North Pacific Tropical Water: its climatology and temporal changes associated with the climate regime shift in the 1970s

North Pacific Tropical Water (NPTW) is characterized as a subsurface salinity maximum flowing in the North Equatorial Current and is the main source of salt for the North Pacific. We briefly describe the climatological features of its formation and circulation, and then examine temporal changes in its properties associated with the climate regime shift in the 1970s. We use a variety of data, which include the repeat hydrographic sections along 130°E, 137°E, 144°E and 155°E meridians, the hydrographic data from the Hawaii Ocean Time-series, the World Ocean Atlas 1994, and available gridded data of wind stress and evaporation. The classical idea that NPTW originates from the zone of the highest sea surface salinity at 20°–30°N centered around the international date line and spreads along the isopycnal geostrophic flow patterns is confirmed. Further, it is shown that the meridional extent of NPTW along 137°E is from 10°N to 23°N on average and the highest salinity core lies at about 15°N and 24.0σ_θ, and that the portion of NPTW north (south) of about 15°N originates from the formation region west (east) of the date line. NPTW in the 137°E section changed remarkably associated with the mid-1970s regime shift. North of 15°N NPTW increased both in its salinity and thickness while to the south of 15°N only its salinity increased and its thickness remained unchanged. The westward geostrophic velocity is increased significantly in both the southern and northern parts of NPTW. The northern thickening and speedup and the southern speedup increased NPTW transport across 137°E. The changes in the thermohaline forcing such as evaporation and Ekman salt convergence in the NPTW formation region possibly contributed to the increases in salinity in the southern part of NPTW, but not to that of the northern part. On the other hand, the increased Ekman pumping accounts for the increase of the NPTW inventory and transport at 137°E. The increased salinity of NPTW at 137°E, especially its northern portion, was presumably caused by an increase in its formation rate rather than changes in the sea surface salinity in its formation region; the thicker the NPTW layer is, the saltier is the core that tends to survive the mixing processes.     

Interdecadal modulation of interannual atmospheric and oceanic variability over the North Pacific

The interdecadal modulation of interannual variability of the atmosphere and ocean is examined over the North Pacific by using Wavelet Transform combined with Empirical Orthogonal Function (EOF) or Singular Value Decomposition (SVD) analysis. For the period of record 1899–1997, the interannual variability of the wintertime Aleutian Low, identified by either the North Pacific Index or the leading eigenvector (EOF-1) of North Pacific sea level pressure (SLP), exhibits an interdecadal modulation. Interannual variance in the strength of the Aleutian Low was relatively large from the mid-1920s to mid-1940s and in the mid-1980s, but relatively small in the periods from 1899 to the mid-1920s and from the mid-1940s to the mid-1970s. The periods of high (low) interannual variability roughly coincide with pentadecadal regimes having a time averaged relatively intense (weak) Aleutian Low. Consistent with this SLP variability the interannual variance in the zonal wind stress is strengthened in the central North Pacific after the 1970s. The SLP EOF-2, which is related to the North Pacific Oscillation, exhibited a strengthening trend from the beginning of this century to the mid-1960s. After the 1970s, the interannual variance of SLP EOF-2 is generally smaller than that in the period from 1930 to 1970. Similar interdecadal changes in interannual variance are found in expansion coefficients for the first two EOFs of the Pacific sector 500 hPa height field for the period 1946–1993. EOF-1 of Pacific sector 500 hPa corresponds to the Pacific/North American (PNA) teleconnection pattern, while EOF-2 is related to the Western Pacific (WP) pattern. The relative influence of the atmospheric PNA and WP interannual variability on North Pacific SSTs appears to have varied at pentadecadal time scales. Results from an SVD analysis of winter season (December–February) 500 hPa and North Pacific spring season (March–May) SST fields demonstrate that the PNA-related SST anomaly exhibited larger interannual variance after the 1970s, whereas the interannual variance of the WP related SST anomaly is larger before the 1970s. Correlations between the coastal North Pacific SST records and gridded atmospheric field data also change on interdecadal time scales. Our results suggest that the SST records from both the northwest and northeast Pacific coasts were more closely coupled with the PNA teleconnection pattern during the periods of 1925–1947 and 1977–1997 than in the regime from 1948 to 1976. Teleconnections between ENSO and preferred patterns of atmospheric variability over the North Pacific also appear to vary on interdecadal time scales. However, these variations do not reflect a unique regime-dependent influence. Our results indicate that ENSO is primarily related to the PNA (WP) pattern in the first (last) half of the present century. Correlation coefficients between indices for ENSO and PNA-like atmospheric variability are remarkably weak in the period from 1948 to 1976.     

