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.     

Summer phytoplankton blooms in the oligotrophic North Pacific Subtropical Gyre: Historical perspective and recent observations

The export of organic matter from the oceanic euphotic zone is a critical process in the global biogeochemical cycling of bioelements (C, N, P, Si). Much of this export occurs in the form of sinking particles, which rain down into the unlit waters of the deep sea. Classical models of oceanic production and export balance this gravitational loss of particulate bioelements with an upward flux of dissolved nutrients, and they describe reasonably well those areas of the ocean where deep winter mixing occurs. The surface waters of the North Pacific Subtropical Gyre (NPSG), however, are strongly stratified and chronically nutrient-depleted, especially in summer. Nevertheless, there is ample evidence that blooms of phytoplankton and subsequent pulses of particle export occur during the height of summer stratification in these waters, especially to the northeast of the Hawaiian Islands. These blooms impact regional bioelemental cycling and act as a food source to the deep-sea benthos. We review here numerous published observations of these events in the NPSG, and present new data collected at Station ALOHA (22.75°N, 158°W) during the first 176 cruises of the Hawaii Ocean Time-series program (1988-2005), along with results from transect cruises conducted in the region in 1996 and 2005. We suggest that the summer phytoplankton bloom can be considered a frequent, perhaps annual feature in the northeastern NPSG, and that its perceived stochastic nature is a manifestation of chronic undersampling in time and space. The bloom is typically dominated by only a few genera of large diatoms and the cyanobacterium Trichodesmium. It appears to be consistently supported by dinitrogen fixation, but the fate of the organic matter produced during the summer depends critically on the species composition of the responsible diazotrophs. We estimate that the summer bloom is responsible for up to 38% of N₂ fixation and up to 18% of N-based new production annually at Station ALOHA. We hypothesize that the spatial distribution, timing and magnitude of the bloom may be determined largely by the physical and biological processes controlling new phosphorus delivery into the euphotic zone during the summer and the preceding winter.     

Depth Distribution of the Subtropical Gyre in the North Pacific

Large-scale aspects of the North Pacific subtropical gyre have been investigated using a climatology of temperature and salinity (World Ocean Atlas 1998). In the central and eastern parts of the basin, the axis of the subtropical gyre, defined as the meridional maximum of dynamic height, tends to move poleward from about 25°N near the surface to about 40°N in the upper intermediate layers. In the western part of the basin, the axis is seen at about 30°N, remaining almost unchanged with depth. Striking features associated with this vertical distribution include a northward shift of the bifurcation latitude of the North Equatorial Current at increasing depth and a barotropic nature of the confluence point between the Kuroshio and Oyashio at their respective western boundaries. The former occurs at about 14°N near the surface and extends north of 20°N at depths around 800 m. The latter, situated at about 36.4°N off Japan, does not appear to have a strong signature of depth-dependence. While some of these results are already known from sporadic hydrographic observations, they have not hitherto been represented in a three-dimensional climatology. This revised version was published online in July 2006 with corrections to the Cover Date.     

西北太平洋副热带逆流、北赤道流、北赤道逆流几个特征的比较

根据137°E断面1967～1995年冬、夏季的温、盐资料,计算和分析该断面的地转流;分析144°E断面上投放卫星跟踪漂流浮标的漂移轨迹;结合CSK图集中的海面重力势分布,对副热带逆流、北赤道流和北赤道逆流几个特征的相同点和不同点进行比较,得出若干有益的结论:(1)副热带逆流、北赤道流、北赤道逆流,并不是单纯的单支海流,而是存在着两支或多支现象;(2)流速结构的带状分布,东、西向流相互交错间隔出现,流层较浅,均为表层流或近表层流;(3)多年平均而言,3支海流的流速以北赤道逆流最强,北赤道流次之,副热带逆流最弱;流量则不同,北赤道流最大,副热带逆流最小,北赤道逆流居中.3支海流的流速夏季均大于冬季,但流量稍有差异:副热带逆流和北赤道流均具有夏强、冬弱的特点,而北赤道逆流为冬强、夏弱;(4)冬季,副热带逆流的"源地",位于巴士海峡和台湾以东"副热带脊"的一个"暖脊"中心附近海域;(5)冬季,卫星跟踪漂流浮标的漂移轨迹,基本上反映出北赤道流和北赤道逆流的路径.但因副热带逆流区为涡旋频繁区,致使浮标漂移轨迹难以反映出副热带逆流的路径.     

