Latitudinal Distribution of CO2 Fugacity along 175°E in the North Pacific in 1992–1996

In order to examine latitudinal distribution and seasonal change of the surface oceanic fCO₂, we analyzed the data obtained in the North Pacific along 175°E during the NOPACCS cruises in spring and summer of 1992–1996. Except for around the equator where the fCO₂ was significantly affected by the upwelling of deep water, the latitudinal distribution of fCO₂ showed distinctive seasonal variation. In the spring, the fCO₂ decreased and then increased going southward with the minimum value of about 300 µatm around 35°N, while in the summer, the fCO₂ displayed high variability, showing minimum and maximum values at latitudes of around 44° and 35°N, respectively. It was also found that the fCO₂ was well correlated with the SST, but the relationship between the two was different for different hydrographic regions. In the subpolar gyre, the frontal regions between the Water-Mass Front and the Kuroshio bifurcation front, and between the Kuroshio bifurcation front and the Kuroshio Extension current, SST, DIC and TA influenced the seasonal fCO₂ change through seasonally-dependent biological activities and vertical mixing and stratification of seawater. In the central subtropical gyre and the North Equatorial current, the seasonal fCO₂ change was found to be produced basically by changes in SST and DIC. The summertime oceanic fCO₂ generally increased with time over the period covered by this study, but the increased rate was clearly higher than those expected from other measurements in the western North Pacific.     

Zonal variability of plankton and particle export flux in the equatorial Pacific upwelling between 165° E and 150° W

Observations made during a “La Niña” situation (April–May 1996) in the equatorial Pacific upwelling, between 165° E and 150° W, show the classic deepening of hydrological isolines from east to west, resulting in zonal gradients for surface temperature and macronutrients. However, contrasting with such a gradient, no clear zonal variation could be seen for integrated planktonic biomasses and carbon fluxes, namely: chlorophyll a, bacterial abundances, particulate organic phosphorus, mesozooplankton ash-free dry weight, primary production, and the sinking flux of particulate organic carbon (POC). Moreover, mean values of these parameters along the zonal equatorial transect, are not significantly different from those of a 7-day-long time series station made at 0°, 150° W in October 1994 during an El Niño period. Such a steady zonal distribution of planktonic parameters seems to be characteristic of equatorial Pacific upwelling west of the Galapagos Islands so that the spatial distributions of nutrient concentrations and planktonic biomass appear to be uncoupled. This is consistent with the High Nutrient-Low Chlorophyll (HNLC) concept, in which primary production is not controlled directly by macronutrient concentrations. The lack of zonal gradient also suggests that carbon budget of the equatorial Pacific is primarily controlled by oscillations in the zonal and meridian extension of the HNLC area, rather than by values of planktonic biomasses and carbon fluxes within the upwelled water, which are quite constant.     

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.     

Fifty years of the 137°E repeat hydrographic section in the western North Pacific Ocean

The 137°E repeat hydrographic section of the Japan Meteorological Agency across the western North Pacific was initiated in 1967 as part of the Cooperative Study of the Kuroshio and Adjacent Regions and has been continued biannually in winter and summer. The publicly available data from the section have been widely used to reveal seasonal to decadal variations and long-term changes of currents and water masses, biogeochemical and biological properties, and marine pollutants in relation to climate variability such as the El Niño-Southern Oscillation and the Pacific Decadal Oscillation. In commemoration of the 50th anniversary in 2016, this review summarizes the history and scientific achievements of the 137°E section during 1967–2016. Through the publication of more than 100 papers over this 50-year span, with the frequency and significance of the publication increasing in time, the 137°E section has demonstrated its importance for future investigations of physical–biogeochemical–biological interactions on various spatiotemporal scales, and thereby its utility in enhancing process understanding to aid projections of the impact of future climate change on ocean resources and ecosystems over the twenty-first century.     

