Yuji Kashino

Pacific western boundary currents and their roles in climate

Pacific Ocean western boundary currents and the interlinked equatorial Pacific circulation system were among the first currents of these types to be explored by pioneering oceanographers. The widely accepted but poorly quantified importance of these currents—in processes such as the El Niño/Southern Oscillation, the Pacific Decadal Oscillation and the Indonesian Throughflow—has triggered renewed interest. Ongoing efforts are seeking to understand the heat and mass balances of the equatorial Pacific, and possible changes associated with greenhouse-gas-induced climate change. Only a concerted international effort will close the observational, theoretical and technical gaps currently limiting a robust answer to these elusive questions.

Intraseasonal Variability in the Indo–Pacific Throughflow and the Regions Surrounding the Indonesian Seas

Intraseasonal oscillations in sea level, sea surface temperature, and upper ocean flow field have recently been observed in various locations surrounding the Indonesian seas. While the observed oscillations at these locations have similar wave periods ranging from 30 to 85 days, their forcing mechanisms can involve different dynamic processes. In order to clarify the dynamic processes essential for the observed intraseasonal signals, the authors adopted in this study a fine-resolution 1‰-layer reduced-gravity model and carried out a series of model experiments by varying coastline geometry and surface wind forcing. From these carefully designed experiments, they show that the intense 50-day oscillations observed at the entrance of Celebes Sea are a result of Rossby wave resonance in which the frequency of cyclonic eddy shedding by the retroflecting Mindanao Current matches that of the gravest Rossby mode of the semi-enclosed Celebes Sea basin. These 50-day oscillations of the Celebes Sea have a large impact on the throughflow in the Makassar Strait and Banda Sea, but do not contribute significantly to the throughflow changes in the Lombok, Ombai, and Timor Straits. The observed intraseasonal signals along the Sumatra/Java coasts in the eastern Indian Ocean are dominated by oscillations with wave periods of 50 and 85 days. The origin of these wave signals can be traced back to the central equatorial Indian Ocean where the surface zonal wind fluctuates intraseasonally with the same wave periods. The remotely induced coastal Kelvin waves along the Sumatra/Java coasts strongly influence the Lombok Strait, but exert no direct influence upon the Ombai and Timor Straits farther to the east; the gap of the Lombok Strait is found to conduit the Kelvin wave signals effectively into the Makassar Strait and the Banda Sea. The intraseasonal signals observed in the Timor Passage have a dominant wave period of 30‐35 days. The model simulation further suggests the presence of a second spectral peak around 85 days. Both of these intraseasonal signals are attributable to the local, along-archipelago wind near the Timor Passage.

The water masses between Mindanao and New Guinea

Water masses between Mindanao and New Guinea are investigated, using hydrographic data, during two R/V Kaiyo World Ocean Circulation Experiment cruises conducted in October 1992 and February 1994. The analysis of this data indicates that the South Pacific Tropical Water (SPTW), which is formed in the southern hemisphere, reached the area of Morotai Island in 1992 and farther north in 1994. The Antarctic Intermediate Water (AAIW) exists in the southwestern area of the southernmost Philippine Sea. There is northeastward flow between Talaud Islands and Morotai Island, which is probably a part of the retroflection of the Mindanao Current. The remnant of the North Pacific Intermediate Water, advected by the flow from the Celebes Sea, was found in 1992. The New Guinea Coastal Undercurrent, transporting SPTW and AAIW from the southern hemisphere, is divided into at least two parts because the retroflection of the Mindanao Current prevents its shallow part from reaching farther north; the shallow part of SPTW turns eastward as a source water of North Equatorial Countercurrent and retroflects toward the southeast. AAIW and the lower part of SPTW flow northward and seem to be linked with the Mindanao Undercurrent.

Current variability at the Pacific entrance of the Indonesian Throughflow

Current variability at the Pacific entrance of the Indonesian Throughflow is investigated using direct current and hydrographic measurements. Two moorings with three current meters (depths of 350, 550, and 1050 m) and one conductivity-temperature-depth profiler (260 m) were deployed at 4°1′N, 127°31′E and 3°11′N, 128°27′E between Talaud Islands and Morotai Island (Indonesia) from February 1994 to June 1995. Data from four hydrographic surveys conducted mainly between Mindanao and New Guinea from 1994 to 1996 are also used. The onset of a strong northwestward flow was observed at the southern mooring during boreal winter. In contrast, a southwestward flow containing salty South Pacific water was observed there during boreal summer. This current pattern change matched monsoon change around the mooring sites, suggesting that this variability is a seasonal signal in this region. This current change may occur because of the meridional shift of the Halmahera Eddy associated with an enlargement/diminishment of the Mindanao Dome. Our observation result during summer (the southwestward flow with the South Pacific water at the southern mooring) suggests that the Maluku Sea is one of the eastern routes of the Indonesian Throughflow. The current data also revealed that intraseasonal variability occurs in 50-day oscillations. Because the coherence between wind variability in the tropics with a period of 40–50 days (Madden-Julian Oscillation) and current variability with this period are >0.4, it is possible that the 50-day oscillation in the ocean current is induced by wind variability associated with the Madden-Julian Oscillation. The ocean eddy activity with an intrinsic period in this region may also be related to this 50-day oscillation.

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...

