Preindustrial A ir-Sea CO2 Flux: Sensitivity to Mixing
P rinceton Univers ity
1
I.Marinov ,
1
A .Gnanadesikan ,
J.
1
Sarmiento ,
R.
1
Slater ,
and N.
2
Gruber
1 Program in A tmospheric and Oceanic Sciences, Princeton University; 2 IGPP & Dept. of A tm. Sciences, UCL A
1.
A bstr act
2.
Understanding the magnitude and spatial distribution of preindustrial carbon sources and sinks
is critical for constructing a correct global carbon budget.
T here are large differences between air-sea fluxes of C O2 obtained with different GC Ms. T hese
could be due to differences in the physical models, and in particular to differences in the
parametrization of mixing processes.
W e separate the air-sea flux into an abiotic component due to the solubility pump and a biotic
component due to the biological pump:
M odel Setup
4.
3.
T he model used is version 3 of MOM with 4o resolution. I t includes Gent-McW illiams
parametrization and the Griffies et al.(98) discretization of isopycnal mixing. B iogeochemistry is
consistent with OC MI P-2 requirements. A t the surface we apply PO4, T and S restoring on a
time scale of 30 days, fluxes of heat and freshwater. T he model is run to equilibrium with an
atmospheric pC O2 of 278 ppm.
Fig 2: A nnual mean sea-air C O2 fluxes from (a)
abiotic and (b) full biology models. Zonal integral.
Positive flux means C O2 comes into the ocean.
Modeled fluxes vary little with mixing,
with largest variation in the Southern Ocean.
M ethods
I n order to distinguish between the abiotic and biotic C O2 fluxes, we set up two models:
• T he solubility pump is mainly a consequence of the heat and water fluxes, which change
the solubility of carbon and lead to large air-sea fluxes.
• T he biological pump is due to the formation of dissolved and particulate organic carbon
and C aC O3 and the transport of these materials to other regions of the world where they are
remineralized or dissolved. I t includes soft tissue and carbonate pump components.
•A B I OT I C model with no biology. T he air-sea flux is determined by the solubility pump and
its interaction with ocean circulation.
•FUL L B I OL OGY model where the air-sea flux is also determined by the biological pump.
Why are abiotic fluxes larger than the
total fluxes over most of the ocean ?
Why does the total flux vary less with
mixing than the abiotic flux ?
F or each model we run four experiments L L , H L , H H , L H , where we vary vertical
diffusivity K v (first index) and along-isopycnal diffusivity Ai (second index).
•
What is the effect of changing the vertical and lateral mixing on the abiotic and biotic
components of the air-sea flux and on the total flux ?
•
What are the mechanisms by which ocean circulation changes the air-sea flux of C O2 ?
5.
L L and HH are runs in which K v and A i are varied together so as to roughly preserve lowlatitude pycnocline. For completeness we add runs where single parameters are varied: an
increased K v run (HL ) and an increased A i run (L H). T he L L run has surface values K v=0.15
cm2/s , A i=1000m2/s, while the HH run has K v=0.6 cm2/s, A i=2000 m2/s. K v increases
hyperbolically with depth to 1.3 cm2/s in all runs. Surface B C s are the same for all runs.
Solubility pump: the effect on C O 2 flux
T he A B I OT I C air-sea flux is due to the solubility pump. W arming of upwelled water is
associated with loss of C O2 in the tropics. C ooling of the surface water results in gain of C O2 in
high latitudes.
Strong E kman transport at the equator moves the water before it can equilibrate with the
atmosphere, resulting in larger spread and lower magnitude of the air-sea flux compared to the
heat flux.
Fig 3: T he abiotic flux of C O2 (b) follows the thermal flux (a), defined as F= Q/C p* dDI C /dT =
=(-Q)/(r* C p* R ) * DI C /C O2* dC O2/dT , where Q is heat flux, C p water heat capacity, R buffer factor
6.
A biotic flux: Sensitivity to M ixing
The meridional heat flux is mostly driven by the meridional overturning. I ncreased vertical
diffusivity implies stronger overturning (F . Bryan'87, Gnandesikan'99, etc.) and therefore
stronger heat fluxes and stronger abiotic flux of C O2.
C onsistent with scaling theories, incr easing K v :
-
increases meridional overturning (Fig 1)
increases C O2 flux into the ocean in the W estern B C (by increasing cooling),
increases C O2 flux out of the tropical ocean (due to increased heating),
increases C O2 flux into the S Ocean (higher convection and increased cooling).
increases carbon transport (Fig. 4).
Figure 3: M eridional overturning in Sv in the standard (L L ) case and differences between
other runs and the L L run. I ncreasing vertical mixing strongly increases the overturning
circulation, while increasing lateral mixing decreases the overturning circulation.
