Isothermal,mixed, and barrier layers in the subtropical and tropical Atlantic Ocean during the ARAMIS experiment

The ARAMIS project (Altimétrie sur un Rail Atlantique et Mesures In Situ) provides a long term survey (2002–2008) for thermohaline structures in the tropical Atlantic Ocean. Here, these data are used to address questions regarding the existence and formation of barrier layers. The analysis presented which uses different criteria to calculate isothermal layer depth and mixed layer depth, shows the sensitivity of different methods to a variety of variables. A 0.5 °C criterion for the isothermal layer and its equivalent in density change for the mixed layer are found to be the most adaptable due to their robustness. Barrier layers observed with ARAMIS data sets are in good agreement with climatologies and in-situ Argo floats along the route. Three systems of barrier layers are observed during the voyages. Their variability and formation as described in previous works concerned with the tropical Atlantic are analyzed. Different processes such as subduction, frontal activity, and precipitation are needed to explain their presence.     

Quasi-isopycnic layer model for large-scale ocean circulation

A hydrothermodynamic model of a multilayer ocean, incorporating the upper mixed layer (UML) is described. The model is based on a system of primitive equations integrated within each layer. All layers are assumed to be horizontally-inhomogeneous, however, the density in each thermocline layer changes within the limits determineda priori by the prescribed basic stratification. It is assumed that the layers may outcrop. Results of the numerical experiment on subduction simulation (downwelling of UML waters toward the main thermocline layers) are given.Translated by V. Puchkin.     

Modelling of the generation and evolution of the cold intermediate layer in the Black Sea

This paper discusses the results of a numerical experiment on modelling the seasonal variability of the water circulation and temperature/salinity fields in the Black Sea. A multi-layered quasi-isopycnic model is used based on primitive equations and incorporating the upper mixed layer (UML). It is shown that during springtime warming, relatively cold layers emerge in some areas, due to subduction, from the UML, which then spread virtually over the entire basin and persist until they become absorbed by the rapidly deepening UML during the period of cooling in winter. Translated by Vladimir A. Puchkin.     

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.     

Response of the North Pacific subtropical countercurrent and its variability to global warming

Response of the North Pacific subtropical countercurrent (STCC) and its variability to global warming is examined in a state-of-the-art coupled model that is forced by increasing greenhouse gas concentrations. Compared with the present climate, the upper ocean is more stratified, and the mixed layer depth (MLD) shoals in warmer climate. The maximum change of winter MLD appears in the Kuroshio–Oyashio extension (KOE) region, where the mean MLD is the deepest in the North Pacific. This weakens the MLD front and reduces lateral induction. As a result of the reduced subduction rate and a decrease in sea surface density in KOE, mode waters form on lighter isopycnals with reduced thickness. Advected southward, the weakened mode waters decelerate the STCC. On decadal timescales, the dominant mode of sea surface height in the central subtropical gyre represents STCC variability. This STCC mode decays as CO₂ concentrations double in the twenty-first century, owing both to weakened mode waters in the mean state and to reduced variability in mode waters. The reduced mode-water variability can be traced upstream to reduced variations in winter MLD front and subduction in the KOE region where mode water forms.     

Simulation and future projection of the mixed layer depth and subduction process in the subtropical Southeast Pacific

The present climate simulation and future projection of the mixed layer depth (MLD) and subduction process in the subtropical Southeast Pacific are investigated based on the geophysical fluid dynamics laboratory earth system model (GFDL-ESM2M). The MLD deepens from May and reaches its maximum (>160 m) near (24°S, 104°W) in September in the historical simulation. The MLD spatial pattern in September is non-uniform in the present climate, which shows three characteristics: (1) the deep MLD extends from the Southeast Pacific to the West Pacific and leads to a "deep tongue" until 135°W; (2) the northern boundary of the MLD maximum is smoothly near 18°S, and MLD shallows sharply to the northeast; (3) there is a relatively shallow MLD zone inserted into the MLD maximum eastern boundary near (26°S, 80°W) as a weak "shallow tongue". The MLD non-uniform spatial pattern generates three strong MLD fronts respectively in the three key regions, promoting the subduction rate. After global warming, the variability of MLD spatial patterns is remarkably diverse, rather than deepening consistently. In all the key regions, the MLD deepens in the south but shoals in the north, strengthing the MLD front. As a result, the subduction rate enhances in these areas. This MLD antisymmetric variability is mainly influenced by various factors, especially the potential-density horizontal advection non-uniform changes. Notice that the freshwater flux change helps to deepen the MLD uniformly in the whole basin, so it hardly works on the regional MLD variability. The study highlights that there are regional differences in the mechanisms of the MLD change, and the MLD front change caused by MLD non-uniform variability is the crucial factor in the subduction response to global warming.     

