Eleven optode-based oxygen sensors were used for shipboard hydrographic casts in the North Pacific. Oxygen data from the optode sensors were compared with high-quality oxygen data obtained with discrete water samples, and the performance of the sensors was evaluated. The response of the sensing foil of the optode decreases with increasing ambient pressure, and this pressure effect was found to decrease the response by 3.2% (1000 dbar) Ϫ1 . A new calibration equation for the optode sensors was proposed. On the basis of oxygen data from water samples, the optode sensors were calibrated so that the reproducibility was less than 1%. High-quality oxygen profiles from the optode were obtained for fast-profiling conductivitytemperature-depth (CTD) observations, by compensating for the temperature-dependent delay in the optode data due to the slow response time of the optode.

Abstract. Recently obtained World Ocean Circulation Experiment (WOCE) sections and pre-WOCE hydrography are used to study the water -mass structure and formation and transformation of North Pacific Intermediate Water (NPIW). Five neutral density surfaces are selected and mapped, encompassing NPIW from 400 to 900 m in the subtropical latitudes with a distance of-100 m between a pair of surfaces. NPIW is defined as a subtropical gyre salinity minimum which is well followed by a neutral density surface ON=26.9.

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[1] Repeat trans-Pacific hydrographic observations along the pathway of Lower Circumpolar Deep Water (LCDW) reveal that bottom water has warmed by about 0.005 to 0.01°C in recent decades. The warming is probably not from direct heating of LCDW, but is manifest as a decrease of the coldest component of LCDW evident at each hydrographic section. This result is consistent with numerical model results of warming associated with decreased bottom water formation rates around Antarctica.

We report a freshening at mid‐depth in the North Pacific subtropical gyre by using long‐term repeat hydrographic data along the 137°E section and one‐time hydrographic data along the World Ocean Circulation Experiment Hydrographic Program (WHP) P2 and P3 sections. North of 15°N along the 137°E section, we estimated a linear freshening trend of 0.0015/yr between the main thermocline and the salinity minimum layer of the North Pacific Intermediate Water, mainly caused by isopycnal surface deepening due to warming, and by westward shifts of the salinity‐minimum tongue due to strengthening of the subtropical gyre. Furthermore, along the WHP‐P2 section, the linear freshening trend could be classified into several groups according to longitude. Such spatial differences in the freshening trend seem to reflect differences in the formation processes and mid‐depth pathways of the salinity minimum waters.

[1] We conducted a trans-Pacific hydrographic section along 24°N in 2005 to investigate the ocean structure and its changes from previous observations in 1985. We detected significant basin-average water property changes from 1985 to 2005. Apparent oxygen utilization increased below the thermocline by up to 6 mmol kg À1 around the density of the central mode water (around 600 m). It appeared that the North Pacific intermediate water (around 800 m) was less dense in 2005 than in 1985 because of warming. From the decrease of the zonal gradient of the temperature and salinity around the North Pacific deep water (2500-4000 m) and lower circumpolar deep water (<4000 m), we suggest that northward bottom water and southward deep water transports became weaker from 1985 to 2005, consistent with the speculation from the observed temperature increase of the bottom water along its main path in previous studies. Although these water property changes suggest a slowdown of the meridional overturn in the North Pacific and large transport changes in the deep layers (below 4000 m) are estimated from an inverse method, significant heat transport changes were not detected. The estimated temperature transport change of 0.1 PW between the two sections was mainly due to shallow overturn changes, especially changes in the Kuroshio. To describe variability due to the Kuroshio changes, we estimated mass and heat transport changes from long-term observations of the Kuroshio in the Okinawa Trough, and we determined decadal variability of temperature transports, which was consistent with the variability estimated from sea-surface flux data sets.

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