Wendong Fang

Seasonal structures of upper layer circulation in the southern South China Sea from in situ observations

[1] Conductivity-temperature-depth and acoustic Doppler current profiler data from six cruises spanning 1989 to 1999 are used to investigate the seasonal structure of the upper layer circulation in the southern South China Sea (SCS). The surveys were made during winter, late spring, summer, and late fall. More detailed structures not presented in previous studies are found. In summer the upper layer circulation of the southern SCS is dominated by an anticyclonic gyre with a strong eastward flow on its northern border, which originates off the southeast coast of Vietnam: the Summer Southeast Vietnam Offshore Current. In winter a stronger cyclonic gyre exists in the western portion of the southern SCS and a weaker anticyclonic circulation in the eastern portion. At the juncture of these two gyres, there is a strong northward upwind flow, called the Winter Natuna Off-Shelf Current. In late spring the anticyclonic gyre begins to form in the northwest, and a trace of the Natuna Off-Shelf Current can still be observed, especially subsurface in the southern part of the study area. Mesoscale eddies are active in this period. In late fall the winter-type pattern begins to appear. The Natuna Off-Shelf Current and the cyclonic gyre in the northwest part of the southern SCS emerge, and the summer anticyclonic gyre vanishes. In the southeast the currents are basically toward the northeast with an anticyclonic trend, revealing a multi-eddy feature. These upper ocean currents appear consistent with the wind forcing.

A review on the South China Sea western boundary current

The advances in understanding the South China Sea (SCS) western boundary current (SCSwbc) have been reviewed since the works of Dale (1956) and Wyrtki (1961) in the middle of the 20th century. The features of the pattern of SCSwbc and the oceanic phenomena associated with it are focused on. The current is driven mainly by monsoon over the SCS and partially by winds over the tropical Pacific governed by the island rule. The SCSwbc exhibits strong seasonal variation in its direction and patterns. In winter, the current is strong and flows southwestward along the South China shelf and slope from the east of Dongsha Islands to the northern central Vietnamese coast, then turns to the south along the central and southern Vietnamese coast, and finally partially exits the SCS through the Karimata Strait. In summer and early fall, the SCSwbc can be divided into three segments based on their characteristics. The southern segment is stable, flowing northward from the Karimata Strait up to about 11°N, where it separates from the coast forming an eastward offshore current. The separation of the current from Vietnamese coast induces some striking features, such as upwelling and cold sea-surface temperature. The middle segment off the central Vietnamese coast may have a bimodal behavior: northward coastal current and meandering current in early summer (June–July), and cyclonic gyre in later summer and early fall (August–September). The northern segment is featured by the summer SCS Warm Current on the South China shelf and a southwestward subsurface current along the continental slope.

Evolution of the semidiurnal (M2) internal tide on the continental slope of the northern South China Sea

Kinetic energy spectra from a site on the continental slope in the South China Sea reveal that significant peaks appear at some nonlinear interaction frequencies, namely M-3 (M-1 + M-2) and fM(1) (M-1 + f), where f is the inertial frequency, M-1 is the diurnal internal wave, and M-2 is the lunar semidiurnal internal tide. A possible generation mechanism of M-3 is explored. Analysis of bicoherence and shear spectra suggests that strong M-3 is indirectly associated with parametric subharmonic instability ( PSI) of M-2. In another word, under the effect of PSI the energy of M-2 is first transferred to M-1; then via other nonlinear coupling, some nonlinear waves (e.g. fM(1), M-3) are generated. Moreover, M-1 is also present at another site near the bottom of the continental slope. The shear spectra from these two sites show, for the first time, that M-1 can be significantly distinguished from lunar diurnal O-1 and lunisolar diurnal K-1.

Low frequency variability of South China Sea surface circulation from 11 years of satellite altimeter data

Eleven years of sea surface height from satellite altimetry data are used to describe the low frequency variability of South China Sea (SCS) surface circulation through Empirical Orthogonal Function (EOF) analysis. The annual variability clearly agrees with previous studies, revealing the SCS response to seasonal fluctuations of the overlaying monsoon winds. The summer eastward jet off west coast of SCS is shown from the second seasonal EOF mode. Two new patterns of year-to-year and decadal variability are presented for the first time from measurements. Long term positive trend appears from 1993 to 2001 and reverses the sign of its trend again from 2001 to 2003. Abnormal events occur in 1994 1995, in 1997 - 1998 and in 2002 with different spatial scales regarding both basin-scale distribution and meso-scale variability located off the western SCS. Association of these events with the El Nino-Southern Oscillation is discussed.

