Study of subsurface eddy properties in northwestern Pacific Ocean based on an eddy-resolving OGCM
A method based on the Okubo–Weiss parameter was used to detect subsurface eddies (SSEs) with an eddy-resolving ocean general circulation model. Statistical analyses showed that SSEs are ubiquitous in the northwestern Pacific Ocean. Three regions were found to have high probability of SSE, which are as follows: the latitudinal band between 9°N and 17°N, the Kuroshio extension region, and the area east of the Ryukyu Islands. Although surface eddies (SEs) were found distributed widely within the zonal band of the Subtropical Counter Current, few SSEs were found there. In contrast, few SEs were found to the east of The Philippines, whereas SSEs were abundant. The kinetic energy contained within SSE was found comparable in magnitude with that of SE. During 1993–2013, about 2569 and 2099 SSEs (at a depth of about 400 m) were observed to be anticyclonic and cyclonic, respectively; thus, SSEs tended to be anticyclonic. The mean radius, lifespan, and propagation speed of SSE in this study were about 60 km, 50 days, and 6.6 cm/s, respectively. The propagation speed showed a wave-like decrease with increasing latitude. Some long-lived SSEs were found to persist for longer than 4 months and to move thousands of kilometers. About 89% of SSEs were nonlinear for at least half their lifespan, which implies that SSE can trap interior fluid during translation. Trajectories revealed that SSEs propagate nearly due west with only small meridional deflection. The findings of this study will contribute to the enrichment of our knowledge regarding SSE in the northwestern Pacific Ocean.
KeywordsSubsurface eddies Eddy characteristics Kinetic energy
Mesoscale eddies are important in the transportation of oceanic heat, salt, freshwater, nutrient, and biological signatures. Mesoscale eddies can be classified as surface-intensified eddies and subsurface-intensified eddies based on the vertical distribution of their hydrographic signals. Subsurface eddies (SSEs), which represent a special class of ocean eddy, are characterized by having their core or maximum velocity in subsurface water (Gordon et al. 2002). The temperature and salinity properties of SSE are reasonably homogeneous but distinct from those of the surrounding waters (Johnson and McTaggart 2010; McWilliams 1985; Nauw et al. 2006). Generally, SSEs are triggered by instability of the undercurrent or subduction of mode water (Oka et al. 2009; Takikawa et al. 2005). Previous studies have proven that SSE can strongly influence intermediate or deeper ocean layers by affecting the subsurface circulation, pathways of water masses, and redistribution of heat, salt, and momentum (Andrade et al. 2014; Colas et al. 2012; Nan et al. 2017; Pelland et al. 2013). Unlike surface eddies (SEs), which can be characterized using satellite altimeter data, SSEs have weak surface expression, and they are poorly understood because of the lack of their systematic measurement (Chaigneau et al. 2011; Gordon et al. 2017; Johnson and McTaggart 2010). Most reported eddies have survived for a significant time (of the order of months) and traveled considerable distance (hundreds of kilometers) (Combes et al. 2015; Takikawa et al. 2005). Based on Argo float data and sporadic in situ observations, SSEs have been detected in numerous oceanic regions, e.g., as “Meddies” in the Mediterranean Sea (McDowell and Rossby 1978; Richardson et al. 2000), “Ruddies” in the Indian Ocean (Shapiro and Meschanov 1991), and “Cuddies” near the California Undercurrent (Collins et al. 2013; Kurian et al. 2011; Pelland et al. 2013). SSEs also exist in the eastern South Pacific Ocean associated with the Peru–Chile Undercurrent (Combes et al. 2015; Hormazabal et al. 2013; Johnson and McTaggart 2010; Thomsen et al. 2016). In contrast, research on SSE in the northwestern Pacific Ocean has been limited, and only a few examples have been observed.
The ocean circulation in the northwestern Pacific Ocean is characterized by a complex western boundary current. In the surface layer, the North Equatorial Current bifurcates into the northward-flowing Kuroshio and the southward-flowing Mindanao Current (MC) as it approaches the coast of The Philippines. A significant portion of the MC veers eastward at the southern tip of the island of Mindanao near 5°N to form the North Equatorial Countercurrent. In the latitudinal band of 18°–24°N, the North Pacific Subtropical Counter Current (STCC), which is a weak and shallow eastward current, penetrates into the open Pacific from 130°–180°E (Chang and Oey 2014). Below the surface, the subsurface circulation is dominated by several important undercurrents. Beneath the MC and the Kuroshio, along the coast of The Philippines, are the southward-flowing Luzon Undercurrent and northward-flowing Mindanao Undercurrent, respectively (Hu and Cui 1989; Hu et al. 1991). The North Equatorial Undercurrent consists of three parallel eastward-flowing jets at the depth of approximately 500–1100 m along 9°N, 13°N, and 18°N. These jets typically have a core velocity of 3–5 cm/s, and they are spatially coherent from the western boundary across the North Pacific basin to about 120°W (Qiu et al. 2015).
