PUBLICATIONS. Journal of Geophysical Research: Oceans. Float observations of an anticyclonic eddy off Hokkaido RESEARCH ARTICLE 10.

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1 PUBLICATIONS RESEARCH ARTICLE Key Points: 16 floats were deployed within a warm-core anticyclonic eddy from June to December 2013 The evolution of an anticyclonic eddy was examined by using time series of the water properties The interaction of an eddy with a front might be important for modifying the water properties of the eddy Correspondence to: R. Inoue, rinoue@jamstec.go.jp Citation: Inoue, R., V. Faure, and S. Kouketsu (2016), Float observations of an anticyclonic eddy off Hokkaido, J. Geophys. Res. Oceans, 121, , doi:. Received 3 FEB 2016 Accepted 8 JUL 2016 Accepted article online 13 JUL 2016 Published online 20 AUG 2016 VC American Geophysical Union. All Rights Reserved. Float observations of an anticyclonic eddy off Hokkaido Ryuichiro Inoue 1, Vincent Faure 1, and Shinya Kouketsu 1 1 Research and Development Center for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan Abstract To clarify the formation process of the salinity minimum in the Kuroshio Oyashio mixed water region and understand the mechanism of meridional heat transport between the subtropical and subpolar gyres, 16 profiling floats were deployed within a warm-core anticyclonic eddy off Hokkaido from June to December Then, the evolution of an anticyclonic eddy was examined using time series of the water properties. The largest fluctuations in water properties were observed in April and May 2013, when the anticyclonic eddy first moved south to interact with a warm front, then back north. Salinity in the salinity minimum layer increased during the interaction. After the eddy detached from the frontal structure, low-salinity water was again observed with small intrusive structures, which eventually converged to a smooth zigzag structure in the potential temperature-salinity diagram, suggesting that both vertical mixing and vertical heaving played a role in the temporal changes observed after the eddy detached from the front. Since the salinity variation during the interaction event was about half the total salinity change during the whole experimental period, the interaction of an eddy with a front might be important for modifying the water properties of the eddy, and, therefore, for the meridional transport of heat and fresh water. 1. Introduction The Oyashio and Kuroshio are the western boundary currents of the subpolar and subtropical gyres in the North Pacific where the Kuroshio in east of Japan is called the Kuroshio Extension. The confluence of the cold fresh Oyashio and warm saline Kuroshio waters east of Japan [e.g., Yasuda, 2003] is called the mixed water region (MWR) and is the formation region of the North Pacific Intermediate Water (NPIW) [e.g., Talley, 1993; Yasuda et al., 1996; Yasuda, 1997] and Transition Region Mode Water (TRMW) [e.g., Saito et al., 2011]. It is one of the richest fishing grounds in the world [e.g., Uda, 1936]. In the MWR, anticyclonic and cyclonic eddies detached from these currents are common [Sugimoto et al., 1992; Yasuda et al., 1992; Itoh and Yasuda, 2010a], and they play an important role in the stirring of the Kuroshio and Oyashio waters. Although open-ocean eddies generally propagate westward and transport heat and salt zonally [e.g., Dong et al., 2014], in the MWR, anticyclonic eddies (ACEs) frequently propagate northward along the eastern coast of Japan [Itoh and Yasuda, 2010a]. Since these ACEs often contain warm, saline Kuroshio water in their core, their northward propagation can potentially contribute to meridional heat and salt transport [Sugimoto et al., 1992; Yasuda et al., 1992; Itoh and Yasuda, 2010a]. On the basis of sea surface height (SSH) and sea surface temperature (SST) data, Itoh and Yasuda [2010a] estimated that the total heat transport around N latitude by northward moving eddies had a significant contribution (20 30%) to the total meridional heat transport over the entire area of the North Pacific. One consequence of this meridional heat transport is that TRMW, which is mainly formed in the deepest mixed layer in the western North Pacific, north of the Kuroshio Extension, is also observed in the warm cores of ACEs in the MWR [Oka et al., 2014]. Furthermore, ACEs interact with other eddies propagating from the east [Yasuda et al., 1992]. Intrusions and blobs of different water masses are often observed around an eddy during these interactions [e.g., Yasuda, 2003]. Using Argo float observations, Itoh and Yasuda [2010b] found that relatively fresh water was often present below the warm and salty core of ACEs, possibly owing to interactions between cold and warm eddies. Itoh et al. [2014] and Kaneko et al. [2015] reported large, abrupt water property changes in an eddy core on the basis of data from a few profiling floats and shipboard measurements across the eddy. As indicated by Itoh and Yasuda [2010b], these interactions might eventually enhance the vertical contrast between upper saline and lower fresh waters. In addition, numerical models have suggested that these interactions and INOUE ET AL. ANTICYCLONIC EDDY OBSERVATION 6103

