Low-frequency variations of the characteristics of pycnocline in the North Atlantic and their correlation with the North-Atlantic oscillation

On the basis of processing of the oceanographic data accumulated for the water area of the North Atlantic in 1950–1999 (∼500,000 stations), we study seasonal and interannual variations of the principal characteristics of pycnocline within the range of σ_t = 25.5–27.5 conventional density units. It is shown that the interannual oscillations of these characteristics in the entire analyzed layer can be regarded as a superposition of fluctuations with periods from 2–3 to 10–12 yr. The typical ranges of these fluctuations for the depths of occurrence of isopycnic surfaces and the corresponding temperature and salinity are equal to 20–25 m, 1–1.5°C, and 0.25‰, respectively. The intensification of atmospheric circulation at middle latitudes is accompanied by the simultaneous deepening of the pycnocline and its heating in the central part of the North Subtropical Anticyclonic Gyre. At the same time, the process of weakening of the atmospheric circulation leads to the rise of the pycnocline and its cooling. The complete cycle of interaction of the North-Atlantic Oscillation with the anomalies of isopycnic characteristics (with regard for the period of their advection) is equal to ∼6–8 yr. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 2, pp. 29–48, March–April, 2007.     

Regional structure of surface-air temperature fluctuations in northern Eurasia in the latter half of the 20th and early 21st centuries

The first empirical orthogonal functions (EOF1s) of surface-air temperature fluctuations for Russia and its neighboring states within the period 1950–2005 are analyzed. The spatial distribution of the EOF1, the first principal components (PC1s) of the observed air temperature (averaged over the summer, July, December–March, and individual winter months), and their time variations (including trend parameters, some spectral characteristics, and the quantitative indices of relation to circulation indices (on the basis of multiple step-by-step regression)) are considered. Significant seasonal differences have been revealed: the winter air-temperature fluctuations are characterized by a higher (when compared to summer) spatial coherence, especially in the latitudinal direction. The EOF1 of the winter air temperature (averaged over December–March) describes its fluctuations for almost all of Russia; in this case, no less than 70% of the PC1 variability is due to variations in several circulation indices; the main contribution (60%) is made by both the North Atlantic Oscillation (NAO) and Scandinavian (SCAND) indices. On the whole, over the periods 1951–2005 and 1971–2005, the NAO contribution exceeds the SCAND contribution to the winter temperature variability; the NAO is associated with a more rapid increase in air temperature in 1968–1997 and with the 1971–2005 trend. In 1951–1970 the main contribution to air temperature fluctuations was made by SCAND; the SCAND contribution exceeds the NAO contribution in the periods 1951–2005 and 1971–2005. The 1971–2005 and 1968–1997 temperature trends are completely described by variations in the NAO (70%) and SCAND (30%) indices for January and February.     

Regional features of the seasonal variability of linear trends of the temperature fields in the Atlantic Ocean and their relation to the large-scale circulation of waters

By using the data array of satellite measurements of the sea-surface temperature (SST), it is shown that the distributions of linear trends of the SST for 17 years (1986–2002) are closely correlated with the specific features of large-scale circulation of waters. The extreme values of trends of the SST are observed in the zones of the Gulf Stream, East-Greenland, Labrador, and North-Atlantic Currents, North Equatorial Countercurrent, South Equatorial and Antarctic Circumpolar Currents. The differences between the distributions of trends of the SST for some months of the year are revealed. In the zones of currents transporting warm surface waters from the tropical areas, positive trends of the SST are observed for the most part of the year. On the contrary, negative trends are predominant in the zones of currents transporting cold upwelling waters. Translated from Morskoi Gidrofizicheskii Zhurnal, No. 4, pp. 32–42, July–August, 2008.     