Takikawa et al. (2005) detected SSE to the southeast of the Ryukyu Islands, which had thickness and width of 300 m and 100 km, respectively. In December 2013, a subsurface lens of water from the Andaman Sea was captured in the Bay of Bengal (Gordon et al. 2017). Nan et al. (2017) detected an extra-large subsurface anticyclonic eddy with horizontal scale of 470 km in the Northwest Pacific subtropical gyre. Their analysis indicated that the SSE formed in the region of Subtropical Mode Water and, then, propagated westward for over 1500 km. In general, these case studies of SSE have focused mainly on a single eddy found at a specific location or along a section. However, such observations of SSE cannot provide information about their spatial distribution or the roles they might play in the ocean. More importantly, most previous studies have focused on subsurface anticyclonic eddies (SSAEs) with low potential vorticity, while the characteristics of subsurface cyclonic eddies (SSCEs) remain unclear. The remainder of this paper is organized as follows: Section 2 briefly describes the model configuration and altimetry dataset used in this study. The eddy detection and tracking method is presented in Section 3. The properties of SSE, i.e., their occurrence frequency, kinetic energy, size, lifetime, propagation characteristics, polarity, and nonlinearity, are presented in Section 4. A summary of the results is provided in Section 5.
2 Model and datasets
The Oceanic General Circulation Model for the Earth Simulator (OFES) used in this study is based on the Modular Ocean Model ver. 3 developed by the Geophysical Fluid Dynamic Laboratory of the National Oceanic and Atmospheric Administration (Masumoto et al. 2004; Sasaki et al. 2008). The model utilizes the z-level coordinate in the vertical, and it solves three-dimensional primitive equations in spherical coordinates under the Boussinesq and hydrostatic approximations. Its domain extends from 75°S to 75°N, excluding the Arctic region, with 0.1° horizontal grid spacing. The vertical level spacing varies from 5 m at the surface to 330 m near the bottom. The model topography is generated using 1/30° bathymetry data provided by the Ocean Circulation and Climate Advanced Modelling project. For further details regarding the configuration and evaluation of this model, readers are referred to Masumoto et al. (2004) and Sasaki et al. (2008).
The OFES outputs have been analyzed in numerous earlier studies (Aoki et al. 2007; Chen et al. 2010; Chiang and Qu 2013; Qu et al. 2012). The results have demonstrated the promising capability of the model in representing realistic variability of different spatial and temporal scales in the ocean, including western boundary currents, mesoscale eddy generation near strong current systems, as well as appropriate water masses in the world’s ocean. This study analyzed snapshot (3d) model outputs for the domain of the northwestern Pacific Ocean (0°–60°N, 120°–180°E) from January 1993 to December 2013.
Two types of satellite data were used to validate the OFES data. Sea level anomaly (SLA) data used in this study were obtained from the French Archiving, Validation, and Interpolation of Satellite Oceanographic (AVISO) data project, which merges the measurements of Jason, TOPEX/POSEIDON, Envisat, GFO, ERS, and Geosat altimeters. The merged data are interpolated onto a global grid with 1/4° resolution, and they are archived in weekly-averaged frames. The entire dataset covers the period 1993–present; however, only the data from 1993 to 2013 were used in this study.
The 4th release of the trajectories of mesoscale eddies produced by the Collecte Localisation Satellites/Data Unification and Altimeter Combination System team is based on the DT-2014 daily “two-sat-merged” SLA fields posted online by AVISO for the 22-year period from January 1993 to April 2015. The eddy dataset provides the amplitude, radius scale, centroid, date of starting point, and number of points along the eddy tracks. The trajectories in this new version of the eddy dataset are available with time steps of 1 day, and only eddies with a lifetime of four weeks or longer are counted. In the new eddy dataset, rather than defining eddies by the outermost closed contour of sea surface height, as in the previous three eddy datasets, each eddy is defined based on connected pixels that satisfy the specified criteria. The procedure is a modified version of the method presented by Williams et al. (2011). A description of the implementation of the eddy identification procedure can be found on the following website: http://wombat.coas.oregonstate.edu/eddies/index.html. In the current study, we used trajectory data of mesoscale eddies for the 21-year period from January 1993 to December 2013.