2 Figure 1. Diagrams illustrating interleaving structures observed during ACE-INBOX. (a, c) h S diagram and (b, d) diapycnal spiciness curvature (s rr ) observed (a, b) near the center of ACE A on 2 March 2013 and (c, d) at the periphery of ACE A on 6 May The peaks of the interleaved layers are indicated by red- and blue-filled circles. The vertical and horizontal lines in Figures 1b and 1d represent the scale and amplitude, respectively, of the intrusions defined by the multiscale peak analysis. subsequent eddy advection of the lower fresh water may be important for the formation of NPIW, which is characterized by a vertical salinity minimum in the North Pacific subtropical gyre [Ishizaki and Ishikawa, 2004; Ishikawa and Ishizaki, 2009]. To clarify the water mass and salinity minimum modifications within an eddy, long-term temporally and spatially dense observations of the eddy core are necessary. In this study, we examined the evolution of an anticyclonic eddy off Hokkaido (hereafter called ACE A), including changes in its shape, movements, and water properties, from June to December During this period, multiple profiling floats were trapped within ACE A. We also investigated the temporal and spatial changes of vertical finestructure associated with ACE A evolution and how they modified the water mass within the ACE A. We also note that our description could be useful to study other part of oceans where the importance of meridional heat transport by eddies has been reported [e.g., Fratantoni et al., 1995]. 2. Data and Method 2.1. ACE-INBOX Profiling Floats As the second component of the western North Pacific Integrated Physical-Biogeochemical Ocean Observation Experiment (INBOX) [e.g., Figure 1 in Inoue et al., 2016], 16 profiling floats with dissolved oxygen (DO) sensors were deployed to circulate within a warm-core ACE beginning in June (ACE-INBOX). The ACE-INBOX profiling floats were released during five hydrographic cruises between June and July 2013 as close to the ACE A center as possible given the weather conditions. On 5 6 June, during cruise MR12-02, we launched one Navigating European Marine Observer (NEMO) float (Optimare, Bremerhaven, Germany) and eight Autonomous Profiling Explorer () floats (Teledyne Webb Research, Falmouth, MA, USA). Each float was equipped with an SBE41 or SBE41CP conductivitytemperature-depth (CTD) sensor (Sea-Bird Electronics, Inc., Bellevue, WA, USA) and an Oxygen Optode 3830 or 4330 sensor (Aanderaa Data Instruments, Nesttun, Norway). On 29 August, during cruise WK1208, four floats equipped with SBE41CP and Oxygen Optode 4330 sensors (all floats subsequently INOUE ET AL. ANTICYCLONIC EDDY OBSERVATION 6104

3 deployed had these sensors) and two Deep Ninja floats equipped with SBE41CP sensors (Tsurumi Seiki Co. Ltd., Yokohama, Japan) [Kobayashi et al., 2013] were deployed in smaller ACE B to the west of ACE A to track eddy merger. On 24 January (during cruise HK1301), 5 April (cruise KH13-3), 26 July 2013 (cruise MR13-04), and 5 October 2013 (cruise HK1310), four, two, and one and one EM- float equipped with electro-magnetic sensors to measure ocean currents [Sanford et al., 2005], respectively, were deployed. We used CTD data from all of the DO floats trapped in ACE A in this study. Floats SN6202 and 6205 (Table 1) were reported in Kaneko et al. [2015]. During ACE-INBOX, the profiling floats drifted at one of several parking depths, then descended to the profiling depth (Table 1). During ascent from the profiling depth to the sea surface, floats measured CTD every 2 dbar, except for one NEMO float typically at 5 dbar. Each float stayed at the sea surface for less than 1 h (the time-out period), during which time data were transmitted via the Iridium satellite system. Then, the float returned to its parking depth and the same procedure was repeated. The sampling interval of the floats was usually once a day, but it was occasionally changed after the float was launched via the two-way Iridium system (Table 1). When floats left ACEs A or B, their profiling mode was changed to be consistent with the International Argo Program, namely a parking depth of 1000 dbar, and a profiling depth of 2000 dbar, repeated every 10 days. The last float, SN6207, left ACE A in early November Eddy Center and Ellipse Parameters We used the eddy center as a reference position to compute the eddy characteristics. The position of the ACE A center was estimated in two ways, first as the location of the local SSH maximum, and second by a calculation based on the trajectories of the profiling floats trapped in ACE A. In this study, we used a daily absolute SSH product provided by the Segment Sol multimissions d AlTimetrie, d Orbitographie et de localisation precise/data Unification and Altimeter Combination System (SSALTO/DUACS) and distributed by the Archiving, Validation, and Interpretation of Satellite Oceanographic Data (AVISO) project, with support from the Centre National d Etudes Spatiales (CNES) of France [Ducet et al., 2000]. The local SSH maximum associated with ACE A was tracked backward and forward in time from the first float deployment. The size of ACE A was also estimated from the SSH, which was defined as the shortest distance between two local speed maxima of geostrophic currents on the ACE A estimated from the SSH. Here, the maximum of the rotational speed of ACE A was selected and another maximum selected so that ACE A center was placed on the diagonal line of those maxima. We defined the average of these maxima as the azimuthal velocity. The ellipse linearity, k 5 (a 2 b 2 )/(a 2 1 b 2 ), where a is the semimajor axis and b is the semiminor axis; the major axis orientation u is defined as the angle between the major axis and the eastward direction are computed by fitting ellipses to a single SSH contour. Here, the single SSH contour was selected to match the average SSH value along a 30 km radius circle (approximately corresponding to the first baroclinic Rossby Radius of deformation [Chelton et al., 1998]). We also used the daily satellite SST maps from the Operational SST and Sea Ice Analysis (OSTIA) [Donlon et al., ] provided by the Group for High Resolution Sea Surface Temperature (GHRSST) to characterize ACE A and the ocean conditions. Float-based center positions were estimated from the float trajectories by using an ellipse decomposition algorithm [Lilly and Gascard, 2006; Lilly et al., 2011], in which the orbit of a float trapped within an eddy is assumed to be a time-evolving ellipse. To determine the slowly varying properties of the ellipse, a wavelet analysis is applied to each trajectory coordinate, and then the wavelength associated with rotating motion is extracted from the wavelet spectrum. Constraints embedded in the algorithm ensure that ellipse properties vary slowly in time. By fitting ellipses to each float trajectory, daily ellipse characteristics, such as k and u, can be computed. If it is assumed that the three ellipse parameters (eddy center, k, and u) computed for each float are independent estimates, then the individual time series can be averaged to produce a time series of the mean ACE A parameters. We found, however, that the ellipse decomposition algorithm lost the eddy center during eddy front interactions. Therefore, in this study we used the SSH method to track the eddy center over the whole period and showed the ellipse method for comparison Spiciness Diapycnal Curvature Calculation To investigate the interleaving structures found in the ACE A, we evaluated the diapycnal curvature of spiciness s rr [Shcherbina et al., 2009], the second derivative of spiciness, s, with respect to potential density along a vertical profile. Spiciness [e.g., Flament, 2002], defined as its differential INOUE ET AL. ANTICYCLONIC EDDY OBSERVATION 6105

