4.3 Buoyancy Forcing

The mechanisms for conveying buoyancy forcing to the ocean have been less well modeled, and possibly less well explored. The response to net loss of buoyancy through heat loss or net evaporation is local convection in the mixed layer, and is likely to be horizontally structured; mixed layer models generally assume penetrative convection which occurs at all locations at the same rate. Recently Jones and Marshall [1993] have begun to re-examine the convection process from theory and modeling, and new field experiments to study convection are underway.

Mixed layer modeling is experiencing a resurgence of interest as a result of the vast improvement in numerical modeling of the general circulation and new attention is being paid to how boundary conditions such as surface forcing are conveyed to the interior. One emphasis is on parameterizing the restratification of mixed layers. Young [1994] examines the effect of separate temperature and salinity fields in a new class of mixed layer theory.

Surface heat fluxes have been computed and mapped for the Pacific over the last decade [ Hsiung, 1985; Oberhuber, 1988], using bulk parameterizations of shipboard measurements. Data are essentially unavailable south of 30S. The maps show the large heat loss region of the Kuroshio Extension, and a fairly strong heat loss in the East Australia Current, with heat gain in the tropics which is strongest in the eastern Pacific equatorial upwelling region, with the heat gain regions extending poleward along the eastern boundaries. Important advances are being made in mapping the various parts of the heat flux using satellite observations, with reduction of the total error to less than 10 W/m using a combination of in situ and satellite data is the goal.

The surface freshwater balance depends on evaporation, precipitation and runoff. The standard work on surface freshwater inputs is that of Baumgartner and Reichel [1975], which formed the basis for Wijffels et al.'s [1992] freshwater transport calculation for the globe. A Pacific-wide updated surface freshwater flux calculation, along the lines of Schmitt et al.'s [1989] for the North Atlantic, has not been made. Royer [1982] has estimated that continental runoff supplies 40% of the freshwater input to the northeastern Pacific; as this is a region where the relatively fresh surface water dominates the surface stratification, discharge rates must be included in any improvements to the Pacific freshwater balance.

The surface mixed layer depth in winter was mapped for the North Pacific by Reid [1982a], using the bottom of the oxygen-saturated surface layer as an indicator of the mixed layer depth. This measure is more robust than indices based on surface-to-depth temperature or even density differences. Temperature is very misleading as an indicator in the subpolar and western tropical regions where salinity dominates the surface stratification. Even use of a standard density difference can be misleading because of variations in strength of stratification. Maps such as Reid's have not been made for other oceans. Salient features of his mixed layer depths are that they do not exceed 200 meters anywhere in the North Pacific. The deepest mixed layers are found just north of the Kuroshio Extension east of 150E, in a band stretching across the Pacific centered just south of 40N. By and large the mixed layer depths do not exceed 150 meters, indicating that the dominant surface mixing is due to the winds.

Another source of buoyancy forcing is sea ice formation and melting at high latitudes, which has particular importance in regions where deep waters are formed. Brine rejection under sea ice formation can create a locally dense water. In the South Pacific, the Ross Sea is an area of sea ice coverage where a dense shelf water is formed which contributes to the dense bottom waters of the Antarctic. In the North Pacific, sea ice formation in the Okhotsk and Bering Seas is important for ventilation of the intermediate depths of the North Pacific. Sea ice formation in the Japan Sea is associated with ventilation of the entire water column, creating the densest surface waters in the North Pacific; these however do not affect the open North Pacific directly due to the limited sill depths between the Japan Sea and the North Pacific.

Geothermal heating of the ocean from below is also a mechanism which has received consideration in the past decade, and is discussed in the section on abyssal circulation below. Its effect on deep temperature profiles has been demonstrated near venting regions on the East Pacific Rise [ Reid, 1982b; Hautala and Riser, 1993] and in the northeastern Pacific near the Juan de Fuca ridge [ Lupton et al., 1985; Talley and Joyce, 1992]. The effect of slow abyssal heating has also been shown in quiescent regions of the northern North Pacific [ Joyce et al., 1986]. This heating might drive a very weak abyssal circulation [ Stommel, 1982].