Tangdong Qu

Upper-Layer Circulation in the South China Sea*

Abstract Upper-layer circulation is investigated by using all available historical temperature profiles combined with climatological temperature–salinity relationships in the South China Sea. Two cyclonic eddies are revealed: one is located east of Vietnam (called the East Vietnam eddy) and the other is off northwest Luzon (called the West Luzon eddy). Both local Ekman pumping and remotely forced basin-scale circulation are important mechanisms controlling these two eddies. The Luzon Strait transport (relative to 400 db) is estimated to be of the order 3.0 Sv (1 Sv = 1 × 106 m3 s−1) in the mean, and has a seasonal cycle dominated by the annual signal, with a maximum (5.3 Sv) in January–February and a minimum (0.2 Sv) in June–July. Pressure gradients are also examined to explore the dynamics of the intrusion of waters from the Pacific into the South China Sea through the Luzon Strait.

Intrusion of the North Pacific waters into the South China Sea

Water mass distribution was studied by analyzing historical hydrographic data in the South China Sea. Despite considerable modification of characteristics as a result of mixing, waters of both salinity maximum and minimum of the North Pacific origin were traced on the density surfaces around 25.0 and 26.73 σθ, respectively. In the salinity maximum layer, property distribution suggests an intrusion into the South China Sea all year-round through the Luzon Strait. The seasonal variation of the intrusion contains a pronounced semiannual signal, with greater strength in winter and summer than in spring and fall. From spring to fall, the intrusion water from the Pacific is narrowly confined in the continental slope south of China; only in winter, when the northeast monsoon becomes fully developed, can it spread in the southern South China Sea. In the salinity minimum layer, water enters the South China Sea only in spring, when the intrusion in the salinity maximum layer is weakest. A combined use of the “island rule” with climatological data suggests a mean Luzon Strait transport of the order 4 Sv (1 Sv = 106 m3 s−1).

Can Luzon Strait transport play a role in conveying the impact of ENSO to the South China sea

The Luzon Strait transport (LST) from the Pacific into the South China Sea (SCS) is examined using results from a high-resolution ocean general circulation model. The LST from the model has a mean value of 2.4 Sv (Sv ( 106 m3 s 21) and reaches its seasonal maximum (6.1 Sv westward) in winter and seasonal minimum (0.9 Sv eastward) in summer. Both the annual mean and seasonal variation of LST compare favorably with earlier observations. On an interannual time scale, LST tends to be higher during El Nino years and lower during La Nina years, with its maximum (minimum) leading the mature phase of El Nino (La Nina) by 1 month. The interannual variation of LST appears to be oppositely phased with the Kuroshio transport east of Luzon, indicating a possible nonlinear hysteresis of the Kuroshio as a driving mechanism of LST. For the annual average, water leaving the SCS in the south is of higher temperature than that with LST, thus producing a cooling advection in the upper 405 m equivalent to a surface heat flux of 2 19 Wm 22. Most of this cooling advection is balanced by the atmospheric heating (17 W m22). From late spring to early fall, surface heat flux is the primary heating process; only a small part of the heat content change can be explained by heat advection. But, in winter, heat advection seems to be the only important process responsible for the cooling in the upper layer of the SCS. The interannual variation of the upper-layer heat content has a strong signature of ENSO, cooling in the development of El Nino and warming in the development of La Nina. An oceanic connection is revealed, in which LST seems to be a key process conveying the impact of the Pacific ENSO into the SCS.

The Bifurcation of the North Equatorial Current in the Pacific

Abstract A new climatology using historical temperature and salinity data in the western Pacific is constructed to examine the bifurcation of the North Equatorial Current (NEC). Integrating dynamically calculated circulation from the sea surface to 1000 m and combining it with surface Ekman transport, it is shown that the bifurcation of the NEC occurs at the southernmost position (14.8°N) in July and the northernmost position (about 17.2°N) in December. This annual signal lags behind the seasonal meridional migration of the zero zonally integrated wind stress curl line by 4–5 months but corresponds pretty well with the local Ekman pumping associated with the Asian monsoon winds. The bifurcation latitude of the NEC is depth dependent. On the annual average, it shifts from about 13.3°N near the surface to north of 20°N at depths around 1000 m. There is a time lag of 1–2 months from the sea surface to the subsurface (300–700 m) for the annual cycle. Below 700 m, the bifurcation of the NEC approaches as far n...

