Search

The eastern part of the Tsugaru Strait (located at ~ 41.5° N) is a remarkable region of the coastal flow system around Japan due to the Tsugaru Warm Current (TgWC), an intensive eastward current that flows over its complex topography and exhibits marked tidal variation. The TgWC has a jetlike structure and demonstrates drastic seasonal changes in its path and vertical current structure. Using a high-resolution dataset obtained from a high-frequency radar system (HFR) and a coastal data assimilation experiment (JCOPE-T DA), we focused on the frontal variation, which is shorter than the seasonal timescale, along the axis of the TgWC in relation to the meanders of the jet. We found variations in the axis with multiple timescales between 1 and 14 days, especially in summer and autumn. The ~ 14-day variation, a remarkable occurrence on the west side of the eastern Tsugaru Strait, was consistent with the tidal variation in the strait. The zonal scale of the axis meander estimated by the HFR matched a scale several times larger than the internal deformation radius, especially in summer and autumn. JCOPE-T DA outputs showed that the crest of such a meander at greater depths progresses in relation to that at the surface. The outputs also suggested instabilities, indicating potential and kinetic energy conversion from the mean to the eddy field. The propagation speed of the several-day variation in the axis estimated from the HFR data was consistent with that of the two-layer baroclinic instability. The several-day variation also showed relatively high coherence with wind variation from summer to autumn, during which time the seasonal wind over the strait switched from easterly to westerly. These variations with multiple timescales imply an effect on north–south water mass mixing in the eastern part of the strait, including rapid acidification in the strait and intensification of stratification. [ABSTRACT FROM AUTHOR]

Chlorofluorocarbon (CFC) and sulfur hexafluoride (SF6) were used to investigate the timescale of Antarctic Bottom Water (AABW) that spreads off Cape Darnley (CD) in East Antarctica. The age of the AABW was estimated based on the observed SF6/CFC-12 ratio while taking into account tracer dilution by Lower Circumpolar Deep Water. Along the western canyons off CD and the ~ 3000 to 3500 m isobaths, the bottom water age was < 5 years, reflecting the spread of newly formed CD Bottom Water. Higher ages of ~ 8 years obtained for areas east of CD and > 20 years in the northwestern offshore region indicate inflows of AABW through the Princess Elizabeth Trough and Weddell Sea Deep Water, respectively. This study determined the age distribution in the region off CD, where three different types of AABW spread. [ABSTRACT FROM AUTHOR]

Seasonal pathway of the Tsugaru Warm Current (TWC), coastal boundary current flowing eastward from the Sea of Japan to the North Pacific Ocean, is investigated using data of surface currents measured by high-frequency ocean radars for the period 2014–2020. The TWC takes a pathway along the center of Tsugaru strait during summer season (41.64ºN), while during winter season the TWC moves to south to flow along the southern coast (41.55ºN). These pathways are associated with seasonal outflow modes known as "Gyre mode" and "Coastal mode," respectively. Seasonal changes in current velocity, characterized by intensified flow in summer and autumn seasons and weakened flow in winter season, are well correlated with seasonal changes of sea level difference between Fukaura and Hakodate. However, this clear relationship is not seen at the eastern exit of the strait (141.5ºE). There is a shallow sill that extends northward from the southern coast. HFR further reveals that Coastal mode is locally intensified and widened when passing through this sill. These TWC's responses are qualitatively explained by mass conservation for barotropic flow that flows in a horizontal two-dimensional channel with sea floor depth change. These responses over the sill are not seen during Gyre mode because the northward shift keeps the pathway away from the sill. During the Coastal mode, there are some cases in which the TWC does not ride on the sill but takes a pathway around the sill. Such patterns are seen when the TWC is weak, thus a weak Coastal mode. [ABSTRACT FROM AUTHOR]

NO concentration and its isotopomer ratios were measured over a wide area from San Diego to Honolulu in the eastern subtropical North Pacific (ESNP). Waters in the study area had an NO maximum (38.2-50.5 nmol kg) at 600-1000 m depth, which is similar to the profiles obtained previously in other areas in the North Pacific. We separated the seawater into five water masses (two for the surface layer, two for the middle layer, and one for the deep layer) and deduced NO production-consumption mechanisms in each water body by use of NO isotopomer ratios. The results showed that the mechanisms differ slightly among water masses. In the 'coastal' surface layer, NO is produced by nitrification (NHOH oxidation). In the 'open ocean' surface layer, it is produced mainly by nitrifier denitrification and to a lesser extent by nitrification under substrate-limited conditions. In both 'upwelling' and 'open ocean' middle layers it is produced mainly by denitrification and to a lesser extent by nitrifier denitrification. It is also partly reduced. In the deep layer, it is produced predominantly by denitrification with partial reduction. In this way, isotopomers aid elucidation of production-consumption mechanisms of NO in the sea even though the mechanisms cannot always be ascertained. [ABSTRACT FROM AUTHOR]