北太平洋经向翻转环流年际变化的数值模拟

北太平洋经向翻转环流(NPMOC)是北太平洋所有经向翻转环流圈的总称,拥有5个环流圈结构.其中,热带环流圈(TC)、副热带环流圈(STC)和深层热带环流圈(DTC)位于北太平洋热带-副热带海域,是该海域间经向物质和能量交换的重要通道.主要运用NEMO模式对这3个经向翻转环流圈的年际变化特征和机理进行了研究.结果表明,TC、STC和DTC的经向流量都具有显著的年际变化特征:在El Nio期间,TC的南、北向流量均减弱,STC的北向流量增强、南向流量减弱,DTC的南向流量减弱;而在La Nia期间则相反.敏感性试验表明,在风应力强迫下得到的TC、STC南、北向流量和DTC南向流量的年际变化特征都很显著,并与在风应力、热通量和淡水通量共同强迫下得到的结果非常一致;而仅在热通量和淡水通量的强迫下,各分支流量的年际变化均较小.由此可见,风场驱动是引起北太平洋经向翻转环流年际变化的主要驱动因素,而热通量和淡水通量的影响却较小.     

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 ...     

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。     

Numerical study of the meridional overturning circulation with “mixing hotspots” in the Pacific Ocean

Using an idealized ocean general circulation model, we examine the effect of “mixing hotspots” (localized regions of intense diapycnal mixing) predicted based on internal wave-wave interaction theory (Hibiya et al., 2006) on the meridional overturning circulation of the Pacific Ocean. Although the assumed diapycnal diffusivity in the mixing hotspots is a little larger than the predicted value, the upwelling in the mixing hotspots is not sufficient to balance the deep-water production; out of 17 Sv of the downwelled water along the southern boundary, only 9.2 Sv is found to upwell in the mixing hotspots. The imbalance as much as 7.8 Sv is compensated by entrainment into the surface mixed layer in the vicinity of the downwelling region. As a result, the northward transport of the deep water crossing the equator is limited to 5.5 Sv, much less than estimated from previous current meter moorings and hydrographic surveys. One plausible explanation for this is that the magnitude of the meridional overturning circulation of the Pacific Ocean has been overestimated by these observations. We raise doubts about the validity of the previous ocean general circulation models where diapycnal diffusivity is assigned ad hoc to attain the current magnitude suggested from current meter moorings and hydrographic surveys.     

Deep-water circulation in the North Pacific deduced from Si-O diagrams

A simple dissolved silica (Si) and dissolved oxygen (O) diagram method was applied to study the deep-water circulation in the North Pacific and the following results and conclusion have been obtained. In the abyssal water flowing northward in the western Pacific Si increases with a constant ratio of Si to decreasing O(Si/O=–0.30). The water is designated as the main sequence. In the eastern Pacific the Si-O diagram is characteristic of the location and reflects the degrees of mixing with older waters and of alteration due to decomposition of biogenic material. The Bay of Alaska is found to be a great source of silica in the North Pacific and its bottom water spreads out to the central North Pacific north of 40°N, called here the abyssal front. The younger abyssal water in the Aleutian Trench flowing to the eastern North Pacific north of 40°N comes through the north end of the Kuril-Kamchatka Trench instead of the gap in the Emperor Seamounts at about 46°N. The deep water is almost completely homogenized by active isopycnal mixing and advection when the deep water reaches its upper boundary by upwelling in the western North Pacific including the Bering Sea. Thus the high productivity in the Bering Sea is principally caused neither by the direct supply of abyssal water rich in nutrients nor by the extremely active vertical mixing reaching depths greater than 500 m, but it may be caused simply by the shallower upper boundary of the deep water mass in the Bering Sea, from which nutrients are easily transported to the surface.     

