Al. New et R. Bleck, AN ISOPYCNIC MODEL STUDY OF THE NORTH-ATLANTIC .2. INTERDECADAL VARIABILITY OF THE SUBTROPICAL GYRE, Journal of physical oceanography, 25(11), 1995, pp. 2700-2714
In a companion paper, a spinup integration of the North atlantic Ocean
with the Miami isopycnic-coordinate model was presented. The winterti
me mixed layer in the central North Atlantic was subject to relatively
little change in salinity or depth but cooled markedly, most probably
because of heat loss associated with a partial surface relaxation to
climatological sea surface temperatures in a region in which the Gulf
Stream was too far to the north. This mixed layer cooling caused the i
sopycnic layers in the model ventilated subtropical gyre to rise and,
surprisingly, to warm. While the experiment was not an attempt to simu
late changes in the real Atlantic Ocean, it nevertheless appears from
observations that, in recent decades, the mixed layer in nature has un
dergone a change similar to that exhibited by the model mixed layer. S
ince it is expected that changes in the ventilated subtropical gyre wi
ll be governed largely by changes in the mixed layer in the central No
rth Atlantic, from where the ventilating water masses are subducted, o
ne might therefore anticipate similarities between the changes in the
ventilated regions of the gyres in the model and the real world, even
though the cause of the mixed layer changes in the real world may have
been different from that in the model. The present paper shows that t
his is indeed so. In particular, the model behavior closely parallels
observed changes in the ventilated subtropical gyre reported by Levitu
s, in a Study of differences between two pentads. The degree of simila
rity between the model and the observations, including in particular w
arming of the isopycnic surfaces, leads to a proposal that the changes
Levitus observed were caused largely by the subduction of water masse
s from a cooler mixed layer. Historical changes in the characteristics
of the warm North Atlantic Central Water may also be explained by thi
s mechanism. Changes in the wind stress or Ekman pumping fields do not
necessarily need to be invoked. Overall, the model provides a framewo
rk in which observations from a number of different sources can be und
erstood in a coherent fashion and allows new insights to be gained int
o the interdecadal variability of the Atlantic Ocean.