A time-series study of particulate matter export in the North Pacific Subtropical Gyre based on Th : U disequilibrium

Depth profiles of total ²³⁴Th (dissolved+particulate) were collected at Station ALOHA (22°45N, 158°00W) in the North Pacific Subtropical Gyre during 9 cruises from April 1999 to March 2000. Samples were collected and processed by a new 2 L technique that enables more detailed depth resolution then previous ²³⁴Th studies. Significant zones of particle export (²³⁴Th deficiency) and particle remineralization (²³⁴Th excess) were measured both temporally and with depth. ²³⁴Th derived particulate carbon (PC) and nitrogen (PN) fluxes were determined with steady-state and non-steady-state models and PC/²³⁴Th and PN/²³⁴Th ratios measured with both in situ pumps and free-drifting particle interceptor traps deployed at 150 m. ²³⁴Th based export estimates of 4.0±2.3 mmol C m⁻² d⁻¹ and 0.53±0.19 mmol N m⁻² d⁻¹, were approximately 60% higher than those measured in PIT style sediment traps from the same time period, 2.4±0.2 mmol C m⁻² d⁻¹ and 0.32±0.08 mmol N m⁻² d⁻¹. Most of this difference is attributable to two large export events that occurred during October and December 1999, when traps undercollected for ²³⁴Th by a factor of 2 to 4. ²³⁴Th export (ThE) ratios based on ²³⁴Th derived PC flux/¹⁴C based primary production ranged from 4% to 22% (average=8.8%). Our results confirm the recent estimates of C export by Emerson et al. (Nature 389 (1997) 951) and Sonnerup et al. (Deep-Sea Research I 46 (1999) 777) and indicate that C export from the oligotrophic ocean must be considered when discussing C sequestration in global climate change.     

北太平洋副热带海洋环流气候变化研究

北太平洋副热带环流的变化在全球气候变化和热量的经向输送中占重要地位。本文对近10年有关北太平洋副热带海洋环流气候变化的研究进行了综述。主要研究成果有：用卫星高度计首次观测到全球海洋Rossby波的传播特征；确定了气候意义下北太平洋副热带逆流为2支．揭示了其中一支与北太平洋模态水的存在有关，另一支是夏威夷群岛附近海洋．大气-陆地相互作用的结果；首次发现了台湾以东黑潮流量有显著的准100天振荡等。本文还提出了在北太平洋副热带环流研究中目前存在的新科学问题。     

西北太平洋热带气旋频数及生成位置的气候变化研究进展

自20世纪70年代末期以来,西北太平洋的热带气旋在全球变暖的背景下主要发生了两种宏观的气候变化：一个是热带气旋生成频数呈现年代际减少,尤其是在东南部海域;另一个则是其生成与活动位置等总体特征有向西北偏移的趋势。本文对这两个方面的研究进展进行了概述。近些年的研究表明,垂直风切变的增强可能是夏秋季热带气旋频数减少的最主要原因,这与太平洋-印度洋海面温度变化导致的大尺度环境变化有密切联系。同样有研究认为北大西洋海面温度的多年代际振荡对近期西北太平洋热带气旋生成频数的减少也非常重要。但西北太平洋西部强热带气旋的频数呈现出增加的趋势,这可能与东亚近海海面温度的显著升高有关,尽管这种变化是否可信仍有争议。近20年来,西北太平洋热带气旋活动普遍出现西北移倾向,包括生成位置和路径位置,这种变化可能受到了ENSO变异及20世纪90年代末期太平洋气候突变的调控。同时,热带环流的极向扩张又导致了热带气旋的有利环境向北扩张,因此西北太平洋热带气旋活动也出现极向迁移的趋势。     