Deep-water circulation at low latitudes in the western North Pacific

Full-depth conductivity-temperature-depth-oxygen profiler (CTDO₂) data at low latitudes in the western North Pacific in winter 1999 were analyzed with water-mass analysis and geostrophic calculations. The result shows that the deep circulation carrying the Lower Circumpolar Water (LCPW) bifurcates into eastern and western branch currents after entering the Central Pacific Basin. LCPW colder than 0.98°C is carried by the eastern branch current, while warmer LCPW is carried mainly by the western branch current. The eastern branch current flows northward in the Central Pacific Basin, supplying water above 0.94°C through narrow gaps into an isolated deep valley in the Melanesian Basin, and then passes the Mid-Pacific Seamounts between 162°10′E and 170°10′E at 18°20′N, not only through the Wake Island Passage but also through the western passages. Except near bottom, dissolved oxygen of LCPW decreases greatly in the northern Central Pacific Basin, probably by mixing with the North Pacific Deep Water (NPDW). The western branch current flows northwestward over the lower Solomon Rise in the Melanesian Basin and proceeds westward between 10°40′N and 12°20′N at 150°E in the East Mariana Basin with volume transport of 4.1 Sv (1 Sv=10⁶ m³ s⁻¹). The current turns north, west of 150°E, and bifurcates around 14°N, south of the Magellan Seamounts, where dissolved oxygen decreases sharply by mixing with NPDW. Half of the current turns east, crosses 150°E at 14–15°N, and proceeds northward primarily between 152°E and 156°E at 18°20′N toward the Northwest Pacific Basin (2.1 Sv). The other half flows northward west of 150°E and passes 18°20′N just east of the Mariana Trench (2.2 Sv). It is reversed by a block of topography, proceeds southward along the Mariana Trench, then detours around the south end of the trench, and proceeds eastward along the Caroline Seamounts to the Solomon Rise, partly flowing into the West Mariana and East Caroline Basins. A deep western boundary current at 2000–3000 m depth above LCPW (10.0 Sv) closes to the coast than the deep circulation. The major part of it (8.5 Sv) turns cyclonic around the upper Solomon Rise from the Melanesian Basin and proceeds along the southern boundary of the East Caroline Basin. Nearly half of it proceeds northward in the western East Caroline Basin, joins the current from the east, then passes the northern channel, and mostly enters the West Caroline Basin (4.6 Sv), while another half enters this basin from the southern side (>3.8 Sv). The remaining western boundary current (1.5 Sv) flows over the middle and lower Solomon Rise, proceeds westward, then is divided by the Caroline Seamounts into southern (0.9 Sv) and northern (0.5 Sv) branches. The southern branch current joins that from the south in the East Caroline Basin, as noted above. The northern branch current proceeds along the Caroline Seamounts and enters the West Mariana Basin.     

Photosynthetic community responses to upwelling in mesoscale eddies in the subtropical North Atlantic and Pacific Oceans

In this paper we summarise the photo-physiological responses of phytoplankton to upwelling of macronutrients in mesoscale eddies in the subtropical North Atlantic (EDDIES project, Sargasso Sea) and subtropical North Pacific (E-FLUX project, Hawaii). The observations, obtained on two sets of cruises over 2 years, occupied six cyclonic eddies and two mode-water eddies. The photosynthetic physiological parameters were measured using a bench-top fluorescence induction and relaxation (FIRe) system and a submersible in situ fast repetition rate fluorometer (FRRF) deployed on an undulating towed vehicle. Both of these instruments were used to provide highly sensitive and well-resolved data on community responses. The responses are dependent on both the type of eddy and its stage of development. Our results indicate that, while cyclonic eddies in the Atlantic and Pacific can increase primary photosynthetic production early in their development, mode-water eddies in the subtropical North Atlantic can support patchy blooms of large diatoms for long periods of time (more than 3 months).     

赤道太平洋近表层上升流的估计

利用1979-2006年间的卫星跟踪漂流浮标资料计算得到多年平均的赤道太平洋近表层流场,并估计赤道中东太平洋上混合层水平流场的散度和上升流流量.主要结论包括:1)赤道中东太平洋上升流区域主要集中在日界线以东以赤道为中心的±2°纬度带内,并且赤道南北约4°处各有一下沉流区域;2)赤道中东太平洋附近(165°E-85°W,2°S-2°N)散度平均约为2.0×10-7s-1,对应30m深处上升流区域整体体积输送约为43Sv,其中大约一半的上升流水体随热带流圈在赤道外4°N/S附近下沉,其余部分向两极输送;3)赤道中东太平洋辐散在春季达到最大值2.1×10-7s-1,而在秋季最弱;4)在El Ni(n)o期间辐散减弱,而在La Ni(n)a期间辐散增强,其中纬向流所致的辐合辐散也起到一定作用.     

The feeding habits of slope dwelling macrourid fishes in the eastern North Pacific

The diet of slope dwelling macrourid fishes in the eastern North Pacific is poorly known. We collected several hundred stomach samples to investigate the feeding habits of Coryphaenoides acrolepis and Albatrossia pectoralis, the two dominant slope dwelling macrourids off the continental United States. Coryphaenoides acrolepis exhibited a pronounced ontogenetic shift in diet. Specimens <15 cm pre-anal fin length (PAF) consumed primarily polychaetes, amphipods, cumaceans and mysids, while larger individuals consumed increasingly larger, more pelagic prey such as fish, squid, and large crustaceans. Scavenging was also very important to specimens >15 cm with scavenged food constituting approximately 20% of the weight of total prey and occurring in approximately 20% of fish 21–29 cm. Albatrossia pectoralis consumed primarily midwater fish and squid, and we believe that it feeds in the water column. There were significant differences between the diets of A. pectoralis and C. acrolepis suggesting some degree of niche separation between macrourid species on the continental slope of the eastern North Pacific. Both species are at the top of the food web on the upper continental slope and, because of their abundance, may exert significant pressures on their prey populations.     

