南海混合层深度的季节变化及年际变化特征

通过分析新的SODA(Simple Ocean Data Assimilation)资料,得到南海混合层时空场的分布特征,剖析了南海混合层深度的季节及年际变化特征。资料分析表明:南海混合层存在着显著的季节和年际变化,且两者的均方差分布存在一定的差异。在季节变化中,冬季混合层在南海北部及西北陆架区深,在南海南部及吕宋冷涡处浅;夏季混合层在南海西北部浅,东南深。南海这种混合层深度分布特征除了与热通量的季节变化有关外,在相当大的程度上与季风引起的Ekman输送及Ekman抽吸有关。混合层深度距平场EOF(Empirical Othorgnal Function)第一模和第二模时间变化的主信号均为周期的年际变化信号,其中第一模态约为3 a,第二模态则有1.8,2.4和4.3 a的3个显著周期。EOF第一模显示混合层深度在南海东南部年际变化幅度最大,且滞后Nino3指数7个月时相关性最好(相关系数为0.422 3);EOF第二模显示在南海南部和北部混合层深度呈反位相变化。     

基于ROMS模型数值研究南海温跃层的季节变化

On the basis of the regional ocean modeling system （ROMS）, the seasonal variations of the thermocline in the South China Sea （SCS） were numerically investigated. The simulated hydrodynamics are in accordance with previous studies： the circulation pattern in the SCS is cyclonic in winter and anticyclonic in summer, and such a change is mostly driven by the monsoon winds. The errors between the modeled temperature profiles and the observations obtained by cruises are quite small in the upper layers of the ocean, indicating that the ocean status is reasonably simulated. On the basis of the shapes of the vertical temperature profiles, five thermocline types （shallow thermocline, deep thermocline, hybrid thermocline, double thermocline, and multiple thermocline） are defined herein. In winter, when the northeasterly monsoon prevails, most shallow shelf seas in the northwest of the SCS are well mixed, and there is no obvious thermocline. The deep region generally has a deep thermocline, and the hybrid or double thermocline often occurs in the areas near the cold eddy in the south of the SCS. In summer, when the southwesterly monsoon prevails, the shelf sea area with a shallow thermocline greatly expands. The distribution of different thermocline types shows a relationship with ocean bathymetry： from shallow to deep waters, the thermocline types generally change from shallow or hybrid to deep thermocline, and the double or multiple thermocline usually occurs in the steep regions. The seasonal variations of the three major thermocline characteristics （the upper bound depth, thickness, and intensity） are also discussed. Since the SCS is also an area where tropical cyclones frequently occur, the response of thermocline to a typhoon process in a short time scale is also analyzed.     

Upper ocean high resolution regional modeling of the Arabian Sea and Bay of Bengal

In this paper, effort is made to demonstrate the quality of high-resolution regional ocean circulation model in realistically simulating the circulation and variability properties of the northern Indian Ocean(10°S–25°N,45°–100°E) covering the Arabian Sea(AS) and Bay of Bengal(BoB). The model run using the open boundary conditions is carried out at 10 km horizontal resolution and highest vertical resolution of 2 m in the upper ocean.The surface and sub-surface structure of hydrographic variables(temperature and salinity) and currents is compared against the observations during 1998–2014(17 years). In particular, the seasonal variability of the sea surface temperature, sea surface salinity, and surface currents over the model domain is studied. The highresolution model's ability in correct estimation of the spatio-temporal mixed layer depth(MLD) variability of the AS and BoB is also shown. The lowest MLD values are observed during spring(March-April-May) and highest during winter(December-January-February) seasons. The maximum MLD in the AS(BoB) during December to February reaches 150 m (67 m). On the other hand, the minimum MLD in these regions during March-April-May becomes as low as 11–12 m. The influence of wind stress, net heat flux and freshwater flux on the seasonal variability of the MLD is discussed. The physical processes controlling the seasonal cycle of sea surface temperature are investigated by carrying out mixed layer heat budget analysis. It is found that air-sea fluxes play a dominant role in the seasonal evolution of sea surface temperature of the northern Indian Ocean and the contribution of horizontal advection, vertical entrainment and diffusion processes is small. The upper ocean zonal and meridional volume transport across different sections in the AS and BoB is also computed. The seasonal variability of the transports is studied in the context of monsoonal currents.     