3 Eddy detection and tracking method
4 Eddy characteristics
4.1 Eddy frequency
4.2 Kinetic energy of SSE
4.3 Subsurface eddy statistics
We tracked SSE at different depth levels and analyzed their properties based on the OFES data. Here, properties of SSE at the depth of 404 m (about 400 m) are considered because most of the undercurrents in the northwestern Pacific Ocean cross this depth. Overall, 2569 SSAEs and 2099 SSCEs were detected in the northwestern Pacific during 1993–2013, which confirms the strong tendency for SSE to be anticyclonic.
This study investigated the characteristics of SSE in the northwestern Pacific Ocean using OFES data. The O–W method was used to detect eddies from the velocity field of the OFES data. Subsequently, the spatial distribution of all eddies (including SE and SSE) at each time step of the model data was determined. We extracted time series of vertical eddy distribution from the surface to the depth of about 2000 m at each grid point, and we estimated the frequency of occurrence of SE and SSE. Comparison of model output and altimeter observations indicated that the model data and our detection algorithm could satisfactorily reproduce eddy activities. A census of SSE at the depth of about 400 m revealed the characteristics of both anticyclonic and cyclonic eddies after eddy tracking.
In general, SSEs were found to exist widely in certain areas, e.g., the Kuroshio extension region, latitudinal band between 9°N and 17°N, and to the east of the Ryukyu Islands, where the frequency of SSE was about 10, 16, and 8%, respectively. Comparison of the frequency of occurrence of SSAE and SSCE revealed that while the STCC is known for abundant SE, the occurrence of SSE in this region is rare. Conversely, to the east of The Philippines, relatively few SEs occur, whereas there are frequent SSEs. The identified SSEs were used to evaluate the kinetic energy contained in SSE, which we found to be comparable in magnitude to that of SE. In region such as to the east of The Philippines and in latitudinal band 9°–17°N, the kinetic energy of SSE was found even larger. The tracks of SSE revealed their propagation characteristics. Most of the detected SSE tended to be anticyclonic. The average radius and lifespan of the SSE were determined as about 60 km and 50 days, respectively. Most of the observed SSEs were found nonlinear, which means that SSE can have considerable impact in the movement of heat and mass transport within the subsurface layer, especially in some regions with abundant SSE. The dynamical mechanism of SSE is complex, and it is often related to the regional background circulation, water mass characteristics, and/or topographic boundaries (Hormazabal et al. 2013; Nan et al. 2017; Takikawa et al. 2005).
Previous research has tended to focus on SE, and the importance of SSE has been underestimated, despite the considerable kinetic energy they contain. Although not visible at the surface, SSE with large spatial structure and long lifetime can accelerate the mixing and exchange of intermediate water. In this research, only eddies with large spatial scale and regular shape were detected, which could have introduced some uncertainty in the identification process and led to underestimation of the number of SSE. Because in situ data of SSE are scarce, model data constitute the only practical resource with which to reveal the characteristics of SSE. The census of the properties of SSE presented here using the model output represents the first step in our analysis of SSE. The results will be verified in future work when additional in situ observational data of eddies become available. Furthermore, the formation mechanisms, vertical structure, and transport of SSE in different regions, which were not investigated here, will be pursued in our future studies.
We are grateful to H. Sasaki and colleagues from the Earth Simulator for assistance in processing the Oceanic General Circulation Model for the Earth Simulator output and to the Segment Sol multi-missions d’ALTimetrie, d’Orbitograpie et de localization précise/Data Unification and Altimeter Combination System (DUACS) for the altimeter products distributed by the Archiving, Validation, and Interpolation of Satellite Oceanographic program. The 4th eddy dataset produced by the Collecte Localisation Satellites/DUACS team is available from http://wombat.coas.oregonstate.edu/eddies/index.
This work was jointly supported by the National Natural Science Foundation of China (41676005), Global Climate Changes and Air–Sea Interaction Program (GASI-IPOVAI-01-06), Chinese Academy of Sciences (CAS) “Huiquan Scholar,” Youth Innovation Promotion Association of CAS, CAS Interdisciplinary Innovation Team, and NSFC Innovative Group Grant (Project No. 41421005).