4 Table 1. Float Serial Number (SN) and Type, Deployment Date (yy/mm/dd) and Position, Deployment Cruise Name, Total Number of Profiles on 31 December 2013, and the Parking and Profiling Depths and Profiling Cycle SN and Float Type NEMO Deployment Date 06/05 06/05 06/05 06/05 06/05 06/06 06/06 06/06 06/06 08/29 08/29 08/29 Deployment Position (degree) N E N E N E N E N E N E N E N E N E N E N E N E Deployment Cruise Name Profile# at 31 Dec 2013 Parking and Profiling Depths (dbar) and Profiling Cycle a MR , 2010 dbar, 1day (1) 1000, 1000 dbar, 1 day (2 127) 1000, 1000 dbar, 3 days ( ) 1000, 1000 dbar, 1 day ( ) 1000, 2010 dbar, 10 days (225) 1000, 2010 dbar, 5 days (226) 1000, 2010 dbar, 10 days ( ) MR , 2010 dbar, 1 day (0 126) 1000, 2010 dbar, 5 days (127, 128) 1000, 2010 dbar, 1 day (129, 130) 1000, 2010 dbar, 5 days ( ) MR , 2010 dbar, 1 day (1) 1000, 1000 dbar, 1 day (2 127) 1000, 1000 dbar, 3 days ( ) 1000, 1000 dbar, 1 day ( ) 600, 1010 dbar, 1 day ( ) MR , 2010 dbar, 1 day (1) 1000, 1000 dbar, 1 day (2 127) 1000, 1000 dbar, 3 days ( ) 1000, 1000 dbar, 1 day ( ) 600, 1010 dbar, 1 day ( ) 1000, 2010 dbar, 10 days ( ) MR , 2010 dbar, 1 day (1) 1000, 1000 dbar, 1 day (2 127) 1000, 1000 dbar, 3 days ( ) 1000, 1000 dbar, 1 day ( ) 600, 1010 dbar, 1 day ( ) 1000, 2010 dbar, 10 days ( ) MR , 2010 dbar, 1 day (1) 1000, 1000 dbar, 1 day (2 127) 1000, 1000 dbar, 3 days ( ) 1000, 1000 dbar, 1 day ( ) 1000, 2010 dbar, 10 days ( ) 600, 1030 dbar, 1 day( ) 1000, 2010 dbar, 10 days ( ) MR , 2000 dbar, 1 day (1) 1000, 1000 dbar, 1 day (2 127) 1000, 1000 dbar, 3 days ( ) 1000, 1000 dbar, 1 day ( ) 1000, 2000 dbar, 10 days ( ) MR , 2010 dbar, 1 day (1) 600, 600 dbar, 1 day (2 127) 600, 600 dbar, 3 days ( ) 600, 600 dbar, 1 day ( ) 600, 600 dbar, 2 days ( ) 600, 1010 dbar, 2 days ( ) 600, 1030 dbar, 1 day ( ) 600, 1030 dbar, 5 days ( ) 600, 1030 dbar, 10 days ( ) MR , 2010 dbar, 1 day (1) 600, 600 dbar, 1 day (2 127) 600, 600 dbar, 3 days ( ) 600, 600 dbar, 1 day ( ) 600, 600 dbar, 2 days ( ) 600, 1010 dbar, 2 days ( ) 600, 1030 dbar, 1 day ( ) WK , 2010 dbar, 5 days (1) 1000, 1000 dbar, 5 days (2 11) 1000, 1000 dbar, 1 day (12 95) 1000, 2010 dbar, 10 days (96) WK , 2010 dbar, 5 days (1) 1000, 1000 dbar, 5 days (2 11) 1000, 1000 dbar, 1 day (12-128) 1000, 2010 dbar, 10 days (129) WK , 2010 dbar, 2 days (1) 1000, 1000 dbar, 2 days (2 25) INOUE ET AL. ANTICYCLONIC EDDY OBSERVATION 6106

5 Table 1. (continued) SN and Float Type Deep Ninja 2 Deep Ninja EM- Deployment Date 08/29 08/29 08/ / / / / / / / /05 Deployment Position (degree) N E N E N E N E N E N E N E N E N E N E N E Deployment Cruise Name Profile# at 31 Dec 2013 Parking and Profiling Depths (dbar) and Profiling Cycle a 1000, 1000 dbar, 1 day (26 110) 1000, 2010 dbar, 10 days (111) WK , 2010 dbar, 2 days (1) 1000, 1000 dbar, 2 days (2 25) 1000, 1000 dbar, 1 day (26 145) 1000, 2010 dbar, 10 days ( ) 600, 1030 dbar, 1 day (152) 600, 1030 dbar, 10 days ( ) 1000, 2010 dbar, 10 days (205) WK , 4000 dbar, 5 days (1) WK , 3800 dbar, 5 days (1 4) 1000, 3800 dbar, 3 days (5 9) 1000, 4000 dbar, 3 days (10 12) 1000, 2000 dbar, 3 days (13) 1000, 4000 dbar, 3 days (14) 1000, 2000 dbar, 3 days (15) 3500, 4000 dbar, 3 days (16) 1000, 2000 dbar, 3 days (17) 4000, 4000 dbar, 3 days (18 19) 1000, 2000 dbar, 10 days (20 21) HK , 1010 dbar, 1 day (1 22) 600, 2010 dbar, 1 day (23 25) 600, 1030 dbar, 1 day (26 187) 600, 2010 dbar, 1 day (188, 189) 600, 1030 dbar, 1 day ( ) 1000, 2000 dbar, 5 days (316) 600, 1030 dbar, 1 day ( ) 1000, 2010 dbar, 10 days ( ) 600, 1030 dbar, 1 day ( ) 1000, 2010 dbar, 10 days (352) HK , 1010d bar, 1 day (1 22) 600, 2010 dbar, 1 day (23 25) 600, 1030 dbar, 1 day (26 59) 600, 2010 dbar, 1 day (60 64) 600, 1030 dbar, 1 day (65 187) 600, 2010 dbar, 1 day (188, 189) 600, 1030 dbar, 1 day ( ) 1000, 2000 dbar, 10 days (283 ) HK , 1010 dbar, 1 day (1 22) 600, 2010 dbar, 1 day (23 25) 600, 1030 dbar, 1 day (26 85) 1000, 2000 dbar, 10 days (86) HK , 1010 dbar, 1 day (1 22) 600, 2010 dbar, 1 day (23 25) 600, 1030 dbar, 1 day (26 187) 600, 2010 dbar, 1 day (188, 189) 600, 1030 dbar, 1 day ( ) 1000, 2010 dbar, 5 days ( ) 1000, 2010 dbar, 10 days (322) KH , 2010 dbar, 1 day (1 4) 500, 1030 dbar, 1 day (5 115) 500, 2010 dbar, 2 days (116, 117) 500, 1030 dbar, 1 day ( ) KH , 2010 dbar, 1 day (1 4) 500, 1030 dbar, 1 day (5 115) 500, 2010 dbar, 2 days (116, 117) 500, 1030 dbar, 1 day ( ) 1000, 2010 dbar, 10 days (265) MR , 2010 dbar, 2 days (1,2) 600, 1030 dbar, 1 day (5 115) 600, 2010 dbar, 1 day (391, 392) 600, 1030 dbar, 1 day ( ) 1000, 2010 dbar, 10 days ( ) HK dbar Continuous mode a Numbers in parenthesis are profile numbers. INOUE ET AL. ANTICYCLONIC EDDY OBSERVATION 6107

