Asymmetry of interoceanic fresh-water and heat fluxes (ocean heat budget/climate/water balance)

Transcription

1 Proc. Nati. Acad. Sci. USA Vol. 77, No. 5, pp , May 1980 Geophysics Asymmetry of interoceanic fresh-water and heat fluxes (ocean heat budget/climate/water balance) HENRY STOMMEL Woods Hole Oceanographic Institution, Woods Hole, Massachusetts Contributed by Henry Stommel, February 25,1980 ABSTRACT According to recent hydrological and meteorological studies, the meridional flux of fresh water and heat is remarkably different from ocean to ocean. These fluxes have been found to be consistent with the temperature and salinity distribution in the northern hemisphere. However, an attempt to map these fluxes on the temperature-salinity plane of a southern latitude leads to such large amplitudes of water-mass volume flux that it seems that there may be something wrong about them. 1. Global scheme of interoceanic fresh-water and heat flux Recently Hastenrath (1) has published a scheme of meridional heat flux in the ocean computed by integrating maps of heat flux across the sea surface. A similar scheme for fresh-water flux can be made of meridional flux of fresh water within the oceans by integrating the hydrological tables of Baumgartner and Reichel (2). Table 1 is composed of both sets of quantities: F, northward fresh water flux in units of 1O3 mn3 sec-1; H, northward heat energy flux in units of 1013 W; and (in parentheses) temperature flux, (H), in units of 106 0C m3 sec1. A graphical display is given in Fig. 1. The reader who refers to the above papers and to refs. 3-5 will appreciate that these fluxes are very approximate and that in the Southern Hemisphere different authors differ sometimes even over signs. Thus Bennett's (5) calculations from hydrographic stations agree with the sign of heat flux shown in Fig. 1 in the South Atlantic and Pacific but not in the South Indian Ocean, and his signs for fresh-water flux (opposite sign to that of salt) are different in both the South Pacific and Indian Oceans (his determination in the South Atlantic is uncertain). Even when the sum of all southern oceans is considered, Bennett's sign for heat flux differs from that of Hastenrath (1) and Trenberth (4). The sign of fresh-water transport in the summed southern ocean obtained from tabulations of Baumgartner and Reichel (2) (Fig. 1) agrees in sign with that expected by reversing Trenberth's sign for atmospheric water vapor. Trenberth's figures for summed southern oceanic transports are obtained as residuals from satellite radiation balance and observed meridional atmospheric transports. 2. Asymmetries of individual oceans' fluxes Let us introduce the idea of a "normal" ocean: one in which the flux of heat is poleward and that of fresh water is equatorward. "Abnormal" then means that the sign is reversed. Thus, the North Atlantic, the North Pacific, and the South Indian Oceans are all normal in this sense. However; the South Atlantic is abnormal in heat flux, the South Pacific is abnormal in fresh-water flux, and the North Indian Ocean (between 30'N and the equator) is abnormal in both fluxes. Of all the possible combi- The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C solely to indicate this fact nations of normality and abnormality in fresh-water and heat flux, the six ocean basins exhibit them all. If we adopt the convention that the normal directions of flux of fresh water and heat are denoted by an ordered pair of letters (for example, NA means normal sign of fresh-water flux but abnormal sign for heat), then the various oceans are as follows: North Atlantic South Atlantic North Pacific South Pacific NA AN North Indian South Indian Sum Northern Ocean Sum Southern Ocean AA It would appear that, in the past, oceanographers have tended to ignore this rather astonishing asymmetry of the interoceanic flux regime. Thus, Sverdrup (ref. 6, pp ), in a discussion of volume and salt flux at the equator and 30 S in the South Atlantic, does not comment upon or publish the abnormal direction of heat flux that his calculations surely must have suggested; and Wfist (7), who made rather painstaking calculations of volume, salt, and oxygen flux in the South Atlantic from the Meteor data, did not publish heat-flux results or even comment upon their abnormal direction. A straightforward computation of heat flux by using Wfist's estimates of mass flux and Meteor's temperature profiles yields equatorward heat flux in fair agreement with Hastenrath's (1) and Bennett's (5). Bennett (5) made both heat and salt flux calculations in all three southern hemisphere oceans and was surprised and plainly concerned about the abnormal directions of heat flux that his results suggested. The abnormalities can be rationalized qualitatively as follows. The Northern Indian Ocean is bounded by land at midlatitudes. Thus, excess evaporation