Origins of the Kuroshio and Mindanao Currents Background

1 Origins of the Kuroshio and Mindanao currents Background The boundary currents off the east coast of the Philippines are of critical importance to the general circulation of the Pacific Ocean. The westward flowing North Equatorial Current (NEC) runs into the Philippine coast and bifurcates into the northward Kuroshio and the southward Mindanao Current (MC) (Figure 1; Nitani, 1972). The partitioning of the flow into the Kuroshio and MC is an important observable. Quantifying these flows and understanding bifurcation dynamics are essential to improving predictions of regional circulation, and to characterizing property transports that ultimately affect Pacific climate. Fluctuations in the Kuroshio and MC can significantly impact variability downstream. For example, the Kuroshio penetrates through Luzon Strait into the South China Sea and onto the East China Sea shelf. The Kuroshio front dramatically alters stratification and may impact internal wave climate. The study proposed here incorporates observation, theory, and modeling to make fundamental advances in our knowledge of the origins of the Kuroshio and Mindanao currents. Figure 1. Region of study. The major currents of the region are identified: the North Equatorial Current (NEC), the Kuroshio, and the Mindanao Current (MC). 2 Figure 2. Tracks of drifters color-coded by instantaneous (6 hourly) speed. 3 Drifter observations of currents at 15 m depth form a comprehensive dataset of direct observations . According to these observations (Figure 2), the roots of the Mindanao Current can be located at approximately 11° N, 1.5° south of the latitude at which the mean NEC takes a southward and a northward bend (Centurioni et al., 2004) while approaching the Philippine Archipelago. The Kuroshio however, appears to become a stable, detectable boundary current (with speeds in excess of 0.8 m s-1) between 16°N and 18°N. Although the drifter data are too sparse to allow a definitive picture of the annual cycle in the region, the available data suggest that the two-dimensional circulation pattern north of 11° N changes seasonally. A region of complex and highly variable near surface flow exists off the western (Philippines) boundary and north of the roots of the Mindanao Current, i.e. between 12°N and 16-18°N. Regions characterized by intermittently high speeds (Figure 2) extend eastward and away from the region’s western boundary from 24°N to 18°N and between12°N and 9°N. Those are also the regions of relatively large, seasonally variable , Eddy Kinetic Energy (EKE), while lower EKE is generally found between those two latitude bands. North Equatorial Current and Bifurcation The bifurcation of the NEC has been the subject of a number of studies, as its position varies seasonally and with depth (Kim et al., 2004: Qu and Lukas, 2003: Yaremchuk and Qu, 2004). The location of the bifurcation also depends on the data and models used in its definition. General statements are difficult to make, but the following description is supported by the weight of the published material. In the annual average, the bifurcation trends north with depth, with the surface expression near 14°N sloping to 17°N near 500 m. The bifurcation is at its northernmost position in the fall and at its southernmost position in spring, with an annual excursion of about 2° in latitude. The mechanisms governing the excursions of the bifurcation and the strengths of the currents are topics of active research. Local wind forcing through the wind stress curl, and remote forcing through Rossby waves are both possibly important (Qiu and Lukas, 1996). Interannual changes may have some relationship to ENSO, shifting northward during El Niño years and southward during La Niña years. The interplay of local and remote forcing, and the rich array of time scales make this region an interesting site for study. Kuroshio The Kuroshio forms in the NEC bifurcation region (12° - 18°N), though energetic mesoscale eddy variability often obscures the current in region east of the Philippine Archipelago. Data availability limits understanding of Kuroshio formation, with the upstream Kuroshio having received far less attention than the regions to the northeast. Although observations reveal an increasingly distinct Kuroshio northward toward Luzon Strait, the formation mechanism remains unclear. Should the upstream Kuroshio be considered as an eddy- driven current or as a laminar boundary flow, and from where does it draw its source waters? The Kuroshio typically flows northward with a slight westward incursion through the deep channels (2400 m sill depth) of Luzon Strait, but occasionally turns westward to form significant intrusions into the South China Sea. These intrusions modify Kuroshio structure through entrainment of South China Sea waters and impact mesoscale and internal wave 4 variability within the South China Sea. The region experiences intense, seasonally reversing wind forcing by the Asian Monsoon, with strong wintertime winds from the northeast, weaker summertime winds from the southwest and relatively calm intermonsoon periods. Although