The vertical structure and intraseasonal variability of the deep currents in the Southern Philippine Basin

Abstract

Based on the four-mooring array measurements along 130°E over 2015–2020, the vertical structure and variability of deep currents in the central Southern Philippine Basin (SPB) are revealed. The deep currents below 1500 m in the central SPB are characterized by alternating westward and eastward zonal jets with strong intraseasonal variability (ISV). These zonal jets have a weak mean velocity of less than 1.5 cm s−1 but vary with a relatively large amplitude of 10 cm s−1 at the intraseasonal period of 20–120 days. The full-depth zonal velocity in the central SPB has three typical vertical structures that are mainly controlled by the first baroclinic mode, the second and higher baroclinic modes, and the barotropic mode in descending order of occurrence frequency. These structures are vertically coherent below ∼2000–2500 m, below ∼1200 m, and over the whole water column, respectively. Further analysis of numerical ocean modeling results demonstrates that the ISV of deep currents is induced by the deep mesoscale eddy. The deep ISV intensifies when a deep eddy passes by and keeps at a low value when there is no deep eddy activity around. The deep eddy extends to the bottom with the upper boundaries at ∼1500 m below the subsurface layer or at ∼300 m including the subsurface layer. These deep eddies arise primarily from barotropic instability and secondarily from baroclinic instability.