Origins and pathways of the subsurface and intermediate water masses of the Indonesian Throughflow derived from historical and Argo data

On the basis of Argo data and historic temperature/salinity data from the World Ocean Database 2001 （ WOD01 ）, origins and spreading pathways of the subsurface and intermediate water masses in the Indonesian Throughflow （ITF） region were discussed by analyzing distributions of salinity on representative isopyenal layers. Results were shown that, subsurface water mostly comes from the North Pacific Ocean while the intermediate water originates from both the North and South Pacific Ocean, even possibly from the Indian Ocean. Spreading through the Sulawesi Sea, the Makassar Strait, and file Flores Sea, the North Pacific subsurface water and the North Pacific Intermediate water dominate the western part of the Indonesian Archipelago. Furthermore as the depth increases, the features of the North Pacific sourced water masses become more obvious. In the eastern part of the waters, high sa- linity South Pacific subsurface water is blocked by a strong salinity front between Halmahera and New Guinea. Intermediate water in the eastern interior region owns salinity higher than the North Pacific intermediate water and the antarctic intermediate water （ AAIW）, possibly coming from the vertical mixing between subsurface water and the AAIW from the Pacific Ocean, and possibly coming from the northward extending of the AAIW from the Indian Ocean as well.     

Argo时代东南印度洋上层盐度变化对海平面年代际演变的影响

In the past nearly two decades, the Argo Program has created an unprecedented global observing array with continuous in situ salinity observations, providing opportunities to extend our knowledge on the variability and effects of ocean salinity. In this study, we utilize the Argo data during 2004–2017, together with the satellite observations and a newly released version of ECCO ocean reanalysis, to explore the decadal salinity variability in the Southeast Indian Ocean(SEIO) and its impacts on the regional sea level changes. Both the observations and ECCO reanalysis show that during the Argo era, sea level in the SEIO and the tropical western Pacific experienced a rapid rise in 2005–2013 and a subsequent decline in 2013–2017. Such a decadal phase reversal in sea level could be explained, to a large extent, by the steric sea level variability in the upper 300 m. Argo data further show that, in the SEIO, both the temperature and salinity changes have significant positive contributions to the decadal sea level variations. This is different from much of the Indo-Pacific region, where the halosteric component often has minor or negative contributions to the regional sea level pattern on decadal timescale. The salinity budget analyses based on the ECCO reanalysis indicate that the decadal salinity change in the upper 300 m of SEIO is mainly caused by the horizontal ocean advection. More detailed decomposition reveals that in the SEIO, there exists a strong meridional salinity front between the tropical low-salinity and subtropical high salinity waters. The meridional component of decadal circulation changes will induce strong cross-front salinity exchange and thus the significant regional salinity variations.     

Significant salinity increase in subsurface waters of the South China Sea during 2016–2017

The South China Sea(SCS) is the largest semi-enclosed marginal sea in the North Pacific. Salinity changes in the SCS play an important role in regional and global ocean circulation and the hydrological cycle. However, there are few studies on salinity changes over the SCS due to lack of high-quality and long-term observations. In the past decade, the deployment of floats from the Argo program in the SCS and their accumulated temperature and salinity profiles have made it possible for us to examine salinity changes over the entire basin. In this study,salinity changes were investigated with Argo and underwater glider temperature and salinity observations and gridded temperature–salinity objective analyses(UK Met Office Hadley Centre EN4.2.1 objective analysis and China Argo Real-time Data Center BOA_Argo). The results indicated that the subsurface water in the entire SCS became significantly saltier during 2016–2017. The most significant salinity increase was found during 2016 in the northeastern SCS. The subsurface water in the northeastern SCS exhibited a salinity maximum above 35, which was recorded by three Argo floats during 2015–2016. Such high salinity water was rarely observed and reported prior to the Argo era. Average salinity of 2016–2017 along the 25.5σ_θ–23.5σ_θ isopycnal surfaces in the whole SCS is 0.014-0.130 higher than the climatology. Increases in subsurface salinity started from the northeastern SCS and extended southwestward gradually. Moreover, the subsurface salinity changes, especially in the northern SCS,exhibited a semiannual lead behind the subsurface Luzon Strait transport. Further analysis indicated that the predominance of advection, driven by subsurface Luzon Strait transport, led to salinification along the western boundary of the SCS. In other parts of the SCS, negative wind stress curl trends tended to preserve the high salinity characteristics of the subsurface water.     

