Trend of ocean acidification in the tropical and subtropical zones of the western North
Pacific along 137˚E
Masao Ishii1, Naohiro Kosugi1, Daisuke Sasano1, Takashi Midorikawa1, Kazutaka Enyo2,
Toshiya Nakano2 and Hisayuki Y. Inoue3
1
Meteorological Research Institute, JMA, Tsukuba, Japan. E-mail: mishii@mri-jma.go.jp
Global Environment and Marine Dept., JMA, Tokyo, Japan
3
Hokkaido University, Sapporo, Japan
2
Tropical and subtropical zones of the western North Pacific accommodate many coral reef
habitats and marine biodiversity hotspots. Many of the 150 million people that live in these
regions depend heavily on fishing and other marine resources. From 3°N to 34°N along 137°E,
Japan Meteorological Agency has been making measurements of pCO2 in the atmosphere and in
surface seawater routinely since 1983 and dissolved inorganic carbon (DIC) since 1994. These
data are combined to analyze the variation of CO2 system variables in space and time. The
amplitude of seasonal variation in CO2 system variables in surface water and SST is larger in the
subtropical zone (max. ~40 µmol kg-1 for salinity-normalized DIC—hereafter sDIC). The
seasonal variation in sDIC is ascribed to the convective mixing in winter and biological
production in winter to summer. However, the net sDIC decrease from spring to summer occurs
under the conditions of very low nitrate and phosphate concentrations. The rate of long-term
surface sDIC increase, as evaluated from multi-parameter regression of sDIC as a function of
SST and time of measurement for each 1-degree latitudinal band, is significantly different
between subtropics (+1.0 – +1.2 µmol kg-1 yr-1) and tropics (+0.7 – +0.9 µmol kg-1 yr-1). The
cause of the meridional variation in the rate of sDIC increase and thereby those of pH and Ω
decreases is likely to be associated with the anthropogenic CO2 uptake/transport pathways in the
shallow overturning cell of the North Pacific, but is yet to be investigated.