Masahide Wakita

Probability of a reduction in the formation rate of the subsurface water in the North Pacific during the 1980s and 1990s

Comparing the apparent oxygen utilization (AOU) and apparent CFC tracer ages (τ) between extensive decadal reobservation data along 47°N (’85–’99) and 165°E (’87–’00) lines, we found that both AOU and τ markedly increased over the North Pacific between 26.4–27.4 σθ. The observed AOU increase was almost consistent with the AOU increase calculated from observed change of τ. Based on a linear trend of increasing AOU over 30 years (’68–’98) in the subpolar region [Ono et al., 2001], we concluded that the formation rate of the subsurface water in the North Pacific has continuously reduced at least during the last fifteen years. In the North Pacific, the recent uptake rate of oceanic anthropogenic carbon was also estimated as reduced by as much as 10% from the efficiency of anthropogenic carbon absorption in the middle of 1980s.

Seasonal variability of primary production and phytoplankton biomass in the western Pacific subarctic gyre: Control by light availability within the mixed layer

A distinct seasonal variation of primary production was revealed from shipboard observations conducted from 2005 to 2013 at time series station K2 in the western Pacific subarctic gyre (WSG). The mean depth-integrated primary production was highest (569 ± 162 mg C m−2 d−1) in summer and lowest (101 ± 16 mg C m−2 d−1) in winter. Strong winter mixing enriched the mixed layer (ML) with nutrients that were not fully consumed during the remainder of the year, the result being that the WSG was a high-nutrient, low-chlorophyll (HNLC) region. The deep ML reduced primary production by reducing light availability in winter, whereas primary production was enhanced by strong light availability in the shallower ML as summer progressed. However, primary production was often attenuated by a reduction of light availability attributable to dense sea fog in summer. We found a significant relationship between primary production and light availability in this HNLC region. However, chlorophyll a was less variable seasonally than primary production. The highest depth-integrated chlorophyll a was observed in summer (54.6 ± 13.4 mg m−2), but chlorophyll a remained high in winter (45.3 ± 7.7 mg m−2). Reduced light availability depressed primary production, but a reduction of the chlorophyll a concentration was prevented by a relaxation of grazing in the deep ML during winter. We found that light availability exerted an important control on the seasonal variability of primary production and phytoplankton biomass in the WSG.

Slow acidification of the winter mixed layer in the subarctic western North Pacific

We used carbon dioxide (CO2) system data collected during 1999–2015 to investigate ocean acidification at time series sites in the western subarctic region of the North Pacific Ocean. The annual mean pH at station K2 decreased at a rate of 0.0025 ± 0.0010 year−1 mostly in response to oceanic uptake of anthropogenic CO2. The Revelle factor increased rapidly (0.046 ± 0.022 year−1), an indication that the buffering capacity of this region of the ocean has declined faster than at other time series sites. In the western subarctic region, the pH during the winter decline at a slower rate of 0.0008 ± 0.0004 year−1. This was attributed to a reduced rate of increase of dissolved inorganic carbon (DIC) and an increase of total alkalinity (TA). The reduction of DIC increase was caused by the decline of surface water density associated with the pycnocline depression and the reduction of vertical diffusion flux from the upper pycnocline. These physical changes were probably caused by northward shrinkage of the western subarctic gyre and global warming. Meanwhile, the contribution of the density decline to the TA increase is canceled out by that of the reduced vertical diffusive flux. We speculated that the winter TA increase is caused mainly by the accumulation of TA due to the weakened calcification by organisms during the winter.