Eddy analysis in the subtropical zonal band of the North Pacific Ocean
Highlights
► An eddy data set from altimeter data is generated and analyzed in the southern band of the Northern Pacific ocean. ► Eddy characteristic parameters and eddy lifetime evolution are investigated. ► Eddies impact on thermoclines and haloclines are studied using the combination of Argo data and the eddy data set.
Introduction
Mesoscale eddy activity is a dominant phenomenon in the upper ocean. However, oceanic eddy generation is not spatially uniform, and is concentrated within specific regions of the oceans. In the Northern Pacific Ocean, there are two zonal bands of strong eddy activity (Aoki and Imawaki, 1996, Wunsch and Stammer, 1998, Qiu, 1999, Qiu and Chen, 2010). The spatial distribution of eddy kinetic energy calculated from altimetry-measured sea surface height anomalies (SSHA) with respect to the long-term mean and after high-pass filtering (shorter than 90 day and longer than 14 day) is shown in Fig. 1. The two high EKE zonal bands can be easily identified. Such bands clearly emerge also in high-pass root-mean-square sea surface height anomalies, e.g., Qiu and Chen (2010). The northern band occurs in correspondence with the Kuroshio Extension. There, the high EKE results from the instability of the Kuroshio after the jet leaves the restraining coastal area and flows to the open ocean (see Qiu and Chen, 2010 for a summary). The southern band is located in the subtropical region, extending from the east of the Luzon Strait all the way to the Hawaiian Islands. This zonal band is the focus of this study.
Previous studies on eddy activity further divide this southern band at roughly 170°E into two distinct areas: the western zone, west of 170°E, corresponds to the subtropical counter-current zone (Qiu, 1999, Hwang et al., 2004, Liu et al., 2005, Qiu and Chen, 2010, Kang et al., 2010); the eastern zone, east of 170°E, corresponds to the lee side of Hawaii Islands (e.g., Yu et al., 2003, Dong et al., 2009, Yoshida et al., 2010). One exception is represented by Kobashi and Kawamura (2002) who studied eddy variations in the entire southern band. The distinction of the two areas is based on the different mechanisms of eddy generation: in the western area eddies are generated through frontal instability associated with the subtropical front: in the eastern area they are mainly due to wind curl and ocean current shear in the wakes of the Hawaiian Islands. However, a significant number of eddies generated in the eastern zone propagates westward and enter the western zone (see Section 3); moreover, the wind curl also works as an important driving force generating eddies in the western zone (Section 5). Therefore, in this study we examine eddy properties in the entire band as a whole.
Previous studies have also shown that eddies within this southern zonal band have direct impacts on the subtropical gyre (Qiu, 1999), while eddies in the northern band affect North Pacific Subtropical Mode Waters (Uehara et al., 2003, Qiu et al., 2007), and North Pacific intermediate waters (Qiu and Chen, 2011), subtropical ventilation (Endoh et al., 2006, Nishikawa et al., 2010), vertical mixing (Pan and Liu, 2005), and even biological processes (Vaillancourt et al., 2003, Johnson and McTaggart, 2010). Several approaches have been used to study the eddy variability in this zonal band. Qiu (1999) and Qiu and Chen (2010) used SSHA anomalies to examine eddy variability at the inter annual and the seasonal time scales. Noh et al. (2007) and Tsujino et al. (2010) applied numerical models to investigate eddy variability. In an early work on eddy statistical analysis from altimeter-measured data in the area, Hwang et al. (2004) obtained eddy statistical features in the western part of the southern band using a dynamical height threshold to identify eddy boundaries. Kang et al. (2010) also generated an eddy dataset in the western part through a different eddy detection scheme based on SSHA data, and provided preliminary statistical results. Two studies on global eddy data mapping and analysis by Chelton et al., 2007, Chelton et al., 2011 include eddy activity information in the area. In particular, Chelton et al. (2011) provides an improved global eddy mapping with more eddies resolved than in Chelton et al. (2007). However, eddy generation and evolution, like any other ocean phenomenon, are strongly affected by the local dynamical conditions. Therefore, a detailed regional eddy analysis would undoubtedly allow to better understand the characteristic eddy features of eddies and their variability in this southern band.
The present study focuses on the area between15–28°N, and 112–150°W, see Fig. 1. An eddy detection scheme based on velocity vector geometry is applied to SSHA-derived geostrophic current anomalies to detect and track eddies, in order to retrieve an eddy dataset for the region. A series of statistical analysis is applied to the dataset to investigate and characterize the regional eddy dynamics.
In addition, vertical profiles from Argo data collected in the study area from 1995 to 2010 are also used in the study. These profiles are then used to examine eddy effects on the thermocline and the halocline.
The rest of the paper is organized as follows: the data and eddy detection scheme used are described in Section 2; statistical analysis of the eddy dataset is presented in Section 3; the application of Argo profiles to study eddy effects on the thermocline and halocline is discussed in Section 4; Section 5 discusses eddy generation mechanisms. Finally, the summary is presented in Section 6.
Section snippets
Data
The following observational data are used in the present paper: satellite measured sea surface height anomalies (SSHA), Argo float-measured temperature (T) and salinity (S) vertical profiles, and sea surface temperature (SST).
