How do differences in density to cause ocean currents

The differences in the density of the water truly cause ocean currents and any other currents. Density may change since it is directly proportional to its temperature and salinity. For example, a cold water with dissolved salts is denser than a warm water with dissolved salts. That is mainly because the dissolved salts were more dissolved by the warm water because of its higher temperature. 

It is the doctrine that the us had the right and duty to expand throughout north america. it just means they believed they had the right to expand westward and they did! they stated with the 13 colonies and then they kept moving gaining different territories throughout the years