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Byline: Toshio Suga (1), Kazunori Motoki (1), Kimio Hanawa (1) Keywords: Subsurface temperature minimum; North Pacific Central Mode Water; dichothermal water; western subarctic gyre; climate regime shift Abstract: A repeat hydrographic section has been maintained over two decades along the 180deg meridian across the subarctic-subtropical transition region. The section is naturally divided into at least three distinct zones. In the Subarctic Zone north of 46degN, the permanent halocline dominates the density stratification, supporting a subsurface temperature minimum (STM). The Subarctic Frontal Zone (SFZ) between 42deg--46degN is the region where the subarctic halocline outcrops. To the south is the Subtropical Zone, where the permanent thermocline dominates the density stratification, containing a pycnostad of North Pacific Central Mode Water (CMW). The STM water colder than 4degC in the Subarctic Zone is originated in the winter mixed layer of the Bering Sea. The temporal variation of its core temperature lags 12--16 months behind the variations of both the winter sea surface temperature (SST) and the summer STM temperature in the Bering Sea, suggesting that the thermal anomalies imposed on the STM water by wintertime air-sea interaction in the Bering Sea spread over the western subarctic gyre, reaching the 180deg meridian within a year or so. The CMW in this section originates in the winter mixed layer near the northern edge of the Subtropical Zone between 160degE and 180deg. The CMW properties changed abruptly from 1988 to 1989 its temperature and salinity increased and its potential density decreased. It is argued that these changes were caused by the climate regime shift in 1988/1989 characterized by weakening of the Aleutian Low and the westerlies and increase in the SST in the subarctic-subtropical transition region. Author Affiliation: (1) Department of Geophysics, Graduate School of Science, Tohoku University, Aoba-ku, Sendai, 980-8578, Japan Article History: Registration Date: 06/10/2004

Byline: Takahiro Miura (1), Toshio Suga (1), Kimio Hanawa (1) Keywords: Bering Sea; dichothermal layer; mixed layer; numerical model; heat flux Abstract: A one-dimensional numerical model with a level-2.5 turbulent closure scheme to provide vertical mixing coefficients has been used to investigate the process by which the dichothermal water is formed in the Bering Sea, the density of which is about 26.6 sigma-theta. The water column to be simulated is assumed to move along a predetermined path. That is, the present model is of the Lagrangian-type. Surface boundary conditions are given using the climatologies of heat, freshwater and momentum fluxes. In order to obtain a plausible moving speed of the water column along the path, pre-liminary experiments were done using the surface fluxes in the central part of the Bering Sea for the initial temperature and salinity profiles at the entrance of the Sea. As a result, it was found that the temperature minimum layer, i.e., the dichothermal water with temperature similar to the climatology at the exit of the Bering Sea, was formed after about two years of integration. Based on the result, the movement speed of the water column along the path was set as 4.5 cm/s in the standard run. It was found that this model could plausibly reproduce the subsurface temperature minimum layer. That is, the dichothermal water was formed in the winter mixed layer process in the Bering Sea. The existence of the subsurface halocline (pycnocline) prohibited the deeper penetration of the winter mixed layer, and therefore water with a temperature colder than that under the mixed layer was formed in the mixed layer due to wintertime surface cooling. In the warming season this water remains as the subsurface temperature minimum layer between the upper seasonal thermocline and the lower halocline. Author Affiliation: (1) Department of Geophysics, Graduate School of Science, Tohoku University, Aoba-ku, Sendai, 980-8578, Japan Article History: Registration Date: 06/10/2004

Byline: Naoto Iwasaka (1,2), Toshio Suga (1), Kensuke Takeuchi (1), Keisuke Mizuno (3), Yasushi Takatsuki (3), Kentaro Ando (2), Taiyo Kobayashi (1), Eitarou Oka (1), Yasuko Ichikawa (1), Motoki Miyazaki (1), Hiroshi Matsuura (1), Kenji Izawa (3), Chan-Su Yang (1), Nobuyuki Shikama (1), Momoko Aoshima (2) Keywords: ARGO project; profiling float; salinity sensor Abstract: We deployed two profiling floats in the region south of the Kuroshio Extension in March 2000. Temperature and salinity profiles from a depth of 1500 x 10.sup.4 Pa to the surface are reported every two and four weeks, respectively. The floats performed very well for first four months after deployment. Later they failed in surfacing for a few months when the sea surface temperature in the region was high. The salinity sensors seemed to suffer from some damage during their failure-in-surfacing period. Despite this trouble, the results clearly demonstrate that the profiling float is a very useful and cost-effective tool for physical oceanographic observation in the open sea. Author Affiliation: (1) Frontier Observational Research System for Global Change, Natsushima, Yokosuka, 237-0061, Japan (2) Tokyo University of Mercantile Marine, 2-1-6 Etchujima, Koto-ku, Tokyo, 135-8533, Japan (3) Japan Marine Science and Technology Center, Natsushima, Yokosuka, 237-0061, Japan Article History: Registration Date: 06/10/2004

