Variability of Upper Ocean Heat Balance in the Shikoku Basin during the Ocean Mixed Layer Experiment (OMLET)

The heat balance of the surface layer in the vicinity of the former Ocean Weather Station “Tango” (OWS-T; 29°N, 135°E), where a large amount of heat is transported by the Kuroshio and transferred to the atmosphere, was studied by during Ocean Mixed Layer Experiment (OMLET) as an oceanographic component of the Japanese World Climate Research Program (1987–1991). Temperature and velocity in the upper ocean measured using a surface moored buoy system deployed by the Ocean Research Institute, the University of Tokyo, in total 668 days of four time series namely the periods of April 1988–November 1988 (OMELET-88), August 1989–February 1990 (OMLET-89), April 1990–September 1990 (OMLET-901) and September 1990–January 1991 (OMLET-902). We have analyzed the moored buoy data of the upper 100 m for the latter three time series (OMLET-89, -901 and -902) and here we discuss the heat balance of the upper 100 m, in combination with surface heat flux and oceanographic data provided by the Japan Meteorological Agency. A large fluctuation of oceanic heat convergence/divergence of 200–300 W/m² in amplitude with predominant period of 20–30 days occurred in the first half of OMLET-89 period, which was just the early stage in the formation process of a large meander path of the Kuroshio. A large amount of heat convergence of 71 and 79 W/m² on average was detected in observation period of OMLET-89 and -901, respectively. During OMLET-902, relatively small heat convergence of 13 W/m² was obtained. It is suggested that these variations of oceanic heat convergence in this region were closely related to the fluctuation of the Kuroshio axis to the south of Japan. This revised version was published online in July 2006 with corrections to the Cover Date.     

Possible source of the antarctic bottom water in the Prydz Bay Region

It has been inferred that the Prydz Bay region is one of the source regions of Antarctic Bottom Water (AABW) based on rather indirect evidence. In order to examine this inference, we investigate the hydrographic condition of the bay based mainly on XCTD data obtained during the Japanese Whale Research Program in the Antarctic (JARPA). The JARPA hydrographic data reveal Circumpolar Deep Water (CDW), which is a salty, warm water mass approaching the shelf break, and capture Modified CDW (MCDW) intruding into the shelf water. AABW production requires mixing of CDW and cold shelf water saltier than 34.6 psu, which is a saltier type of Low Salinity Shelf Water (LSSW). Saltier LSSW is observed near the bottom over the shelf, being mixed with MCDW. We further identify saltier LSSW near the shelf break. This saltier LSSW appears close enough to unmodified CDW to be mixed with it over the continental slope, indicating a possible source of AABW in Prydz Bay.     

Hydrographic Features off the Southeast Coast of Kyushu during the Kuroshio Small Meanders: A Case Study for Small Meanders that Occurred in 1994 and 1995 Spring

Three Kuroshio small meanders off the southeast coast of Kyushu that occurred during 1994 to 1995 were investigated by using satellite-derived sea surface temperature (SST) and sea surface height (SSH) maps, World Ocean Circulation Experiment (WOCE) Hydrographic Program (WHP) repeat section and Japan Meteorological Agency (JMA) hydrographic observations. Based on the satellite data, we observed that the three small meanders are formed by different processes: the triggering and growth of these meanders are caused by a cyclonic eddy propagating from the Kuroshio recirculation region or Kuroshio front meanders traveling from the East China Sea. Investigation of the two small meanders in 1994 and 1995 spring that are captured by the WHP observation showed quite consistent hydrographic features. On the nearshore side of the meandering Kuroshio, a countercurrent appears, associated with vertically uniform upward lifts of the isopycnals from sea surface to bottom at the boundary between the countercurrent and the Kuroshio. In the countercurrent region, the waters in the density ranges of the North Pacific subtropical mode water (NPSTMW) and the North Pacific Intermediate Water (NPIW) are more saline and less saline than typical waters that the Kuroshio carries in a non-small meander state, respectively. There are indications that high-salinity NPSTMW and low-salinity NPIW distributed off the Kuroshio was supplied to the countercurrent region. In the meandering Kuroshio flow, while there is no notable change in properties around the NPSTMW density range, salinity of the NPIW is significantly higher than that carried by the Kuroshio in a non-small meander state, but not higher than that in the Kuroshio at the Tokara Strait, which suggests that saline NPIW from the Tokara Strait, less mixed with low-salinity NPIW off the Kuroshio, may be carried by the meandering Kuroshio. This revised version was published online in July 2006 with corrections to the Cover Date.     

Zonally asymmetric response of the Japan Sea to synoptic pressure forcing

We investigated the sea level response of the Japan Sea to changes in atmospheric pressure using barotropic shallow water models driven by idealized synoptic pressure forcing. The regional response lags behind the synoptic pressure forcing because the adjustment is slowly established by water exchange through narrow, shallow straits. The sea level response of the realistic Japan Sea to the idealized forcing varies with geographical location and shows zonally asymmetric variations in amplitude and phase. The simulated response is in good agreement with the observed response of sea level recorded at Japanese coastal tide gauges. The results of a simple one-dimensional model indicate that the zonally asymmetric pattern, with an eastward-propagating pressure system, is essentially caused by bottom friction in shallow straits. This asymmetry arises if the typical wavelength of the synoptic pressure system is slightly larger than the spatial scale of the Japan Sea.     

