GEOSS Asia-Pacific Symposium
12 January 2017 Tokyo International Exchange Center

Trend of Ocean Acidification for the past three decades
in the western North Pacific Subtropical zone
and in the western equatorial Pacific warm pool

Masao ISHII
Oceanography and Geochemistry Research Dept.,
Meteorological Research Institute, Japan Meteorological Agency

Special thanks to

H. Y. Inoue (Hokkaido U.)
T. Midorikawa, D. Sasano, N. Kosugi, K. Toyama, H. Nakano, N.Usui, Y. Fujii (JMA/MRI)T. Nakano, Y. Takatani, Y. Iida, A. Kojima, S. Masuda, K. Enyo, K. Nemoto et al. (JMA)
K. Ando, T. Kawano (JAMSTEC), I. Asanuma (TUIS)
C. Cosca, R. A. Feely (NOAA/PMEL), A.G. Dickson (SIO)
K.B. Rodgers (Princeton U.), D. Iudicone (SZAD), B. Blanke (Ifemer), O. Aumont (LOCEAN)
SOCAT team, PACIFICA team
Officers and crew of RV Ryofu Maru, RV Keifu Maru (JMA),RV Natsushima, RV Kaiyo, RV Mirai (JAMSTEC), RV Hakuho Maru (U. Tokyo, JAMSTEC)

Subtropics and tropics of the western North Pacific

Kuroshio Extension

Subtropical Gyre
Okinawa

Ogasawara

North Equatorial Current
GuamPalau

Pohnpei
Chuuk– Accommodating many coral reefhabitats and marine biodiversity
hotspots,e.g., 75% of the world’s coral species
and more than 3000 species of fish.

MajuroKiribatiSouth Equatorial Current

Coral Triangle
Photo by WWF・ Coral reef habitats

“Is ocean acidification in fact occurring in the western North Pacific?”Contents
Trends of ocean acidification in surface layers for the past decades in :(1) 137ºE repeat line between Japan and Indonesia (3ºN – 34ºN) acrosssubtropical and tropical zones.
(2) Western equatorial Pacific warm pool
(130ºE – 180º, 5ºS – 5ºN).

137ºE Repeat Line by Japan Meteorological Agency (JMA)

Kuroshio Extension

Subtropical Gyre
137ºE

Okinawa

PalauOgasawara

Repeat measurements at 137ºE
1967 – Temp., Salinity, oxygen and nutrients.
1983 – Partial pressure of CO2 in surface water(pCO2sw) and in the atmosphere (pCO2air) .

NECGuamPohnpei

MajuroChuukSEC
Kiribati

1994 – total dissolved inorganic carbon (DIC) atdepths.
2010 – total alkalinity (TA) and pH at depths Saturation level of CO2 in surface water> sea-air CO2 flux

RV Ryofu Maru III

RV Keifu Maru II DIC increase in surface water and in theocean interior> Anthropogenic CO2 accumulation andcarbon cycle / ocean circulation changes> Ocean acidification

Trends in the surface of the subtropics at 137ºE, 27ºN
pCO2sw
Kuroshio Extension(µatm)

Subtropical Gyre
137ºE

Okinawa

PalauOgasawara

nDIC(µmol/kg)

NEC+1.74 µatm yr-1

±0.21

+1.16 µmol kg-1 yr-1
±0.07

GuamPohnpei

MajuroChuukSECKiribati

pH@SST-0.0019 yr-1±0.0002

Ωarag-0.012 yr-1

±0.001 Despite large seasonal and year-to-year
variations, trends of CO2 increase andacidification are significant.

Temp.

(ºC)

SalinityΩarag : Calcium carbonate mineral “aragonite” saturation index

Trends in the surface of the tropics at 137ºE, 7ºN
pCO2sw
Kuroshio Extension(µatm)

Subtropical Gyre
Okinawa

Ogasawara

nDIC+1.06 µatm yr-1±0.26+0.66 µmol kg-1 yr-1±0.08(µmol/kg)

NECGuam
Palau

Pohnpei

MajuroChuukSEC

Kiribati

pH@SST-0.0010 yr-1±0.0003

Ωarag-0.007 yr-1±0.002

 Trends of CO2 increase and acidification
are clear, but their rates of change arelower than in the subtropics and than therates under the condition of air-sea CO2equilibrium.

