Ocean variability east of Mindanao: Mooring ... - Wiley Online Library

PUBLICATIONS Journal of Geophysical Research: Oceans RESEARCH ARTICLE 10.1002/2015JC010703 Key Points: Mindanao Undercurrent was not confirmed at the 7 N line Weak Mindanao Current during 2011–2012 Westward propagation signals with intraseasonal periods along 7 N

Correspondence to: Y. Kashino, [email protected] Citation: Kashino, Y., I. Ueki, and H. Sasaki (2015), Ocean variability east of Mindanao: Mooring observations at 7 N, revisited, J. Geophys. Res. Oceans, 120, 2540–2554, doi:10.1002/ 2015JC010703. Received 6 JAN 2015 Accepted 9 MAR 2015 Accepted article online 13 MAR 2015 Published online 2 APR 2015

Ocean variability east of Mindanao: Mooring observations at 7 N, revisited Yuji Kashino1, Iwao Ueki2, and Hedeharu Sasaki3 1 Center for Earth Information Science and Technology, Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan, 2Research and Development Center for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan, 3Application Laboratory, Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan

Abstract Two subsurface moorings were deployed east of Mindanao Island, the Philippines, at 7 010 N, 126 550 E and 7 010 N, 127 460 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 s21 at 300 m depth and of 60–72 days and 0.11–0.14 m s21 at 960 m depth.

1. Introduction In the 15 years between 1985 and 2000, the Pacific low-latitude western boundary currents (LLWBCs) received special attention because of their important role in supplying water to the warm water pool [Lukas et al., 1996]. Therefore, several ocean observation cruises were carried out during this period under the Western Equatorial Pacific Ocean Circulation Study (WEPOCS), the United States-People’s Republic of China Cooperative Studies (US-PRC), and the World Ocean Circulation Experiment (WOCE) projects with the aim of observing the Pacific LLWBCs [e.g., Lukas et al., 1991; Toole et al., 1990; Kashino et al., 1996]. Through these observations, a zero-order (mean) description of the Pacific LLWBCs was completed [Lukas et al., 1996]. The interest of many tropical oceanographers shifted to the Indian Ocean when the Indian Ocean Dipole event (IOD) was found in 1999 [Saji et al., 1999]. Subsequently, observational researches of the Pacific LLWBCs were mostly carried out under the Tropical Ocean Climate Study (TOCS) project [e.g., Kashino et al., 2001, 2005, 2007, 2009; Kuroda, 2000; Ueki et al., 2003] of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), until the North Pacific Ocean Circulation and Climate Experiment (NPOCE) [Tang et al., 2013] and the Southwest Pacific Ocean Circulation and Climate Experiment (SPICE) [Ganachaud et al., 2014] projects were launched in the 2010s. Despite these shifts, the first-order description of the Pacific LLWBCs was improved during the decade of 2000–2010 by the TOCS project. For example, the first mooring observations of the Mindanao Current (MC) were carried out near the Mindanao Coast (6 500 N, 126 430 E) during 1999–2002 under the TOCS project, and its intraseasonal, seasonal and interannual variations were described by Kashino et al. [2005]. Kuroda [2000], Ueki et al. [2003], and Kashino et al. [2007] described the New Guinea Coastal Current/Undercurrent (NGCC/NGCUC) using data from the moorings deployed at 2.5 S, 142 E, and others near the New Guinea coast. From onboard observations, the retroflection of the MC in the Celebes and Maluku Seas was shown by Kashino et al. [2001]. Changes in the North Equatorial Current (NEC), MC, and Kuroshio Current System between the ~o and the 2007/2008 La Nin ~a were discussed by Kashino et al. [2009]. 2006/2007 El Nin C 2015. American Geophysical Union. V

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Despite these advances via the TOCS project, however, the Mindanao Undercurrent (MUC) has remained a point of controversy. The MUC, which is thought to flow northward below a depth of 300 m east of

