Eitarou Oka

Progress of North Pacific mode water research in the past decade

This article reviews the progress in research on North Pacific mode waters of the past decade from the physical oceanographic perspective. The accumulation of satellite altimeter sea surface height data, the rapid growth of the Argo profiling float array, and the advancement in eddy-resolving ocean general circulation models have greatly improved the traditional views on the mode waters that were formed prior to the 1990s based on the analyses of historical temperature/salinity data. Areas where significant progress was made include: (1) descriptions of the mode waters’ distributions and properties with fine spatial scales, particularly in their formation regions in winter where observational data had been insufficient; (2) clarifications of the mode waters’ formation and subduction processes relating to the large-scale mean circulation, as well as to the time-varying mesoscale eddy field; (3) impacts of the mode waters’ circulation and dissipation processes on the climate and biogeochemical processes; and (4) dynamic versus thermodynamic causes underlying the mode waters’ decadal changes. In addition to the review, future directions for mode water research are also presented.

The role of meso-scale eddies in mixed layer deepening and mode water formation in the western North Pacific

Distributions of mixed layer depths around the centers of anti-cyclonic and cyclonic eddies in the North Pacific Ocean were composited by using satellite-derived sea surface height anomaly data and Argo profiling float data. The composite distributions showed that in late winter, deeper mixed layers were more (less) frequently observed inside the cores of the anti-cyclonic (cyclonic) eddies than outside. This relationship was the clearest in the region of 140°E–160°W and 35°N–40°N, where the temperature and salinity of the deep mixed layers were similar to those of the lighter variety of central mode water (L-CMW). A simple one-dimensional bulk mixed layer model showed that both strong sea-surface heat and momentum fluxes and weak preexisting stratification contributed to formation of the deep mixed layer. These conditions were associated with the anti-cyclonic eddies, suggesting that these eddies are important in the formation of mode waters, particularly L-CMW.

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...

Characteristics of variations of water properties and density structure around the Kuroshio in the East China Sea

Quarterly data of CTD at the PN line in the East China Sea during 1988–94 were analyzed to examine the variations of water properties and density structure in relation to the Kuroshio. The Kuroshio flows over the continental slope at the PN line. Water properties in the surface layer less than 100 db change greatly and show a clear seasonal cycle, while those in the subsurface layer are much less variable. The small isobaric variations in the subsurface layer are almost due to the vertical movement of isopycnals, on which the water properties vary little. The subsurface variations of salinity, temperature and isopycnal depth are classified into four groups occurring in the four regions, divided vertically by the middle of the main pycnocline and horizontally by the offshore edge of the Kuroshio, named Groups 1 (upper Kuroshio), 2 (upper offshore region), 3 (lower Kuroshio), and 4 (lower offshore region). The difference in averaged isopycnal depth between Groups 1 and 2 (3 and 4) is highly correlated with the vertical shear of the Kuroshio velocity in the upper (lower) pycnocline. The isopycnal depth of Groups 1 and 3 has little annual cycle (with large intraseasonal variations in Group 3), while that of Groups 2 and 4 shows a clear seasonal variation with the minimum in fall. As a result, the Kuroshio velocity is smallest in fall almost every year, although the amplitude of seasonal variation and the season of maximum velocity are different from year to year. Interannual variations of isopycnal depth are characterized by a large amplitude of Group 2 and an opposite phase between Groups 3 and 4, so that the variations of difference in isopycnal depth between Groups 1 and 2 and Groups 3 and 4, i.e., the upper and lower shear of the Kuroshio velocity, are comparably significant.

Formation region of North Pacific subtropical mode water in the late winter of 2003

[1] The mixed layer structure and properties in the formation region of the North Pacific Subtropical Mode Water (STMW) east of Japan in the late winter of 2003 were examined using temperature and salinity data primarily from Argo profiling floats. The formation region extends south of the Kuroshio Extension, between 30N and 35N and as far east as 175E. It is characterized by a mixed layer with depths greater than 200db and meridionally uniform temperature between 16.5C and 18.2C, associated with a mixed layer front at its southern edge. The mixed layer in the formation region becomes colder, fresher, and denser toward the east, leading to the formation of warmer, saltier, and lighter STMW in the western part of the region. The spatial variation of the mixed layer depth in the formation region corresponds well with the underlying permanent thermocline depth. INDEX TERMS: 4223 Oceanography: General: Descriptive and regional oceanography; 4283 Oceanography: General: Water masses; 4572 Oceanography: Physical: Upper ocean processes. Citation: Oka, E., and T. Suga, Formation region of North Pacific subtropical mode water in the late winter of 2003, Geophys. Res. Lett., 30(23), 2205, doi:10.1029/2003GL018581, 2003.

