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A total of 8218 pelagic microplastic samples from the world’s oceans were synthesized to create a dataset composed of raw, calibrated, processed, and gridded data which are made available to the public. The raw microplastic abundance data were obtained by different research projects using surface net tows or continuous seawater intake. Fibrous microplastics were removed from the calibrated dataset. Microplastic abundance which fluctuates due to vertical mixing under different oceanic conditions was standardized. An optimum interpolation method was used to create the gridded data; in total, there were 24.4 trillion pieces (8.2 × 104 ~ 57.8 × 104 tons) of microplastics in the world’s upper oceans.

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The location data set of 31 surface drifters deployed in the Indian Ocean between 1990 and 1993 are analyzed to illustrate the surface current field and its seasonal changes in the tropical Indian Ocean and the area of Indo-Pacific throughflow. Trajectories of drifters, daily drifting velocities, and grid-averaged velocities are described and compared with published climatologies of the surface currents in the Indian Ocean and numerically simulated flows in the Indonesian Seas. The surface current field and its seasonal changes in the Indian Ocean derived from drifter data are generally consistent with the published climatologies. Zonal velocities computed from the drifter data set in the equatorial jet, which appears twice a year in the transition periods of the monsoon, are greater than previous estimations. Surface currents flowing from the Indian Ocean toward the Indonesian Seas in boreal winter, in the opposite direction from that observed during other seasons, are traced with drifters deployed in the region south of the Indonesian archipelago. In particular, a drifter passing from the Indian Ocean to the Flores Sea through Lombok Strait in December 1993 is consistent with numerical modeling which shows northward flow in the Lombok Strait in boreal winter. 1. Introduction Water circulation in the eastern Indian Ocean and the seas adjacent to Indonesia (the Flores Sea, the Banda Sea, and the Arafura Sea) has been an issue for oceanographers interested in mass/heat/salt exchange between the Pacific Ocean and the Indian Ocean through the Indonesian Waters or interested in seasonal change of the surface circulation of the Indian Ocean caused by the strong monsoon signal in the wind field over the Indian Ocean. Several maps of the surface currents in the Indian Ocean have been presented by previous studies; for example, Wyrtki [1971] described the replacement of the southwest monsoon current, dominant eastward flow in the equatorial area in boreal summer, by the westward flowing North Equatorial Current in boreal winter. Wyrtki [1973] also pointed out that an equatorial jet, a strong eastward current along the equator, is observed during the transition periods of the monsoon. Molinari et al. [1990] have proposed a schematic representation of the current system in the Indian Ocean, including its seasonal differences, and presented a monthly climatology of surface currents there by analyzing surface buoy trajectories. As for the currents along the south coast of Indonesia, Quadfasel and Cresswell [1992] determined the seasonal variability of the South Java Current through an analysis of the tracks of surface drifters and ship drifts. Semtner and Chervin [!988], Masumoto and Yamagata [1993], and others have conducted numerical experiments assessing the current field, transport, and variability of the Indo-Pacific throughflow. Japanese Science and Technology Agency national research program, the Japanese Experiment on Asian Monsoon (JEXAM), studying the surface current system of the tropical Indian Ocean and the seas...

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