Toshio Suga

The North Pacific Climatology of Winter Mixed Layer and Mode Waters

Abstract A climatology of the winter mixed layer in the North Pacific Ocean was constructed using hydrographic data from historical archives and recent observational programs, including the World Ocean Circulation Experiment. The main aim was to provide better knowledge about source areas of upper water masses. The authors have endeavored to preserve water properties near the frontal regions by keeping the smoothing scale as small as possible. The resulting climatology shows considerable differences in the mixed layer depth and its water properties from those derived from the World Ocean Atlas (WOA). Maps of the potential vorticity field of the North Pacific pycnocline are presented using the isopycnally averaged climatology, HydroBase. Three distinct lateral minima of potential vorticity are identified as Subtropical Mode Water (STMW), Central Mode Water (CMW), and Eastern Subtropical Mode Water (ESTMW), in the western, central, and eastern parts of the subtropical gyre, respectively. The HydroBase isopy...

Thermostad Distribution in the North Pacific Subtropical Gyre: The Central Mode Water and the Subtropical Mode Water

Abstract The upper thermal structure of the midlatitude North Pacific is studied with the use of all the bathythermograph data compiled in the global ocean temperature and salinity profiles released by the National Oceanographic Data Center. Climatological temperature data are prepared for each 2.5° × 5° (latitude × longitude) rectangle. The upper layer of the subtropical gyre is characterized by two types of thermostads or mode waters: one in the western basin known as North Pacific subtropical mode water (NPSTMW) and the other in the central basin to be named North Pacific central mode water (NPCMW). The NPCMW thermostad lies centered around 160°W between the Kuroshio Extension and the Kuroshio bifurcation front. Its local core temperature ranges from 10° to 13°C with a somewhat zonally elongated pattern, in contrast to the more uniform core temperature at 16°–17°C of the NPSTMW thermostad lying centered at 150°–160°E south of the Kuroshio Extension. The climatological map of the wintertime mixed layer ...

Subtropical Mode Water in the 137°E Section

Abstract The subtropics mode water (STMW) in the North Pacific Ocean has been investigated based on the data of long-term observations along the 137°E meridian, which have been performed by the Japan Meteorological Agency since 1967 for winter and since 1972 for summer. STMW cores were identified as vertical potential vorticity minima, and examined by the use of apparent oxygen utilization as an indicator of the age of STMW. The main results can be summarized as follows. 1) A major post of the STMW appearing in the summer (or the winter) sections is the water formed in the immediately previous winter, its age being half a year (or one year). 2) Within half a year after its formation the STMW can be advected to the 137°E section only as far south as about 26°N and as far as about 23°N within one year. 3) Typical potential temperature in summer was higher than in winter, with salinity higher and potential density lower. 4) Less STMW was observed during the period of the typical large meander of the Kuroshio...

The Subtropical Mode Water Circulation in the North Pacific

Abstract Historical hydrographic data are analyzed to investigate the seasonal variation of the subsurace pycnostad in the northwestern part of the North Pacific and to relate it to the circulation of subtropical mode water (STMW). A gyre-scale scheme of STMW circulation is deduced based on its observed climatological formation area and climatological geostrophic flow fields. The deduced circulation is supported by climatological features of the subsurface hydrography presented separately for the non-large-meander period and the large-meander period of the Kuroshio: zonal and meridional sections and isopycnal maps of potential vorticity and apparent oxygen utilization. The Kuroshio Countercurrent composing the Kuroshio recirculation system advects STMW formed in the wintertime thick mixed layer immediately off the Kuroshio and the Kuroshio Extension. During the non-large-meander period, the recirculation system has a single anticyclonic gyre centered near 30°N, 137°E and advects STMW formed off the Kurosh...

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

Abstract 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 24.0 σ 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.

An Isopycnally Averaged North Pacific Climatology

Abstract Approximately a quarter of a million hydrographic stations extracted from the North Pacific World Ocean Atlas 1994 have been subjected to a statistical quality control to produce a Pacific climatology in the spirit of the Atlantic hydrobase. The CTD casts from the publicly available World Ocean Circulation Experiment (WOCE) and pre-WOCE cruises have also been included, and where available, nutrient data have been retained. Particular attention has been paid to the quality control of the 200 000 stations that lie within the region surrounding Japan as it was determined that much of these data contained suspicious salinity values. A comparison with the gridded World Ocean Atlas 1994 confirms expectations that within deep waters over much of the North Pacific the relatively flat isopycnal surfaces produce only small differences between the two climatologies. Closer to surface, however, the differences between the datasets are far more apparent. The major differences (as large as 1.3°C and 0.2 psu at...

