Interdecadal variability of the Western Subarctic Gyre in the North Pacific Ocean

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Highlights

  • The Western Subarctic Gyre weakened gradually from the late 1990s to mid-2010s.

  • A sea level peak between the WSAG and Alaskan Gyre lowered abruptly around 2001.

  • The WSAG shape expanded abruptly to the east when the sea level peak lowered.

  • Wind-forced baroclinic Rossby waves caused WSAG weakening and eastward expansion.

  • Increased disappearance of Aleutian eddies in WSAG after 2005 may affect weakening.

Abstract

Interdecadal variations of the Western Subarctic Gyre (WSAG) in the North Pacific were examined mainly by analysis of absolute dynamic topography (ADT) during 1993–2017. The ADT was based on altimetry-derived sea level anomalies superimposed on state-of-the-art mean dynamic topography. We specified geostrophic surface streamlines of the WSAG associated with closed isolines of the ADT. The WSAG intensity was the strongest in the late 1990s, when the WSAG was zonally contracted, and it decreased to the mid-2010s in a linear manner with time. The WSAG expanded abruptly toward the east around 2001 in a step function–like manner. After this abrupt shift, the WSAG was connected most strongly to the Alaskan Gyre. This condition persisted even into the late 2010s. The shift to the very elongated pattern was also accompanied by a drastic lowering of the ADT around 170°W, 50°N, where the WSAG and Alaskan Gyre had been distinctly separated by a peak of the ADT during the previous period when the shape of the gyre was less elongated. A reduced-gravity model of wind-induced, long, baroclinic Rossby waves with weak dissipation could account for both the interdecadal weakening of the WSAG and the step function–like lowering of the ADT peak at the time of the shift of the WSAG to the very elongated pattern. In addition to the baroclinic response of the subarctic ocean to basin-scale wind stress, mesoscale clockwise eddies contributed to sea level rise around the center of the WSAG. In particular, there was an interdecadal increase in the frequency of appearance of strong, mesoscale, clockwise eddies that were generated south of the Aleutian Islands at longitudes of 170°E−180°. Those eddies propagated approximately to the west-southwest, and they disappeared around the center of the WSAG.

Introduction

The subarctic gyre of the North Pacific extends zonally north of the subtropical gyre and is divided into western and eastern cyclonic subcirculation cells: the Western Subarctic Gyre (hereafter, “WSAG”) and the Alaskan Gyre (Fig. 1a) (Dodimead et al., 1963; Favorite et al., 1976; Ohtani, 1991; Nagata et al., 1992; Michida et al., 2002; Fukuwaka et al., 2004). A part of the Alaskan Stream, which originates in the Alaskan Gyre, joins the WSAG around its eastern edge. The northern part of the WSAG penetrates into the interior of the Bering Sea west of the Bering Sea Gyre. The western boundary currents of the WSAG, the East Kamchatka Current and the Oyashio, flow along the coasts of the Kamchatka Peninsula, the Kuril Islands, Hokkaido, and Honshu Island (Fig. 1a and b). Some of the water transported by the East Kamchatka Current flows into the Okhotsk Sea (Matsuda et al., 2015); the rest is greatly modified by mixing with Okhotsk Sea water along the Kuril Islands and is then transported by the Oyashio (Ohtani, 1989; Yasuda, 1997). The Oyashio intrudes southward and meanders to the east of Honshu Island, typically with two peaks: the First Oyashio Intrusion, which is the closest to the coast, and the Second Oyashio Intrusion, which is the second-closest (Fig. 1b) (Kawai, 1972; Kuroda et al., 2017a). About 5 sverdrups (1 sverdrup = 106 m3 s−1) of the Oyashio water in the intermediate layer is directly entrained into the subtropical gyre (Yasuda, 2004), but the rest returns to the northeast in a highly stable path along the Subarctic Current (Kawai, 1972; Kuroda et al., 2017a; Sugimoto et al., 2014), which is almost parallel to the quasi-stationary western Isoguchi Jet (Fig. 1b) (Isoguchi et al., 2006; Wagawa et al., 2014; Kakehi et al., 2017; Miyama et al., 2018; Mitsudera et al., 2018). The Subarctic Current then bifurcates; one branch returns to the WSAG, and the other extends eastward to the Alaskan Gyre.

