Mean annual poleward energy transports by the oceans in the southern hemisphere

Dynamics of Atmospheres and Oceans, 4 ( 1 9 7 9 ) 5 7 - - 6 4 © Elsevier Scientific P u b l i s h i n g C o m p a n y , A m s t e r d a m - - P r i n t e d in T h e N e t h e r l a n d s

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MEAN ANNUAL POLEWARD ENERGY TRANSPORTS BY THE OCEANS IN T H E S O U T H E R N H E M I S P H E R E

KEVIN

E. T R E N B E R T H

Laboratory for Atmospheric Research, University of Illinois, Urbana, Illinois61801 (U.S.A.) (Received 20 November 1978; accepted 30 January 1979)

ABSTRACT

Trenberth, K.E., 1979. Mean annual poleward energy transports by the oceans in the Southern Hemisphere. Dyn. Atmos. Oceans, 4: 57--64. A survey is m a d e of the published estimates of the components of the poleward flux of energy by the atmosphere in the Southern Hemisphere in order to determine the total atmospheric transport. Together with recent measurements by satelliteof the Earth's radiation budget this allows a new estimate of the required poleward energy transport by the oceans in the Southern Hemisphere for mean annual conditions. Results show that the ocean and atmosphere each contribute similar amounts for 0--30°S and that the ocean probably also transports about one third of the total at 60°S. The latter is in contrast to similar latitudes in the Northern Hemisphere where the ocean transport is negligible, but consistent with the different distribution of land and sea in the two hemispheres.

I. INTRODUCTION

The availability of detailed measurements, by satellite, of the earth's radiation budget has enabled new estimates of the contribution of the total oceanic poleward energy transports to be made for the Northern Hemisphere (Vonder Haar and Oort, 1973; Oort and Vonder Haar, 1976). The method used computes the oceanic transports indirectly as a residual, but probably gives more reliable results than previous techniques. The results reveal that the oceanic transports are much greater than had previously been assumed and are, in fact, comparable in magnitude to the atmospheric transports in the region of peak poleward transport (30--35°N). However, poleward of about 55°N the atmospheric transports account for almost all of that required by the radiation imbalance and oceanic transports are small. Perhaps this is not altogether surprising in view of the distribution of land and sea in the Northern Hemisphere. The percentage area covered by water drops to less than 50% north of 45°N and is as low as 28.7% for 65--70°N (List, 1968). In contrast, the ocean occupies over 90% of the area in the region 35--65°S and the land area constitutes less than 1% from 55--65 ° S.

58 The question therefore arises as to whether the role of the oceans is similarly small in the Southern Hemisphere high latitudes. The predominant mechanism of atmospheric transport in middle and high latitudes is through eddy transports of sensible heat by the stationary and transient eddies. In the Northern Hemisphere in winter, the contribution of stationary and transient eddies are similar in magnitude but the former dominate at 50--60°N (Oort, 1971). On the other hand, although stationary waves are present in the middle and high latitudes of the Southern Hemisphere (most notably wave number 1), the poleward heat flux by the stationary waves is very small (Robinson, 1970; van Loon and Williams, 1977}. Nevertheless, it seems that the contribution to the poleward heat flux in winter by the transient eddies is nearly the same in each hemisphere (compare the estimates of Oort and Rasmusson for the Northern Hemisphere with transient eddy fluxes of sensible heat given by Newell et al. (1969), Fig. 12). Further, since it appears that the required total poleward transport as determined from radiation measurements is nearly the same in each hemisphere (Stone, 1978) the question of whether oceanic heat fluxes make up the difference comes sharply to the fore. This paper therefore utilizes published estimates of the components that make up the total atmospheric transport along with the estimates of the total required transport from radiation measurements to estimate the oceanic transport in the Southern Hemisphere using the residual method. Undoubtedly the atmospheric transports are not as well known as in the Northern Hemisphere, but the residuals appear to be sufficiently large to establish with reasonable confidence that oceanic transports are indeed greater in high latitudes of the Southern Hemisphere than in the Northern Hemisphere. 2. METHOD The method used is that of Vonder Haar and Oort (1973) and the reader is referred to there for details. The energy balance for a polar cap for periods of a complete annual cycle can be approximated by (1) where energ~y storage terms and conduction within the solid earth are ignored. Measurements of the mean emitted thermal radiation and mean albedo at latitude ~ may be used to determine the net radiational heating ( R F ) and thus the total flux of energy across a latitude circle, which can then be partitioned into the atmospheric ( A T ) and oceanic ( O T ) transports. If we can estimate A T from rawinsonde observations and associated analyses then O T may be determined as a residual. The atmospheric energy flux across latitude ~ into the polar cap is given by AT + OT + RF = 0

