Wu Dexing

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  • Article
    Explaining the global distribution of peak-spectrum variability of sea surface height
    (American Geophysical Union, 2008-07-19) Lin, Xiaopei ; Yang, Jiayan ; Wu, Dexing ; Zhai, Ping
    A 14-year satellite observation of sea surface height (SSH) reveals an interesting pattern. Along any latitude, there is a frequency at which the SSH power spectrum peaks, regardless of which hemisphere or oceanic basin. This peak-spectrum frequency is nearly identical to the critical frequency at which the zonal energy propagation of Rossby waves becomes stagnant. The interior ocean adjusts to atmospheric forcing by radiating energy away through Rossby waves. There are two distinct groups of Rossby waves, long ones carry the energy to the west while short ones send the energy to the east. At the critical frequency, these two waves merge and their zonal energy propagation becomes stagnant. Consequently, the energy from atmospheric forcing may accumulate in the ocean interior, and thus result in a spectrum peak.
  • Article
    On the dynamics of the seasonal variation in the South China Sea throughflow transport
    (John Wiley & Sons, 2013-12-16) Yang, Jiayan ; Lin, Xiaopei ; Wu, Dexing
    The Luzon Strait transport (LST) of water mass from the Pacific Ocean to the South China Sea (SCS) varies significantly with seasons. The mechanisms for this large variability are still not well understood. The steady-state island rule, which is derived from a steady-state model, is not applicable to seasonal time scale variations in a large basin like the Pacific Ocean. In this paper, we will use a theoretical model that is based on the circulation integral around the Philippines. The model relates the LST variability to changes in the boundary currents along the east coast of the Philippines, including the North Equatorial Current (NEC) Bifurcation Latitude (NECBL), the transports of Kuroshio and Mindanao Currents (KC and MC), and to the local wind-stress forcing. Our result shows that a northward shift of the NECBL, a weakening of the KC or a strengthening of the MC would enhance the LST into the SCS. This relationship between the LST and the NEC-KC-MC is consistent with observations. The analytical result is tested by a set of idealized numerical simulations.
  • Article
    An open-ocean forcing in the East China and Yellow seas
    (American Geophysical Union, 2010-12-21) Ma, Chao ; Wu, Dexing ; Lin, Xiaopei ; Yang, Jiayan ; Ju, Xia
    Recent studies have demonstrated that the annual mean barotropic currents over the East China and Yellow seas (ECYS) are forced primarily by the oceanic circulation in the open-ocean basin through the Kuroshio Current (KC), the western boundary current of the subtropical gyre in the North Pacific Ocean. The local wind stress forcing plays an important but secondary role. Those previous results were mainly qualitative and from a simple barotropic model forced by a steady wind stress field. They remain to be tested in a more complete 3-D model with both wind stress and buoyancy fluxes. In addition, the seasonal variability of major ECYS currents may involve different forcing mechanisms than their annually averaged fields do, and this can only be addressed when a seasonally varying forcing is used in the model. In this paper, we will address these issues by using a 3-D baroclinic model. Our results confirm the finding from the previous studies that the KC is the primary forcing mechanism for major annually mean currents in the ECYS, which include the Taiwan Strait Current, the Tsushima Warm Current, and the Yellow Sea Warm Current (YSWC), etc. However, the local monsoonal forcing plays a prominent role in modulating the seasonal variability of all major currents in the region. A deep northwestward intrusion of the YSWC in winter, for instance, is mainly due to a robustly developed China Coastal Current and Korea Coastal Current, which draw water along the Yellow Sea Trough to feed the southward flows along the west and east coasts of the Yellow Sea.
  • Article
    On the mechanism of the cyclonic circulation in the Gulf of Tonkin in the summer
    (American Geophysical Union, 2008-09-18) Wu, Dexing ; Wang, Yue ; Lin, Xiaopei ; Yang, Jiayan
    The circulation in the Gulf of Tonkin had been traditionally considered to be anticyclonic in the summer. This view was challenged recently by results from reanalyzing observational data, which clearly revealed that the circulation is cyclonic in all seasons. The surface wind stress is monsoonal, southwesterly in the summer and reversed in the winter. It remains unexplained why the circulation is always cyclonic, while the surface forcing reverses seasonally. In this study, we hypothesize that the inflow through Qiongzhou Strait, a shallow and narrow channel between Hainan Island and the Chinese mainland, is responsible for maintaining the cyclonic circulation in the summer. Besides the requirements of mass conservation and bathymetry constraint, this flow, even with a rather small transport, carries a considerable amount of potential vorticity (PV) into the gulf, and the integral constraint of PV requires the presence of a frictional torque to be associated with a cyclonic circulation. Several numerical experiments with a three-dimensional model have been conducted to test this hypothesis. When the westward flow through Qiongzhou Strait is blocked, the model simulates an anticyclonic circulation in the summer. When the westward flow through Qiongzhou Strait is allowed, the circulation changes to a cyclonic one, consistent with our hypothesis.