Pathway and variability of deep circulation around 40°N in the northwest Pacific Ocean

To clarify the global deep-water circulation in the northwest Pacific, we conducted current observations with seven moorings at 40°N east of Japan from May 2007 to October 2008, together with hydrographic observations. By analyzing the data, while taking into consideration that the deep circulation has a northward component in this region and carries low-silica, high-dissolved-oxygen water, we clarified that the deep circulation flows within the region between 144°30′ and 146°10′E at 40°N on and east of the eastern slope of the Japan Trench with marked variability; the deep circulation flows partly on the eastern slope of the trench and mainly to the east during P1 (10 May–24 November 2007), is confined to the eastern slope of the trench during P2 (25 November 2007–20 May 2008), and flows on and to the immediate east of the eastern slope of the trench during P3 (21 May–15 October 2008). Previous studies have identified two branches of the deep circulation at lower latitudes in the western North Pacific; one flows off the western trenches and the other detours near the Shatsky Rise. It was thus concluded that the eastern branch flows westward at 38°N and then northward to the east of the trench, finally joining the western branch around 40°N during P1 and P3, whereas the eastern branch passes westward south of 38°N, joins the western branch around 38°N, and flows northward on the eastern slope of the trench during P2.     

Fronts and surface zonal geostrophic current along 115°E in the southern Indian Ocean

Altimeter and in situ data are used to estimate the mean surface zonal geostrophic current in the section along 115°E in the southern Indian Ocean,and the variation of strong currents in relation to the major fronts is studied.The results show that,in average,the flow in the core of Antarctic Circumpolar Current(ACC) along the section is composed of two parts,one corresponds to the jet of Subantarctic Front(SAF) and the other is the flow in the Polar Front Zone(PFZ),with a westward flow between them.The mean surface zonal geostrophic current corresponding to the SAF is up to 49 cm · s-1 at 46°S,which is the maximal velocity in the section.The eastward flow in the PFZ has a width of about 4.3 degrees in latitudes.The mean surface zonal geostrophic current corresponding to the Southern Antarctic Circumpolar Current Front(SACCF) is located at 59.7 °S with velocity less than 20 cm · s-1.The location of zonal geostrophic jet corresponding to the SAF is quite stable during the study period.In contrast,the eastward jets in the PFZ exhibit various patterns,i.e.,the primary Polar Front(PF1) shows its strong meridional shift and the secondary Polar Front(PF2) does not always coincide with jet.The surface zonal geostrophic current corresponding to SAF has the significant periods of annual,semi-annual and four-month.The geostrophic current of the PFZ also shows significant periods of semi-annual and four-month,but is out of phase with the periods of the SAF,which results in no notable semi-annual and fourmonth periods in the surface zonal geostrophic current in the core of the ACC.In terms of annual cycle,the mean surface zonal geostrophic current in the core of the ACC shows its maximal velocity in June.     

Long-term variations of surface and intermediate waters in the southern Indian Ocean along 32°S

Variations of water properties in surface and intermediate layers along 32°S in the southern Indian Ocean were examined using a 50-year (1960–2010) time series reproduced from historical hydrographic and Argo data by using optimum interpolation. Salinity in the 26.7–27.3σ_θ density layer decreased significantly over the whole section, at a maximum rate of 0.02 decade⁻¹ at 26.8–26.9σ_θ, for the 50-year average. Three deoxygenating cores were identified east of 75°E, and the increasing rate of apparent oxygen utilization in the most prominent core (26.9–27.0σ_θ) exceeded 0.05 ml l⁻¹ decade⁻¹. The pycnostad core of Subantarctic Mode Water (SAMW) and the salinity minimum of Antarctic Intermediate Water shifted slightly toward the lighter layers. Comparisons with trans-Indian Ocean survey data from 1936 suggest that the tendencies found in the time series began before 1960. Interestingly, cores of many prominent trends were located just offshore of Australia at 26.7–27.0σ_θ, which is in the SAMW density range. Spectrum analysis revealed that two oscillation components with time scales of about 40 and 10 years were dominant in the subsurface layers. Our results are fairly consistent with, and thus support, the oceanic responses in the southern Indian Ocean to anthropogenic climate change predicted by model studies.     

Nitrous oxide and methane in the Atlantic Ocean between 50°N and 52°S: Latitudinal distribution and sea-to-air flux