Numerical investigation of the South China Sea deep circulation

Based on a two-level nested model from the global ocean to the western Pacific and then to the South China Sea（SCS）, the high-resolution SCS deep circulation is numerically investigated. The SCS deep circulation shows a basin-scale cyclonic structure with a strong southward western boundary current in summer（July）, a northeastsouthwest through-flow pattern across the deep basin without a western boundary current in winter（January）,and a transitional pattern in spring and autumn. The sensitivity ...     

Recent progress in studies of the South China Sea circulation

The South China Sea (SCS) is a semi-enclosed marginal sea with deep a basin. The SCS is located at low latitudes, where the ocean circulations are driven principally by the Asia-Australia monsoon. Ocean circulation in the SCS is very complex and plays an important role in both the marine environment and climate variability. Due to the monsoon-mountain interactions the seasonal spatial pattern of the sea surface wind stress curl is very specific. These distinct patterns induce different basin-scale circulation and gyre in summer and winter, respectively. The intensified western boundary currents associated with the cyclonic and anticyclonic gyres in the SCS play important roles in the sea surface temperature variability of the basin. The mesoscale eddies in the SCS are rather active and their formation mechanisms have been described in recent studies. The water exchange through the Luzon Strait and other straits could give rise to the relation between the Pacific and the SCS. This paper reviews the research results mentioned above.     

Seasonal variability of the mixed layer in the central Bay of Bengal and associated changes in nutrients and chlorophyll

Hydrographic data from National Oceanographic Data Center (NODC) and Responsible National Oceanographic Data Centre (RNODC) were used to study the seasonal variability of the mixed layer in the central Bay of Bengal (8–20°N and 87–91°E), while meteorological data from Comprehensive Ocean Atmosphere Data Set (COADS) were used to explore atmospheric forcing responsible for the variability. The observed changes in the mixed-layer depth (MLD) clearly demarcated a distinct north–south regime with 15°N as the limiting latitude. North of this latitude MLD remained shallow (∼20 m) for most of the year without showing any appreciable seasonality. Lack of seasonality suggests that the low-salinity water, which is perennially present in the northern Bay, controls the stability and MLD. The observed winter freshening is driven by the winter rainfall and associated river discharge, which is advected offshore under the prevailing circulation. The resulting stratification was so strong that even a 4 °C cooling in sea-surface temperature (SST) during winter was unable to initiate convective mixing. In contrast, the southern region showed a strong semi-annual variability with deep MLD during summer and winter and a shallow MLD during spring and fall intermonsoons. The shallow MLD in spring and fall results from primary and secondary heating associated with increased incoming solar radiation and lighter winds during this period. The deep mixed layer during summer results from two processes: the increased wind forcing and the intrusion of high-salinity waters of Arabian Sea origin. The high winds associated with summer monsoon initiate greater wind-driven mixing, while the intrusion of high-salinity waters erodes the halocline and weakens the upper-layer stratification of the water column and aids in vertical mixing. The deep MLD in the south during winter was driven by wind-mixing, when the upper water column was comparatively less stable. The deep MLD between 15 and 17°N during March–May cannot be explained in the context of local atmospheric forcing. We show that this is associated with the propagation of Rossby waves from the eastern Bay. We also show that the nitrate and chlorophyll distribution in the upper ocean during spring intermonsoon is strongly coupled to the MLD, whereas during summer river runoff and cold-core eddies appear to play a major role in regulating the nutrients and chlorophyll.     

