The time series of cross-sectional area of STMW in the 137ºE section (Oka et al. 2019) was extended to the latest observation in summer 2020 (Fig. 1). Since mid-1970s, the cross-sectional area had increased (decreased) during stable (unstable) KE periods with a time lag of about one year, as demonstrated by Oka et al. (2019). Since 2016, however, it has declined greatly in spite of the prolonged stable-KE period. Such decadal variations were commonly observed in the winter and summer 137ºE sections.
Decomposition of the cross-sectional area into θ bins reveals that the STMW in the winter 137ºE section consisted mainly of 16–17 ºC and 17–18 ºC varieties, the sum of which well explained the decadal variability of the total cross-sectional area (Fig. 2a). In addition, there are smaller contributions by the 15–16 ºC variety, mainly prior to mid-1990s, and the 18–19 ºC variety after mid-1990s, which also reflects the long-term warming of STMW. To examine in which region each variety of STMW was formed in the past 15 years, zonal distribution of core θ in spring was plotted over the formation region using Argo profiles (Fig. 3). As demonstrated by previous studies (Suga and Hanawa 1990; Oka and Suga 2003), colder STMW tended to be formed toward the east, but the zonal change of core θ was not continuous. In most years, relatively cold STMW of 16–18 ºC was formed east of 140ºE, while relatively warm STMW of 18–19.5 ºC was formed west of 150ºE and intensively west of 140ºE. There seems to be a gap of core θ near 140ºE, as demonstrated by Sugimoto and Hanawa (2014) for LM and offshore NLM periods of the Kuroshio during which the Kuroshio recirculation gyre split into two parts. Thus, STMW appearing in the winter 137ºE section consists mainly of that colder than 18 ºC formed south of the KE.
In the summer 137ºE section (Fig. 2b), the 16–17 ºC and 17–18 ºC varieties dominated the KE-related decadal variability, as in the winter section. In addition, the 18–19 ºC variety originating near the 137ºE section occupied a comparable cross-sectional area to the 16–17 ºC and 17–18 ºC varieties, except in LM periods during which the 18–19 ºC variety almost disappeared. Argo float data supportively exhibit that in the late winters of 2005 and 2018–2020 during the LM period, the 19–19.5 ºC variety was formed south of Japan and the 16–17 ºC and 17–18 ºC varieties were formed south of the KE, while the 18–19 ºC variety was absent (Fig. 3).
Since 2016, not only the 18–19 ºC variety in the summer 137ºE section but also the 16–17 ºC and 17–18 ºC varieties in both the winter and summer sections have shrunk (Fig. 2). This suggests that the westward advection of STMW from the region south of the KE has been interrupted by the LM since summer 2017, as demonstrated by the previous studies (Suga et al. 1989; Suga and Hanawa 1995a,b). There is also a possibility that the STMW formation south of the KE has decreased, possibly in relation to the LM. Therefore, Argo float data will be analyzed next to examine the formation and advection of STMW over its entire distribution region in the past 16 years.
The STMW thickness from individual Argo float profiles (if there are more than one STMW layer in a profile, their thickness was summed up) was horizontally interpolated and mapped for each month in the same manner as Oka et al. (2015). Due to the change in the STMW definition (Sect. 2), the total volume of STMW in the study region of 15–40ºN, 120ºE − 170ºW during 2005–2014 (black curve in Fig. 4) is larger than that presented by Oka et al. (2015; their Fig. 7a) by 9% on average, while the two time series exhibit similar seasonal and decadal variability (correlation coefficient = 0.98), including a decrease in 2006 − 2009 in the unstable KE period and an increase after 2010 in the stable KE period. Since 2016, the total volume has gradually declined in spite of the extended stable-KE period, exhibiting the same tendency as observed in the cross-sectional area in the 137ºE section (Fig. 1).
Oka et al. (2015) divided the STMW volume into two regions north and south of 28ºN where STMW is ventilated and unventilated, respectively (Oka and Suga 2003; Oka 2009). We further divide these regions zonally at 140ºE where the gap of core θ existed (Fig. 3), defining four regions: the NE region at 28–40ºN, 140ºE–170ºW, the NW region at 28–40ºN, 120–140ºE, the SE region at 15–28ºN, 140ºE–170ºW, and the SW region at 15–28ºN, 120–140ºE (Fig. 4). In the NE region, the STMW volume exhibited a distinct seasonal variation with a maximum in February–April and a minimum in November–December. The volume in the NW region showed a less clear seasonal variation with a maximum around February–May and a minimum around September–December. Seasonal variations were much smaller in the SE and SW regions, indicating that STMW observed in these regions were formed in the NE and NW regions and advected southward. As the formation location within the NE and NW regions critically depends on the θ of STMW (Fig. 3), it is instructive to also decompose the STMW volume into θ bins.
The volume of the 16–18 ºC variety (Fig. 5a) accounted for the great majority of the whole STMW layer (16–19.5 ºC; Fig. 4), as in the 137ºE section (Fig. 2), and exhibited a similar variation to the whole STMW layer in each region, except in the NW region where the 16–18 ºC variety lacked seasonality. Seasonal variations were seen only in the NE region where the volume reached a maximum in March–April and a minimum in November–December. The 16–18 ºC variety is formed in this region and advected westward and southward to the other three regions.
