2013 Vol. 32, No. 5

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Observations of the Karimata Strait througflow from December 2007 to November 2008
SUSANTO R Dwi, WEI Zexun, ADI Rameyo T, FAN Bin, LI Shujiang, FANG Guohong
2013, 32(5): 1-6. doi: 10.1007/s13131-013-0307-3
In order to quantitatively estimate the volume and property transports between the South China Sea and Indonesian Seas via the Karimata Strait, two trawl-resistant bottom mounts, with ADCPs embedded, were deployed in the strait to measure the velocity profile as part of the South China Sea-Indonesian Seas transport/exchange (SITE) program. A pair of surface and bottom acoustic modems was employed to transfer the measured velocity without recovering the mooring. The advantage and problems of the instruments in this field work are reported and discussed. The field observations confirm the existence of the South China Sea branch of Indonesian throughflow via the Karimata Strait with a stronger southward flow in boreal winter and weaker southward bottom flow in boreal summer, beneath the upper layer northward (reversal) flow. The estimate of the averaged volume, heat and freshwater transports from December 2007 to March 2008 (winter) is (-2.7±1.1)×106 m3/s, (-0.30±0.11) PW, (-0.18±0.07)×106 m3/s and from May to September 2008 (summer) is (1.2±0.6)×106 m3/s, (0.14±0.03) PW, (0.12±0.04)×106 m3/s and for the entire record from December 2007 to October 2008 is (-0.5±1.9)×106 m3/s, (-0.05±0.22) PW, (-0.01±0.15)×106 m3/s (negative/positive represents southward/northward transport), respectively. The existence of southward bottom flow in boreal summer implies that the downward sea surface slope from north to south as found by Fang et al. (2010) for winter is a year-round phenomenon.
Effects of the surface wave-induced mixing on circulation in an isopycnal-coordinate oceanic circulation model
HUANG Chuanjiang, QIAO Fangli, WEI Zexun
2013, 32(5): 7-14. doi: 10.1007/s13131-013-0308-2
The influence of the nonbreaking surface wave-induced mixing under the mixed layer on the oceanic circulation was investigated using an isopycnal-coordinate oceanic circulation model. The effect of the waveinduced mixing within the mixed layer was eliminated via a bulk mixed layer model. The results show that the wave-induced mixing can penetrate through the mixed layer and into the oceanic interior. The waveinduced mixing under the mixed layer has an important effect on the distribution of temperature of the upper ocean at middle and high latitudes in summer, especially the structure of the seasonal thermocline. Moreover, the wave-induced mixing can affect the oceanic circulation, such as western boundary currents and the North Equatorial Currents through changes of sea surface height associated with the variation of the thermal structure of the upper ocean.
The low-frequency variance of the ocean surface wave field in the area of the Antarctic Circumpolar Current
ZHANG Xiaoshuang, WU Kejian, WANG Bin, WANG Zhifeng
2013, 32(5): 15-21. doi: 10.1007/s13131-013-0309-1
The low-frequency variance of the surface wave in the area of the Antarctic Circumpolar Current (ACC) and its correlation with the antarctic circumpolar wave (ACW) are focused on. The analysis of the series of 44 a significant wave height (SWH) interannual anomalies reveals that the SWH anomalies have a strong periodicity of about 4-5 a and this signal propagates eastward obviously from 1985 to 1995, which needs about 8 a to complete a mimacircle around the earth. The method of empirical orthogonal function (EOF) is used to analyze the filtered monthly SWH anomalies to study the spatio-temporal distributions and the propagation characteristics of the low-frequency signals in the wave field. Both the dominant wavenumber-2 pattern in space and the propagation feature in the south Pacific, the south Atlantic and the south Indian ocean show strong consistency with the ACW. So it is reasonable to conclude that the ACW signal also exists in the wave field. The ACW is important for the climate in the Southern Ocean, so it is worth to pay more attention to the large-scale effect of the surface wave, which may also be important for climate studies.