North Pacific Subtropical Mode Waters Observed in Long XBT Cross Sections along 32.5°N Line

The S/V Shoyo, of the Hydrographic Department, Japan Coast Guard, has conducted high-density expendable bathythermograph (XBT) measurements along the 32.5°N line in the North Pacific every year from 1990 to 1993 as a part of the Japanese-World Ocean Circulation Experiment (WOCE). These XBT data are analyzed here, focusing on year-to-year variations of the inventory and core layer temperature (CLT) of the North Pacific subtropical mode water (NPSTMW). Large year-to-year changes are found in the NPSTMW CLTs estimated in longitudes between 140°E and 160°E. CLT values were found of 17.4°C in 1990, 17.1°C in 1991, 17.3°C in 1992 and 17.6°C in 1993. Inspection of the wintertime westerlies over the formation area and sea surface temperature distribution revealed that this change in CLT can be qualitatively attributed to the strength of atmospheric cooling in the formation area in the previous winter. Although a large year-to-year variation of NPSTMW inventory was also found, it is hard to state any relationship between CLT and atmospheric forcing. There is a possibility that different observational seasons may affect the inventory. It has also been found that the thermocline depth in 1991 was shallower in the sea area east of 180° than in 1992 and 1993. Associated with this change, the North Pacific central mode water (NPCMW), characterized by thermostad with temperatures ranging from 14°C to 11°C, appears in the sea area east of 180° in the 1992 and 1993 cross sections. The 1993 cross section, which ranged from the Japanese coast to the west coast of North America, possessed another thermostad in the surface layer, with a temperature of about 17°C in the eastern part of the cross section, off California. This revised version was published online in August 2006 with corrections to the Cover Date.     

北太平洋副热带模态水盐度的年代际变化

副热带模态水(Subtropical Mode Water;STMW)在气候变化中起着重要作用。本文利用全球高分辨率数值模拟结果,研究了北太平洋STMW核心层盐度(Core Layer Salinity;CLS)的年代际变化及其物理机制。结果表明,CLS存在显著的年代际变化,其空间分布则与背景流场分布特征有关。侵蚀区CLS滞后生成区CLS约1~2年,这主要是海流平流输运引起的。生成区内,STMW的季节循环一般可分为生成期(12-4月)、隔离期(5-6月)和侵蚀期(7-11月),生成期混合层盐度(Mixed Layer Salinity;MLS)决定着隔离期和侵蚀期的CLS,而MLS年代际变化则主要由同太平洋年代际涛动存在负相关性的海表面淡水通量的变化引起。     

北太平洋850和500hPa夏季副热带高压的年际和年代变化

利用NCEP\NCAR再分析位势高度场资料和Hadley中心SST资料,将北太平洋500和850hPa夏季副热带高压异常变化结合在一起进行了分析,发现夏季500和850hPa副高的位置和强度的年际和年代变化都有明显差异,并且850hPa副高远比500hPa副高稳定。上下两层除了同时增强和同时减弱外,还存在着500hPa增强850hPa减弱,以及500减弱850hPa增强等不同类型。相关分析表明,夏季500和850hPa副热带高压年际和年代异常变化与同期不同海区海表温度(SST)异常之间存在明显的关系。     