Decadal-Scale Climate and Ecosystem Interactions in the North Pacific Ocean

Decadal-scale climate variations in the Pacific Ocean wield a strong influence on the oceanic ecosystem. Two dominant patterns of large-scale SST variability and one dominant pattern of large-scale thermocline variability can be explained as a forced oceanic response to large-scale changes in the Aleutian Low. The physical mechanisms that generate this decadal variability are still unclear, but stochastic atmospheric forcing of the ocean combined with atmospheric teleconnections from the tropics to the midlatitudes and some weak ocean-atmosphere feedbacks processes are the most plausible explanation. These observed physical variations organize the oceanic ecosystem response through large-scale basin-wide forcings that exert distinct local influences through many different processes. The regional ecosystem impacts of these local processes are discussed for the Tropical Pacific, the Central North Pacific, the Kuroshio-Oyashio Extension, the Bering Sea, the Gulf of Alaska, and the California Current System regions in the context of the observed decadal climate variability. The physical ocean-atmosphere system and the oceanic ecosystem interact through many different processes. These include physical forcing of the ecosystem by changes in solar fluxes, ocean temperature, horizontal current advection, vertical mixing and upwelling, freshwater fluxes, and sea ice. These also include oceanic ecosystem forcing of the climate by attenuation of solar energy by phytoplankton absorption and atmospheric aerosol production by phytoplankton DMS fluxes. A more complete understanding of the complicated feedback processes controlling decadal variability, ocean ecosystems, and biogeochemical cycling requires a concerted and organized long-term observational and modeling effort. This revised version was published online in July 2006 with corrections to the Cover Date.     

Anthropogenic CO2 distribution in the North Pacific Ocean

The depth of penetration of anthropogenic CO₂ in the North Pacific Ocean based on carbonate data in the literature is discussed. The results indicate that the deepest penetration (over 2000 m) is found in the northwest North Pacific. The shallowest penetration (to less than 400 m) is found in the eastern equatorial Pacific. Depth of penetration of anthropogenic CO₂ appears to have been controlled by such factors as deep water formation in the Northwest Pacific; upwelling in the equatorial Pacific and; vertical mixing in the western boundary areas. These results compare well with results implied from tritium, C-14, and freons distributions. The total inventory of excess carbon in the North Pacific was 14.7±4×10¹⁵ g around 1980.     

Climate variability and marine ecosystem impacts: a North Atlantic perspective

In recent decades it has been recognized that in the North Atlantic climatic variability has been largely driven by atmospheric forcing related to the North Atlantic Oscillation (NAO). The NAO index began a pronounced decline around 1950 to a low in the 1960s. From 1970 onward the NAO index increased to its most extreme and persistent positive phase during the late 1980s and early 1990s. Changes in the pattern of the NAO have differential impacts on the opposite sides of the North Atlantic and differential impacts in the north and south. The changes in climate resulting from changes in the NAO appear to have had substantial impacts on marine ecosystems, in particular, on fish productivity, with the effects varying from region to region. An examination of several species and stocks, e.g. gadoids, herring and plankton in the Northeast Atlantic and cod and shellfish in the Northwest Atlantic, indicates that there is a link between long-term trends in the NAO and the productivity of various components of the marine ecosystem. While broad trends are evident, the mechanisms are poorly understood. Further research is needed to improve our understanding of how this climate variability affects the productivity of various components of the North Atlantic marine ecosystem.     

The distribution of chlorophylla,protein, RNA and DNA in the North Pacific Ocean

Chla, protein, RNA and DNA were measured in 400 samples taken from the surface down to 5,000 m at 27 oceanographical stations in the North Pacific Ocean. Two section diagrams of these cellular constituents were given along 155°E and 155°W meridians, and several vertical profiles of the four constituents were also given at some stations near Japan. The average concentration ranges of Chla, protein, RNA and DNA obtained in this study area were 0.025–0.862, 11.4–88.1, 1.36–35.3 and 0.13–5.24g/l, respectively. Chla was distributed mostly in the photic zone as we would expect. However protein, RNA were in high concentrations within the photic zone and sometimes extremely high concentrations in the deep aphotic zone.     

