We have looked at evidence for horizontal currents that move along the surface of the ocean to redistribute heat. Vertical currents also move water from place to place. Upwelling currents driven by the wind are one mechanism for bringing deeper water to the ocean surface. Vertical motions can also be induced by density instabilities in the water column. An unstable water column occurs where denser water overlies less dense water. Throughout much of the ocean, vertical motions are inhibited by a stable density structure.
In the open ocean, temperature has the largest effect on water density. Based on temperature variations, the ocean can be divided into three depth zones:
(A) Mixed or surface zone of uniformly warm water.
(B) Thermocline where temperature decreases rapidly.
(C) Deep zone of uniformly cold water.
The three zones produce a stable water column where less dense (warm) water overlies more dense (cold) water. It is primarily at high (polar) latitudes, where the three zones are poorly developed, that the density instabilities that stimulate vertical motions occur.
The graphs below (see also Depth Profiles, attached) show water-temperature measurements sampled in a vertical column along the equatorial Pacific Ocean, at 110 W longitude, 140 W longitude, and 180 W longitude.. The data were collected on 22 and 23 February 1996 and four to six sampling sites are included on each diagram. The sites range from 9 degrees North to 8 degrees South Latitude. On the graphs below, mark the depths of the boundaries between the surface zone, thermocline, and deep zone. Label the 3 zones.
19. What changes do you observe from east to west along the equator in the Pacific Ocean?
20. Why do you think these changes occur? (Hint: this reason is similar to the reason for your observations in question 5?)