Effect of a current trapped by a continental slope on the pathway of a coastal current crossing Toyama Trough, Japan

Abstract

To clarify the Tsushima Warm Current (TWC) system around a downstream region of Noto Peninsula (NP) where Toyama Trough (TT) causes a discontinuity of the along shelf-current, numerical experiments were performed using two-layered ocean model with simplified bottom topography. When a current trapped by the continental slope representing an offshore branch of TWC (OB) encounters the NP and TT, the OB was trapped by the coast facing the TT. A clockwise lee-eddy developed between a current axis and a coastline over the TT, which results in a current path transition offshore (CPT). The OB finally adjusted to a current path which crosses TT by shifting from the discontinuity point of the continental slope. If a coastal current representing a coastal branch of the TWC (CB) developed under this situation, the CB also formed lee-eddy within the identical region of the OB’s eddy, resulting in CPT. Period for the CPT of the CB was shorter with increase of the OB volume transport. These results indicate that the OB acted to accelerate CPT of the CB within the TT. To support these results, observational data comprising mooring current, tidal, and CTD measurements obtained in and around the TT, along with the output from high-resolution data-assimilated ocean model DR_C were analyzed. Although the lee-eddy of the CBs was generated in every summer, the CPT events did not fully develop when the OB was absent over the continental slope because of the slow growth rate of the lee-eddy.

Appendix

Interannual variation of the current path transition of the coastal branch found in observation data.

To verify the calculated result by the DR_C, we attempt to extract signals of the lee-eddy generation, i.e., complete or incomplete CPT from several kinds of observation data. We can use the d/dt (∆η_ok)_min (Fig. 9) calculated from the sea-level data as an index of the complete and incomplete CPT to compare model results against the observations. First, we simply explain signals of the CPT which occurred in 2012 and are shown by IG17 (refer to IG17 about the details).

Figure 

Fig. 11

a Time series of 15-day running averaged alongshore current velocities at St. 1. b The same as (a) but for St.2. Clockwise angles defined as positive values are 30° at both stations (with the coast on the right). c Temporal variations of 15-day running averaged sea level at Ogi. (d) The same as (c) but for Kashiwazaki. (e) Time series of 15-day running averaged sea level difference obtained by subtracting Ogi from Kashiwazaki (∆η_ok, line in black). Line in red indicate time derivative of the sea level difference (d/dt (∆η_ok)). f–j and k–o are the same as (a)–(e), respectively, but for 2013 and 2014

11a and b depict running averaged currents over a 15-day time period with the coast on their right at St.1 and 2, respectively. The evidence of the CPT, shown by IG17, consists of the wave-like perturbation of the current that was found at St.1 (arrows 1–3 in Fig. 11a). This signal was accompanied by strengthening of the alongshore current at St.2 (an arrow in Fig. 11b) due to seasonality of the Tsushima Warm Current around the Noto Peninsula region. The strengthening of the current also emerged as an increase of the ∆η_ok, that is, the sea level rising around the coastal area (arrow 1 in Fig. 11e). This increase of the ∆η_ok diminished around the time when wave-like current perturbation was observed at St.1 (arrow 2 in Fig. 11e) because of the sea level increase around Ogi (an arrow in Fig. 11c). This resulted from the arrival of high sea level event at Ogi due to the lee eddy development as shown on day 1840 in Fig. 3. These are the signals of typical CPT of the CB.

Next, we check whether or not these characteristics can be identified in 2013 and 2014. In 2013, we can find (1) wave-like perturbation of the current at St.1 despite its weakness (arrows 1–3 in Fig. 11f), (2) an amplification of the alongshore current accompanied by sea level rising at the coastal region (an arrow in Fig. 11g and an arrow 1 in Fig. 11j), and (3) a rapid decrease of the ∆η_ok in September (arrow 2 in Fig. 11j). These three notions are important points in 2012 with respect to whether the lee-eddy generation results in the complete CPT or not. Although there are several discrepancies, e.g., current amplitude, the three characteristics in 2013 were fundamentally identical with those of 2012. This suggests that the CPT occurred in 2013.