Summer Sea Surface Temperature Variability off Vancouver Island From Satellite Data

Satellite-sensed advanced very high resolution radiometer (AVHRR) sea surface temperature (SST) data over eight summers (1984-1991) were used to analyze the summer SST patterns of variability off the west coast of Vancouver Island. Empirical orthogonal function (EOF) analysis of the spatial variance for 133 nearly cloud-free summer images was performed. The first EOF mode, which resembled the mean of all images, showed a strong cool water band located at the northwest comer of Vancouver Island, a cool tongue extending seaward from the Strait of Juan de Fuca and a warm patch off Barkley Sound. The second mode revealed topographically controlled upwelling: cool water over the shelf region with its seaward boundary roughly following the 200-m depth contour, plus a cold eddy located just north of the Juan de Fuca Canyon. The third mode displayed cool water extending southwestward off Brooks Peninsula, while the fourth mode showed a cool water plume extending off Cape Scott at the northern tip of Vancouver Island. These four modes accounted for 33, 12, 10, and 5% of the SST variance, respectively. The temporal amplitude of these EOF modes revealed how the SST features changed as summer progressed. From these images, we also constructed an overall seasonal coolness index, which revealed the summers of 1986 and 1991 to have the coolest coastal water, with both summers immediately preceding an E1 Nifio.

Seasonal characteristics of internal tides on the continental shelf in the northern South China Sea

Three across-shelf Acoustic Doppler Current Profiler (ADCP) moorings were deployed on the continental shelf in the northern South China Sea in 2006 and 2007, in order to obtain time series of ocean currents. During each of the three observational periods, in spring, autumn, and winter seasons, the observation was sustained for at least 1 month. Conductivity-temperature-depth data were also collected in the vicinity of these ADCP moorings. The two diurnal baroclinic constituents (O-1 and K-1) are found to be more prominent than the semidiurnal baroclinic ones (M-2 and S-2) at each mooring site, which are different from the barotropic tides. The highest diurnal (semidiurnal) baroclinic kinetic energy density exceeded 10 (2.7) kJ/m(2) during the observational periods. In each observational period, the vertical structures of baroclinic tidal ellipses indicate mode-1 characteristics for O-1 and K-1, whereas some of the semidiurnal internal tides show mode-2 characteristics in autumn. Propagation directions of the diurnal and semidiurnal internal tides are basically across-shelf, and their phase speeds estimated from phase changes and buoyancy frequencies indicate deceleration of internal tides, when the internal tidal waves propagate from deeper shelf break to shallower shelf. The increased stratification in autumn is favorable for maintaining internal tidal waveform and preserving internal tidal energy, while the intensity of the internal tidal flow decreases significantly in winter.

Spatiotemporal Variations of Mesoscale Eddies in the Sulu Sea

Mesoscale eddies have been observed in the Sulu Sea, but their characteristics have not been well described. This study investigates the eddy population in the Sulu Sea using 22 years of satellite altimeter data with high spatiotemporal resolution. On average, there are approximately 1.6 eddies observed in the Sulu Sea each day and 1.8 eddy tracks generated each month. Two of the main eddy genesis regions are west of Negros Island and the Zamboanga Peninsula. The mean radius, lifespan and propagation speed of the eddies are 76.6 km, 32 days and 4.5 cm/s, respectively. The eddy radius and amplitude are generally large in the central Sulu Sea but small on its margin. The mean eddy kinetic energy and vorticity generally monotonically decrease from south to north, consistent with the distributions of background current kinetic energy. Over the seasonal cycle, there are more cyclonic eddies during boreal winter, and they tend to have a larger amplitude and radius than the other 3 seasons, while there are more anti-cyclonic eddies during boreal summer, and they tend to have a larger amplitude and radius than the other 3 seasons. The instability of the mean current and the island gap wind jets are the two key eddy genesis mechanisms in the Sulu Sea.

Interannual to decadal variation of spring sea level anomaly in the western South China Sea

Satellite observations of sea level anomalies (SLA) from January 1993 to December 2012 are used to investigate the interannual to decadal changes of the boreal spring high SLA in the western South China Sea (SCS) using the Empirical Orthogonal Function (EOF) method. We find that the SLA variability has two dominant modes. The Sea Level Changing Mode (SLCM) occurs mainly during La Niña years, with high SLA extension from west of Luzon to the eastern coast of Vietnam along the central basin of the SCS, and is likely induced by the increment of the ocean heat content. The Anticyclonic Eddy Mode (AEM) occurs mainly during El Niño years and appears to be triggered by the negative wind curl anomalies within the central SCS. In addition, the spring high SLA in the western SCS experienced a quasi-decadal change during 1993–2012; in other words, the AEM predominated during 1993–1998 and 2002–2005, while the La Niña-related SLCM prevailed during 1999–2001 and 2006–2012. Moreover, we suggest that the accelerated sea level rise in the SCS during 2005–2012 makes the SLCM the leading mode over the past two decades.