Deep-circulation flow at mid-latitude in the western North Pacific

Direct current measurements with five moorings at 27–35°N, 165°E from 1991 to 1993 and with one mooring at 27°N, 167°E from 1989 to 1991 revealed temporal variations of deep flow at mid-latitude in the western North Pacific. The deep-circulation flow carrying the Lower Circumpolar Deep Water from the Southern Ocean passed 33°N, 165°E northwestward with a high mean velocity of 7.8 cm s⁻¹ near the bottom and was stable enough to continue for 4–6 months between interruptions of 1- or 2-months duration. The deep-circulation flow expanded or shifted intermittently to the mooring at 31°N, 165°E but did not reach 35°N, 165°E although it shifted northward. The deep-circulation flow was not detected at the other four moorings, whereas meso-scale eddy variations were prominent at all the moorings, particularly at 35°N and 29°N, 165°E. The characteristics of current velocity and dissolved oxygen distributions led us to conclude that the deep-circulation flow takes a cyclonic pathway after passing through Wake Island Passage, passing 24°N, 169.5–173°E and 30°N, 168–169°E northward, proceeds northwestward around 33°N, 165°E, and goes westward through the south of the Shatsky Rise. We did not find that the deep-circulation flow proceeded westward along the northern side of the Mid-Pacific Seamounts and eastward between the Hess Rise and the Hawaiian Ridge toward the Northeast Pacific Basin.     

Direct velocity measurements of deep circulation southwest of the Shatsky Rise in the western North Pacific

Direct velocity measurements undertaken using a nine-system mooring array (M1–M9) from 2004 to 2005 and two additional moorings (M7p and M8p) from 2003 to 2004 reveal the spatial and temporal properties of the deep-circulation currents southwest of the Shatsky Rise in the western North Pacific. The western branch of the deep-circulation current flowing northwestward (270–10° T) is detected almost exclusively at M2 (26°15′N), northeast of the Ogasawara Plateau. It has a width less than the 190 km distance between M1 (25°42′N) and M3 (26°48′N). The mean current speed near the bottom at M2 is 3.6±1.3 cm s^?1. The eastern branch of the deep-circulation current is located at the southwestern slope of the Shatsky Rise, flowing northwestward mainly at M8 (30°48′N) on the lower part of the slope of the Shatsky Rise with a mean near-bottom speed of 5.3±1.4 cm s^?1. The eastern branch often expands to M7 (30°19′N) at the foot of the rise with a mean near-bottom speed of 2.8±0.7 cm s^?1 and to M9 (31°13′N) on the middle of the slope of the rise with a speed of 2.5±0.7 cm s^?1 (nearly 4000 m depth); it infrequently expands furthermore to M6 (29°33′N). The width of the eastern branch is 201±70 km on average, exceeding that of the western branch. Temporal variations of the volume transports of the western and eastern branches consist of dominant variations with periods of 3 months and 1 month, varying between almost zero and significant amount; the 3-month-period variations are significantly coherent to each other with a phase lag of about 1 month for the western branch. The almost zero volume transport occurs at intervals of 2–4 months. In the eastern branch, volume transport increases with not only cross-sectional average current velocity but also current width. Because the current meters were too widely spaced to enable accurate estimates of volume transport, mean volume transport is overestimated by a factor of nearly two, yielding values of 4.1±1.2 and 9.8±1.8 Sv (1 Sv=10⁶ m³ s^?1) for the western and eastern branches, respectively. In addition, a northwestward current near the bottom at M4 (27°55′N) shows a marked variation in speed between 0 and 20 cm s^?1 with a period of 45 days. This current may be part of a clockwise eddy around a seamount located immediately east of M4.     