A hydrothermodynamic quasi-isopycnic model of the ocean thermocline

The results of a numerical experiment on the formation of the large-scale circulation and the main thermocline in the temperate latitudes of an ocean at steady external factors are discussed. A multilayer model [1] is used which involves the upper mixed layer (UML). The boundaries of the areas where the internal layers reach the UML are calculated. The zones where seawater is entrapped from the thermocline to the UML are revealed, as well as the subduction zones where seawater is brought from the UML to the internal layers and moves there, ventilating the thermocline.Translated by Mikhail M. Trufanov. UDK 551.465.     

Morphology of a Pre-collisional, Salt-bearing, Accretionary Complex: The Mediterranean Ridge (Eastern Mediterranean)

The Eastern Mediterranean Sea is a remnant of a deep Mesozoic oceanic basin, now almost totally consumed as a result of long-term plate convergence between Eurasia and Africa. The present-day surface morphology of the Eastern Mediterranean relates both to the early history of formation of the deep basins and the recent geodynamic interactions between interfering microplates. Among the most conspicuous morphologic features of the basin is an arc-shape, elongated and wide, bathymetric swell bisecting the entire basin from the Ionian to Levantine areas, known as the Mediterranean Ridge. During the last decade this tectono-sedimentary accretionary prism, which results from the Hellenic subduction, has been intensively surveyed by swath mapping, multichannel seismic profiling and deep dives. We present here, and briefly discuss, the main morphological characteristics of this feature as derived from swath bathymetric data that considerably help to better assess the lateral and north–south morphostructural variability of the Mediterranean Ridge. This study reveals that the characteristics and morphostructural variability of the Mediterranean Ridge are related to: (1) a specific incipient collision geodynamic setting south of Crete, where the African and Aegean continental margins are nearly in contact, (2) a unique regional kinematics, controlled by frontal convergence south of Crete (central Mediterranean Ridge) and oblique subduction with opposite sense of shear for the western (Ionian) and eastern (Levantine) domains of the Mediterranean Ridge, that explain the lateral variations of deformation and (3) particularities of its sedimentary cover, which includes massive salt layers within the outer Mediterranean Ridge and local salt deposits within the inner domains, that control the north–south morphostructural variability of the sedimentary wedge.     

副热带东北太平洋混合层深度及其对潜沉的影响

The present climate simulations of the mixed layer depth(MLD) and the subduction rate in the subtropical Northeast Pacific are investigated based on nine of the CMIP5 models. Compared with the observation data,spatial patterns of the MLD and the subduction rate are well simulated in these models. The spatial pattern of the MLD is nonuniform, with a local maximum MLD(140 m) region centered at(28°N, 135°W) in late winter. The nonuniform MLD pattern causes a strong MLD front on the south of the MLD maximum region, controls the lateral induction rate pattern, and then decides the nonuniform distribution of the subduction rate. Due to the inter-regional difference of the MLD, we divide this area into two regions. The relatively uniform Ekman pumping has little effect on the nonuniform subduction spatial pattern, though it is nearly equal to the lateral induction in values. In the south region, the northward warm Ekman advection(–1.75×10~(–7) K/s) controls the ocean horizontal temperature advection(–0.85×10~(–7) K/s), and prevents the deepening of the MLD. In the ensemble mean, the contribution of the ocean advection to the MLD is about –29.0 m/month, offsetting the sea surface net heat flux contribution(33.9 m/month). While in the north region, the southward cold advection deepens the MLD(21.4 m/month) as similar as the heat flux(30.4 m/month). In conclusion, the nonuniform MLD pattern is dominated by the nonuniform ocean horizontal temperature advection. This new finding indicates that the upper ocean current play an important role in the variability of the winter MLD and the subduction rate.     

