Geoscience Reference
In-Depth Information
Fig. 2.12
Diagram of
in situ
density of the two-layer system minus the density of an upper ocean
with uniform upper layer characteristics. Displacement of a water parcel with upper layer char-
acteristics (square) downward makes it denser than its surrounding, while upward displacement
from below the interface has the opposite tendency. The system is thermobarically unstable (see
Akitomo1999)
limits mixing drivenfrom the surface, forminga new, shallower near surface layer.
Thermobaricmixing below this layer may continue, drivenby the nonlinearity,but
isnolongeraffectingsurfaceexchanges.
5
Thefactremainsthatifmeltingattheice/oceaninterfaceistooweakortooslow
to counteract the combined effects of surface buoyancy loss from cooling and the
cabbeling/thermobaricity mechanism at the base of the mixed layer, then convec-
tion will continue (McPhee 2003). Once the ice cover is gone and the air remains
cold,thereisnothingexcepthorizontaladvectionoficeorfreshwatertoquelldeep
mixing, and essentially a direct connection between the abyssal ocean and the at-
mosphereisestablished.TheWeddellPolynyademonstratedthatsuchaneventcan
havelarge,evenglobal,impact.
Afactoroftenignoredinthe“mixingline”argumentforinstabilitiesarisingfrom
mixing of adjacent water masses with similar density but different
T/S
characteris-
ticsisthatevenina fairlyturbulentregime,diffusivitiesofheatandsaltmaydiffer.
In Section 2.7 we described an upwelling event observed in March 1998, at the
5
The conjecture that subsurface well mixed layers like that in the ANZFLUX profile in Fig.2.10
between 100 and 180m are remnants of mixing events where thermobaricity contributed is dis-
cussed further in Chapter 8.
