## The Coriolis Effect And The Oceans

So far we have considered ocean temperature and salinity, two of the factors controlling currents in the sea, which in turn govern coastal climates. A third factor is the 'Coriolis effect', the 'apparent deflection of moving objects, due to the observer being on a rotating Earth', named after Gaspard de Coriolis (1792-1843).

Unfortunately, it is not easy to understand immediately, being different from commonsense observation, and so various explanations are offered in what follows.

As a preliminary, think of the Earth rotating once each day, and an observer looking at the Sun. There is a paradox, since we know that it is the Earth (and observer) which actually turn, yet the observer sees the Sun as moving. Reality differs from appearance. Likewise, if you sit on a roundabout, it is the rest of the world which seems to spin. In brief, the 'rotation of the observer is perceived as the rotation of the observed'.

In the same way, a rotating observer, looking at an object which is really moving in a straight line, sees the object as turning. This is illustrated in Figure 11.10. Drawing along a straight ruler creates a curved line on a rotating disc. Laying the ruler in any direction over a disc turning clockwise, and moving the pencil in either direction along the ruler, always causes the trace to bend to the left, whereas turning the disc counter-clockwise instead always causes sidling to the right. These different directions of rotation correspond to conditions in the two hemispheres of the globe, where an observer at the South Pole sees the Earth turning clockwise (i.e. the left hand advances towards the Sun), whereas an observer's right hand moves forward at the North Pole (Figure 11.11). In between, at the equator, there is no turning round by the observer, i.e. neither the left hand moves to take the place of the right nor vice versa.

The Coriolis effect can also be understood in terms of a projectile from a cannon at A fixed facing a target B (Figure 11.12), all on a disc representing the Earth. The cannon and the target turn a little while the shell is in the air, and during that time the target moves from B to B', so that when the shell lands at B it is behind the target. As a result, the actually straight-line trajectory AB (as seen from space) seems to curve (A'B), deflected to the left when observed where the Earth rotates clockwise. Figure 11.12 and Note 11.D show that this is true whichever way the cannon is pointing, whether zonally (i.e. east-west) or meridionally (i.e. north-south).

Theory shows that the effect depends solely on latitude and the object's velocity (Note 11 .D). The outcome is that any straight-line motion (as observed from a fixed point in space, such as the Sun) appears circular to a person on Earth,

Figure 11.10 Demonstration of the Coriolis effect. A line drawn along the straight ruler registers a curve on the rotating disc, i.e. straight-line motion appears curved when viewed from a rotating platform.

as though the moving object is continually pushed to one side by a force. This hypothetical force is called the Coriolis force and is most important in understanding oceanic and atmospheric motions. It is summarised in Ferrel's Law, that 'all motion suffers a bias towards the left in the southern hemisphere (and right in the northern)'.

The Coriolis force is negligible near the equator and on the scale of water going down a plug-hole, for instance (Note 11.D). It is significant only on a scale of many kilometres, affecting global winds and large-scale currents in the oceans, particularly at high latitudes.

### Upwelling

One important consequence of the Coriolis effect is the way in which winds over the ocean move the surface water. Friction at the surface drags the uppermost water along with the wind, but simultaneously the Coriolis effect operates, deflecting the moving water to the left (in the southern hemisphere). As a result, that top layer of the ocean slowly moves at an angle approaching 45 degrees to the wind. The top

Figure 11.11 Explanation of the opposite directions of rotation in the two hemispheres of the Earth. The left hand advancing in the southern hemisphere means a clockwise rotation there. At the equator, there is no rotation about an axis perpendicular to the surface, and therefore no Coriolis effect. (A radian is an angle of 57.3°.)

Figure 11.11 Explanation of the opposite directions of rotation in the two hemispheres of the Earth. The left hand advancing in the southern hemisphere means a clockwise rotation there. At the equator, there is no rotation about an axis perpendicular to the surface, and therefore no Coriolis effect. (A radian is an angle of 57.3°.)

