Shigeki Hosoda

Interdecadal Thermocline Variability in the North Pacific for 1958–97: A GCM Simulation*

An ocean general circulation model is forced with the NCEP reanalysis wind stress for 1958‐97 to understand mechanisms of ocean subsurface variability. With relatively high horizontal (1 83 18) and vertical (41 levels) resolutions, the model produces mode waters on a range of density surfaces in the western, central, and eastern North Pacific, in qualitative agreement with observations. These mode waters appear as a thermostad or a region of weak stratification in the upper thermocline as they flow southward from their formation regions in the Kuroshio and its extension. In the model, subsurface temperature variability in the central subtropical gyre reaches a maximum within the thermostad, in contrast to what might be expected from the linear baroclinic Rossby wave theory. This variance maximum is associated with the longitudinal shift in the path of mode waters. In particular, deepened mixed layer and accelerated eastward currents in the Kuroshio Extension by wind changes in the mid-1970s act cooperatively to move the central mode waters toward the east, causing large subsurface temperature anomalies. Besides the local maximum in the central North Pacific subtropical gyre, two additional maxima of the subsurface anomaly are identified in the northwestern and southern parts of the gyre, respectively. Among these subsurface anomaly centers, the one in the northwestern North Pacific has a strong effect on the model sea surface temperature, suggesting that the Kuroshio and its extension are a key region to decadal/interdecadal ocean‐atmosphere interaction. Finally, possible effects of atmospheric thermal forcing are discussed.

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.

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...

Observed Ocean Variability in the Mindanao Dome Region

Abstract Ocean variations at semiannual, annual, and interannual time scales in the Mindanao Dome (MD) region of the southern Philippine Sea were examined using data derived from underwater sensors on Triangle Trans-Ocean Buoy Network (TRITON) buoys at 8°N, 137°E; 5°N, 137°E; and 8°N, 130°E. Annual signal dominated above 300-m depth in the MD region. At 5°N, 137°E, saline water exceeding 35 psu was observed at 100–200-m depth from boreal winter to spring, seemingly associated with the meridional migration of the North Equatorial Countercurrent during these seasons. Thermocline ascent, probably related to the MD, was also observed from boreal winter to spring. An important mechanism of the annual variation of the MD at 5°N seems to be the annual variability of local wind, as mentioned in past studies. However, annual variability at 8°N seems to be due to Rossby waves originating west of 150°W rather than to local wind effects. Semiannual variation was also observed, with its amplitude reaching 40%–70% of t...

Eastern North Pacific Subtropical Mode Water in a general circulation model: Formation mechanism and salinity effects

The Eastern North Pacific Subtropical Mode Water (ESTMW) is a water mass of low potential vorticity (PV) and appears as a weak pycnostad or thermostad. Distinct from other subtropical mode waters, it forms in the absence of a deep winter mixed layer. The formation mechanism of this ESTMW is investigated using an ocean general circulation model that is forced by monthly climatological temperature, salinity, and wind stress at the sea surface. An equation based on the ventilated thermocline theory is used to diagnose the initial PV of a water mass right after its subduction. In this equation, three factors affect the initial PV: the spacing of density outcrop lines, the mixed layer depth gradient, and the vertical velocity at the bottom of mixed layer. Among them the wide spacing between outcrop lines is the most important for ESTMWs low PV instead of the deep mixed layer, which is most important for classical mode waters. It is found that weak gradients in both sea surface temperature and salinity in the direction of mixed layer flow are important for the low PV formation. A low-salinity tongue that extends southeastward off North America is responsible for the small surface density gradient in the eastern North Pacific and contributes to the formation of the ESTMW. An additional experiment forced with observed freshwater flux demonstrates that the southward advection of fresher water from the high latitude along the eastern boundary is the cause of this low-salinity tongue.

Seasonal temperature variation below the thermocline detected by Argo floats

[1] Strong seasonal temperature variation below the thermocline is detected by Argo profiling floats. The variation is associated with a long baroclinic Rossby wave. The results of this study show that seasonal signals can be captured at 1000-2000 dbar not only in basin boundary regions or deep convection regions but also in the interior ocean. A clear, westward propagation signal is observed. The phase speed is of the same order as a theoretically derived speed. Below the thermocline, the first mode of empirical orthogonal function analysis of potential density variations in the strong signal region shows a clear seasonal variation. Even in the middle layer, the amplitude of the temperature anomaly is 0.05-0.12° C at 1200 dbar, which is equivalent to the annual standard deviation from the WOA01. These results are obtained using the dense spatial and temporal distributions provided by the Argo observations.

