基于高频地波雷达观测的粤西近海潮流潮能分析

利用粤西海域高频地波雷达观测得到的表层海流资料进行潮流调和分析。结果表明:粤西近海主要属于不正规半日分潮,浅水分潮较强。以M_2分潮为例,潮流运动形式主要为逆时针的往复流为主,方向沿西北—东南方向。粤西近海的潮能主要由东部陆架输送进来,潮能自东向西传播,在大潮期间,粤西的潮能出现向岸方向分量,表现为从东南向西北方向传播,在近岸区域潮能通量传播的方向会发生一个向岸的偏转。通过潮能收支方程计算潮能耗散,发现粤西近海潮能耗散的高值区在西部岛屿密集区域,与琼州海峡的存在和琼州海峡东北处地形变化存在明显的相关关系。

菲律宾海底层水体在1990s—2010s之间的暖化

跨菲律宾海的重复断面水文观测揭示:菲律宾海底层水体从1990s到2010s增暖了0.002～0.01℃。在西马里亚纳海盆和四国海盆,较冷的下层绕极深层水(Lower Circumpolar Deep Water,LCDW)减少,较暖的LCDW增加;而在菲律宾海盆,较冷的变性LCDW减少,较暖的变性LCDW增加。菲律宾海盆4000dbar的热通量是0.0413W·m^(–2),而西马里亚纳海盆和四国海盆的是0.0221W·m^(–2)。菲律宾海盆由于深层海洋热膨胀引起的局地海平面上升速度是0.0621mm·yr^(–1),而西马里亚纳海盆和四国海盆的是0.0333mm·yr^(–1)。

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 downweUing jets of LC accelerate and increase the downward transport of energy as compared to ST. The vertical eddy viscosity Km of LC is much larger than that of ST. Strong mixing induced by LC has two locations. They are located in the 26s-36s （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 Krn 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 Sz induced by LC is changing all the time, which needs to be fully considered in the parameterization of LC.

Formation of an anticyclonic eddy and the mechanism involved: a case study using cruise data from the northern South China Sea

Data from satellite altimetry and in situ observations together with the Hybrid Coordinate Ocean Model (HYCOM) reanalysis data were used to investigate the mechanism and formation of an anticyclonic eddy in the northeastern South China Sea (SCS). Analysis of water mass using cruise data indicated that the water captured in the eddy diff ers from those in the SCS, the Kuroshio intrusion, and the eddy-forming region. Data from sea surface height (SSH) and sea level anomaly (SLA) indicate that the eddy formed due both to the Kuroshio intrusion and the local circulation in the SCS. The Kuroshio intrusion is present at the start of the eddy growth (March 5-9) before Kuroshio leaps the Luzon Strait. The eddy then becomes larger and stronger in the absence of the Kuroshio intrusion. From the eddy budget of the HYCOM reanalysis data, the formation of the eddy goes in three steps. By the third step, the eddy had become aff ected by variations of local SCS circulation, which is more strongly than in the fi rst step in which it is aff ected more by the Kuroshio intrusion. The variability of the temperature and salinity inside the eddy provide a support to this conclusion. The water in the SCS intruded into the eddy from the southeast, which decrease the salinity gradually in the southern part of the eddy during the growth period.

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.

Observation system simulation experiments using an ensemble-based method in the northeastern South China Sea

An ensemble-based method for the observation system simulation experiment(OSSE)is employed to design optimal observation stations and assess the present observation stations in the northeastern South China Sea(SCS).We employed the 20-year(1992-2012)sea surface height(SSH)data to design an array to monitor the intraseasonal to interannual variability.The results show that the most key region was found located at the northwest of Luzon Island(LI)where the energetic Luzon cyclonic gyre(LCG)occurs;other key regions include the edge of the LCG,the northwest of the Luzon Strait(LS),and the southwest of Taiwan,China.By contrast,we found that the present observation stations might oversample at the northwest of the LS and undersample at the northwest of LI.In addition,the optimal stations perform better in a larger area than the present stations.In vertical direction,the key layer is located within the upper 200-m depth,of which the surface and subsurface layers are most valuable to the observing system.

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.

Progress on deep circulation and meridional overturning circulation in the South China Sea

The deep overflow through the Luzon Strait drives the cyclonic deep circulation in the South China Sea(SCS). In the mean time, the intruding Pacific deep water transforms and upwells due to enhanced diapycnal mixing in the SCS. Both processes greatly contribute to the SCS meridional overturning circulation(SCSMOC). At the same time, both the deep circulation and meridional overturning circulation are modulated by rough topography in the SCS. Furthermore, the spatial structure of the SCSMOC infers a link between the upper-layer circulation and deep circulation in the SCS. This paper reviews recent advances in the SCS deep circulation and meridional overturning circulation, including the driving mechanism of the SCS deep circulation and its modulation by topography, as well as the spatial structure of the SCSMOC and its dynamical mechanism.

Progress of regional oceanography study associated with western boundary current in the South China Sea

Recent progress of physical oceanography in the South China Sea (SCS) associated with the western boundary current (WBC) and eddies is reviewed in this paper. It includes Argo observations of the WBC, eddy detection in the WBC based on satellite images, cross-continental shelf exchange in the WBC, eddy-current interaction, interannual variability of the WBC, air-sea interaction, the SCS throughflow (SCSTF), among others. The WBC in the SCS is strong, and its structure, variability and dynamic processes on seasonal and interannual time scales are yet to be fully understood. In this paper, we summarize progresses on the variability of the WBC, eddy-current interaction, air-sea interaction, and the SCSTF achieved in the past few years. Firstly, using the drifting buoy observations, we point out that the WBC becomes stronger and narrower after it reaches the central Vietnam coast. The possible mechanisms influencing the ocean circulation in the northern SCS are discussed, and the dynamic mechanisms that induce the countercurrent in the region of northern branch of WBC in winter are also studied quantitatively using momentum balance. The geostropic component of the WBC was diagnosed using the ship observation along 18°N, and we found that the WBC changed significantly on interannual time scale. Secondly, using the ship observations, two anti-cyclonic eddies in the winter of 2003/2004 in the northern SCS, and three anti-cyclonic eddies in the summer of 2007 along 18°N were studied. The results show that the two anti-cyclonic eddies can propagate southwestward along the continental shelf at the speed of first Rossby wave (～0.1 ms-1 ) in winter, and the interaction between the three anti-cyclonic eddies in summer and the WBC in the SCS is preliminarily revealed. Eddies on the continental shelf of northern SCS propagated southeastward with a maximum speed of 0.09 ms-1 , and those to the east of Vietnam coast had the largest kinetic energy, both of which imply strong interaction between eddy activity and WBC in the SCS. Thirdly, strong intraseasonal variability (ISV) of sea surface temperature (SST) near the WBC regions was found, and the ISV signal of SST in winter weakens the ISV signal of latent heat flux by 20%. Fourthly, the long-term change of SCSTF volume transport and its connection with the ocean circulation in the Pacific were discussed.