A T =f [(C~T + gz + Lq + c2]2)v] 2~ra cos ~ dp g

(2)

A T = S H + PE + L H + K E

(3)

59 where: [()] = (1/2~)~() dX; Cp is specific heat at constant pressure; T is temperature (K); g is acceleration due to gravity; z is geopotential height; L is latent heat of condensation; q is specific humidity; c is wind speed; v is northward c o m p o n e n t of wind velocity; a is radius of the earth; p is pressure; )~ is longitude. The individual parts of A T consist of, respectively, the fluxes of sensible heat, SH; potential energy, PE; latent heat, LH; and kinetic energy, KE. In turn, these fluxes may be broken up into contributions from the mean meridional circulation and the eddies (stationary and transient combined). 3. DATA SOURCES We make use of published calculations for all estimates of the atmospheric transports. Generally these do not include estimates of errors or sampling uncertainties b u t we have attempted to provide some indication of the levels of uncertainty in individual components, expressed as a probable error, by comparing calculations of the same quantity by different authors whenever possible. The required total flux o f energy across a latitude circle deduced from radiation measurements is taken from Stone (1978). Error bars given b y Stone indicate probable errors of 4 × 10 i4 W at 45 ° latitude, decreasing to zero at the pole. In low latitudes (0--30°S) estimates of most of the components o f AT have been made by NeweU et al. (1974, see Table 7.3) which we will use. Largest contributions come from the mean meridional circulation and it seems that the sum PE + SH + L H may be more reliable than estimates of the individual components since there are large conversions between the three forms of energy (Oort and Rasmusson, 1971). This points to the need to use estimates which are consistent with one another and are derived from the same observations. A comparison o f the results of Oort and Rasmusson with those of Newell et al. for the region where t h e y overlap shows large discrepancies between individual terms but less disagreement in their sum. The contribution from the poleward flux of kinetic energy b y the mean meridional circulation is shown by Oort and Rasmusson to be negligible (less than 0.1 X 10 t4 W). The uncertainty in the estimates for the total contribution from the mean cells 0--30°S is probably a b o u t 2 × 1014 W. In high latitudes of the Southern Hemisphere it is impossible to estimate the strength of the mean meridional circulations accurately from the current observational n e t w o r k (Oort, 1977). However, it seems possible to make reasonable estimates of the total contribution by the mean meridional cells to the poleward flux o f energy b y assessing the strength of the Ferrel cell based on m o m e n t u m balance considerations (Newton, 1972). We adopt Newton's results, which he assesses as " n o t likely to be o f f b y more than a b o u t 2 X 10 i4 W" although comparison with mean cell mass fluxes calculated b y NeweU et al. (1969) indicates that his results may be slightly high.