Intercomparison of three South China Sea circulation models

1IntroductionTheSouthChinaSeaisthelargesttropicalmarginaldeepsealocatingbetweenthewesternPacificOceanandtheeasternIndianOcean.AsapartofAsia-Australiamaritimecontinent,monsoonisaprimaryfactorforcingtheSouthChinaSeaCurrent(SCSC)variation.Drivenbynortheasterlymonsooninwinterandsouth-westerlymonsooninsummer,respectively,theSCSCbehavesacyclonicgyreandananticy-clonicgyre,correspondingly(Wyrtki,1961;Xuetal.,1982).Owingtotheshortageandexpen-sivenessofdirectobservationsintheSCS,fur-therunder…     

利用HOAPS资料研究南海海气界面热通量时空分布

基于第二版本HOAPS(Hamburg Ocean Atmosphere Parameters and Fluxes from Satellite data)潜热、感热和海表温度(SST)3个参量的15 a(1988~2002年)逐月平均资料,利用经验正交方法分解分析了这3个参量在南海的时空分布.结果表明,在夏季模态,潜热表现为南高北低,感热表现为中间低两边高,两者主要都是海洋向大气输送热量,但大气有时也向南海中部输送感热;在冬季模态,潜热和感热的高值区都在南海北部,东北部有一强中心,该中心主要是由风场引起的;夏季SST的变化导致全年SST呈准半年周期变化.冬季SST的变化滞后于潜热变化1个月;除夏季和冬季模态外,冬夏转换季节模态也十分明显;HOAPS与NCEP(National Center of Environment Prediction)资料相比,两者3个参量的时空分布大体一致,区别在于HOAPS资料能更好地反映参量的一些细微特征.     

A study of the mixed layer of the South China Sea based on the multiple linear regression

Multiple linear regression(MLR) method was applied to quantify the effects of the net heat flux(NHF),the net freshwater flux(NFF) and the wind stress on the mixed layer depth(MLD) of the South China Sea(SCS) based on the simple ocean data assimilation(SODA) dataset.The spatio-temporal distributions of the MLD,the buoyancy flux(combining the NHF and the NFF) and the wind stress of the SCS were presented.Then using an oceanic vertical mixing model,the MLD after a certain time under the same initial conditions but various pairs of boundary conditions(the three factors) was simulated.Applying the MLR method to the results,regression equations which modeling the relationship between the simulated MLD and the three factors were calculated.The equations indicate that when the NHF was negative,it was the primary driver of the mixed layer deepening;and when the NHF was positive,the wind stress played a more important role than that of the NHF while the NFF had the least effect.When the NHF was positive,the relative quantitative effects of the wind stress,the NHF,and the NFF were about 10,6 and 2.The above conclusions were applied to explaining the spatio-temporal distributions of the MLD in the SCS and thus proved to be valid.     

Seasonal and interannual evolution of the mixed layer in the Antarctic Zone south of Tasmania

Seasonal and interannual variations of the mixed layer properties in the Antarctic Zone (AZ) south of Tasmania are described using 7 WOCE/SR3 CTD sections and 8 years of summertime SURVOSTRAL XBT and thermosalinograph measurements between Tasmania and Antarctica. The AZ, which extends from the Polar Front (PF) to the Southern Antarctic Circumpolar Current Front (SACCF), is characterized by a 150 m deep layer of cold Winter Water (WW) overlayed in summer by warmer, fresher water mass known as Antarctic Surface Water (AASW). South of Tasmania, two branches of the PF divide the AZ into northern and southern zones with distinct water properties and variability. In the northern AZ (between the northern and southern branches of the PF), the mixed layer depth (MLD) is fairly constant in latitude, being 150 m deep in winter and around 40–60 m in summer. In the southern AZ, the winter MLD decreases from 150 m at the S-PF to 80 m at the SACCF and from 60 to 35 m in summer. Shallower mixed layers in the AZ-S are due to the decrease in the wind speed and stronger upwelling near the Antarctic Divergence. The WW MLD oscillates by ±15 m around its mean value and modest interannual changes are driven by winter wind stress anomalies.The mixed layer is on annual average 1.7 °C warmer, 0.06 fresher and 0.2 kg m⁻³ lighter in the northern AZ than in the southern AZ. The Levitus (1998) climatology is in agreement with the observed mean summer mixed layer temperature and salinity along the SURVOSTRAL line but underestimates the MLD by 10–20 m. The winter MLD in the climatology is also closed to that observed, but is 0.15 saltier than the observations along the AZ-N of the SR3 line. MLD, temperature and density show a strong seasonal cycle through the AZ while the mixed layer salinity is nearly constant throughout the year. During winter, the AZ MLD is associated with a halocline while during summer it coincides with a thermocline.Interannual variability of the AZ summer mixed layer is partly influenced by large scale processes such as the circumpolar wave which produces a warm anomaly during the summer 1996–1997, and partly by local mechanisms such as the retroflection of the S-PF which introduces cold water across the AZ-N.     

