"Current" Events - Forces and Pressure on Currents
Wind energy is converted to water movements called "currents" by friction between the wind and the water surface. The surface currents resemble the surface winds. Once these surface currents are set in motion they are influenced by three other factors: Coriolis effect, presence of coasts, and horizontal pressure gradients.
Earth's rotation produces a force on winds and currents. The force is perpendicular to the wind and current. It acts to the right of the flow looking downstream in the northern hemisphere. This effect is referred to as the CORIOLIS EFFECT.
Almost always the Coriolis force is balanced by the pressure gradient and the force does not cause any movement. Remember only unbalanced force cause movement. While you are sitting and reading this page, gravity pulls you towards the center of Earth but you don't fall because the seat of your chair pushes up with an equal and opposite force. The two forces are in balance.
Coasts and other masses, like dams of granite, deflect the flow of currents. Currents bumping into the continents must change direction and flow toward the equator or the pole. Only the Antarctic circumpolar current can flow unimpeded around Earth.
A pressure gradient is a change of pressure across a horizontal distance. Water that is piled up in a mound creates a zone of high pressure. The water responds by flowing down the pressure gradient. The steeper the gradient is, the faster the flow of water. The less steep, the less speed the water or you will have. When the water first starts to flow from high to low pressure, the coriolis force is weak. But as the flow goes faster, the Coriolis force becomes strong enough to balance the pressure gradient. When this happens, the flow goes around the high pressure area.
You might ask, " How does water mound up? Isn't the ocean always flat? " The answer to your second question is, " No! " If you could walk across the watery surface of the ocean you would find yourself moving up hills and down into valleys. Far too small to see by casual observance, the hills are usually no more than 1 m (3 ft) high over a space of 100 km (60 miles). The hills might be only 1 meter high measured from the valley trough, but they have a very important effect on ocean currents. Now, back to your first question--How does water in the ocean mound up? Read on, please.
Steady winds blowing on the sea surface produce a thin, horizontal layer at the sea surface called the Ekman layer. This layer may be 10 m, 50 m, or at most 100 m thick. This may not sound " thin " to you, but when compared to the depths of the ocean--it is thin. Each layer of the ocean affects the layer beneath it through its movement. Each successively deeper layer moves at a slower speed than the layer above and moves, in the northern hemisphere, slightly to the right of the layer above. The change in direction is created by the coriolis effect and is known as the Ekman Spiral.