Water Property Changes of North Pacific Intermediate Water in the Subtropical Gyre

We investigate property changes of NPIW after 1980s in the subtropical gyre of the North Pacific by comparing properties along observation sections of P2 (along 24N°), P3 (along 30N°), and P10 (along 149E°) in World Ocean Circulation Experiment (WOCE) and its revisit. A typical pattern of salinity changes on neutral surfaces (γ) between WOCE and the revisit, that salinity decreased above NPIW salinity minimum and increased below the salinity minimum, were detected along all the sections west of 170W°. The salinity changes above the salintiy minimum is typically about 0.02 along P2 and P10, and up to 0.07 along P3. We compare potential temperature (θ) and salinity (S) diagrams averanged in the region where the pattern is detected. Along P10 and P2, θ-S diagrams seem to shift in the direction of high θ during the term from WOCE to the revist. The averaged θs of NPIW core defined as salinity less than 34.2 increased by 0.10 °C and 0.17 °C, although the salinity of NPIW core did not change largely (by about 0.003). On the other hand, the θ-S diagram along P3 looks like shifting in the directions of low salinity as well as high θ. The changes of θ and salinity of NPIW core are about 0.17°C 0.003, respectively. This suggests that the pattern of the salinity changes on neutral surfaces around NPIW is mainly due to the density decrease of the NPIW core, because salinity in new observations could be lower (higher) than in previous observations in the upper (lower) layer of salinity minimum, if the density of the salinity minimum becomes lighter. The density decrease might be caused by NPIW warming. Along P3, the θ-S diagram of P3 seems to shift in the low-salinity direction as well as in the high-θ direction. This freshening of NPIW core along 24°N might be the same trend as Wong et. al. (1999) reported.