Polymorphic Zooids in Deitaic Species Populations of Conopeum seurati (CANU, 1928) (Bryozoa, Cheilostomata)

Abstract. Deltaic populations of Conopeum seurati from diverse environmental settings have variable phenotypes related to strong competitive regimes and to dissolved oxygen minima. Under extremely stressful conditions, the poorly canalized phenotype of C. seurati displays new additional features such as kleistozooids ( kleistós = closed) and chaotic celleporoid layers. Kleistozooids are clearly distinguishable polymorphs characterized by the presence of a compact, imperforate frontal membrane and by a strongly reduced lumen of the "opesium". They commonly aggregate in clusters of regularly-arranged individuals that store a large quantity of proteic material. The inferred function of these cell-like structures is to supply nutrients for basic colony activities. Celleporoid layers are, possibly, a developmental relaxation of the colony of C. seurati for a more efficient use of space in competitive interactions.
Both celleporoid layers and kleistozooids reveal the magnitude of intracolony variability of a widespread opportunistic bryozoan species.     

南太平洋和南印度洋亚南极模态水潜沉率的长期变化趋势

亚南极模态水(sub-Antarctic mode water,SAMW)的潜沉过程与全球变暖减缓现象密切相关。为了增进对亚南极模态水长期变化特征的认识,使用一个高分辨率长时间序列的海洋模式数据对SAMW的潜沉率变化趋势的空间分布进行了系统地分析。结果显示,在1958~2016年间,SAMW的潜沉量在南太平洋和南印度洋在长时间段上存在着相反的趋势变化,即在南太平洋增大,在南印度洋减少,这与已有研究结果相符。但进一步的分析发现,SAMW潜沉量的空间分布存在着明显的差异。在南印度洋,其北部潜沉区的潜沉率仅有很微弱的上升趋势,而位于南部潜沉区的潜沉率则有明显的下降趋势。与此同时,在南太平洋中,其西部潜沉区的潜沉率趋势非常小,而东部潜沉区的水的潜沉有明显上升的长期趋势。总体而言,密度较大的SAMW潜沉水团比密度较小的潜沉水团表现出更显著的长期变化的趋势。南部变化趋势明显的潜沉水量大概占总潜沉水量的60%,由此可知SAMW的总体趋势更多地来自其南部密度更大的潜沉区的贡献。进一步的分析表明,SAMW潜沉区的混合层的长期变化趋势与潜沉率的长期变化趋势之间存在较为一致的空间分布。其中,在南太平洋,东侧潜沉区的混合层的长期增大趋势,主要由于风应力增大的作用,而西侧潜沉区的混合层的长期减小趋势,则主要因为海表浮力强迫的控制;在南印度洋,南侧潜沉区的潜沉率长期减小趋势更多的是受到浮力强迫的影响,而西北部的潜沉率长期增加趋势则主要由风应力增强导致的。     

A synthesis of the circulation and hydrography of the South Aegean Sea and the Straits of the Cretan Arc (March 1994–January 1995)

Four seasonal oceanographic cruises were carried out in the Eastern Mediterranean Sea, within the framework of the CEC/MAST-MTP Project PELAGOS, during 1994–1995. The surveys covered the South Aegean Sea and the adjacent open sea regions (southeastern Ionian, northwestern Levantine). Analysis of CTD data revealed that a multiscaled circulation pattern prevails in the area. It differs from the circulations detected during the 1986–87, thus indicating interannual variability. Cyclonic and anticyclonic gyres and eddies are interconnected by currents and jets variable in space and time. Most of the features are persistent, others seem transitional or recurrent. The hydrological structure is also complex and apart from the upper layer does not present basinwide any significant seasonality. Dynamical and hydrological regimes are variable in the upper and intermediate layers at the Straits of the Cretan Arc, while the deep regime seems rather constant. Topographic control is evident on the flows through the straits. The new very dense deep water mass, namely the Cretan Deep Water (CDW) and a well-defined intermediate layer of minimum temperature and salinity, the so-called Transition Mediterranean Water (TMW), consists the new important structural elements of the South Aegean Sea. The CDW outflows towards the deep and bottom layers of the Eastern Mediterranean, thus considerably contributing to the formation of the new, denser Deep and Bottom Water of the Eastern Mediterranean, which sinks and displaces the Eastern Mediterranean Deep Water of Adriatic origin in the adjacent sea regions outside the Aegean Sea.     