北太平洋经向翻转环流和热盐输送研究综述

北太平洋经向翻转环流是北太平洋所有经向翻转环流圈的总称,目前它拥有五个环流圈,即副热带环流圈(the subtropical cell,STC)、热带环流圈(the tropical cell,TC)、副极地环流圈(the subpolar cell,SPC)、深层热带环流圈(the deep tropical cell,DTC)和温跃层环流圈(the thermohaline cell,THC)。这些环流圈是北太平洋经向物质和能量交换的重要通道,它们的变化对海洋上层热盐结构和气候变化皆有重要影响。迄今,人们已对STC、TC和DTC的结构形态、变化特征与机理开展了广泛而深入的研究,并对STC的极向热输送特征也做了一些初步分析。但应指出的是,关于SPC和THC的研究仍较少,迄今尚不清楚这两个环流圈的三维结构和变异机理;而且,对北太平洋经向翻转环流的热盐输送研究尚处于起步阶段,目前对各环流圈的热盐输送特征、变化规律和变异机理仍知之甚少,这些科学问题亟待深入研究。     

North Pacific Intermediate Water: Progress in SAGE (SubArctic Gyre Experiment) and Related Projects

We survey the recent progress in studies of North Pacific Intermediate Water (NPIW) in SAGE (SubArctic Gyre Experiment), including important results obtained from related projects. Intensive observations have provided the transport distributions relating to NPIW and revealed the existence of the cross-wind-driven gyre Oyashio water transport that flows directly from the subarctic to subtropical gyres through the western boundary current as well as the diffusive contribution across the subarctic front. The anthropogenic CO₂ transport into NPIW has been estimated. The northern part of NPIW in the Transition Domain east of Japan is transported to the Gulf of Alaska, feeding the mesothermal (intermediate temperature maximum) structure in the North Pacific subarctic region where deep convection is restricted by the strong halocline maintained by the warm and salty water transport originating from NPIW. This heat and salt transport is mostly balanced by the cooling and freshening in the formation of dense shelf water accompanied by sea-ice formation and convection in the Okhotsk Sea. Intensive observational and modeling studies have substantially altered our view of the intermediate-depth circulation in the North Pacific. NPIW circulations are related to diapycnal-meridional overturning, generated around the Okhotsk Sea due to tide-induced diapycnal mixing and dense shelf water formation accompanied by sea-ice formation in the Okhotsk Sea. This overturning circulation may possibly explain the direct cross-gyre transport through the Oyashio along the western boundary from the subarctic to subtropical gyres. This revised version was published online in July 2006 with corrections to the Cover Date.     

Formation, Distribution and Volume Transport of the North Pacific Intermediate Water Studied by Repeat Hydrographic Observations

In order to examine the formation, distribution and transport of North Pacific Intermediate Water (NPIW), repeated hydrographic observations along several lines in the western North Pacific were carried out in the period from 1996 to 2001. NPIW formation can be described as follows: (1) Oyashio water extends south of the Subarctic Boundary and meets Kuroshio water in intermediate layers; (2) active mixing between Oyashio and Kuroshio waters occurs in intermediate layers; (3) the mixing of Oyashio and Kuroshio waters and salinity minimum formation around the potential density of 26.8σ_θ proceed to the east. It is found that Kuroshio water flows eastward even in the region north of 40°N across the 165°E line, showing that Kuroshio water extends north of the Subarctic Boundary. Volume transports of Oyashio and Kuroshio components (relative to 2000 dbar) integrated in the potential density range of 26.6–27.4σθ along the Kuroshio Extension across 152°E–165°E are estimated to be 7–8 Sv (10⁶ m³s⁻¹) and 9–10 Sv, respectively, which is consistent with recent work. This revised version was published online in July 2006 with corrections to the Cover Date.     