Antarctic Intermediate Water circulation in the tropical and subtropical South Atlantic

Recent hydrographic data from the South Atlantic Ventilation Experiment cruises and others are combined with historical data and used to map the isopycnal properties corresponding to the Antarctic Intermediate Water (AAIW) in the Atlantic Ocean. The low salinity of the AAIW extends eastward across the South Atlantic just south of the equator (3–4°S). Evidence of a weak eastward flow just north of the equator (1–2°N) is also shown. Lateral and vertical homogenization of properties in the AAIW is found at the equator between 2°S and 2°N; there is no clear zonal gradient in salinity just along the equator. These observations suggest enhanced mixing within the equatorial baroclinic deformation radius. The South Atlantic tropical gyre is shown to consist of the following three cells: one cyclonic cell centered at about 7°S, another centered at about 19°S in the west and 23°S in the east, and one anticyclonic cell centered at about 13°S. These cells are associated with a westward extension at 10°S of high salinity and low oxygen which originates in the eastern tropical South Atlantic and a front in these properties at about 15°S in the west and about 20°S in the east.

Interannual Variations of North Pacific Subtropical Mode Water in the 137°E Section

Abstract Examination of interannual variations of North Pacific subtropical mode water (STMW) in the 137°E repeat hydrographic section by Suga et al. is extended to seek a relation between changes in STMW renewal and the wintertime cooling. Time series of isopycnal potential vorticity (PV) and apparent oxygen utilization (AOU) demonstrate substantial interannual variations of STMW. It is suggested that the interannual variations are closely related to the changes both in the Kuroshio recirculation system and in the formation rate. For the non-large-meander period of the Kuroshio, during which renewed STMW is advected vigorously from east of the section, the monsoon index as an indicator of cooling in a given winter, and PV and AOU at an STMW isopycnal surface in the following summer show significant correlation, indicating a strong relation between the wintertime cooling and the STMW formation.

Subsurface Subtropical Fronts of the North Pacific as Inherent Boundaries in the Ventilated Thermocline

The two subsurface temperature‐density fronts and the accompanying eastward currents in the central and southern part of the North Pacific subtropical gyre are identified using the repeat hydrographic sections in the western part of the gyre and the sections over the whole gyre from the World Ocean Circulation Experiment (WOCE) Hydrographic Program (WHP). The northern eastward current corresponds to what has been known as the subtropical countercurrent (STCC). The countercurrent/front is located typically near 24 8N and extends from the western boundary approximately to the international date line. The previous suggestion that the STCC is located at the southern edge of North Pacific Subtropical Mode Water (STMW) is confirmed; the front appears as the southern boundary of the lower potential vorticity (PV)/apparent oxygen utilization (AOU) waters on the isopycnals within the STMW layer. The southern eastward current corresponds to what was recognized earlier but has not been documented in detail. The southern countercurrent/front is located near 188N west of the date line, but shifts to the north east of the date line. Its eastern limit is around 165 8 Wa t 258N. The southern front corresponds to the southern boundary of the lower PV/AOU waters on the isopycnal surfaces of the wide su range including those of North Pacific Central Mode Water (CMW). While CMW seems to contribute largely to this low PV/AOU layer, the contribution from other waters is still significant. Judging from the associated PV/AOU features in the thermocline, both fronts can be regarded as inherent boundaries in the ventilated thermocline, dividing the regions ventilated to distinctive degrees. Correspondence between the observational features of the countercurrents/fronts and some of the previously proposed theories concerning the STCC are discussed.

Differential formation and circulation of North Pacific central mode water

Abstract A repeat hydrographic section along 165°E was analyzed to verify a westward extension of the formation region of the North Pacific Ocean Central Mode Water (CMW) suggested by previous synoptic observations, and to investigate the relation between the formation region and thermohaline fronts. The CMW formation region extends at least as far west as 155°E, much farther than recognized in a previous study based on climatology. It is located in two interfrontal regions between the Kuroshio Extension front and the Kuroshio Bifurcation front (KBF), and between the KBF and the subarctic front, where two types of CMW—namely, the lighter variety with potential density of 25.8–26.2 kg m−3 and the denser one of 26.3–26.4 kg m−3—are formed. How this differential formation of CMW is reflected in its gyrewide distribution was examined using one-time sections of the World Ocean Circulation Experiment (WOCE) Hydrographic Program in the North Pacific. The main circulation paths of the two types of CMW diverge eas...