2017年春夏季西北太平洋浮游植物的区域差异及其与不同水团的关系

The West Pacific Ocean is considered as the provenance center of global marine life and has the highest species diversity of numerous marine taxa. The phytoplankton, as the primary producer at the base of the food chain,effects on climate change, fish resources as well as the entire ecosystem. However, there are few large-scale surveys covering several currents with different hydrographic characteristics. This study aimed to explore the relationships between the spatio-temporal variation in phytoplankton community structure and different water masses. A total of 630 water samples and 90 net samples of phytoplankton were collected at 45 stations in the Northwest Pacific Ocean(21.0°–42.0°N, 118.0°–156.0°E) during spring and summer 2017. A total of 281 phytoplankton taxa(5 μm) belonging to 61 genera were identified in the study area. The distribution pattern of the phytoplankton community differed significantly both spatially and temporally. The average abundances of phytoplankton in spring and summer were 797.07×10~2 cells/L and 84.94×10~2 cells/L, respectively. Whether in spring or summer, the maximum abundance always appeared in the northern transition region affected by the Oyashio Current, where nutrients were abundant and diatoms dominated the phytoplankton community;whereas the phytoplankton abundance was very low in the oligotrophic Kuroshio region, and the proportion of dinoflagellates in total abundance increased significantly. The horizontal distribution of phytoplankton abundance increased from low to high latitudes, which was consistent with the trend of nutrient distributions, but contrary to that of water temperature and salinity. In the northern area affected by the Oyashio Current, the phytoplankton abundance was mainly concentrated in the upper 30 m of water column, while the maximum abundance often occurred at depths of 50–75 m in the south-central area affected by the Kuroshio Current.Pearson correlation and redundancy analysis(RDA) showed that phytoplankton abundance was significant negatively correlated with temperature and salinity, but positively correlated with nutrient concentration. The phytoplankton community structure was mainly determined by nutrient availability, especially the N:P ratio.     

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.     

Variations of oceanic fronts and their influence on the fishing grounds of Ommastrephes bartramii in the Northwest Pacific

Two predominant currents, the warm Kuroshio Current and the cold Oyashio Current, meet in the Northwest Pacific Ocean. The dynamics of physical oceanographic structures in this region, including frontal zones and meandering eddies, result in a highly productive habitat that serves as a favorable feeding ground for various commercially important species. Neon flying squid, Ommastrephes bartramii, is an important oceanic squid, which is widely distributed in the North Pacific Ocean. Based on the catch data collected by Chinese squid jigging fleets and relevant environmental data, including sea surface temperature(SST) and fronts(represented by gradients of SST and thermocline) during 1998–2009, the variations of oceanic fronts and their influence on the fishing grounds of O. bartramii were evaluated, and the differences in distribution of fishing grounds of O. bartramii in 2000 and 2002 were compared by describing the differences in vertical temperature between 0–300 m. It was found that the preferred horizontal temperature gradient of SST for O. bartramii tended to be centered at 0.01–0.02°C/nm, which attracted nearly 80% of the total fishing effort, and the preferred horizontal temperature gradients at the 50 m and 105 m layers were mainly located at 0.01–0.03°C/nm, which accounted for more than 70% of the total fishing effort during August–October. The preferred vertical temperature gradient within the 0–50 m layer for O. bartramii tended to be centered at 0.15–0.25°C/m during August and September and at 0.10–0.15°C/m in October, implying that the mixed surface layer was distributed at depths of 0–50 m. It was concluded that the vertical temperature gradient was more important than the horizontal temperature gradient in playing a role in forming the fishing ground. The results improved our understanding of the spatial dynamics of the O. bartramii fishery.     

