water in the oceans is in constant motion the major surface currents are driven by energy transferred from the wind by friction the direction water travels is determined by how the transferred wind energy interacts with Coriolis deflection other currents and geological features such as continents and island arcs a major feature of the current systems in all of Earth's large ocean basins is a central gyre these gyres move water near the surface enlarged roughly circular patterns around the center of each ocean basin given the orientation of the wind in these areas relative to the direction the currents travel it may look as if the wind directly creates the currents but due to the influence of the Earth's rotation the process that forms gyres is more complicated this circulation pattern is an example of geostrophic flow which is a type of movement that occurs when the forces acting on objects are so weak relative to the influence of the rotation of the earth that Coriolis deflection is the factor that determines the direction of motion to understand this it helps to start with a more familiar situation and consider a ball sitting at the top of an inclined plane the force of gravity pulling down on the ball will cause it to roll down the plane in this situation the force of gravity is strong and the motion of the ball is fast relative to other forces acting on the ball allowing the ball to roll down the plane as expected the dynamics change if we alter the situation so that the slope of the plane is very shallow and the ball is tiny as the slope of the plane and the mass of the ball decrease the force of gravity acting on the ball becomes weaker with a small enough ball in a shallow enough plane Coriolis deflection becomes so influential that it overwhelms the orientation of gravity's pull down the slope and the ball will actually move across the plane instead of down it this can create some seemingly counterintuitive behavior if instead of a plane the ball was on a small hill it would roll around the hill instead of down it this is geostrophic movement or as used in fluid dynamics geostrophic flow it is important to understand that for this type of motion to occur the slope has to be extremely small and be extended over a very large area like for exam a large portion of an ocean basin while the ocean surface does look flat to the unaided eye there are hills of water in the open ocean these hills form at mid-latitudes in both hemispheres where the low latitude easterly trade winds are replaced by westerlies at higher latitudes the surface currents created by these winds are turned 90 degrees by Coriolis deflection the resulting Ekman transport drives the formation of regions of convergence where water actually piles up forming small hills these hills of water are only about a meter in height but this change in elevation is enough to generate a difference in pressure across the basin with the elevated region and center that is slightly higher pressure than the surrounding area this creates a small pressure gradient across the basin the pressure gradient acts like the hill in the example with the ball generating a force that pushes water down the gradient away from the center of the hill since the elevation difference is small and is spread across a large distance the pressure gradient force is weak creating conditions for geostrophic flow to occur so rather than moving down the gradient water flows along lines of equal pressure around the hill instead of down it to summarize the formation of the large Ocean gyre starts with tradewind driven Ekman transport piling water in the middle of the basins this pile of water generates a pressure gradient to pushes the water back out away from the center of the pile since the pressure gradient is small and spread across a large distance the flowed down the pressure gradient is deflected by the rotation of the earth and the water actually flows around the pile instead of down it this is a steady-state situation with energy from Ekman transport in balance with the force of pressure gradient pushing back out resulting in a stable circulation of water around the gyre if you found this video helpful please consider sharing it and giving it a thumbs up feel free to comment with any questions or suggestions and if you want to keep up with the content here at science primer click the subscribe button click on the video on the left if you'd like to learn more about Ekman transport if you'd like to learn about the Coriolis effect click on the video on the right thank you for watching