Frontal Waves in the East of the Tsugaru Strait Revealed by the High-Frequency Radar Observation

Surface velocity observations of the eastern part of the Tsugaru Strait made by the high-frequency radar revealed frequent occurrence of frontal waves along the axis of the Tsugaru Warm Current in 2017–2019. The current axis (maximum of the zonal velocity in the meridional direction) disturbed in the north–south direction with period of ~ 13.7 days that is dominant timescale of tide modulation in the strait, in addition to that of ~ 27.3 days. The amplitude of the axis fluctuation increased in the downstream direction, from the eastern neck of the channel (~ 141.0°E) to the outlet of the strait adjacent to the Pacific Ocean (~ 141.5°E). The propagation speed of the disturbance was slower than that due to surface advection especially in the seasons when the stratification was developed, and agreed well with that estimated from the theory based on the two-layer baroclinic instability model except for winter. The north–south modulation of the axis at the outlet of the strait (~ 141.5°E) could cause short-term (from 20 days to 1 month) variations of an anticyclonic gyre of the Tsugaru Warm Current that is developed in the east of the outlet from summer to autumn reported by the previous studies.

Centennial- to millennial-scale change of Holocene shallow marine environments recorded in ostracode fauna, northeast Japan

Using the record of shallow-marine ostracode fauna and sediment grain size data from an 865-cm-long piston core obtained from Mutsu Bay, northeast Japan, paleoceanographic changes of the bay were determined at high resolution for the early to middle Holocene. Changes in the relative frequencies of several species showed periodicities of 1300–1800 years similar to Bond cycles. At around 10,300 cal yr BP and again, at 9500–9300 cal yr BP, cold water strongly influenced the bay owing to cooling events. Since at least 10,200 cal yr BP the Tsugaru Warm Current influenced the surface waters, and since ca. 7400 cal yr BP, also the bottom waters of the bay. Since ca. 8400 cal yr BP the water depth rapidly increased and peaked at 7000–5900 cal yr BP due to global sea-level rise. Subsequently, a drop of water temperature and sea level in the bay at 5900 and around 4000 cal yr BP influenced the composition of the ostracode assemblages. These millennial-scale oscillations in relative sea level and bay temperature during the Holocene can be correlated to paleoclimate records elsewhere.