Saturday, 20 August 2011

Inertia Currents

You may have got the impression from some of more recent blog posts that I have been spending quite a lot of my summer holidays trying to learn some of the basics of oceanography. Unfortunately, as our EPQ's can only be 5000 words long and have to be very specific, there are lots of interesting things, especially with regards to the thereotical side of physical oceanography, that I have to leave out. Instead I thought I would perhaps do a few more posts on here about some of the stuff I have been learning as, you never know, it may come in handy when we do the climate module as the oceans and atmosphere are very closely linked. So, first up is inertia currents........

Winds are not always constant in strength or direction and so when a wind that has been driving a current ceases to provide sufficient energy to do so, inertia currents are created. Momentum will not leave the water immediately as, in open ocean, it takes a while to dissipate and whilst in motion, frictional forces and Coriolis force will continue to act upon them, thus resulting in a circular motion that characterises inertia currents. The extent of this circular motion is determined by the influence of Coriolis over other forces with the most circular inertia currents being generated when Coriolis is the only horizontally acting force on a current whose journey involves minimal latitudinal movement. The energy in the oceans is both kinetic, by virtue of its motion, and potential due to the displacement of isopycnic and isobaric surfaces and it is this huge store of potential energy in the oceans that ensures that the ocean circulation would take a few decades to completely cease if global winds stopped blowing.
Various possible paths for inertia currents

There is, as with oceanography as a whole, an awful lot of maths involved with inertia current to calculate things like the centripetal force (which is essentially the Coriolis force in this case) so you can then move on to work out the period of the inertia current. I will spare you all the maths - I am currently staring at a bunch of equations that work this all out in a textbook and trust me it doesn't look very pleasant at all!!!
Plan view demonstrating inertial motion in the
Northern Hemisphere

Anyway, apparently the equations demonstrate that, in an ideal situation, the only variable affecting the period of the inertia current (basically the time taken for a water parcel to complete one circuit) is latitude so, approximately, at latitude 45 degrees, it theorectically, the period of the inertia current is 17 hours whereas, at the Equator, it becomes infinite.
Thats some of the theory, but these currents have actually be identified and observed in real life from the current measurements taken in many parts of the oceans. Here is an example of one observed in the Baltic Sea. The wind-driven current flowing to the north-north-west has been superimposed on by the inertial motion which, at this latitude, has a period of about 14 hours and, therefore, the inertia current died out after around 9 or 10 rotations.
Plan view showing the inertial motion
observed in the Baltic Sea
So, I think thats about it really on inertia currents - I hope some of you found it interesting!

No comments:

Post a Comment