Upper-Ocean Salinity Variability in the Tropical Pacific: Case Study for Quasi-Decadal Shift during the 2000s Using TRITON Buoys and Argo Floats

AbstractUpper-ocean salinity variation in the tropical Pacific is investigated during the 2000s, when Triangle Trans-Ocean Buoy Network (TRITON) buoys and Argo floats were deployed and more salinity data were observed than in previous periods. This study focuses on upper-ocean salinity variability during the warming period of El Nino–Southern Oscillation (ENSO)-like quasi-decadal (QD)-scale sea surface temperature anomalies over the central equatorial Pacific (January 2002–December 2005; hereafter “warm QD phase”). It is shown that strong negative salinity anomalies occur in the western tropical Pacific and the off-equatorial Pacific in the upper ocean at depths less than 80 m, showing a horseshoe-like pattern centered at the western tropical Pacific during the warm QD phase. TRITON mooring buoy data in the western equatorial Pacific show that low-salinity and high-temperature water could be transported eastward from the western equatorial Pacific to the central equatorial Pacific during the warm QD phase...

Impact of downward heat penetration below the shallow seasonal thermocline on the sea surface temperature

Observational data are used to investigate summer heat penetration into the subsurface ocean in order to quantify the heat capacity of the upper ocean with respect to surface heat exchange. Sea surface temperature is strongly modulated by the change in heat capacity, which could influence the overlying atmosphere and hence trigger climate variations, even during the warming season, when the ocean has been regarded as being rather passive. Few studies have focused on the heat exchange process in surface and subsurface layers because of the existence of a strong seasonal thermocline at the bottom of thin summer mixed layers (ML). By introducing the concept of the heat penetration depth (HPD), defined as the depth to which the downward net heat flux (Qnet) distinctly penetrates, we here characterize the heat capacity in terms of the heat content above the HPD using a simple, one-dimensional vertical model during the warming season. Seasonal changes in the HPD indicate that the thermal effects of Qnet gradually penetrate below the shallow seasonal thermocline due to vertical eddy diffusivity. Downward heat penetration into the layer below the shallow seasonal thermocline occurs widely throughout the North Pacific, and two-thirds of Qnet penetrates below the ML. In a hypothetical analysis of the case where the observed Qnet accumulates only within the ML, the change in SST is unrealistically larger than that of the observed SST. These results indicate that heat penetration plays a crucial role in climate variations during the warming season.

Evolution and modulation of a poleward‐propagating anticyclonic eddy along the Japan and Kuril‐Kamchatka trenches

To investigate the relationships between the movement of an eddy and its interior structure and water properties, four profiling floats were deployed in an anticyclonic eddy in the western North Pacific in 2013 (April–October). Daily float profiles showed rapid changes in temperature and salinity corresponding to strong interactions between eddies north of the subtropical Kuroshio Extension. After the first interaction with a warm-core eddy in April, the isolation of the winter mixed layer from the surface was observed, forming a subsurface remnant layer. Another interaction with a cold fresh eddy at middepths in May resulted in the formation of a multilayer structure. The eddy then moved poleward along the Japan and Kuril-Kamchatka trenches, indicating changes in its propagation pattern coupled to its interior structure. The eddy then moved northward (June–July), stalled (July–August), and moved eastward (August–October). In addition to a general declining trend, the properties of the warm saline core changed over a short time period, coinciding with changes in propagation. A density anomaly at middepths of the eddy changed location during the stalled period; however, denser waters were continuously observed in the southeast part of the eddy during its northward and eastward movement. This unidirectional density anomaly pattern was consistent with the structure of the poleward-propagating eddy, which interacted with the western topographic boundary. Meridional exchanges of heat and material were potentially elevated by the eddys advection and movement, as well as by water modifications in the eddy associated with exchanges along its perimeter.

Long-term variability of surface nutrient concentrations in the North Pacific

We present the spatial distributions and temporal changes of the long-term variability of surface nutrient concentrations in the North Pacific by using nutrient samples collected by volunteer ships and research vessels from 1961 to 2012. Nutrient samples are optimally interpolated onto 1° × 1° monthly grid boxes. When the Pacific Decadal Oscillation is in its positive phase, nutrient concentrations in the western North Pacific are significantly higher than the climatological means, and those in the eastern North Pacific are significantly lower. When the North Pacific Gyre Oscillation is in its positive phase, nutrient concentrations in the subarctic are significantly higher than the climatological means. The trends of phosphate and silicate averaged over the North Pacific are −0.012 ± 0.005 µmol l−1 decade−1 and −0.38 ± 0.13 µmol l−1 decade−1, whereas the nitrate trend is not significant (0.01 ± 0.13 µmol l−1 decade−1).