中国南海表层叶绿素a季节内变化特征及成因

对中国南海表层叶绿素a季节内变化的研究有助于深入认识其海洋特征,满足渔情预报等实际应用需求。利用卫星观测资料分析南海表层叶绿素a不同季节的季节内变化特征,结果表明南海表层叶绿素a季节内振荡强度冬季最高。冬季和春季的季节内振荡最强区域都位于吕宋岛西北侧海区,夏季和秋季振荡较强的区域偏向菲律宾群岛一侧。分析表明研究海区表层温度和表层叶绿素a 存在负相关,冬强夏弱,北强南弱。大部分海区海面高度和叶绿素a 相关性不显著,但南海东南边缘海区海面高度和叶绿素a在季节内存在正相关。冬季海盆尺度逆时针旋转的环流结构应是这些现象产生的原因。除南海东南边缘海区、海南岛东南海区和吕宋岛西侧海区之外,风应力大小和热通量均与叶绿素a 在季节内呈正相关。这显示非局地风场和海流等因素、海洋动力调整过程可能在吕宋海峡以西和南海东南边缘的表层叶绿素a 季节内变化中起到重要作用。     

The seasonal cycle of surface chlorophyll in the Peruvian upwelling system: A modelling study

The seasonal variability of surface chlorophyll in the northern Humboldt Current System is studied using satellite data, in situ observations and model simulations. The data show that surface chlorophyll concentration is highest in austral summer and decreases during austral winter, in phase opposition with coastal upwelling intensity. A regional model coupling ocean dynamics and biogeochemical cycles is used to investigate the processes which control this apparently paradoxical seasonal cycle. Model results suggest that the seasonal variability of the mixed layer depth is the main controlling factor of the seasonality. In winter, the mixed layer deepening reduces the surface chlorophyll accumulation because of a dilution effect and light limitation. In summer, biomass concentrates near the surface in the shallow mixed layer and nitrate limitation occurs, resulting in a biomass decrease in the middle of summer. Intense blooms occur during the spring restratification period, when winter light limitation relaxes, and during the fall destratification period, when the surface layer is supplied with new nutrients. Model sensitivity experiments show that the seasonal variations in insolation and surface temperature have little impact on the surface chlorophyll variability.     

Patterns of upper layer circulation variability in the South China Sea from satellite altimetry using the self-organizing map

Patterns of the South China Sea (SCS) circulation variability are extracted from merged satellite altimetry data from October 1992 through August 2004 by using the self-organizing map (SOM). The annual cycle, seasonal and inter-annual variations of the SCS surface circulation are identified through the evolution of the characteristic circulation patterns. The annual cycle of the SCS gener- al circulation patterns is described as a change between two opposite basin-scale SW-NE oriented gyres embedded with eddies: low sea surface height anomaly (SSHA) (cyclonic) in winter and high SSHA (anticyclonic) in summer half year. The transition starts from July--August (January--February) with a high (low) SSHA tongue east of Vietnam around 12°～14° N, which de- velopa into a big anticyclonic (cyclonic) gyre while moving eastward to the deep basin. During the transitions, a dipole structure, cyclonic (anticyclonic) in the north and anticyclonic (cyclonic) in the south, may be formed southeast off Vietnam with a strong zonal jet around 10°～12° N. The seasonal variation is modulated by the interannual variations. Besides the strong 1997/1998 e- vent in response to the peak Pacific El Nino in 1997, the overall SCS sea level is found to have a significant rise during 1999～ 2001, however, in summer 2004 the overall SCS sea level is lower and the basin-wide anticyclonic gyre becomes weaker than the other years.     

