4.10. Heat, Mass, and Freshwater Transports

The oceans contribute up to about half of the total transport of heat from the tropics to the poles which is required to maintain the current climate conditions of the earth. A typical mid-latitude heat transport for an ocean basin in on the order of W. The errors in computing the ocean heat transport are large, but much progress has been made in the last decade towards a consistent picture of the ocean's role. Hsiung [1985] and Talley [1984] used an indirect method, based on integration of surface heat fluxes, to calculate the ocean's heat transport. Several of the surface heat flux data sets used in these papers yielded a fairly symmetric pattern of poleward export in both hemispheres of the Pacific. However, the error bounds appeared to be on the order of the quantity to be estimated [ Talley, 1984]. At the same time, direct estimates of Pacific heat transport, based on temperatures and velocities in the ocean, yielded a confusing picture which did not match the indirect estimates at all well. Since then, a number of new direct estimates of heat transport have been made for the Pacific Ocean, which agree with the expected symmetrically-poleward pattern. These estimates are in Roemmich and McCallister [1989] for 47N, 35N and 24N, Bryden et al. [1991] for 24N, Wijffels [1993] for 10N and 15S, and Toole [personal communication] for 33S. According to Wijffels [personal communication], an important remaining source of error is the amount of the Indonesian throughflow from the Pacific to the Indian, rather than improvements in western boundary current transport estimates.

The ocean circulation which accomplishes this heat transport has been clarified also in the last decade. Wunsch et al. [1983] show quantitatively that the meridional overturning cell in the South Pacific does consist of northward flow in the upper kilometer and in the bottommost kilometer, with southward return flow in the layer in between. These are the basic layers described above: the upper ocean and Antarctic Intermediate Water, the Circumpolar Water and Antarctic Bottom Water, and the returning Pacific Deep Water. Roemmich and McCallister [1989] quantified the transports across 24N in the North Pacific, showing northward flow only in the Ekman layer at the surface and in the bottom water, with southward flow concentrated just below the sea surface and just above the bottom layer. The sense of the South and North Pacific transport pictures is that the southern hemisphere heat transport is more dominated by a large vertical scale overturning cell, while the overturning vertical scale in the North Pacific is that of the shallow wind-driven gyre, and a near-bottom upwelling.

Freshwater transport estimates have also been greatly improved in the last few years. Wijffels et al. [1992] present a new analysis which indicates the importance of northward flow of relatively fresh water through Bering Strait from the Pacific to the Arctic (Fig. 7). This small net transport of water has an enormous effect on the freshwater balance, and results in the freshwater transport being northward throughout the Pacific and southward throughout the Atlantic. The balance is maintained by the relative evaporation in the Atlantic and precipitation in the Pacific.

A commonly-used simplified picture of the global conveyor belt for the North Atlantic Deep Water (NADW) shows the NADW forming in the northern North Atlantic, flowing southward and eastward around Antarctica, and rising to the surface in the Indian and Pacific Oceans [ Broecker, 1987]. Current understanding is that such upwelling to the surface may in fact occur in the Indian Ocean [ Fu, 1986; Toole and Warren, 1993], but not in the Pacific (see preceding paragraph). Broecker [1991] discusses more completely the circulation and mixing involved in the global thermohaline circulation, which also involves the Antarctic Bottom Water cell, large scale excursions from the meridional direction, and several passes through the Antarctic region during the long time required to upwell to the surface. Roemmich and McCallister [1989] tied their North Pacific transport picture to the global overturning circulation, showing the connections between the various layers of each of the oceans (Fig. 8). Schmitz [1994] has attempted a new set of cartoons of the global overturning circulation, with much more detail and emphasis on the pathways in the Atlantic than were given by Roemmich and McCallister [1989]. However, the Pacific pathways appear much more tenuously in his picture, and a number of fundamental questions about the vigor and location of upwelling, residence time of deep and bottom waters, and role of intermediate water formation remain.

Acknowledgments. The opportunity to present this material at the Oceanography Society's Pacific Basin meeting in July, 1994, is gratefully acknowledged. Presentations by D. Rudnick (abyssal circulation and Samoan Passage), S. Wijffels (heat transport), S. Hautala (Sverdrup balance), J. Lupton (hydrothermal forcing), D. Roemmich (circulation variability) and P. Niiler (surface velocity) at that meeting were extremely helpful in compiling this material. Discussions with J. Reid, A. Mantyla, T. Chereskin, P. Niiler, S. Hautala, and M. Morris were very helpful, particularly in allowing access to unpublished materials.