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Mindanao, was first reported by Hu et al. [1991]. They showed that this current had a double-core structure with speed of 20–30 cm s21 using three geostrophic current velocity sections along the 7.5 N line east of Mindanao, and its core existed 80–200 km off the Mindanao coast (127 E–128.5 E). Qu et al. [1998] showed by geostrophic calculation that the permanent MUC was observed along 8 N line with a speed of >0.08 m s21 at about 900 dbar and 75–100 km off shore. They also commented that the MUC may be mainly a component of local recirculation associated with the Halmahera Eddy (HE). In contrast to the above geostrophic calculation results, some direct current measurements did not support the existence of a permanent northward undercurrent in this region. For example, Wijffels et al. [1995] did not find the MUC by eight surveys of shipboard Acoustic Doppler Current Profiler (ADCP) along 8 N. Firing et al. [2005] first found subthermocline eddies east of Mindanao below 500 m depth rather than the MUC using data from shipboard and lowered ADCPs during two TOCS cruises in this region. Recent onboard observation results [Kashino et al., 2013] also showed that the northern tip of an anticyclonic eddy, which was the Halmahera Eddy, reached the 7 N line below 300 m depth east of Mindanao and they concluded that no stationary northward undercurrent was found at that latitude. In recent years, several scientists described the MUC using numerical simulation results [Qu et al., 2012], mooring observation results [Zhang et al., 2014], and geostrophic calculation results from climatological data [Wang et al., 2015] (hereafter, we refer these three papers as ‘‘QZW’’). Results from the first two studies showed that the MUC is highly variable relative to the mean flow. Qu et al. [2012] suggested a doublecore structure of the MUC at 126.8 E (inshore core) and 127.7 E (offshore core) along 7 N, similar to Hu et al. [1991]. However, they missed the zonal component of the current velocity along this line; according to their Figure 2a, the MUC as they call it, seems to be part of the anticyclonic eddies east of the Philippines. Zhang et al. [2014] termed the northward flows of the western part of these anticyclonic eddies the MUC in reference to the simulation results by Qu et al. [2012] because the northward flow of the western part of the eddies was stronger than the southward flow of the eastern part of the eddies. This flow bears little resemblance, however, to the original definition of the MUC proposed by Hu et al. [1991], which has a double-core structure with a speed of 20–30 cm s21 and flows 80–200 km away from off the Mindanao coast. Zhang et al. [2014] also suggested a northward mean flow below 600 m depth flow from mooring results at 8 N, 127 30 E and also called this mean flow the MUC, but the vertical structure and magnitude of this mean flow are an artifact of the sampling. The curve bends sharply near 800 m because the deeper measurements are available only from a small and unrepresentative fraction of the whole record (compare their Figures 2a, 2c, and 4). Furthermore, Zhang et al. [2014] did not observe the offshore core of the MUC, which is supposedly stronger than the inshore core [Qu et al., 2012]. Wang et al. [2015] indicated that they had confirmed the MUC from multisections of geostrophic velocity, but the MUC shown in Wang et al. [2015] also appears to be part of eddies according to their maps of acceleration potential (see their Figure 6), and they missed the possibility of eddies because, like Qu et al. [2012], they did not consider the zonal velocity component. Thus, we think that the MUC proposed by Hu et al. [1991] is not still confirmed by their studies. The MUC is thought to be important for the ocean circulation in the intermediate layer of the Pacific because it is possible that the Antarctic Intermediate Water (AAIW) is advected to 15 N along the east coast of the Philippines by this current [Qu and Lindstrom, 2004]. Therefore, features of the MUC should be clarified from observations; in particular, it should be determined whether this is a permanent current by time series observations at the MUC location. Before Zhang et al. [2014], Kashino et al. [2005] found intraseasonal variability which seems to be associated with ocean intrinsic eddies at 700 m depth at the MC axis. However, they could not determine whether the MUC is a permanent current because its location appears to be too near the Mindanao coast. Like Zhang et al. [2014], they did not also carry out mooring observations at the location of the offshore core of the MUC. The NPOCE program was recently endorsed by Climate Variability and Predictability (CLIVAR) to understand ocean circulation in the northwestern Pacific and its role in low-frequency modulations of regional and global climate [Tang et al., 2013]. Observations of the Pacific LLWBCs below the thermocline are important parts of this program. In part to address the controversy regarding the MUC, we again deployed two subsurface moorings with an ADCP and five or six single-point current meters east of Mindanao at the 7 N line from 2011 to 2013 in collaboration with the NPOCE project. In this paper, we report the mooring results focusing on the MUC.

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The following section presents the data source used in this study. In section 3, we describe mooring observation results and their analysis. We discuss observed MC weakening, intraseasonal variability, and the MUC using sea surface height (SSH) data from satellite measurement and numerical simulation results, which Qu et al. [2012] and Zhang et al. [2014] used, in section 4. A summary is provided in section 5.