Impact of El Niño events and climate regime shift on living resources in the western North Pacific

Features of El Niño events and their biological impacts in the western North Pacific are reviewed, focusing on interactions between ENSO and the East Asian monsoon. Impacts of El Niño on the climate in the Far East become evident as ‘cool summers and warm winters’. Effects of climate regime shift on ENSO activities, western boundary currents and upper-ocean stratification, as well as their biological consequences are summarized. These have been:

1. In the western equatorial Pacific, an eastward extension of the warm pool associated with El Niño events induces an eastward shift of main fishing grounds of skip jack and big eye tunas.

2. The surface salinity front in the North Equatorial Current region retreats southward, associated with El Niño events. This leads to a southward shift of the spawning ground of Japanese eel, which is responsible for a reduction in the transport of the larval eels to the Kuroshio and Japanese coastal region, causing poor recruitment.

3. Intensification of winter cooling and vertical mixing associated with La Niña (El Niño) events in the northern subtropical region of the western (central) North Pacific reduces surface chlorophyll concentration levels and larval feeding condition for both Japanese sardines and the autumn cohort of Neon squid during winter–early spring. The semi-decadal scale calm winter that occurred during the early 1970s triggered the first sharp increase of sardine stock around Japan.

4. A remarkable weakening of southward intrusion of the Oyashio off the east coast of Japan during 1988–91, resulted in a decrease in chlorophyll concentrations and mesozooplankton biomass in late spring–early summer of the Kuroshio-Oyashio transition region. Changes occurred in the dominant species of small pelagic fish, through successive recruitment failures of Japanese sardine.

北太平洋大气沉降的时空特征及其对副极区海洋生态系统的影响

北太平洋副极地海区作为全球海洋三个高营养盐低叶绿素(high nutrient and low chlorophyll, HNLC)海区之一, 其浮游植物生长受到微量元素铁的限制。对于开阔大洋, 大气沉降是海洋表层铁的一个重要来源, 铁元素沉降进入海洋后能够促进浮游植物生长, 进而引起海洋初级生产力和生物泵的响应。本文利用SPRINTARS(Spectral Radiation-Transport Model for Aerosol Species)模式的时长为20a的日均大气沉降数据, 对北太平洋海区大气沉降的时空特征进行了分析。结果表明, 进入北太平洋海区的大气沉降量为26.81Tg·a^-1, 并且存在显著的季节变化: 春季最高, 冬季最低, 5月份进入海洋的沉降量达到峰值。大气沉降主要来源于陆地区域, 在风场的驱动下向海洋传输, 因此大气沉降量的空间分布呈现出西高东低的特征。本文以2010年8月中旬卫星观测到的一次强沙尘(即高大气沉降量)事件为例, 研究了大气沙尘的传播路径。进一步结合2001年4月9—12日及2008年4月20—22日的沙尘事件, 分析了西北太平洋K2站位(47°N, 160°E)附近海域海洋初级生产力对大气沉降——沙尘事件的响应。结果表明, 三次沙尘事件后, K2站位的颗粒有机碳通量、叶绿素浓度均有明显增加, 即沙尘事件对北太平洋副极区海洋初级生产力存在促进作用。     

Distribution of pelagic blue-green algae in the North Pacific Ocean

The distribution of pelagic blue-green algae, especially ofTrichodesmium thiebautii, was investigated on the basis of the collection of theHakuhō Maru Cruise KH-69-4 along 155°W (50°N-15°S) in the North Pacific Ocean from September to November 1969.

1. Five species were identified:Trichodesmium thiebautii (most predominant),T. erythraeum, Oscillatoria sp.,Katagnymene spiralis andRichelia intracellularis.
2. T. thiebautii was most abundant in the western North Pacific central water and abundant next to it in the equatorial water, but it did not occur in the subarctic water.
3. T. thiebautii was ubiquitously distributed in the lower layer of 100–200 m in the equatorial water, though not in a large quantity.
4. T. thiebautii inhabited only the water warmer than 20°C. In its main habitat, nitrate and nitrite were almost zero, but ammonia and phosphate were present. There was not found any correlation between its occurrence and the salinity.
5. Blue-green algae were generally thinly populated in the water rich in diatoms.

