Observed Warming and Weakening of the Philippine Sea Deep Circulation Over the Past Decade

Deep circulation in the Philippine Sea, an important component of the deep limb of the Pacific Meridional Overturning Circulation, redistributes heat and carbon, as the circulation carries cold dense water originating from Antarctic bottom water (AABW). On the basis of hydrographic observations, the water mass properties and circulation in the deep Philippine Sea were investigated. The cold water entering the deep Philippine Sea through the Yap-Mariana Junction gradually warms as it invades the interior of the deep basin, and the intrusion path exhibits a cyclonic structure. From 2015 to 2023, deep water in the Philippine Basin warmed by 0.18 ± 0.20 × 10⁻³ °C yr⁻¹ below 4,000 m, featuring the deepening of potential temperature isotherms due to the continued loss of cold water. Further analyses of the satellite-observed ocean bottom pressure data revealed anticyclonic anomalies in the deep currents, indicating a weakening of the deep circulation in the Philippine Sea. Warming of the deep water and weakening of the deep circulation both imply a contraction of cold dense water entering the deep Philippine Sea, which is believed to be relevant to the reduced formation rates of AABW.