Geostrophic Motion

Geostrophic motion is a fluid flow that occurs in a direction parallel to the lines of equal pressures/isobaric in a rotating system, such as the Earth. 

A Geostrophic flow occurs by the balance of the Coriolis force (a force caused by the Earth’s rotation), and the pressure-gradient force (when the friction is low).

Hence in a geostrophic flow, instead of water moving from a high-pressure region to a low-pressure region, it moves along with the lines of equal pressure and this happens because of the Earth’s rotation.

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On this page, we will understand what Geostrophic motion, pressure-gradient  force is and Geostrophic flow is all about.

What is a Geostrophic Motion?

From the above text, we understand that water does not flow from a high sea level to a low sea level, it just gets along with the lines of equal pressure. 

The velocity of the flow varies directly with the pressure gradient and conversely with the latitude. 

In practical, observed fluid flow is not strictly geostrophic, though large-scale oceanic and atmospheric movements approach the ideal stage. It means that the geostrophic current usually portrays the actual current within around 10 percent, provided the comparison is made over large areas and there is a little curve in the isobars.

Pressure-Gradient Force

The pressure gradient quantifies the lowering of the atmospheric pressure in an area at a specific time. For instance, gale force winds turn into a light breeze in a specific city after an hour. 

A pressure-gradient force is a relative force that is calculated when there is a difference in pressures. The below diagrams shows the relative pressure difference:

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Geostrophic Flow

A geostrophic current is an oceanic current in which the pressure-gradient force is balanced by the Coriolis effect or the Earth’s rotational force. 

The direction of geostrophic flow is parallel to the lines of equal pressure/isobars, with the high-pressure to the right of the flow in the Northern Hemisphere, and the high-pressure to the left in the Southern Hemisphere. 

The concept of Geostrophic current is taken from weather maps, whose isobars show the direction of geostrophic flow in the atmosphere. 

The below image shows that surface currents generally mirror average planetary atmospheric circular patterns:

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Geostrophic flow can either be barotropic or baroclinic. A geostrophic current can be assumed as a rotating shallow-water wave with a zero frequency. 

The geostrophic principle is useful for oceanographers because it helps them infer ocean currents from measurements of the sea surface height (by combined satellite altimetry and gravimetry) or from vertical profiles of seawater density taken by ships or autonomous buoys. 

Do You Know the Examples of Geostrophic Currents?

Examples of Geostrophic Currents

The major currents of the world's oceans, like the Gulf Stream, the Agulhas Current, and the Antarctic Circumpolar Current,  the Kuroshio Current are all approximately in geostrophic balance and hence they are considered examples of geostrophic currents.

Concept of Geostrophic Motion

You may think about how an oceanographer changes overestimations of the surface slope into a current speed. The premise supposition will be that when we take a gander at the huge flows oversized of 100 km or more there is a considerable balance between two forces – the pressure gradient and the Coriolis force.

Now, let’s understand the concept of Geostrophic flow through ocean currents.

Ocean Currents

Now, talk about the ocean current.

Imagine for a moment (an ideal situation) that there is a ‘high’ and a ‘low’ level in the sea surface (an altimeter can measure this( and that there is no Coriolis effect. 

In the absence of Coriolis force, water would naturally flow from the high to the low region in order to restore the equilibrium. In other words, there is a force that pushes the water from the high level to the low level – and if this force lies proportionally to the difference in levels, then it is the ‘pressure-gradient ’.

Now, considering that Coriolis force occurs on the water. Now, it will pull the current to the right in the Northern hemisphere (as shown in the figure below) and to the left in the Southern hemisphere.

Geostrophic Balance

A time comes when the pressure-gradient force becomes equal to the Coriolis force, the balance between these two forces on a parcel of the water is what we state as the Geostrophic balance. The below image represents the above statement:

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So, when the situation is the same as depicted in the figure above, we say that there is a geostrophic balance and that the current is purely geostrophic.

The best part is, an oceanographer can compute the current by the measurement of the slope.

So, let’s understand the Geographic Wind in brief.

Geostrophic Wind                   

The geostrophic wind is a theoretical wind directed along with isobars, i.e., the lines of constant pressure at a given height. This balance rarely holds exactly in nature. 

However, the real wind somewhat differs from the geostrophic wind (imaginary wind) because of the other forces such as friction from the ground. 

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From the above diagram, we see the deviation of a real wind from its original path; however, geostrophic wind seamlessly flows without getting affected by any force.

Do You Know?

The suspicion that there is geostrophic balance is just precise when we take a gander at the large-scale flows, for example at scales bigger than a few tens of km. All the significant currents can be considered geostrophic to a first estimate. 

At more limited sizes, the geostrophic (non-geostrophic) segments of the flows, for example, because of the force by the neighborhood wind, become increasingly significant. 

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In several coastal areas, the dissemination is to a great extent geostrophic. An altimeter can anyway still be utilized to measure the geostrophic part.