     

Interdecadal temperature variations in the North Pacific Central Mode Water simulated by an OGCM

The North Pacific Central Mode Water (CMW) is a water mass that forms in the Kuroshio-Oyashio Extension (KOE) region with characteristic low potential vorticity. Recent studies have suggested that the CMW, as low potential vorticity water, plays an important role in the adjustment of the subtropical gyre and subsurface variability on decadal to interdecadal timescales. We have forced a realistic ocean general circulation model (OGCM) with observed wind stress and sea surface temperature (SST) forcing to investigate the decadal variations of the CMW. Associated with the large atmospheric changes after the mid-1970s climate regime shift, the upper thermocline experiences a cooling as negative SST anomalies in the central North Pacific are subducted and advected southward. In addition to this thermodynamic response, the CMW’s path shifts anomalously eastward in response to anomalous Ekman pumping. This eastward shift of the core of the CMW produces a lowering of the isotherms, and a consequent warming, on the path of the CMW core. This warming partially counteracts the cooling associated with subducted surface anomalies, and it may be responsible for the reduced temperature variations at the climatological position of the CMW when both anomalous wind and heat fluxes are given. Lateral induction across the sloping bottom of the winter mixed layer in the KOE is critical to the formation of the low potential vorticity CMW. Coarse resolution models, which are widely used in climate modeling, underestimate the horizontal gradient of the mixed layer depth and form only a weak CMW or none at all. We have conducted a coarse resolution experiment with the same OGCM, showing that the subsurface response is much reduced. In particular, there is no dynamic warming in the CMW and the thermodynamic response to the SST cooling dominates. The resultant total response differs substantially from that in the finer resolution run where a strong CMW forms. This sensitivity to the model resolution corroborates the important dynamical role that the CMW may play with its distinctive low potential vorticity character and calls for its improved simulation.     

北太平洋大洋钻探研究进展

北太平洋作为全球大洋环流的重要组成部分,在高低纬间热量和物质的传输与再分配方面起到重要的调控作用,进而影响到地球气候系统。基于过去50多年来的大洋钻探工作,前人在北太平洋地球科学的研究上取得了一系列的成果。本文回顾了北太平洋古海洋和古气候方面的研究进展,包括：（1）东亚夏季风和西部边界流演化,以及其对高低纬热量、水汽的传输;（2） 北太平洋中层水和深层水的性质变化、分布范围和驱动机制,以及冰期旋回中水体垂直交换作用的气候响应;（3） 风尘输入对亚洲内陆古环境的反映,及其对北太平洋生产力的铁肥效应。尽管前人针对上述科学问题都开展了相应的研究工作,但目前在对北太平洋上述几方面的认识上仍然存在着分歧。基于对前人研究的总结概括,本文最后提出了未来北太平洋研究的关键科学问题,强调了多圈层、多系统角度对深入认识过去地球气候系统变化的重要性,并对未来大洋航次开展的理想靶区进行了展望。     

Distribution of cyanobacteria and other picophytoplankton in the western North Pacific Ocean,Summer 1989

Cell densities of phycoerythrin-fluorescing cyanobacteria and other chlorophyll-fluorescing picophytoplankton in the 0.2–2.0 µm size fraction were investigated, using an epifluorescence microscope, in the western North Pacific Ocean (36.5–44.0 °N, 155.0°E) in the summer of 1989. Cyanobacteria were most abundant in the surface of the subtropical water (36.5–38.0°N) and less in the northern sea area (39.5–44.0°N). The cell density of other picophytoplankton was, however, high in the northern part and low in the subtropical water. Results showed that algae other than cyanobacteria may significantly contribute to the picophytoplankton community under the low water temperature conditions of open waters. Chlorophylla concentration represented well the abundance of picophytoplankton other than cyanobacteria, but had no significant correlation with the cyanobacteria cell density. Chlorophylla-based data must be interpreted with caution, since the abundances of cyanobacteria were often considerably different even though the chlorophylla concentrations were the same level.     

North Pacific Synthesis of the Joint Global Ocean Flux Study: Overview

This special issue is comprised of 13 papers, including this overview, and focuses on the synthesis of the Joint Global Ocean Flux Study (JGOFS) in the North Pacific which took place from 1997 through 2003. The effort was led by the JGOFS North Pacific Synthesis Group, with the aim of quantifying CO₂ drawdown by physical and biological pumps in the North Pacific by identifying and studying the regional, seasonal to inter-annual variations in the key processes, and understanding their regulating mechanisms. Emphasis was placed on the similarities and differences of the biogeochemical regimes in the eastern and western subarctic Pacific. Effort was also made to address the future research directions which arose from the scientific findings during the North Pacific JGOFS process study. A brief overview of the papers from view points of CO₂ drawdown by physical and biological pumps, spatial variability, and temporal variability from seasonal to decadal scales is made, followed by suggestions for the directions of future research. This revised version was published online in July 2006 with corrections to the Cover Date.     

