CO2 flux in the
North Pacific
Alan Cohn
May 10, 2006
Introduction
• Oceans contain ~50x as much CO2 as atmosphere
• Mean annual rate of oceanic CO2 uptake by oceans for past few
decades is estimated at about 2 Pg-C yr-1 (Takahashi et al., 2005)
• Only a few stations in the ocean CO2 monitoring network
 lack of ocean CO2 time-series limits scientists’ ability to
estimate interannual changes in oceanic CO2 uptake
• Researchers have looked at atmospheric time series of CO2, 13CO2, and
O2 to infer interannual changes in oceanic and terrestrial CO2 uptake
Distribution of climatological mean annual sea-air CO2 flux (moles CO2 m-2 yr-1)
for reference year 1995 representing non-El Niño conditions. This map yields
an annual oceanic uptake flux for CO2 of 2.2 ± 0.4 Pg C yr-1.
http://www.pmel.noaa.gov/pubs/outstand/feel2331/mean.shtml
Why the North Pacific?
• I concentrate on North Pacific as it is a region of strong climate variability
with implications for variability of atmospheric CO2
• One of most frequently sampled regions of oceans for CO2 variability and
nutrient chemistry
• Strongly influenced by strength of wintertime Aleutian Low through
changes in surface wind stress, Ekman advection, surface ocean mixing,
and heat fluxes
• In winter, surface water pCO2 values are governed primarily by physical
processes because of reduced biological activity (McKinley et al., 2006)
• Photosynthesis has significant effects on pCO2 come spring and summer
pCO2 sensitivity
• pCO2 of seawater is sensitive function of temperature as well as total
concentration of CO2
 TCO2 depends on net biological
community production, rate of
upwelling of CO2rich subsurface
waters, and air-sea CO2 flux
 Revelle factor measures sensitivity
of pCO2 to changes in total CO2
SST vs. Biology
• Influences of SST on surface ocean pCO2 oppose effects of biological
and physical influences on dissolved inorganic carbon (DIC)
• Temperatures lead to low pCO2 in winter and high pCO2 in summer, i.e.
they’re positively correlated (McKinley et al., 2006)
• In mixed layer, lower total CO2 from photosynthesis counteracts effect of
seasonal warming on pCO2
 often evident
during spring blooms
Mixed Layer Variability
• Upwelling of CO2-rich subsurface waters in winter counteracts effect of
cooling on pCO2 (Takahashi et al., 2005)
• Interannual variability of CO2 in surface ocean strongly correlated with
changes in mixing depth during winter
 Deep surface-mixed layers can
lead to increased CO2 uptake and
higher levels of photosynthesis
than during normal years (Quay,
2002).
http://www.pmel.noaa.gov/~cronin/encycl/cartoon.jpg
Aleutian Low is a wintertime semi-permanent cyclone
Strong Low:
 strong westerly winds in central N. Pacific
 cooler SSTs, deeper mixed layer
 enhanced southerly winds in eastern N. Pacific
 warmer SSTs, upwelling supressed
Strength of Low associated with
PDO and ENSO.