6 ds5qðadh1bdsþ; (1) is a convenient water property for characterizing the contrast between eddy core water with the surrounding waters. Here, q is density, h is potential temperature, S is salinity, a52q 21 ð@q=@hþ is the thermal expansion coefficient, and b5q 21 ð@q=@sþ is the saline contraction coefficient. Before the curvature was calculated, the profiles were made stable with respect to density by removing density inversions due to either instrument sampling noise or small-scale overturning processes. Each spiciness profile was then interpolated onto density coordinates at a rate of 0.01 kg m 23. s rr was computed using second-order finite differences in a density coordinate system. The use of this coordinate system means that the effects of isopycnal heaving due to internal waves are removed, but intrusions created by low-frequency internal waves shear cannot be specified. A low-pass Blackman filter with a half-width of 0.1 kg m 23 (10 grid points) was applied in the diapycnal direction to minimize a noise effect before the second derivative s rr was calculated [Shcherbina et al., 2009]. A density difference of 0.01 kg m 23 was achieved for 97.38% (57.37%) of pressures larger than 2 dbar (6 dbar), suggesting that the vertical resolution of the floats was sufficient to resolve intrusions larger than 0.01 kg m 23, except for one NEMO float which had a typical resolution of 5 dbar. We compared the root mean square (RMS) values of s rr with changes in the water properties of ACE A in the float time series. In the MWR studied here, we expected that the relatively large-scale vertical salinity minimum within the ACE A core around r h kg m 23 and the intrusive structures associated with the mixing of different water masses would lead to large RMS values of s rr. Therefore, to separate the effects of those two phenomena, we also used a multiscale peak analysis [Shcherbina et al., 2009] to obtain the scale and amplitude of s rr peaks, which can be used to characterize water mass structures (Figure 1). The multiscale peak analysis is the method to find robust peaks, which are not sensitive to the observation noise and obtained by tracking back the original peak from the peak of the smoothed profiles (for details, see Shcherbina et al. [2009]). In this analysis, the scale (thickness in the density coordinate) of a peak is defined as the difference in density between a maximum s rr value and the nearest extremum of opposite sign. The amplitude is the difference in s rr between a s rr maximum and the successive s rr minimum (Figures 1b and 1d). A profile with a single salinity minimum on a h S diagram (Figure 1a) will have a vertical profile of s rr characterized by peaks with a broad density range (i.e., a large scale) and a large s rr amplitude (Figure 1b). If the profile on the h S diagram has many intrusive structures (Figure 1c), then the scale of the peaks on the vertical profile will be smaller, though the amplitudes may still be large (Figure 1d). 3. Time Series Data In this section, we begin by examining the interactions between ACE A and large-scale features in the monthly averaged satellite data. Then, we relate those interactions to the changes in water properties occurring on shorter temporal and finer spatial scales within ACE A, as observed by the profiling floats Sea Surface Measurements ACE A was observed around E, N by floats from June to December Before the floats were deployed, in May, ACE A seemed to be detached from a weak local front that stretched from 1468E, 388N to 1508E, 408N (along a 0.4 m SSH contour in Figure 2a); it was moving swiftly northward, as indicated by the track of its central position, and the SST in its core was relatively high (>108C). After the deployment of profiling floats, from June to August, ACE A moved slowly toward the east and northeast. The relatively high SST within ACE A was obscured in August, mainly by strong sea surface heating in the area. Almost all of the floats deployed in June remained trapped in the eddy core through August (Figures 2b 2d). Monitoring of ACE B, in the area west of ACE A around E, 40.58N, was started in September to capture the eddy eddy interaction (Figure 2e). In September ACE A moved swiftly northeastward (Figure 2e), and in October it moved back southwestward at a speed of up to 120 km month 21 (0.05 m s 21 ) and the two eddies were closest together (Figure 2f). During these months, the eddy shape became less circular and several floats were ejected from the ACE A core, but none was exchanged between the two eddies. In November, ACE A began moving slowly eastward again, and the two ACEs began to separate (Figure 2g). During November and December, SST was relatively high along the western edge of ACE A (around 1468E, 41.58E; Figures 2g and 2h). From December to February 2013, ACE A remained relatively stationary (Figures 2h 2j), and it was isolated from local fronts associated with the INOUE ET AL. ANTICYCLONIC EDDY OBSERVATION 6108

7 Figure 2. (a t) Monthly mean SSH (black contours; contour interval, 0.05 m) and SST (color scale) from May to December 2013, respectively. White lines denote the trajectory of the eddy center, and the white star and square denote the position of the center at the beginning and end of each month, respectively. Float trajectories are shown by the yellow curves. INOUE ET AL. ANTICYCLONIC EDDY OBSERVATION 6109