A dipole mode at thermocline layer in the tropical Indian Ocean

Temperature data at different layers of the past 45 years were studied and we found adiploe mode in the thermocline layer (DMT): anomalously cold sea temperature off the coast of Sumatra and warm sea temperature in the western Indian Ocean. First, we analyzed the temperature and the temperature anomaly (TA) along the equatorial Indian Ocean in different layers. This shows that stronger cold and warm TA signals appeared at subsurface than at the surface in the tropical Indian O-cean. This result shows that there may be a strong dipole mode pattern in the subsurface tropical Indian Ocean. Secondly we used Empirical Orthogonal Functions (EOF) to analyze the TA at thermocline layer. The first EOF pattern was a dipole mode pattern. Finally we analyzed the correlations between DMT and surface tropical dipole mode (SDM), DMT and Nino 3 SSTA, etc. and these correlations are strong.     

Exploring thermocline and water masses variability in southern South China Sea from the World Ocean Database (WOD)

Study about water characteristics(temperature and salinity) from the World Ocean Database(WOD) was conducted in the area of southern South China Sea(SSCS), covering the area of 0°–10°N, 100°–117°E. From interannual analysis, upper layer(10 m) and deep water temperature(50 m) increased from 1951 until 2014. Monthly averaged show that May recorded the highest upper layer temperature while January recorded the lowest. It was different for the deep water which recorded the highest value in September and lowest in February. Contour plot for upper layer temperature in the study area shows presence of thermal front of cold water at southern part of Vietnam tip especially during peak northeast season(December–January). The appearances of warm water were obviously seen during generating southwest monsoon(May–June). Thermocline study revealed the deepest isothermal layer depth(ILD) during peak northeast and southwest monsoon. Temperature threshold at shallow area reach more than 0.8°C during the transitional period. Water mass study described T-S profile based on particular region. Water mass during the southwest monsoon is typically well mixed compared to other seasons while strong separation according to location is very clear. During transitional period between northeast monsoon to southwest monsoon, the increasing of water temperature can be seen at Continental Shelf Water(CSW) which tend to be higher than 29°C and vice versa condition during transitional period between southwest monsoon to northeast monsoon. Dispersion of T-S profile can be seen during southwest monsoon inside Tropical Surface Water(TSW) where the salinity and temperature become higher than during northeast monsoon.     

Low 210Pb in the upper thermocline in the Canadian Basin: scavenge process over the Chukchi Sea

210Pb was measured during the 3rd Chinese National Arctic Research Expedition cruise to investigate its spatial pattern in the western Arctic Ocean, as well as its relation with the thermocline in the Canadian Basin. The specific activities varied from 0.04 to 2.72, 0.013 to 4.37, and 0.1 to 4.85 Bq/m3 for dissolved, particulate, and bulk 210Pb, respectively, corresponding to respective averages of 0.65, 0.43, and 1.08 Bq/m3. In the Canadian Basin, the minimum 210Pb activities occurred in the thermocline, which was characterized by low temperature of-1.52°C and salinity of 33.1. Combining the spatial distribution of 210Pb and hydrographical characteristics in the western Arctic Ocean, this scenario was ascribed to the effective scavenging of 210Pb when the Pacific water flowed across the Chukchi Shelf. Quantitatively, this interpretation was supported by both the shorter residence times and higher scavenging efficiencies(SE) of dissolved 210Pb over the Chukchi Shelf. The highest SE values were observed in the Herald Shoal and bottom waters over the slope.     