Surface geostrophic velocity anomalies are derived from SSHA data using the following formula: and , where and are the zonal and meridional components of the geostrophic velocity anomalies, and is the sea surface height anomaly (SHHA);
Eddy analysis
In the subtropical zonal band, the EKE estimated from the SSHA-derived geostrophic currents varies on seasonal and inter annual time scales (see Fig. 2). The EKE in later spring and early summer (May and June) is much higher than in other seasons. The level of EKE in the periods 1993–1995 and 1999–2002 is much lower than normal. Anomalies of both sea surface EKE and vorticity propagate westward with a speed of about 10 cm/s (see Fig. 3). These results are similar to those obtained by Qiu and
Eddy impact on thermocline and halocline
With altimetry data, we can only see eddy activity at the sea surface. Argo T/S vertical profiles provide the much needed information for the subsurface ocean. In total, 43,733 Argo vertical profiles are found in the northern Pacific subtropical band from Sep. 1995 to Dec. 2010. Most of the AGRO floats were deployed after 2000. To analyze the data, we first interpolate the recorded temperature and salinity vertical profiles into vertical levels evenly separated from 10 to 1000 m with an interval
Eddy generation mechanisms
What mechanisms drive eddy generation? As discussed in the Introduction, the northern Pacific subtropical band can be separated into two regions regulated by different dynamics. The western region is the subtropical frontal zone, and it is associated with a weak eastward counter current. Using T/S vertical profile along a section at 137°E, Qiu and Chen (2010) suggested that eddy generation in the western part of the zonal band is due to the baroclinic instability associated with the front.
Summary
Using the observational data: SSHA, SST, Argo T/S vertical profiles, this paper analyzes coherent eddy activity in the north Pacific subtropical band. This region is characterized by the second largest eddy activity of the whole North Pacific Ocean (the largest activity occurring in the Kuroshio extension region). The geometry-based eddy detection scheme by Nencioli et al. (2010) is applied to the SSHA-derived geostrophic currents to identify and track eddies. An eddy dataset is retrieved. The
Acknowledgments
YL and YPG appreciate supports from the Knowledge Innovation Program of the Chinese Academy of Sciences (Grant KZCX1-YW-12-4) and National Basic Research Program of China (2007CB411801), and YPG is supported in part by JIFRESSE, UCLA. CD appreciates the support from the National Aeronautics and Space Administration (grant NNX08AI84G). The work was partially done when YL visited CD at UCLA and working with CD in 2010. YL and CD appreciate the support from the State Key Laboratory of Satellite
References (39)
- et al.
Eddy activity in the Lee of the Hawaiian Islands
Deep Sea Res. Part II
(2008) - et al.
Global observations of nonlinear mesoscale eddies
Prog. Oceanogr.
(2011) - et al.
Blocking and westward passage of eddies in the Luzon Strait
Deep Sea Res. Part II
(2010) - et al.
Impact of a cyclonic eddy on phytoplankton community structure and photosynthetic competency in the subtropical North Pacific Ocean
Deep Sea Res. Part I
(2003) - et al.
Eddy activities of the surface layer in the Western North Pacific detected by satellite altimeter and radiometer
J. Oceanogr.
(1996) Hydrodynamical modeling of oceanic vortices
Surv. Geophys.
(2001)- et al.
Global observations of large oceanic eddies
Geophys. Res. Lett.
(2007) - et al.
An oceanic cyclonic eddy on the lee side of Lanai Island, Hawaii
J. Geophys. Res.
(2009) - et al.
Three-dimensional eddy analysis in the Southern California Bight
J. Geophys. Res.
(2012) - et al.
Sensitivity of the ventilation process in the North Pacific to eddy-induced tracer transport
J. Phys. Oceanogr.
(2006)
Pattern and velocity of propagation of the global ocean eddy variability
J. Geophys. Res.
The correlation of the surface circulation between the Western Pacific and the South China Sea from satellite altimetry data
Int. J. Remote Sens.
Propagation of big island eddies
J. Geophys. Res.
TOPEX/Poseidon observations of mesoscale eddies over the subtropical Countercurrent: kinematic characteristics of an anticyclonic eddy and a cyclonic eddy
J. Geophys. Res.
Generation of mesoscale eddies in the lee of the Hawaiian Islands
J. Geophys. Res.
Equatorial Pacific 13 °C water eddies in the eastern subtropical south Pacific Ocean
J. Phys. Oceanogr.
Eddy generation and evolution in the North Pacific Subtropical Countercurrent (NPSC) zone
Chin. J. Oceanol. Limnol.
Sensitivity study of the generation of mesoscale eddies in a numerical model of Hawaii islands
Ocean Sci.
Seasonal variation and instability nature of the North Pacific Subtropical Countercurrent and the Hawaiian Lee Countercurrent
J Geophys. Res.
Cited by (156)
Detection of materially coherent eddies from satellite altimetry in the Bay of Bengal
2023, Deep-Sea Research Part I: Oceanographic Research PapersAnalysis of mesoscale Eddy in the Nordic seas and Barents Sea using multi-satellite data
2023, Journal of Sea ResearchMarine heatwaves in the Western North Pacific Region: Historical characteristics and future projections
2023, Deep-Sea Research Part I: Oceanographic Research PapersReview of oceanic mesoscale processes in the North Pacific: Physical and biogeochemical impacts
2023, Progress in OceanographyA comparative analysis of the mesoscale thermohaline features across subarctic frontal zones in the Northern Hemisphere
2023, Deep-Sea Research Part I: Oceanographic Research PapersCitation Excerpt :Below these water depths, both the AE and CE salinity anomaly profiles in the OE change their signs. Similar profile shape, i.e., signs changing at certain water depth, of eddy-induced vertical salinity anomaly in the North Pacific has been reported in previous studies (Liu et al., 2012; Sun et al., 2017). Unlike the profile shape of the salinity anomaly in the OE mentioned above, the profile shapes of vertical salinity anomaly for both the CE and AE in the GS region are approximately similar to their respective temperature profiles in both subregions, comparing Fig. 8d with Fig. 8b.