8 Figure 2. (continued) INOUE ET AL. ANTICYCLONIC EDDY OBSERVATION 6110

9 Figure 3. Times series of (a) latitude and (b) longitude of the ACE A center; (c) horizontal size of ACE A; (d) ellipse linearity k, a measure of ellipticity, where k 5 0 indicates a circle and k 5 1 indicates a straight line; (e) major axis orientation / (08 5 east west, north south) of ACE A, (f) relative vorticity averaged over 40 km radius from the ACE A center, normalized by the local Coriolis parameter and (g) azimuthal speed (thin line) and translation speed (thick line). Time axis starts in May. Red lines show values estimated from the SSH. Hann filter with a window length of 20 days is applied for Figure 3c the eddy size and Figure 3g azimuthal speed. Black lines show the average of those estimated from the float data, and the gray shading shows standard deviations. Oyashio, along 1448E and observed from 1468E, 388N to 1508E, 408N. During this period, the eddy core shrank (e.g., the area within the closed 0.4 m contour became smaller; Figures 2h 2j), and some floats were ejected from ACE A. In March 2013, ACE A moved slightly southward, and then in April it shifted suddenly southwestward (Figures 2k and 2l), where it started to interact with high-sst water. In May 2013, ACE A was still small and located around E, 40.88N (Figure 2m), and it moved northeastward in June to E, 41.58N (Figure 2n). From July to October 2013, the warm core was again obscured by sea surface heating, but relatively high SSTs were observed around the western edge of the eddy (Figures 2o 2r). In July and August 2013, the eddy was almost stationary (Figures 2o and 2p), but in September 2013, it moved eastward (Figure 2q) and it was observed around E, 41.78N in October (Figure 2r). Similar to in October, in October 2013, the eddy shape became flattened and a few floats were ejected from the ACE A core during an eddy eddy interaction. After November 2013, the warm ACE A core (>128C in November, Figure 2s, and 108C in December, Figure 2t) again became clear and distant from the local fronts (Figure 2s), and its diameter gradually decreased. It is notable that the eddy behavior was similar in May December (Figures 2a 2h) and May December 2013 (Figures 2m 2t), but we have no obvious explanation for this repeated behavior. ACE A, with a relatively weak warm core, was detected south of 418N in May (Figures 2a, 2m, 3a, and 3f), then moved northward and reached around 41.58N in June (Figures 2b, 2n, and 3a). During the summer (July September), the eddy core defined by SSH became clear and high-temperature advection was frequently observed around the western or southwestern edge of the eddy (Figures 2c 2e and 2o 2q). In September or October, ACE A moved eastward (Figures 2e, 2f, 2q, 2r, and 3b), eddy eddy interactions were observed (Figures 2f and 2r), and ACE A become more elliptical (Figures 3c 3e). Floats were ejected from the ACE cores and, as a result, the eddy properties estimated from the float trajectories had relatively large standard deviations (Figures 3a, 3b, 3d, and 3e). Note that eddy properties could not be calculated from the float trajectories during the large southward movement of ACE A that occurred around May The warm core of the ACE A was clear during November and December, but the isoline of SSH gradually shrank and the eddy size estimated from a distance between the speed maxima of a rotation speed of ACE A became smaller (Figures 2g, 2h, 2s, 2t, and 3c). The normalized relative vorticity (Figure 3f) indicated that ACE A became weaker after April 2013, but its size and behavior were nevertheless similar in and Water Properties During the first deployment of the profiling floats (cruise MR12-02), CTD and DO were measured on 6 June along a transect across ACE A (Figure 4) at each deployment position. The CTD and DO profiles along this transect, which did not pass through the eddy center, revealed that a warm core (8 98C) pycnostad existed between r h and 26.5 kg m 23, and a salinity minimum layer (33.6) between r h and 26.8 corresponded to a local low-temperature layer. Below the salinity minimum layer, salinity increased INOUE ET AL. ANTICYCLONIC EDDY OBSERVATION 6111

10 Figure 4. (a) Map of SSH (cm) and cross section of (b) h, (c) S, and (d) DO (color scales) during the first float deployment by R/V Mirai (cruise MR12-02) on 5 June. Red stars in Figure 4a show the deployment stations, which correspond to the inverted red triangles at the top of Figures 4b 4d. Black contours in Figures 4b 4d show rh (kg m23). with depth. The oxygen minimum layer was between rh and In this study, we defined three isopycnal layers and tracked water property changes observed by the profiling floats inside ACE A by the thickness and average salinity of these layers. The first layer was the pycnostad layer (rh to 26.7 kg m23), which was defined as that between the density at the top of the weakly stratified water and that of the densest wintertime mixed layer. This density range corresponds to that of the Transient Mode Water found in the MWR [e.g., Oka et al., 2014]. The second layer was the salinity minimum layer (rh to 26.9 kg m23) observed in the CTD cross section; this density range corresponds to that of NPIW, which has been shown to be renewed within eddies in the MWR [e.g., Itoh et al., 2014; Kaneko et al., 2015]. The third layer was the main thermocline layer (rh and 27.2 kg m23); this layer, which corresponded to subarctic water, was located between the salinity minimum and oxygen minimum layers. Since changes observed in the time series of the profiles of each float included changes due to physical processes such as water advection and mixing as well as those due to the float s movements, we also examine spatial averages relative to the center position of ACE A. At the time of the first float deployment, the time series show an averaged salinity of in the pycnostad and salinity minimum layers (Figures 5a and 5b), and a salinity of nearly 34.1 in the main thermocline layer (Figure 5c). In each layer srr was highest at INOUE ET AL. ANTICYCLONIC EDDY OBSERVATION 6112

