Showing papers by "Quanan Zheng published in 2006"

Winter‐spring fronts in Taiwan Strait

Abstract: [1] Fine structures of surface fronts during winter-spring in the Taiwan Strait and their evolution over time are studied using satellite and in situ data. At times, different water masses are interleaved into each other. The northward intruding warm water bifurcates into several branches, causing complicated multiple cross strait fronts with maximum surface temperature gradients at the fronts to be greater than 2°C/km. The mean surface velocity of the fronts estimated from a sequence of satellite images is between 0.15 and 0.3 m/s during the February–March 2001 frontal event. The speed appears to have accelerated during the latter stages from 0.2 m/s during March 2–4 to 0.3 m/s during March 4–6. The total northward transport associated with the passage of the fronts is estimated to be 0.6–1 Sv between February 16 and March 6, 2001. The timescale of the passage of the fronts through the entire Taiwan Strait is therefore at least four weeks. Hydrographic observations in February–March of 1998 and 2000 also demonstrate similar frontal events with strong vertical gradients of water temperature and salinity on the western side of the strait. The 1998 data also show clearly that the low temperature and low salinity Zhemin Coastal Current separates from the coast and enters into the middle of the strait, a feature consistent with the satellite observations.

Mechanism of internal waves in the Luzon Strait

Abstract: [1] This study suggests a western boundary current instability generation mechanism for the ocean internal waves. A linear model describing this instability is developed, and an analytical solution is derived from a zero-order complex frequency–wavenumber relation. The real part of the relation represents a dispersion relation of the generated internal waves, and the imaginary part is an exponential growth rate. The dispersion relation describes the wave propagation characteristics, while the growth rate represents the instability properties. The theoretical results are used for the case of the Kuroshio east of the Luzon Strait. Based on the analysis, it is found that for the westward propagating disturbance, the Kuroshio west wing is unstable and the east wing is stable; while the reverse is true for the eastern propagating disturbance. The results are used to interpret satellite SAR images of the ocean internal waves, which were generated in the Luzon Strait and propagated westward. Reasonable estimates for the growth rate and propagation velocity are derived from a scale analysis of the Kuroshio east of the Luzon Strait.

Introduction to special section: Dynamics and Circulation of the Yellow, East, and South China Seas

Abstract: . The seas aresurrounded by 12 countries: China, North Korea, SouthKorea, Japan, the Philippines, Indonesia, Brunei, Malaysia,Singapore, Thailand, Cambodia, and Vietnam, which ac-count for over 2.0 billion of the human inhabitants (mid-2005 estimate), nearly one third of the world population.These populations are impacted by all coastal manifesta-tions of global climate change, such as rising sea level andmore frequent and severe storms [Anderson et al., 2001].Since ancient times, the seas have served as a convenientnavigation waterway for the East Asian and Southeast Asiannations to communicate with each other and with nations ofthe outside world [Guo et al., 2004; Su and Yuan, 2005].Even today, the seas are among the busiest waterways in theworld because of the size and the high growth rates of theregion in the world economy and trade.[