Long-Term Variability of North Pacific Subtropical Mode Water in Response to Spin-Up of the Subtropical Gyre

The Meteorological Research Institute's ocean general circulation model (MRI-OGCM) has been used to investigate the temperature variability of the North Pacific Subtropical Mode Water (NPSTMW) over a time series longer than 5 years via the spin-up of the subtropical gyre. Besides an interannual variation, the wintertime sea surface temperature in the area where the NPSTMW is formed, and the temperature of the NPSTMW itself, both change remarkably in a >5-year time scale. An analysis of heat budgets showed that the long-term changes in NPSTMW temperature are due mainly to a leading advection of heat by the Kuroshio Extension and compensating surface heat flux. As a result of a dynamical adjustment to the wind stress fields, the transports of the Kuroshio and the Kuroshio Extension increased in the mid 1970s with a lag of 3 years after the wind stress curl in the central North Pacific. The increased heat advection by the Kuroshio Extension induces a warming in the mixed layer in the NPSTMW formation area, followed by a warming of the NPSTMW itself. Both these warming actions increase the heat release to the atmosphere. These results imply that the surface heat flux over the Kuroshio Extension area varies in response to the change in the ocean circulation through the spin-up of the subtropical gyre. This revised version was published online in August 2006 with corrections to the Cover Date.     

Excess CO2 and pHexcess in the Intermediate Water Layer of the Northwestern Pacific

Excess CO₂ and pH_excess showing an increase in dissolved inorganic carbon and a decrease in pH from the beginning of the industrial epoch (middle of the 19th century) until the present time have been calculated in the intermediate water layer of the northwestern Pacific and the Okhotsk Sea. It is concluded that: (1) The Kuril Basin (Okhotsk Sea) and the Bussol' Strait areas are characterized by the greatest concentrations of excess CO₂ at isopycnal surfaces due to the processes of formation and transformation of intermediate water mass. (2) The largest difference in excess CO₂ concentration between the Okhotsk Sea and the western subarctic Pacific (about 8 µmol/kg) is found at the = 27.0. (3) The difference in excess CO₂ between the western subarctic Pacific and subtropical regions is significant only in the upper part of the intermediate water layer ( = 26.7–27.0). (4) About 10% of the excess CO₂ accumulation in the subtropical north Pacific is determined by water exchange with the subarctic Pacific and the Okhotsk Sea.     

The three-dimensional structure and seasonal variation of the North Pacific meridional overturning circulation

The three-dimensional structure and the seasonal variation of the North Pacific meridional overturning circulation (NPMOC) are analyzed based on the Simple Ocean Data Assimilation data and Argo profiling float data.The NPMOC displays a multi-cell structure with four cells in the North Pacific altogether.The TC and the STC are a strong clockwise meridional cell in the low latitude ocean and a weaker clockwise meridional cell between 7°N and 18°N,respectively, while the DTC and the subpolar cell are a weaker ...     

Temporal Evolution of the North Pacific CO2 Uptake Rate

The recent changes in the North Pacific uptake rate of carbon have been estimated using a number of different techniques over the past decade. Recently, there has been a marked increase in the number of estimates being submitted for publication. Most of these estimates can be grouped into one of five basic techniques: carbon time-series, non-carbon tracers, carbon tracers, empirical relationships, and inverse calculations. Examples of each of these techniques as they have been applied in the North Pacific are given and the estimates summarized. The results are divided into three categories: integrated water column uptake rate estimates, mixed layer increases, and surface pCO₂ increases. Most of the published values fall under the water column integrated uptake rate category. All of the estimates varied by region and depth range of integration, but generally showed consistent patterns of increased uptake from the tropics to the subtropics. The most disagreement between the methods was in the sub-arctic Pacific. Column integrated uptake rates ranged from 0.25 to 1.3 mol m⁻²yr⁻¹. The mixed layer uptake estimates were much more consistent, with values of 1.0–1.3 μmol kg⁻¹yr⁻¹ based on direct observations and multiple linear regression approaches. Surface pCO₂ changes showed the most obvious regional variability (0.5–2.5 μatm yr⁻¹) reflecting the sensitivity of these measurements to differences in the physical and biological forcing. The different techniques used to evaluate the changes in North Pacific carbon distributions do not completely agree on the exact magnitude or spatial and temporal patterns of carbon uptake rate. Additional research is necessary to resolve these issues and better constrain the role of the North Pacific in the global carbon cycle. This revised version was published online in July 2006 with corrections to the Cover Date.     