Circulation of Intermediate and Deep Waters in the Philippine Sea

The circulation of intermediate and deep waters in the Philippine Sea west of the Izu-Ogasawara-Mariana-Yap Ridge is estimated with use of an inverse model applied to the World Ocean Circulation Experiment (WOCE) Hydrographic Program data set. Above 1500 m depth, the subtropical gyre is dominant, but the circulation is split in small cells below the thermocline, causing multiple zonal inflows of intermediate waters toward the western boundary. The inflows along 20°N and 26°N carry the North Pacific Intermediate Water (NPIW) of 11 × 10⁹ kg s⁻¹ in total, at the density range of 26.5σ_θ–36.7σ₂ (approximately 500–1500 m depths), 8 × 10⁹ kg s⁻¹ of the NPIW circulate within the subtropical gyre, whereas the rest is conveyed to the tropics and the South China Sea. The inflow south of 15°N carries the Tropical Salinity Minimum water of 35 × 10⁹ kg s⁻¹, nearly half of which return to the east through a narrow undercurrent at 15–17°N, and the rest is transported into the lower part of the North Equatorial Countercurrent. Below 1500 m depth, the deep circulation regime is anti-cyclonic. At the density range of 36.7σ₂, – 45.845σ₄ (approximately 1500–3500 m depths), deep waters of 17 × 10⁹ kg s⁻¹ flow northward, and three quarters of them return to the east at 16–24°N. The remainder flows further north of 24°N, then turns eastward out of the Philippine Sea, together with a small amount of subarctic-origin North Pacific Deep Water (NPDW) which enters the Philippine Sea through the gap between the Izu Ridge and Ogasawara Ridge. The full-depth structure and transportation of the Kuroshio in total and net are also examined. It is suggested that low potential vorticity of the Subtropical Mode Water is useful for distinguishing the net Kuroshio flow from recirculation flows. This revised version was published online in August 2006 with corrections to the Cover Date.     

Nitrogen, Oxygen and Argon Dissolved in the Northern North Pacific in Early Summer

Major gases dissolved in seawater were accurately determined with a shipboard gas chromatographic method. The standard deviations were 0.28, 0.34 and 0.36% for N₂, O₂ and Ar, respectively. The method was applied to water from the northwestern North Pacific Ocean collected in May to June 2000. We got 127 duplicate seawater samples from the surface 200 m layer at 11 stations. The O₂ concentrations obtained by this method agreed with those given by the Winkler method. All the seawater samples from the surface 200 m, especially those from the upper 30 m, were supersaturated with respect to atmospheric N₂ and Ar concentrations. In the topmost 30 m layer, the degrees of supersaturation in the inventory were 2.7–4.3% for N₂ (ΔN₂) and 1.7–2.6% for Ar (ΔAr), and their ratios, ΔN₂/ΔAr, ranged from 1.53 to 1.81. This supersaturation seems to be chiefly due to air bubbles injected into the water and dissolved due to the water pressure, because the N₂/Ar ratio of the air is around 2. The amounts of air bubbles dissolved in the upper 30 m water were relatively large, with mean value of 0.41 ml/kg or 18.4 μmol/kg. The ΔN₂, ΔAr and ΔN₂/ΔAr values were all positively well correlated with the wind velocities averaged for the last 24 hours prior to sampling, allowing the conclusion to be drawn that the weaker the wind velocity, the dissolved gas composition approaches in equilibrium with the air; while the stronger the wind velocity, it approaches in the air composition. This revised version was published online in July 2006 with corrections to the Cover Date.     

Variation of the southward interior flow of the North Pacific subtropical gyre,as revealed by a repeat hydrographic survey

Baroclinic variations of the southward flow in the interior region of the North Pacific subtropical gyre are presented with five hydrographic sections from San Francisco to near Japan during 2004–2006. The volume transport averaged temperature of the interior flow, which varies vigorously by a maximum of 0.8°C, is negatively correlated with the transport in the layer of density 24.5–26.5σ _θ, associated with changes in the vertical current structure. Transport variation in this density layer is thus mainly responsible for the thermal impact of the interior flow on the heat transport of the subtropical gyre.     

Possible Density Change of the Origin of North Pacific Intermediate Water Due to Double Diffusion in the Mixed Water Region

In this study we test Talley's hypothesis that Oyashio winter mixed-layer water (26.5–26.6σ _θ) increases its density to produce the North Pacific Intermediate Water (NPIW) salinity minimum (26.7– 26.8σ_θ) in the Mixed Water Region, assuming a combination of cabbeling and double diffusion. The possible density change of Oyashio winter mixed-layer water is discussed using an instantaneous ratio of the change of temperature and salinity along any particular intrusion (R _l ). We estimate the range of R _l ^DD required to convert Oyashio winter mixed-layer water to the NPIW salinity minimum due to double diffusion, and then assume double-diffusive intrusions as this conversion mechanism. A double-diffusive intrusion model is used to estimate R _l ^DD in a situation where salt fingering dominates vertical mixing, as well as to determine whether Oyashio winter mixed-layer water can become the NPIW salinity minimum. Possible density changes are estimated from the model R _l ^DD by assuming the amount of density change due to cabbeling. From these results, we conclude that Oyashio winter mixed-layer water contributes to a freshening of the lighter layer of the NPIW salinity minimum (around 26.70σ_θ) in the MWR.     