Interannual variations in low potential vorticity water and the subtropical countercurrent in an eddy-resolving OGCM

Interannual-to-decadal variations in the subtropical countercurrent (STCC) and low potential vorticity (PV) water and their relations in the North Pacific Ocean are investigated on the basis of a 60-year-long hindcast integration of an eddy-resolving ocean general circulation model. Although vertically coherent variations are dominant for STCC interannual variability, a correlation analysis shows that an intensified STCC vertical shear accompanies lower PV than usual to the north on 25.5- to 26.1-σ_θ isopycnal surfaces, and intensified meridional density gradient in subsurface layers, consistent with Kubokawa’s theory (J Phys Oceanogr 29:1314–1333, 1999). The low-PV signals appear at least 2 years before peaks of STCC, propagating southwestward from the subduction region.     

Full-range equation for wave boundary layer thickness

Full-range equation covering all the flow regimes in a wave boundary layer is proposed for the boundary layer thickness. The results are compared with the available experimental data and good agreement has been found. In case of wave boundary layers, there are three definitions of boundary layer thickness in use. Therefore, the full-range equation is derived for three of the definitions. The findings of this study may be useful in calculating suspended sediment transport in coastal environments and studying wave–current combined motion.     

Seasonal and annual variability of vertically migrating scattering layers in the northern Arabian Sea

A 30-month time series of mean volume backscattering strength (MVBS) data obtained from moored acoustic Doppler current profilers (ADCPs) is used to analyze the evolution of vertically migrating scattering layers and their seasonal and annual variability in the Arabian and Oman Seas. Substantial diel vertical migration (DVM) is observed almost every day at all three mooring sites. Two daytime layers (Layers D1 and D2) and one nighttime layer (Layer E1) are typically present. The greatest biomass is observed near the surface during the night in Layer E1 and at depth between 250 and 450 m during the daytime in Layer D2. All layers are deepest during the spring inter-monsoon and shallowest during the summer/fall southwest monsoon (SWM). Seasonal modulation of the D2 biomass change is evident in our high-resolution data. The lowest biomass in D2 is measured in the early summer (May or June) followed by a rapid biomass increase during the SWM (June–November) until the biomass reaches a maximum at the end of the SWM season. Short-period oscillations in D2 biomass are often seen with periods ranging from days to one month. Occasionally, a lower nighttime layer E2 is formed between 180 and 270 m, mostly near the time of full moons. The upper daytime layer D1 is centered at 200 m and densely concentrated. It is only formed during the winter northeast monsoon (NEM) and the spring inter-monsoon. The influence of physical processes on layer distribution is also investigated. Interestingly, the two daytime layers are found to be formed at the two boundaries of the Persian Gulf outflow water (PGW) and follow the seasonal depth change of the PGW. The timing of the DVM and the formation, persistence, decay and reformation of the deep scattering layers seem to be governed by light, both solar and lunar. The scattering strength, the layer depth and the layer thickness are likewise closely related to the Moon phase at night. Cloud coverage, the isotherm and the isohaline also appear to affect the distribution and depth of the scattering layers. The continuous multiple-year acoustic data from ADCPs allow us, for the first time, to study the seasonal and annual variations of scattering layers in this region.     

Reconciling thermodynamic and dynamic methods of computation of water-mass transformation rates

The computation of the water-mass transformation rate in a particular density range from thermodynamic and dynamic methods are compared and reconciled by diagnosis of the Atlantic sector of a global integration of an ocean model driven by analyzed air–sea fluxes. In the absence of diffusive processes, the rate of subduction of fluid between two density surfaces across a fixed control surface, and integrated across the ocean from one solid boundary to another, must be equal to the rate of formation of fluid at the sea surface induced by surface fluxes in that density range. But due to the action of mixing on the body of fluid between the control surface and the sea-surface, transformation may differ from the integrated subduction. We find that vertical diffusive fluxes at the base of the winter mixed layer and in the seasonal thermocline can substantially modify transformation due to air–sea interaction and bring about an accommodation between it and the subduction rate. In high latitudes, an additional accommodation is achieved by lateral diffusive fluxes directed across the almost vertical isopycnals, typical of the deep, end-of-winter mixed layers of the sub-polar gyre. Finally we speculate on the likely nature and intensity of the mixing processes at work in the boundary layer of the ocean and their role in subduction and transformation.     