http://www.ecy.wa.gov/programs/sea/coast/storms/weather.html
PDO
• Pacific Decadal Oscillation (PDO) is measure of climate variability with
possible impacts on CO2 flux; it has a 20 – 30 year periodicity
Positive Phase:
• SSTs cold, mixing layer deep in central and western North Pacific
• Warm SSTs in Alaska Gyre, along coast of North America, and into
tropics
http://tao.atmos.washington.edu/pdo/
PDO
Aleutian Low
Strong Low
Weak Low
PDO
• In positive phase, upwelling of high CO2 waters suppressed due to
anomalously northward wind off of Canada
• Patra et al. (2005) finds that sea-air CO2 flux over North Pacific is
significantly associated with PDO at 5 months lag
• Believe that delayed effect may be result of slow response of marine
ecosystems and other environments to changes in climate mode
PDO
• May also influence pCO2 via changes in ocean circulation
• station located near Hawaii is believed to have shifted from a
weak CO2 sink to weak source due to increased transport of
high salinity waters from the north (Keeling et al., 2004)
• shift may be linked to a possible 1997 regime shift in the PDO
http://kela.soest.hawaii.edu/ALOHA/images/hawaii.jpg
ENSO
• El Nino-Southern Oscillation (PDO) has its primary signature in tropics;
it has a 3-7 year periodicity
• Linked to PDO through the variability of the Aleutian Low
• Patra et al. find that CO2 flux over North Pacific is significantly
associated with ENSO at three months lag
http://tao.atmos.washington.edu/pdo/
PDO
ENSO
La Nina predominates when
PDO is in negative phase
El Nino predominates when
PDO is in positive phase
http://tao.atmos.washington.edu/pdo/
http://www.pmel.noaa.gov/~kessler/ENSO/soi-1950-98.gif
Physical Mechanisms
• Upwelling regions in central and eastern equatorial Pacific are a strong
source of CO2 throughout year
• Kuroshio Current and extension are strong CO2 sink in winter due
primarily to cooling, and a weak source in summer due to warming
• Western subarctic areas are strong CO2 source in winter because of
convective mixing of waters rich in respired CO2 and nutrients
 become strong sink in winter since nutrients help fuel
intense photosynthesis
Takahashi et al., 2005
pCO2 variability
• Many studies have shown increased uptake in tropics and subtropics in
recent decades, but their temporal structures are inconsistent
• A few areas show decreasing pCO2; these are in or near the Bering
and Okhotsk Seas due to increased biological activity
 may be result of changing
nutrient supplies caused by
changes in land hydrology or
by increases in river or
airborne inputs of nutrients
http://www.pmel.noaa.gov/np/images/maps/npacific6.gif
Summary
• Seasonal temperature changes are primary cause for seasonal changes
of pCO2 in subtropical gyres
• Changes in total CO2 concentration caused by winter upwelling and
springtime plankton blooms are primary cause for seasonal changes in
sub-polar and polar regions (Takahashi et al., 2006)
• Takahashi et al. (2006) find that observed increase in pCO2 is not
affected significantly by SST changes, but is primarily due to change in
seawater chemistry most likely by uptake of atmospheric CO2
Conclusion
• Important to study seawater chemistry as well as temperature and
circulation changes throughout world’s oceans, as these can affect future
uptake or outgassing of CO2
• Vital to understand role of various mechanisms for changes in CO2 flux in
order to accurately quantify potentially changing role of the ocean as a sink
for future climate scenarios
References
Keeling, C.D., H. Brix, and N. Gruber (2004), Seasonal and long-term dynamics
of the upper ocean carbon cycle at Station ALOHA near Hawaii, Global
Biogeochem. Cycles, 18¸ GB4006, doi:10.1029/2004GB002227.
McKinley, G.A., T. Takahashi, E. Buitenhuis, F. Chai, J.R. Christian, S.C. Doney,
M.-S. Jiang, K. Lindsay, J.K. Moore, C. Le Quéré, I. Lima, R.
Murtugudde, L. Shi, and P. Wetzel (2006), North Pacific Carbon Cycle
Response to Climate Variability on Seasonal to Decadal Timescales,
submitted to J. Geophys. Res. Oceans
Patra, P., S. Maksyutov, M. Ishizawa, T. Nakazawa, T. Takahashi, and J. Ukita
(2005), Interannual and decadal changes in the sea-air CO2 flux from
atmospheric CO2 inverse modeling, Global Biogeochem. Cycles, 19,
GB4013, doi:10.1029/2004GB002257.
Quay, P. (2002), Ups and Downs of CO2 Uptake, Science, 298, 2344.
Takahashi, T., S.C. Sutherland, R.A. Feely, and R. Wanninkhof (2005), Decadal
Change of the Surface Water pCO2 in the North Pacific: A Synthesis of
35 Years of Observations, submitted to J. Geophys. Res.