Harry L. Bryden

Southampton Oceanography Centre

Empress Dock, Southampton SO14 3ZH

and

Shiro Imawaki

Kyushu University

Kasaga, Fukuoka 816-8580

April 1999

Submitted as Chapter 6.1 of Ocean Circulation and Climate edited by Gerold Siedler and John Church

A fundamental issue in the global climate system is the relative amounts of heat carried by the oceans and the atmosphere to balance the global heat budget in which radiational heating from the sun occurs predominantly in tropical regions while radiational cooling back to space occurs more uniformly over the globe. Progress in estimating ocean heat flux by three methods is reviewed.

The residual method subtracts atmospheric energy transport from the radiation budget requirement for total ocean plus atmosphere energy transport to determine ocean heat transport as a residual. Previously this method was applied to zonal averages. New studies have derived spatial patterns of atmosphere-surface heat flux by subtracting atmospheric energy flux divergence from the net radiation at the top of the atmosphere. Such patterns exhibit substantial and unrealistic annually averaged heat fluxes over the land but the patterns of heat flux over the oceans appear realistic. Masking the land to consider only these air-sea heat fluxes yields ocean heat transports in broad agreement with existing direct estimates of ocean heat transport. Dipole patterns of heat gain and heat loss over land topography, however, suggest that refinements in atmospheric energy transport are still needed.

The bulk formula method for determining air-sea heat exchange from marine observations has been used to produce several new global air-sea flux climatologies. Depressingly, each exhibits an overall global imbalance such that on average the ocean gains heat at a rate of 30 to 50 W m-2. There is evidence that spatially uniform corrections are not appropriate. Refinement of the air-sea fluxes will require accurate area-averaged estimates of ocean heat flux divergence over a variety of oceanic regions and additional buoy measurements of air-sea heat exchange in inhospitable subpolar and polar regions.

Making direct estimates of ocean heat transport from transoceanic sections in each ocean basin was a primary objective in designing the World Ocean Circulation Experiment (WOCE) field programme. While many zonal hydrographic sections were taken during WOCE, there are only a few published estimates of heat transport from WOCE sections to date. Most of the recent estimates are for the Atlantic Ocean, for which there is general agreement that there is northward ocean heat transport at all latitudes. There is also general understanding that this northward heat transport is associated with the thermohaline circulation in which North Atlantic Deep Water is formed in the polar and subpolar North Atlantic and subsequently flows southward throughout the Atlantic. For the Pacific and Indian oceans, analyses of pre-WOCE transoceanic sections have concentrated on estimating the heat fluxes across the southern boundary at approximately 30°S. There is general confusion in that there appears to be only a small poleward heat transport across 30°S out of the Pacific Ocean and the estimates of poleward heat transport across 30°S in the Indian Ocean range from 0.4 to 1.3 PW. Uncertainty in the size of the Indonesian Throughflow clouds the determination of ocean heat transports across 30°S in the Indian and Pacific Oceans.

A variety of new measurements were made during WOCE with the aim of providing accurate reference velocities for geostrophic calculations. Such new measurements include satellite altimeter measurements of sea surface height, direct current measurements by underway and lowered acoustic Doppler current (ADCP) profilers, by surface drifters and subsurface floats, and a variety of geochemical tracer measurements. Because experience suggests that measurements of western boundary currents provide the strongest constraints to estimates of ocean heat transport, careful analysis of WOCE observations of shallow and deep boundary currents in the Pacific, Indian and Atlantic Oceans likely holds the key to direct determinations of ocean heat transport in each ocean basin and the subsequent understanding of the role of the oceans in maintaining the global heat balance.