Subsurface eddies east of the Philippines: Geographic characteristics, vertical structures, volume and thermohaline transport

Abstract

Energetic subsurface eddies (SSEs) play a significant role in regulating the subthermocline circulation east of the Philippines. However, due to the paucity of targeted observations, they remain largely unexplored. By analyzing the outputs from an eddy-resolving ocean general circulation model (OGCM), this study investigated the statistics of SSEs east of the Philippines, including their geographic characteristics, vertical structures, and eddy-induced transport. During the period of 2009–2019, approximately 1927 and 1176 SSEs were detected to be anticyclonic and cyclonic, respectively, indicating the predominance of subsurface anticyclonic eddies (SSAE). The hotspot area of SSEs is in the latitude band of 6° N–15° N off the Philippine coast, especially around 10° N and 14° N–15° N prevailing abundant SSEs. Most SSEs originate at ∼138° E furthest with a mean lifespan of 55 days and a westward translation speed of 6 cm/s. The composite SSAE exhibits a typical subsurface-intensified feature with a velocity core at ∼520 m, while the subsurface cyclonic eddy (SSCE) has a relatively shallow core at ∼420 m and exhibits considerable strength near the surface with a rotating speed is around 5 cm/s. Temperature anomalies induced by SSEs show a dual-core structure associated with lens-like isopycnal undulations, and salinity anomalies are characterized by an alternating positive and negative signal owing to the complexity of the water masses. SSEs-induced meridional volume transport mainly occurs off the Philippine coast, where the northward transport is approximately 0.3 Sv and the southward transport is approximately 0.5 Sv. The zonal volume transport of SSEs is 1–2 Sv per latitude and mainly occurs in the North Equatorial Current (NEC) region. SSEs-induced stirring heat/salt transport is 1–2 orders larger than the trapping component and is mainly concentrated near the Mindanao coast. The meridional stirring heat transport is equatorward, which is up to 2.7 × 10⁷ W/m, and the salt transport is northward with a magnitude of ∼100 kg·m⁻¹·s⁻¹ between 200 m and 2000 m. At the 8°N section, the mean salt transport associated with SSEs is about 24% of that induced by the Mindanao Undercurrent (MUC).