北极径流增加对大西洋经向翻转环流影响的模拟研究

观测显示过去几十年间北极入海径流呈现增加趋势,CMIP5耦合模式预测表明21世纪北极入海径流仍会增加,在RCP8.5路径下,21世纪末北极入海径流量将会是1950年的1.4倍。本文利用冰-海耦合数值模式研究了北极径流增加对大西洋经向翻转环流的影响。基于两个数值实验的结果表明,如果北极入海径流按每年0.22%的速度（与RCP8.5路径下的速度相当）增加,大西洋经向翻转环流的强度在100、150和200年后将会分别减弱0.6（3%）、1.2（7%）和1.8（11%） Sv。北极入海径流增加导致大西洋经向翻转环流减弱的主要原因是,北极入海径流增加的淡水被输运到北大西洋后,会抑制北大西洋深层水的生成,这也会导致北大西洋深层水海水年龄的增加。     

上新世/更新世之交太平洋经向翻转流演化

随着北半球冰盖的发育,全球气候环境发生了显著变化。太平洋经向翻转流（PMOC）对全球海洋热量分配和大气CO₂在深海的封存起举足轻重的作用,但是关于PMOC与北半球冰盖的形成之间的关系还欠缺研究。本文收集了太平洋海山富钴结壳的Nd同位素记录,通过对比不同区域Nd同位素记录的演化特征,分析风尘输入、水团演化等因素对Nd同位素记录的影响,探讨了太平洋经向翻转流演化及其与全球气候变化之间的关系,认为北太平洋深层水下沉的停滞和亚洲风尘输入增加可能是导致深层水Nd同位素从距今3～4 Ma开始降低的原因。同时,因北太平洋深层水下沉停滞,PMOC改组,使得更多的CO₂在深水封存,从而对全球气候变冷和北半球冰盖形成产生了积极的贡献。     

Decadal variability of the shallow Pacific meridional overturning circulation: Relation to tropical sea surface temperatures in observations and climate change models

In this study, we use existing observational datasets to evaluate 20th century climate simulations of the tropical Pacific. The emphasis of our work is decadal variability of the shallow meridional overturning circulation, which links the tropical and subtropical Pacific Ocean. In observations, this circulation is characterized by equatorward geostrophic volume transport convergence in the interior ocean pycnocline across 9°N and 9°S. Historical hydrographic data indicate that there has been a decreasing trend in this convergence over the period 1953–2001 of about 11 Sverdrup (1 Sv = 10⁶ m³ s⁻¹), with maximum decade-to-decade variations of 7–11 Sv. The transport time series is highly anti-correlated with sea surface temperature (SST) anomalies in the central and eastern tropical Pacific, implying that variations in meridional overturning circulation are directly linked to decadal variability and trends in tropical SST. These relationships are explored in 18 model simulations of 20th century climate from 14 state-of-the-art coupled climate models. Significant correlation exists between meridional volume transport convergence and tropical SST in the majority of the models over the last half century. However, the magnitude of transport variability on decadal time scales in the models is underestimated while at the same time modeled SST variations are more sensitive to that transport variability than in the observations. The effects of the meridional overturning circulation on SST trends in most the models is less clear. Most models show no trend in meridional transport convergence and underestimate the trend in eastern tropical Pacific SST. The eddy permitting MIROCH model is the only model that reasonably reproduces the observed trends in transport convergence, tropical Pacific SST, and SST gradient along the equator over the last half century. If the observed trends and those simulated in the MIROCH model are ultimately related to greenhouse gas forcing, these results suggest that the Bjerknes feedback, by affecting pycnocline transport convergences, may enhance warming that arises from anthropogenic forcing in the eastern tropical Pacific.     