  • Preprint
    The Kuroshio Extension : a leading mechanism for the seasonal sea-level variability along the west coast of Japan
    ( 2009-10-08) Ma, Chao ; Yang, Jiayan ; Wu, Dexing ; Lin, Xiaopei
    Sea level changes coherently along the two coasts of Japan on the seasonal time scale. AVISO satellite altimetry data and OFES (OGCM for the Earth Simulator) results indicate that the variation propagates clockwise from Japan's east coast through the Tsushima Strait into the Japan/East Sea (JES) and then northward along the west coast. In this study, we hypothesize and test numerically that the sea level variability along the west coast of Japan is remotely forced by the Kuroshio Extension (KE) off the east coast. Topographic Rossby waves and boundary Kelvin waves facilitate the connection. Our 3-d POM model when forced by observed wind stress reproduces well the seasonal changes in the vicinity of JES. Two additional experiments were conducted to examine the relative roles of remote forcing and local forcing. The sea level variability inside the JES was dramatically reduced when the Tsushima Strait is blocked in one experiment. The removal of the local forcing, in another experiment, has little effect on the JES variability. Both experiments support our hypothesis that the open-ocean forcing, possibly through the KE variability, is the leading forcing mechanism for sea level change along the west coast of Japan.
  • Article
    On the dynamics of the South China Sea Warm Current
    (American Geophysical Union, 2008-08-05) Yang, Jiayan ; Wu, Dexing ; Lin, Xiaopei
    The South China Sea Warm Current (SCSWC) flows northeastward over the shelf and continental slope in the northern South China Sea (SCS). This current persists in its northeastward direction in all seasons despite the fact that the annually averaged wind stress is decisively southwestward against it. Two major mechanisms have been proposed in previous studies, one attributing it directly to the wind stress forcing within the SCS and the other to the Kuroshio intrusion through the Luzon Strait. In this study we use a simple model to demonstrate that neither of them is the leading forcing mechanism. Instead, the SCSWC is a source- and sink-driven flow induced by the Taiwan Strait Current (TSC), a year-round northward flow through the Taiwan Strait. The two previously suggested mechanisms are important but secondary. The model simulations show that the local wind stress alone would force a current in the opposite direction to the SCSWC. Blocking the Kuroshio intrusion through the Luzon Strait, on the other hand, only weakens the SCSWC. The SCSWC vanishes when the Taiwan Strait is closed in the model.
  • Article
    Wind-driven exchanges between two basins : some topographic and latitudinal effects
    (John Wiley & Sons, 2013-09-18) Yang, Jiayan ; Lin, Xiaopei ; Wu, Dexing
    This study examines some topographic effects on the island rule. We use an idealized and barotropic model to investigate the throughflow between a semienclosed marginal sea and a larger oceanic basin that are connected to each other by two channels. Two sets of experiments are conducted in parallel, one with a flat bottom and the other with a ridge between two basins. The model results show that the ridge affects the island rule considerably in several ways. First, the ridge blocks geostrophic contours and restricts a free exchange between two basins. The bottom pressure torque (or the form drag) is a dominant term in the balance of the depth-integrated vorticity budget and always results in a significant reduction of the throughflow transport. Second, horizontal friction promotes cross-isobathic flows and enhances the throughflow transport over the ridge. This is the opposite of what friction does in the original island rule in which a friction tends to reduce the throughflow transport. Third, the forcing region in the open ocean for the marginal-sea throughflow is shifted meridionally. Last, the topographic effect becomes small near the equator due to its dependence on f. This may explain why the PV barrier effect is smaller in the South China Sea than in the Japan/East Sea. The limitation of the barotropic model and some baroclinic effects will be discussed.