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Mindanao Current and Undercurrent: Thermohaline Structure and Transport from Repeat Glider Observations

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Mindanao Current and Undercurrent: Thermohaline Structure and Transport from Repeat Glider Observations AUGUST 2017 S C H Ö NAU AND RUDNICK 2055 Mindanao Current and Undercurrent: Thermohaline Structure and Transport from Repeat Glider Observations MARTHA C. SCHÖNAU AND DANIEL L. RUDNICK Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California (Manuscript received 15 December 2016, in final form 3 May 2017) ABSTRACT Autonomous underwater Spray gliders made repeat transects of the Mindanao Current (MC), a low- latitude western boundary current in the western tropical North Pacific Ocean, from September 2009 to 2 2 October 2013. In the thermocline (,26 kg m 3), the MC has a maximum velocity core of 20.95 m s 1, weakening with distance offshore until it intersects with the intermittent Mindanao Eddy (ME) at 129.258E. In 2 the subthermocline (.26 kg m 3), a persistent Mindanao Undercurrent (MUC), with a velocity core of 2 0.2 m s 1 and mean net transport, flows poleward. Mean transport and standard deviation integrated from the 2 coast to 1308Eis219 6 3.1 Sv (1 Sv [ 106 m3 s 1) in the thermocline and 23 6 12 Sv in the subthermocline. Subthermocline transport has an inverse linear relationship with the Niño-3.4 index and is the primary in- fluence of total transport variability. Interannual anomalies during El Niño are greater than the annual cycle for sea surface salinity and thermocline depth. Water masses transported by the MC/MUC are identified by subsurface salinity extrema and are on isopycnals that have increased finescale salinity variance (spice variance) from eddy stirring. The MC/MUC spice variance is smaller in the thermocline and greater in the subthermocline when compared to the North Equatorial Current and its undercurrents. 1. Background and introduction Pacific and Indian Oceans has led to several initiatives in the last two decades to observe and model the MC/MUC The Mindanao Current (MC) is a low-latitude western system. Observations of the MC/MUC include sea level boundary current (LLWBC) in the western tropical gauges (Lukas 1988), hydrographic surveys (Hacker North Pacific, formed by the bifurcation of the North et al. 1989; Toole et al. 1990; Hu et al. 1991; Lukas et al. Equatorial Current (NEC) in the Philippine Sea (Fig. 1). 1991; Wijffels et al. 1995; Kashino et al. 1996; Qu et al. The MC flows equatorward along the coast of Mindanao 1998; Firing et al. 2005; Kashino et al. 2009, 2013), until splitting to contribute to the global overturning moorings (Kashino 2005; Zhang et al. 2014; Kashino circulation through the Indonesian Throughflow (ITF; et al. 2015; Hu et al. 2016), and Argo floats (Qiu et al. Gordon 1986) and to close the interior North Pacific 2015; Wang et al. 2015). However, MC/MUC water mass Sverdrup transport via the North Equatorial Counter transport, coastal structure, and variability are not well Current (NECC; Wyrtki 1961). The MC is the northern understood owing to strong currents, eddy activity, and source of subtropical water in the western Pacific warm large annual and interannual changes in surface forcing. pool (Fine et al. 1994; Lukas et al. 1996) and thus im- From hydrographic surveys, the MC current is narrow pacts the tropical heat and freshwater budgets and the at 108N, then widens, shoals, and increases in speed as it El Niño–Southern Oscillation (ENSO) phenomena flows equatorward until it separates into the ITF and (Gu and Philander 1997; Zhang et al. 1998). In the NECC between 78N and 68N(Lukas et al. 1991). Upper- subthermocline, the MC transports North Pacific In- ocean cyclonic recirculation in the Philippine Sea, such termediate Waters equatorward while a Mindanao as that associated with the quasi-permanent Mindanao Undercurrent (MUC; Hu et al. 1991) flows poleward. Eddy (ME), centered near 78N, 1308E, causes the MC The confluence of subtropical, tropical, and inter- transport to increase as it flows equatorward (Lukas mediate water masses of North and South Pacific origin et al. 1991). At 88N, the mean MC is in geostrophic and their role in salt and heat exchange of the tropical balance, extending approximately 200 km offshore, 2 with a subsurface velocity core of 0.8 to 1 m s 1 (Wijffels Corresponding author: Martha Schönau, [email protected] et al. 1995; Qu et al. 1998). Although the ME has been DOI: 10.1175/JPO-D-16-0274.1 Ó 2017 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). Unauthenticated | Downloaded 10/07/21 09:18 AM UTC 2056 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 47 FIG. 1. (a) Schematic of circulation in the tropical northwestern Pacific. Red is circulation from the surface to the bottom of the thermocline, typically 300-m depth. Main features are the NEC, MC, the Kuroshio, ITF, NECC, and