Physical mechanism and numerical simulations of surface layer temperature inversion in tropical ocean

1 Introduction Ocean upper mixed layer, with nearly uniform temperature, salinity and density, is formed by sea sur- face forces such as wind stress, buoyancy flux and sur- face waves, etc. Under the mixed layer, thermocline and halocline often exist. Usually the depth of them is almost equal. But in the tropical ocean, the halocline is often shallower than the thermocline. Then between the layer of uniform salinity and the thermocline, there is a layer which has a rather strong density gradi…     

The effect of a northward shift in the southern hemisphere westerlies on the global ocean

We examine the effect of a northward shift in the position of the southern hemisphere subpolar westerly winds (SWWs) on the vertical and horizontal distribution of temperature and salinity in the world ocean. A northward shift of the SWWs causes a latitudinal contraction of the subpolar gyres in the southern hemisphere (SH). In the Indian and Pacific, this leads to subsurface warming in the subtropical thermocline. As the southern margins of the gyres move into latitudes characterised by warmer surface air temperature (SAT), the layers at mid-depth below 400 m depth become ventilated by warmer water. We characterize the approximation of the ventilated thermocline in our coarse resolution model using a set of passive tracer experiments, and illustrate how the northward shift in the SWWs causes an equatorward shift in the latitude of origin of water ventilating layers deeper than 400 m in the Indian and Pacific, leaving the total surface ventilation of the upper 1200 m unchanged. In contrast, the latitudinal constraint on the Antarctic Circumpolar Current posed by the Drake Passage causes a cooling and freshening throughout the Atlantic thermocline; here, subsurface thermocline water originates from higher latitudes under the wind shift. On longer timescales Atlantic cooling and freshening is reinforced by a reduction in North Atlantic Deep Water (NADW) formation and surface salinification of the Indian and Pacific Oceans. In effect, the latitude of zero wind stress curl in the SWWs regulates the relative importance of the “cold water route” via the Drake Passage and the “warm water route” associated with thermocline water exchange via the Indian Ocean. Thus, a more northward location of the SWWs corresponds with a reduced salinity contrast between the Indian/ Pacific Oceans and the Atlantic. This results in reduced NADW formation. Also, a more northward location of the SWWs facilitates the injection of cool fresh Antarctic Intermediate Water into the South Atlantic subtropical gyre. Beyond these changes, on a millennial timescale, the deep ocean warms throughout the water column in response to the wind shift. Global salinity stratification also becomes less stable, as more saline water remains at the surface and accumulates in the Indian and Pacific thermocline. The freshening of the deep ocean reflects a reduced stirring of the global ocean due to reduced net circulation arising from a misalignment between the westerlies and the topographically constrained ACC. Our results lend support to the idea that a more equatorward location of the SWW maximum during glacial climates contributed to cooler and fresher conditions in the Atlantic, inhibiting NADW.     

热带太平洋盐度变化:2015?2017年盐度异常

本文利用Argo海水盐度资料、海流同化数据和同期大气再分析数据，探讨热带太平洋盐度趋势变化和相关动力过程。Argo资料显示，2015?2017年热带太平洋出现显著的盐度异常(SAE)，这是改变长期趋势的主要原因，表现为表层显著淡化和次表层咸化特征。这种盐度异常具有明显的区域性特征和垂直结构的差异，体现在热带太平洋北部海区(NTP)和南太平洋辐合区(SPCZ)表层淡化，盐度最大变幅为0.71～0.92，淡化可以达到混合层底；热带太平洋南部海区(STP)次表层咸化，最大变幅为0.46，主要发生在温跃层附近，期间盐度异常沿着等位密面从西向东扩展。平流和挟卷是与SAE密切相关的海洋动力过程，两者在NTP淡化海域有着持续而较为显著的影响，在SPCZ淡化、STP咸化海域后期贡献也较大，其中盐度平流对热带太平洋海区盐度变化起主要贡献。NTP淡化海区表层淡水通量和STP咸化海区密度补偿引起的混合也是SAE的重要影响因素。     