We discuss nitrous oxide (N₂O) and methane (CH₄) distributions in 49 vertical profiles covering the upper ∼300 m of the water column along two ∼13,500 km transects between ∼50°N and ∼52°S during the Atlantic Meridional Transect (AMT) programme (AMT cruises 12 and 13). Vertical N₂O profiles were amenable to analysis on the basis of common features coincident with Longhurst provinces. In contrast, CH₄ showed no such pattern. The most striking feature of the latitudinal depth distributions was a well-defined “plume” of exceptionally high N₂O concentrations coincident with very low levels of CH₄, located between ∼23.5°N and ∼23.5°S; this feature reflects the upwelling of deep waters containing N₂O derived from nitrification, as identified by an analysis of N₂O, apparent oxygen utilization (AOU) and NO₃⁻, and presumably depleted in CH₄ by bacterial oxidation. Sea-to-air emissions fluxes for a region equivalent to ∼42% of the Atlantic Ocean surface area were in the range 0.40–0.68 Tg N₂O yr⁻¹ and 0.81–1.43 Tg CH₄ yr⁻¹. Based on contemporary estimates of the global ocean source strengths of atmospheric N₂O and CH₄, the Atlantic Ocean could account for ∼6–15% and 4–13%, respectively, of these source totals. Given that the Atlantic Ocean accounts for around 20% of the global ocean surface, on unit area basis it appears that the Atlantic may be a slightly weaker source of atmospheric N₂O than other ocean regions but it could make a somewhat larger contribution to marine-derived atmospheric CH₄ than previously thought.     

Latitudinal trends in morphological characteristics of <Emphasis Type="Italic">Neogloboquadrina pachyderma</Emphasis> (Ehrenberg) along a north–south transect in the south-western Indian Ocean

A total of 25 surficial sediment samples (Peterson grab, gravity and piston cores), collected during the Pilot Expedition to Southern Ocean (PESO) 2004 cruises 199C and 200 onboard the ORV Sagar Kanya along a N–S transect between 9.69°N and 55.01°S, and 80 and 40°E in the Indian Ocean sector of the Southern Ocean (SW Indian Ocean), have been investigated for various morphological features—test size, mean proloculus size and coiling direction (dextral/sinistral forms)—of the planktic indicator species Neogloboquadrina pachyderma (Ehrenberg). The results show that the coiling directions co-vary with temperature and salinity, the abundances of sinistrally coiled forms increasing towards higher latitudes (south of 40°S), whereas dextrally coiled forms show a reverse trend. Similarly, overall test and proloculus sizes depend largely on the physicochemical properties (salinity, temperature, nutrients, calcium saturation) of the ambient water masses. These observations suggest that, particularly at the boundaries between different water masses, variations in morphological features of N. pachyderma can meaningfully be used to reconstruct paleoceanographic conditions from Indian Ocean sediments.     

The slanting property of semi-diurnal internal tide propagation in the Pacific Ocean at 1°45′S, 156°E

1Introduction DuringtheTOGA COARE,extensivejointin ternationaloceanicobservationswerecarriedoutin thewesternequatorialPacificOceanandavasta mountofvaluabledatawereobtained.Themaindata usedhereincludethedatafromasinglemooringin strumentarrayat1°45′S,156°E,deployedbythe WoodsHoleOceanographicInstitution(WHOI)and theCTD(profilerofconductivity-temperature-depth)datacollectedbytheChineseRVXiangy anghongNo.5.Fangetal.(2000)described brieflytheseobservationdataandanalyzedthedis persion…     

The slanting property of semi-diurnal internal tide propagation in the Pacific Ocean at 1°45′S, 156°E

By analyzing a data set collected using a moored instrument array and CTD during TOGA-COARE, it is found that there exist remarkable internal tides in the western equatorial Pacific Ocean around 1°45′S,156°E, whose horizontal wavenumber (wavelength), vertical wavenumber, h 156° orizontal propagation speed and vertical propagation speed are 3.3×10-2km-1(210 km),-1.6×10-3m, 2.0 m/s and -3.8 cm/s, respectively, that is, the waveform propagates downwards slantingly. Moreover, the propagating direction rotates statistically clockwise as the depth increases and its cause is unclear.     

Bottom fauna associated with mussel beds and alvinellid communities in the hydrothermal field at 9° N of the East Pacific Rise

Samples from mytilid mollusk and alvinellid polychaet communities and associated fauna were collected during cruise 49 of R/V Akademik Mstislav Keldysh at 9° N of the East Pacific Rise. The mytilids Bathymodiolus thermophylus occupied zones of diffuse releases of hydrothermal solutions with temperature anomalies a few degrees in amplitude. The alvinellid assemblages were located in the hottest parts of hydrothermal vents (～ 40°C) over sulfide mounds. The taxonomic compositions of these kinds of assemblages are different. The taxonomic richness of mytilid communities is almost twice higher. A comparison of the samples using Jaccard’s index showed a greater similarity among the alvinellid samples than that between the mytilid ones. With respect to the species number, polychaets formed the most diverse group, while gastropods were the second most diverse. The dominant species had greater proportions in alvinellid communities than in mytilid ones. This study showed great differences between the mytilid and alvinellid associations in their species composition and community structure.     