- Aoki S, Hariyama M, Mitsudera H, Sasaki H, Sasai Y (2007) Formation regions of subantarctic Mode Water detected by OFES and Argo profiling floats. Geophys Res Lett 34(10). https://doi.org/10.1029/2007gl029828
- Chaigneau A, Le Texier M, Eldin G, Grados C, Pizarro O (2011) Vertical structure of mesoscale eddies in the eastern South Pacific Ocean: a composite analysis from altimetry and Argo profiling floats. J. Geophys. Res.-Oceans 116(16). https://doi.org/10.1029/2011jc007134
- Chelton DB, Schlax MG, Samelson RM, de Szoeke RA (2007) Global observations large oceanic eddies. Geophys Res Lett 34:L15606. https://doi.org/10.1029/2007GL030812
- Gordon AL, Giulivi CF, Lee CM, Furey HH, Bower A, Talley L (2002) Japan/East Sea intrathermocline eddies. J Phys Oceanogr 32(6):1960–1974. https://doi.org/10.1175/1520-0485(2002)032<1960:jesie>2.0.co;2 CrossRefGoogle Scholar
- Gordon AL, Shroyer E, Murty VSN (2017) An intrathermocline eddy and a tropical cyclone in the Bay of Bengal. Sci Rep 7. https://doi.org/10.1038/srep46218
- Hu D, Cui M (1989) The western boundary current in the far-western Pacific Ocean. In: Picaut J, Lukas R, Delcroix T (eds) Proceedings of Western Pacific International Meeting and Workshop on TOGA-COARE, May 24–30, 1989, Noum ea, New Caledonia. Inst. Fr. de Rech. Sci. pour le De ev. en Coop, Noum ea, pp 123–134Google Scholar
- Hu D, Cui M, Qu T, Li Y (1991) A subsurface northward current off Mindanao identified by dynamic calculation. In: Takano K (ed) Oceanography of Asian marginal seas, Elsevier oceanography series, vol 54. Elsevier, Amsterdam, pp 359–365. https://doi.org/10.1016/S0422-9894(08)70108-9 CrossRefGoogle Scholar
- Isern-Fontanet J, Garcia-Ladona E, Font J (2003) Identification of marine eddies from altimetric maps. J Atmos Ocean Technol 20(5):772–778. https://doi.org/10.1175/1520-0426(2003)20<772:iomefa>2.0.co;2 CrossRefGoogle Scholar
- Isern-Fontanet J, Font J, Garcia-Ladona E, Emelianov M, Millot C, Taupier-Letage I (2004) Spatial structure of anticyclonic eddies in the Algerian basin (Mediterranean Sea) analyzed using the Okubo-Weiss parameter. Deep-Sea Res Part II-Top Stud Oceanogr 51(25–26):3009–3028. https://doi.org/10.1016/j.dsr2.2004.09.013 CrossRefGoogle Scholar
- Kurian J, Colas F, Capet X, McWilliams JC, Chelton DB (2011) Eddy properties in the California Current System. J Geophys Res-Oceans 116. https://doi.org/10.1029/2010jc006895
- Masumoto Y et al (2004) A fifty-year eddy-resolving simulation of the world ocean—preliminary outcomes of OFES (OGCM for the Earth Simulator). J Earth Simulator 1:35–56Google Scholar
- Nan F, Yu F, Wei C, Ren Q, and Fan C (2017) Observations of an extra-large subsurface anticyclonic eddy in the Northwestern Pacific subtropical gyre. https://doi.org/10.4172/2155-9910.1000234
- Nauw JJ, van Aken HM, Lutjeharms JRE, and de Ruijter WPM (2006) Intrathermocline eddies in the southern Indian Ocean. J Geophys Res-Oceans 111(C3):14. https://doi.org/10.1029/2005jc002917
- Qiu B (1999) Seasonal eddy field modulation of the North Pacific subtropical countercurrent: TOPEX/Poseidon observations and theory. J Phys Oceanogr 29(10):2471–2486. https://doi.org/10.1175/1520-0485(1999)029<2471:sefmot>2.0.co;2 CrossRefGoogle Scholar
- Qu TD, Chiang TL, Wu CR, Dutrieux P, Hu DX (2012) Mindanao current/undercurrent in an eddy-resolving GCM. J. Geophys. Res.-Oceans 117(16). https://doi.org/10.1029/2011jc007838
- Sasaki H, Nonaka M, Masumoto Y, Sasai Y, Uehara H, Sakuma H (2008) An eddy-resolving hindcast simulation of the Quasiglobal Ocean from 1950 to 2003 on the earth simulator. In: Hamilton K, Ohfuchi W (eds) chapter 10High resolution numerical modelling of the atmosphere and ocean. Springer, New York, pp 157–185CrossRefGoogle Scholar
- Thomsen S, Kanzow T, Krahmann G, Greatbatch RJ, Dengler M, Lavik G (2016) The formation of a subsurface anticyclonic eddy in the Peru-Chile Undercurrent and its impact on the near-coastal salinity, oxygen, and nutrient distributions. J. Geophys. Res.-Oceans 121(1):476–501. https://doi.org/10.1002/2015jc010878 CrossRefGoogle Scholar
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