The formation volumeFootnote 1 of the 16–18 ºC variety in the NE region was small (0.21 × 1015 m3 on average) during 2006–2009 in the unstable KE period and large (0.38 × 1015 m3) during 2010–2015 in the stable KE period (Fig. 5b). It tended to be smaller (larger) than the erosion volume during the former (latter) period, causing a decadal variation of volume in this region with a minimum in 2009 and a maximum in 2014 (Fig. 5a). This decadal volume variation spread to the other regions through advection; the annual-mean volume in the NW, SE, and SW regions during 2005–2015 was highly correlated (coefficient > 0.9) with that in the NE region when lagged by 1 year.
In 2016, the formation volume dropped to 0.23 × 1015 m3 despite the KE still being in the stable state (Fig. 5b). Late winter mixed layers in the NE region of this year were anomalously shallow (Fig. 6) and warm (Fig. 3), and became relatively thin STMW over an unventilated, colder STMW layer formed in the previous year. (Such insufficient development of mixed layers in the NE region in the late winter of 2016 will be an interesting theme for future studies.) The formation volume increased to 0.35 × 1015 m3 in 2017 in the short unstable-KE period, but did not recover any further in the following LM-forced stable-KE period (Fig. 5b). The formation volume during 2018–2020 was 0.28 × 1015 m3 on average, an intermediate value between the 2006–2009 unstable-KE and 2010–2015 stable-KE periods.
Furthermore, the lagged relationship between the annual-mean volume in the NE region and that in the three other downstream regions observed in 2005–2015 seems to have failed since the current LM period began. During the unstable-KE period of 2006–2009, the annual-mean volume of the 16–18 ºC variety dropped by 26% from 2006 to 2008 in the NE region and by 32, 48, and 50% from 2007 to 2009 in the SE, NW, and SW regions, respectively (Fig. 5a). At the beginning of the stable-KE period since 2010, the annual-mean volume increased by 68% from 2010 to 2012 in the NE region and by 32, 181, and 185% from 2011 to 2013 in the three other regions. After the current LM began, however, the annual-mean volume in the NE region increased by 6% from 2017 to 2019, while that in the SE, NW, and SW regions dropped by 22, 36, and 39%, respectively, from 2018 to 2020. The unanticipated volume decline in the downstream regions, particularly the larger decrease in the NW and SW regions west of 140ºE, suggests that the westward advection of STMW was interrupted by the LM, as demonstrated by the previous studies. When we look at the thickness distributions of the 16–18 ºC variety in recent years (Fig. 7), the 16–18 ºC variety thicker than 100 dbar had been advected westward from the region south of the KE to the region south of Japan prior to mid-2017. Since the current LM began, the 16–18 ºC variety has had to make a southern detour around the LM to enter the region south of Japan, which likely led to the decreasing advection with time. Thus, the reduced formation in the NE region since 2016 and the decreasing advection to the downstream regions since 2018 have contributed to the decline of annual-mean total volume of the 16–18 ºC variety from 0.74 × 1015 m3 in 2015 to 0.55 × 1015 m3 in 2020 (Fig. 5a).
The 18–19 ºC variety occupied a much smaller volume than the 16–18 ºC variety (Fig. 8). Its volume in the NE region exhibited a seasonal variation with a maximum in January–February (Fig. 8a), which is 2 months earlier than the peak of the 16–18 ºC variety in this region (Fig. 5a). This implies that some portion of the 18–19 ºC variety observed in January–February was further cooled to become the 16–18 ºC variety in March–April. Consistently, the 18–19 ºC variety widely spread over the NE region in January–February (not shown), but was found mostly west of 150ºE in April (Fig. 9).
The 18–19 ºC variety was also formed in the NW region and, to the less extent, in the SE and SW regions south of 28ºN, where its volume was at a peak in March–April of particular years such as 2006, 2008, 2011–2015, and 2017 (Figs. 8b–d, 9). On the other hand, its formation in the NW region almost ceased in 2005 and 2018–2020 during the LM period, as observed in the 137ºE section (Fig. 2). After spring, the 18–19 ºC variety shrank before being substantially advected southward (Fig. 9). Its volume in the NW and SW regions tended to be larger in July than in the following January (Fig. 8b, d), which is consistent with its cross-sectional area in the 137ºE section being larger in summer than in winter (Fig. 2).
The 19–19.5 ºC variety also occupied a small volume in the whole STMW layer (Fig. 10). In the NE region, its volume reached a maximum mostly in January and was small after spring (Fig. 10a). Such a seasonal variation probably reflected the formation process of colder varieties of STMW and not that of the 19–19.5 ºC variety. In the NW region, the 19–19.5 ºC variety had the largest volume in March–May (in other words, it was formed) in 2005, 2007, 2009, 2010, 2016, 2018, and 2020 (Figs. 10b, 11). Interestingly, these were the years in which the 18–19 ºC variety was not formed in this region (Fig. 8b). The formation of the 19–19.5 ºC variety in 2018 and 2020 in the current LM period supports Sugimoto and Hanawa (2014), who analyzed Argo float data in 2005–2011 to demonstrate that STMW warmer than 19 ºC was formed in the recirculation gyre south of Japan when the Kuroshio took an LM or offshore NLM path.
Like the 18–19 ºC variety, the 19–19.5 ºC variety shrank after spring and almost disappeared by fall, but exceptionally survived for more than one year south of Japan in 2018–2019 (Figs. 10b, 11). Its formation and temporal evolution in relation to colder varieties of STMW in the current LM period will be detailed in our accompanying paper (Nishikawa et al. in prep.).