Byline: Takahiro Miura (1), Toshio Suga (1), Kimio Hanawa (1) Keywords: Bering Sea; dichothermal layer; mixed layer; subarctic gyre; halocline Abstract: The temperature minimum layer, called 'dichothermal water', is a characteristic feature of the North Pacific subarctic gyre. In particular, dichothermal water having a density of approximately 26.6 sigma-theta ([sigma].sub.I,), which corresponds to the densest water outcropping in winter in the North Pacific, is seen in the Bering Sea. In order to clarify the water properties, and the area in which and the process by which the dichothermal water is formed, a new seasonal mean gridded climatological dataset with a fine resolution for the Bering Sea and adjacent seas has been prepared using historically accumulated hydrographic data. Although the waters of the Alaskan Stream have temperature minimum layers, their temperature inversions are very weak in climatologies and the core densities of the temperature minimum layers are much lighter than 26.6[sigma].sub.I,. On the other hand, in the Bering Sea one can see the robust structure of temperature minimum layers, the core density of the dichothermal water being around 26.6[sigma].sub.I,. In addition, it has been found that the properties of the dichothermal water observed in the warming season are almost the same as those in the winter mixed layer. That is, the dichothermal waters are formed in the winter mixed layer in the Bering Sea. Since these waters are found in the Kamchatka Strait, i.e., the main exit of the Bering Sea waters, it can be supposed that the dichothermal waters are exported from the Bering Sea to the Pacific Ocean by the Kamchatka Current. Author Affiliation: (1) Department of Geophysics, Graduate School of Science, Tohoku University, Aoba-ku, Sendai, 980-8578, Japan Article History: Registration Date: 12/10/2004

It is necessary for air–sea interaction and climate research to evaluate heat and salinity transport between the mixed layer (ML) and the permanent pycnocline and its contribution to the ML properties. The authors have calculated the obduction and subduction rates with the Eulerian definition using a 1°-grid Argo dataset and incorporated the obduction rate into the ML budget analysis in order to assess the impact of the entrained permanent pycnocline water on the ML temperature and salinity. They have successfully separated the effect of obduction on the ML from the entrainment of the seasonal pycnocline, and specified the regions where the obduction impact is noticeable. The ML heating rate due to obduction was smaller than ± 0.1 K/yr on average over most of the domain, but it was larger than − 0.5 K/yr in places around the Circumpolar Current, in the northeastern tropical Pacific, the southwestern tropical Indian Ocean, and the northeastern North Atlantic. The salinizing rate due to obduction exceeded ± 0.02 psu/yr in these areas, and in the subarctic North Pacific and the eastern South Pacific. The waters entrained from the permanent pycnocline warm and salinize the ML in the high latitudes due to the dichothermal structure. The method applied in this study also enabled us to assess the residence time and route of obducted and subducted water parcels below the ML. Furthermore, the authors examined the changes of obduction and subduction related with the marine heatwave that occurred in the northeastern North Pacific around 2014. [ABSTRACT FROM AUTHOR]

Inter-annual variability in the formation and subduction of the lighter variety of the Central Mode Water (L-CMW) in the North Pacific is investigated using Argo profile data from 2003 to 2013. The deep mixed layer (ML) in winter (February-April) with the L-CMW property (potential temperature of 10-16 °C and salinity of 34.2-34.6) shows a zonal shift inter-annually. In winters of 2003-2005 and 2010, deep MLs are more frequently generated in the eastern part of the formation region (30°-40°N, 180°-160°W), which is mainly caused by large oceanic heat loss associated with strengthening of the westerlies. Relatively large parts of the L-CMW that formed in this eastern region survive to subduct into the subsurface layer and, in the following summer, spread widely over the North Pacific subtropical gyre. On the other hand, during the winters of 2006-2009 and 2011-2013, deep MLs were more frequently formed in the western part of the formation region (36°-40°N, 140°-155°E). These were induced by weakening of surface ocean stratification attributable to anti-cyclonic meso-scale eddies pinched off northward from the Kuroshio Extension. In contrast to the L-CMW formed in the eastern part of the formation region, the L-CMW formed in the western part is hardly detected in the subsurface layer during the following summer. [ABSTRACT FROM AUTHOR]