60 TABLE [ Poleward flux of sensible heat by eddies for each 10° of latitude in 1014 W from Newell et al. (1974) (N) and Robinson (1970) (R) Lat (°S) N R

0

10

20

30

40

50

60

70

80

90

--0.4 0.0

0.6 1.4

2.9 3.8

11.9 11.8

22.8 14.3

28.4 12.6

19.7 9.6

7.2 5.6

1.8 1.5

0.0 0.0

We use the ed dy transports for all terms compiled by Newell et al. {1974} for 0--30 ° S, and also use their estimates o f the eddy transports of sensible heat for 30---90°S {derived from their Table 7.2). These are com pared with similar estimates by Robinson {1970) in Table I. I n d e p e n d e n t estimates of the e d d y fluxes o f sensible heat in the Sout hern Hemisphere at 700 mb have recently been made by van L oon {personal c o m m u n i c a t i o n ) based on five years o f Australian hemispheric analyses and these show t hat Robinson's values are low by nearly a factor of 2 for 40--60° S but show good agreement with values o f Newell et al. Uncertainty also exists owing to lack o f information above 100 mb which could affect results by up to 2 X 10 i4 W and the overall level o f u n c e r t ai nt y is possibly a bout 4 X 10 TM W in mid-latitudes. Estimates o f the poleward flux of latent heat can be made from the total meridional transport of water vapor given by Starr et al. (1969). Comparison with values o f Newell et al. in low latitudes indicates discrepancies as high as 4 × 1014 W at 30°S but the errors should be less at higher latitudes. In order to assess the c o n t r i b u t i o n from the eddies poleward of 30 ° S, we have a p p or tio n ed the total LH flux into contributions from the mean meridional circulation and the eddies in the ratio given by Oort {1971) for the Northern Hemisphere. This is not a critical step since the c o n t r i b u t i o n of the eddies dominates south o f 30 ° S. The remaining terms are the eddy fluxes of potential energy and kinetic energy south o f 30°S. Both have a m a x i m u m of about 0.7 X 1014 W in the Northern Hemisphere but the f o r m e r are mainly equatorward while the latter are poleward. Together t hey are negligible with a m axi m um cont ri but i on of about 0.3 × 1014 W. 4. RESULTS Table II shows the contributions o f the various terms t hat make up AT (positive denotes southward transport). In Table II values have been given to 0.1 X 1014 W to indicate the smallness o f some terms but clearly the last figure is o f no significance. Our crude estimates o f the u n c e r t a i n t y combine to give a probable error of 5 × 104 W although errors could be as large as 10 X 10 i4 W. The u n c e r t a i n t y is less at high latitudes owing to the constraint t ha t the flux is zero at the pole.

SHE AT ATNH

KEy.

MMC LH E PEE

Lat. (°S)

2.1 0.1 --0.1 (0.0) --0.4 1.7 0.2

0

8.2 4.0 --0.5 (0.0) 0.6 12.3 10

10

10.2 8.2 --0.5 (0.1) 2.9 20.9 11

20 1.7 12.6 --1.3 0.9 11.9 25.8 28

30 --3 13.8 (--0.7) (0.6) 22.8 33.5 30

40 --5 10.9 (--0.3) (0.4) 28.4 34.4 31

50 --3 3.1 (0.2) (0.1) 19.7 20.1 31

60

7 0.5 (--0.2) (0.0) 7.2 14.5 16

70

3 0.0 (--0.0) (0.0) 1.8 4.8 5

80

0 0 0 0 0 0 0

90

Poleward transports of energy by the atmosphere, AT, decomposed into contributions from the mean meridional circulation (MMC) and eddy contributions of LH, PE, KE, and SH as a function of latitude, in 1014 W. Values in parentheses are Northern Hemisphere transports (Oort and Rasmusson, 1971) and total A T values for the Northern Hemisphere are also given for comparison (from Oort and Vonder Haar 1976)

