Life in the Open Ocean. Joseph J. Torres

Life in the Open Ocean - Joseph J. Torres


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equator. Let us stick with the missile. Because it was fired from the equator, the missile has an eastward velocity of 1668 km h−1 when it leaves the ground, along with its northward velocity. As it speeds north the Earth is rotating beneath it, so the velocity at the Earth’s surface is declining with increasing distance from the equator. The result is that the missile, when viewed from the perspective of missile control at the equator, has veered to the right or clockwise (Figure 1.8). Imagine now a similar experiment with the missile being fired from latitude 45 °N toward the equator. The missile has an eastward velocity of 1180 km h−1 when it leaves the launch pad and is heading south toward the equator, which is moving east at 1668 km h−1. In this case, the Earth is literally moving east more quickly than the missile is moving as it heads south and, once again, the missile appears to veer to the right or clockwise.

      This brings us to three general rules about Coriolis force. In the northern hemisphere, the Coriolis force deflects moving bodies, including fluids, in a clockwise direction: to the right. In the southern hemisphere, deflection is counterclockwise, to the left. Third, Coriolis force is nonexistent at the equator and strongest at the poles. Consider also that the influence of the Coriolis force will be very much greater on slow‐moving bodies such as parcels of water than on a quickly moving object such as our imaginary missile, which spends only minutes in the air.

Schematic illustration of coriolis effect. Differences in velocity of Earth's surface as a function of latitude.

       Ekman Transport

Schematic illustration of coriolis effect. Apparent curved path of an object not coupled to the Earth's surface, moving in the northern hemisphere.

      Source: Brown et al. (1989), figure 1.2a (p. 7). Reproduced with the permission of Pergamon Press.

Schematic illustration of Ekman transport. Net spiral pattern of wind-driven motion down through a water column due to Coriolis effect and drag.

      Geostrophic currents are the result of a dynamic balance between the driving force of the wind, the turning effects of the Coriolis force, and pressure gradients caused by differences in sea‐surface height. Ekman Transport and wind stress act to create a slight hill of water, or topographic high, roughly in the middle of a gyre. Water in the high attempts to flow downhill but is offset by the Coriolis force so that the current in the gyre becomes parallel to the elevated sea surface, flowing clockwise in the northern hemisphere and counterclockwise in the southern.

      Ocean Gyres and Geostrophic Flow

      Six great circuits are found in the world ocean, four in the southern hemisphere (South Atlantic, South Pacific, Indian, and Antarctic Circumpolar) and two in the northern (North Atlantic, North Pacific). The gyres correspond fairly well to the biogeographic distribution of oceanic species.

      Upwelling

      Deep‐Ocean Circulation


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