Climate fluctuations of tropical coupled systems : The role of ocean dynamics

The tropical oceans have long been recognized as the most important region for large-scale ocean–atmosphere interactions, giving rise to coupled climate variations on several time scales. During the Tropical Ocean Global Atmosphere (TOGA) decade, the focus of much tropical ocean research was on understanding El Nino–related processes and on development of tropical ocean models capable of simulating and predicting El Nino. These studies led to an appreciation of the vital role the ocean plays in providing the memory for predicting El Nino and thus making seasonal climate prediction feasible. With the end of TOGA and the beginning of Climate Variability and Prediction (CLIVAR), the scope of climate variability and predictability studies has expanded from the tropical Pacific and ENSO-centric basis to the global domain. In this paper the progress that has been made in tropical ocean climate studies during the early years of CLIVAR is discussed. The discussion is divided geographically into three tropical ocean basins with an emphasis on the dynamical processes that are most relevant to the coupling between the atmosphere and oceans. For the tropical Pacific, the continuing effort to improve understanding of large- and small-scale dynamics for the purpose of extending the skill of ENSO prediction is assessed. This paper then goes beyond the time and space scales of El Nino and discusses recent research activities on the fundamental issue of the processes maintaining the tropical thermocline. This includes the study of subtropical cells (STCs) and ventilated thermocline processes, which are potentially important to the understanding of the low-frequency modulation of El Nino. For the tropical Atlantic, the dominant oceanic processes that interact with regional atmospheric feedbacks are examined as well as the remote influence from both the Pacific El Nino and extratropical climate fluctuations giving rise to multiple patterns of variability distinguished by season and location. The potential impact of Atlantic thermohaline circulation on tropical Atlantic variability (TAV) is also discussed. For the tropical Indian Ocean, local and remote mechanisms governing low-frequency sea surface temperature variations are examined. After reviewing the recent rapid progress in the understanding of coupled dynamics in the region, this study focuses on the active role of ocean dynamics in a seasonally locked east–west internal mode of variability, known as the Indian Ocean dipole (IOD). Influences of the IOD on climatic conditions in Asia, Australia, East Africa, and Europe are discussed. While the attempt throughout is to give a comprehensive overview of what is known about the role of the tropical oceans in climate, the fact of the matter is that much remains to be understood and explained. The complex nature of the tropical coupled phenomena and the interaction among them argue strongly for coordinated and sustained observations, as well as additional careful modeling investigations in order to further advance the current understanding of the role of tropical oceans in climate.

Observation of Luzon Strait transport

Using recently collected current and hydrographic data, we provide a high resolution picture of the subinertial flow and estimate the volume transport through the Luzon Strait. The distribution of the subinertial flow shows a strong westward flow around 100 m in the northern part of the Luzon Strait, while the eastward flow is confined to the deeper layers, mostly at depths around 1000 m. The total volume transport is estimated to be 6 +/- 3 Sv during the period of observations from October 4 to 16, 2005. The observations also confirm that the Luzon Strait transport has a sandwiched vertical structure. The net westward volume transport in the deep (> 1500 m) layer of the Luzon Strait reaches 2 Sv.

On the western boundary currents in the Philippine Sea

In this study we describe the velocity structure and transport of the North Equatorial Current (NEC), the Kuroshio, and the Mindanao Current (MC) using repeated hydrographic sections near the Philippine coast. A most striking feature of the current system in the region is the undercurrent structure below the surface flow. Both the Luzon Undercurrent and the Mindanao Undercurrent appear to be permanent phenomena. The present data set also provides an estimate of the mean circulation diagram (relative to 1500 dbar) that involves a NEC transport of 41 Sverdrups (Sv), a Kuroshio transport of 14 Sv, and a MC transport of 27 Sv, inducing a mass balance better than 1 Sv within the region enclosed by stations. The circulation diagram is insensitive to vertical displacements of the reference level within the depth range between 1500 and 2500 dbar. Transport fluctuations are, in general, consistent with earlier observations; that is, the NEC and the Kuroshio vary in the same phase with a seasonal signal superimposed with interannual variations, and the transport of the MC is dominated by a quasi-biennial oscillation. Dynamic height distributions are also examined to explore the dynamics of the current system.

A Climatology of the Circulation and Water Mass Distribution near the Philippine Coast

Abstract This study provides a climatology of the circulation and water mass distribution by using historical data combined with observations from dozens of recent cruises near the Philippine coast. The most striking results are related to the poleward contraction of the subtropical gyre on denser surfaces, with the bifurcation of the North Equatorial Current moving from about 15°N in the upper thermocline to about 20°N at intermediate depths. Though time variability and the possible errors in the data are rather large, the Halmahera eddy (HE) is clearly seen in the climatic mean fields, lying at about 3°N, 130°E near the surface and reaching the Mindanao coast on density surfaces around 27.2σθ. It seems that the previously observed Mindanao Undercurrent is merely a component of the recirculation associated with the HE. North Pacific Tropical Water (NPTW) and Intermediate Water (NPIW) enter the western ocean with their extreme properties centered at 15° and 20°N, respectively, and continue southward as fa...

Connecting the tropical Pacific with Indian Ocean through South China Sea

Analysis of wind data over the past 40 years and results from a high-resolution general circulation model has revealed the existence of a previously undescribed circulation that connects the tropical Pacific with Indian Ocean. As a direct response to the Pacific wind, water of the Pacific origin enters the South China Sea through Luzon Strait, and from there part of the water continues southward into the Java Sea and returns to the Pacific through Makassar Strait. This circulation contains a strong signal of El Nino and Southern Oscillation and appears to have a notable impact on the Indonesian Throughflow heat transport.

Seasonal Variability of the Large-Scale Currents near the Coast of the Philippines*

Abstract The mean seasonal cycle of the western boundary currents in the tropical North Pacific Ocean is studied diagnostically by combining atmospheric climatologies with drifter, satellite altimetry, and hydrographic data in the framework of a simplified numerical model incorporating geostrophy, hydrostatics, continuity, and tracer conservation. The approach enables the authors to diagnose the absolute 3D velocity field and to assess the seasonal cycle of sea surface height (SSH)/total transports. Errors are estimated by considering multiple datasets and averaging over the results of the corresponding diagnostic computations. Analysis shows that bifurcation of the North Equatorial Current (NEC) occurs at 14.3° ± 0.7°N near the Philippine coast. Meridional migration of the NEC bifurcation latitude is accompanied by quantitative changes in the partitioning of the NEC transport between the Kuroshio and Mindanao Current. In February–July, when the NEC transport is 58 ± 3 Sv (Sv ≡ 106 m3 s−1), the Kuroshio t...