Analysis of the possibilities of aGALS-3 hydrophysical towed complex

We describe the characteristics of a towed complex used for measurements in the upper layer of the ocean under the conditions of periodic deepening and lifting (scanning) of a carrier with sensors connected with the ship by a weight-carrying cable of constant lengt. For a maximum scanning range of 0–200 m and a towing speed of up to 12 knots, the measurements were performed every 1.5–2.0 km. The minimum vertical scale of recorded temperature and conductivity inhomogeneities is 0.05–0.08 m. We present the results of measurements carried out by the towed complex in a section of the frontal zone in the north-east part of the Tropical Atlantic. Translated by Peter V. Malyshev and Dmitry V. Malyshev     

Downflow of the Antarctic shelf water at the shelf and continental slope of the Commonwealth Sea in the summer season and its effect on the bottom water formation in the Southern Ocean

In the summer seasons of 2004–2007, the intensive runoff (cascading) of the Antarctic shelf water (ASW) down the shelf and continental slope was revealed thanks to the recording of numerous thermohaline profiles across the shelf and continental slope of the Commonwealth Sea and Prydz Bay. The quickly executed profiles (4–10 h) with submesoscale resolution (near the shelf’s edge, the scale was even eddy-determinative, i.e., within 1.9–5.6 km), in combination with the fine-structure sounding and fine vertical resolution of the near-bottom boundary layer, provided a qualitatively new level of understanding the natural data. The detailed analysis of the temperature, salinity, and density patterns revealed the regularities and peculiarities of the ASW shelf and slope cascading. The intensive ASW cascading near the shelf break and lower part of the slope can be forced (appearing as discrete frontal meanders) or free (appearing as discrete plumes) and often has a wave-eddy character. The field observational data confirmed the obtained representative estimates of the elements of the ASW slope cascading. The basic area of the ASW formation is near the Amery Shelf Ice, from where the ASW spreads to the northwest, goes around the Fram Bank, and flows down the continental slope. The evaluative contribution of the ASW slope cascading to the ventilation of the deep and slope water of the Southern Ocean (near the shelf break 70 km long where the ASW cascading was observed) is Q _K = 0.04–0.24 Sv, which agrees well with the analogous estimates obtained in other regions of the Antarctic.     

Water characteristics and current structure at 65°E during the southwest monsoon

Hydrographic data collected aboard R. V. Anton Bruun along 65°E between 18°N and 42°S from 17 May to 4 July 1964 are used to investigate water characteristics and current structure in the upper 500 m in the Indian Ocean. The water characteristics indicate the occurrence of three main water masses,viz., warm, saltier, low-oxyty and nutrient-rich Arabian Sea Surface Water, relatively fresh and high-oxyty Equatorial Indian Ocean Water, and more saline, high-oxyty and nutrient-poor Tropical Water of the South Indian Ocean. The recently discovered South Equatorial Countercurrent and Subtropical Countercurrent (renamed Tropical Countercurrent, at the suggestion of Dr. R. B.Montgomery) are observed in the current structure at 13°S and 22°–26°S respectively, and these could also be identified on the vertical sections of temperature, thermosteric anomaly and salinity. Contrary to the existing concept, the North Equatorial Current continues to be present even after the onset of the southwest monsoon. The Equatorial Undercurrent could not be traced in the Indian Ocean during this period.     

Helium and oxygen isotope analyses of hydrothermal fluids from the East Pacific Rise between 17°S and 19°S

 Hydrothermal vent fields south of the Garret Fracture zone were sampled for the isotope composition of helium and oxygen ([¹⁸O]H₂O/[¹⁶OH₂O). The helium isotopes end-member (³He / ⁴He=8.3×R _a and [⁴He]≈1.2–2.4×10^-5 cm³ STP g^-1) is quite similar to other known hydrothermal sites pointing to the homogeneous helium composition of the upper mantle. The δ¹⁸O end-member value (δ¹⁸O≈0.5–0.6‰) confirms previous suggestions from other sites and from isotope modeling, that hydrothermal fluids are slightly enriched in ¹⁸O relative to the ocean as a result of water–rock interactions at high temperature. Received: 11 December 1995/Revision received: 20 December 1996     

The circularly propagating pattern of the low-frequencyfluctuations of monthly MSL in the tropical PacificOcean and its correlation with El Nino

