Geoscience Reference
In-Depth Information
8.3 Marginal Static Stability, MaudNESS
The last example of applying the one-dimensionalLTC model arises from another
practical requirementencounteredin planningfor the MaudNESS experimentnear
Maud Rise in the Weddell sector of the Southern Ocean. The basic plan for Maud-
NESS was to perform a fast, relatively shallow CTD survey across the seamount,
concentrating on the margins, and use a combination of special weather forecasts
and ice concentration analyses to estimate the most likely regions for thermobaric
instability and deep-reaching convection based on CTD stations made at different
timesandplaces.Tothisendaforecastmodelwasneededthatwassimpleenoughto
update several candidate profiles with every new (daily) weather forecast, with the
goal of enhancing operational planning by identifying areas where mixing would
occurintheleaststabledensityenvironment.Overall,ourobjectivewastomeasure
turbulent exchange in a low stability environment to understand what conditions
mightremovethethinicecovercompletelyovera substantialarea.
A comparison of upper ocean conditions during late summer at SHEBA in the
western Arctic
9
◦
N
6
◦
W
(
79
.
,
161
.
)
with late winter at MaudNESS in the Weddell
5
◦
S
1
◦
E
(
illustrates a striking contrast in static stability of the water
column (Fig. 8.18). In the former (used to initialize the model run described in
Section 8.2 above), potential density (Fig. 8.18c) increases by nearly 3
65
.
,
001
.
)
5kgm
−
3
in the upper 150m, while for the latter (MaudNESS Station 91), the increase is
two orders of magnitude less, about 0
.
03kgm
−
3
, and is barely perceptible when
drawn at the same scale. Thus by comparison the Weddell profile is near to being
statically neutral; nevertheless, there is a steep thermocline (Fig. 8.17a) starting at
around 100m with far more heat content close to the surface than in the Arctic.
A much less obvious halocline (again when drawn at the scale appropriate for the
Arctic) contributes to a slightly stable pycnocline (in potential density, less stable
for
in situ
density) that separatesthe uppercold layer fromthe underlyingWeddell
Deep Water. Because of the low stability, it is relatively easy to mix heat upward
from the large WDW reservoir, whenever there is vigorous stirring at the surface
(which is common in the Weddell in winter). But this is the source of the
thermal
barrier
(Martinson 1990) that melts ice when basal heat flux exceeds conduction
through the ice cover, forming a shallower halocline that severely inhibits deeper
convection. As the profiles indicate, the system is very delicately balanced. Note
thatbelowabout120mintheMaudNESSprofile,temperatureandsalinityarevery
uniform,indicatingsomeactivemixingactivity.
In the operationalmode, the modelstrategy was to use the weather forecast and
ice concentration data provided daily via satellite communicationsfrom the Arctic
MesoscalePredictionSystem(AMPS)MM-5regionalmodel(Powersetal.2003)to
projectaheadfivedaysfromthecurrenttime,keepingbackwardtrackataparticular
locationfromthetimeatwhichaninitialupperoceanstructurewasmeasured.This
was done by accepting the “nowcasts” that initialized each daily weather forecast
as valid analyses. Comparison of the nowcasts with ship observations were gener-
allyfavorable,althoughtheMM5temperaturesoftenappearedtobebiasedhighby
a few degrees. In this way, we could constructfor a given location in the operation
.