Meridional carbon dioxide transport in the northern North Atlantic

Combination of estimated water transport and accurate measurements of total carbon dioxide (TCO₂) on a hydrographic section at 58 °N allows the assessment of meridional inorganic carbon transport in the northern North Atlantic Ocean. The transport has been decomposed into contributions from the large-scale baroclinic overturning, the Ekman transport, baroclinic and a barotropic eddy terms, and an estimated contribution of the East Greenland Current. These terms are −0.27 · 10⁶, +0.03 · 10⁶, +0.03 · 10⁶, +0.10 · 10⁶, and +0.05 · 10⁶ mol s⁻¹, respectively, which result in a total southward inorganic carbon transport of only −0.06 · 10⁶ mol s⁻¹. An order of magnitude estimate of the meridional transport of dissolved organic carbon (DOC) has shown that in general this term cannot be ignored in the total carbon flux, this being +0.04 · 10⁶ to +0.16 · 10⁶ mol s⁻¹ at 58 °N. A simple carbon budget has been formulated for the temperate North Atlantic, using our flux estimates as well as those of Brewer et al. (1989). This budget shows that the divergence of the meridional carbon flux, connected with the freshwater balance of the ocean may be of the same order of magnitude as the divergence of the total inorganic carbon flux. For an accurate estimate of the total carbon budget of the ocean it will be necessary to take both the DOC transport and the effects of the freshwater balance into account.     

热带北太平洋西部固氮速率及固氮微生物组成

In the present study, we report N_2 fixation rate(~(15)N isotope tracer assay) and the diazotroph community structure(using the molecular method) in the western tropical North Pacific Ocean(WTNP)(13°–20°N, 120°–160°E). Our independent evidence on the basis of both in situ N_2 fixation activity and diazotroph community structure showed the dominance of unicellular N_2 fixation over majority of the WTNP surface waters during the sampling periods.Moreover, a shift in the diazotrophic composition from unicellular cyanobacteria group B-dominated to Trichodesmium spp.-dominated toward the western boundary current(Kuroshio) was also observed in 2013. We hypothesize that nutrient availability may have played a major role in regulating the biogeography of N_2 fixation.In surface waters, volumetric N_2 fixation rate(calculated by nitrogen) ranged between 0.6 and 2.6 nmol/(L·d) and averaged(1.2±0.5) nmol/(L·d), with 10 μm size fraction contributed predominantly(88%±6%) to the total rate between 135°E and 160°E. Depth-integrated N_2 fixation rate over the upper 200 m ranged between 150 μmol/(m~2·d)and 480 μmol/(m~2·d)(average(225±105) μmol/(m~2·d). N_2 fixation can account for 6.2%±3.7% of the depthintegrated primary production, suggesting that N_2 fixation is a significant N source sustaining new and export production in the WTNP. The role of N_2 fixation in biogeochemical cycling in this climate change-vulnerable region calls for further investigations.     

The destiny of the North Pacific Intermediate Water in the South China Sea

The previous studies show that the spreading path of the subtropical salinity minimum of the North Pacific Intermediate Water (NPIW) is southwestward pointing to the Luzon Strait. Based on the P -vector method and generalized digital environmental model (GDEM) data, the volume transport of NPIW through Luzon Strait and the upward transport on the NPIW lower and upper boundaries are calculated to examine the destiny of NPIW in the South China Sea (SCS). On the annual mean, the estimation of NPIW transport into the SCS through the Luzon Strait is 1.72 Sv (1Sv=10 6 m 3 /s). The upward transport over the SCS is 0.31 Sv on the NPIW upper boundary and 1.31 Sv on the NPIW lower boundary. There is no strait or passage deeper than the surface for the NPIW to extend, except for the Luzon Strait. For the volume balance in the SCS NPIW, the volume transport of 2.72 Sv has to flow out of the SCS NPIW layer through the Luzon Strait.     

