The oceanic heat balance

The exchange of heat between the ocean and atmosphere principally consists of the four terms discussed above. Solar radiation provides the input to the ocean, and, in most situations, the net long-wave radiation, latent heat and sensible heat result in transfer of energy to the atmosphere. The net heat exchange, Q (in Wm-2), at a location is therefore

where QI is the incident solar radiation that is absorbed by the ocean and positive Q indicates addition of energy to the ocean. This expression neglects latent heat from the condensation of water onto the ocean surface and heat transfer due to mixing of precipitation with sea water. These contributions are both extremely small, however, and can be neglected in most circumstances. Note that if a full

Longitude

Fig. 2.7. Mean February air-sea temperature difference (Ts — Ta) over the Atlantic Ocean. Contour interval is 0.5°C; solid, or positive, contours indicate a warmer ocean than atmosphere while dashed, or negative, contours indicate a warmer atmosphere. [Data from Oberhuber, 1988.]

Longitude heat budget, rather than just the heat exchange, for a particular location in either the atmosphere or ocean was computed then the additional heat supplied (or lost) due to advection would need to be considered. This can be considerable, particularly for some of the regions of the Atlantic already highlighted.

An estimate of the distribution of the annual average air-sea energy exchange is shown in Fig. 2.8. There are extensive regions in which the ocean loses heat to the atmosphere. Referring to the surface ocean circulation shown in Fig. 1.15 it can be seen that these are principally regions with strong, poleward-flowing, warm currents, where we have already seen that latent and sensible heat fluxes increase. Typical areas are in the Gulf Stream of the North Atlantic, and the Kuroshio Current of the North Pacific.

If we examine the surface atmospheric circulation, shown in Fig. 1.6, it can be seen that these areas of heat input to the atmosphere tend to be close to consistent cyclone centres. The latent and sensible heat energy is responsible for recurring frontal development. We shall explore these associations further in §§2.13 and 5.1.4.

There are also areas in Fig. 2.8 where the ocean takes a considerable quantity of energy from the atmosphere. These tend to be close to the equator, and on the eastern margins of ocean basins. Upwelling of cold water from perhaps several

Fig. 2.7. Mean February air-sea temperature difference (Ts — Ta) over the Atlantic Ocean. Contour interval is 0.5°C; solid, or positive, contours indicate a warmer ocean than atmosphere while dashed, or negative, contours indicate a warmer atmosphere. [Data from Oberhuber, 1988.]

180°W 150°W 120°W 90°W 60°W 30°W 0° 30°E 60°E 90°E 120°E 150°E 180°E

Longitude

Fig. 2.8. Annual average oceanic heat gain (in Wm-2) over the global ocean. Contour interval is 20 Wm-2; dotted (or negative) contours indicate a net loss of heat by the ocean. [Data from Oberhuber, 1988.]

180°W 150°W 120°W 90°W 60°W 30°W 0° 30°E 60°E 90°E 120°E 150°E 180°E

Longitude hundred metres below the sea surface occurs in these regions. This reverses the sensible heat flux, lessens the long-wave radiation, and, because both the overlying air and ocean surface are cooled, lessens the latent heat loss. This interaction will be discussed further in §§2.11.2 and 2.11.3, and Chapter 5.

The estimates in Fig. 2.8 are climatological averages. They provide a qualitatively correct picture of the global balance of heat exchange between ocean and atmosphere. Globally, the net ocean heat budget is not in balance: the ocean gains heat at a rate of about 30 Wm-2. However, there are considerable uncertainties within the basic observations, and also within the terms of the empirical formulae amounting to 20-30% uncertainties in individual terms, or several tens of Wm-2. In order to predict climatic change the terms in the heat balance need to be known to high accuracy, because small changes of a few Wm-2, if consistent over a large region, can push the delicate balances within the climate system towards a new state. The net change to the long-wave radiation balance of the atmosphere due to a doubling of the CO2 concentration is estimated to be only about 4 Wm-2. The ramifications of this potential change, as will be discussed in Chapter 7, may be substantial, so accuracy in our estimates of the air-sea heat exchange is important.

There are a number of components of the empirical formulae that require more investigation. The transfer coefficients, cH and cE, are actually functions of wind speed or atmospheric stability, rather than constants. At low wind speeds the transfer processes are under-estimated because gusts of stronger winds produce short, but vigorous, periods of enhanced evaporation and conduction. Estimates of wind speed are unreliable. Chapter 6 will discuss the problems with changing instrumentation over the past century; it is, in any case, difficult to obtain a representative open ocean wind speed on board ship. Should the wind at 10 m be used, as is standard, or conditions closer to the sea surface where the actual exchange processes are occurring? Estimates of sea surface temperature have an instrumental bias (see §6.4.1), while air temperature measurements are subject to distortion by ship heating. Another poorly parameterized variable is the cloud cover. The radiation formulae, such as (2.2), use the proportion of

Fig. 2.8. Annual average oceanic heat gain (in Wm-2) over the global ocean. Contour interval is 20 Wm-2; dotted (or negative) contours indicate a net loss of heat by the ocean. [Data from Oberhuber, 1988.]

sky covered by cloud, as this is what is commonly measured. However, the radiational properties of different thicknesses, and heights, of clouds vary considerably. The properties of ice clouds will also differ strongly from those of water clouds.

This short discussion shows that significant uncertainties will remain in estimates of the heat exchange between ocean and atmosphere for some time. The climatic importance of this flux, however, means that considerable international effort is being put into improving satellite and surface observations, and also models, both theoretical and numerical. Recent research programmes have included WOCE (World Ocean Circulation Experiment), JGOFS (Joint Global Ocean Flux Study), and TOGA (Tropical Ocean, Global Atmosphere programme). Ongoing programmes include IGBP (International Geosphere-Biosphere Programme), CLIVAR (CLImate VARiability), PAGES (Palaeoclimate and Global Environmental Study), GEWEX (Global Energy and Water cycle Experiment), and ISCCP (International Satellite Cloud Climatology Project). Most of these are coordinated by the WCRP (World Climate Research Programme) of the World Meteorological Organization.

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