基于Argo浮标和历史资料的热带西太平洋次表层与中层水年代变化分析

利用20世纪80年代和90年代WOD01(World Ocean Database2001)中的CTD温盐剖面资料和2000年以后Argo资料,对比分析了热带西太平洋次表层和中层水团分布的年代变化特征。分析结果表明,在这两个时期,起源于南北太平洋中高纬度海域的各次表层水和中层水,在热带西太平洋分布特征和交织在一起的总体态势基本一致,水团性质的年代变化不大。这与上述两个时段全球海洋-大气耦合系统趋于正常状态相吻合。通过辨识和跟踪表征次表层水性质的盐度极大值,发现南太平洋热带水沿西边界向北扩散程度有所加大,由前一时期的5°N,进一步扩散到6°~7°N;北太平洋热带水在西边界附近的向南扩散程度有所削弱,在2002-2005年间只向南扩散到4°N,而前一个时期则可向南扩散到2°N。通过辨识表征中层水性质的盐度极小值,南极中层水在西边界附近向北扩散程度有所加大,在2002-2005年到达13°N附近,而前一个时期只到达11°N;同期,北太平洋中层水在西边界附近的向南扩散程度有所削弱。上述年代变化与全球水循环强度的变化之间有何关系有待进一步研究。     

Long-term change and variation of salinity in the western North Pacific subtropical gyre revealed by 50-year long observations along 137°E

The 137°E repeat hydrographic section for 50 winters during 1967–2016 has been analyzed to examine interannual to interdecadal variations and long-term changes of salinity and temperature in the surface and intermediate layers of the western North Pacific, with a particular focus on freshening in the subtropical gyre. Rapid freshening on both isobars and isopycnals began in the mid-1990s and persisted for the last 20 years in the upper main thermocline/halocline in the western subtropical gyre. In addition, significant decadal variability of salinity existed in the subtropical mode water (STMW), as previously reported for the shallower layers. An analysis of the 144°E repeat hydrographic section during 1984–2013 supplemented by Argo profiling float data in 2014 and 2015 revealed that the freshening trend and decadal variability observed at 137°E originated in the winter mixed layer in the Kuroshio Extension (KE) region and was transmitted southwestward to 137°E 1–2 years later in association with the subduction and advection of STMW. The mechanism of these changes and variations in the source region was further investigated. In addition to the surface freshwater flux in the KE region pointed out by previous studies, the decadal KE variability in association with the Pacific Decadal Oscillation likely contributes to the decadal salinity variability through water exchange between the subtropics and the subarctic across the KE. Interdecadal change in both the surface freshwater flux and the KE state, however, failed to explain the rapid freshening for the last 20 years.     

Characteristics and variability of the Indonesian throughflow water at the outflow straits

Property structure and variability of the Indonesian Throughflow Water in the major outflow straits (Lombok, Ombai and Timor) are revised from newly available data sets and output from a numerical model. Emphasis is put on the upper layers of the Indonesian Throughflow that impacts the heat and freshwater fluxes of the South Equatorial Current in the Indian Ocean. During the April–June monsoon transition the salinity maximum signature of the North Pacific thermocline water is strongly attenuated. This freshening of the thermocline layer is more intense in Ombai and is related to the supply of fresh near-surface Java Sea water that is drawn eastward by surface monsoon currents and subject to strong diapycnal mixing. The freshwater exits to the Indian Ocean first through Lombok Strait and later through Ombai and Timor, with an advective phase lag of between one and five months. Because of these phase lags, the fresher surface and thermocline water is found in the southeast Indian Ocean from the beginning of the monsoon transition period in April through until the end of the southeast monsoon in September, a much longer time period than previously estimated.     

Ocean variability North of New Guinea derived from TRITON buoy data

We investigated variability in the ocean surface-subsurface layer north of New Guinea using Triangle Trans-Ocean Buoy Network (TRITON) buoys at 2°N, 138°E and 0°N, 138°E during the period from October 1999 to July 2004. Both North and South Pacific waters were observed below the subsurface at these stations. The variability in the subsurface waters was particularly high at 2°N, 138°E. Clear interannual variability occurred near the surface; the water type differed before and after onset of the 2002–03 El Niño. Before summer 2001, water that appeared to be advected from the central equatorial Pacific occupied the near surface layer. After autumn 2001, waters advected by the New Guinea Coastal Current were observed near the surface. Intraseasonal and seasonal variations were also observed below the subsurface. With regard to seasonal variability, the salinity of the subsurface saline water, the South Pacific Tropical Water, was generally high during the boreal summer-autumn, when the New Guinea Coastal Undercurrent was strong. Intraseasonal fluctuations on a scale of 20 to 60 days were also seen and may have been associated with intrinsic oceanic variability, such as ocean eddies, near the stations. Ocean variability in the thermocline layer between 100 and 200 m greatly affects the surface dynamic height variability; water variability before 2001 and variability in the pycnocline depth after 2002 are important factors affecting the thermocline.     