can be supplied only by fresh water flowing poleward, and the excess of heat input through the surface can only escape equatorward; thus, this ocean is doubly abnormal. The North Atlantic (a normal-sign ocean), because it extends to high latitudes, loses so much heat in its northern portion that it actually causes the heat flux through the South Atlantic to be reversed in sign (thus accounting for the abnormality in sign of heat flux there), but in the South Pacific the heat flux that supplies the South Atlantic is in the normal direction and simply larger than normal. The large excess of precipitation at the Pacific equator supplies much of the fresh water required by the excessive evaporative regions in the southern oceans. According to the scheme in Fig. 1, this causes the massive abnormal fresh-water flow in the South Pacific. 3. Distribution of salinity and temperature on vertical sections near 30'S latitude In order to discuss the oceanographic implications of these fluxes in the southern hemisphere, it is necessary to examine the distribution of salinity and temperature in the different oceans at about 30'S latitude. Fortunately, there are sections available: R/V Atlantis at 320S in the Atlantic (ref. 8), RRS

2 Geophysics: Stommel Proc. Natl. Acad. Sci. USA 77 (1980) Table 1. Meridional northward flux of fresh water, F (103 m3 sec'), and heat, H (1013 W), and temperature flux, (H) (106 0C m3 sec-1) Atlantic Ocean Indian Ocean Pacific Ocean F H (H) F H (H) F H (H) From North Polar Ocean N (62) (2) 300N (370) (272) (344) (-114) (-55) 300S (275) (-117) (-458) 600S (253) (-14) (-284) From Antarctica Discovery II at 320S in the Indian (ref. 9), and R/V Eltanin at 280S in the Pacific (ref. 10). Coarse resolution versions of the salinity and temperature on these sections are depicted in Fig. 2. The reader who wishes to examine more detail than can be rendered on this scale is referred to the above publications. It is convenient to think of the water-mass volume flux with amplitude 30 X 106 m3 sect as a standard ocean current. To achieve the fresh-water and heat fluxes of Fig. 1 and Table 1 at 300S with such a standard ocean current would require that the poleward (S) flowing water be dt 0C warmer and ds parts per thousand (%o) saltier than the equatorward (N) flowing water, as shown in Table 2. If one chooses a water-mass volume flux of M X 106 m3 sec1 instead of the standard current, then the numbers dt and ds in Table 2 should be multiplied by 30/M. We use Fig. 2 to try to locate water masses that might be able to carry the values of dt and&ds required in each ocean as given in Table 2. As shown in Fig. 2, the main feature of all three oceans is a warm salty surface layer some 800 m thick (central water) underlain, between 800 and 1800 m, by a-layer of low salinity (Antarctic intermediate water) and from 1800 to the bottom, a cold and medium salinity deep or bottom water (called NADW and AABW in the Atlantic and common water in the South Indian and Pacific Oceans, but without marked differences in temperature and salinity). These' water masses define a temperature-salinity relationship that by and large is similar on the sections shown for all three oceans. Envelopes containing most of the water in these temperature-salinity relationships are shown in Fig. 3; they are taken from ref. 6. The three southern oceans are so well connected by the Circumpolar Current that the water-mass properties, as seen on a temperature-salinity diagram, are' approximately alike. In the South Indian Ocean, the near parallelism of the isotherms and isohalines (SI) ensures a fairly tight temperaturesalinity relationship for- the whole section. By contrast, there is a major departure from this parallelism in the low-salinity layer of the South Pacific, which intensifies and penetrates to the surface (and higher temperature) at the eastern end of the section. Eltanin station 91 is representative of this low-salinity portion of the eastern South Pacific and is shown separately in Fig. 3, with depths marked in meters. It lies within the flowing Peru Current, generally accepted to be flowing northward. A similar but much weaker departure from parallelism at the boundary between the central and intermediate water occurs in the South Atlantic. Atlantis station 5837 is typical of this low-salinity eastern'atlantic water and is shown separately in Fig. 3. It lies in the Benguela Current, which oceanographers believe flows northward. In order to relate the distribution of water masses in Fig. 3 to the differences dt and ds required in Table 2 to carry the fresh-water and heat fluxes in Fig. 1 and Table 1, the four oblique line segments shown in Fig. 3 are introduced. They are FIG. 1. Fresh-water flux, F, (in 103 m3 sec1) on left and heat flux, H, (in 1013 W) on right, at various latitudes, positive northward. Data are from refs. 1 and 2.

