The subsurface water in the North Pacific tropical gyre

The subsurface water (22.5–25.5 σθ) in the North Pacific tropical gyre (NPTG; 130°E–150°W, 5°–10°N), addressed as the North Pacific tropical subsurface water (TSSW), features a lateral salinity minimum and vertical salinity maximum in the western and central Pacific Ocean. In this study this water body is investigated using Argo float profiles and Word Ocean Atlas 2009 (WOA09) dataset. Comparing with the North Pacific Tropical water (NPTW), the TSSW is of lower salinity, lower oxygen, higher nutrient concentration, and denser vertical salinity maximum Smax. Subtropical ventilation and local ocean dynamics are both important in setting up its unique characteristics. Our analysis shows that its properties, structure, and seasonal variations are diverse at different longitudes. In the western Pacific, the TSSW is mainly of North Pacific water origin. Its high salinity core is formed by the southward intrusion of the NPTW. Diapycnal mixing with the surface-layer fresh water reduces its salinity and lowers its Smax to denser isopycnals. The TSSW in the western Pacific can be regarded as a diluted portion of the NPTW. In the central Pacific, advection of salty water masses from both hemispheres and westward invasion of the eastern Pacific fresh water are all important in regulating its characteristics. The TSSW in the central Pacific is a mixture of various water sources formed under highly variable flow pattern and intensive mixing. The TSSW should be regarded as an important subsurface water body connecting North/South Pacific thermocline water, although it might not be a water mass by traditional definition.