7.
A biotic T r anspor t
- decreases overturning circulation
- inhibits the large cooling of the ocean associated with the western boundary currents,
- decreases equatorial upwelling.
B iological pump: the effect on C O 2 flux
10.
B iotic F lux: Sensitivity to M ixing
T he annual mean of the air-sea C O2 flux due to biology follows the inverse of the soft tissue
pump. T he soft tissue pump is defined as the salinity normalized PO4 minus the average surface
PO4.
A s we go from the equator towards the poles, higher surface PO4 means a reduction in the
efficiency of the biological pump resulting in biotic flux of C O2 out of the ocean.
A s K v incr eases,
-tropical waters are warmed up, increasing
the abiotic flux out of the ocean.
-larger biological productivity at tropics
(Gnanadesikan '01) increases the biotic flux of
C O2 into the ocean.
Thus, prescribed nutrients at the surface severely constraint the biotic air-sea flux.
Q = B iotic air-sea C O2
Fig 6: B iotic air-sea flux and (-1)* surface soft tissue pump. a. A nnual means. b. L inear Fit, r2=0.76
I ncreasing vertical mixing changes both
abiotic and biotic fluxes such that the
compensation between the two increases.
Abiotic transport increases with increasing
vertical diffusivity. Highest differences in the
DI C transports are in the S Hemisphere,
where we see a 50% increase in DI C transport
for a four fold increase in K v.
B iological pump is inefficient at high
latitudes, resulting in loses of C O2 to the
atmosphere. T his biotic flux is offset by the
abiotic flux into the ocean due to cooling.
Figure 5: A nnual mean fluxes, L L case. Zonal
integrals. T he biotic flux is the difference between C O2
fluxes in the model with full biology and in the abiotic
11.
B iotic T r anspor t
12.
C onclusions
•
T he biotic transport opposes the abiotic
transport of DI C .
T ransport of DI C is more sensitive to mixing than the air-sea C O2 flux. L argest sensitivity to
mixing is seen in the Southern Ocean. First order sensitivity to vertical mixing obeys known
scaling theories.
I ncreasing K v increases the biotic transport
of DI C (due to stronger overturning
circulation).
•
A ir-sea C O2 flux and transport are explained to first-order by air-sea heat flux and transport
and by boundary conditions (i.e., phosphate, temperature, salinity, wind) (Fig 3 and Fig 6).
I ncreasing A i decreases biotic transport of
DI C in the N hemisphere (weaker circulation).
• Strong compensation between biotic and abiotic C O2 fluxes scales with mixing.
Stronger
vertical mixing implies a stronger compensation mechanism (Fig 8).
•
F urther Questions:
1. What processes set the varying degrees of compensation between biotic and
abiotic air-sea C O2 fluxes?
2. What determines the large Southern Ocean sensitivity to mixing and the
compensation mechanism in this region?
Figure 7: Differences in annual mean biological
fluxes of C O2. Zonal integral.
P = (-1)* (35/salinity* PO4-[PO4surface])
Thus, the solubility and biological
pumps partially compensate each other.
Figure 4: (a) Heat transport. (b) Global abiotic
transport of DI C . A nnual means.
Q=P* 1.150e-11+2.934e-4
L A T I T UDE
C ompensation M echanism
T he abiotic DI C transport varies with latitude
just like the heat transport, transporting DI C
northward S of the equator and southward N
of the equator.
The compensation effect explains the
small sensitivity of total air-sea CO2 flux and
total transports with mixing (Fig 2, Fig 8).
r2=0.7631
8.
B etween 40S and 40N, fast biological
uptake strips out surface nutrients and C O2
and takes up additional C O2 from the
atmosphere. T his biotic flux is compensated
by an abiotic C O2 flux out of the ocean due
to warming of tropical waters.
while incr easing A i has the opposite effect:
9.
M otivating questions
Figure 8: (a) B iotic transport of DI C in the ocean.
(b) T otal transport of DI C . A nnual means.
R eferences
B ryan, F (1987) "Parameter sensitivity of primitive equation ocean general circulation models," JPO 17.
Gnanadesikan et al. (2001) "Oceanic vertical exchange and new production: A comparison between models and observa
tions," Deep Sea R esearch (submitted).
Gnanadesikan (1999) "A simple predictive model for the structure of the Oceanic Pycnocline," Science, vol 283.
M urnane, Sarmiento, L e Quere (1999) "Spatial distribution of air-sea C O2 fluxes and the interhemispheric transport of
carbon by the oceans," GB C 13, no2.
Najjar, Orr (1998) "Design of OC M I P-2 simulations of chlorofluorocarbons, the solubility pump and biogeochemistry"
corresponding author: imarinov@ princeton.edu