2. Data Source We deployed two subsurface moorings at 7 010 N, 126 550 E (Mindanao West (MIW), water depth: 4826 m) and at 7 010 N, 127 460 E (Mindanao East (MIE), 5833 m), where straddle the Philippine Trench (Figure 1). The MIW mooring was located near the axis of the MC, 51 km from the Mindanao coast. This position was near the location of the mooring of Kashino et al. [2005]; the distance between these moorings was 30 km. Because of its location, it was expected that not only the MC but also the inshore core of the MUC (P1 of Figure 3a of Qu et al. [2012]) would be observed at the MIW location. In contrast, the MIE was located 145 km from the Mindanao coast, and it was expected that the offshore core of the MUC shown by Qu et al. [2012] (P2 of Figure 3a of Qu et al. [2012]) would be observed by this mooring. The distance between MIW and MIE was 94 km. Figure 1. Map of bottom topography near Mindanao Island and mooring sites of Mindanao West (MIW) and Mindanao East (MIE) denoted by yellow stars.

Mooring information is summarized in Table 1. We used data from an upward-looking ADCP (RD Instrument) with a 75 kHz frequency and a conductivity–temperature–depth profiler (CTD, SBE-16) installed at a depth of 350 m. We also used data from single-point current meters installed at depths of 560 m (DW-Aquadopp, JFE Advantech Co. Ltd.), 960 m (RCM-9, Aanderaa Instruments), and 1460 m (3D-ACM, Falmouth Scientific, Inc.) at MIW, and at 960 m (RCM-9) at MIE. These moorings were deployed on 10 (MIE) and 11 September 2011 (MIW) during the MR11-06 cruise of R/ V Mirai. The mooring at MIW was successfully recovered on 16 March 2013 during the MR13-01 cruise. Although observations by the 3D-ACM at 1460 m depth stopped before its recovery, data from the ADCP, CTD, DW-Aquadopp, and RCM-9 of the MIW mooring were acquired until their recovery. The mooring line of the MIE was cut in October 2012. Fortunately, it was recovered at sea by Philippine fishermen in January 2013. We used only data from the ADCP, and RCM-9 at a depth of 960 m at MIE. Because data between depths of these instruments at MIE were not available, the velocity maximum of the offshore core of the MUC was probably missed. However, as shown in section 3, patterns of current variations at 300 and 960 m depths were similar, and Zhang et al. [2014] and Wang et al. [2014] showed that vertical coherent structure was observed in current variability below the thermocline at 8 N, 127.03 E. Therefore, we think that it is not serious concern. We used daily data after tidal signal removal using a 48 h tide-killer filter [Hanawa and Mitsudera, 1985] based on the analysis of Thompson [1983]. For the averaging shown in Figures 2 and 3, we used data from a

Table 1. Information on Moorings Deployed East of Mindanao MIW Location Water depth Instrument type, mean depth of instrument, and measurement period

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ADCP/CTD Aquadopp RCM-9 3D-ACM

7 00.890 N, 126 54.980 E 4826 m 350 m 11 Sep 2011–15 Mar 2013 560 m 11 Sep 2011–15 Mar 2013 960 m 11 Sep 2011–15 Mar 2013 1460 m 11 Sep 2011–14 Sep 2012


MIE

ADCP/CTD RCM-9

7 00.610 N, 127 46.120 E 5833 m 350 m 10 Sep 2011–17 Oct 2012 960 m

10 Sep 2011–17 Oct 2012

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period of 1 year, from 17 September 2011 to 16 September 2012. All available time series data were used for coherence/phase analyses shown in section 3. We also used numerical simulation results from the Ocean General Circulation Model for the Earth Simulator (OFES) to compare with the mooring results and discuss the intraseasonal variability and MUC. Model descriptions were given in Masumoto et al. [2004] and Sasaki et al. [2008]. We used the results of 3 day snapshots over the period of 2011–2012 for this paper. The model output from the OFES has also been used in several other studies focused in this region [Qu et al., 2012; Chiang and Qu, 2013; Zhang et al., 2014; Wang et al., 2014]. Chiang and Qu [2013] considered OFES to be one of the best available models for studies in this region. Our results also presently support their suggestion; for example, the northwestward shift of the HE increasing with depth, which was shown by Kashino et al. [2013], is well reproduced in the OFES results (see section 3). We also used sea surface height (SSH) data from Archiving, Validation, and Interpretation of Satellite Oceanographic (AVISO) data to check SSH variability and currents in this region. The data used in this paper have a horizontal resolution of 1/3 on a Mercator grid and a 7 day interval during the period from October 1992 to March 2013.