The long-term time series analysis of the SST (sea surface temperature) in the Eastern Equatorial Pacific Ocean and the monthly MSL (mean sea level) in the tropical Pacific Ocean is conducted. Their quasiperiodic and low-frequency oscillation features are revealed. The significant periods of low-frequency fluctuations for monthly MSL in the area of 20°N-20° S are between 43. 5 months and 50. 0 months, approximating closely to 47. 6 months which is the significant period of SST in the Eastern Equatorial Pacific Ocean. From the results of space-spectral analysis, the low-frequency fluctations of monthly MSL in the tropical Pacific Ocean appear to have a anticlockwise circularly-propagating pattern, which is, the Eastern Pacific Ocean (off-shore of Mexico) →the area of NEC (North Equatorial Current) →the Western Equatorial Pacific Ocean→the area of NECC (North Equatorial Counter-Current)→the Eastern Equatorial Pacific Ocean. The phases of the pattern correspond to those of El Nino cycle. On the basis     

CHARACTERISTICS ON INTERANNUAL VARIATIONS OF NORTH PACIFIC SST AND ITS RELATION TO EAST ASIA CLIMATE

The interannual variations of the monthly sea surface temperature (SST) in the North Pacific (including Equatorial East Pacific) during 1951-1980 are analysed by means of EOF method. The findings are:(1) In the cold and warm ocean current areas, such as the North Pacific Current, the California Current and the Equatorial East Pacific areas, the convergence speeds are the fastest, while in the Kuroshio and the western part of the North Equatorial Current areas they are fast only in winter.(2) The physical features of the first 3 eigenvectors are obvious. The first eigenvector shows that the SST values are high in the south and low in the north in the latitudinal distribution of the SST field. The warm current area, i.e. the northwestern part of the North Pacific is positive and the cold current area, i.e. the southeastern part of the North Pacific including the Eastern Equatorial Pacific is negative. The zero line of the 2nd eigenvector field runs from northeast to southwest, in the same direction as the     

Interannual and seasonal variability of hydrometeorological fields in some regions of the Atlantic-European sector as a manifestation of the abnormal behaviour of the North-Atlantic Oscillation

We study the interannual and seasonal variability of hydrometeorological fields in some regions of the Atlantic-European sector. These low-frequency processes are analyzed depending on the index ΔP (an analog of the index of the North-Atlantic Oscillation). It is shown that the average value of the index ΔP over the winter natural synoptic season can be used for the determination of stable locations of the centre of the Azorean maximum in 1971–1980 and 1981–1990, the typical state of cloudiness in the Atlantic-European sector, and the fields of precipitations and atmospheric temperature in some regions of the Crimea. Typical anomalies of the fields of precipitations and atmospheric temperature over the west and south coasts of the Crimea are described. The signs of the anomalies of precipitations in the winter natural synoptic season are in good agreement with the signs of the corresponding anomalies of the field of cloudiness in the Black-Sea region. Translated by Peter V. Malyshev and Dmitry V. Malyshev     

Time-series observation of dissolved inorganic carbon and nutrients in the northwestern North Pacific

Time-series measurements of dissolved inorganic carbon (DIC) and nutrient concentrations were conducted in the northwestern North Pacific from October 2002 to August 2004. Assuming that data obtained in different years represented time-series seasonal data for a single year, vertical distributions of DIC and nutrients showed large seasonal variabilities in the surface layer (∼100 m). Seasonal variabilities in normalized DIC (nDIC) and nitrate concentrations at the sea surface were estimated to be 81–113 μmol kg⁻¹ and 12.7–15.7 μmol kg⁻¹, respectively, in the Western Subarctic Gyre. The variability in nutrients between May and July was generally at least double that in other seasons. In the Western Subarctic Gyre, estimations based on statistical analyses revealed that seasonal new production was 39–61 gC m⁻² and tended to be higher in the southwestern regions or coastal regions. The seasonal new productions in the northwestern North Pacific were two or more times higher than in the North Pacific subtropical gyre and the northeastern North Pacific. It is likely that this difference is due to spatial variations in the concentrations of trace metals and the species of phytoplankton present. In addition, from estimations of surface pCO₂ it was verified that the Western Subarctic Gyre is a source of atmospheric CO₂ between February and May and a sink for CO₂ between July and October.     