西北太平洋台风频数异常及其与海气通量的关系

利用1984-2002年联合台风预警中心(JTWC)最佳路径台风资料、全球海洋客观分析海气通量(OAFluxes)资料和NCAR/NCEP-2再分析资料,使用SVD等统计方法,对西北太平洋台风频数与低层大气环流及海气通量异常之间的关系进行了研究,结果发现150°E以东的低纬海区是台风频数年际异常变化最显著的区域,台风频数与低层大气环流异常、潜热通量和短波辐射通量变化有着密切的联系:当副热带高压强度减弱(增强)、脊线偏北(南)、主体东移(西伸)时,季风槽加深东进至160°E(西退至140°E),低纬的纬向西风加强(减弱),海洋输送给大气较多(少)潜热通量,盛行的纬向西风携带着季风槽南侧的暖湿水汽与副热带高压南缘偏东气流的水汽输送在150°E以东(以西)的低纬海区辐合,150°E以东辐合上升的暖湿气团吸收的短波辐射偏多(少),有利于(不利于)形成高温高湿的不稳定结构,台风能量不断(不易)积累,在低层强(弱)辐合、高层强(弱)辐散的环境场作用下,有利于(不利于)台风在150°E以东的低纬海区的生成。     

Selective depressions of surface silicic acid within cyclonic mesoscale eddies in the oligotrophic western North Pacific

To reveal spatial dynamics of silicic acid [Si(OH)₄] in the poorly sampled oligotrophic western North Pacific, we investigated the surface distribution of Si(OH)₄ and associated biogeochemical parameters by using an underway survey system with a highly sensitive nutrient analyzer along the 138°E transect (between 30 and 34°N) and the 155°E transect (between 10 and 35°N) during the summers of 2007 and 2008. Surface Si(OH)₄ concentrations ranged from the detection limit (11 nmol L⁻¹) to 2462 nmol L⁻¹. High Si(OH)₄ concentrations (>1000 nmol L⁻¹) and dynamic fluctuations were generally observed north of 23°N, while consistently stable low concentrations of 415–751 nmol L⁻¹ were observed south of 23°N. Surface nitrate+nitrite (N+N) and phosphate (PO₄³⁻) were typically depleted to <20 nmol L⁻¹, except for PO₄³⁻ in the area south of 16°N. The majority of the study area was characterized by high-Si(OH)₄ and low-N+N and PO₄³⁻. However, submesoscale/mesoscale depressions of Si(OH)₄ were locally observed in the cyclonic eddy fields north of 23°N. Among a total of six Si(OH)₄ depressions within the eddies, a complete Si(OH)₄ depletion (<11 nmol L⁻¹) was observed on the cyclonic side near the Kuroshio axis (33.1°N, 138°E). This depletion was closely coupled with a diatom bloom, suggesting that Si(OH)₄ was exhausted by diatoms. All of the Si(OH)₄ depressions were selective and not accompanied by local depressions of N+N and PO₄³⁻. This unique phenomenon might be driven by biogeochemical processes such as selective Si export (Si pump), anomalous Si uptake associated with diatom physiology, and/or Si uptake supported by N₂ fixation.     

冬季北太平洋流场异常主要模态与PDO及NPGO的关系

采用复经验正交函数(EOF)分解和小波分析,对冬季北太平洋上层海流异常进行了统计动力诊断,并讨论了主要模态与北太平洋年代际振荡(PDO)模态和北太平洋环流振荡(NPGO)模态的关系。结果显示,冬季北太平洋上层洋流异常复EOF分解的第一模态是PDO在流场异常上的反映,第二模态则包含了NPGO的明显信息。主要依据有:(1)第一和第二模态的实时间系数序列分别有准20a和准13a的年代际变化周期,与PDO和NPGO模态的年代际变化周期相同;(2)第一和第二模态实时间系数与北太平洋海表面温度异常(SSTA)的回归系数场的空间分布分别与PDO和NPGO模态的空间结构相近。根据第一和第二模态上层洋流异常计算得到的垂直运动异常的分布,与SSTA的PDO和NPGO模态的空间分布类似,表明海盆尺度流场异常造成的垂直运动是形成PDO和NPGO模态的重要原因。     