Amino acid and hexosamine composition and flux of sinking particulate matter in the equatorial Pacific at 175°E longitude

Sinking matter collected by sediment traps, which were deployed in the equatorial Pacific Ocean at 175°E for about 11 months during 1992–1993, were analyzed for their flux and labile components in terms of amino acids and hexosamines. The samples provided a temporal resolution of 15 days and were collected from 1357 (shallow trap) and 4363 m (deep trap) depths where sea floor depth was 4880 m. Particle flux along with major components (carbonate, organic matter, biogenic opal and lithogenic material) and amino acid parameters showed distinct temporal variations, which were more pronounced in the shallow trap relative to deep trap. A coupling between the fluxes in the shallow and deep traps was more evident during the period of maximum particle flux, which seems to be connected with the short reappearance of non-El Niño conditions in equatorial Pacific during the 1991–1993 El Niño event. The biogeochemical indicators C/N, Asp/Bala, Glu/Gaba, Bala+Gaba mol%, THAA-C% and THAA-N% implied that the increase in sinking flux was associated with upwelling and enhanced surface production. Degradation of sinking particulate organic matter between the shallow and deep traps was also evident. Occasionally higher mass and major component fluxes in the deep trap relative to the shallow trap are attributed to contribution of resuspended particulates from sea floor (nepheloid layer) or to laterally advected particulates from nearby areas. Carbonate and opal composition of the sinking flux showed a predominance of calcareous plankton; however, Asp/Gly mol ratio and Ser+Thr mol% indicated enhanced occurrence of diatoms during the periods of higher flux.     

Long baroclinic Rossby waves with periods of about 500d near 20°N in the northwest Pacific Ocean

1IntroductionThe oceanic Rossby waves play an importantrole in the large-scale oceanic circulations.In the o-ceans baroclinic Rossby waves have surface manifes-tations of just a fewcentimeters,and wavelengths atmid-latitudes of hundreds to thousands of ki…     

Long baroclinic Rossby waves with periods of about 500d near 20°N in the northwest Pacific Ocean

On the basis of maps of sea level anomalies data set from October 1992 to January 2004, pronounced low frequency variations with periods of about 500 d are detected in the area near 20°N from 160°W to 130°E. A linear two-layer model is employed to explain the mechanism. It is found that the first-mode long baroclinic Rossby waves at 20°N in the northwest Pacific propagate westward in the form of free waves at a speed of about 10.3 cm/s. This confirms that the observed low frequency variabilities appear as baroclinic Rossby waves. It further shows that these low frequency variabilities around 20°N in the northwest Pacific can potentially be predicted with a lead up to 900 d.     

Improved Method for Calculating Anthropogenic CO2 in the Upper Layer of the North Pacific Subtropical Gyre along 175°E

Seawater samples were collected in the North Pacific along 175°E during a cruise of the Northwest Pacific Carbon Cycle Study (NOPACCS) program in 1994. Many properties related to the carbonate system were analyzed. By using well-known ratios to correct for chemical changes in seawater, the CO₂ concentration at a given depth was back calculated to its initial concentration at the time when the water left the surface in winter. We estimated sea-surface CO₂ and titration alkalinity (TA) in present-day winter, from which we evaluated the degree of air-sea CO₂ disequilibrium in winter was. Using a correction factor for air-sea CO₂ disequilibrium in winter, we reconstructed sea-surface CO₂ in pre-industrial times. The difference between the back-calculated initial CO₂ and sea-surface CO₂ in pre-industrial times should correspond to anthropgenic CO₂ input. Although the mixing of different water masses may cause systematic error in the calculation, we found that the nonlinear effect induced by the mixing of different water masses was negligible in the upper layer of the North Pacific subtropical gyre along 175°E. The results of our improved method of assessing the distribution of anthropogenic CO₂ in that region show marked differences from those obtained using the previous back-calculation method.     

