Abstract Operational ocean nowcast/forecast systems require real-time sampling of oceanic data for representing realistic oceanic conditions. Satellite altimetry plays a key role in detecting mesoscale variability of the ocean currents. The 10-day sampling period and horizontal gaps between the altimetry tracks of 100 km cause difficulties in capturing shorter-term/smaller-scale ocean current variations. Operational systems based on a three-dimensional variational method (3dVar) do not take into account temporal variability of the data within data assimilation time windows. Four-dimensional data assimilation technique is considered as a possible tool for more efficient utilization of the observations arriving from satellite altimeters by the dynamically constrained interpolation. In this study, we develop and test the performance of the adjoint-free four-dimensional variational method (a4dVar) for operational use in regional models. Numerical experiments targeting the Kuroshio path variations south of Japan demonstrate that the a4dVar scheme dynamically corrects the initial condition so as to effectively reduce the satellite altimetry data misfit during a 9-day time window. The corrected initial condition further contributes to improvements in the skill of reconstruction of the Kuroshio path variation in a 30-day lead hindcast run.
Abstract The Kuroshio and Gulf Stream (GS), Northern Hemisphere warm ocean currents, have narrow bands of intense jets and vigorous eddies. While the importance of them in regulating physical and biological ocean systems and climate has been increasingly recognized, the operational predictability of year‐to‐year variations in these jets and eddies has not been quantitatively assessed yet. Through a series of experiments using an eddy‐resolving dynamical ocean forecasting system, here we show that observed year‐to‐year variations in the intensities of the jets and eddies in the Kuroshio Extension and GS regions can be skillfully predicted up to a lead time of about 2 years. Sources of predictability mostly come from initial ocean memories, and variations in local atmospheric conditions play a secondary role. Our results demonstrate that the observed multiyear variations in ocean jets and eddies in these regions are operationally predictable when ocean models are properly initialized with sufficient spatial resolution.
The possible effect of directional larval swimming on the recruitment success of the Japanese eel, Anguilla japonica, was examined with a three-dimensional particle-tracking ocean circulation model using horizontal northwestward swimming and diel vertical migration (DVM). Four separate experiments included virtual larvae (v-larvae) movement from the spawning area over 290 days (total migration) and 160 days (stage A), from the STCC eddy region in 70 days (stage B), and from the origin of the Kuroshio in 60 days (stage C) to evaluate the effect of directional swimming and DVM compared to simple drifting. Passive or random swimming were not the most effective strategies for larvae dispersing from the spawning area because most v-larvae remained south of 20°N without entering the Kuroshio. Northwestward swimming resulted in wider dispersion and a better chance of successful recruitment, with v-larvae becoming widely distributed in the STCC eddy zone, arriving at the east coast of the Philippines (stage A), escaping the STCC eddy area and reaching the Kuroshio (stage B), and crossing the Kuroshio into the East China Sea shelf (stage C). DVM slightly shortened the migration period due to faster shallow layer ocean currents during nighttime. The NEC transported non-swimming v-larvae westward to the Kuroshio and occasionally northward into the Subtropical Countercurrent (STCC) area where eddies transported v-larvae westward into the Kuroshio, but less than with swimming. Directional swimming increased recruitment success, northwestward swimming was more effective than other directions, and a slower swimming speed was still better than no/random swimming in sensitivity tests. The present study demonstrated a first view of the possibility that Japanese eel larvae might be able to use a strategy of single-direction swimming to increase arrival at their recruitment areas.
Abstract 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.