11 Figure 5. Averaged salinity within the (a) pycnostad layer (r h and 26.7 kg m 23 ), (b) salinity minimum layer (r h and 26.9 kg m 23 ), and (c) main thermocline layer (r h and 27.2). The black lines and error bars show the average and standard deviation of salinity within km of the eddy center calculated monthly. This range is arbitrary because the floats did not stay near the eddy center, but it is smaller than the eddy size (Figure 3c). The color scale indicates the distance from the ACE A center of each profile position. The inverted triangles on the top indicate the float deployment cruises (Table 1). the time of the float deployment (June ), and it decreased with time, reaching a minimum in October (Figure 6). As described in section 3.1, the SSH and SST maps (Figure 2) indicated that an eddy eddy interaction event occurred in October. Although ACE A might have interacted with ACE B during this event, salinity of ACE A apparently decreased at this time in the salinity minimum layer: Near the eddy center, fresher water (33.55) was observed, and the standard deviation of salinity was relatively large. A large fall in salinity occurred in the pycnostad layer during December to February 2013, because of the mixed layer deepening (Figures 5a and 7a). The water properties of the pycnostad layer in February 2013 were close to those of TRMW (S and h C). The largest eddy movement was observed in April and May 2013 (Figures 3a and 3b), and this movement caused salinity to increase in the pycnostad layer in April (Figure 5a), possibly because of advection of the saltier water near surface described in next section. In the salinity minimum layer, the spatially averaged salinity (30 50 km) decreased (but the standard deviation was large) during the movement in April, and then increased in May (to ), after which it became almost constant (33.55), with small standard deviations. Data collected by the four floats, which were deployed in late January 2013 (Table 1) and tracked the water near the eddy center (Figure 5b), indicated that salinity in the salinity minimum layer might have increased slightly in the eddy core during the large eddy movement in April and May s rr also increased in each layer during this period (Figure 6). The salinity of the pycnostad layer decreased in September 2013, when ACE A moved eastward and was surrounded by Oyashio water (Figure 3b), before the eddy eddy interaction. In October 2013, an interaction event occurred and the standard deviation of salinity in the pycnostad layer increased (Figure 5a). s rr gradually increased after October 2013 in the salinity minimum layer (Figure 6b). Finally, the change in the average salinity was small at this time in the main thermocline layer (Figure 5c). Figure 6. Root mean square values of the diapycnal spiciness curvature (s rr ) averaged over the (a) pycnostad layer, (b) salinity minimum layer, and (c) main thermocline layer, as defined in Figure 5. The black lines and color scale are the same as in Figure 5. When the first floats were deployed, the pycnostad layer was the thickest (over 300 dbar) among the isopycnal layers, whereas the thicknesses of the other two layers were similar ( dbar) (Figure 7). The pycnostad layer was thicker near INOUE ET AL. ANTICYCLONIC EDDY OBSERVATION 6113

12 Figure 7. Layer thickness (h) averaged over the (a) pycnostad layer, (b) salinity minimum layer, and (c) main thermocline layer, as defined in Figure 5. The black lines and color scale are the same as in Figure 5. The pycnostad layer cropped out between January and March the center of the eddy and became thinner and outcropped in January where a near-surface mixed layer density was r h kg m 23.Itsstandard deviations were large during the period of large eddy movement between April and June After this eddy movement event, the pycnostad layer thickness decreased (to 200 dbar), and the thickness of the other layers increased above 200 dbar. These changes might be related to the eddy vorticity weakening (Figure 3f), where the normalized relative vorticity was reduced by 0.02 (20% of the normalized relative vorticity and 2% of planetary vorticity). However, the relative contribution of thickness changes in different layers to the vorticity change was unclear (e.g. 30% of the thickness change in the pycnostad layer) and it may deserve future studies. 4. Detailed Description of the Eddy Movement Event In the time series of the salinity, s rr, and isopycnal layer thickness (Figures 5 7), the largest fluctuations occurred in April and May 2013 when ACE A moved a considerable distance southward and then back northward. During this period, the profiling floats were trapped inside of ACE A and were recording modifications of its water properties. The averaged salinity in the salinity minimum layer near the eddy center (<20 km) was in April, in May, and in June (Figure 5b). In this section, we show how well the float array captured the detailed spatial structure of ACE A during this movement event. From 22 March to 10 April 2013 (Figures 8a and 8b), when ACE A was starting to move southward with a translation speed of 0.05 m s 21 (Figure 3g), the water surrounding the eddy center was colder and less saline in the pycnostad and main thermocline layers and warmer and saltier in the salinity minimum layer (Figure 8b), similar to the conditions reported by Itoh and Yasuda [2010a]. During April (Figures 8c and 8d), ACE A was skewed possibly due to its interaction with a front associated with the Kuroshio Extension and apparently moved the farthest (e.g., Figure 3g) with the translation speed of 1.5 m s 21 at 12 April. During this event, two SSH maxima were appeared within ACE A and the new maximum at south became the eddy center. Low-salinity water was advected to the periphery of ACE A, and much saltier water (33.7) with intrusive structures, which surrounded ACE A from west to north, was also observed in the salinity minimum layer. During 1 20 May (Figures 8e and 8f), ACE A was attached to the frontal structure at 39.58N, E and started to move northward. Time series of the daily SSH map indicated that ACE A merged with a relatively small anticyclonic eddy in the north while a part of ACE A was absorbed into the front (not shown). During this period, the profiling floats did not observe any salinity values as low as in the salinity minimum layer, and there were many intrusive structures that corresponded to the maximum averaged salinities shown in Figure 5b. From 21 May to 9 June (Figures 8g and 8h), the profiling floats were trapped within ACE A, which detached from the frontal structure and was moving further northeastward, and observed the lowest salinity water during the event (33.2) around the outside of the core, low-salinity water (33.4), and relatively saltier water (33.6) in the salinity minimum layer. During June (Figures 9a and 9b), ACE A moved further north and again became isolated from the frontal structures and the h-s profiles started to converge to one zigzag structure. A salinity minimum and maximum formed in the pycnostad layer, above the salinity minimum at r h After 30 June, ACE A stayed at around 41.58N, 1478E. The water near its center had fewer intrusive structures, and its salinity increased to slightly above 33.4 (Figures 9c 9h). Therefore, the salinity values close to the ACE A center in the salinity minimum layer after the event were almost the same as before the event, as is also shown in Figure 5b. INOUE ET AL. ANTICYCLONIC EDDY OBSERVATION 6114