Currents in the Celebes and Maluku Seas, February 1999

Upper ocean currents in the Celebes Sea, the northern Maluku Sea, and the adjacent region to the east were measured during February 1999 using a shipboard acoustic Doppler current profiler (ADCP) on R/V Kaiyo. The new data provide the first quasisynoptic observations of the circulation within the central Celebes Sea. South of Mindanao, the Mindanao Current split into three branches. Only the westernmost branch entered the Celebes Sea, where part retroflected around a cyclonic eddy in the western Celebes Sea and part took a shorter route through the eastern basin before the two paths joined and returned to the Pacific. The remainder of the Celebes Sea branch of the Mindanao Current continued toward Makassar Strait. In the central and eastern Celebes Sea, the circulation pattern below 150 m differed substantially from that at the surface. The strongest feature was a northward flow that exceeded 60 cm s -1 near 230 m in the center of the basin, where shallower currents were weak. A northward transport of 6 Sv in the top 300 m was measured across 1.75°N in the Maluku Sea, carrying Indonesian Seas waters northeast toward the Pacific. Water properties and other current sections in the northern Maluku Sea confirm shallow northeastward flow.

Ocean variability east of Mindanao: Mooring observations at 7°N, revisited

Two subsurface moorings were deployed east of Mindanao Island, the Philippines, at 7°01′N, 126°55′E and 7°01′N, 127°46′E, at the location of the inshore and offshore cores of the Mindanao Undercurrent (MUC) suggested by past studies, from September 2011 to October 2012 and March 2013. A steady northward undercurrent, the MUC, was not confirmed by these observations, not only at the location of its inshore core but also of the offshore core. The observed mean flow at the mooring sites seems to be part of an anticyclonic eddy rather than the MUC. A particle-tracking experiment using a high-resolution general circulation model output showed that the northward mean flow, called the MUC by past studies, was too weak to advect water to the north. The Mindanao Current during 2011–2012 was weaker than during 1999–2002 because the sea surface height in the Philippine Sea during 2011–2012 was lower than that during 1999–2002. Intraseasonal variability with periods of 50–100 days was observed at the mooring sites, comparable to the previous observations during 1999–2002. Westward signal propagations were observed with periods and speeds of 50 days and 0.20 m s−1 at 300 m depth and of 60–72 days and 0.11–0.14 m s−1 at 960 m depth.

A New Paradigm for the North Pacific Subthermocline Low-Latitude Western Boundary Current System

AbstractSubthermocline western boundary circulation along the low-latitude North Pacific Ocean (2°–25°N) is investigated by using profiling float and historical CTD/expendable CTD (XCTD) data and by analyzing an eddy-resolving global OGCM output. In contrast to the existing paradigm depicting it as a reversed pattern of the wind-driven circulation above the ventilated thermocline (i.e., depth < 26.8 σθ), the subthermocline western boundary circulation is found to comprise two components governed by distinct dynamical processes. For meridional scales shorter than 400 km, the boundary flows along the Philippine coast exhibit convergent patterns near 7°, 10°, 13°, and 18°N, respectively. These short-scale boundary flows are driven by the subthermocline eastward zonal jets that exist coherently across the interior North Pacific basin and are generated by the triad instability of wind-forced annual baroclinic Rossby waves. For meridional scales longer than 400 km, a time-mean Mindanao Undercurrent (MUC) is obs...

Variability and Linkages of New Guinea Coastal Undercurrent and Lower Equatorial Intermediate Current

Abstract Velocity at depths of 700–800 m was measured between September 1998 and October 2002 at 2.5°S, 142°E off the New Guinea coast and at 0°, 138°E to examine the New Guinea Coastal Undercurrent (NGCUC) and the current on the equator carrying Antarctic Intermediate Water (AAIW). Velocity characteristics before November 1999 were markedly different from those after November 1999. The typical state occurred during the second period: the intermediate NGCUC and the Lower Equatorial Intermediate Current (LEIC) varied markedly with an annual cycle in opposite phases. In austral winter, the NGCUC flowed west-northwestward strongly (14 cm s−1, 285°T), especially in May–July during which the LEIC disappeared and eddylike equatorial variations with periods of 20–60 days were significant. In austral summer, the LEIC flowed westward strongly (12 cm s−1, 270°T), especially in October–December, whereas the NGCUC reversed its direction repeatedly to flow east-southeastward in November–February. Thus, the intermediat...

Shear‐generated turbulence in the equatorial Pacific produced by small vertical scale flow features

We investigate the characteristics of shear-generated turbulence in the natural environment by considering data from a number of cruises in the western equatorial Pacific. In this region, the vertical shear of the flow is dominated by flow structures that have a relatively small vertical scale of O(10 m). Combining data from all cruises, we find a strong relationship between the turbulent dissipation rate, ϵ, vertical shear, S, and buoyancy frequency, N. Examination of ϵ at a fixed value of Richardson number, Ri = N2∕S2, shows that ϵ∝ut2N for a wide range of values of N, where ut is an appropriate velocity scale which we assume to be the horizontal velocity scale of the turbulence. The implied vertical length scale, lv = ut∕N, is consistent with theoretical and numerical studies of stratified turbulence. Such behavior is found for Ri < 0.4. The vertical diffusion coefficient then scales as κv∝ut2/N at a fixed value of Richardson number. The amplitude of ϵ is found to increase with decreasing Ri, but only modestly, and certainly less dramatically than suggested by some parameterization schemes. Provided the shear generating the turbulence is resolved, our results point to a way to parameterize the unresolved turbulence.