Indices of strength and location for the North Pacific Subtropical and Subpolar Gyres
JIANG Hua, JIN Qihua, WANG Hui, HUANG Ruixin
2013, 32(5): 22-30. doi: 10.1007/s13131-013-0310-8
The adjustment of the North Pacific Subtropical and Subpolar Gyres towards changes in wind stress leads to different time-scale variabilities, which plays a significant role in climate changes. Based on the Simple Ocean Data Assimilation (SODA) and Global Ocean Data Assimilation System (GODAS) datasets, the variations of the Subtropical and Subpolar Gyres are diagnosed using "three-dimension Ocean Circulation Diagnostic Method", and established three types of index series describe the strength, meridional and depth center of the Subtropical and Subpolar Gyres. The above indices present the seasonal, interannual and interdecadal variabilities of the Subtropical and Subpolar Gyres, which proves well. Both the Gyres are the strongest in winter, but the Subtropical Gyre is the weakest in summer and the Subpolar Gyre is the weakest in autumn. The Subtropical Gyre moves northward from February to March, southward in October, and to the southernmost in around January, while the Subpolar Gyre moves northward in spring, southward in summer, northward again in autumn and reaching the extreme point in winter to the south. The common feature of the interannual and interdecadal variabilities is that the two gyres were weaker and to the north before 1976-1977, while they were stronger and to the south after 1976-1977. The Subpolar Gyre has made a paramount contribution to the variability on interdecadal scales. As is indicated with the Subpolar Gyre strength indices, there was an important shift from weak to strong around 1976-1977, and the correlation coefficient with the North Pacific Decadal Oscillation (PDO) indices was 0.45, which was far better than that between the Subtropical Gyre strength indices and the PDO. Tests show that influenced by small and mesoscale eddies, the magnitude of large-scale gyres strength is strongly dependent on data resolution. But seasonal interannual and interdecadal large-scale variabilities of the two gyres presented with indices is less affected by model resolution.
The numerical simulation of the Kuroshio frontal eddies in the East China Sea using a hybrid coordinate ocean mode
JIA Yongjun, ZHANG Youguang, LIN Mingsen
2013, 32(5): 31-41. doi: 10.1007/s13131-013-0311-7
A hybrid coordinate ocean model (HYCOM) is used to simulate the Kuroshio frontal eddies in the East China Sea (ECS). The research area is located (20°-32°N, 120°-132°E). Using the simulating data, it is figured out that the Kuroshio frontal eddies occur in summer as well as in the other season in this area. The life cycle of the Kuroshio and its frontal eddies is different with the position. The life-cycle of the Kuroshio frontal eddies of the northwest Diaoyu Islands is about 14 d; and the life cycle of the Kuroshio frontal eddies of southwest Yakushima about 20 d. This result extends the in situ researching results greatly. In addition, the vertical impact depth of the Kuroshio frontal eddies is also changing with the position. On the whole, in the ECS, the maximum impact depth of the Kuroshio frontal eddies of the northwest Taiwan Islands is about 75 m; the maximum impact depth of the Kuroshio frontal eddies of the northwest Diaoyu Islands is more than 125 m, but no more than 200 m; and the maximum impact depth of the Kuroshio frontal eddies of southwest Yakushima is up to 100 m.
Study of the propagation direction of the internal waves in the South China Sea using satellite images
WANG Juan, HUANG Weigen, YANG Jingsong, ZHANG Huaguo, ZHENG Gang
2013, 32(5): 42-50. doi: 10.1007/s13131-013-0312-6
Internal wave propagation carries considerable vertical shear which can lead to turbulence and mixing. Based on the analysis of more than 2 500 synthetic aperture radar (SAR) and optical satellite images, the internal wave propagation in the whole South China Sea was investigated systematically. The results show that (1) in the northeastern South China Sea, most internal waves propagate westward from the Luzon Strait and are diffracted by coral reefs near the Dongsha Islands. Some impinge onto the shelf and a few are reflected; (2) in the northwestern South China Sea, most internal waves are generated at the shelf and propagate northwestward or westward to the coast; (3) in the western South China Sea, most internal waves propagate westward to the Vietnamese coast, except a few propagate southward to the deep sea; and (4) in the southern South China Sea, most internal waves propagate southwestward to the coast. Some propagate southeastward to the coast of Kalimantan Island, and a few propagate southeastward because of the influence of the Mekong River.
An experimental and numerical study of the freak wave speed
CUI Cheng, ZHANG Ningchuan, KANG Haigui, YU Yuxiu
2013, 32(5): 51-56. doi: 10.1007/s13131-013-0313-5
The propagation speed is one of the most important characteristics for describing freak waves. The research of freak wave speed is not only helpful for understanding the generation mechanism and evolution process of freak waves, but also applicable to the prediction. A stable and accurate method is proposed for the calculation of the freak wave speed, in which physical model tests are carried out to measure the motion of the largest wave crest along the wave tank. The linear regression relationship between the spatial position of the largest wave crest and instantaneous moment is established to calculate the speed of totally 248 cases of experimental freak waves and 312 supplementary cases of numerical freak waves. Based on the calculate results, a semitheoretical and semiempirical formula is proposed by using a regression analysis method to predict the speed of the freak wave, and the nonlinear characteristic of the freak wave speed is also investigated.