海浪对北太平洋海-气二氧化碳通量的影响

利用4种海-气界面气体传输速率公式对比研究了北太平洋气体传输速率及其CO₂通量的季节变化特征。与单纯依赖风速的算法相比, 考虑波浪影响的气体传输速率和CO₂通量在空间分布和季节变化上具有明显差异。在低纬度地区(0°～30°N), 波浪参数使气体传输速率下降, 海洋对大气CO₂的吸收减少, 而在30°N以北范围内则出现新的气体传输速率高值区, 海洋对大气的吸收增加。进一步研究了黑潮延伸体区域的气候态月平均气体传输速率和CO₂通量。结果表明, 该区域气体传输速率和CO₂通量最大值分别出现于冬季和春季, 引入波浪参数后, 虽然该区域气体传输速率和CO₂通量平均值没有明显差异, 但季节变化强度显著增强。     

北太平洋西边界流研究综述

综述了近20年来国内外学者在研究北太平洋西边界流的平均结构及NEC分叉动力机制、NM K流系平均输运的分配及变化、NM K流系季节及年际变化规律及其与EN SO之间的关系、NM K流系在热带和亚热带水交换中的作用以及水团的平均分布特征等方面所取得的主要成果。通过分析,发现东亚季风、R ossby波和K e lv in波等是影响北太平洋西边界流的主要因素;而缺乏长期直接的海流观测资料是深入研究北太平洋西边界流遇到的最大障碍。     

A primary study of the correlation between the net air-sea heat flux and the interannual variation of western North Pacific tropical cyclone track and intensity

A singular value decomposition (SVD) analysis is carried out to reveal the relationship between the interannual variation of track and intensity of the western North Pacific tropical cyclones (WNPTCs) in the tropical cyclone (TC) active season (July-November) and the global net air-sea heat flux (Q net ) in the preceding season (April-June). For this purpose, a tropical cyclone track and intensity function (TIF) is defined by a combination of accumulated cyclone energy (ACE) index and a cyclone track densit...     

Influence of Kuroshio Flow on the Horizontal Distribution of North Pacific Intermediate Water in the Shikoku Basin

The influence of the Kuroshio flow on the horizontal distribution of North Pacific Intermediate Water (NPIW) in the Shikoku Basin is examined based upon observational data collected by the training vessel “Seisui-maru” of Mie University together with oceanographic data compiled by the Japan Oceanographic Data Center (JODC). Although it has been stated that the NPIW with salinity less than 34.2 psu had been confined to the south of the Kuroshio main axis along the PT (KJ) Line on the eastern side of the Izu Ridge, a similar tendency can be detected on the western side of the Izu Ridge. Namely, the NPIW on the southern side of the Kuroshio main axis in the Shihoku Basin does not indicate a tendency to go northward across the Kuroshio main axis without an increase in salinity of more than 34.2 psu. However, the JODC data show that less saline water (<34.2 psu) was present on the northern side of the Kuroshio main axis south of the Kii Peninsula in May 1992. Satellite observed sea surface temperature (SST) data suggested that the Kuroshio approaches the Kii Peninsula after forming a small meander off Kyushu and some intrusions of the NPIW into the northern coastal side of the Kuroshio main axis occurred in this period. It is concluded that intrusion of the NPIW with salinity less than 34.2 psu to the northern coastal side through the Kuroshio main axis occurred during the decay period of the small meander path in May 1992. Based on these observational results, the source of the salinity minimum water on the northern coastal side of the Kuroshio main axis is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.