This study focused primarily on long-term variability of the intensity and shape of the WSAG, which is an essential contributor to the lateral transport of heat and materials in the North Pacific (Endoh et al., 2004; Yasunaka et al., 2014; Nakanowatari et al., 2017; Kunamoto et al., 2019), including their exchange with marginal seas (Nishioka and Obata, 2017). The WSAG also has a large impact on the vertical distributions of stratification and chemical materials linked with biological productivity around the center of the WSAG (Nagano et al., 2015; Wakita et al., 2017). On the basis of satellite-derived altimetry data, Qiu (2002) has reported the presence of multi-year variability of the WSAG. The shape of the WSAG changed from an elongated pattern in 1993–1995 to a contracted pattern in 1997–1999. The East Kamchatka Current and Oyashio were intensified during the time when the shape was contracted as a result of the baroclinic rather than barotropic response of the subarctic ocean to basin-scale wind stress on the North Pacific (e.g., Sekine, 1988a, Sekine, 1988b; Isoguchi et al., 1997; Isoguchi and Kawamura, 2006). Ito et al. (2004) have also reported a decrease and increase of the net Oyashio transport across the Oyashio Intensive observation line off Cape Erimo (OICE line) (Fig. 1b) during 1994–1996 and 1997–2000, respectively, which they estimated by combining altimetry with conductivity-temperature-depth (CTD) data. Their results are consistent with those of Qiu (2002), except for a time lag of about one year.

Nagano et al. (2015) have reported that the WSAG shrank in the north to just south of the western Bering Sea east of the Kamchatka Peninsula from the late 1990s to the mid-2000s. However, the Oyashio and Subarctic Current never disappeared, despite the northward shrinkage (Kuroda et al., 2015; Nakano et al., 2018). In fact, the meridional position of the Subarctic Current was very stable in 1993–2015 (Kuroda et al., 2017a; Nakano et al., 2018), and the Oyashio transport across the A-line (Fig. 1b) on the slope north of the Kuril-Kamchatka Trench decreased from the 1990s to the 2010s on an interdecadal (about 20-year) timescale because of the combined effects of the baroclinic response of the ocean to the basin-scale wind stress and frequent stagnation of the clockwise mesoscale eddies near the trench (Kuroda et al., 2015; Kuroda and Yokouchi, 2017).

A question arises to as whether and how the whole of the WSAG, including the East Kamchatka Current, Oyashio, and Subarctic Current, changed systematically after 1999, i.e., following the contracted pattern during 1997–1999. The WSAG is expected to weaken in response to westerlies and the Aleutian Low, which weakened from the 1990s to the 2010s (e.g., England et al., 2014; Kuroda et al., 2015; Wang et al., 2016; Wu et al., 2019). That weakening was related to the recent slowdown of global surface warming (Yan et al., 2016), which was tightly linked with La Niña–like cooling at the sea surface in the equatorial Pacific (Kosaka and Xie, 2013). It should be remembered that a regime shift occurred in the North Pacific in 1998/1999 (Hare and Mantua, 2000; Minobe, 2000, 2002; Bond et al., 2003; Jo et al., 2013). Moreover, the North Pacific Gyre Oscillation (NPGO), the second most important mode of the empirical orthogonal function (EOF) with respect to Sea Level Anomaly (SLA), tended to frequently exhibit positive values after the regime shift of 1998/1999, except during 2005–2006 and after 2014 (Fig. 2a) (Di Lorenzo et al., 2008). The weak negative correlation between the WSAG and the Alaskan Gyre suggested by the correlation of the NPGO index with altimetry-derived SLA data (Fig. 2b) implies that after 1998/1999, the WSAG weakened while the Alaskan Gyre strengthened. However, it has been unclear how the weakening of the westerlies, the regime shift, and the NPGO were linked to interdecadal variations of the WSAG throughout the period from the 1990s to the 2010s.

