Jinglong Yao

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.

Mesoscale eddies cases study at Xisha waters in the South China Sea in 2009/2010

Analyzing the observed currents at Xisha (110.3899 degrees E, 17.1038 degrees N) during May 2009 to May 2010, it is found that the kinetic energy has significant mesoscale variability, and each peak responds to large positive/negative ocean surface current curl caused by mesoscale eddies. Compared the kinetic energy with the wind stress work and the pressure work, it is also found that the barotropic pressure work which is mainly contributed by the sea surface height (SSH) corresponding to the mesoscale eddies behaves like the kinetic energy. The contribution of the mesoscale eddies to the kinetic energy can be up to 90% sometimes and reach deep level every time. Using the satellite altimeter data, the paths of mesoscale eddies contributing to the kinetic energy variability are traced back. In the winter half of the year, the mesoscale eddies propagating along the northern South China Sea shelf or across the basin from the west of the Philippines toward Xisha arrive at Xisha, influencing the kinetic energy. In the summer half of the year, the mesoscale eddies are mainly from the south, which were shed from the Vietnam coast current. And the cause for eddy shedding may be related to the relaxation of the Ekman transport anomalies.

Synoptic-scale disturbances over the northern South China Sea and their responses to El Nino

Using surface and aerological meteorological observations obtained at the Xisha Automatic Weather Station and three moored buoys along the continental slope, characteristics of the synoptic-scale disturbances over the northern South China Sea (NSCS) are extensively studied. The power spectra of surface and aerological observations suggest a synoptic feature with a pronounced energy peak at a period of 5–8 d and a weak peak at 3–4 d. The standard deviation of the synoptic temperature component derived at Xisha Station from 1976 to 2011 indicates that the strongest variability normally exists in August all through the whole troposphere. At the interannual scale, it is found that El Niño plays an important role in regulating the synoptic disturbances of atmosphere. The vertical synoptic disturbances have a double active peak following El Niño condition. The first peak usually occurs during the mature phase of El Niño, and the second one occurs in the summer of decay year. Comparing with the summer of developing years, the summer of the decaying year of El Niño has more active and stronger synoptic disturbances, especially for the 5–8 d period variations.

Assimilating temperature and salinity profiles using Ensemble Kalman Filter with an adaptive observation error and T-S constraint

Temperature (T) and salinity (S) profiles from conductivity-temperature-depth data collected during the Northern South China Sea Open Cruise from August 16 to September 13, 2008 are assimilated using Ensemble Kalman Filter (EnKF). An adaptive observational error strategy is used to prevent filter from diverging. In the meantime, aiming at the limited improvement in some sites caused by the T and S biases in the model, a T-S constraint scheme is adopted to improve the assimilation performance, where T and S are separately updated at these locations. Validation is performed by comparing assimilated outputs with independent in situ data (satellite remote sensing sea level anomaly (SLA), the OSCAR velocity product and shipboard ADCP). The results show that the new EnKF assimilation scheme can significantly reduce the root mean square error (RMSE) of oceanic T and S compared with the control run and traditional EnKF. The system can also improve the simulation of circulations and SLA.

Comparison of summer chlorophyll a concentration in the South China Sea and the Arabian Sea using remote sensing data

The South China Sea (SCS) and the Arabian Sea (AS) are both located roughly in the north tropical zone with a range of similar latitude (0°–24°N). Monsoon winds play similar roles in the upper oceanic circulations of the both seas. But the distinct patterns of chlorophyll a (Chl a) concentration are observed between the SCS and the AS. The Chl a concentration in the SCS is generally lower than that in the AS in summer (June–August); the summer Chl a concentration in the AS shows stronger interannual variation, compared with that in the SCS; Moderate resolution imaging spectroradiometer (MODIS)-derived data present higher atmospheric aerosol deposition and stronger wind speed in the AS. And it has also been found that good correlations exist between the index of the dust precipitation indicated by aerosol optical thickness (AOT) and the Chl a concentration, or between wind and Chl a concentration. These imply that the wind and the dust precipitation bring more nutrients into the AS from the sky, the sub-layer or coast regions, inducing higher Chl a concentration. The results indicate that the wind velocity and the dust precipitation can play important roles in the Chl a concentration for the AS and the SCS in summer. However aerosol impact is weak on the biological productivity in the west SCS and wind-induced upwelling is the main source.