TABLE II

62

A comparison of AT in each hemisphere shows largest differences at 20 and 60 ° latitude. However we note that these latitudes in the Northern Hemisphere are also those where the latest values given in Table II differ markedly from earlier estimates given by Oort (1971). At 20°N other estimates by Oort (1971) of 14.5 X 1014 W and Newell et al. (1974) of 16.5 × 1014 W show less of a discrepancy. At 60 ° N, the earlier estirhate showed the northward flux of energy b y the atmosphere to be 6 X 1014 W less than that at 50°N whereas more recent valuesindicate that they are the same. However, part of the differences in AT between the hemispheres is u n d o u b t edly real. The a s y m m e t r y of the Hadley cells with respect to the equator ensures that the flux by the mean meridional circulation is poleward year round at 20°S b u t not at 20°N in the northern summer. At 60 ° latitude the main difference appears to arise through the extra contribution of the stationary eddies in winter in the Northern Hemisphere. In particular, significant differences between the hemispheres would be expected at high latitudes in the lower stratosphere, in keeping with the warmer polar night temperatures in the Northern Hemisphere and the marked differences in intensity and frequency of sudden stratospheric warming events. This may explain where the relative hemispheric differences at 50 and 60 ° latitude arises. There is also a difference in the mean meridional circulation at 60 ° , since in the Northern Hemisphere the direct polar cell apparently dominates (Oort, 1971) whereas the Ferrel cell is present year round in the Southern Hemisphere (Newell et al., 1969, Fig. 17). The remaining differences are all within the general uncertainties in the data and the methods of computation, and may not be significant in spite of obvious marked differences in the seasonal cycles within each hemisphere. Using the values of R F taken from Stone (1978) and A T from Table II, estimates of OT have been found using (1). These are shown in Fig. 1. At 45°S the probable error in OT, based on the previously discussed estimates of uncertainty, is 6.3 X 1014 W. Poleward of 70°S A T a p p e a r s to account for all of the required transport, as it must south of about 75°S where there is no

6

RF

0

10

20

30 40 LATLTUDE

50 °S

60

70

80

90

Fig. 1. P o [ e w a r d f l u x o f total energy as a f u n c t i o n o f l a t i t u d e f o r the Southern Hemisphere. R F is the total required flux inferred from the satellite measured radiation imbalance; A T and O T are the atmospheric and oceanic transport respectively.

63

open ocean. The ocean transport is nearly the same as the atmospheric transport in low latitudes (0--30 ° S) b u t is less important where the baroclinic eddies are capable of producing large eddy heat fluxes. Nonetheless, compared to the Northern Hemisphere, the required oceanic poleward heat flux is significantly larger at 60 ° S. Although this is only marginally greater than our assessment of the total uncertainty, it is plausible physically in view of the known differences in atmosphere fluxes b y the stationary waves and the differences in the land--ocean distribution. 5. CONCLUDING REMARKS

The estimates of the total oceanic poleward energy flux in the Southern Hemisphere show that the role played by the oceans in low latitudes is much greater than earlier estimates {Sellers, 1965), consistent with Vonder Haar and Oort's (1973) conclusions for the Northern Hemisphere. For the region 10--60 ° S, the oceanic transport averages 43% of the total. This result is in marked contrast to very recent estimates of the poleward heat fluxes at 30 ° S by Bennett (1978) who finds an equatorward transport of 1--2 X 10 i5 W using a direct method. However, the direct method uses scanty data in different seasons of different years and therefore probably includes significant errors. Nor does it take into account the effects of mid-ocean eddies and it seems possible that the transient eddies m a y be an important c o m p o n e n t of the total flux. At 60°S a unique situation exists since it is the only area where the ocean is continuous around the earth. This allows the strong Antarctic Circumpolar Current to exist and is associated with the location and intensity of the Antarctic Convergence (e.g. Hamon and Godfrey, 1978). In spite of the absence of b o u n d a r y currents in the region it would be surprising indeed, if transient eddy heat fluxes within the ocean were not responsible for a significant poleward flux of energy. Bryden (1979) has used current-meter records in the Drake Passage to estimate a poleward eddy heat flux of 0.67 W cm -2 which, if distributed uniformly over an ocean depth of 4 km at 60 ° S, would produce a poleward heat flux of 5.5 X 10 i4 W, or a substantial fraction of the required flux estimated here. The strong negative feedback that must exist between the oceanic and atmospheric poleward energy fluxes (Stone, 1978), ensures that any significant change in either will have further consequences. Such changes presumably t o o k place some 30 000 000 years ago when the Antarctic Circumpolar Current was first established. Also, the present day absence of significant stationary heat fluxes in the atmosphere of the Southern Hemisphere ensures that the atmospheric transient eddies together with the oceanic transports should make up the difference. Although better data should be forthcoming for the Southern Hemisphere over the next few years and will no d o u b t change the details o f the relative contributions b y the ocean and atmosphere to the total transport as presented here, the important role of the oceans in climate is well established.