layer in turn drags the layer beneath, which again is affected by the Coriolis force, so that the second layer moves at a greater angle to the wind. Similarly for lower layers, each moving more slowly and more at an angle than the layer above. With each layer's movement represented by an arrow of appropriate direction and a length proportional to the speed, we have the arrangement shown in Figure 11.13. The tips of the arrows trace an 'Ekman spiral', named after the Swedish oceanographer Vagn Ekman (1874-1954). He proposed this spiral in 1902 to explain Fridtjof Nansen's observation that icebergs move at an angle of around 30° to the right of the wind in the northern hemisphere. The outcome of a complete spiral is an average movement, called Ekman transport, which for the whole spiral amounts to motion at right angles to the wind, and the top layer of the ocean (i.e. the Ekman layer) is driven towards that direction. This is a surprising result, that the Coriolis effect causes the ocean to move perpendicular to the wind—towards the left in the southern hemisphere. But a complete spiral develops only in deep water. Often a shallow thermocline, or the sea-bed in shallow waters, limits downwards transfer of momentum, and then surface ocean currents are more closely aligned with the wind.

The result of Ekman transport is upwelling of deeper, cold water to the surface near some coasts, as illustrated in Figure 11.14. The upwelling happens wherever there is either a polewards wind parallel to an east coast, or an equatorwards wind parallel to a west coast, as in north Chile or Namibia. The wind creates Ekman transport of the warm surface water away from the land, and cold deep water rises to takes its place at a rate of a metre per day or so. This is one explanation for the low

Figure 11.12 Demonstrations of the apparent deflections of actually straight-line motions (relative to the Sun, for instance) which are either (a) radial (corresponding to movement along a line of longitude), or (b) circumferential (along a line of latitude). In both cases, the rotation is shown as clockwise (as in the southern hemisphere—see Figure 11.11), and in both cases the apparent deflection is to the left.

Figure 11.12 Demonstrations of the apparent deflections of actually straight-line motions (relative to the Sun, for instance) which are either (a) radial (corresponding to movement along a line of longitude), or (b) circumferential (along a line of latitude). In both cases, the rotation is shown as clockwise (as in the southern hemisphere—see Figure 11.11), and in both cases the apparent deflection is to the left.

Figure 11.13 The Ekman spiral in the southern hemisphere. The wind is towards the top left (parallel to the right-hand long side of the base), and Ekman transport is towards the bottom left, parallel to the nearest short side of the base.

temperature of waters within 20 km of the Peruvian coast, and for the climate of Lima at 12°S (Note 11.E, Table 11.3). The upwelling off Lima results in an SST which is 3 K less than that 2,400 km nearer the South Pole at Antafagasta in Chile. Similarly, pleasantly cool conditions near Rio de Janeiro are induced by the upwelling caused by occasional north winds, even though they come from the equator. Coastal upwelling off south-west

upwelling

Figure 11.14 Wind-induced ocean currents and upwelling at a coast in the southern hemisphere.

upwelling

Figure 11.14 Wind-induced ocean currents and upwelling at a coast in the southern hemisphere.

Africa, south of 15°S, leads to SSTs near the shore which are as much as 5 K lower than at 320 km out to sea, particularly in summer and at 26°S. This leads to low temperatures in the atmospheric PBL, under a strong inversion (Figure 7.10).

Upwelling also occurs along the equator in the eastern Pacific, because winds from the east deflect the water southward (to the left) just south of the equator, and northward (to the right) just north of the equator, where the Coriolis force though weak acts in the opposite direction. This creates a furrow in the equatorial surface water, which in turn causes upwelling there, and consequently surface temperatures of only about 19°C off the Galapagos Islands, on the equator in the Pacific ocean. Decline of the easterly winds prior to an El Niño (Note 11.C) leads to a rapid end to upwelling and

Table 11.3 Comparison of the average climates of Darwin and Lima, both at 1 2°S

Darwin Lima

Table 11.3 Comparison of the average climates of Darwin and Lima, both at 1 2°S

Darwin Lima

 July January July January Daily mean temp. (°C) 25 28 15 21 Sunshine (hours/day) 9.8 6.1 3.4 5.1 Raindavs/month <1 19 1 <1 Rainfall (mm/mo) 1 391 2 1.2 Wind direction at 3 p.m. E NW S S Wind speed (m/s) 2.5 2.6 2.5 3.5

consequently a dramatic rise of sea-surface temperature.

There are plenty of fish where there is upwelling, partly because the deep water which is brought up contains the deposited nutritious debris of previous generations of fish. Another reason is that cold waters contain more oxygen, e.g. water at 0°C can hold twice as much as water at 25°C. Ninety per cent of the world's fish are caught in the 15 per cent of the oceans where upwelling takes place.

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### Responses

• tobias
Does coriolis effect cause lower temperatures?
12 months ago