observational evidence remains scarce, Kuroshio loop current events appear more common during the winter monsoon (Wang and Chern, 1987). Competing theories attempt to explain the dynamics governing these intrusions, the simplest invoking westward Ekman transport produced by the winter monsoon to drive the Kuroshio through the Strait (Farris and Wimbush, 1996). Weak (strong) wintertime (summertime) density contrasts across the Kuroshio-South China Sea front may accelerate (retard) westward translation of the front (Chern and Wang, 1998). Numerical investigations suggest that strong meridional windstress curl gradients across Luzon Strait may generate sharp contrasts in thermocline depth and enhance Kuroshio penetration (Metzger and Hurlburt, 2001), but find little direct correlation between winds and loop current formation. Sheremet (2001) finds that inertia carries western boundary currents across gaps for strong flows, but at slower speeds effect dominates, driving a westward turn. These dynamics exhibit several characteristics consistent with observed loop current formation, including preferential wintertime formation (when monsoon winds may weaken the Kuroshio) and lack of direct correlation with local winds, possibly the result of response hysteresis. Both observations and theoretical results feature energetic meandering and mesoscale eddy generation. Though mesoscale variability complicates quantification of Kuroshio transports south of Luzon Strait, Gilson and Roemmich (2002) employ an eight-year record of repeated XBT surveys to characterize transports off the southern end of Taiwan, after interactions with the South China Sea but prior to passage over the Ilan Ridge. Annual mean volume transport was 22±1.5 Sv with 8±6 Sv seasonal variation, with the strongest currents confined close to the Taiwan coast, in the upper 700 m of the water column. Kuroshio transport is strongest in winter/spring and weakest in autumn, in phase with NEC seasonal variations. Kuroshio transport exhibits 12±6 Sv interannual variability, well in excess of its seasonal range. The Kuroshio often exhibits a dual-core structure east of Taiwan (Chern and Wang, 1998), collapsing to a single current prior to passing over Ilan Ridge. Analysis of historical hydrographic measurements suggests that this two-core structure is a consistent feature of the Kuroshio offshore of Taiwan (Lien, personal communication). After leaving the Luzon Strait region, the Kuroshio flows along the east coast of Taiwan, eventually encountering the Ilan Ridge where it can enter the East China Sea though the East Taiwan Channel or turn northeastward along the east side of the Ryukyu Islands. East of Taiwan, the Kuroshio exhibits annual mean transport of roughly 20 Sv with large 10 Sv variations at timescales of days to months (Johns et al., 2001). The mean transport profile is significantly sheared in the upper 500 m, with 40% (60%) of the transport occurring in the upper 100 (200) m. Meanders induced by anticyclonic eddies impinging from the Philippine Sea drive strong transport variability at ~100 day timescales (Zhang et al., 2001). During strong transport periods, the Kuroshio passes through the East Taiwan Channel and enters the East China Sea, impacting circulation and internal wave variability within the marginal sea. Zhang et al. (2001) associate low transport periods with impinging eddies that steer the current toward the eastern side of the Ryukyu Islands. Mesoscale-induced transport variability exceeds seasonal fluctuations (Johns et al., 2001), and eddy interactions may thus exert a controlling influence on the Kuroshio’s interactions with the East China Sea. 5 Mindanao Current The MC has not been as well observed as has the Kuroshio. The most extensive series of observations took place during 1987-1990 as part of two efforts: the Western Equatorial Pacific Ocean Circulation Study (WEPOCS) and a joint United States/People’s Republic of China Tropical Ocean Global Atmosphere (TOGA) program. Hydrographic and Acoustic Doppler Current Profiler (ADCP) measurements were made in a series of 8 cruises. These observations were synthesized by Wijffels et al. (1995), which found a remarkably steady MC. While the core of the MC was stable, the flows offshore of the MC were extremely variable. For example, recirculation indicative of the Mindanao Eddy was found in only 2 of the 8 cruises, and the northward flowing Mindanao Undercurrent (Hu et al., 1991: Qu et al., 1998) was not apparent in the mean. The possibility remains that the Mindanao Eddy is strongly modulated on seasonal or shorter time scales (Toole et al., 1990). A single mooring observation of the MC was made over the period from 1999-2002 by Kashino et al. (2005). They reported that the existence of a strong shallow surface current with a speed of over 1.3 m/s at 100 m, with a remarkably low variability of less than 0.2 m/s. The velocity was highest during boreal summer, and the strength was apparently modulated by the onset of the 2002 El Nino. However, the mooring observations left open several unanswered questions.

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