Paleomagnetic studies combined with fission-track datings on the western arc of Sulawesi, east Indonesia

Paleomagnetic studies in conjunction with fission-track dating on the western arc of Sulawesi yield important evidence bearing on the tectonic history of the area. During the Paleogene to Early Miocene time interval the paleomagnetic pole for southwestern Sulawesi was situated at 36.5°E 44.8°N. This pole position is significantly different from that in the time interval Middle Miocene to Recent, which is consistent with the north pole of the axial geocentric dipole. This fact suggests that subsequent to the Paleogene to Early Miocene period, possibly 19–13 m.y. B.P., a major tectonic event occurred which caused about 40 degrees of anticlockwise rotation of the area. It is suggested by the present work that the postulated collision followed by welding of eastern Sulawesi with western Sulawesi during the Pliocene (Katili, 1978) may be the tectonic event mentioned above. In addition, our data does not support the hypothesis that western Sulawesi has been derived from the dispersal of Gondwanaland.     

Further evidence of normal mode Rossby waves

Further observational evidence of normal mode Rossby waves with higher meridional mode numbers is presented with the aid of global data from the troposphere to the stratosphere over the period November 1979 through April 1986.It is shown, without using ana priori assumption of meridional structure, that the third antisymmetric modes of zonal wavenumbers 1 and 2,i.e., (1,4) and (2,4) modes, substantially exist in the real atmosphere. These modes are, however, easily influenced by the nonuniform background field even in the equinoctial season; amplitude submaxima near the equator are apt to be dubious in the upper stratosphere so that the prototype meridional structure becomes obscure. The period of the (1,4) mode often falls into that of the (1,3) mode, about 16 days. Hence, these two modes cannot be classified simply by their periods, but the separation is made by their meridional structure.An appearance calendar of various modes is also presented for the analysis period. It is found that each mode appears irregularly throughout the year and that the year-to-year variation is fairly large.     

New detection method for XBT depth error and relationship between the depth error and coefficients in the depth-time equation

New CTD-XBT (T-7 probe) comparison data are analyzed, which provide additional evidence of XBT depth error and support previous results (Hanawa and Yoritaka, 1987; Hanawa and Yoshikawa, 1991). The depth difference between the corrected and uncorrected data is about 26 m at 750 m. In the present study, new data processing procedures by which the depth errors are automatically detected, are developed and adopted. In the new method, first, temperature gradients (TG) of XBT and CTD profiles are calculated. Then, 20 m segment of the XBT-TG profile which should fit to the CTD-TG profile of 20 m segment to be referred to is searched in the XBT-TG profile. Actually, this is achieved by shifting the XBT-TG profile of 20 m segment so as to minimize the area surrounded by both TG profiles. The shifted depth of XBT-TG profile for CTD-TG profile can be regarded as the XBT depth error. This processing is repeated at intervals of 5 m from 10 m to 790 m of CTD-TG profile. The relationship between the scatter of the quadratic depth-time equation coefficients and the depth error is also discussed. It is shown that when the two coefficients have a certain relationship, the depth differences between the plural depth-time equations are small, even if the two coefficients of those equations have apparently very different values.This paper was presented and discussed in the Ad Hoc Meeting of the IGOSS Task Team on Quality Control for Automated System, held in Marion, Massachusetts,U.S.A. in June 3–6, 1991.     

Subtropical Mode Water south of Honshu,Japan in the spring of 1988 and 1989

The distribution and characteristics of Subtropical Mode Water (STMW) south of Honshu, the main island of Japan, were investigated using CTD, XBT, and dissolved oxygen data taken by the research vessels in the spring of 1988 and 1989. A comparatively low inventory of STMW was shown in spring 1988 during the large-meander period of the Kuroshio south of Honshu, while in spring 1989 during the non-large-meander period, the observation showed a considerable inventory of STMW which had outcropped east of 140°E in the preceding winter. These observations, together with published temperature maps, surface current charts, time series of vertical temperature profiles along 140°E, and wintertime Monsoon Index consistently support the climatology of the STMW circulation recently presented by the authors. That is, the change of the Kuroshio Countercurrent associated with the large meander of the Kuroshio most likely cuts off the westward/southwestward advection of STMW from its formation area east of 140°E.     

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.     

Low-frequency variations of the Eastern Subtropical Front in the North Pacific in an eddy-resolving ocean general circulation model: roles of central mode water in the formation and maintenance

Temporal variations (1960–2005) of the Eastern Subtropical Front (ESTF) in the North Pacific are investigated using historical-run output data of the eddy-resolving Meteorological Research Institute Community Ocean Model, forced by atmospheric reanalysis dataset. Simulated ESTF is distributed in a region of zonal band of 24°N–30°N east of the International Date Line, and is located at the southern boundary of the central mode water (CMW) north of the front. The ESTF intensity clearly shows an interdecadal variation with a timescale of about 20?years. This variation is associated with that in the potential vorticity of CMW, which originates in the CMW formation region farther north about 3?years earlier due to changes in the surface wind forcing.