Observed characteristics of atmospheric ducts over the South China Sea in autumn

The observed characteristics of lower atmospheric ducts over the South China Sea (SCS) were analyzed based on Global Position Systerm (GPS) radiosonde data collected four times daily during autumn open cruises from 2006 to 2012. Duct occurrence, thickness, and strength over the SCS were about 40%, 150-m thick, and 8 M units, respectively, which were larger than during the summer monsoon period. Most ducts occurred at heights <1 500 m and these ducts easily trap electromagnetic wave clusters with wavelengths <2 m. Diurnal variation of the SCS ducts appeared evident. They occurred more often at midnight at higher altitudes (about 1 100 m), with a thickest layer of about 145 m and less frequently during the evening at lower altitudes (about 800 m), with a thinnest layer of about 125 m. Moreover, ducts during the daytime at a mean height of about 900 m, with the greatest strength of about 10 M units. Furthermore, all duct variables observed over the SCS in autumn decreased from north to south. These findings are useful not only in the design of radar and communication systems, but also for evaluating possible effects of anomalous propagation on meteorological radar and military applications.

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.

Statistical characteristics of the surface ducts over the South China Sea from GPS radiosonde data

Based on the global position system (GPS) radiosonde data near the sea surface, the surface duct characteristics over the South China Sea (SCS) were statistically analyzed. The annual surface duct occurrence over the SCS was about 64%. Of the observed surface ducts, duct heights mainly distributed between 18 and 42 m, with M slopes in the range of–0.3 to–0.2 M units/m. Those ducts accounted for about 80% of the ducting cases. For the total profiles, the duct occurrences in a day changed slowly and were more than 60% in all times. The surface ducts formed more easily in the daytime than in the nighttime and most of the duct height were at bellow about 32 m. Additionally, The seasonal variation of the SCS ducts appeared to be evident, except that the mean duct thickness was almost constant, about 33 m for all seasons. The highest occurrence was about 71% in the autumn, followed by in the summer, spring and winter. In spring, their top-height existed more often at a height of more than 48 m. Their mean duct strength became stronger trend from spring to winter, with the M-slope in the range between–0.26 and–0.18 M units/m. Those results agreed well with other studies, provided considering the data resolution. The statistical analysis was reliable and gave the duct estimation for the SCS. Such duct climatology not only has important implications for communication systems and the reliability of the radar observation, but also can provide useful information to improve the accuracy of the meteorological radar measurements.

The relationship between significant wave height and Indian Ocean Dipole in the equatorial North Indian Ocean

Based on reanalysis data, we find that the Indian Ocean Dipole (IOD) plays an important role in the variability of wave climate in the equatorial Northern Indian Ocean (NIO). Significant wave height (SWH) in the equatorial NIO, especially over the waters southeast to Sri Lanka, exhibits strong interannual variations. SWH anomalies in the waters southeast to Sri Lanka correlate well with dipole mode index (DMI) during both summer and autumn. Negative SWH anomalies occur over the oceanic area southeast to Sri Lanka during positive IOD events and vary with different types of IOD. During positive prolonged (unseasonable) IOD, the SWH anomalies are the strongest in autumn (summer); while during positive normal IOD, the SWH anomalies are weak in both summer and autumn. Strong easterly wind anomalies over the southeast oceanic area of Sri Lanka during positive IOD events weaken the original equatorial westerly wind stress, which leads to the decrease in wind-sea waves. The longer wave period during positive IOD events further confirms less wind-sea waves. The SWH anomaly pattern during negative IOD events is nearly opposite to that during positive IOD events.

Preliminary analysis of the intraseasonal air–sea interaction influenced by Xisha warm eddy

A study of the Xisha eddy is a good supplement to comprehend the local oceanic and atmospheric currents. Our work aims at analyzing the intraseasonal Air–Sea interaction process during the occurring of the Xisha warm eddy. Using a Complex Empirical Orthogonal Function statistical technique, we found an eddy that propagated southwestward along the continental slope of the northwestern South China Sea, at a similar phase speed to that of Rossby waves, at 0.12 m s−1. Intraseasonal variability in spring is related to the ocean-to-atmosphere interaction, and involves the cloud-radiation effects on surface sea temperature, as well as its impact on lower-level convergence over the South China Sea. This process, induced by the warm eddy, results in abnormal surface sea temperature, downward shortwave/longwave radiation flux, surface latent heat flux, and wind changes. We used composite results and the Weather Research and Forecasting Model to explore how the observed surface sea temperature anomalies influence precipitation in the South China Sea. The results showed that the atmospheric transport of heat and moisture improved with respect to the surface sea temperature max and the air active events have caused lower atmosphere instability, along with the lower pressure and enhanced precipitation frequency in spring.