南太平洋副热带偶极子对南太平洋辐合带的影响

为研究南太平洋副热带偶极子的局地气候效应,利用Hadley中心的海温数据集Had ISST以及NCEP-NCAR的大气再分析数据,分析了南太平洋副热带偶极子(South Pacific Subtropical Dipole,SPSD)对南太平洋辐合带(South Pacific Convergence Zone,SPCZ)的影响,并探讨了相应的物理过程。研究结果显示,南太平洋副热带偶极子事件线性独立于ENSO(El Ni?o-Southern Oscillation)事件,有明显的季节锁相,于12~2月达到峰值并显著影响SPCZ降水带的位置。其中,正偶极子事件(简称"正事件")期间偶极子东北极区域(暖海温)水汽辐合上升,降水增多;而在偶极子西南极区域(冷海温)水汽辐散下沉,降水减少,因此SPCZ降水带偏北。负偶极子事件(简称"负事件")则相反,东北极降水减少的同时西南极降水增多,从而SPCZ降水带偏南。本研究关于SPSD与SPCZ关系的分析,将有助于更好地理解南太平洋的年际气候变异和海气相互作用。     

西太平洋副热带高压与海表温度的关系

利用超前滞后相关分析研究了西太平洋副热带高压与海表面温度异常的关系。选取各关键海区分析海温与西太副高在不同时段上的超前滞后相关, 结果表明,冬季东太平洋海温与滞后其2—3个月的副高异常达最大正相关,热带印度洋海温异常与冬季同期副高异常的正相关最显著;西太平洋海温在冬春季与同期的副高负相关最显著;北太平洋海温在冬春季滞后副高1—2个月时存在负相关,大西洋暖池区6月与西太副高的同期正相关最大;对南太平洋来说,冬季的西太副高与从前秋到春季的SST都存在最大负相关。海表温度的异常主要解释冬春季的西太副高异常,而对于夏秋季副热带高压,SST的作用比较有限     

Diatoms in the desert: Plankton community response to a mesoscale eddy in the subtropical North Pacific

As part of the E-Flux project, we documented spatial variability and temporal changes in plankton community structure in a cold-core cyclonic eddy in the lee of the Hawaiian Islands. Cyclone Opal spanned 200 km in diameter, with sharply uplifted isopycnals (80–100 m relative to surrounding waters) and a strongly expressed deep chlorophyll a maximum (DCM) in its central core region of 40 km diameter. Microscopic and flow cytometric analyses of samples from across the eddy revealed dramatic transitions in phytoplankton community structure, reflecting Opal's well-developed physical structure. Upper mixed-layer populations in the eddy resembled those outside the eddy and were dominated by picophytoplankton. In contrast, the DCM was composed of large chain-forming diatoms dominated by Chaetoceros and Rhizosolenia spp. Diatoms attained unprecedented levels of biomass (nearly 90 μg C l⁻¹) in the center of the eddy, accounting for 85% of photosynthetic biomass. Protozoan grazers displayed two- to three-fold higher biomass levels in the eddy center as well. We also found a distinct and persistent layer of senescent diatom cells overlying healthy populations, often separated by less than 10 m, indicating that we were sampling a bloom in a state of decline. Time-series sampling over 8 days showed a successional shift in community structure within the central diatom bloom, from the unexpected large chain-forming species to smaller forms more typical of the subtropical North Pacific. The diatom bloom of Cyclone Opal was a unique, and possibly extreme, example of biological response to physical forcing in the North Pacific subtropical gyre, and its detailed study may therefore help to improve our predictive understanding of environmental controls on plankton community structure.     

Calculation of Interannual Variations of Sea Level in the Subtropical North Pacific

The interannual variations of sea level at Chichi-jima and five other islands in the subtropical North Pacific are calculated for 1961–95 with a model of Rossby waves excited by wind. The Rossby-wave forcing is significant east of 140°E. Strong forcing of upwelling (downwelling) Rossby wave occurs during El Niño (La Niña) and warm (cold) water anomaly in the eastern equatorial Pacific. The first and second baroclinic modes of Rossby wave are more strongly generated than the barotropic mode in the study area. A higher vertical mode of Rossby wave propagates more slowly and is more decayed by eddy dissipation. The best coefficient of vertical eddy dissipation is determined by comparing the calculated sea level with observation. The variation in sea level at Chichi-jima is successfully calculated, in particular for the long-term change of the mean level between before and after 1986 with a rise in 1986 as well as the variations with periods of two to four years after 1980. It is concluded that variations of sea level at Chichi-jima are produced by wind-forced Rossby waves, the first baroclinic wave primarily and the barotropic wave secondly. The calculation for other islands is less successful. Degree of the success in calculation almost corresponds to a spatial difference in quantity of wind data, and seems to be determined by quality of wind data.     

