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Superposition of coastal-trapped waves and Kuroshio warm water intrusions caused unusually high sea levels around the southern coasts of Japan in early September 1971

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Abstract

Unusually high sea level (UHSL) in early September 1971, which caused coastal flooding around the southern coasts of Japan despite no severe weather conditions, is examined using a coastal assimilation system with 2-km resolution. The observed duration of high sea-level anomalies (SLAs) for the UHSL was successfully represented by assimilation results. Through sensitivity experiments, we investigated the contribution of two factors, the wind and the Kuroshio effects, as suggested by previous research. The northeasterly winds associated with Typhoon 7123 induced coastal-trapped waves (CTWs) along the Kashima-nada Sea. The CTWs propagated clockwise along the coast at a speed of about 2.3 m/s and caused the sea level to increase by about 15 cm in Sagami and Tokyo Bays. A cold eddy associated with a Kuroshio meander was formed, and as a result, the warm Kuroshio water intruded into the Enshu-nada Sea due to the northward flow in the eastern flank of the eddy. The warm water intrusion significantly contributed to the duration of SLAs of about 30 cm around the Enshu-nada and Kumano-nada Seas. It was also shown that the high SLAs for the UHSL were quantitatively explained by the sum of the two factors. Thus, we conclude that the UHSL event was caused by the superposition of the wind-induced CTWs and the Kuroshio-induced warm water intrusions.

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Acknowledgements

We thank the members of the Department of Atmosphere, Ocean, and Earth System modeling Research in Meteorological Research Institute (MRI) for fruitful discussions and helpful comments. Thanks are extended to two anonymous reviewers for their variable comments that greatly helped improve an early version of the manuscript. This work was funded by MRI and partly supported by JSPS KAKENHI Grant Numbers 19K03978 and 20H01968. The tide-gauge data and sea level pressure data were provided from Japan Meteorological Agency. In-situ temperature and salinity profiles were obtained from the World Ocean Database (https://www.ncei.noaa.gov/data/oceans/woa/WOD/). The authors would like to thank Enago (https://www.enago.jp/) for the English language review.

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Correspondence to Nariaki Hirose.

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Appendices

Appendix 1

1.1 Assessment of DSJRA-55 wind and SLP

We assessed the SLP and wind in DSJRA-55 compared with the data observed at the weather stations (Fig. 1b). The time interval is 1 h in DSJRA-55 and 3 or 6 h in observation. Two typhoons passed southeast of Japan Island in early September 1971, as shown in Fig. 1a. Both observations and DAJRA-55 show sea-level depression at the end of August and the beginning of September. Around the 7th of September, the SLP at Choshi is quite similar to that of observation, though at Tokyo and Omaezaki, the SLP in DSJRA-55 is slightly lower than that of observation. Strong northerly winds due to typhoons 7123 around September 1st and 7125 around September 8th are quantitatively reproduced by DSJRA-55. Therefore, DSJRA-55 will be appropriate to simulate the UHSL in 1971. See Fig. 15.

Fig. 15
figure 15

Sea-level pressure, wind speed, and wind direction at weather stations close to tide-gauge stations, Choshi (CS) (a, d, g), Tokyo (TK) (b, e, h), and Omaezaki (OM) (c, f, i). The black circle is an observation and the gray line is the DSJRA-55 obtained at grid points near the observation stations.

Appendix 2

2.1 An estimation method of model sea-level anomaly

The SLA calculated from the tide-gauge data is the anomaly from the astronomical tide and the mean sea level (Hiyajo et al. 2011). Harmonic analysis of the previous 10 years is used to determine the amplitude and phase of the 60 tidal components for each year, and the averaged amplitudes and phases are used to reconstruct the astronomical tide. The redeveloped sea level is subtracted from the original sea level. The annual mean sea level for the previous 5 years is used to calculate the mean sea level. However, it is difficult to obtain the model SLA based on the same procedure described above, because it requires a long-term simulation. Instead, we estimated the equivalent amount of model SLA described below.

We chose to use the major eight tidal components (M2, S2, K2, N2, O1, K1, N1, and Q1), which were applied as tidal forcing (Sakamoto et al. 2013). A 30-min interval snapshot was used to perform harmonic analysis on the model SSH, including the effects of tide, wind, and SLP. The data period is from January 1st to December 31st, 1971 on the experiment driven by the wind and SLP of JRA55-do. CNTL in Sect. 3 was applied to the wind and SLP of DSJRA-55. However, the same tidal forcing was used, so that the harmonic analysis amplitudes and phases were nearly identical between the different atmospheric conditions. Using the obtained harmonic constants, we reconstructed the SSH which is made up of the major eight tides at 30-min intervals.

One of the large components of the 60 tidal components, except for the major 8 tidal components, is the annual tide of Sa. We assumed that Sa is explained by seasonal variation. To obtain the seasonal variation from the observation, a 30-day running mean was applied to the tide-gauge data at 1-h intervals (

Fig. 16
figure 16

a Seasonal component of observed sea-levels from 1956 to 1965 at Uchiura subtracted by 30-day running the mean filter, and b the same with (a) but from August to September. Red line in (b) is the average of a seasonal component over 10 years. c Sea level (gray), sea level with SLP correction (black), and sea-level with SLP and seasonal correction (red) in a model in 1971. An offset of the seasonal component is set to 1st August

Fig. 16a). Seasonal variations are only a few centimeters from the 1st August to 30th of September (Fig. 16b). The averaged values over the last 10 years are removed from the model as a seasonal variation. The averaged period was chosen from 1956 to 1965, based on the method of Hiyajo et al. (2011) (Fig. 3 in their literature). Here the mean value on August 15th is chosen as the reference value to be zero for seasonal variation, and the seasonal variation is subtracted from the model (Fig. 16c).

The mean sea level in the model differs from observation but the data for the same period is difficult to obtain. Since the sea-level variation was small, we assumed that the average sea level from August 15th to August 25th could be used as a reference (Fig. 3). The average of the model results during this period is subtracted from the model, and the average of SLA in observations is added to the model. Figure 3 depicts the model’s SLA of after accounting for the tidal component and the reference level.

In addition, the model does not consider the eustatic sea level changes due to mass changes caused by glacier melting and thermal expansion for reasons of volume conservation by assuming the Boussinesq approximation. They only have variations of a few mm to cm on an annual scale (e.g., Kuragano et al. 2014), so they are considered to be sufficiently small compared to the scale of variability of the UHSL in this study and were ignored.

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Hirose, N., Usui, N., Sakamoto, K. et al. Superposition of coastal-trapped waves and Kuroshio warm water intrusions caused unusually high sea levels around the southern coasts of Japan in early September 1971. J Oceanogr 78, 475–493 (2022). https://doi.org/10.1007/s10872-022-00655-4

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