In the beginning of the lecture, there was an explanation about how ocean can circulate at one place to another in every part of this world. Wind pressure is one of the factor which is driving the ocean currents. It happens when the wind moves through the sea and causes the molecules of the air and the fluid having a collision where the energy is transferred. If there is a deflection happened between more than one different directions of large wind driven the ocean, this results a circular motion of current called a gyre. Gyres are generally caused by planetary motion of vorticity along with horizontal and vertical friction, Coriolis effect, and landmasses which is blocking the gyre to circle in one particular place. There are five most known gyres, those are Indian Ocean Gyre, North Atlantic Gyre, North Pacific Gyre, South Atlantic Gyre, and South Pacific Gyre.
As the wind is driving the surface ocean current, the total of water transported end up moving in a complete different direction from the wind. This phenomenon can be explained by Ekman spiral and transport theory. The theory clarifies that the drag at wind-water interface and Coriolis effect caused the surface current to be deflected and moves continuously in different direction around 45° to 90° as the current becoming weak in greater deep of the ocean. This illustration can be pictured as Ekman spiral, where there is a wind moving straight and the surface current is moving in 45° from the wind and Ekman current is deflected into 90° from the wind. The total of deflected currents is called Ekman transport.
Ekman current theory can explain how the surface current and the current below are having an interaction called upwelling and downwelling. Upwelling occurs where Ekman transport moves surface waters away from the coast and surface waters are replaced by water that wells up from below. In contrast, downwelling occurs where Ekman transport moves surface waters toward the coast then water piles up and sinks below the surface current. In the place where upwelling takes place, waters in the surface is becoming colder than the surface waters which have been replaced, while downwelling transports heat into the current below the surface. Apart from the temperature influence, upwelling could increase the biological productivity in the surface by helping the growth of phytoplankton and downwelling transports the dissolved oxygen from the surface to below.