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Interannual Variability of the Mindanao Current/Undercurrent in Direct Observations and Numerical Simulations

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Interannual Variability of the Mindanao Current/Undercurrent in Direct Observations and Numerical Simulations FEBRUARY 2016 H U E T A L . 483 Interannual Variability of the Mindanao Current/Undercurrent in Direct Observations and Numerical Simulations SHIJIAN HU AND DUNXIN HU Institute of Oceanology, and Key Laboratory of Ocean Circulation and Wave, Chinese Academy of Sciences, and Laboratory for Ocean and Climate Dynamics, Qingdao National Laboratory for Marine Science and Technology, Qingdao, China CONG GUAN Institute of Oceanology, and Key Laboratory of Ocean Circulation and Wave, Chinese Academy of Sciences, Qingdao, and University of Chinese Academy of Sciences, Beijing, China FAN WANG,LINLIN ZHANG,FUJUN WANG, AND QINGYE WANG Institute of Oceanology, and Key Laboratory of Ocean Circulation and Wave, Chinese Academy of Sciences, and Laboratory for Ocean and Climate Dynamics, Qingdao National Laboratory for Marine Science and Technology, Qingdao, China (Manuscript received 18 May 2015, in final form 30 November 2015) ABSTRACT The interannual variability of the boundary currents east of the Mindanao Island, including the Mindanao Current/Undercurrent (MC/MUC), is investigated using moored acoustic Doppler current profiler (ADCP) measurements combined with a series of numerical experiments. The ADCP mooring system was deployed east of the Mindanao Island at 78590N, 127830E during December 2010–August 2014. Depth-dependent in- terannual variability is detected in the two western boundary currents: strong and lower-frequency variability dominates the upper-layer MC, while weaker and higher-frequency fluctuation controls the subsurface MUC. Throughout the duration of mooring measurements, the weakest MC was observed in June 2012, in contrast to the maximum peaks in December 2010 and June 2014, while in the deeper layer the MUC shows speed peaks circa December 2010, January 2011, April 2013, and July 2014 and valleys circa June 2011, August 2012, and November 2013. Diagnostic analysis and numerical sensitivity experiments using a 2.5-layer reduced- gravity model indicate that wind forcing in the western Pacific Ocean is a driving agent in conditioning the interannual variability of MC and MUC. Results suggest that westward-propagating Rossby waves that generate in the western Pacific Ocean (roughly 1508–1808E) are of much significance in the interannual variability of the two boundary currents. Fluctuation of Ekman pumping due to local wind stress curl anomaly in the far western Pacific Ocean (roughly 1208–1508E) also plays a role in the interannual variability of the MC. The relationship between the MC/MUC and El Niño is discussed. 1. Introduction the El Niño–Southern Oscillation (ENSO) cycle and in regulating the western Pacific warm pool that is a key Low-latitude western boundary currents (LLWBCs) in factor in the tropical Pacific Ocean (Hu and Cui 1991; the Pacific Ocean are of much importance in the tropical Lukas et al. 1996; Jin 1997; Hu and Hu 2012; Hu et al. climate system for their role in the recharge/discharge of 2015). The Mindanao Current (MC) and the underlying Mindanao Undercurrent (MUC) are remarkable com- ponents of the LLWBCs in the Pacific Ocean (Hu and Corresponding author address: Shijian Hu, Institute of Ocean- ology, Chinese Academy of Sciences, 7 Nanhai Road, Qingdao Cui 1989). They are suggested to be important in the 266071, China. global thermohaline circulation for their significant con- E-mail: [email protected] tribution to the Indonesian Throughflow (ITF) that DOI: 10.1175/JPO-D-15-0092.1 Ó 2016 American Meteorological Society Unauthenticated | Downloaded 10/04/21 07:08 AM UTC 484 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 46 connects different ocean basins (e.g., Gordon and Fine suggested that ‘‘the stationary northward undercurrent, 1996; Sprintall et al. 2014). The MC is also a crucial the Mindanao Undercurrent, was also not found at 78N pathway between the midlatitudes and equatorial region east of Mindanao’’ on the basis of observations shallower and influences the interdecadal climate variability (e.g., than 600 m (thus shallower than the MUC layer). Gu and Philander 1997). The significance of LLWBCs Kashino et al. (2015,p.1)proposedthatthe‘‘Mindanao in the Pacific Ocean has led to international research ef- Undercurrent was not confirmed at the 78N line,’’ but the forts such as the Northwestern Pacific Ocean Circulation measurements in that paper are confined to the upper and Climate Experiment (NPOCE; Hu et al. 2011)and 350-m layer and depths of 560, 960, and 1460 m, which Southwest Pacific Ocean Circulation and Climate Exper- expectedly might miss the MUC core. Lack of enough iment (SPICE; Ganachaud et al. 2014). observations gives rise to some studies using model out- The MC and MUC