13 The time series of float observations from late March to late June 2013 demonstrate how intrusive structures evolved during the event (Figure 10). In March and early April 2013 (corresponding to Figures 8a and 8b), two floats (SN6197 and SN6204) were positioned close together near the eddy center, and they circled clockwise around the center approximately every 5 days (Figures 10a and 10b). There was a strong salinity minimum at around 400 dbar (Figures 10c and 10d). When ACE A was skewed and the eddy center moved rapidly to the southwest in April (Figures 8c and 8d), the two floats were left behind, so that they were northeast of the eddy center. Subsequently, the floats approached the eddy center and interleaving features with smaller srr peaks started to be observed. Near the sea surface, the high salinity water appeared at around 16 April. Smaller srr peaks were frequently observed in May when, after a rapid southward shift, the floats again became trapped within ACE A (Figures 8e and 8f). In June, SN6197 (SN6204) was located Figure 8. (left) Twenty day averaged SSH, float trajectories within each period, and salinity inside (outside) of ACE A and in each profile averaged over rh kg m23. The color scale on the top indicates averaged salinity at each profile position. (right) h-s diagrams within 60 km of the ACE A was circling around the center in center, based on float data. The color scale on the top indicates the distance from the ACE 5 (15) days. Near the eddy cena center of each profile position. The dashed lines are isopycnal surfaces defining the three ter (Figure 10c), several curvalayers (see Figure 5) (a and b) 22 March to 10 April 2013; (c and d) April 2013; (e and f) 1 20 May 2013; and (g and h) 21 May to 9 June ture peaks were observed that corresponded to the zigzag structure seen in Figure 9. In contrast, outside of ACE A, fresher water was occasionally observed owing to the float s position relative to Oyashio water (Figure 10d). To examine the spatial distributions of the intrusive structures, we compared composite plots of RMS srr between 26.7 and 26.9 rh among three periods (before the event, 1 March to 10 April; during the event, 11 April to 30 June; and after the event, 1 July to 31 August) in Figure 11. Large RMS srr values were observed near the eddy center during all three periods. The RMS srr values and the area in which high values were found were largest during the event (compare Figures 11a, 11e, and 11 i). Large RMS values outside of the eddy were associated with the eddy movement during the interaction with a front (northeast and southwest; Figure 11e). Before the event, intrusions with the larger amplitude and scale (Figures 11b and 11c) were associated with the salinity minimum near the center of the eddy (Figure 1a). Otherwise, the amplitudes were small (with peaks close to 0 m3 kg21). During the event (Figures 11f and 11g), larger scale intrusions were disappeared, but smaller scale intrusions were more frequent. The peak seen at low absolute amplitude values before the event broadened, and the frequency of larger amplitudes (both positive and INOUE ET AL. ANTICYCLONIC EDDY OBSERVATION 6115

14 negative) increased because of the many intrusive structures (e.g., Figure 1b). After the event (Figures 11j and 11k), when ACE A moved northward, the frequencies of scale and amplitude were intermediate between those before and those during the event, although two new amplitude peaks were associated with the zigzag structure above the salinity minimum layer. The thickness of intrusions (Figures 11d, 11h, and 11l) also showed the largest vertical scales for intrusions with the large amplitude before the event, smaller vertical scales for both large and small amplitudes during the event, and relatively larger vertical scales after the event. 5. Discussion The float data showed that the water near the eddy center, which had had a smooth salinity minimum layer, was modified during the large southward movement of ACE A (Figure 8). The layer averaged salinity, and therefore temperature (not shown), increased near the eddy center, and many intrusive structures Figure 9. Same as Figure 8 but for (a and b) June 2013; (c and d) 30 June to 19 July 2013; (e and f) 20 July to 8 August 2013; and (g and h) 9 28 August were observed. Subsequently, the floats observed a relatively fresher salinity minimum layer near the eddy center, which gradually became saltier as ACE A moved northward. On the h-s diagram, intrusive structures converged to a zigzag structure during this northward movement. Here, we first discuss how much heat could be transported to the subarctic gyre during the interaction with a front. Then, we discuss how the vertical mixing process affected the averaged salinity structure around the eddy center between 1 June and 31 August 2013, when the ACE A was isolated and moved northward or remained north. When the ACE A interacted with a front, an average salinity in the core of the salinity minimum layer temporarily increased 0.1 (Figure 5b) where a corresponding temperature increase Dh was 18C (not shown). If this water had flowed into the subpolar gyre, a bulk heat flux within the salinity minimum layer could be written, HFbulk 5Dh3h3Cp3 q : Dt (2) Here, h is a thickness of the salinity minimum layer ( 200 m from Figure 7b), Cp is a specific heat ( 4000 J kg21 K21), q is density of seawater kg m23 and Dt is a time scale of the event ( 1 month). Then, HFbulk became 317 W m22. Finally, the diameter of the ACE A during this period was about 80 km (Figure 3c), the heat transport became 1.6 TW. For comparison, Qiu and Chen [2005] estimated the zonally integrated INOUE ET AL. ANTICYCLONIC EDDY OBSERVATION 6116