Three-dimensional characteristics of mesoscale eddies simulated by a regional model in the northwestern Pacific Ocean during 2000–2008

Mesoscale eddies play vital roles in ocean processes. Although previous studies focused on eddy surface features and individual three-dimensional (3D) eddy cases in the northwestern Pacific Ocean, the analysis of unique eddy 3D regional characteristics is still lacking. A 3D eddy detection scheme is applied to 9 years (2000–2008) of eddy-resolving Regional Ocean Modeling System (ROMS) output to obtain a 3D eddy dataset from the surface to a depth of 1 000 m in the northwestern Pacific Ocean (15°–35°N, 120°–145°E). The 3D characteristics of mesoscale eddies are analyzed in two regions, namely, Box1 (Subtropical Countercurrent, 15°–25°N, 120°–145°E) and Box2 (Southern Kuroshio Extension, 25°–35°N, 120°–145°E). In Box1, the current is characterized by strong vertical shear and weak horizontal shear. In Box2, the current is characterized by the strong Kuroshio, topographic effect, and the westward propagation of Rossby waves. The results indicate the importance of baroclinic instability in Box1, whereas in Box2, both the barotropic and baroclinic instability are important. Moreover, the mesoscale eddies’ properties in Box1 and Box2 are distinct. The eddies in Box1 have larger number and radius but a shorter lifetime. By contrast, Box2 has fewer eddies, which have smaller radius but longer lifetime. Vertically, more eddies are detected at the subsurface than at the surface in both regions; the depth of 650 m is the turning point in Box1. Above this depth, the number of cyclonic eddies (CEs) is larger than that of anticyclonic eddies (AEs). In Box2, the number of CEs is dominant vertically. Eddy kinetic energy (EKE) and mean normalized relative vorticity in Box2 are significantly higher than those in Box1. With increasing depth, the attenuation trend of EKE and relative vorticity of Box1 become greater than those of Box2. Furthermore, the upper ocean (about 300 m in depth) contains 68.6% of the eddies (instantaneous eddy). Only 16.6% of the eddies extend to 1 000 m. In addition, about 87% of the eddies are bowl-shaped eddies in the two regions. Only about 3% are cone-shaped eddies. With increasing depth of the eddies, the proportion of bowl-shaped eddies gradually decreases. Conversely, the cone- and lens-shaped eddies are equal in number at 700–1 000 m, accounting for about 30% each. Studying the 3D characteristics of eddies in two different regions of the northwestern Pacific Ocean is an important stepping stone for discussing the different eddy generation mechanisms.     

Quasi-decadal modulations of North Pacific Intermediate Water area in the cross section along the 137°E meridian: impact of the Aleutian Low activity

Temporal variations of area of the North Pacific Intermediate Water (NPIW), in the repeat hydrographic section along 137°E meridian conducted by the Japan Meteorological Agency, are investigated using the de-trended variables from 1972 to 2008. Variations of NPIW area show a clear quasi-decadal (about 10 years) modulation and it is caused by the vertical displacement of isopycnal surfaces in the lower portion of NPIW around the northern boundary of its distribution (30–32°N): The downward (upward) movement of isopycnal surfaces in the lower portion of NPIW as a result of the first-mode baroclinic ocean response stretches (shrinks) the density layer equivalent of NPIW and causes strengthening (weakening) of westward flows associated with the Kuroshio Counter Current, and then it can induce an increase (decrease) of volume transport of NPIW from the east. Consequently, the NPIW northern boundary shifts northward (southward) and an increase (decrease) of the NPIW area is induced. Large-scale atmospheric forcing controlling the vertical displacements of isopycnal surfaces is explored using a wind-driven hindcast ocean model. The vertical displacements stem from the first-mode baroclinic ocean response to the two types of Aleutian Low (AL) activities: in particular, the meridional movement of the AL imparts more potential influence on them than the AL intensity variation does.     

Deep scattering layers resolved by narrow‐beam echo sounder along 35°s in the south pacific

The narrow‐beam echo sounder (3.5° half angle, 20 kHz) is capable of resolving the configuration of deep scattering layers to 750 m and was used to study the ecology and population density of organisms that make up the layers. Several layers to 1,000 m depth were identified in the South Pacific Ocean, and the population densities calculated. Swimming speeds of possible predators are estimated at 0.2–0.3 m.sec‐¹. Among organisms calculated to range in length from 0.01 m to 0.07 m, the larger were interpreted as grazing on smaller organisms, after comparing results from two echo sounders of 12 kHz and 20 kHz frequencies respectively.