Physical oceanography of the Caroline M4 seamount in the tropical western Pacific Ocean in summer 2017.

Physical oceanography plays an important role in the formation of submarine sediments, and the distribution of nutriments and biocenoses in seamounts. The M4 seamount is located in the Caroline Island Ridge of the western Pacific Ocean. The physical properties around M4 seamount are preliminarily analyzed based on the in-situ data obtained in summer 2017 in Caroline M4 seamount and open-sourced data. We found that the water in the upper 200 m is controlled by the westward North Equatorial Current (NEC), while the water between 300–1 000 m is dominated by the eastward North Equatorial Undercurrent (NEUC). The current direction fluctuates significantly below 300 m at upstream stations. At the same depth of the lee sides, the current direction changes with the distance from seamount. These are likely caused by the obstacle of M4 seamount. The calculation results show that there is an anticyclonic cap above M4 seamount caused by tidal rectification. Tidal currents in M4 seamount are squeezed by the topography and amplified, and the amplified tidal currents play a dominant role in M4 seamount. First, the circulation system generated by the interaction of the amplified tidal current and M4 seamount drives the upward/downward movement of the isotherms. Secondly, the thickness of the surface turbulent layer is changed with the tidal phase. Thirdly, high turbulent diffusivities are found in the bottom of M4 seamount, and these are most likely attributed to the turbulent mixing induced by the mutual effect between semidiurnal tidal currents and steep bathymetry. This article of physical oceanography provides scientific basis for further analysis of the distribution of biological community and deposition mechanism in M4 seamount. [ABSTRACT FROM AUTHOR]