Revisit of WHP P17N in 2001

During the WHP P17N revisit cruise by R/V Mirai in 2001 summer, we carried out hydrographic observations in the eastern North Pacific from 30 to 55 N degrees. A total of 78 CTDO/rosette stations were conducted and more than 3,500 seawater samples for salinity, oxygen, phosphate, silicate, nitrate, nitrite, dissolved inorganic carbon (DIC), pH, and alkalinity were measured on board. Additionally, vertical profiles of radiocarbon and carbon-13 of DIC were also measured at 21 stations (about 700 samples). Comparison of hydrographic data between the first visit in the early 1990s (P17N in 1993 and P17C in 1991) and the re-visit elucidated temporal changes of intermediate water properties in the eastern North Pacific in the 1990s. From the subtropical region to the Alaskan Gyre, apparent oxygen utilization (AOU) in intermediate layers (26 - 27 sigma-theta) increased by up to 60 micro-mol/kg with increased nutrients and DIC concentrations. Patchy AOU increases in the subtropical region (30 - 40 N degrees) suggest that high AOU water was brought by mesoscale eddies from the east, off of California, where oxygen concentration is low. Between 41 and 53 N degrees, a tongue of AOU increase was measured around 26.6 sigma-theta isopycnal. This AOU increase is in agreement with previous studies showing AOU and CFC age increases along 152 W degrees from 30 to 45 N degrees between 1991 and 1997 (Emerson et al., 2001) and along 47 N degrees from 160 E to 145 W degrees between 1985 and 1999 (Watanabe et al., 2001). Furthermore, the AOU increase was larger in the Alaskan Gyre from 44 to 53 N degrees and was associated with intensification of stratification. These results suggest that ventilation rate of intermediate water slowed down in the subpolar and the subpolar/subtropical transition regions of the eastern North Pacific during the 1990s. Measured changes of the carbon-13 and radiocarbon of DIC also imply stagnation of the subpolar intermediate water.