什么是地转流？
当物体位于重力位能最低处时，会处于最安定的静止状态。海水会趋向等重力位面，已达到静止的状态，所以当海水的位能高度不同时，也就是绝对重力高度不同时，会产生一个压力梯度，海水由压力 梯度大的区域流向压力梯度小的区域。同时，由于地球的自转，运动中物体会受到科氏力作用的影响 (科氏力即为改变物体运动方向的假想力) ，进行直线运动的质点，在旋转坐标系中质点运动轨迹会有 所偏移。地球上的力处于平衡状态，称为地转平衡，在这个状态下，压力梯度和科氏力会达到平衡，海 水会以平衡状态下流动，即称为地转流。
以下内容为英语，本人翻译会失真，于是决定给出原文：
You may wonder how an oceanographer converts measurements of the surface slope into a measure of the current speed. The basis assumption is that when we look at the large currents over scales of 100 km or more there is a substantial equilibrium between two forces – the pressure gradient and the Coriolis force .
Imagine for a moment that there is a ‘high’ and a ‘low’ in the sea surface (an altimeter can measure these!) and that there is no Coriolis effect. Water would naturally flow from the high to the low 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 this force if proportional to the difference in levels i.e. the ‘pressure gradient’.
But now consider that in the real world the Coriolis force IS there – and it will pull the current to the right in the Northern hemisphere (as you see in the figure below) and to the left in the Southern hemisphere.
When the situation is as depicted in the figure above, we say that there is geostrophic balance, and that the current is purely geostrophic. The nice thing is that the current can be readily computed from the measurement of the slope.
In fact the Coriolis force is given by the current speed v times a parameter f called the Coriolis parameter (the Coriolis parameter varies with latitude: at 45° its value is approximately 10-4 s-1), while the pressure gradient is the slope dh/dx (where h is height and x is distance) times g, the acceleration due to gravity (9.81 m s-2).
So if we simply write the geostrophic balance, i .e. we write
Coriolis force=pressure gradient ⇔ fv=g dh dx
We can immediately compute v as
v= g f ⋅ dh dx
and the direction of the current will be perpendicular to the slope, as shown in the figure.
Not all currents are geostrophic
The assumption that there is geostrophic balance is only accurate when we look at the large-scale currents, i.e. at scales larger of a few tens of km. All the major currents can be considered geostropic to a first approximation. At smaller scales, the ageostrophic (non-geostrophic) components of the currents, for instance due to the forcing by the local wind, become more and more important. So for instance in several coastal areas the circulation is largely ageostrophic. An altimeter can however still be used to measure the geostrophic component.
参考文献：
https://seos-project.eu/oceancurrents/oceancurrents-c06-s02-p01.html
常乐，中国近海海平面变化及 ENSO 的影响【D】, 2016.
