concentration increases with time. This effect was
corrected to the reference year using the observed
increase in the atmospheric CO 2 concentration.
During El Nin ˜ o periods, sea–air pCO 2 differences
over the equatorial belt of the Pacific Ocean, which
are large in normal years, are reduced significantly
and observations are scarce. Therefore, observations
made between 101N and 101S in the equatorial Pacific for these periods were excluded from the maps.
Accordingly, these maps represent the climatological
means for non-El Nin ˜ o period oceans for the past 40
years. The purple-blue areas indicate that the ocean
is a sink for atmospheric CO 2 , and the red-yellow
areas indicate that the ocean is a source.
Strong CO 2 sinks (blue and purple areas) are
present during the winter months in the Northern
(Figure 1A) and Southern (Figure 1B) Hemispheres
along the poleward edges of the subtropical gyres,
where major warm currents are located. The Gulf
Stream in the North Atlantic and the Kuroshio
Current in the North Pacific are both major CO 2
sinks (Figure 1A) due primarily to cooling as they
flow from warm tropical oceans to subpolar zones.
Similarly, in the Southern Hemisphere, CO 2 sink
areas are formed by the cooling of poleward-flowing
currents such as the Brazil Current located along
eastern South America, the Agulhus Current located
south of South Africa, and the East Australian Current located along south-eastern Australia. These
warm water currents meet with cold currents flowing
equator ward from the Antarctic zone along the
northern border of the Southern (or Antarctic)
Ocean. As the sub Antarctic waters rich in nutrients
flow northward to more sunlit regions, CO 2 is drawn
down by photosynthesis, thus creating strong CO 2
sink conditions, as exemplified by the Falkland
Current in the western South Atlantic (Figure 1A).
Confluence of subtropical waters with polar waters
forms broad and strong CO 2 sink zones as a result of
the juxta position of the lowering effects on pCO 2 of
the cooling of warm waters and the photosynthetic
drawdown of CO 2 in nutrient-rich subpolar waters.
This feature is clearly depicted in azone between
401S and 601S in Figure 1A representing the austral
summer, and between 201S and 401S in Figure 1B
representing the austral winter.
During the summer months, the high latitude areas
of the North Atlantic Ocean (Figure 1A) and the
Weddell and Ross Seas, Antarctica(Figure 1B), are
intense sink areas for CO 2 . This is attributed to the
intense biological utilization of CO 2 within the
strongly stratified surface layer caused by solar
warming and ice melting during the summer. The
winter convective mixing of deep waters rich in CO 2
and nutrient seliminates the strong CO 2 sink and
replenishes the depleted nutrients in the surface
waters.
The Pacific equatorial belt is a strong CO 2 source
which is caused by the warming of upwelled deep
waters along the coast of South America as well as by
the upward entrainment of the equatorial under
current water. The source strengths are most intense
in the eastern equatorial Pacific due to the strong
upwelling, and decrease to the west as a result of the
biological utilization of CO 2 and nutrients during the
westward flow of the surface water.
Small but strong source areas in the north-western
subArctic Pacific Ocean are due to the winter convective mixing of deep waters (Figure 1A). The
lowering effect on pCO 2 of cooling in the winter is
surpassed by the increasing effect of highCO 2 concentration in the upwelled deep waters. During the
summer (Figure 1B), however, these source areas
become a sink for atmospheric CO 2 due to the intense biological utilization that overwhelms the increasing effect on pCO 2 of warming. A similar area is
found in the Arabian Sea, where upwelling of deepwaters is induced by the south-west monsoon during
July–August(Figure 1B), causing the area tobecome a
strong CO 2 source. This source area is eliminated by
the photosynthetic utilization of CO 2 following the
end of the upwelling period (Figure 1A).
As illustrated in Figure 1A and B, the distribution
of oceanic sink and source areas for atmospheric
CO 2 varies over a wide range in space and time.
Surface ocean waters are out of equilibrium with
respect to atmospheric CO 2 by as much as
7200 matm (or by760%).The large magnitudes of
CO 2 disequilibrium between the sea and theair is in
contrast with the behavior of oxygen, another biologically mediated gas, that shows only up to 710%
sea–air disequilibrium. The large CO 2 disequilibrium
may be attributed to the fact that the internal ocean
processes that control pCO 2 in sea water, such as the
temperature of water, the photosynthesis, and the
upwelling of deep waters,occur at much faster rates
than the sea–air CO 2 transfer rates. The slow rate of
CO 2 transfer across the sea surface is due to the slow
hydration rates of CO 2 as well as to the large solubility of CO 2 in sea water attributable to the formation of bicarbonate and carbonate ions. The latter
effect does not exist at all for oxygen.
Net CO 2 Flux Across the Sea Surface
The net sea–air CO 2 flux over the global oceans may
be computed using eqns [2] and [3]. Figure 2 shows
the climatological mean distribution of the annual
sea–air CO 2 flux for the reference year 1995 using
CARBON DIOXIDE (CO 2 ) CYCLE 509
corrected to the reference year using the observed
increase in the atmospheric CO 2 concentration.