In the present study, we used geostrophic streamlines specified along the WSAG at the sea surface from altimetry-derived SLA data combined with state-of-the-art mean dynamic topography. Our goal was to use the streamlines to identify interdecadal variations of the entire WSAG during 1993–2017, with a particular focus on the intensity and shape of the WSAG, and to clarify the possible dynamics for these interdecadal variations in association with the basin-scale wind stress, the regime shift, and the NPGO.

This paper is structured as follows. The data that were analyzed and the methods used to determine the geostrophic surface streamlines are explained in Section 2. Section 3 describes interdecadal variability of the intensity and shape of the WSAG. In Section 4, we examine the possible dynamics of the interdecadal variability in terms of not only the baroclinic response of the subarctic ocean to the basin-scale wind stress but also mesoscale eddy activity, which changed in response to interdecadal variations of the WSAG. Section 5 provides a brief comparison with previous studies that have included the contribution of barotropic responses to basin-scale wind stress. The new findings from this study and brief remarks are summarized in Section 6.

Section snippets

Data

Daily maps of altimetry-based SLA data, referred to as “MSLA” data, were produced by the Archiving, Validation and Interpolation of Satellite Oceanographic data group (AVISO) (SSALTO/DUACS, 2014). The MLSA products were given at cartesian grid points with a horizontal resolution of 0.25°. The period of the data analysis was 1993–2017. We used MSLA products based on two-mission measurements using pairs of satellites with the same ground track, the sampling schedules of which did not change over

Overview of the WSAG

First, we briefly describe the general features of the WSAG based on the climatological mean and seasonality of the GS streamlines. Fig. 4 shows the distribution of all GS streamlines generated from daily MSLAs during 1993–2017 in terms of the number of GS streamlines per day in each 0.25 ° × 0.25 ° grid cell (i.e., the density of GS streamlines) together with the CNES-CLS13 MDT mean dynamic topography. A high density of GS streamlines is associated with a stable stream. The density tended to

Possible causes

In this section, the interdecadal variations of the WSAG are explained in terms of the baroclinic responses of the subarctic ocean to basin-scale wind stress in the North Pacific. In addition, we note that mesoscale eddies may have contributed to the interdecadal increase of SLA that was localized around the center of the WSAG and to the interdecadal decrease of the WSAG intensity (Fig. 7a).

Mean dynamic topography

This study revealed that the WSAG weakened gradually on an interdecadal timescale and that the WSAG shape changed abruptly around 2001 from a contracted pattern to a very elongated pattern. That abrupt change was manifested by basin-scale variations over the WSAG. It should be noted that our study did not identify the reported northward shrinkage of the WSAG; according to Nagano et al. (2015), the WSAG shrank in the north to just south of the western Bering Sea east of the Kamchatka Peninsula

Conclusions and remarks

We examined interdecadal variations of the Western Subarctic Gyre in the North Pacific mainly by analyzing ADT, specifically altimetry-derived SLAs during 1993–2017 combined with state-of-the-art mean dynamic topography. We specified GS streamlines of the WSAG associated with enclosed isolines of the ADT. The GS streamlines provided results consistent with previous studies for the climatological mean and seasonality of the WSAG. The WSAG intensity was the strongest in the late 1990s and

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by funds provided by the Fisheries Agency of Japan and the Japan Ministry of Education, Culture, Sports, Science and Technology (KAKEN grant 17H00775). However, the contents of this study do not necessarily reflect the views of the Fisheries Agency. We also extend a special thanks to the Deep-Sea Research associate editor, the editor-in-chief, and three reviewers for many supportive and constructive comments.

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