3 O-0Q-07 Geophysics: Stommel E T C ATLANTIC 320S -F -X7,5W 2 W ~~ ~ ~ ~ ~ ~ ~ ~ 40OW 20OW o Proc. Natl. Acad. Sci. USA 77 (1980) 2379 ~~ ~ ~ ~ ~ ~ ~ E W 20 FIG. 2. Coarse resolution versions of ocea nographic sections near 30'S in all three oceans. drawn so that the difference in temperature and salinity of the two ends corresponds to that in Table 2. This fixes their length and slope. The sign convention tells us the direction of flow at each end (N or 5). Because they are defined by differences, they can be freely translated anywhere on the plane but, of course, the only meaningful positions will be ones where the end points lie within water-populated regions or in regions where mixtures could be concocted. The lengths of the segments shown in Fig. correspond to water-mass volume transports of 30 x 106 m3 sec1l. A larger transport M permits them to be shortened by the ratio 30/M. An idea of the errors in assigned length and slope of the line segments, assuring error in F to be x io6 m3 sec1 errors in (H) to be + 50 x 106 and m3 sec-', is shown by the error bars at the S end of the Atlantic segment. Examiination of Fig. 3 reveals the following. The line segment for the sum of all oceans (at approximately 300S) can be placed (as shown) so that both ends lie in the main ridge of probability distribution of temperature and salinity in all three oceans. Thus, 30 X 106 m3 sec- of central water flowing southward Table 2. Excess temperature, dt, and salinity, ds, of poleward flowing water over that of equatorward flowing water required to produce fresh-water and heat fluxes of Table 1 at 300 latitude, with standard volume flux of 30 X 106 m3 sec1 Ocean dt, 0C ds, %o North Atlantic North Pacific South Atlantic -9.2* 0.62 South Indian South Pacific * Sum Southern Oceans * Values have abnormal sign.