Formation and Subduction of Central Mode Water Based on Profiling Float Data, 2003–08

Abstract Temperature and salinity data from Argo profiling floats in the North Pacific during 2003–08 have been analyzed to study the structure of winter mixed layer north of the Kuroshio Extension and the subsurface potential vorticity distribution in the subtropical gyre in relation to the formation and subduction of the central mode water (CMW). In late winter, two zonally elongated bands of deep mixed layer extend at 33°–39° and 39°–43°N, from the east coast of Japan to 160°W. These correspond to the formation region of the lighter variety of CMW (L-CMW) and that of the denser variety of CMW (D-CMW) and the recently identified transition region mode water (TRMW), respectively. In the western part of the L-CMW and D-CMW–TRMW formation regions west of 170°E, the winter mixed layer becomes deeper and lighter to the east (i.e., to the downstream). As a result, the formed mode water is reentrained into the mixed layer in the farther east in the following winter and modified to the lighter water and is thus...

Formation and Subduction of North Pacific Tropical Water and Their Interannual Variability

AbstractFormation and subduction of the North Pacific Tropical Water (NPTW), its interannual variability, and its associated mechanisms were investigated by using gridded Argo-profiling float data and various surface flux data in 2003–11. The NPTW has two formation sites in the center of the North Pacific subtropical gyre, corresponding to two regional sea surface salinity maxima. Mixed layer salinity variations in these two NPTW formation sites were found to be significantly different. While seasonal variation was prominent in the eastern formation site, interannual variation was dominant in the western site. The mixed layer salinity variation in the eastern site was controlled mainly by evaporation, precipitation, and entrainment of fresher water below the mixed layer and was closely related to the seasonal variation of the mixed layer depth. In the western site, the effect of entrainment is small due to a small vertical difference in salinity across the mixed layer base, and excess evaporation over pre...

Decadal seesaw of the Central and Subtropical Mode Water formation associated with the Kuroshio Extension variability

Available Argo profiling float data from 2002 to 2011 were analyzed to examine the effect of the Kuroshio Extension (KE) current system variability on the formation of the Central Mode Water. Just north of the upstream portion of the KE at 140–152°E, formation of a lighter variety of the Central Mode Water in winter was active during the unstable period of the upstream KE in 2006–2009 and was reduced when the upstream KE was in the stable period of 2002–2005 and 2010–2011. This decadal formation variability is out of phase with that of the Subtropical Mode Water just south of the KE.

Eddy Resolving Observation of the North Pacific Subtropical Mode Water

AbstractHydrographic data obtained by high-resolution shipboard observations and Argo profiling floats have been analyzed to study the mesoscale structure and circulation of the North Pacific Subtropical Mode Water (STMW). The float data show that in the late winter of 2008, STMW having a temperature of approximately 18.8°, 17.7°, and 16.6°C formed west of 140°E, at 140°–150°E, and east of 150°E, respectively, in the recirculation gyre south of the Kuroshio Extension. After spring, the newly formed STMW gradually shift southward, decreasing in thickness. Simultaneously, the STMWs of 16.6° and 17.7°C are gradually stirred and then mixed in terms of properties. In late fall, they seem to be integrated to form a single group of STMWs having a temperature centered at 17.2°C. Such STMW circulation in 2008 is much more turbulent than that in 2006, which was investigated in a previous study. The difference between the two years is attributed to the more variable state of the Kuroshio Extension in 2008, associate...

Stability of Temperature and Conductivity Sensors of Argo Profiling Floats

After recalibration of the temperature and conductivity sensors of three Argo profiling floats recovered after operations for four to nine months, the results indicate that the floats basically showed no significant drift, either in temperature or salinity, and adequately fulfilled the accuracy requirement of the Argo project (0.005°C for temperature and 0.01 psu for salinity). Only the third float showed a significant offset in salinity of about −0.02 psu, as expected from comparison between the float data and the shipboard conductivity-temperature-depth data. This offset was caused by the operational error of the PROVOR-type float, in which the surface water was pumped immediately after the launch, fouling the conductivity sensor cell.