南海暖水季节和年际变化的初步研究

南海暖水具有明显的季节和年际变化。利用气候平均的COADS资料和NCEP大气资料分析了南海暖水的季节变化及其与海面净热通量的关系,以及由此引起的南海地区大气环流的变化。发现海面净热通量在南海暖水的季节变化过程中起到了主要的作用;冬季无暖水存在时,最大上升气流位于赤道及以南地区的印尼群岛附近,夏季最大上升气流北移到了南海暖水上空,南海暖水上空对流强烈,成为大气的对流活动中心。利用50年逐月的SODA海温资料进行垂直方向的3次样条插值,定义并计算南海暖水的强度指数,分析南海暖水的年际变化,并对南海暖水的几个异常暖年份作了合成分析,探讨了暖水年际变化的形成因素。     

南海混合层近惯性能通量的时空变化

On the basis of the QSCAT/NCEP blended wind data and simple ocean data assimilation(SODA), the wind-induced near-inertial energy flux(NIEF) in the mixed layer of the South China Sea(SCS) is estimated by a slab model, and the model results are verified by observational data near the Xisha Islands in the SCS. Then, the spatial and temporal variations of the NIEF in the SCS are analyzed. It is found that, the monthly mean NIEF exhibits obvious spatial and temporal variabilities, i.e., it is large west of Luzon Island all the year, east of the Indo-China Peninsula all the year except in spring, and in the northern SCS from May to September. The large monthly mean NIEF in the first two zones may be affected by the large local wind stress curl whilst that in the last zone is probably due to the shallow mixed layer depth. Moreover, the monthly mean NIEF is relatively large in summer and autumn due to the passage of typhoons. The spatial mean NIEF in the mixed layer of the SCS is estimated to be about 1.25 m W/m2 and the total wind energy input from wind is approximately 4.4 GW. Furthermore, the interannual variability of the spatial monthly mean NIEF and the Ni?o3.4 index are negatively correlated.     

副热带东北太平洋混合层深度及潜沉率季节循环对全球变暖的响应

本文利用第五次国际耦合模式比较计划（CMIP5）中的地球系统模式（ESM2M）,结合Argo观测数据和由Ishii等整理的再分析数据集,分析现在气候背景和辐射强迫极端增强下副热带东北太平洋海域（10°～40°N,110°～160°W）混合层深度（MLD）和潜沉率的季节变化特征,研究其对全球变暖的响应。在现在气候背景下,二者最大值均出现在冬季。潜沉率的主要贡献项存在显著的季节变化差异,1?5月主要受侧向潜沉率的变化控制,6?12月则由风应力旋度导致的埃克曼抽吸速度变化主控。全球变暖后,季节循环信号的主控要素不变。但受风应力旋度等要素变化的影响,各季节的MLD减小,大值区范围收缩。由于冬季减小幅度远大于夏季,MLD季节波动幅度（振幅）显著变小。长期看,MLD呈现持续变浅的趋势,其空间不均匀性减弱引起的MLD锋面减弱是控制侧向潜沉率减弱,最终导致总潜沉率减弱的关键。由于埃克曼抽吸速度的季节变化信号对全球变暖的响应较小,因此总潜沉率在冬季受全球变暖的影响最为强烈。上述结果表明,构成潜沉率的两个关键要素对总潜沉率的贡献比例是随着季节变化而改变的:冬季MLD锋面强盛时期,侧向潜沉率的影响将显著增强。全球变暖前后二者截然不同的变化会显著改变潜沉率的季节循环振幅,可能对该区域模态水的形成和输运产生深远的影响。     

南海东北部亚中尺度过程时空分布特征

基于高分辨率模型2009-2012年的模拟结果，本文对南海东北部亚中尺度过程的时空分布特征进行了研究。模拟结果表明，南海东北部上层广泛存在着相对涡度接近于局地行星涡度的亚中尺度过程。统计结果发现，亚中尺度过程的相对涡度的分布具有着明显的非对称性，即正涡度明显强于负涡度。这意味着相比于负涡度，具有正涡度的亚中尺度过程要更为活跃，而这主要是由离心不稳定导致。同时，亚中尺度过程在时间分布上表现出明显的冬强夏弱的季节变化特征。通过对该海区亚中尺度过程可能生成机制的分析发现，该季节变化与流场拉伸和混合层的厚度有着密切关系，冬季更强的流场拉伸和更深的混合层有利于通过锋生过程和混合层不稳定为亚中尺度过程生成提供更多的能量。     

