The Motion of the Ocean: Part II

What does the spinning of water in the toilet when you flush it have to do with fisheries in Peru? 

Both exist because of the Coriolis effect.

The Coriolis “force” isn’t really a force as such, but a consequence of the centrifugal force, which acts outward from the centre of the Earth. This force exists because the Earth is rotating. It has it’s greatest effect at the equator. As it acts outward, at the equator this acts solely against gravity. This means that at the equator, because of the centrifugal force you weigh less than at the equator. This isn’t a great way to lose weight and cheat a diet as it only affects your weight by ~ 0.5% than from the poles when there is no effect from the centrifugal force. 

Anyway, interesting quirks aside, the centrifugal force component that acts against gravity is not constant; it diminishes with latitude, as such the Coriolis effect itself is not constant. Therefore, when things move with increasing latitude, they appear to be deflected. It’s hard to perceive, but take it as this - in the Northern hemisphere, everything deflects to the right, and deflects to the left in the southern hemisphere, with increasing latitude. 

Visualizing the deflection caused by the Coriolis Effect

The Coriolis force is weak and its effect only visible over large scales. The oceans have the size required for this as they cover many degrees of latitude, and form the gigantic oceanic gyres.

The 5 oceanic gyres on Earth. Note the clockwise (right deflection) in the
Northern hemisphere,and anti-clockwise (left deflection) in the Southern
hemisphere, due to the Coriolis effect.

The Coriolis effect is responsible for generating some of the most important features on the planet. Wind stress on the ocean surface moves subsequent layers beneath it, with gradually less momentum as a consequence of friction. However, in the oceans, the Coriolis effect causes a deflection in the motion of the water known as Ekman motion. Do to gravity and the Coriolis “force” acting on the Ekman motion, the water moves in a spiral, called the Ekman spiral. In the Northern hemisphere, Ekman motion is to the right of the wind stress and to the left in the southern hemisphere. 

The Ekman spiral

Like the oceans, the winds are deflected by the Coriolis effect and so create atmospheric gyres. Atmospheric anti clockwise (cyclonic) motion in the Northern hemisphere induces anti-clockwise wind stress on the ocean, which will therefore result in a Ekman motion to the right, out of the atmospheric gyre. This movement is termed Ekman pumping. This will remove water from the gyre. Divergence of this water results in upwelling, as deeper water is drawn to the surface. The opposite, downwelling, is seen when anticyclonic stress is applied to the oceans, and Ekman pumping moves water into the gyre and is forced down. This is summarized in the diagram below. 

Upwelling is particularly important, as it draws up valuable nutrients from the sediments on the sea floor to the surface. Below is another diagram showing all the areas in the world where upwelling occurs. 

The must intense upwelling occurs off the western coast of South America, where the combination of a strong wind stress and the strong cold Humboldt current along the eastern edge of the South Pacific gyre create ideal conditions for very intense upwelling. 

The water that upwells is also very nutrient rich and comes from the nutrient rich Southern Ocean. This combination of factors has led to very rich marine ecosystems. It is estimated that ~20% of the world’s fish supplies are harvested from this area of the world. It's importance is felt during the years of El Nino in the Southern hemisphere, when the system weakens, and the economies of several South American countries who depend on fisheries suffer chaotic and catastrophic losses; El Nino itself is named by Peruvian fishermen who noticed a severe drop in their catches during this period. In 1997-1998, the economic damages were estimated to be somewhere in the order of $3.5bn. Recent articles indicate that Peru will suffer a weak El Nino event into 2013.