Water‐Mass Properties and Circulation in the Deep and Abyssal Philippine Sea

Abstract

AbstractWater‐mass properties and circulation in the deep and abyssal Philippine Sea are investigated using repeatedly occupied full‐depth hydrographic data. The abyssal water‐mass, the Lower Circumpolar Deep Water (LCDW), is captured. After crossing the Yap Mariana Junction, the LCDW spreads to the Parece Vela Basin, the southern part of the Shikoku Basin, the Minami‐Daito Basin, and the northern Philippine Basin, with its upper interface sinking from ∼4,000 m to ∼5,000 m. Blocked by the Central Basin Fault, the LCDW cannot penetrate further southward into the southern Philippine Basin. An abyssal current across the Kyushu‐Palau Ridge connecting the eastern and western parts of the Philippine Sea is identified at 22.5°N–29.5°N, which is suggested to be the main branch transporting LCDW from the Yap Mariana Junction westward into the northern Philippine Basin. Synoptic analysis of hydrographic observations suggests the existence of a cyclonic circulation in the abyssal Philippine Sea. During 1990–2020, a warming trend of 0.33 ± 0.19 × 10−3°C yr−1 is identified at depths below 2,000 m, while a SiO2 maximum core at 30°N implies a possible cooling of the Mixed Deep Water (more influenced by North Pacific Deep Water) in the Philippine Sea. Under 4,000 m, freshening together with warming south of 20°N indicate contraction of LCDW after entering the Yap Mariana Junction.