Atmospheric vs oceanic control of the surface

Following Juillet-Leclerc et al. (1987), we try to separate the effects of atmospheric fluxes and oceanic circulation on determining both the surface fields and their relationship. We again base our reasoning on the salinity, which can be compared to observed values over the whole ocean, by contrast to the less documented oxygen 18. Both fields respond to similar forcings and are qualitatively very close. Figure III.10a shows the influence of atmospheric fluxes alone on salinity, 10c includes the full oceanic circulation, and 10e represents climatological observations of Levitus (1982). Note that realistic values are not expected from the simple parameterisation of the box model. Although the box model (Figure III.10a) captures some basic features of the salinity field, ie the zonal distribution and the Atlantic enrichment, the GCM-simulated field is much more closer to the observations. The ocean dynamics readily explains the difference. The tropical gyres smooth and displace the maximal subtropical values by transporting depleted water i) from the equatorial band along the western part of the basins and ii) from the high latitudes along the eastern parts. Upwelling of deep waters characterised by mean values contributes to strongly smooth surface waters, either depleted as in the equatorial Pacific or enriched as along the western coasts of Africa and South America. In the north Atlantic basin, the net transport of surface waters originating in the Gulf of Mexico increases the salinity, and the north Atlantic drift brings enriched waters northern than 60¡N. The same differences between figures III.10b and III.10d suggest similar mechanisms for the oxygen 18.

Figure III.15. Estimation of the surface oxygen 18 - salinity slope from our simulations. a. (top) Spatial slope calculated from the OPA GCM outputs, by a linear fit of the 15 closest points around each grid point. No value is shown when the relative deviation on the slope is higher than 20%, ie for a poor correlation, which happens in the intertropical Pacific and Indian oceans. b. (bottom) Box model slope calculated at each grid point, which represents the sensitivity of oxygen 18 to salinity, and not a spatial slope. It is calculated as [.22 - (P.dp-E.d^E)/(P- E)]/34.6 according to the model of Craig and Gordon, and the parameters of Juillet- Leclerc et al. (1997), where 0.22ä and 34.6ä stand for the deep water d¹⁸O and salinity mean values. The extreme values (blue and red colors) are found in area where P~E.

The box model allows to clarify the origin of the oxygen 18 - salinity relationship, in two different ways. In each grid box, it allows to estimate an oxygen 18 - salinity sensitivity, based on the ratio (P.dp-E.d^E)/(P-E), shown by Figure III.15b. The extreme values (red and blue boxes) correspond to region where P~E, especially at 40¡ of latitude, along the coasts, and in the central tropical Pacific. Alternatively to this sensitivity, a spatial slope can be estimated by computing separately the oxygen 18 and salinity fields (Figures III.10b and a) and plotting them in a d¹⁸O-S diagram (Figures III.12b and III.13b). Both modelling ways give similar values and trend with latitude when compared with observations (Figures III.12a and III.12a) and with the full oceanic simulation (Figure III.15a). But we note two differences : (i) the scatter of the points in Figures III.12b and III.13b is higher than with the ocean dynamics, and (ii) the slopes are lower, especially for the highest salinities. These differences can be conceptually explained by the oceanic advection, as described above, which (i) limits the range of values created by spatially contrasted atmospheric fluxes, and (ii) forces these values towards a general fit (slope~0.5). Thus the box model concept of an oxygen 18 - salinity relationship as a mixing line, between a ÔpureÕ oceanic source (close to the average deep ocean) and a ÔpureÕ freshwater source (consisting of the atmospheric isotopic flux) seems the main control of this relationship compared to the oceanic advection, which increases slightly the slope and the spatial correlation.

Practical determination of the freshwater isotopic content (the intercept of this mixing line) is not trivial, since this includes both precipitation and evaporation. Continental runoff can also play an important role, but only in coastal region and not in the open ocean, as explained by Craig and Gordon (1965) for the high latitudes. This can be simulated with our model by shutting off the runoff (see next paragraph).

Modelling the distribution of oxygen 18 also allows to separate the different terms of its oceanic transport, diffusion, advection, which must balance the atmospheric flux. Although the advective fluxes entering and leaving each surface box are individually three orders of magnitude higher than the other fluxes, they compensate each other. It happens that the net advective flux is of the same order of magnitude, but generally lower, than the net diffusive flux (mainly vertical) which thus balances the atmospheric flux, as in the box model (Figure IIII.16). However, the advective term can be higher than the diffusive one in area where the oxygen 18 concentration presents strong horizontal gradients : this happens (Figure III.10b) at temperate and equatorial latitudes where the atmospheric flux sign changes from positive (tropical zone) to negative (high/equatorial latitudes) values. This advective mixing mainly explains the differences between Figures III.10b and 10d.

Since the surface is close to equilibrium, the horizontal fluxes must compensate each others. This is particularly easy to understand for the subtropical gyres, which are enriched in salt and oxygene 18 in the tropical latitudes by the strong evaporation, and depleted in higher latitudes by the precipitation. As schematised by Figure III.17, this oceanic advection brings enriched water to higher latitudes and is continuously depleted by the precipitation formed by exhaustion of the vapour formed at tropical latitudes.

Figure III.16. Comparison of the net advective, diffusive and atmospheric fluxes driving the isotopic composition of the surface in the simulation, after 2200 years.

These are annual averages expressed in isotopic ratio (¹⁸O/¹⁶O) x water flux (m³/s).

TROPIQUES MOYENNES LATITUDES advection ocŽanique

fractionnement atmosphŽrique

F=15

F=500

F=15 DF=0.05

DF=0.05

DF=0.05

DF=0.05

bord ouest bord est

Figure III.17. Sketch of the isotopic equilibrium taking place in the subtropical gyres. Isotopic fluxes F are expressed in isotopic ratio (¹⁸O/¹⁶O) x water flux (m³/s).