are of particular interest to the puts (e.g., Kashino et al. 2015), but unfortunately few of oceanography community for their spatial complexity these simulations can be validated by observations. Re- (e.g., Hu and Cui 1989, 1991; Wijffels et al. 1995; Wang cently, Qiu et al. (2015) pointed out that a time-mean et al. 2015) and multi-time-scale variabilities (e.g., Lukas MUC is observed from 68 to 138N and extends from the 1988; Qiu and Lukas 1996; Tozuka et al. 2002; Kashino about 400- to 1200-m layer at 68N to the about 800- to et al. 2005, 2009, 2011; Zhang et al. 2014). On an in- 1200-m layer at about 128N, on the basis of 14-yr Argo terannual time scale, Lukas (1988) found that fluctua- float profiling data (obviously with depth up to 2000 m) tions of the MC transport are at least 50% of the mean from 2001 to 2014. The study by Qiuetal.(2015)together transport, with a period of 2 yr, but have no apparent with other previous observations seems to confirm that relationship to the strength of ENSO, on the basis of sea the MUC is a mean flow along the east coast of Mindanao level observations at the islands of Mindanao in the Island (e.g., Zhang et al. 2014; Hu et al. 2015). Philippines and Malakal in Palau. Using hydrographic Previous studies have tremendously facilitated our observations from eight cruises, Wijffels et al. (1995) understanding of the structure and variability of these suggested that the MC is a stable coastal jet with a currents. However, in a situation of a lack of sustained 2 maximum speed of 1 m s 1 and speculated that the measurements of these boundary currents, the feature variation of MC is related to the eddy or meanderlike and mechanism of the interannual variability of MC are anomalous circulation. Qiu and Lukas (1996) pointed still controversial. For example, we do not know much out that the interannual variability of the MC is influ- about the vertical feature of the interannual variability, enced by both the ENSO wind on a time scale of 3–7 yr and it is unclear yet what the relative contributions are and the quasi-biennial wind that is confined to the from remote forcing and local wind forcing. Geostrophic tropical gyre of the North Pacific. Kashino et al. (2005) calculation in the MC/MUC region is possibly of non- examined the variability of the MC using mooring ob- ignorable error, and thus direct observations of the un- servations conducted east of the Mindanao Island and dercurrents in this region are extremely needed. The suggested that the MC was enhanced during the onset of interannual variability of MUC in the direct observa- 2002 El Niño. tions is even nearly unknown yet. With regard to the MUC, direct observations suggest Here, we present the direct observations of the MC that the MUC around 88N is probably below 600 m to and MUC from December 2010 to August 2014 with the depth deeper than 1000 m and features significant emphasis on an interannual time scale. Spatial and intraseasonal variability (Wang et al. 2014; Zhang et al. temporal features of the mean current and interannual 2014; Qiu et al. 2015; Wang et al. 2015). It seems that the variability will be described in sections 2 and 3. Dynamic MUC is a relatively weak northward continuous mean mechanisms about the MC/MUC interannual variability flow with a mean speed of several centimeters per sec- will be explored in sections 4 and 5 by combining dy- ond, possesses a double-core structure [a major inshore namic diagnostic analysis and sensitivity numerical ex- core and a secondary offshore core (e.g., Qiu et al. periments using a 2.5-layer reduced-gravity model. 2015)], and probably is related to the subthermocline Section 6 will discuss the relationships between the eddies. But debate persists as to if it is a transient currents and ENSO and between currents and in- or permanent current (Hu and Cui 1989, 1991; terannual modulation of mesoscale subthermocline Lukas et al. 1991; Wijffels et al. 1995; Wang and Hu eddies. Results will be summarized in section 7. 1998; Qu et al. 1998; Firing et al. 2005; Kashino et al. 2005; Qu et al. 2012; Kashino et al. 2015). This debate 2. Observations and mean structure might be due to the absence of enough observations of the MUC, especially the shortage in the depths of mea- To monitor the boundary currents east of Mindanao surements. For example, Kashino et al. (2013, p. 1) Island, a subsurface mooring has been sequentially Unauthenticated | Downloaded 10/04/21 07:08 AM UTC FEBRUARY 2016 H U E T A L . 485 FIG. 1. Topography (color) around the subsurface mooring (black dot) and mean current during 2011–13 in the upper 410-m layer from the ECCO2 dataset. Major surface currents, including the NEC, NECC, ITF, and the MC, are labeled. deployed and recovered at 78590N, 127830E since De- documented methods (Hu et al. 2013). All the time cember 2010 (Fig. 1), as a part of the observation project series are low-pass filtered with a cutoff period of 3 days of the NPOCE.
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