Ocean Gyres and Geostrophic Flow

Water in the oceans is in constant motion driven by tidal forces, density^* differences, and winds. Winds create surface currents by transferring energy to the water by friction. The direction of a wind-driven surface current 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 surface current systems in all of Earth's large ocean basins is a central gyre. These gyres move water in large, roughly circular patterns around each ocean basin's center.

In many areas, water flows around the gyres in the same direction as the wind. This may give the impression that the winds create the currents directly by simply pushing water along in the same direction as the wind is blowing, but due to the influence of the Earth's rotation, the process that forms these gyres is more complicated.

Mid-basin Ocean gyres are examples of geostrophic flow, a type of movement that occurs when the forces acting on an object are weak relative to the influence of the Earth's rotation, allowing Coriolis deflection to be THE factor that determines the direction of motion.

To understand this, it helps to start with a familiar situation. Consider a regular-sized 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 ball's motion is fast relative to the Coriolis force, resulting in the ball rolling down the plane as expected.

The dynamics change if we alter the situation so that the slope of the plane is very shallow. As the slope of the plane decreases, the force of gravity acting on the ball becomes weaker. With a shallow enough plane, Coriolis deflection becomes so influential that it overwhelms the orientation of gravity's pull, and the ball ends up moving across the plane instead of down it.

The movement of an object along the plane instead of it down it is an example of geostrophic movement. Or, as used in fluid dynamics, geostrophic flow.

Geostrophic flow can create some seemingly counterintuitive behavior. If the ball is on a small hill instead of a plane, the ball will roll around the hill instead of down it.

It is important to understand that for this type of motion to occur, the slope has to be extremely small and extended over a sizable area, like, for example, a large portion of an ocean basin.

The ocean surface looks flat to the unaided eye, but there are hills of water in the open ocean. These hills exist at mid-latitudes in both hemispheres. The hills form because Ekman transport turns currents driven by low latitude easterlies away from the equator. Ekman transport also turns currents caused by higher latitude westerly winds towards the equator. This creates regions of convergence in the transition zone between the easterlies and the westerlies.

In the regions of convergence, water piles up, forming "hills of water." These hills are only about a meter high, but the elevation change is enough to generate a small pressure gradient back down the hill. This pressure gradient acts like the hill in the example with the ball, creating a force that pushes water down the slope away from the hill's center.

Since the elevation difference is small and spread across a considerable distance, the pressure gradient force is weak, creating conditions for geostrophic flow. Resulting in water flows along the lines of equal pressure around the hill instead of down it.

To summarize, the formation of the large ocean gyres starts with trade wind-driven Ekman transport piling water in the middle of the basin. This water pile generates a pressure gradient that pushes the water back out away from the pile's center. Since the pressure gradient is small and spread across a large distance, the flow down the pressure gradient is deflected by the Earth's rotation, and the water flows around the pile instead of down it.

This set of forces creates a steady-state situation with energy from Ekman transport in balance with the pressure gradient force pushing back out, resulting in a stable circulation of water around the gyre.