Volume transport and distribution of deep circulation at 165°W in the North Pacific

We conducted full-depth hydrographic observations between 8°50′ and 44°30′N at 165°W in 2003 and analyzed the data together with those from the World Ocean Circulation Experiment and the World Ocean Database, clarifying the water characteristics and deep circulation in the Central and Northeast Pacific Basins. The deep-water characteristics at depths greater than approximately 2000 dbar at 165°W differ among three regions demarcated by the Hawaiian Ridge at around 24°N and the Mendocino Fracture Zone at 37°N: the southern region (10–24°N), central region (24–37°N), and northern region (north of 37°N). Deep water at temperatures below 1.15 °C and depths greater than 4000 dbar is highly stratified in the southern region, weakly stratified in the central region, and largely uniform in the northern region. Among the three regions, near-bottom water immediately east of Clarion Passage in the southern region is coldest (θ<0.90 °C), most saline (S>34.70), highest in dissolved oxygen (O₂>4.2 ml l^?1), and lowest in silica (Si<135 μmol kg^?1). These characteristics of the deep water reflect transport of Lower Circumpolar Deep Water (LCDW) due to a branch current south of the Wake–Necker Ridge that is separated from the eastern branch current of the deep circulation immediately north of 10°N in the Central Pacific Basin. The branch current south of the Wake–Necker Ridge carries LCDW of θ<1.05 °C with a volume transport of 3.7 Sv (1 Sv=10⁶ m³ s^?1) into the Northeast Pacific Basin through Horizon and Clarion Passages, mainly through the latter (～3.1 Sv). A small amount of the LCDW flows northward at the western boundary of the Northeast Pacific Basin, joins the branch of deep circulation from the Main Gap of the Emperor Seamounts Chain, and forms an eastward current along the Mendocino Fracture Zone with volume transport of nearly 1 Sv. If this volume transport is typical, a major portion of the LCDW (～3 Sv) carried by the branch current south of the Wake–Necker and Hawaiian Ridges may spread in the southern part of the Northeast Pacific Basin. In the northern region at 165°W, silica maxima are found near the bottom and at 2200 dbar; the minimum between the double maxima occurs at a depth of approximately 4000 dbar (θ～1.15 °C). The geostrophic current north of 39°N in the upper deep layer between 1.15 and 2.2 °C, with reference to the 1.15 °C isotherm, has a westward volume transport of 1.6 Sv at 39–44°30′N, carrying silica-rich North Pacific Deep Water from the northeastern region of the Northeast Pacific Basin to the Northwest Pacific Basin.     

Review on North Pacific Subtropical Countercurrents and Subtropical Fronts: role of mode waters in ocean circulation and climate

A Subtropical Countercurrent (STCC) is a narrow eastward jet on the equator side of a subtropical gyre, flowing against the broad westward Sverdrup flow. Together with theories, recent enhanced observations and model simulations have revealed the importance of mode waters in the formation and variability of North Pacific STCCs. There are three distinct STCCs in the North Pacific, maintained by low potential vorticity (PV) that mode waters carry from the north. Model simulations show that changes in mode water ventilation result in interannual to interdecadal variations and long-term changes of STCCs. STCCs affect the atmosphere through their surface thermal effects, inducing anomalous cyclonic wind curl and precipitation along them. Thus, mode waters are not merely passive water masses but have dynamical and climatic effects. For temporal variability, atmospheric forcings are also suggested to be important in addition to the variability of mode waters. STCCs exist in other oceans and they are also flanked by mode waters on their poleward sides, suggesting that they are maintained by similar dynamics.     

A pycnostad on the bottom of the ventilated portion in the central subtropical North Pacific: Its distribution and formation

A water mass characterized by the pycnostad on the bottom of the ventilated portion in the central subtropical North Pacific is described through the comparison with the Subtropical Mode Wate (STMW). In this paper, this water mass is called the North Pacific Central Mode Water (CMW), because of its vertical homogeneity. The distribution of CMW is examined based on the climatological maps of annual mean potential vorticity. On the other hand, its formation area is examined based on the climatological winter temperature data set and the STD sections across the Kuroshio Extension in early spring of individual years. The main results are summarized as follows: 1) STMW is formed in the deep winter mixed layer south of the main path of the Kuroshio Extension (termed 12°C Front in this paper). On the other hand, CMW is formed in the deep winter mixed layer in the east-west band surrounded by a branch of the Kuroshio Extension (termed 9°C Front in this paper) and the boundary of two water masses representing the subtropical and subpolar gyres. 2) The winter mixed layer between the 12°C Front and the 9°C Front is shallower than that in the CMW and STMW formation areas. 3) These geographical features of the winter mixed layer depths near the subarcticsubtropical transition zone result in two pycnostads (STMW and CMW) in the main thermocline of the subtropical North Pacific through the advection caused by the subtropical gyre.     

