Kimio Hanawa

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 ...

Decadal Changes in the Mode Waters in the Midlatitude North Pacific

Abstract Temporal changes in the properties of the North Pacific subtropical mode water (NPSTMW) and the North Pacific central mode water (NPCMW), which occurred around the mid-1970s, are investigated using temperature data composited for the two decades bounded by the mid-1970s: 1966–75 and 1976–85. Properties of these mode waters changed greatly after the mid-1970s. The colder NPCMW was formed and widely distributed during 1976–85. In the NPSTMW formation area, warmer water occupied the southwestern part, and colder water occupied the northeastern part during 1976–85. The cause of this change is discussed with regard to the heat flux and wind stress data. The cooling can be explained as a result of changes in surface heat flux and heat divergence in the Ekman layer, that is, a larger amount of heat released from the ocean surface and an increased southward Ekman transport of cold water due to intensification of the westerlies. In particular, the latter plays a dominant role in the observed cooling. On t...

Characteristic Variations of Sea Surface Temperature with Multiple Time Scales in the North Pacific

Abstract Temporal evolution and spectral structure of sea surface temperature (SST) anomalies in the North Pacific over the last 37 years are investigated on the three characteristic time scales: shorter than 24 months (HF), 24–60 months (ES), and longer than 60 months (DC). The leading empirical-orthogonal function (EOF) for the DC time scale is characterized by a zonally elongated monopole centered at around 40°N, 180°. The leading EOF for the HF time scale is somewhat similar to that for the DC time scale, although there are two centers of action with the same polarity at the mid and western Pacific. The leading EOF for the ES time scale, however, exhibits a different pattern whose center of action at the mid Pacific is located farther southeastward. In the time evolution of the SST anomalies associated with the leading EOF of the DC time scale, several anomaly periods can be identified that last five years or longer. The transition from a persistent period to another with the opposite polarity is gene...

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...

Variation of water system distribution in the Sanriku Coastal Area

We analyzed hydrographic data obtained monthly by the Iwate Prefectural Fisheries Laboratory during 1977–1981. Our attention was focussed on the classification of waters based on the Temperature-Salinity (T-S) scatter diagram analyses, and on the extraction of the “mean state” of their distribution for each season. The “water system” was defined as “all waters occupying some specific region on the T-S plane”, and the mean state for each season was expressed by the distribution of the “mode water systems” (i.e., the most commonly occuring water system).Because of the coexistence of the Tsugaru Warm Current, the Oyashio and the Kuroshio, and their large seasonal variability, the sea waters had a wide variety of T-S combinations. The detailed T-S scatter diagram analyses allowed us to classify the waters into six water systems. A time-longitude diagram of water systems and the distribution of mode water systems are presented. The latter showed clear seasonal variations of the Tsugaru Warm Current and the Oyashio.Though the water system analysis was successfully able to present the distribution of the various waters in the Sanriku Coastal Area and to clearly extract the mean state of sea conditions, it was found that the classification of water systems on the T-S plane was not complete in all cases for this area.

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.

Mesoscale eddies observed by TOLEX/ADCP and TOPEX/POSEIDON altimeter in the Kuroshio recirculation region south of Japan

Mesoscale eddies in the Kuroshio recirculation region south of Japan have been investigated by using surface current data measured by an Acoustic Doppler Current Profiler (ADCP) installed on a regular ferry shuttling between Tokyo and Chichijima, Bonin Islands, and sea surface height anomaly derived from the TOPEX/POSEIDON altimeter. Many cyclonic and anticyclonic eddies were observed in the region. Spatial and temporal scales of the eddies were determined by lag-correlation analyses in space and time. The eddies are circular in shape with a diameter of 500 km and a temporal scale of 80 days. Typical maximum surface velocity and sea surface height anomaly associated with the eddies are 15–20 cm s−1 and 15 cm, respectively. The frequency of occurrence, temporal and spatial scales, and intensity are all nearly the same for the cyclonic and anticyclonic eddies, which are considered to be successive wave-like disturbances rather than solitary eddies. Phase speed of westward propagation of the eddies is estimated as 6.8 cm s−1, which is faster than a theoretical estimate based on the baroclinic first-mode Rossby wave with or without a mean current. The spatial distribution of sea surface height variations suggests that these eddies may be generated in the Kuroshio Extension region and propagate westward in the Kuroshio recirculation region, though further studies are needed to clarify the generation processes.

Trajectory of Mesoscale Eddies in the Kuroshio Recirculation Region

Trajectories of mesoscale eddies in the Kuroshio recirculation region were investigated by using sea surface height (SSH) anomaly observed by the TOPEX/POSEIDON and ERS altimeters. Cyclonic and anticyclonic eddies have been traced on maps of the filtered SSH anomaly fields composed from the altimeter observations every ten days. Both the cyclonic and anticyclonic eddies propagate westward in the Kuroshio recirculation region from a region south of the Kuroshio Extension. The propagation speed of these eddies has been estimated as about 7 cm s−1, which is much faster than the phase speed theoretically estimated for the baroclinic first-mode Rossby wave in the study area. It was also found that in the Izu-Ogasawara Ridge region, most of eddies pass through the gap between the Hachijojima Island and Ogasawara (Bonin) Islands, and some of the eddies decay around the Izu-Ogasawara Ridge. It seems that the trajectory of the eddies is crucially affected by the bottom topography. In the region south of Shikoku and east of Kyushu, some of the eddies coalesce with the Kuroshio. It is also suggested that this coalescence may trigger the path variation of the Kuroshio in the sea south of Japan.

Interannual variations of the winter‐time outcrop area of subtropical mode water in the western north pacific ocean

Abstract Seasonal and interannual variations of the SST 16–19°C zone in the western North Pacific are described. Temperatures ranging from 16 to 19°C correspond with those of the Subtropical Mode Water (SMW) first reported and named by Masuzawa (1969). In the cooling season, this zone gradually moves southward and about December crosses latitudes 35–37°N where the Kuroshio axis lies. From January to April, the zone stagnates and spreads from the Kuroshio axis to about 28°N, i.e. to a width of about 700 km at 145°E in midwinter. This stagnation and widening are a manifestation of the existence of a thick mixed layer of SMW, i.e. the formation of a large amount of SMW, which is confirmed by several examples of the subsurface temperature distribution. In the heating season, the zone migrates northward with a narrow width as a result of the warming of the surface layer through the air‐sea interface. SST maps in March, and other related data, show the large interannual variations of the zone, especially in the...