Ferromanganese nodule fauna in the Tropical North Pacific Ocean: Species richness,faunal cover and spatial distribution

The poorly known ferromanganese nodule fauna is a widespread hard substratum community in the deep sea that will be considerably impacted by large-scale nodule mining operations. The objective of this study was to analyze the spatial distribution of the fauna attached to nodules in the Clarion-Clipperton Fracture Zone at two scales; a regional scale that includes the east (14°N, 130°W) and the west (9°N, 150°W) zones and a local scale in which different geological facies (A, B, C and west) are recognizable. The fauna associated with 235 nodules was quantitatively described: 104 nodules from the east zone (15 of facies A, 50 of facies B and 39 of facies C) and 131 nodules from the west zone. Percent cover was used to quantify the extent of colonization at the time of sampling, for 42 species out of the 62 live species observed. Fauna covered up to 18% of exposed nodule surface with an average of about 3%. While species richness increased with exposed nodule surface, both at the regional and at the facies scales (except for facies A), total species density decreased (again except for facies A). When all nodules were included in the statistical analysis, there was no relation between faunal cover and exposed nodule surface. Nevertheless, faunal cover did decrease with exposed nodule surface for the east zone in general and for both facies B and C in particular. Species distributions among facies were significantly different but explained only a very small portion of the variance (∼5%). We identified two groups of associated species: a first group of two species and a second group of six species. The other species (34) were independently distributed, suggesting that species interactions play only a minor role in the spatial distribution of nodule fauna. The flux of particulate organic carbon to the bottom is the only major environmental factor considered to vary between the two zones within this study. We conclude that the higher species richness and higher percent faunal cover of the east zone can be partially attributed to greater food availability derived from surface inputs. Moreover, the surfaces of facies B and C nodules had a complex, knobby micro-relief, creating microhabitat heterogeneity that may also have contributed to the greater species richness observed in the east zone.     

东太平洋海隆深海热液区沉积物微生物多样性的研究

提取东太平洋海隆区深海热液系统沉积物样品的总DNA,构建沉积物中的细菌16S rDNA 克隆文库,通过PCR-RFLP分析与序列测定,对沉积物中的微生物类群及其与环境的关系进行了分析.结果表明,该海区沉积物中的36个克隆代表的22种基因型分别属于7个主要类群,其中变形菌(Proteobacteria)的γ-亚群为优势菌群,α-和β-亚群也均有分布;而硫氧化相关共生菌的属(sulfur-oxidizing symbionts)为优势种属.系统发育分析表明,在该沉积物中细菌主要是跟共生有关、跟C、S代谢相关,大多还能在无氧和高温环境的条件下生存,说明采样点具有典型的深海热液生态系统的特点,甲烷代谢和硫代谢在该区域的深海物质能量循环中占据着重要地位.另外大量新的极端微生物的存在,预示着该区域的微生物资源有着非常大的开发潜力.     

New computation of the wind stress over the North Pacific Ocean

The wind-stress field in the North Pacific Ocean during 1961–75 is computed from nearly five million ship reports. With a drag coefficient having a linear relation to wind speed, annual mean and monthly mean wind-stress fields are obtained, and their features are described.Compared with the stress fields obtained byHellerman (1967) andWyrtki andMeyers (1976), the eastward component of the stress in the present study is larger in magnitude and the northward one smaller in magnitude, especially in the trade wind region. Differences in the drag coefficient do not have a pronounced effect on the estimated stress field. Long-period inter-annual variations in the wind field are the most likely cause of the discrepancies between the present study and those of the above authors.The maximum of the wind-stress curl, estimated from the annual mean wind-stress fields, is as large as 1.0×10^–8dyn cm^–3 around 30°N, and is larger than that estimated byEvenson andVeronis (1975). The discrepancy is considered to be mainly due to differences in the computed stress field itself rather than due to differences in the grid size used in the stress computations.The Sverdrup transports integrated from the eastern boundary on the basis of the present stress field have a maximum greater than 40×10^–12cm³ s^–1 (Sv.) near the western boundary around 30°N. This value is closer to the observed transport of the Kuroshio than that based on Hellerman's stress field.