Enhancement of subduction/obduction due to hurricane-induced mixed layer deepening

Ventilation, including subduction and obduction, in the North Pacific is re-examined, based on SODA outputs and the Eulerian definition. The annual subduction rate averaged from 2001 to 2004 is estimated at 49.8 Sv, whereas the annual obduction rate is 26.7 Sv.Furthermore, the annual subduction/obduction rate enhancement induced by tropical cyclones in the North Pacific, defined as the difference between the annual subduction/obduction rate for the cases including the mixed layer depth perturbations induced by tropical cyclones and that for the cases without the perturbations, is estimated. Based on SODA outputs and the mixed layer deepening obtained from a hurricane-ocean coupled model, the annual tropical cyclone-induced subduction rate enhancement averaged from 2001 to 2004 is estimated at 4.4 Sv and the obduction rate enhancement 5.2 Sv, and such enhancement is mainly concentrated in the latitudinal band from 10°N to 30°N.     

Distribution of surface-active substances in the northern Adriatic Sea

Spatial distribution and seasonal variability of surfactant activity (SA) of seawater were studied in the northern Adriatic Sea and compared to the temperature and salinity patterns in the 2-year period from February 1998 to January 2000, based on monthly surveys. Surface-active substances (SAS) were determined with alternating current voltammetry (in-phase mode) using o-nitrophenol as an electrochemical probe. A general characteristic of the SA seasonal variability for the northern Adriatic is the sinusoidal change of SA within the period of 1 year; similar behaviour was observed for the seasonal variations of temperature. Maximum SA values appeared during the summer period, while minimum SA values were measured in winter, during the period of well-mixed water layers and lower phytoplankton production. In May and October, the highest SA values were obtained in the upper seawater layer, which was ascribed to the influence of high inflow of nutrient-rich Po River freshwater. Riverine inputs indirectly favour autochthonous production processes, which result in increased concentration of organic matter (OM), particularly surface-active substances.     

伊豆−博宁−马里亚纳岛弧地壳厚度分布及其对岩浆活动的指示

数值模拟研究认为洋底高原/洋脊俯冲和弧后扩张能够有效影响俯冲带岩浆活动和岛弧地壳增生。本文以伊豆?博宁?马里亚纳(IBM)俯冲带为实例,论证该结论的有效性。以卫星测高反演重力异常为基础,通过构建地球不同圈层密度模型,反演得到IBM俯冲带莫霍面埋深。本文的莫霍面埋深反演结果与地震解释结果具有一致的分布趋势。结合开源水深和沉积层厚度数据,给出了IBM俯冲带地壳厚度分布。IBM岛弧地壳体积沿走向的分布特征表明:①小笠原洋底高原和相对较小规模达顿洋脊的俯冲,都能够使得相应位置的岛弧变窄、地壳变厚、体积增大;②马里亚纳海槽扩张显著降低了岛弧地壳体积的增生量。     

气候变化情景下东南印度洋亚南极模态水的演变趋势研究

基于参与第六次耦合模式比较计划（CMIP6）的8个地球系统耦合模式所输出的历史模拟结果,本文通过与观测对比,评估了CMIP6模式对东南印度洋亚南极模态水的模拟能力,并预估了在中等强迫情景和高强迫情景下,该模态水潜沉率、体积及性质的变化趋势。结果表明:与Argo观测相比,CMIP6模式中南印度洋混合层偏深且上层海洋的位势密度偏小,因此其模拟的东南印度洋亚南极模态水潜沉率偏大而位势密度偏小。不同CMIP6模式之间模拟的东南印度洋亚南极模态水潜沉区存在差异,混合层侧向输入是导致这一差异的主要原因。此外,在历史模拟和两种情景试验中,东南印度洋亚南极模态水均呈现出潜沉率和体积减小、温度升高、盐度和密度降低的趋势。其中,在高强迫情景下,变化趋势最大,中等强迫情景次之,历史模拟中的变化趋势最小。这表明,辐射强迫越强,东南印度洋海表温度升高和淡水输入增加的趋势越大,导致混合层变浅及其南北梯度减小的趋势越快,东南印度洋亚南极模态水潜沉率、体积和性质变化的趋势也随之增大。