Eddy analysis in the subtropical zonal band of the North Pacific Ocean

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Abstract

There are two zonal bands of eminently high eddy kinetic energy (EKE) in the North Pacific Ocean. The highest one is located in the Kuroshio Extension and the second one is in the subtropical area. This paper focuses on the latter. An eddy detection scheme based on velocity vector geometry is applied to the SSHA-derived geostrophic currents to identify and track eddies, and to generate an eddy dataset which includes spatial and temporal information on eddy generation, evolution and termination. Analysis of this dataset allowed the investigation of a broad range of eddy parameters. From 1993 to 2010, about six thousand eddies with lifetime longer than eight weeks were generated within the band. All eddies moved westward, and both cyclonic and anticyclonic eddies deflected northward south of 21°N and southward north of 21°N, respectively. Statistically, three different stages of an eddy's lifetime can be identified: the first one-fifth of its lifetime corresponds to the growing period; the successive three-fifths after that to its stable stage; the last one-fifth to its decaying period. Observed Argo vertical profiles collected within the detected eddy areas are used to investigate the eddy-induced vertical displacement of the thermocline and the halocline. Frontal intensity derived from the SST data is used to explain the mechanism modulating temporal and spatial eddy variations within the zonal band.

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: u=g/f(h/y) and v=g/f(h/x), where u and v are the zonal and meridional components of the geostrophic velocity anomalies, and h is the sea surface height anomaly (SHHA); g

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

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