15 Figure 10. Time series of (a) distance between a float and the eddy center, (b) direction of each float relative to the eddy center. Blue line shows SN6197 float and red is SN6204. The black and gray bar and figure numbers at the top indicate the periods and corresponding figure numbers shown in Figures 8 and 9. Time series of salinity (color scale) measured by (c) SN6197 and (d) SN6204. Dots indicate diapycnal spiciness curvature (s rr ) peaks (e.g., Figures 1b and 1d). The white (black) line indicates the r h (26.9) kg m 23 isopycnal surface. poleward eddy heat transport over the entire basin between 328 and 388N to be 0.1 PW.Itoh and Yasuda [2010a] estimated that from 1708E to the western boundary at around 358N to be PW, which was one order higher than our estimate. Itoh and Yasuda [2010b] estimated that from 1508E to the western boundary between 40 and 448N above 200 dbar to be 0.8 TW. Although the similarity with the previous regional estimates was encouraging, we caution that the amount of warm/salty water entrains into an eddy could be highly variable, depending on the strength of the interaction. However, it could suggest that an interaction between an eddy and front is an important mechanism for a poleward heat transport around this latitude. Our float measurements further showed that intrusions were associated with the event, which could have resulted in the poleward heat transport, and could be important to mix anomalies caused by an interaction between an eddy and front. After ACE A detached from the frontal structure, a large change in the monthly averaged salinity below 300 dbar between June and July 2013 (Figures 12a and 12d) may reflect vertical heaving of the isopycnal layer (Figures 12b and 12f). Since the salinity minima in the monthly averaged h-s diagram (Figure 12c) became fresher and saltier with time on the r h and kg m 23 surfaces, respectively, isopycnal and diapycnal processes further modified the structure. If we assume that diapycnal mixing was the dominant cause of salinity changes after ACE A became isolated, then the dominant balance for averaged salinity within a radius of 40 km of the eddy center (Figures 8, 9, 10a, and 11i) could @z K 2 2 : (3) Here, S represents the 10-dbar boxcar-averaged salinity, and the square brackets indicate the horizontal and monthly average over the eddy core. The pressure coordinate is used to calculate vertical gradients. K z is vertical diffusivity (unknown variable). When we compared the left hand side and the first term on the right hand sides of equation (3) above 500 dbar (e.g., around the salinity minimum), we found that both terms had the same sign, except above the salinity minimum, around 400 dbar ½S Š tended to be neg- h ative (Figure 12d) 2 the must be negative i was positive (Figures 12a and 12e). To compensate the sign inconsistency in h i was positive, indicating that mixing was INOUE ET AL. ANTICYCLONIC EDDY OBSERVATION 6117

16 Figure 11. (a, e, i) RMS values (color scale) of diapycnal spiciness curvature s rr within r h kg m 23, plotted relative to the eddy center. Histograms of (b, f, j) the intrusion scale (shown as vertical lines in Figures 1b and 1d) and (c, g, k) amplitude (horizontal lines in Figures 1b and 1d); (d, h, l) intrusion thickness (the projection of the scale onto the pressure coordinate). Gray bars show the frequency of thicknesses for intrusion with amplitudes larger than 50 m 3 kg 21. Figures 11 (a d) 1 March to 10 April, (e h) 11 April to 30 June, and (i l) 1 July to 31 August Histogram data are for r h kg m 23 to include the zigzag structure and a km 2 area around the center of ACE A in Figures 11a, 11e, and 11i. just above the salinity minimum. By ½S h i (Figures 12d and 12e) we found that a vertical 2 diffusivity of m 2 s 21 could explain 100% of the temporal salinity increase around 100 dbar and 25% of the decrease around 350 dbar during June and July, and 50% of the decrease around 300 dbar during July and August. It should be emphasized that the assumed balance might not hold below 300 dbar, where the vertical heaving would appear on the right-hand side and might play a role as 2 2 cussed above (Figures 12a and 12d). Therefore, vertical mixing might be less important because both the h i h i r terms were close to 0 (Figures 12e and 12g) below 400 dbar. However, the apparent impor- 2 tance of mixing to the observed temporal changes of the averaged profiles indicates that detailed studies of the highly intermittent diapycnal mixing process and its importance relative to other processes [e.g., Armi et al., 1989; Hebert et al., 1990; Ruddick, 1992] should be carried out in the future. 6. Summary In this study, we examined the evolution of a single anticyclonic eddy (called ACE A) in the mixed water region from June to December Satellite measurements showed that the behavior of ACE A from May to December was similar between and ACE A was located south of 418N in May and started INOUE ET AL. ANTICYCLONIC EDDY OBSERVATION 6118

17 Figure 12. Vertical profiles of monthly and spatially (within a radius of 40 km from the eddy center) averaged (a) salinity and (b) r h with standard deviations. (c) h S diagram for the averaged profiles. (d and f) Temporal change rates of salinity and r h. (e and g) Monthly and spatially averaged second vertical partial derivatives of salinity and r h with standard deviations. The blue (red) line represents the average in June (July) in Figures 12a 12c, 12e, and 12g. The black line in Figures 12a 12c represents the August 2013 average. In Figures 12 d and 12f, the blue (red) line represents the difference between June and July (July and August). The 10 dbar boxcar average was calculated for each float profile. to move northward in June to around 41.58N. During the summer, high-temperature advection was frequently observed around the western or southwestern edge of the eddy. In September or October, ACE A moved eastward and an eddy eddy interaction was observed. The warm core feature was clear, and the eddy core gradually shrank, during November and December. Float time series data showed that the averaged salinity was 33.6 in the salinity minimum layer at the time of the float deployment. Relatively fresher water (33.55) was observed near the eddy center during the eddy eddy interaction in October. The largest fluctuations of h S properties were observed in April and May 2013, when ACE A moved a considerable distance southward and then back northward. During this movement event, the southward movement led to the interaction of ACE A with a front derived from the Kuroshio Extension and a large salinity change (to ). After ACE A detached from the frontal structure, it moved northeastward. The profiling floats again observed low-salinity water (33.55) with small intrusive structures, which eventually converged to a smooth zigzag structure with slightly increased salinity (33.55). The variability of the vertically averaged salinity during the event was about half the total salinity change in the salinity minimum layer between June and December Thus, it is possible that the interaction between an eddy and a front is an important cause of property changes as well as meridional heat and fresh water exchanges. Our results suggest that the combination of the amplitude and scale of the diapycnal spiciness curvature can be used to describe water mass modifications. Before the event, intrusions with larger amplitude and scale were associated with the salinity minimum core near the center. During the event, the frequency of intrusions with smaller scale and larger amplitudes, both positive and negative, increased. After the event, INOUE ET AL. ANTICYCLONIC EDDY OBSERVATION 6119