In 2014, the aforementioned points were identified in the current and sea-level data (arrows 1–2 in Fig. 11k for (1), an arrow in Fig. 11l and an arrow 1 in Fig. 11o for (2), an arrow 2 in Fig. 11o for (3)). However, these signals are slightly different from those of 2012 and 2013. First, the alongshore current reverses its direction only once (from an arrow 1 to an arrow 2. This might imply incomplete CPT in 2014, since the current with the coast on its right shown by an arrow 3 in Fig. 11a was interpreted as the downstream current of the CCTT in 2012 by IG17. Second, the signal of ∆η_ok diminishing is not clear. This is considered to be caused by the small amplitude of the sea level fluctuation at Ogi and Kashiwazaki and resulted in the slow depression of the ∆η_ok in 2014 compared to those of 2012 and 2013. These characteristics could be expressed by the rate of the ∆η_ok depression, that is, the d/dt (∆η_ok)_min discussed in the subsection 4a and 4b. The value of d/dt (∆η_ok)_min in 2014 was about a half of those of 2012 and 2013 (values in red beside arrows in Fig. 11e, j, o). The d/dt (∆η_ok)_min obtained from observational data in 2012, 2013, and 2014 are plotted as a black line in Fig. 9. Variation pattern of the observation and the model have qualitatively common characteristic that d/dt (∆η_ok)_min in 2014 is smaller than those in 2012 and 2013. This fact agrees with the behavior of the CB shown by the model, i.e., the CB completed its CPT in 2012 and 2013, whereas the CPT ended incompletely in 2014. The current fluctuations at St.1also support these interannual fluctuations of the CPT as mentioned above, although the current signals are not well resolved spatially.

To compensate for the lack of spatial current structures, the statuses of CPTs are discussed using CTD transect observations. Figure 

Fig. 12

Cross sections of sigma theta (contour lines) along CTD transect (Fig. 1) observed at each day. Geostrophic currents (color hatch) which are calculated from the sigma theta are superposed (unit: in cm s⁻¹). Lines in red indicate zero-contour lines of the geostrophic current. Arrows with and without numerals are used for explanations in Appendix. Bidirectional arrows indicate location of the Sado Island shown in Fig. 1

12 shows vertical cross sections of sigma–theta and geostrophic current along the CTD transect obtained in 2012, 2013, and 2014. Here, the geostrophic calculations were made using sigma–theta, and we chose the shallower bottom depth between the two stations as the reference layers. In 2012, the current with the coast on its right was stronger in September (arrow 1 in Fig. 12b) relative to the current in August (Fig. 12a). The northeastward current perpendicular to the transect around Sado Island (arrow 2 in Fig. 12b) was amplified simultaneously and accompanied by a southwestward current (arrow 3 in Fig. 12b) just off Sado Island. This structure indicates clockwise eddy which developed over Toyama Trough as explained by IG17. These results support the fact that the lee eddy was formed in association with the CB amplification as identified by mooring observations shown in Fig.11. Finally, despite a decrease of the coastal current, the northeastward current remained around the Sado Island which expressed the CCTT (an arrow in Fig. 12c).

Characteristics of the geostrophic current system in August of 2013 indicated by arrows 1–3 in Fig. 12d has common features with the current pattern in September of 2012 shown by arrows 1–3 in Fig. 12b suggesting the existence of a clockwise eddy just off Sado Island. The northeastward current evoking the CCTT was recognized clearly around more off the Sado Island in September and October in 2013 (arrows in Fig. 12e, f). The current in October was not accompanied by coastal current with the coast on its right. These same characteristics can be seen during the event that occurred in October of 2012 (an arrow in Fig. 12e). These facts support the speculation that the lee eddy event over Toyama Trough occurred in 2013 in a very similar fashion as the 2012 event.

In 2014, although a coastal current with the coast on its right was found in a cross section in August (an arrow in Fig. 12g), the coastal current was not amplified in September (arrow 1 in Fig. 12h) unlike the feature found in 2012 and 2013. On the other hand, the current system found in September indicated by arrows 1–3 in Fig. 12h confirms the existence of the lee-eddy as shown in September in 2012 and August in 2013 (arrows 1–3 in Fig. 12b, d).

The geostrophic currents in 2014 were slower than those of 2012 and 2013 throughout the period. This feature supports the fact that the ∆η_ok was relatively small in 2014 (Fig. 11o). In October, the weak coastal current remained (Fig. 12i), which alludes that the CPT event ended incompletely despite of the lee-eddy formation as shown in Fig. 7i, that is, the CCTT was not formed in 2014. Unfortunately, we were not able to obtain an information about the geostrophic current field around offshore region because of data lacking in October of 2014 (Fig. 12i). Additionally, we can find the southwestward jet in the offshore region in Fig. 12h. This might be an eddy, since this area is known to be an eddy-rich area (Isoda 1994; Kawaguchi et al. 2020; Wagawa et al. 2020).

The results obtained from the CTD transects prove the interannual variations of the CPT shown as d/dt (∆η_ok)_min from observation in Fig. 9. Therefore, we can conclude that the CPTs of the CB were adequately promoted in 2012 and 2013, whereas the CPT ended incompletely in 2014. In addition, we can point out high accuracy of the DR_C in temporal variations. The CTD observations suggest that the lee-eddy can be found in 2013, 1 month earlier than that in 2012. If we recheck the time series of the ∆η_ok and its time derivative in Fig. 11e and j, the decreasing of ∆η_ok were recognized to start at early and middle of September in 2013 and 2012, respectively (arrows in red in Fig. 11e and j). These results express a possibility that the CPT in 2013 occurred about a half—1 month earlier than that of the 2012. The features were found in the results from the DR_C as shown in Fig. 8.