topography of the southern TS. This reflection enhances the amplitudes of the incident semidiurnal tides and contributes to the formation of a nearly standing wave in the eastern TS. The southward-propagating diurnal tides in the TS are connected by the diurnal tides in the northern SCS when the amplitudes of the two tide systems are comparable and their phases nearly equal at the step. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Mapping distortion of detectors in UVIT onboard AstroSat observatory     

V Girish  Shyam N. Tandon  S. Sriram  Amit Kumar  Joe Postma 《Experimental Astronomy》2017,43(1):59-74

Ultraviolet Imaging Telescope (UVIT) is one of the payloads onboard AstroSat, India’s first multi-wavelength Astronomy mission. UVIT is primarily designed to make high resolution images in wide field, in three wavelength channels simultaneously: FUV (130–180 nm), NUV (200–300 nm) and VIS (320–550 nm). The intensified imagers used in UVIT suffer from distortions, and a correction is necessary for these to achieve good astrometry. In this article we describe the methodology and calculations used to estimate the distortions in ground calibrations.  相似文献   

Sedimentary environments of the east-central Bering Sea continental shelf     

Harley J. Knebel  Joe S. Creager 《Marine Geology》1973,15(1):25-47

A factor analysis of 180 bottom sediment samples from the east-central Bering Sea continental shelf identifies five factors that account for 95% of the variation in the 17 whole ø size classes that were used as variables. Factor I represents coarse sediments that have been bypassed in areas of active water circulation. Factors II and III represent fine and very fine sands that have been hydraulically sorted, reworked, and mixed. Factor IV represents coarse to medium silt that has been segregated from areas of relatively high energy. Factor V represents both the production of sediments finer than medium silt and deposition within the lowest-energy environment in this area.Modern and palimpsest sediments are areally prevalent over this section of the shelf. Relict sediments occur in only a few small areas. The dispersal of sediments is affected by surface and tidal currents as well as wave action. Ice rafting is not an important geological agent. Data from the eastcentral Bering Sea shelf indicate that sediments on subarctic continental shelves are not necessarily characterized by an abundance of rocky sediments or gravel.  相似文献   

Beta gyres in global analysis fields     

Sun-Hee KIM  H. Joe KWON  R. L. ELSBERRY 《大气科学进展》2009,26(5):984-994

A three-component decomposition is applied to global analysis data to show the existence of a beta gyre, which causes Tropical Cyclone (TC) to drift from a large-scale environmental steering current. Analyses from the Global Data Assimilation and Prediction System (GDAPS) of the Korea Meteorological Administration (KMA), the Global Forecast System (GFS) of NCEP, and the Navy Operational Global Atmospheric Prediction System (NOGAPS) are used in this study. The structure of the beta gyre obtained in our analyses is in good agreement with the theoretical structure, with a cyclonic circulation to the southwest of the TC center, an anticyclonic circulation to the northeast, and a ventilation flow directed northwestward near the center. The circulation of the beta gyre is strongest at the 850-hPa level where the cyclonically swirling primary circulation is strongest, and decreases with height, in a pyramid shape similar to the primary circulation. The individual structure of the beta gyre is case- and model-dependent. At a certain analysis time, one model may clearly reveal a well-defined beta gyre, but the other models may not. Within one model, the beta gyre may be well defined at some analysis times, but not at other times. The structure of the beta gyre in the analysis field is determined by the nature of the vortex initialization scheme and the model behavior during the 6-h forecast in the operational data assimilation cycle.  相似文献   

Energy Balance Closure Using Eddy Covariance Above Two Different Land Surfaces and Implications for CO<Subscript>2</Subscript> Flux Measurements     

Joe Kidston  Christian Brümmer  T. Andrew Black  Kai Morgenstern  Zoran Nesic  J. Harry McCaughey  Alan G. Barr 《Boundary-Layer Meteorology》2010,136(2):193-218

Components of the surface energy balance of a mature boreal jack pine forest and a jack pine clearcut were analysed to determine the causes of the imbalance that is commonly observed in micrometeorological measurements. At the clearcut site (HJP02), a significant portion of the imbalance was caused by: (i) the overestimation of net radiation (R _n ) due to the inclusion of the tower in the field of view of the downward facing radiometers, and (ii) the underestimation of the latent heat flux (λE) due to the damping of high frequency fluctuations in the water vapour mixing ratio by the sample tube of the closed-path infrared gas analyzer. Loss of low-frequency covariance induced by insufficient averaging time as well as systematic advection of fluxes away from the eddy-covariance (EC) tower were discounted as significant issues. Spatial and temporal distributions of the total surface-layer heat flux (T), i.e. the sum of sensible heat flux (H) and λE, were well behaved and differences between the relative magnitudes of the turbulent fluxes for several investigated energy balance closure (C) classes were observed. Therefore, it can be assumed that micrometeorological processes that affected all turbulent fluxes similarly did not cause the variation in C. Turbulent fluxes measured at the clearcut site should not be forced to close the energy balance. However, at the mature forest site (OJP), loss of low-frequency covariance contributed significantly to the systematic imbalance when a 30-min averaging time was used, but the application of averaging times that were long enough to capture all of the low-frequency covariance was inadequate to resolve all of the high-frequency covariance. Although we found qualitative similarity between T and the net ecosystem exchange (NEE) of carbon dioxide (CO₂), forcing T to closure while retaining the Bowen ratio and applying the same factor to CO₂ fluxes (F _C ) cannot be generally recommended since it remains uncertain to what extent long wavelength contributions affect the relationship between T, F _C and C.  相似文献   

High resolution remote sensing observations for missions to the Jovian system: Io as a case study   总被引：1，自引：0，他引：1  

Gregory T. Delory  Conor Laver  Joe Pitman 《Planetary and Space Science》2010,58(13):1699-1707

We present modeled images of Io at a variety of distances from the surface as a function of imager aperture size and wavelength. We consider the science objectives that could be achieved from missions engaged in long range remote-sensing of Io during the approach to the Jovian system and subsequently from orbit around Europa or Ganymede, in both the visible and near infrared wavelength ranges. We find that basic global mapping objectives in the visible can be met with a traditional 0.5 m telescope design. A more ambitious 1.5 m telescope could accomplish much more detailed objectives such as topographical measurements, and determination of flow patterns and thermal sources for individual active regions on Io.  相似文献   

Marine ecosystem management: Fish abundance and size under exploitation     

Joe Horwood 《Marine Policy》2010

This study is targeted on policy makers and environmental scientists to illustrate the typical historical scale of depletion of our fish stocks, and what current and emerging legislation might mean for fisheries management and the metrics of fish stocks. The population demography of the Bristol Channel sole is described since 1820. Their decline in abundance, and change in length compositions, are modelled. By 2000, the mature stock had been depleted to less than 5% of its original size, and larger sole were rarely caught. The implications of maximum sustainable yield targets, and of good environmental status, are examined.  相似文献