2011年3月日本福岛核电站核泄漏在海洋中的传输

使用全球版本的迈阿密等密度海洋环流模式对2011年3月日本福岛核电站泄漏在海洋中的传输以及扩散进行了数值模拟。数值模式中核废料(示踪物)排放情景采取等通量连续排放,排放时间从3月25日开始,分别持续20 d以及1 a,两种情形分别积分20 a。为了减少海洋环流年际变化带来的数值模拟的的不确定性,20 a的模式积分分别用2010年、1991-2011年、1971-1991年以及1951-1971年4个不同时段的NCEP/NCAR逐日再分析资料作为大气强迫场,因此每种排放情形包含4个数值试验。模拟结果的分析表明,不同核废料排放情景及其在不同时段大气资料对海洋模式的驱动下,模拟的示踪物总体的传输扩散路径(包括表层以及次表层)、传输速率以及垂直扩展的范围没有显著的差异。集合平均数值模拟的结果显示:在两种排放情景下,日本福岛核泄漏在海洋的传输路径受北太平洋副热带涡旋洋流系统主导,其传输路径首先主要向东,到达东太平洋后,再向南向西扩散至西太平洋,可能在10~15 a左右影响到我国东部沿海海域,且海洋次表层的传输信号比表层信号早5 a左右。通过进一步分析模式积分过程中最大示踪物浓度随时间变化发现,在积分第20 a(2031年3月),海洋表层和次表层浓度的最高值分别只有模式积分第一年浓度的0.1%和1%。在积分的20 a里,排放的核废料主要滞留在北太平洋海域(超过86%±1.5%的核废料在积分结束时,滞留在北太平洋),而在积分的前10 a(2021年之前),几乎所有的核废料滞留在北太平洋;在核废料的垂直分布上,主要集中在海洋表层至600 m的深度,在积分的20 a时间里,没有核废料信号扩散至1 000 m以下的深度。数值模拟的结果也表明核废料浓度减弱的强度以及演变的时间特征主要受洋流系统的影响,与排放源的排放时间长短关系不大。值得指出的是,更加准确地评估一个真实的核泄漏事故对海洋环境所造成的可能影响,还需要考虑大气中的放射性物质的沉降以及海洋生态对核物质的响应。     

2011年3月日本福岛核电站核泄漏在海洋中的传输

使用全球版本的迈阿密等密度海洋环流模式对2011年3月日本福岛核电站泄漏在海洋中的传输以及扩散进行了数值模拟。数值模式中核废料(示踪物)排放情景采取等通量连续排放,排放时间从3月25日开始,分别持续20 d以及1 a,两种情形分别积分20 a。为了减少海洋环流年际变化带来的数值模拟的的不确定性,20 a的模式积分分别用2010年、1991-2011年、1971-1991年以及1951-1971年4个不同时段的NCEP/NCAR逐日再分析资料作为大气强迫场,因此每种排放情形包含4个数值试验。模拟结果的分析表明,不同核废料排放情景及其在不同时段大气资料对海洋模式的驱动下,模拟的示踪物总体的传输扩散路径(包括表层以及次表层)、传输速率以及垂直扩展的范围没有显著的差异。集合平均数值模拟的结果显示:在两种排放情景下,日本福岛核泄漏在海洋的传输路径受北太平洋副热带涡旋洋流系统主导,其传输路径首先主要向东,到达东太平洋后,再向南向西扩散至西太平洋,可能在10~15 a左右影响到我国东部沿海海域,且海洋次表层的传输信号比表层信号早5 a左右。通过进一步分析模式积分过程中最大示踪物浓度随时间变化发现,在积分第20 a(2031年3月),海洋表层和次表层浓度的最高值分别只有模式积分第一年浓度的0.1%和1%。在积分的20 a里,排放的核废料主要滞留在北太平洋海域(超过86%±1.5%的核废料在积分结束时,滞留在北太平洋),而在积分的前10 a(2021年之前),几乎所有的核废料滞留在北太平洋;在核废料的垂直分布上,主要集中在海洋表层至600 m的深度,在积分的20 a时间里,没有核废料信号扩散至1 000 m以下的深度。数值模拟的结果也表明核废料浓度减弱的强度以及演变的时间特征主要受洋流系统的影响,与排放源的排放时间长短关系不大。值得指出的是,更加准确地评估一个真实的核泄漏事故对海洋环境所造成的可能影响,还需要考虑大气中的放射性物质的沉降以及海洋生态对核物质的响应。     

