Geoscience Reference
In-Depth Information
50
0
50
150
100
50
50
100
150
Longitude
-6
-4
-2
0
(W m -3 )
log 10 ε
Figure 11.7 See colour plates version . Tidal energy dissipation during the last glacial maximum.
From (Green, 2010) with permission from Springer.
ocean which is necessary for the maintenance of the Meridional Overturning
Circulation (MOC) (Wunsch and Ferrari, 2004 ).
Recent model studies indicate that important changes in dissipation have taken
place, for example in the period since the last ice age when, at the time of the peak
glaciation, sea level was
130 metres lower than now and much of the present shelf
sea area was dry. In this situation, we might expect that the removal of large areas of
the shelf, where a considerable proportion of the existing dissipation occurs, would
reduce the total dissipation. In fact, a number of different models (e.g. Egbert et al.,
2004 ; Griffiths and Peltier, 2008 ; Arbic et al., 2009 ) concur that total dissipation was
4.7 TW, of which 4.2 TW
could be found in the semi-diurnal band (Egbert et al., 2004 ; Green et al., 2009 ). At
the same time, the rate of energy dissipation in the deep ocean was higher by a factor
of
33% higher in the Pleistocene with a total dissipation of
2.5 relative to the present situation. In consequence, considerably more power
was available to produce mixing in the deep ocean with stronger forcing of the MOC
during the glacial era (Green et al., 2009 ). In spite of this increase in forcing, the
evidence available suggests that the MOC was not increased but rather decreased
during the last ice age, because of the suppression of deepwater formation by the
presence of the increased freshwater content of the surface waters of the northern
North Atlantic.
How can removal of the damping area (the shelf) increase dissipation? The answer
seems to be that the reduction in damping changes the dynamical response of the
ocean and moves it closer to resonance in one, or more, of the ocean basins. For
example, in Fig. 11.7 the distribution of tidal dissipation at the peak of the last
glaciation is seen to be very different from the current situation ( Fig. 2.13 ), most
especially in the North Atlantic where the tides are considerably enhanced. The
 
Search WWH ::




Custom Search