North Atlantic Deep Water and the World Ocean 
Arnold L. Gordon 
Lamont-Doherty Geological Observatory 
Palisades, New York 10964 
North Atlantic Deep Water (NADW) by being warmer and more saline than the average 
abyssal water parcel introduces heat and salt into the abyssal ocean. The source of these pro- 
perties is upper layer or thermocline water considered to occupy the ocean less dense than 
sigma-y of 27.6 (Gordon and Piola, 1983: Fig. 1). That NADW convects even though it's warmer 
than the abyssal ocean is obviously due to the high salinity. In this way, NADW formation may 
be viewed as haline convection. The counter "force" removing heat and salinity (or introducing 
fresh water) is usually considered to take place in the Southern Ocean where upwelling deep 
water is converted to cold fresher Antarctic water masses. The Southern ocean convective pro- 
cess is driven by low temperatures and hence may be considered as thermal convection. A 
significant fresh water source may also occur in the North Pacific where the northward flowing 
of abyssal water from the Southern circumpolar belt is saltier and denser than the southward 
flowing, return abyssal water. The source of the low salinity input may be vertical mixing of 
the low salinity surface water or the low salinity Intermediate water. 
It is likely that heat and salt are also introduced into abyssal water by vertical mixing, 
particularly for these thermocline regions susceptible to salt finger instability. The 
Intermediate water mass of low salinity is a participating factor. If NADW were turned off and 
another convective transfer of water from upper to abyssal layer is not established, it is reaso- 
nable to conclude that the thermocline water would become saltier (assuming the net atmosphere 
fresh water forcing remains unchanged). In this situation further enhancement of salt finger 
activity which drives large Kz values would increase the transfer of thermocline heat and salt 
into abyssal water, compensating to some measure for the absence of NADW. In this case the 
abyssal ocean may not be altered drastically in regard to heat and salinity, though some changes 
in the non-conservative properties may occur, since the transfer mechanism switches from point 
source convection to broad regional diffusion. 
e 
° 
ATLANTIC 
WATER VAPOR 
EXPORT 
3 
420 —_—P = 
< ARCTIC 
THERMOCLINE OUTFLOW 
Om, : 
aad => =p —_> = 
H UPPER LAYER 
& 
3 i 
a 
= ABYSSAL LAYER He OS 
sa ATLANTIC j S 
SALINITY x 
uJ EXPORT } = 
o —@mme NORTH ATLANTIC S 
TO INDIAN = 
DEEP WATER 
THERMOCLINE ABYSSAL 
TRANSFER 
OCEAN 
YY MWWWWTC@ V!! tlt ee eEEE@EeqCq@q@E@q@@@@Z@¥¥VIZ 
LATITUDE 
Fig. 1. Schematic diagram of the meridional circulation model for the Atlantic Ocean. The 
upper layer is required to flow northward from 35°S across the equator and into the northern 
North Atlantic in order to conserve the mass lost by formation of North Atlantic Deep Water. 
The salinity of the upper layer is altered by freshwater exchange with the atmosphere. The 
Atlantic abyssal layer exports the relatively salty NADW into the Antarctic Circumpolar Current 
and atmospheric water vapor is exported southward across the 35°S and eventually contributes in 
decreasing salinity of the upper layer south of Africa. The parameters Ms, My, Ma are 
described in the text. 