During El Nin ˜ o periods, sea–air pCO 2 differences
over the equatorial belt of the Pacific Ocean, which
are large in normal years, are reduced significantly
and observations are scarce. Therefore, observations
made between 101N and 101S in the equatorial Pacific for these periods were excluded from the maps.
Accordingly, these maps represent the climatological
means for non-El Nin ˜ o period oceans for the past 40
years. The purple-blue areas indicate that the ocean
is a sink for atmospheric CO 2 , and the red-yellow
areas indicate that the ocean is a source.
Strong CO 2 sinks (blue and purple areas) are
present during the winter months in the Northern
(Figure 1A) and Southern (Figure 1B) Hemispheres
along the poleward edges of the subtropical gyres,
where major warm currents are located. The Gulf
Stream in the North Atlantic and the Kuroshio
Current in the North Pacific are both major CO 2
sinks (Figure 1A) due primarily to cooling as they
flow from warm tropical oceans to subpolar zones.
Similarly, in the Southern Hemisphere, CO 2 sink
areas are formed by the cooling of poleward-flowing
currents such as the Brazil Current located along
eastern South America, the Agulhus Current located
south of South Africa, and the East Australian Current located along south-eastern Australia. These
warm water currents meet with cold currents flowing
equator ward from the Antarctic zone along the
northern border of the Southern (or Antarctic)
Ocean. As the sub Antarctic waters rich in nutrients
flow northward to more sunlit regions, CO 2 is drawn
down by photosynthesis, thus creating strong CO 2
sink conditions, as exemplified by the Falkland
Current in the western South Atlantic (Figure 1A).
Confluence of subtropical waters with polar waters
forms broad and strong CO 2 sink zones as a result of
the juxta position of the lowering effects on pCO 2 of
the cooling of warm waters and the photosynthetic
drawdown of CO 2 in nutrient-rich subpolar waters.
This feature is clearly depicted in azone between
401S and 601S in Figure 1A representing the austral
summer, and between 201S and 401S in Figure 1B
representing the austral winter.
During the summer months, the high latitude areas
of the North Atlantic Ocean (Figure 1A) and the
Weddell and Ross Seas, Antarctica(Figure 1B), are
intense sink areas for CO 2 . This is attributed to the
intense biological utilization of CO 2 within the
strongly stratified surface layer caused by solar
warming and ice melting during the summer. The
winter convective mixing of deep waters rich in CO 2
and nutrient seliminates the strong CO 2 sink and
replenishes the depleted nutrients in the surface
waters.
The Pacific equatorial belt is a strong CO 2 source
which is caused by the warming of upwelled deep
waters along the coast of South America as well as by
the upward entrainment of the equatorial under
current water. The source strengths are most intense
in the eastern equatorial Pacific due to the strong
upwelling, and decrease to the west as a result of the
biological utilization of CO 2 and nutrients during the
westward flow of the surface water.
Small but strong source areas in the north-western
subArctic Pacific Ocean are due to the winter convective mixing of deep waters (Figure 1A). The
lowering effect on pCO 2 of cooling in the winter is
surpassed by the increasing effect of highCO 2 concentration in the upwelled deep waters. During the
summer (Figure 1B), however, these source areas
become a sink for atmospheric CO 2 due to the intense biological utilization that overwhelms the increasing effect on pCO 2 of warming. A similar area is
found in the Arabian Sea, where upwelling of deepwaters is induced by the south-west monsoon during
July–August(Figure 1B), causing the area tobecome a
strong CO 2 source. This source area is eliminated by
the photosynthetic utilization of CO 2 following the
end of the upwelling period (Figure 1A).
As illustrated in Figure 1A and B, the distribution
of oceanic sink and source areas for atmospheric
CO 2 varies over a wide range in space and time.
Surface ocean waters are out of equilibrium with
respect to atmospheric CO 2 by as much as
7200 matm (or by760%).The large magnitudes of
CO 2 disequilibrium between the sea and theair is in
contrast with the behavior of oxygen, another biologically mediated gas, that shows only up to 710%
sea–air disequilibrium. The large CO 2 disequilibrium
may be attributed to the fact that the internal ocean
processes that control pCO 2 in sea water, such as the
temperature of water, the photosynthesis, and the
upwelling of deep waters,occur at much faster rates
than the sea–air CO 2 transfer rates. The slow rate of
CO 2 transfer across the sea surface is due to the slow
hydration rates of CO 2 as well as to the large solubility of CO 2 in sea water attributable to the formation of bicarbonate and carbonate ions. The latter
effect does not exist at all for oxygen.
Net CO 2 Flux Across the Sea Surface
The net sea–air CO 2 flux over the global oceans may
be computed using eqns [2] and [3]. Figure 2 shows
the climatological mean distribution of the annual
sea–air CO 2 flux for the reference year 1995 using
CARBON DIOXIDE (CO 2 ) CYCLE 509