Deep ocean currents are too slow to be measured directly but can be inferred from measurements of density and salt concentrations; similarity at two places suggests that water flows
from one to the other. In this way, we deduce that Bottom Water, even at the equator and off Alaska, all comes from the Antarctic.
A consequence of the deep-sea currents is the global circulation shown in Figure 11.19, a huge 'conveyor belt' driven largely by convection in the north Atlantic, caused by an increase of surface-water density due to evaporation and to chilling on contact with Arctic winds and waters. The circuit includes the Indonesian Throughflow to the north of Australia, and a sub-surface westwards flow to the south. It seems possible that shutting down or reversal of this massive circulation was responsible for past Ice Ages, and might affect future climate change. In other words, even the deep ocean affects the atmosphere.
Thus we conclude our consideration of the hydrologic cycle, which repeats itself endlessly with evaporation from the oceans, cloud, rain and then runoff back to the ocean. The cycle is linked intimately with energy balances (Chapters 2-5) which determine rates of evaporation and the atmospheric heating which creates instability and hence clouds. It is also closely connected with the patterns of winds over the Earth which distribute the water vapour and its associated latent-heat energy. These patterns comprise the 'general circulation' of the atmosphere, to be considered in the next chapter.
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