4 2380 Geophysics: Stommel S, %o FIG. 3. The temperature (T)-salinity (S) distribution of each of three oceans at about 30'S (ref. 6): South Atlantic (SA), South Indian (SI), and western South Pacific (WSP) and eastern South Pacific (ESP). The distribution at two stations is also shown: Eltanin (El) 91 in the Peru Current of the eastern South Pacific and Atlantis (At) 5837 in the Benguela Current of the eastern South Atlantic, with depths indicated in meters. The stars numbered 1, 2, and 3 correspond to points in the three-point model of Section 4. The four line segments are computed from Table 2; N (S) denotes northward (southward) flow. The error bars (at the S end of the Atlantic segment) are computed on the assumption that the error in F is +0.2 X 106 m3 sec-1 and the error in (H) is +50 X 106 OC m3 sec-1. and the same amount of a mixture of deep and intermediate water flowing northward can carry the fresh-water and heat flux for all three oceans combined. It can occur anywhere over most of the sections, at any longitudes except the most eastern portion of the South Pacific. It would be difficult to reduce the transport much below 20 X 106 m3 sec1 without lengthening the segment beyond the limits of the distribution of properties on the temperature-salinity plane. Geostrophic constraints might require a somewhat larger volume transport, but the line segment, if shortened, can easily be shifted toward the left to keep both ends on the main ridge; in any case, the inverse method of Wunsch (11) is very accommodating. There is no obvious inconsistency between the summed fresh-water fluxes, the distribution of water masses on the temperature-salinity plane, and the location or magnitude of the volume fluxes of summed oceanic currents. A similar statement can be made for the Indian Ocean: as one can see, its line segment also, within indicated errors, can easily be fit to the main ridge of the South Indian (SI) water-mass distribution. There is some indication that most of the northward flow should be in the intermediate water, and this is consistent with the conventional oceanographic idea that South Indian deep water may actually be flowing weakly southward. The fitting of the Atlantic segment presents more difficulty. If it is shortened and made more vertical (within the errors), it can be translated so that the S end lies within deep water and the N (northward flowing) end in the shallow water of the Proc. Natl. Acad. Scd. USA 77 (1980) Table 3. Distribution of properties on the temperature (T)-salinity (S) plane (from Fig. 3) Si, Ti, Point %o 0C Water mass Central water Intermediate water Deep water Benguela Current (Atlantic station 5837) or even on the main ridge between the intermediate and central waters. There is no clear inconsistency in fitting flows on the temperature-salinity plane to the abnormal direction of heat flux and normal direction of fresh-water flux in the South Atlantic beyond a suggestion that the magnitude of fresh-water flux indicated in Table 1 and Fig. 1 may be too large. The South Pacific is another matter. It is true that the Pacific line segment can be translated and shortened within error so that the S end can be placed at shallow levels on Eltanin station 91 and the N end in deep water, but this requires a reversal in direction of the Peru Current, which from other evidence (geostrophic and the distribution of water masses at other latitudes) almost certainly flows northward. The segment can be shortened to one-third its length, the S end placed on the main western South Pacific ridge in the intermediate water and the N end in the deep water, but this implies a massive recirculation of deep and intermediate water inconsistent with conventional descriptive studies (12, 13). I can only conclude that there is something wrong with the abnormal direction of the South Pacific's fresh-water flux shown in Table 1 and Fig A formal three-point model The graphical discussion based on Fig. S can be stated formally as follows, but actually does not convey much more meaning. Let us suppose that the distribution of properties on the temperature-salinity plane can be represented crudely by three points common to all three oceans, indicated by the stars in Fig. 3 (see Table 3). Clearly better representations can be made for each individual ocean [for example, in the eastern South Pacific, the central water has significantly lower (ts34.5) salinity at 10WC], but our purpose here is to emphasize the similarity and discover how wild the computed water-mass fluxes may be. If we define northward mass flux by mi, then mass conservation, vanishing salt flux, and heat conservation statements can be written. mi + m2 + M3 = F ml Sl + m2 S2 + m3 S3 = 0 ml tl + M2 t2 + m3 t3 = (H) Table 4 shows the volume fluxes mi for each ocean and for all oceans summed at 300S, as computed from these equations and the fluxes F and (H) given in Table 1. The fluxes mi in Table 4 for the intermediate water and Table 4. Fresh-water flux, F, temperature flux, (H), and computed water-mass volume flux, mi, at 300S South South South Atlantic Indian Pacific Sum F, 106 m3sect (H), 106 IC m3secl mi, 106 m3 sect ml: Central m2: Intermediate m3: Deep