64 REFERENCES Bennett, A.F., 1978. Poleward heat fluxes in the Southern Hemisphere oceans. J. Phys. Oceanogr., 8: 785--798. Bryden, H.L., 1979. Poleward heat flux and conversion of available potential energy in the Drake Passage. J. Mar. Res., 37: 1--22. Hamon, B.V. and Godfrey, J.S., 1978. The role of oceans. In: A.B. Pittock, L.A. Frakes, D. Jenssen, J.A. Peterson, J.W. Zillman (Editors), Climatic Change and Variability: A Southern Perspective. Cambridge University Press, Cambridge, pp. 31--52. List, R.J., 1968. Smithsonian Meteorological Tables. Sixth revised edition. Smithsonian Misc. Collections Vol. 114. Fourth reprint. Smithsonian Inst. Press, Washington, D.C., 527 pp Newell, R.E., Vincent, D.C., Dopplick, T.G., Ferruza, D. and Kidson, J.W., 1969. The energy balance of the global atmosphere. In: G.A. Corby (Editor), The global circulation of the atmosphere. R. Meteorol. Soc., London, pp. 42--90. Newell, R.E., Kidson, J.W., Vincent, D.G. and Boer, G.J., 1974. The general circulation of the tropical atmosphere and interactions with extratropical latitudes. Vol. 2. M.I.T. Press, Cambridge, Mass., 371 pp. Newton, C.W., 1972. Southern Hemisphere general circulation in relation to global energy and m o m e n t u m balance requirements. In: C.W. Newton (Editor), Meteorology of the Southern Hemisphere, Met. Mongr., 13, No. 35, pp. 215--246. Oort, A.H., 1971. The observed annual cycle in the meridional transport of atmospheric energy. J. Atmos. Sci., 28: 325--339. Oort, A.H., 1977. Adequacy of the rawinsonde network for global circulation studies tested through numerical model output. Mon. Weather Rev., 106; 174--195. Oort, A.H. and Rasmusson, E.M., 1971. Atmospheric circulation statistics. NOAA Prof. Paper No. 5, U.S. Govt. Print Office, Wash., D.C., 323 pp. Oort, A.H. and Vonder Haar, T.H., 1976. On the observed annual cycle in the ocean-atmosphere heat balance over the Northern Hemisphere. J. Phys. Oceanogr., 6: 781-800. Robinson, J.B., Jr., 1970. Meridional eddy flux of enthalpy in the Southern Hemisphere during the IGY. Pure Appl. Geophys., 80: 319--334. Sellers, W.D., 1965. Physical Climatology. University of Chicago Press, 272 pp. Starr, V.P., Peixoto, J.P. and McKean, R.G., 1969. Pole-to-pole moisture conditions for the IGY. Pure Appl. Geophys., 75: 300--331. Stone, P.H., 1978. Constraints on dynamical transports of energy on a spherical planet. Dyn. Atmos. Oceans, 2: 123--139. van Loon, H. and Williams, J., 1977. The connection between trends of mean temperature and circulation at the surface: Part IV. Comparison of the surface changes in the Nortt~ern Hemisphere with the upper air and with the Antarctic in winter. Mon. Weather Rev., 105: 636--647. Vonder Hear, T.H. and Oort, A.H., 1973. New estimate of annual poleward energy transport by Northern Hemisphere oceans. J. Phys. Oceanogr., 2: 169--172.