Ju Chen

The 1997¸1998 warm event in the South China Sea

A strong warm event happens during spring 1997 to spring 1999 in the South China Sea. Its intensity and duration show that it is the strongest event on the record over the past decades. It also corresponds with the severe flood over the valley of the Yangtze River and a couple of marine environmental events. This note addressed the evolution process by using several data sets, such as sea surface temperature, height and wind stress in addition to subsurface temperature. The onset of the warm event almost teleconnects with the El Niño event in the tropical Pacific Ocean. Summer monsoon is stronger and winter monsoon is weaker in 1997 so that there are persistent westerly anomalies in the South China Sea. During the development phase, the warm advection caused by southerly anomalies is the major factor while the adjustment of the thermocline is not obvious. Subsequently, the southerly anomalies decay and even northerly anomalies appear in the summer of 1998 resulting from the weaker than normal summer monsoon in 1998 in the South China Sea. The thermocline develops deeper than normal, which causes the downwelling pattern and the start of the maintaining phase of the warm event. Temperature anomalies in the southern South China Sea begin to decay in the winter of 1998–1999 and this warm event ends in the May of 1999.

Relative contributions of local wind and topography to the coastal upwelling intensity in the northern South China Sea

Ministry of Science and Technology of China [2011CB403504]; National Natural Science Foundation of China [41006011, 41006012]; South China Sea Institute of Oceanology [SQ201001]; Recruitment Program of Global Experts

Thermal variations in the South China Sea associated with the eastern and central Pacific El Nino events and their mechanisms

In this study, we investigate the interannual variability of the sea surface temperature (SST) in the South China Sea (SCS) associated with two types of El Nino, namely, the eastern Pacific (EP) El Nino and the central Pacific (CP) El Nino. First, double warm peaks can occur during both types of El Nino events in the SCS. However, the strong warm basin mode can only develop in the EP El Nino, while the warm semibasin mode exists during the CP El Nino. Associated with an anomalous positive (negative) net surface heat flux in the EP (CP) El Nino, along with a shallower thermocline with weaker (stronger) northeasterly wind anomalies, the SST anomalies become warmer (cooler) in the developing autumn. Over the background of cooling SST in autumn of CP El Nino, therefore, only a weak warming can occur in the subsequent years, which is limited in the western boundary area under the forcing of warm ocean advection. Second, the SST oscillation periods are different in these two types of El Nino. The SST evolution in the EP El Nino is negative-positive with a quasi-biennial oscillation, but that in the CP El Nino is positive-negative-positive-negative with an annual oscillation. It seems that the double cooling in the CP El Nino is phase-locked to the late autumn season.

Contrasting dynamic characteristics of shear turbulence and Langmuir circulation in the surface mixed layer

Large eddy simulation (LES) is used to investigate contrasting dynamic characteristics of shear turbulence (ST) and Langmuir circulation (LC) in the surface mixed layer (SML). ST is usually induced by wind forcing in SML. LC can be driven by wave-current interaction that includes the roles of wind, wave and vortex forcing. The LES results show that LC suppresses the horizontal velocity and greatly modifies the downwind velocity profile, but increases the vertical velocity. The strong downwelling jets of LC accelerate and increase the downward transport of energy as compared to ST. The vertical eddy viscosity Km of L is much larger than that of ST. Strong mixing induced by LC has two locations. They are located in the 2δs–3δs (Stokes depth scale) and the lower layer of the SML, respectively. Its value and position change periodically with time. In contrast, maximum Km induced by ST is located in the middle depth of the SML. The turbulent kinetic energy (TKE) generated by LC is larger than that by ST. The differences in vertical distributions of TKE and Km are evident. Therefore, the parameterization of LC cannot be solely based on TKE. For deep SML, the convection of large-scale eddies in LC plays a main role in downward transport of energy and LC can induce stronger velocity shear (S2) near the SML base. In addition, the large-scale eddies and S2 induced by LC is changing all the time, which needs to be fully considered in the parameterization of LC.

Toward a mesoscale hydrological and marine meteorological observation network in the South China Sea

Air-sea interaction in the South China Sea (SCS) has direct impacts on the weather and climate of its surrounding areas at various spatiotemporal scales. In situ observation plays a vital role in exploring the dynamic characteristics of the regional circulation and air-sea interaction. Remote sensing and regional modeling are expected to provide high-resolution data for studies of air-sea coupling; however, careful validation and calibration using in situ observations is necessary to ensure the quality of these data. Through a decade of effort, a marine observation network in the SCS has begun to be established, yielding a regional observatory for the air-sea synoptic system.Earlier observations in the SCS were scarce and narrowly focused. Since 2004, an annual series of scientific open cruises during late summer in the SCS has been organized by the South China Sea Institute of Oceanology (SCSIO), carefully designed based on the dynamic characteristics of the oceanic circulation and air-sea interaction in the SCS region. Since 2006, the cruise carried a radiometer and radiosondes on board, marking a new era of marine meteorological observation in the SCS. Fixed stations have been established for long-term and sustained records. Observations obtained through the network have been used to study regional ocean circulation and processes in the marine atmospheric boundary layer. In the future, a great number of multi-institutional, collaborative scientific cruises and observations at fixed stations will be carried out to establish a mesoscale hydrological and marine meteorological observation network in the SCS.

SCSPOD14, a South China Sea physical oceanographic dataset derived from in situ measurements during 1919-2014.