5 Geophysics: Stommel bottom water in the South Atlantic and South Pacific appear to be awkwardly large, and we, as oceanographers, would expect much smaller values to be appropriate. In the South Atlantic we can reduce the 23 m' from 14,744 to a minimum of about 2222 by choosing F as instead of [in which case the m vector is (34, -1, -33)]. These are still quite large, but more acceptable oceanographically. However, they suggest that the fresh-water flux ought to be reversed in Fig. 1. Similarly, if the figure for heat flux of the South Pacific is accepted, then the water-mass fluxes can be reduced from (-34, 92, 125) to somewhat smaller size (-57, 0, 57) by a change in sign of fresh-water flux there. The mass-flux vector for the sum of all southern oceans at 30'S m = (-44, 22, 21) is much more manageable in size. 5. Conclusions An earlier study of fresh-water and heat flux estimates in the North Atlantic and Pacific (14) found that they are consistent with moderate meridional volume fluxes of water masses as mapped on the temperature-salinity plane at a midlatitude (450N). The striking interoceanic asymmetries of sign in the fresh-water and heat fluxes of the South Atlantic and South Pacific, found in the works of Baumgartner and Reichel (2) and Hastenrath (1) and displayed in Fig. 1, are more problematical (at 300S) because, with the available temperature-salinity distribution, volume fluxes of uncomfortably large magnitude are required. The South Indian Ocean alone and the sum of all three southern oceans are symmetrical to the North Atlantic and Pacific and sum of all northern oceans and present no problem of excessive magnitude of volume fluxes. The doubly abnormal northern Indian Ocean can transport fresh water and heat in the unusual directions indicated without excessive volume fluxes. The apparent, and possibly real, discrepancies between the fluxes shown in Fig. 1 and the magnitude of computed mass fluxes on the temperature-salinity plane are localized in the South Atlantic and Pacific Oceans. Can we identify the source of discrepancy a little more closely? In the South Atlantic the abnormal direction of Hastenrath's Proc. Natl. Acad. Sci. USA 77 (1980) 2381 (northward) heat flux is confirmed by Bennett's (5) direct computations and by simple sums from Wust's (7) estimates of mass flux from analysis of the Meteor data. If we could reduce the magnitude of the fresh-water flux in the South Atlantic and reverse its sign in the South Pacific (for example, by a more uniform global equatorial precipitation), the large oceanic volume fluxes in Table 4 could be reduced. Hydrologists will be able to judge whether this is possible. That there can still be some uncertainty about the signs of these oceanic fluxes is a measure of the level of our present knowledge about these globally important processes. Dr. Harry Bryden, Dr. Dean Roemmich, and Mr. L. V. Worthington offered helpful advice at an early stage. The work was done under National Science Foundation Grant OCE This is Woods Hole Oceanographic Institute Contribution Hastenrath, S. (1980) J. Phys. Oceanogr. 10, Baumgartner, A. & Reichel, E. (1975) The World Water Balance (Elsevier/North-Holland, Amsterdam). 3. Bryden, H. L. & Hall, M. M. (1980) Science 207, Trenberth, K. E. (1979) Dyn. Atmos. Oceans 4, Bennett, A. F. (1978) J. Phys. Oceanogr. 4, Sverdrup, H. U., Johnson, M. & Fleming, R. (1942) The Oceans (Prentice-Hall, Englewood Cliffs, NJ). 7. Wust, G. (1957) Wissenschaftliche Ergebnisse der deutschen atlantischen Expedition "Meteor" (Walter de Gruyter, Berlin), Vol. 6, part 2, pp Fuglister, F. C. (1960) Atlantic Ocean Atlas (I. G. V ) (Woods Hole Oceanographic Institution, Woods Hole, MA). 9. Wyrtki, K. (1971) Oceanographic Atlas of the International Indian Ocean Expedition (National Science Foundation, Washington, DC). 10. Stommel, H., Stroup, E. D., Reid, J. L., & Warren, B. A. (1973) Deep-Sea Res. 20, Wunsch, C. (1978) Rev. Geophys. Space Phys. 16, Warren, B. A. (1973) Deep-Sea Res. 20, Reid, J. L. (1973) Deep-Sea Res. 20, Stommel, H. & Csanady, G. T. (1980) J. Geophys. Res. 85, CI,