2.1.5.2 Heat transport

Locally, heat storage by the climate system cannot compensate for the net radiative flux imbalance at the top of the atmosphere and, annually, the balance is nearly entirely achieved by heat transport from regions with a positive net radiative flux to regions with a negative net radiative flux. When the balance is averaged over latitudinal circles (zonal mean), this corresponds to a meridional heat transport from equatorial to polar regions (Fig. 2.17). This poleward heat transport at a latitude $\phi$ can be estimated by integrating the net radiative balance at the top of the atmosphere from the South Pole to latitude $\phi$ :

The heat transport obtained is nearly zero at the equator, rising to more than 5 PW at latitudes of about 35°, before declining again towards zero at the poles (Fig. 2.17). It can be divided into an oceanic and an atmospheric contribution, the horizontal transport on continental surface being negligible. This shows that, except in tropical areas, the atmospheric transport is much larger than the oceanic transport.

The energy can be transported as sensible heat (c_pT, which is related to internal energy), potential energy (gz), latent heat (Lq) and kinetic energy (0.5 u²) and is expressed per unit of mass as:

where z is the altitude (or depth), L_v the latent heat of vaporisation of water, q the specific humidity and u the velocity of the media. The first term is often called sensible heat. The transport of kinetic energy is much weaker than the other transports and is generally neglected. In the atmosphere, the three remaining terms must be taken into account, but in the ocean the transport of sensible heat is clearly dominant. Moreover, in some special cases, an additional term representing the transport of latent heat by sea ice and icebergs must be considered for local or regional analyses at high latitudes.

In the tropics, the majority of the atmospheric poleward heat transport is achieved by the Hadley circulation. By contrast, the mean circulation plays a much weaker role at mid to high latitudes where nearly all the transport is effected by eddies. In the ocean, both the wind-driven and deep-oceanic circulation are responsible for a significant part of the oceanic poleward heat transport, the latter having a dominant influence in the tropics. The role of the oceanic eddies is less well known, but they can be significant in at least some regions (such as the Southern Ocean).

In addition to its dominant role in the reduction of the temperature contrast between the equator and the poles on Earth (compared to a planet without an ocean and an atmosphere), horizontal heat transport is also responsible for some temperature differences on a regional scale. This can be illustrated by analysing the departure of the local temperature from to the zonal mean temperature. At first sight, Fig 2.18 emphasises the mountainous area such as the Tibetan Plateau, the Rocky Mountains and Greenland, where the temperature is much lower than at other locations at the same latitude. However, the influence of the atmospheric circulation is also clearly apparent with, for instance, in cold areas such as north-east Canada. This is because the dominant winds have a strong northerly component in this region, while the North Atlantic is warmer, partly because of the south-westerly winds in this region (see section 1.2.2). Over the ocean, the influence of the northward western boundary currents (see section 1.3.2) results in generally warmer surface oceanic temperature at about 30-40°N in the western part of the basin than in the eastern part (where the oceanic currents are generally bringing colder water from the North).

The thermohaline circulation is an additional source of longitudinal asymmetry as, in the Northern Hemisphere, deep water formation only occurs in the North Atlantic and not in the North Pacific (see section 1.3.2). The associated circulation transports cold water southward at great depths with the mass balance ensured by a corresponding northward transport of warmer water in the surface layer. This results in a net oceanic transport in the North Atlantic of about 0.8 PW at 30^oN, (i.e. more than twice the estimated transport in the wider Pacific at the same latitude). The thermohaline circulation is also responsible for the northward oceanic heat transport at all latitudes in the Atlantic, even in the Southern Hemisphere.

This oceanic heat transport contributes to the fact that higher temperatures are observed in the North Atlantic than in other oceanic basins. Its influence is particularly large in the Barents Sea, north of Norway. Thanks to the oceanic heat transport, this area located north of 70°N (i.e., at the same latitude as the northern part of Alaska) remains free of sea ice all year long. Climate model calculations have shown that, if deep water formation was suppressed in the North Atlantic, the temperature in the North Atlantic and in Western Europe would be reduced by about 3°C at 45°N, while the annual mean temperature would decrease by more than 15° C in northern Norway and the Barents Sea.