For the purpose of estimating nitrate concentration using the dissolved oxygen concentration and seawater temperature recorded in an instrument such as a profiling float with an oxygen sensor, we examined the relationship between oxygen and nitrate concentrations in the Kuroshio Extension, Oyashio, and mixed water regions (34-43ºN, 140-150ºE) using vertical profile data, which includes temperature, salinity, oxygen, and nitrate, from the World Ocean Database 2005. Examining the NO/O ratio along isotherms, we found a clear linear regression between the oxygen and nitrate concentrations in each potential temperature range, irrespective of water mass and seasonal variation. The slopes represented by the NO/O ratio in each temperature range varied from −0.07 to −0.12. The benefit of analyzing data on constant potential temperature surfaces arises because of the strong dependence of the oxygen saturation of surface waters on temperature. This sets the end point of subducted waters and helps distinguish waters that may have similar preformed nitrate values. Our results suggest that subsequent reminearlization in the subducted water retained the Redfield ratio between N and O. The estimate of nitrate concentration, by using the oxygen-nitrate relationship discovered in the present study, showed good accuracy with in general a mean error of less than 2.0 μmol/kg. The present method has been demonstrated to be a useful tool for estimating the nitrate concentration from temperature and oxygen. [ABSTRACT FROM AUTHOR]

A shipboard high-resolution hydrographic survey in the subtropical region of the western North Pacific conducted from October to November 2008 detected part of a cyclonic eddy around 30°N, 145°E. This eddy had propagated westward in the region south of the Kuroshio extension for at least 6 months as a wavelike disturbance. Within this eddy, isopycnals shallowed between a depth of 600 m and just below the surface mixed layer. In addition, maximal dissolved oxygen concentrations were observed in the subsurface layer between depths of 50 and 100 m. Nitrate was depleted within this subsurface maximal oxygen layer. These results suggest that nutrients in the deeper layers were supplied into the euphotic layer as a result of the uplift of isopycnals in the eddy, fueling the photosynthesis of phytoplankton in the subsurface and emitting an excess of oxygen due to new production. Compared with the outside of the eddy, the enhancement of oxygen and the decrease of nitrate in the center of the eddy were estimated to be 2.7 mol O m and 0.22 mol N m, respectively. The primary productivity calculated using the eddy transition speed of 5.1 km day was 548 mg C m day at the center of the eddy. The enhanced primary productivity due to the passage of the eddy is likely to have an important role in the ecosystem and on material cycling in the subtropical region. [ABSTRACT FROM AUTHOR]

This study describes the three-dimensional distributions of the Turner angle (Tu) and the potential vorticity (PV) of the main pycnocline water in the subtropical North Pacific (10-50°N, 120°E-120°W) using a large in situ CTD data set taken by the Argo profiling floats during June to October of 2001-2009 to clarify the detailed distribution of the central water and the mode waters as well as the relationship between these water masses. The ventilated part of the main pycnocline water ( σ < 26.7 kg m) in the subtropical gyre generally displays a sharp peak in Tu value of 59° in the histogram. The Tu histograms for 10° × 10° geographical boxes mostly show that the mode for the Tu value is 59° too, but they also show some regional differences, suggesting some types of relations with the North Pacific mode waters. To further investigate this relationship, the appearance probability density function of the central water (defined as the main pycnocline water with Tu = 56°-63°) and those of the mode waters with PVs lower than the critical value on each isopycnal surface were analyzed. The distribution area of the central mode water (CMW) corresponds so well with that of the central water that a direct contribution of the CMW to the formation and maintenance of the central water is suggested. On the other hand, the distribution areas of subtropical mode water (STMW), Eastern STMW, and transition region mode water do not correspond to that of the central water. Nevertheless, indirect contributions of these mode waters to the formation and maintenance of the central water through salt finger type convection or diapycnal mixing are suggested. [ABSTRACT FROM AUTHOR]