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it pure 3.3. The difference between the annual mean CO? concentrations lor Mauna I xxi, Hawaii (\1I.Q), and the South Pole (SFO) for 1980-1994.

In Equations 2 and 3, Aitt and Ms are the CO? mass in the Northern and Southern Hemispheres, respectively, r is the iriterhemispheric exchange time, taken to be 1,2 years, and Sn and Ss are the sink in the Northern and Southern 1 lemispheres, respectively. We have assumed that all the fossil fuel OO2 is released in the Northern 1 lemisphere.

We have investigated the interannual variations of r, using the Goddard Institute for Space Studies global three-dimensional climate model forced by the observed sea surface temperatures, volcanic optical depths, greenhouse gases, and ozone variations for 198(1-1994. An inert tracer with a time-invariant Northern Hemisphere industrial source pattern is transported in the climate model. When the model results were reduced to a two-box representation as in Equations 2 and 3, the effective r was found to vary by a factor of 20%. This magnitude of variation turns out to be unimportant for the CO? source/sink problem, because the MLO-SPO difference varies by a factor of 2 over the same period.

Equations 2 and 3 together with the requirement that Sn + 55 = Sink yield the interannual variations of the Northern Hemisphere and Southern Hemisphere sinks. These are shown in Figure 3A> together with the southern oscillation index (SOI), a measure of the state of the El Nino/southern oscillation. The SOI displayed are annual means for each calendar year, consistent with the way the CO2 data have been averaged. We have not included the deforestation source explicitly, because the contemporary deforestation source is located in the Tropics and makes no contribution to the hemispheric gradient In this way, the hemispheric sinks described below do not include half the deforestation source (M).8 PgC/y),

Averaged 1980-1988, the NH sink was 2+5 ± 1 PgC/y, whereas the SH sink was 0.5 ±0.5 PgC/y. From 1988 to 1990, the total sink increased by 2 PgC/y (compare Figure 3,1) with approximately equal contributions from the NH and SH.

Figure 3.4 shows that variations in the NH sink are larger than the variations in the SH sink. This is not unexpected, because interannual climate perturbations are more

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Figure 3-4. V ariations of the Northern Hemisphere (top panel) and Southern í lemisphere (bottom panel) sinks and the southern oscillation index (SOI, dashed line) for I9RÍM W, as determined by a two-box model. Note that the calculation does not include deforestation, and so half the deforestation release should be added to the displayed sink strengths to obtain the actual sink. Negative values of the SOI indicate El Nino years,

Year

Figure 3-4. V ariations of the Northern Hemisphere (top panel) and Southern í lemisphere (bottom panel) sinks and the southern oscillation index (SOI, dashed line) for I9RÍM W, as determined by a two-box model. Note that the calculation does not include deforestation, and so half the deforestation release should be added to the displayed sink strengths to obtain the actual sink. Negative values of the SOI indicate El Nino years, pronounced in the Northern Hemisphere, The dominant mode of interannual climate variations is the El Niño/southern oscillation, captured in the SOI, the normalized anomalous pressure difference between Tahiti and Darwin, Australia. During an El Nino, there is weaker east-west pressure gradient in the equatorial Pacific (negative SOI ) leading to weaker easterly winds. Anomalously warm sea surface temperatures during the El Nino lead to global-scale perturbations in temperature and precipitation. The NH CO2 sink is out of phase with the SOI; the NH sink is strong during strong El Niños (SOI < 0) and is weak during La Niñas (SOI > 0). The SH sink has the opposite phase relationship with the SOI; the sink lags the SOI by a year (the resolution of the investigation). The phase lag suggests alteration of total carbon concentrations resulting from circulation changes.

There was a steep increase in the global sink strength from 1988 to 1990. In our calculation, half of this sink was in the NH. This NH sink was predominantly terrestrial.

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atmospheric; t;o2 variations

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