Читать книгу Exploring the Solar System - Peter Bond - Страница 78

The other major conveyor belts for heat from the tropics are the warm ocean currents (Figure 3.15). These fairly narrow ribbons of surface water display marked differences in temperature and salinity from the surrounding ocean.⁷ They are also associated with differences in sea height, which can be measured by satellites. Warm currents are seen as rises and cold currents as valleys in the ocean surface.

Many of the currents are driven by surface winds blowing around the subtropical high pressure zones. This means that they flow clockwise in the northern oceans and the opposite direction in the southern oceans, traveling at speeds of between 0.4 and 1.2 m/s (35–105 km per day).

Not only do the warm currents carry heat absorbed by sea water, but they also warm the air above them. (The oceans redistribute about half as much heat as the atmosphere.) The effects can be very noticeable at mid‐latitudes. The most famous example is the North Atlantic Drift (commonly known as the Gulf Stream) which carries warm water from the Gulf of Mexico to the shores of Norway. As a result, winters in northwestern Europe are much milder than would be expected from their latitude (50–70°N).

Other places are cooled by the oceanic circulation. Cold currents flowing from high latitudes towards the equator tend to cool nearby coastal areas. Since cool air is relatively dense and stays near the surface, condensation of water vapor to form fog is quite common, but clouds and rain are rare. As a result, places such as southern California, northern Chile, and southwestern Africa experience desert conditions.

The foggiest places on Earth occur where warm and cold currents meet, e.g. off the coast of Newfoundland, where the interaction between warm, moist air and cooler air (also fairly moist) results in 150–200 days of sea fog per year.

Vertical motions in the oceans are also important in places such as Peru. Earth's rotation and strong winds push surface water away from some western coasts, so that cold water rises from the depths to replace it. This upwelling of nutrient‐rich waters is a bonanza for marine life.

The cooling and sinking of cold water in the polar regions drive a much deeper, global circulation. This oceanic “conveyor belt” is set in motion when cold, dense water in the North Atlantic sinks and moves south (Figure 3.16). It circulates around Antarctica, and then moves into the Indian and Pacific basins, where it returns to the surface. Once at the surface it is carried back to the North Atlantic, and the cycle begins again. This circulation is extremely slow – water from the North Atlantic may take 1,000 years to find its way into the North Pacific.