Shin'ichiro Kako

High‐resolution ASCAT wind vector data set gridded by applying an optimum interpolation method to the global ocean

[1]xa0A new global 0.25° longitude by 0.25° latitude gridded daily wind vector data set was produced by applying an optimum interpolation method to Advanced Scatterometer (ASCAT) data as a quick recovery of the QuikSCAT/SeaWinds (QSCAT) mission that ended in November 2009. ASCAT measures the wind speed and direction over the global ocean twice a day as QSCAT did. However, the area covered by ASCAT accounts only for 60% of that covered by QSCAT during the same period because its spatial resolution measured along two parallel 550-km swaths separated by 700 km is coarser than that of QSCAT with a single 1800-km width swath along its orbit. The accuracy of gridded wind data sets constructed using ASCAT or QSCAT is therefore validated by comparing with wind data sets obtained from meteorological buoy observations. The comparisons show that both satellite wind observations agree well with the buoy observations, and the accuracy of these satellite-derived wind data sets mostly meets mission requirements (biases and root-mean square differences are less than 0.5 and 2.0 m/s, respectively) except for coastal and heavy-rain regions. In addition, intercomparison of ASCAT and QSCAT products demonstrates that the spatiotemporal wind variations derived from ASCAT are in reasonable agreement with those derived from QSCAT. Thereby, the data lost in the absence of QSCAT can be recovered using the ASCAT wind data set.

Transient and local weakening of surface winds observed above the Kuroshio front in the winter East China Sea

To confirm whether surface winds strengthen above warm waters around oceanic fronts using in situ data, a field measurement was conducted using both expendable bathythermographs and Global Positioning System sondes released concurrently across the Kuroshio front in the East China Sea in December 2010. In contrast to previous studies mainly based on satellite observations, the finding of the present field survey is the local weakening of surface winds at the northern flank of the Kuroshio front. From the above field observation in conjunction with a regional numerical model experiment, it is suggested that the northwesterly winds crossing the Kuroshio front from the cooler side first weaken at the northern flank of the front because of the onset of upward transfer of the “nonslip” condition at the sea surface. Thereafter, as the atmospheric mixed layer with warm and humid air mass develops gradually downwind over the Kuroshio region, the surface winds are gradually accelerated by the momentum mixing with strong winds aloft. The surface winds remain strong over the cool East China Sea shelf, and it is thus considered that the surface winds only weaken at the northern flank of the Kuroshio front. However, numerical modeling indicates that this local weakening of the surface winds occurs as a transient state with a short duration and such a structure has thus rarely been detected in the long-term averaged wind fields observed by satellites.

The potential of oceanic transport and onshore leaching of additive-derived lead by marine macro-plastic debris.

The long-distance transport potential of toxic lead (Pb) by plastic marine debris was examined by pure water leaching experiments using plastic fishery floats containing high level of additive-Pb such as 5100±74.3mgkg(-1). The leaching of Pb ended after sequential 480-h leaching experiments, and the total leaching amount is equivalent to approximately 0.1% of total Pb in a float. But it recovered when the float was scratched using sandpaper. We propose that a low-Pb layer, in which Pb concentration is negligibly small, be generated on the float surface by the initial leaching process. Thickness of the layer is estimated at 2.5±1.2μm, much shallower than flaws on floats scratched by sandpaper and floats littering beaches. The result suggests that the low-Pb layer is broken by physical abrasion when floats are washed ashore, and that Pb inside the floats can thereafter leach into beaches.

Fate of microplastics and mesoplastics carried by surface currents and wind waves: A numerical model approach in the Sea of Japan

A numerical model was established to reproduce the oceanic transport processes of microplastics and mesoplastics in the Sea of Japan. A particle tracking model, where surface ocean currents were given by a combination of a reanalysis ocean current product and Stokes drift computed separately by a wave model, simulated particle movement. The model results corresponded with the field survey. Modeled results indicated the micro- and mesoplastics are moved northeastward by the Tsushima Current. Subsequently, Stokes drift selectively moves mesoplastics during winter toward the Japanese coast, resulting in increased contributions of mesoplastics south of 39°N. Additionally, Stokes drift also transports micro- and mesoplastics out to the sea area south of the subpolar front where the northeastward Tsushima Current carries them into the open ocean via the Tsugaru and Soya straits. Average transit time of modeled particles in the Sea of Japan is drastically reduced when including Stokes drift in the model.

Sequential webcam monitoring and modeling of marine debris abundance

The amount of marine debris washed ashore on a beach in Newport, Oregon, USA was observed automatically and sequentially using a webcam system. To investigate potential causes of the temporal variability of marine debris abundance, its time series was compared with those of satellite-derived wind speeds and sea surface height off the Oregon coast. Shoreward flow induced by downwelling-favorable southerly winds increases marine debris washed ashore on the beach in winter. We also found that local sea-level rise caused by westerly winds, especially at spring tide, moved the high-tide line toward the land, so that marine debris littered on the beach was likely to re-drift into the ocean. Seasonal and sub-monthly fluctuations of debris abundance were well reproduced using a simple numerical model driven by satellite-derived wind data, with significant correlation at 95% confidence level.