Water masses and currents of deep circulation southwest of the Shatsky Rise in the western North Pacific

We conducted full-depth hydrographic observations in the southwestern region of the Northwest Pacific Basin in September 2004 and November 2005. Deep-circulation currents crossed the observation line between the East Mariana Ridge and the Shatsky Rise, carrying Lower Circumpolar Deep Water westward in the lower deep layer (θ<1.2 °C) and Upper Circumpolar Deep Water (UCDW) and North Pacific Deep Water (NPDW) eastward in the upper deep layer (1.3–2.2 °C). In the lower deep layer at depths greater than approximately 3500 m, the eastern branch current of the deep circulation was located south of the Shatsky Rise at 30°24′–30°59′N with volume transport of 3.9 Sv (1 Sv=10⁶ m³ s⁻¹) in 2004 and at 30°06′–31°15′N with 1.6 Sv in 2005. The western branch current of the deep circulation was located north of the Ogasawara Plateau at 26°27′–27°03′N with almost 2.1 Sv in 2004 and at 26°27′–26°45′N with 2.7 Sv in 2005. Integrating past and present results, volume transport southwest of the Shatsky Rise is concluded to be a little less than 4 Sv for the eastern branch current and a little more than 2 Sv for the western branch current. In the upper deep layer at depths of approximately 2000–3500 m, UCDW and NPDW, characterized by high and low dissolved oxygen, respectively, were carried eastward at the observation line by the return flow of the deep circulation composing meridional overturning circulation. UCDW was confined between the East Mariana Ridge and the Ogasawara Plateau (22°03′–25°33′N) in 2004, whereas it extended to 26°45′N north of the Ogasawara Plateau in 2005. NPDW existed over the foot and slope of the Shatsky Rise from 29°48′N in 2004 and 30°06′N in 2005 to at least 32°30′N at the top of the Shatsky Rise. Volume transport of UCDW was estimated to be 4.6 Sv in 2004, whereas that of NPDW was 1.4 Sv in 2004 and 2.6 Sv in 2005, although the values for NPDW may be slightly underestimated, because they do not include the component north of the top of the Shatsky Rise. Volume transport of UCDW and NPDW southwest of the Shatsky Rise is concluded to be approximately 5 and 3 Sv, respectively. The pathways of UCDW and NPDW are new findings and suggest a correction for the past view of the deep circulation in the Pacific Ocean.     

Chlorofluorocarbons in the western North Pacific in 1993 and formation of North Pacific intermediate water

Chlorofluorocarbons (CFC-11 and CFC-12) in the intermediate water having between 26.4 and 27.2 were determined at 75 stations in the western North Pacific north of 20°N and west of 175.5°E in 1993. The intermediate water of 26.4–26.6 was almost saturated with respect to the present atmospheric CFC-11 in the zone between 35 and 45°N around the subarctic front. Furthermore, the ratios of CFC-11/CFC-12 of the water were also of those formed after 1975. These suggest that the upper intermediate water (26.4–26.6) was recently formed by cooling and sinking of the surface water not by mixing with old waters. The water below the isopycnal surface of 26.8 contained less CFCs and the area containing higher CFCs around the subarctic front was greatly reduced. However, the CFC age of the lower intermediate water (26.8–27.2) in the zone around the subarctic front was not old, suggesting that the water was formed by diapycnal mixing of the water ventilated with the atmosphere with old waters not containing appreciable CFCs, probably the Pacific Deep Water. The southward spreading rate decreased with depth and it was one sixth of its eastward spreading rate of the North Pacific Intermediate Water (NPIW).     

印度洋海温异常对西北太平洋台风的影响及机制研究

基于近40 a NCEP/NCAR再分析月平均高度场、风场、涡度场、垂直速度场以及NOAA重构的海面温度(sea surface temperature,SST)资料和美国联合台风预警中心(Joint Typhoon Warning Center,JTWC)热带气旋最佳路径资料,利用合成分析方法,研究了前期春季及同期夏季印度洋海面温度同夏季西北太平洋台风活动的关系。结果表明:1)前期春季印度洋海温异常(sea surface temperature anomaly,SSTA)尤其是关键区位于赤道偏北印度洋和西南印度洋地区对西北太平洋台风活动具有显著的影响,春季印度洋海温异常偏暖年,后期夏季,110°～180°E的经向垂直环流表现为异常下沉气流,对应风场的低层低频风辐散、高层辐合的形势,这种环流形势使得低层水汽无法向上输送,对流层中层水汽异常偏少,纬向风垂直切变偏大,从而夏季西北太平洋台风频数偏少、强度偏弱,而异常偏冷年份则正好相反。2)春季印度洋异常暖年,西北太平洋副热带高压加强、西伸;而春季印度洋异常冷年,后期夏季西北太平洋副热带高压减弱、东退,这可能是引起夏季西北太平洋台风变化的另一原因。