ME. Subthermocline currents (blue) that oppose the direction of thermocline flow are the LUC, MUC, and NEUCs. (b) MDT from AVISO with contours at 0.015-m intervals. Glider observations are objectively mapped onto a mean line from 8.158N, 126.618E to 8.778N, 1308E (thick black line). The mean line is perpendicular to coastal bathymetry and MDT near the coast. observed by surface drifters (Lukas et al. 1991), its of the East Asian monsoon and the arrival of annual sporadic presence is difficult to observe in hydrographic Rossby waves from the central Pacific (Qiu and Lukas transects (Wijffels et al. 1995; Qu et al. 1998; Firing et al. 1996; Kim et al. 2004; Qiu and Chen 2010). Interannual 2005; Kashino et al. 2013). The velocity variability of the variability of the MC is forced by basinwide changes MC, its structure near the coast, and the variability of during ENSO. Numerical models suggest an increase in the ME remain inadequately observed. MC transport when the NEC bifurcates at higher latitude: In the subthermocline, the poleward MUC is roughly 50 annually in the fall and interannually during El Niño to 80 km offshore of the coast of Mindanao near 500-m (Qiu and Lukas 1996; Kim et al. 2004; Qiu and Chen 2 depth and potential density of 27.2 kg m 3 (Hu et al. 1991; 2010). Observations have both verified (Lukas 1988; Lukas et al. 1991; Qu et al. 1998), with a second velocity Kashino et al. 2009)andconflicted(Toole et al. 1990) core occasionally reported farther offshore (Hu et al. 1991; as to the relationship of transport with ENSO. Less is Qu et al. 1998). The MUC could be a quasi-permanent known about the transport variability of the MUC. recirculation or a poleward current transporting South Transport variability of the MC/MUC impacts the Pacific water masses; intermittent subthermocline eddies heat and salt balance in the tropical Pacific. Water that populate the region on intraseasonal time scales masses transported equatorward by the MC are the make previous observations inconclusive (Hu et al. 1991; North Pacific Tropical Water (NPTW), a subsurface Qu et al. 1998, 1999; Firing et al. 2005; Dutrieux 2009; salinity maximum in the thermocline, and the North Chiang and Qu 2013; Wang et al. 2014; Zhang et al. Pacific Intermediate Water (NPIW), a salinity minimum 2014; Chiang et al. 2015; Kashino et al. 2015; Schönau et al. in the subthermocline (Gordon 1986; Bingham and 2015; Hu et al. 2016). The forcing, persistence, and con- Lukas 1994; Fine et al. 1994; Kashino et al. 1996; nectivity of the MUC, and its relationship to the thermo- Schönau et al. 2015; Wang et al. 2015). Antarctic In- cline circulation are areas of ongoing research. termediate Water (AAIW), saltier than NPIW and of The transport of the MC/MUC forms a closed mass South Pacific origin, is also present in the Philippine Sea system with the NEC and Kuroshio Current (KC), and and may be carried northward by the MUC (Fine et al. their respective undercurrents, the North Equatorial 1994; Wijffels et al. 1995; Qu et al. 1999; Qu and Undercurrents (NEUCs) and the Luzon Undercurrent Lindstrom 2004; Schönau et al. 2015; Wang et al. 2015). (LUC) (Toole et al. 1990; Qiu and Lukas 1996; Qu et al. The volume transport and modification of these water 1998). The relative transports of the MC and KC depend masses as they transit the Philippine Sea are less well on the bifurcation latitude of the wind-driven NEC. The understood. Isopycnal stirring of large-scale salinity bifurcation latitude, dictated by Sverdrup theory as the gradients introduced by subsurface water masses creates zero line of zonally integrated wind stress curl, is affected finescale salinity variance that leads to isopycnal by local Ekman pumping and suction caused by shifts and diapycnal diffusion (Ferrari and Polzin 2005). Unauthenticated | Downloaded 10/07/21 09:18 AM UTC AUGUST 2017 S C H Ö NAU AND RUDNICK 2057 Quantifying the relationship between stirring, which from March to August (Fig. 2b). The ENSO phase, as increases isopycnal salinity variance, and diffusion, measured by the Niño-3.4 index, was positive (El Niño) which decreases it, will give insight into heat and salt from September 2009 to April 2010 and negative (La transfer in the ocean interior and improve parameteri- Niña) from June 2010 to April 2011 and August 2011 to zations of mixing in models. March 2012. Repeat glider observations of the MC/MUC provide An example section from the return of mission mean water mass transport and variability during a 4-yr 11A03601 is shown in Fig. 3. Salinity with potential period. Spray gliders made 16 sections of the MC/MUC temperature contours are plotted on depth and density from September 2009 to October 2013 under the Origins surfaces as a function of along-track distance from the of Kuroshio and Mindanao Current (OKMC) initiative nearest position to the coast at 9.38N, 126.318Eto (Rudnick et al.
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