Ocean Circulation & Climate
When talking about the greenhouse effect, we have considered the fact that water on Earth occurs in all its three phases — gaseous, fluid and solid. As vapor in the air, water provides the strongest warming effect of all the greenhouse gases. As a fluid, it covers some seventy percent of the Earth's surface, and if spread evenly over the planet, it would make a layer two miles thick. As a solid, it whitens the polar regions and the high latitudes in winter, and greatly increases the reflectivity of Earth's surface wherever it covers the ground. Ice also forms a protective layer on water wherever it covers the sea surface.
As we learned in the last lesson, the physical properties of water make it an ideal means for conditioning the climate on Earth. It stores heat better than most other substances – that’s why tea takes some time to make, but then stays hot for quite a while. It takes up heat when moving from solid to fluid or from fluid to gas, and releases that heat again when moving through the phases in the opposite direction. The "latent" heat thus stored in water vapor is freed up with precipitation (and can then drive storms). This transfer of energy by latent heat is a hundred times more efficient than transfer by sensible heat, gram for gram.
Quite generally, the ocean surface is a source for heat in the atmosphere. The sea surface tends to stay at the same temperature for a long time, because of the high specific heat off water and the thickness of the mixed layer. Thus, the ocean surface stabilizes the atmospheric flow patterns, which depend on heat supply. Without this stabilizing influence of the ocean, the weather would be much more fickle.
The general circulation of the atmosphere discussed esrlier is closely related to the general circulation of the ocean. Winds blowing over the sea surface produce ocean currents. Winds also evaporate water, which precipitates elsewhere as rain. A large amount of heat is transferred to the atmosphere in this process. Evaporation and precipitation also affect the patterns of water density, which are inseparably tied to ocean circulation.
But how will the ocean's circulation respond to climate change? Although this sounds like a simple question, answering it is difficult. The ocean responds largely to winds, and wind fields respond to the distribution of heat, which depends on the ocean circulation. Within a logic loop like this, where the final answer depends on the initial guess, any small errors can be readily amplified into large errors in the prediction game.