A B S T R A C TWe estimated long-term trends of ocean acidification in surface waters in latitudinal zones from 3 • N to 33 • N along the repeat hydrographic line at 137 • E in the western North Pacific Ocean. Estimates were based on the observational records of oceanic CO 2 partial pressure and related surface properties over the last two decades. The computed pH time series both for 25 yr in winter (late January-early February) and for 21 yr in summer (June-July) exhibited significant decreasing trends in the extensive subtropical to equatorial zones, with interannual variations that were larger in summer. The calculated rates of pH decrease ranged from 0.0015 to 0.0021 yr −1 (average, 0.0018 ± 0.0002 yr −1 ) in winter and from 0.0008 to 0.0019 yr −1 (average, 0.0013 ± 0.0005 yr −1 ) in summer. The thermodynamic effects of rising sea surface temperature (SST) accounted for up to 44% (average, 15%) of the trend of pH decrease in the subtropical region in winter, whereas a trend of decreasing SST slowed the pH decrease in the northern subtropical region (around 25 • N) in summer. We used the results from recent trends to evaluate future possible thermodynamic changes in the upper ocean carbonate system.

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[1] The 2-decade records of the partial pressure of CO 2 in surface waters (pCO 2 sea ) and the resulting air-sea CO 2 flux in the extensive subtropical to equatorial area along 137°E in the western North Pacific in winter exhibited significant interannual variations that differed in different regions. The pCO 2 sea varied largely in the equatorial region of 3°N to 6°N, depending on the oceanographic conditions related to the El Niño-Southern Oscillation (ENSO) events. The magnitude of the variations differed year by year, corresponding to the different degrees of compensation between the sea surface temperature (SST) and dissolved inorganic carbon (DIC) effects on pCO 2 sea during the events. Small pCO 2 sea variations in the subtropical gyre north of 23°N were due to highly counteracting effects between anticorrelated SST and DIC anomalies through the entrainment process. In contrast, a low negative correlation existed between SST and DIC, associated with the lateral advection in the region restricted around 15°N to 18°N in the North Equatorial Current, resulting in a large amplitude of variations of pCO 2 sea and hence CO 2 influx. The interannual variations of CO 2 flux depended predominantly on the difference in pCO 2 between air and sea south of 18°N, but on wind speed in the northern region. It is important to monitor how the contributions of different properties to the CO 2 flux could change in the western North Pacific, responding to possible shifts of the ENSO conditions and the future progress of global warming.

[1] The long-term trend of the partial pressure of CO 2 in surface seawater (pCO 2 sea ) in late-January to early-February during the past two decades was examined in the western North Pacific along the repeat line at 137°E from 3°N to 34°N. The growth rate of pCO 2 sea at each 1°in latitude ranged from +1.3 ± 0.2 to +2.1 ± 0.3 matm yr À1, and the average was +1.7 ± 0.2 matm yr À1

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Impact of climate change on marine biogeochemical parameters and ecosystem is one of the important issues of our environment. Direct evidence of marine pelagic ecosystem changes is found with warming of sea water and sea-level rise in the main stream of the Kuroshio in the East China Sea and the western North Pacific during these three decades based on the analysis of long-term comprehensive hydrographic observations. In terms of annual mean, the warming rate of surface air temperature and sea surface temperature ranged from 0.15 to 0.21°C per decade in and around the main stream of the Kuroshio in the East China Sea, which exceed the global mean warming rate of 0.128 ± 0.026°C per decade during the period from 1956 to 2005 reported in IPCC 2007. One of the features in this rapid warming region is an increase of number of Pterosagitta draco, a cosmopolitan warmwater zooplankton. Biogeochemical parameters, such as wet weight of zooplankton, plant pigment and nutrients concentration in the upper 200 m have been decreasing while dissolved oxygen content and seawater temperature have been increasing in the upper 200 m in the main stream of the Kuroshio in the East China Sea. These observed linear trends of the biogeochemical parameters would be foresights for temperate oceans in the future.

We observed the partial pressure of oceanic CO 2 , pCO 2 sea , and related surface properties in the westernmost region of the subarctic North Pacific, seasonally from 1998 to 2001. The pCO 2 sea in the Oyashio region showed a large decrease from winter to spring. In winter, pCO 2 sea was higher than 400 µatm in the Oyashio region and this region was a source of atmospheric CO 2 . In spring, pCO 2 sea decreased to extremely low values, less than 200 µatm (minimum, 139 µatm in 2001), around the Oyashio region with low surface salinity and this region turned out to be a strong sink. The spatial variations of pCO 2 sea were especially large in spring in this region. The typical Oyashio water with minimal mixing with subtropical warm water was extracted based on the criterion of potential alkalinity. The contribution of main oceanic processes to the changes in pCO 2 sea from winter to spring was estimated from the changes in the concentrations of dissolved inorganic carbon and nutrients, total alkalinity, temperature and salinity observed in surface waters in respective years. These quantifications indicated that photosynthesis made the largest contribution to the observed pCO 2 sea decreases in all years and its magnitude was variable year by year. These year-to-year differences in spring biological contribution could be linked to those in the development of the density stratification due to the decrease in surface salinity. Thus, the changes in the surface physical structure could induce those in pCO 2 sea in the Oyashio region in spring. Furthermore, it is suggested that the direction and magnitude of the air-sea CO 2 flux during this season could be controlled significantly by the onset time of the spring bloom.

The role of spring biological production for the air-sea CO 2 flux was quantified in the Western Subarctic Gyre (48 • N, 165 • E), where the vertical profile of temperature revealed the existence of a temperature minimum (T min ) layer in the North Pacific. The vertical profiles of temperature, salinity, dissolved oxygen, nutrients and dissolved inorganic carbon, DIC, in the upper water column were significantly variable year by year in spring, 1996-2000. Correspondingly, surface seawater at this site in spring was supersaturated with CO 2 in 1997, 1999 and 2000, but was undersaturated in 1996 and 1998. The concentrations of DIC and nutrients in the winter mixed layer were estimated from those in the T min layer in spring with a correction for particle decomposition based on the apparent oxygen utilization. The net community production (NCP) and air-sea CO 2 flux from winter to spring were calculated from the vertically integrated deficits of DIC and nutrients in the upper water column between the two seasons. The calculation of the carbon budget indicated large interannual variations of NCP (0-13 mmol m −2 d −1 ) and CO 2 efflux (4-16 mmol m −2 d −1 ) for this period. The CO 2 efflux was generally low in the year when NCP was high. The close coupling between biological production and CO 2 efflux suggested the important role of the changes in the mixed-layer depth, as a key process controlling both processes, especially of the timing, so that a decrease in the mixed-layer depth could result in the activation of biological production. The early biological consumption of the surface DIC concentration could shorten the period for acting as a source for atmospheric CO 2 and depress the CO 2 efflux in the Western Subarctic Gyre from winter to spring in 1996 and 1998. On the contrary, in 1997, persistently deep vertical mixing until late spring could suppress the biological activity and give rise to long-lasting CO 2 efflux.