北太平洋副热带逆流区海流的准90天振荡

根据 POCM模式输出的流场资料 (1993年 1月 1日～ 1998年 12月 31日 ) ,运用小波分析方法分析北太平洋副热带逆流区海流的季节内振荡特征 ,并用功率谱方法作了验证。结果表明 :在以 19.81°N为中心 ,18～ 2 2°N,16 0°E以西的西太平洋海域 ,海流具有显著的准 90 d振荡特征 ;该振荡对应的波长约为 86 5 km,沿 19.81°N该振荡信号自东向西传播的相速度约为 0 .0 9m/ s。从能量角度看 ,该振荡信号主要存在于 15 0 m以内的海洋次表层 ,2 2 2 .5 m深度能量是 6 2 .5 m能量的13%。     

北太平洋海洋带状流的正压特性

海洋中包含着多种时空尺度的运动过程,海洋带状流是近年来发现的一种新的流动形式。这种流动现象介于地转湍流和准定常流之间。上层海洋中大尺度运动的强度远大于海洋带状流而将其遮盖,因而带状流需要借助滤波方法提取出来。本研究基于纬向速度数据,通过切比雪夫高通滤波提取了北太平洋海洋带状流的三维结构,定义了“流向稳定度”指数,量化分析了北太平洋海域带状流的正压特性,从而进一步加深了对北太平洋海洋带状流结构特征的认识。     

赤道外北太平洋上层洋流异常分析

采用复EOF分析方法.对全年各月份除赤道太平洋外的北太平洋海域的上层洋流异常做了统计动力诊断,并与赤道太平洋洋流异常做了比较:得到的主要结论有:北太平洋海域上层洋流的明显异常发生在日本本州岛以东、以南范围不大的海域(关键区)内,此关键区仅占整个北太平洋海域很小的一部分,而在该海域之外,洋流异常均非常小,虽该洋流异常的第...     

Composite Analysis of North Pacific Subtropical Mode Water Properties with Respect to the Strength of the Wintertime East Asian Monsoon

The interannual variation of the thermal structure of North Pacific subtropical mode water (NPSTMW) is investigated by means of composite analysis with respect to the wintertime Monsoon Index (MOI) which can represent the strength of the wintertime East Asian monsoon. The wind stress field over the NPSTMW formation area has significant variation over the interannual (2–4 year) and the decadal (10–20 year) bands. Changes in interannual variation are well correlated with the intensity of the wintertime East Asian monsoon. By means of composite analysis, it is found that significant differences occur in the thermal structure of the NPSTMW between stronger and weaker monsoon years. That is, colder and thicker NPSTMW is formed in years with stronger monsoons. Analysis of the heat flux through the sea surface and horizontal heat divergence in the Ekman layer shows that colder and thicker NPSTMW in stronger monsoon years can be attributed to a larger amount of heat release through the sea surface in the formation area. A larger horizontal divergence of the heat transport in the upper Ekman layer is considerably responsible for this increased heat loss.