黑潮延伸体海域次中尺度过程的季节变化研究

近年来的现场观测和理论研究发现, 次中尺度现象广泛存在于上层海洋, 其产生与锋生作用及混合层斜压不稳定存在密切联系。本文利用高分辨率的数值模拟结果并结合动力学及能量诊断分析, 对黑潮延伸体海域次中尺度过程的季节变化进行了探讨。探讨结果表明, 黑潮延伸体海域次中尺度过程具有冬季最强, 春季和秋季次之, 夏季最弱的显著季节变化特征。基于冬、夏季次中尺度能量源的诊断可以看到, 这些季节变化特征主要与上层海洋的斜压不稳定和锋生作用有关。冬季, 黑潮延伸体海域的中尺度能量较弱, 但次中尺度过程在季节尺度上表现最为活跃, 这主要与混合层斜压不稳定的作用有关; 夏季, 黑潮延伸体海域的混合层较浅, 次中尺度过程较弱, 但中尺度涡旋活跃, 中尺度流场变形引起的锋生作用对夏季次中尺度现象的产生具有重要影响。在次中尺度能量的季节变化方面, 冬季次中尺度过程从中尺度过程汲取能量的速率远高于夏季, 这是冬季次中尺度过程比夏季更为活跃的主要原因。本文研究结果有助于加深对黑潮延伸体海域次中尺度过程季节性变化及其动力机制的理解。     

Effect of Langmuir circulation on upper ocean mixing in the South China Sea

Effect of Langmuir circulation (LC) on upper ocean mixing is investigated by a two-way wave-current coupled model. Themodel is coupled of the ocean circulationmodel ROMS (regional ocean modeling system) to the surface wave model SWAN (simulating waves nearshore) via the model-coupling toolkit. The LC already certified its importance by many one-dimensional (1D) research andmechanismanalysis work. This work focuses on inducing LC’s effect in a three-dimensional (3-D) model and applying it to real field modeling. In ROMS, theMellor-Yamada turbulence closuremixing scheme is modified by including LC’s effect. The SWAN imports bathymetry, free surface and current information fromthe ROMS while exports significant wave parameters to the ROMS for Stokes wave computing every 6 s. This coupled model is applied to the South China Sea (SCS) during September 2008 cruise. The results show that LC increasing turbulence and deepening mixed layer depth (MLD) at order of O (10 m) in most of the areas, especially in the north part of SCS where most of our measurements operated. The coupled model further includes wave breaking which will bringsmore energy into water. When LC works together with wave breaking,more energy is transferred into deep layer and accelerates the MLD deepening. In the north part of the SCS, their effects aremore obvious. This is consistent with big wind event in the area of the Zhujiang River Delta. The shallow water depth as another reasonmakes themeasy to influence the oceanmixing as well.     

CMPI5多模式对南海上层海洋热含量的模拟比较

Seventeen models participating in the Coupled Model Intercomparison Project phase 5（CMIP5） activity are compared on their historical simulation of the South China Sea（SCS） ocean heat content（OHC） in the upper 300 m. Ishii＇s temperature data, based on the World Ocean Database 2005（WOD05） and World Ocean Atlas 2005（WOA05）, is used to assess the model performance by comparing the spatial patterns of seasonal OHC anomaly（OHCa） climatology, OHC climatology, monthly OHCa climatology, and interannual variability of OHCa. The spatial patterns in Ishii＇s data set show that the seasonal SCS OHCa climatology, both in winter and summer, is strongly affected by the wind stress and the current circulations in the SCS and its neighboring areas. However, the CMIP5 models present rather different spatial patterns and only a few models properly capture the dominant features in Ishii＇s pattern. Among them, GFDL-ESM2 G is of the best performance. The SCS OHC climatology in the upper 300 m varies greatly in different models. Most of them are much greater than those calculated from Ishii＇s data. However, the monthly OHCa climatology in each of the 17 CMIP5 models yields similar variation and magnitude as that in Ishii＇s. As for the interannual variability, the standard deviations of the OHCa time series in most of the models are somewhat larger than those in Ishii＇s. The correlation between the interannual time series of Ishii＇s OHCa and that from each of the 17 models is not satisfactory. Among them, BCC-CSM1.1 has the highest correlation to Ishii＇s, with a coefficient of about 0.6.