In addition to the oceanographic data available for the South China Sea (SCS) from the World Ocean Database (WOD) and Array for Real-time Geostrophic Oceanography (Argo) floats, a suite of observations has been made by the South China Sea Institute of Oceanology (SCSIO) starting from the 1970s. Here, we assemble a SCS Physical Oceanographic Dataset (SCSPOD14) based on 51,392 validated temperature and salinity profiles collected from these three datasets for the period 1919–2014. A gridded dataset of climatological monthly mean temperature, salinity, and mixed and isothermal layer depth derived from an objective analysis of profiles is also presented. Comparisons with the World Ocean Atlas (WOA) and IFREMER/LOS Mixed Layer Depth Climatology confirm the reliability of the new dataset. This unique dataset offers an invaluable baseline perspective on the thermodynamic processes, spatial and temporal variability of water masses, and basin-scale and mesoscale oceanic structures in the SCS. We anticipate improvements and regular updates to this product as more observations become available from existing and future in situ networks.

Observed evidence of the anomalous South China Sea western boundary current during the summers of 2010 and 2011

Seven years of directly measured current data from a mooring in the Xisha area of the South China Sea (SCS), together with shipboard ADCP and satellite data, have shown the western boundary current (WBC) anomaly and its vertical structure during the summers of 2010 and 2011. The observed WBC presented obvious year-to-year variability, especially in the summer. Overall, the summer mean velocity at the mooring site over 7-year (2007-2013) was northeastward. The moored ADCP showed that the northeastward velocity was particularly strong in the summer of 2010, but the increase was confined in the upper 120 m. In contrast, the northeastward current disappeared throughout the observed depth range (from 50 to 450 m) in the summer of 2011. Even at the deepest observed position, the monthly velocity anomalies reached 14 cm s(-1) westward and 12 cm s(-1) southward in the zonal and meridional directions, respectively. Both the Vietnam offshore current (VOC) and double gyres in the western SCS disappeared and the southern anticyclonic gyre expanded to strengthened the northward WBC in the summer of 2010. However, in summer of 2011, the VOC intensified, and the northern cyclonic gyre enlarged with its northern edge reaching 18 degrees N, slightly north of mooring site, which weakened the northeastward WBC. The observed SCS circulation anomalies during 2010 and 2011 were mainly induced by the basin-scale wind field anomalies associated with the 2009/2010 El Nino and 2010/2011 La Nina.

Persistent and energetic bottom-trapped topographic Rossby waves observed in the southern South China Sea.

Energetic fluctuations with periods of 9–14 days below a depth of 1400 m were observed in the southern South China Sea (SCS) from 5 years of direct measurements. We interpreted such fluctuations as topographic Rossby waves (TRWs) because they obey the dispersion relation. The TRWs persisted from May 24, 2009 to August 23, 2013, and their bottom current speed with a maximum of ~10 cm/s was one order of magnitude greater than the mean current and comparable to the tidal currents near the bottom. The bottom-trapped TRWs had an approximate trapping depth of 325 m and reference wavelength of ~82 km, which were likely excited by eddies above. Upper layer current speed that peaked approximately every 2 months could offer the energy sources for the persistent TRWs in the southern SCS. Energetic bottom-trapped TRWs may have a comparable role in deep circulation to tides in areas with complex topography.

Intercomparison of GPS radiosonde soundings during the eastern tropical Indian Ocean experiment

Temperature and relative humidity profiles derived from two China-made global positioning system (GPS) radiosondes (GPS-TK and CF-06-A) during the east tropical Indian Ocean (ETIO) experiment were compared with Vaisala RS92-SGP to assess the performances of China-made radiosondes over the tropical ocean. The results show that there have relative large biases in temperature observations between the GPSTK and the RS92-SGP in the low troposphere, with a warm bias of greater than 2 K in the day and a cooling bias of 0.6 K at night. The temperature differences of the CF-06-A were small in the troposphere both in daytime and nighttime, and became large peak-to-peak fluctuations in the stratosphere. The intercomparison of the relative humidity showed that the CF-06-A had large random errors due to the limitation of sensors and the lack of correction scheme, and the GPS-TK had large systematic biases in the low troposphere which might be related to the temperature impact. GPS height measurements are clearly suitable for China-made radiosonde systems operation. At night, the CF-06-A and the GPS-TK could provide virtual potential temperature and atmospheric boundary layer height measurements of suitable quality for both weather and climate research. As a result of the intercomparison experiment, major errors in the China-made radiosonde systems were well indentified and subsequently rectified to ensure improving accuracy for historical and future radiosondes.

Decadal variation and trends in subsurface salinity from 1960 to 2012 in the northern South China Sea

Observations suggest that subsurface waters in the northern South China Sea (NSCS) exhibited substantial low-frequency variability, with a striking decadal change in the southern limit of the 34.6-psu isohaline. Long-term freshening of the subsurface waters started in 1960, was followed by salinification from 1975, and freshening occurred again from 1993 to 2012. The linear trends were –0.0076, 0.0100, and –0.0078 psu/yr, respectively. An analysis of the subsurface salinity budget reveals that the main underlying contributors to subsurface salinity are horizontal advection and vertical entrainment. In particular, advection driven by the Luzon Strait transport and vertical entrainment from the mixed layer are the key factors controlling variations on subsurface salinity. Diagnosis of the salinity budget further suggests that entrainment from the mixed layer played a more important role in the freshening periods than in the salinifying period.