Temp.

(ºC)

Salinity

3-D view of nDIC, pCO2sw, pH@SST, and Ωarag at 137ºE, 3ºN - 34ºN
nDIC

pCO2SW

pH @SSTΩarag Large seasonal variation in the northern subtropics under nitrate depletion.
 Large meridional gradient at around 20ºN.
 Long-term changes with significantly different rates between subtropics and tropics.

Meridional distributions of the linear rate ofocean CO2 increase and acidification at 137ºE
nDIC change(µmol kg-1 yr-1)

pCO2sw change(µatm yr-1)

pH change(yr-1)

Ωarag change(yr-1)

North Equatorial Current

Mean rate of atmospheric
CO2 increase (1985- 2015)
 Rates of DIC increase and acidification are consistent with those expected from the rate ofatmospheric CO2 increase in the subtropics (20ºN-34ºN), but are lower in the tropics (3ºN-19ºN).
 From the biogeochemical point of view, marine ecosystems in the subtropics are morethreatened by the ocean acidification.

Oceanic CO2 measurements in the western equatorial Pacific
Kuroshio Extension

Subtropical Gyre
Okinawa

Ogasawara

1987 –2003; MRI-JAMSTEC collaborative
studies on ocean CO2 in the westernequatorial Pacific.
R/V Natsushima

R/V Kaiyo

R/V Mirai

NECGuamPalau

Pohnpei

MajuroChuuk

SEC Warm Pool
Kiribati

1996 – JMA’s repeat line at 165E.
and many others cruises by Japan, USA andFrance. Data of pCO2sw taken in thesecruises have been stored in“Surface Ocean CO2 Atlas”

 Here, we define the “warm pool” as theregion where t>28.7ºC, S<34.6 in thewestern equatorial Pacific.

The pCO2sw measurements in the equatorial
Pacific have originally been made in order tounderstand the variability in sea-air CO2 flux
associated with El Nino Southern Oscillation.

Trends in the warm poolpCO2sw
Kuroshio Extension(µatm)

Subtropical Gyre
Okinawa

Ogasawara

nDIC+1.37 µatm yr-1

±0.09

+0.81 µmol kg-1 yr-1±0.06(µmol/kg)

NECGuam
Palau

Pohnpei

MajuroChuuk

SEC Warm Pool
Kiribati

pH@SST-0.0013 yr-1±0.0001

Ωarag-0.0083 yr-1±0.001

 Rate of CO2 increase and acidification in thewarm pool are also slower than the rates
expected under the conditions of air-sea
CO2 equilibrium.
 Why slower?

Temp.

(ºC)

Salinity

Why ocean acidification is slower in the warm pool?

1) Excess CO2 uptake in the surface ofthe vast extratropics
2) Subduction into the ocean interior
10 ~ 15 years3) Equatorial Undercurrent (EUC)< 2 years4) Upwelling to the surface in thecentral equatorial Pacific+ excess CO2 uptake
in the equatorial zone5) Westward transport to the warm poola) Smaller CO2 buffering capacityb) Time-lag of CO2 increase in the warm
pool after the atmospheric CO2 growthc) Increasing transport from the South
Pacific with naturally lower preformednDIC?
= Negative feedback from theclimate variability/change?

Summary Progress of OA for the past decades have been distinctly observed at avariety of site in the subtropical and tropical zones of the western
North Pacific.
 Rate of OA in surface layer has been primarily controlled by the growthrate of the atmospheric CO2 concentration.
 In the tropics, rate of OA appears to have a time-lag of 10-15 years
behind the rate corresponding to the growth of the atmospheric CO2
because of the transport of anthropogenic CO2 from the extratropics
through the ocean interior.
 Climate variability/change is likely to have a significant impact on therate of OA in the tropics.