西太平洋暖池变异及其对西太平洋次表层海温场的影响

应用热带太平洋上层XBT温度资料,分析研究了西太平洋暖池区(0°～16°N,125°～145°E)上层海洋的变化特征以及与西太平洋次表层海温场之间的关系.研究表明,西太平洋暖池区的垂向温度存在显著的年际变化,尤其在次表层(120～200m)的变化最为明显.西太平洋暖池区的次表层冷暖信号明显早于西太平洋次表层的海温异常.分析发现,西太平洋暖池区的海温异常是导致整个西太平洋次表层海温场变异的关键区,当西太平洋暖池区的次表层冷暖信号加强时,3～4个月后西太平洋海温场出现大范围的冷暖异常.     

基于历史资料和Argo资料的印尼贯通流次表层和中层水起源与路径探讨

利用Argo资料和《世界海洋数据集2001版》(WOD01)温盐历史资料,通过对代表性等位势面上盐度分布的分析,探讨了次表层和中层等不同层次上印尼贯通流(ITF)的起源与路径问题.分析结果表明,ITF的次表层水源主要来自北太平洋,中层水源地既包括北太平洋、南太平洋,同时也不能排除有印度洋的可能性.在印度尼西亚海域西部,ITF的次表层和中层水源分别为北太平洋热带水(NPTW)和中层水(NPIW),经苏拉威西海、望加锡海峡到达弗洛勒斯海,层次越深特征越明显.在印度尼西亚海域东部,发现哈马黑拉-新几内亚水道附近存在次表层强盐度锋面,阻隔了南太平洋热带水(SPTW)由此进入ITF海域;中层水具有高于NPIW和来自南太平洋的南极中层水(AAIW)的盐度值,既可能是AAIW和SPTW在当地发生剧烈垂直混合而形成,也可能是来自印度洋的AAIW向北延伸进入ITF的结果.     

Spreading and salinity change of North Pacific Tropical Water in the Philippine Sea

Newly formed North Pacific Tropical Water (NPTW) is carried to the Philippine Sea (PS) by the North Equatorial Current (NEC) as a subsurface salinity maximum. In this study its spreading and salinity change processes are explored using existing hydrographic data of the World Ocean Database 2009 and Argo floats. Spreading of NPTW is closely associated with the transports of the NEC, Mindanao Current (MC), and Kuroshio. Estimated for subsurface water with salinity S greater than 34.8?psu, the southward (northward) geostrophic transport of NPTW by the MC (Kuroshio) at 8°N (18°N) is about 4.4 (5.7)?Sv (1?Sv?=?10⁶?m³?s^?1), which is not sensitive to reference level choice. Fields of salinity maximum, geostrophic current, sea level variation, and potential vorticity suggest that the equatorward spreading of NPTW to the tropics is primarily afforded by the MC, whereas its poleward spreading is achieved by both the Kuroshio transport along the coast and open-ocean mesoscale eddy fluxes in the northern PS. The NPTW also undergoes a prominent freshening in the PS. Lying beneath fresh surface water, salinity decreases quicker in the upper part of the NPTW, which gradually lowers the salinity maximum of NPTW to denser isopycnals. Salinity decrease is especially fast in the MC, with along-path decreasing rate reaching O (10^?7?psu?s^?1). Both diapycnal and isopycnal mixing effects are shown to be elevated in the MC owing to enhanced salinity gradient near the Mindanao Eddy. These results suggest intensive dispersion of thermal anomalies along the subtropical-to-tropical thermocline water pathway near the western boundary.