SAR imaging and hydrodynamic analysis of ocean bottom topographic waves

Abstract: [1] The satellite synthetic aperture radar (SAR) images display wave-like patterns of the ocean bottom topographic features at the south outlet of Taiwan Strait (TS). Field measurements indicate that the most TS water body is vertically stratified. However, SAR imaging models available were developed for homogeneous waters. Hence explaining SAR imaging mechanisms of bottom features in a stratified ocean is beyond the scope of those models. In order to explore these mechanisms and to determine the quantitative relations between the SAR imagery and the bottom features, a two-dimensional, three-layer ocean model with sinusoidal bottom topographic features is developed. Analytical solutions and inferences of the momentum equations of the ocean model lead to the following conditions. (1) In the lower layer, the topography-induced waves (topographic waves hereafter) exist in the form of stationary waves, which satisfy a lower boundary resonance condition σ = kC0, here σ is an angular frequency of the stationary waves, k is a wavenumber of bottom topographic corrugation, and C0 is a background current speed. (2) As internal waves, the topographic waves may propagate vertically to the upper layer with an unchanged wavenumber k, if a frequency relation N3 < σ < N2 is satisfied, here N2 and N3 are the Brunt-Waisala frequencies of middle layer and upper layer, respectively. (3) The topographic waves are extremely amplified if an upper layer resonance condition is satisfied. The SAR image of topographic waves is derived on the basis of current-modulated small wave spectra. The results indicate that the topographic waves on SAR images have the same wavelength of bottom topographic corrugation, and the imagery brightness peaks are either inphase or antiphase with respect to the topographic corrugation, depending on a sign of a coupling factor. These theoretical predictions are verified by field observations. The results of this study provide a physical basis for quantitative interpretation of SAR images of bottom topographic waves in the stratified ocean.

Satellite chlorophyll retrievals with a bipartite artificial neural network model

Abstract: An artificial neural network (ANN) model with a bipartite classification scheme is developed to retrieve the chlorophyll‐a concentration (Chl) from sea‐viewing wide field‐of‐view sensor (SeaWiFS) data. Bio‐optical data derived from the SeaWiFS bio‐optical algorithm mini‐workshop (SeaBAM) are used to verify this bipartite artificial neural network (BANN) model. In comparison with SeaWiFS operational algorithms and a general ANN model, the BANN model significantly increases the accuracy of Chl retrieval not only on a log scale but also on a normal scale. The BANN model can significantly improve the accuracy of Chl especially in the high Chl region. The model also performs well in a test with in situ measurements from Taiwan coastal waters. The biases induced by errors in atmospheric correction are also reduced in the coastal water case.

Satellite-derived chlorophyll-a concentration in the Taiwan Strait

Abstract: A series of the Orbview-2/SeaWiFS (Sea-viewing Wide Field-of-view Sensor) images during the period from 1997 to 2003 is used to understand the spatial and temporal distribution of the chlorophyll-a concentration (Chl-a) in the Taiwan Strait (TS) It is found that the area with higher Chl-a is mainly along the western TS; it extends more offshore in cold seasons The lowest Chl-a is always inside the deep Pe ng-Hu Channel, it can spread fu rther northward in summer From mode 1 results of the Empirical Orthogonal Function (E OF) analysis, we find the Chl- a in La Nina years (during the period from June 1998 to May 2001) showing greater variation than the other El Nino or normal years The EOF1 results also indicate the highest Chl-a always in fall Me anwhile, the peak in the 1997/1998 El Nino fall was the lowest maximum, while the lowest Chl-a is mainly in winter, but its interannual variation is not so clear Keywords: Taiwan Strait, OrbView-2/SeaWiFS chlorophyll-a concentration, Empirical Orthogonal Function Analysis, El Nino/La Nina

Detecting Internal Waves From Satellite Ocean Color Imagery

Abstract: Internal waves have been observed by lots of high resolution satellite images, such as Synthetic Aperture Radar (SAR) and optical images of SPOT and Landsat. These images are usually expensive. In this study, some free but lower spatial resolution satellite images are applied to observe the internal wave phenomena. The internal waves in the Sulu Sea are detected from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) onboard the Orbview-2 satellite. The SeaWiFS image has a spatial resolution of 1.1 km. It is acceptable to observe the internal wave phenomena while the soliton width is larger than the image resolution. The results show that the internal solitary in the Sulu Sea can be observed successfully with SeaWiFS chlorophyll images. The internal waves in the Sulu Sea have amplitudes of 10 to 90 m and wavelengths of 5 to 16 km. The large-amplitude internal solitary waves may significantly influence the near-surface chlorophyll concentration. The chlorophyll concentration would be lower when the depression internal waves passed through. A theoretic model is proposed and tested to estimate the amplitudes of internal waves from chlorophyll concentration images.© 2006 ASME