Why does the frequency of a siren get higher as an ambulance using that siren gets closer?

How do temperature and salinity affect deepwater currents? they create changes in wind direction, moving denser water in the same direction as the wind and causing the deepwater circulation patterns found in the ocean. as temperatures and salinity levels of water increase, the water rises to the surface where it creates currents as it moves to colder regions. they create density differences that cause dense deepwater currents to flow toward the equator where they displace less dense, warmer water above them. they equalize the forces on undersea currents caused by the coriolis effect as they replace more dense water with less dense water.