New type of pycnostad in the western subtropical-subarctic transition region of the North Pacific: Transition Region Mode Water

A new type of pycnostad has been identified in the western subtropical-subarctic transition region of the North Pacific, based on the intensive hydrographic survey carried out in July, 2002. The potential density, temperature and salinity of the pycnostad were found to be 26.5–26.7 σ _θ , 5°–7°C and 33.5–33.9 psu respectively. The pycnostad is denser, colder and fresher than those of the North Pacific Central Mode Water and different from those of other known mode waters in the North Pacific. The thickness of the pycnostad is comparable to that of other mode waters, spreading over an area of at least 650 × 500 km around 43°N and 160°E in the western transition region. Hence, we refer to the pycnostad as Transition Region Mode Water (TRMW). Oxygen data, geostrophic current speed and climatology of mixed layer depth in the winter suggest that the TRMW is formed regularly in the deep winter mixed layer near the region where it was observed. Analysis of surface heat flux also supports the idea and suggests that there is significant interannual variability in the property of the TRMW. The TRMW is consistently distributed between the Subarctic Boundary and the Subarctic Front. It is also characterized by a wide T-S range with similar density, which is the characteristic of such a transition region between subtropical and subarctic water masses, which forms a density-compensating temperature and salinity front. The frontal nature also tends to cause isopycnal intrusions within the pycnostad of the TRMW.     

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.     

黑潮入侵南海的强弱与太平洋年代际变化及厄尔尼诺-南方涛动现象的关系

在黑潮入侵南海强弱的问题上,到底是太平洋年代际变化(Pacific Decadal Oscillation,PDO)还是厄尔尼诺-南方涛动(El Nio-Southern Oscillation,ENSO)现象在起关键作用,目前还存在着较大争议。本文先以高盐水作为黑潮入侵强弱的示踪物,用120°E断面的高盐水数据和北赤道流分叉点(North Equator Current Bifurcation,NEC-Y)的南北变动进行相关分析,接着,进一步用学者所用的黑潮入侵指数(KI指数,Kuroshio intrusion index和NEC指数,North Equatorial Current index)与北赤道流分叉点南北变动进行相关分析。最后,用EMD(Empirical Mode Decomposition)方法和相关关系分析法分别分析了PDO指数、Nio3.4指数与北赤道流分叉点南北变动的关系并用NECP风场数据探讨其影响机制。结果表明:(1)通过对120°E断面的高盐水的KI指数、NEC指数与NEC-Y的相关分析,表明了北赤道流分叉点的南北变动能够很好地指代黑潮入侵南海的强弱;(2)通过PDO指数和Nio3.4指数与北赤道流分叉点的南北变动的相关性分析,发现PDO指数、Nio3.4指数与北赤道流分叉点的南北变动都具有较好的相关性,都在0.5水平。这些良好的相关性表明了PDO和ENSO对黑潮入侵南海的强弱都具有重要的影响;(3)当处于厄尔尼诺年(拉尼娜)时,赤道太平洋发生西(东)风异常,使得北赤道流分叉点偏北(南),使吕宋岛东侧的黑潮流速减弱(加强),黑潮入侵南海增强(减弱);当PDO处于暖(冷)阶段时,会加强热带太平洋的西(东)风异常,使得黑潮入侵南海增强(减弱)。     

Interannual variations in the components of heat budget in the upper layer of the North Atlantic in different seasons

Seasonal variability of interannual fluctuations of the heat balance components of the upper quasi-homogeneous ocean layer (UQL) in the North Atlantic is analyzed by processing the reanalysis data set for the period of 1959–2011. It is shown that interannual variations in the components of the UQL heat budget are characterized by pronounced regional features in all seasons. In the tropics and subtropics, heat balance is quasistationary and is determined by the nonlocal processes, such as heat advection and horizontal mixing. In the subpolar latitudes, nonstationarity (in the spring) and heat fluxes at the UQL boundary (in the autumn and in the winter) are also important. A major role in the interannual variability of the UQL temperature in the vicinity of jet currents of the Gulf Stream type is played in all seasons by the fluctuations of horizontal heat advection. However, the contribution of interannual fluctuations of the individual components of the heat budget to variability of the UQL temperature varies considerably in different seasons. The interannual fluctuations of the local variation in the UQL temperature are characterized by the largest variability in the spring and the lowest one in the autumn. The greatest contribution of the variations in the horizontal heat advection to the change in the UQL temperature at the interannual scale is recorded in the winter, and the lowest one is in the summer. The contribution of the interannual variations in the heat fluxes at the UQL upper boundary to the variability of the UQL temperature is the highest in the summer and the lowest in the autumn. Fluctuations of the heat fluxes at the UQL lower boundary do not have a significant impact on the interannual variations in the UQL temperature for the whole water area. The exception is small areas in the region of the formation of the North Atlantic deep water in the autumn–winter period and in the vicinity of the Equatorial Counter Current in the spring–summer period.