Etudes et évaluation de processus océaniques par des hiérarchies ...

225
Chapter 9
Abyssal and Overturning Circulation
tel-00545911, version 1 - 13 Dec 2010
The study of the deep circulation of the world ocean has historically relied on the analysis of
water masses. The reasons are that: (i) in the deep water masses change very slowly in time
as they are not subject to boundary forcing and as they give an integrated view of the velocity
field which mostly weakens when descending into the depth of the ocean; (ii) it is technically
difficult to measure the moderate but highly variable velocities in the deep ocean, especially
from a ship that is transported by the stronger currents at the ocean surface.
In 1751 Stephen Hales constructed a “bucket sea-gage” and asked Henry Ellis, the captain
of the Earl of Halifax, to perform temperature measurements in the deep North Atlantic. Ellis
found that temperature decreases with depth and noted: “This experiment, which seemed at
first but mere food for curiosity, became in the interim very useful to us. By this means we
supplied our cold bath, and cooled our wines or water at pleasure; which is vastly agreeable to
us in this burning climate.”
It was Count Rumford who noted in 1800 that this cold water can only originate from high
latitudes and called it “[...] an inconvertible proof of the existing of cold water at the bottom
of the sea, setting from the poles towards the equator.” This picture was then refined and the
zones of formation of the deep waters were identified to lie in the high latitudes of the North
Atlantic and the Antarctic Ocean. There is no formation of deep waters in the Indian and
Pacific Ocean. The deep waters are upwelling in the rest of the ocean counter balancing the
diffusion of heat into the deep ocean and thus forming the thermocline , that is a more or less
sharp boundary between the warm surface waters and the cold deep waters in the mid and low
latitudes. These processes are schematised in fig. 9.1.
North Pole
✻
Z
convection
✛
✛
North Atlantic
surface layer (warm)
Equator
thermocline
✻ ✻ ✻ ✻ ✻ ✻ ✻ ✻ ✻ ✻ ✻ ✻ ✻ ✻ ✻ ✻ ✻ ✻ ✻
upwelling upwelling upwelling
❄❄❄
✲
deep layer (cold)
✲
Figure 9.1: Overturning Circulation
51