Showing papers by "Quanan Zheng published in 2004"

Wave-induced mixing in the upper ocean: Distribution and application to a global ocean circulation model

Abstract: [1] From the Reynolds stress expression, the wave-induced vertical viscosity (or diffusivity) Bv is defined, which can be used as a parameter to estimate the strength of wave-induced mixing. In addition, a parameter D5 is introduced to represent a wave-induced mixing penetration depth. The global distribution of Bv averaged over the upper 20 m is calculated and its latitudinal transects in boreal summer and winter is discussed. The results show that in summer the wave-induced mixing is strong in the southern oceans south of 30°S, and in winter it is strong in the north Pacific and the north Atlantic north of 30°N, as well as in the southern oceans south of 40°S. Adding Bv to the vertical diffusivity in a global ocean circulation model yields a temperature structure in the upper 100 m that is closer to the observed climatology than a model without the wave-induced mixing.

Remote sensing evidence for baroclinic tide origin of internal solitary waves in the northeastern South China Sea

Abstract: [1] Evidence for baroclinic tide origin of internal solitary waves (ISWs) in the northeastern South China Sea is presented, based on 116 internal wave packets observed in satellite images from 1995 to 2001. These wave packets can be divided into two types, a single-wave ISW packet containing only one ISW with/without an oscillating tail, and a multiple-wave ISW packet composed of a group of rank-ordered ISWs. All of the 22 single-wave ISW packets occur in the deep water zone. It is suggested that the ISWs, instead of being generated by the lee-wave mechanism, are developed by nonlinear steepening of the baroclinic tides, which are produced by the strong tidal currents flowing over a ridge in Luzon Strait. This suggestion is verified by an ERS-2 SAR image, which records such an evolution process from a baroclinic tide to a single ISW in its spatial domain.

Observation of the Kuroshio intrusion region in the South China Sea from AVHRR data

Abstract: The variability of sea surface temperature in the region of the Kuroshio intrusion into the South China Sea (SCS) through the Luzon Strait was studied using sea surface temperature (SST) derived from Advanced Very High Resolution Radiometer (AVHRR) from 1985 to 2002. The covariance empirical orthogonal function (CEOF) method was applied for analysing the temporal and spatial variability in the study area. The results show that the Kuroshio intrusion during El Nino periods is weaker than that in La Nina periods. The calculation of surface layer heat of the Kuroshio intrusion region also shows response to the El Nino-La Nina events. The variation is attributed to the changes in wind fields during those events.

SeaWiFs observations of upwelling south of Madagascar: long-term variability and interaction with East Madagascar Current

Abstract: Sea-viewing Wide Field-of-view Sensor data from September 1997 to November 2001 were used to observe upwelling south of Madagascar. From Empirical Orthogonal Function analysis, we found that the upwelling is enhanced in austral winter each year except 1999 and in austral summer each year except 2001. Current and wind stress are the mechanisms to cause the upwelling. The upwelling borders on the East Madagascar Current in the east and south. The movement and deformation of the boundary between the two contain the information of their interaction. Mechanism of the interaction can be explained by the shear wave propagation theory.

Satellite synthetic aperture radar detection of Delaware Bay plumes: Jet-like feature analysis

Abstract: current structure, sech 2 h, constitutes a determining factor for the radar image. The modeled image intensity depends on the axial velocity, decreasing as x � 1 along the jet axis, and is sensitive to the wind direction with respect to the jet axis. The model was used to interpret synthetic aperture radar (SAR) images of Delaware Bay plumes taken by the RADARSAT-1 (Canadian Radar Satellite) and ERS-2 (European Remote Sensing satellite) satellites during a period from summer 1996 to spring 1997. In all seasons the low-salinity plumes appear as relatively bright, jet-like patterns on SAR images. From a morphological interpretation of the image features, both summer and winter plumes can be divided into three sections along the axis: source, jet, and dispersion region. Along the transverse direction, SAR image interpretations indicate that the plumes have a twin jet structure, which conforms to decomposition of field measurements. In summer the typical axial velocity is estimated at 6 � 10 � 1 ms � 1 with a Reynolds number of 18. At about 5 km downstream from the source the plume behaves like a turbulent jet, and beyond that range down to about 10 km downstream, it behaves like a laminar jet. In winter the plumes become weaker than in summer. The typical axial velocity is estimated at 4 � 10 � 1 ms � 1 with a Reynolds number of 3. The jet behaves like the laminar jet off the source down to about 4 km downstream. In both summer and winter cases the SAR images were taken at maximum flood tide, and the plumes appeared as jets. In the spring case the SAR image was taken at early flood tide; the plume appeared as an integrated body with relatively uniform bright tunes. In all cases the plume disperses within about 25 km downstream. INDEX TERMS: 4235 Oceanography: General: Estuarine processes; 4528 Oceanography: Physical: Fronts and jets; 4546 Oceanography: Physical: Nearshore processes; 4568 Oceanography: Physical: Turbulence, diffusion, and mixing processes; KEYWORDS: Delaware Bay, estuarine plume, radar image

A New Method for Estimation of the Sensible Heat Flux under Unstable Conditions Using Satellite Vector Winds

Abstract: It has been difficult to estimate the sensible heat flux at the air‐sea interface using satellite data because of the difficulty in remotely observing the sea level air temperature. In this study, a new method is developed for estimating the sensible heat flux using satellite observations under unstable conditions. The basic idea of the method is that the air‐sea temperature difference is related to the atmospheric convergence. Employed data include the wind convergence, sea level humidity, and sea surface temperature. These parameters can be derived from the satellite wind vectors, Special Sensor Microwave Imager (SSM/I) precipitable water, and Advanced Very High Resolution Radiometer (AVHRR) observations, respectively. The authors selected a region east of Japan as the test area where the atmospheric convergence appears all year. Comparison between the heat fluxes derived from the satellite data and from the National Centers for Environmental Prediction (NCEP) data suggests that the rms difference between the two kinds of sensible heat fluxes has low values in the sea area east of Japan with a minimum of 10.0 W m22. The time series of the two kinds of sensible heat fluxes at 10 locations in the area are in agreement, with rms difference ranging between 10.0 and 14.1 W m22 and correlation coefficient being higher than 0.7. In addition, the National Aeronautics and Space Administration (NASA) Goddard SatelliteBased Surface Turbulent Flux (GSSTF) was used for a further comparison. The low-rms region with high correlation coefficient (.0.7) was also found in the region east of Japan with a minimum of 12.2 W m22. Considering the nonlinearity in calculation of the sensible monthly means, the authors believe that the comparison with GSSTF is consistent with that with NCEP data.

Technical note: Evidence of the coexistence of upstream and downstream solitary wavetrains in the real atmosphere

Abstract: From a true colour image of the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) onboard the Orbview-2 satellite, we observed two packets of orderly wave clouds on two sides of Hainan Island in the South China Sea. A packet of 23 wave clouds stretches southward from the island. A second packet of more than 20 wave clouds stretches north-eastward off the north-east coast of the island. The concave orientation of the wave cloud lines implies that both packets are propagating away from the island. A chart of geopotential height and velocity at 850 mbar shows a south-westerly air flow over the island; hence the two wave cloud packets propagate upstream and downstream, simulta- neously. Thus, we have found new evidence of the coexistence of both upstream and downstream solitary wavetrains generated in the real atmosphere by land topographic disturbances. Comparison with theoretical results supports this conclusion.