The physical structures of snow and sea ice in the Arctic section of 150°-180°W during the summer of 2010
HUANG Wenfeng, LEI Ruibo, ILKKA Matero, LI Qun, WANG Yongxue, LI Zhijun
2013, 32(5): 57-67. doi: 10.1007/s13131-013-0314-4
The physical structures of snow and sea ice in the Arctic section of 150°-180°W were observed on the basis of snow-pit, ice-core, and drill-hole measurements from late July to late August 2010. Almost all the investigated floes were first-year ice, except for one located north of Alaska, which was probably multi-year ice transported from north of the Canadian Arctic Archipelago during early summer. The snow covers over all the investigated floes were in the melting phase, with temperatures approaching 0℃ and densities of 295-398 kg/m3. The snow covers can be divided into two to five layers of different textures, with most cases having a top layer of fresh snow, a round-grain layer in the middle, and slush and/or thin icing layers at the bottom. The first-year sea ice contained about 7%-17% granular ice at the top. There was no granular ice in the lower layers. The interior melting and desalination of sea ice introduced strong stratifications of temperature, salinity, density, and gas and brine volume fractions. The sea ice temperature exhibited linear cooling with depth, while the salinity and the density increased linearly with normalized depth from 0.2 to 0.9 and from 0 to 0.65, respectively. The top layer, especially the freeboard layer, had the lowest salinity and density, and consequently the largest gas content and the smallest brine content. Both the salinity and density in the ice basal layer were highly scattered due to large differences in ice porosity among the samples. The bulk average sea ice temperature, salinity, density, and gas and brine volume fractions were -0.8℃, 1.8, 837 kg/m3, 9.3% and 10.4%, respectively. The snow cover, sea ice bottom, and sea ice interior show evidences of melting during mid-August in the investigated floe located at about 87°N, 175°W.
On the parameterization of drag coefficient over sea surface
WANG Juanjuan, SONG Jinbao, HUANG Yansong, FAN Conghui
2013, 32(5): 68-74. doi: 10.1007/s13131-013-0315-3
Six parameterization schemes of roughness or drag coefficient are evaluated on the basis of the data from six experiments. They present great consistency with measurement when friction velocity u*<0.5 m/s (approximately corresponding to 10 m wind speed U10<12 m/s) and large deviation from measurement when u*≥0.5 m/s (approximately U10≥12 m/s). In order to improve the deviation, a new parameterization of drag coefficient is derived on the basis of the similarity theory, Charnock relationship and Toba 3/2 power law. Wave steepness and wind-sea Reynolds number are considered in the new parameterization. Then it is tested on the basis of the measurements and shows significant improvement when u* ≥0.5 m/s. Its standard errors are much smaller than the ones of the other six parameterizations. However, the new parameterization still needs more tests especially for high winds.
Research Notes
FY-3A/MERSI, ocean color algorithm, products and demonstrative applications
SUN Ling, GUO Maohua, ZHU Jianhua, HU Xiuqing, SONG Qingjun
2013, 32(5): 75-81. doi: 10.1007/s13131-013-0316-2
A medium resolution spectral imager (MERSI) on-board the first spacecraft of the second generation of China's polar-orbit meteorological satellites FY-3A, is a MODIS-like sensor with 20 bands covering visible to thermal infrared spectral region. FY-3A/MERSI is capable of making continuous global observations, and ocean color application is one of its main targets. The objective is to provide information about the ocean color products of FY-3A/MERSI, including sensor calibration, ocean color algorithms, ocean color products validation and applications. Although there is a visible on-board calibration device, it cannot realize the on-board absolute radiometric calibration in the reflective solar bands. A multisite vicarious calibration method is developed, and used for monitoring the in-flight response change and providing post-launch calibration coefficients updating. FY-3A/MERSI ocean color products consist of the water-leaving reflectance retrieved from an atmospheric correction algorithm, a chlorophyll a concentration (CHL1) and a pigment concentration (PIG1) from global empirical models, the chlorophyll a concentration (CHL2), a total suspended mater concentration (TSM) and the absorption coefficient of CDOM and NAP (YS443) from China's regional empirical models. The atmospheric correction algorithm based on lookup tables and ocean color components concentration estimation models are described. By comparison with in situ data, the FY-3A/MERSI ocean color products have been validated and preliminary results are presented. Some successful ocean color applications such as algae bloom monitoring and coastal suspended sediment variation have demonstrated the usefulness of FY-3A/MERSI ocean color products.
“Digital earth” in support of an online oceanic educational public service and popularization
ZHANG Xin, DONG Wen, JIANG Xiaoyi, SHI Suixiang, CHI Tianhe
2013, 32(5): 82-86. doi: 10.1007/s13131-013-0317-1
According to the initial vision of "digital earth" (DE), the public should constitute a significant proportion of its users. However, to date, most of the studies and applications have focused on science, the private sector and government. A DE-supported online oceanic educational public service and popularization system, iOcean, is studied. First, the vision for the public's engagement with "digital ocean" is described:an analysis is presented from four aspects, i.e., the space dimension, the time dimension, the state dimension and its relationship with human beings. Second, the technical framework of iOcean is discussed, including data updating and model computing, the data, the function, and the application layers. Third, two key technologies are studied in detail that will enable the construction of iOcean. More than half a million public viewers have used the current version's website. Practical demonstrations show that iOcean can bring virtual oceans to web browsers and desktops and construct a bridge between government departments and the general public.