3. Results Figure 2a shows that the strong southward current, MC, was well captured at the MIW. As in the results shown by Kashino et al. [2005], there was a subsurface velocity maximum at approximately 100 m depth in this current (not shown). It is interesting that the mean current speed of the MC during 2011–2012 is clearly slower than that during 1999–2002 [Kashino et al., 2005]; the mean current speeds at 100 m depth were 0.99 m s21 (our observations) and 1.38 m s21 [Kashino et al., 2005], respectively. The current speed of 0.99 m s21 is in the same order as that at 8 N [Wijffels et al., 1995]; the current velocity of the MC downstream is increased [Lukas et al., 1991]. We will discuss weakening of the MC in the next section. Although variability of the MC is a little greater than that found by Kashino et al. [2005], the Figure 2a. (left) Stick diagrams of current velocity at depths of 100, 200, 300, 560, 960, and 1460 m, and (right) the averages (arrows) and standard deviations (ellipses) of current velocity at these depths at MIW. Sticks are plotted every 2 days. Small circle in the right plots denotes the tail of mean velocity vector.

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MC above 200 m depth was a stable current because of the small variability compared with the mean velocity. There was not a stable southward current associated with the MC below 560 m depth at MIW. Mean current speeds (direction) from 17 September 2011 to 16 September 2012 at the depths of 560, 960, and 1460 m were 0.06 m s21 (southeastward), 0.05 m s21 (northeastward), and 0.03 cm s21 (southeastward), respectively. These speeds were much smaller than the variations, as reported by Kashino et al. [2005]; again, we could not find a steady northward current (MUC) at this position, the location of the inshore core of the MUC [Qu et al., 2012]. Rather than a steady current, dominant intraseasonal variability with periods of 50–100 days was observed as it was by Kashino et al. [2005]. This result also agrees with Qu et al. [2012] and Zhang Figure 2b. Same as Figure 2b but at depths of 100, 200, 300, and 960 m at MIE. et al. [2014]. Current variability was large along the direction of the MC even below 560 m depth. Patterns of current variability below 560 m were similar to that at the depth of 960 and 1460 m, that is, vertical coherent pattern in the current variability; this result seems to differ from Zhang et al. [2014], who showed a bend in the vertical profile of mean meridional velocity component and standard deviation [see Zhang et al., 2014, Figure 2a] and reported that the northward mean flow below 800 m depth was the MUC with a speed of 10 cm s21. As already noted, however, this high speed and the corresponding shear near 800 m are an artifact of averaging without taking into account the extensive datagaps at depth. In fact, Figure 4 of Zhang et al. [2014] shows vertical coherent pattern in the current variability below 600 m depth. Therefore, we consider our result not inconsistent with Zhang et al. [2014]. It should be noted that current variability at 1460 m depth was similar to that at 960 m depth, i.e., large intraseasonal variability with a maximum current velocity of 0.4 m s21 was observed. This result indicates that a large error would be involved in geostrophic calculations for snapshot observations in this region [e.g., Hu et al., 1991]; even at a reference level below 1000 m, there may be substantial intraseasonal velocity, and internal tides may perturb the dynamic height profile [Firing et al., 2005]. At MIE, the current was highly variable during the first half of the record at 100 m depth, suggesting that the MC was not consistently present there during this period (Figure 2b, left). After that, a strong southeastward flow with a velocity exceeding 0.5 m s21, which seems part of the MC, was observed. The southward flow associated with the MC was not observed at or below 300 m depth at this location. The magnitude of variability was the same order as that at MIW, and the main axis of the variability ellipse was oriented northeast–southwest above 300 m, but there was no directivity in the current variability at 960 m. As described in section 2, MIE is located at the proposed offshore core of the MUC. According to Qu et al. [2012], the velocity maximum of the offshore core of the MUC is at 600–700 m depth; our observations missed current variability at this depth due to instrument troubles. However, because patterns of current variability at 300 and 960 m depths at MIE were similar (correlation for meridional velocity component between these depths exceeds 0.5), we think that the current variability at 600–700 m depth is not so different from those at 300 and 960 m depths. The mean current speed (direction) at 300 and 960 m depths were 0.09 m s21 (east-northeastward), and 0.05 m s21 (eastward), respectively. The magnitude of standard deviation was comparable to the mean speed at 300 m depth, but it was double the mean speed at 960 m.

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Figure 3. Mean current velocity vectors at depths of (a) 300 m and (b) 960 m at MIE and MIW superimposed on the OFES mean velocity map at these depths. These vectors are plotted using averages for the period from 17 September 2011 to 16 September 2012. Red circles denote the locations of MIE and MIW.

The mean speed of the meridional velocity component at 300 m (960 m) depth was 0.04 m s21 (