Yunkai He

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.

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.

A case study of the near-inertial oscillations near the Xisha Islands in the South China Sea during the passage of typhoon Conson 2010

Near-inertial oscillations in the northern South China Sea, close to the Xisha Islands, during the passage of typhoon Conson, were investigated using 52-day observational data and a damped slab model. Using spectral analyses, we found that these near-inertial oscillations were dominated by clockwise components. The observed, as well as simulated, inertial currents matched well before and after the passage of the typhoon, which revealed that significantly enhanced oscillations were induced by the typhoon. They have a peak frequency of 0.0237 cycles per hour, which is 2% lower than the local inertial frequency, meaning a red-shift phenomenon exists, even in shallow water. The red-shift might be attributed to the background negative vorticity. Additionally, we found that the barotropic near-inertial currents were more significant after the passage of the typhoon. The position of the maximum intensity of near-inertial current shear with a period of 20.8 h was found to propagate upward slowly with time before the passage of the typhoon.

Observation and numerical simulation of the marine meteorology elements and air-sea fluxes at Yongxing Island in September 2013

The variation of marine meteorology elements and the heat flux at Yongxing Island were investigated in September 2013 using observational data collected by the Yongxing Island Air–Sea Flux Tower. Heat flux was measured through eddy covariance and estimated using the bulk flux method. The traditional thirty-minute average method was employed to analyze the eddy covariance data. A TOGA–CORE 3.0 Algorithm was used to estimate the bulk flux. Capacity of the Advanced Weather Research and Forecasting model to simulate the variations in marine meteorological elements was evaluated using the observational data obtained during the same period. Observations suggested a response by the different parameters that were synchronous to the retreat of the monsoon around 7 September 2013. Surface air masses became cool and dry, and the downward short (long) wave radiation was reduced (increased). The Weather Research and Forecasting model simulated the variation of the atmospheric elements well, as observed by the Air-Sea Flux Tower in September 2013. The model was very accurate in simulating surface wind, radiation, and scalar–humidity. However, it failed to simulate the dramatic variations of surface air temperature, though it successfully reproduced the retreat of the summer monsoon, but weakened during the onset of the typhoon. The sensible flux simulated through the model was between the fluxes estimated through the bulk flux and eddy covariance flux. It also overestimated the latent heat flux at times, particularly after 20 September.

Interannual variation of the South China Sea circulation during winter: intensified in the southern basin

Surface geostrophic current derived from altimetry remote sensing data, and current profiles observed from in-situ Acoustic Doppler Current Profilers (ADCP) mooring in the northern South China Sea (NSCS) and southern South China Sea (SSCS) are utilized to study the kinetic and energetic interannual variability of the circulation in the South China Sea (SCS) during winter. Results reveal a more significant interannual variation of the circulation and water mass properties in the SSCS than that in the NSCS. Composite ananlysis shows a significantly reduced western boundary current (WBC) and a closed cyclonic eddy in the SSCS at the mature phase of El Niño event, but a strong WBC and an unclosed cyclonic circulation in winter at normal or La Niña years. The SST is warmer while the subsurface water is colder and fresher in the mature phase of El Niño event than that in the normal or La Niña years in the SSCS. Numerical experiments and energy analysis suggest that both local and remote wind stress change are important for the interannual variation in the SSCS, remote wind forcing and Kuroshio intrusion affect the circulation and water mass properties in the SSCS through WBC advection.