Movements of Intermediate Water by Subthermocline Eddies in the Philippine Sea

Abstract

Intermediate waters which are originated from both hemispheres, i.e., the North Pacific Intermediate Water (NPIW) and the Antarctic Intermediate Water (AAIW), cross in the southern Philippine Sea. In addition to these well-known water masses, an intermediate water mass characterized by relatively high salinity and high dissolved oxygen was observed from the survey conducted in November 2017. The newly discovered intermediate water showed the salinity maximum with higher dissolved oxygen at a density of 26.8 between the NPIW and AAIW. Comparing this water mass with the data from Argo and previous observations, it is concluded that it originated from the southern hemisphere and moved to the observation region by the New Guinea Coastal Under Current (NGCUC). The vertical velocity section along 6.5°N shows the opposite meridional velocity pattern at the depths below the thermocline in the east-west direction around 128.5°E where this water mass is found, suggesting the existence of a subthermocline eddy. The reanalysis data of the eddy resolving ocean circulation model support that the NGCUC forms eddies and moves to the Philippine Sea. This is an evidence that the subthermocline eddy plays an important role in the exchange of water masses between the North and South Pacific Oceans.