R, Muench, L' Padman, S' Howard Presentation OS51J10 at the

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Diapycnal mixing near Marguerite Bay During
Winter 2002 ... some results from Southern Ocean
GLOBEC, and a contribution to an ONR project that
aims to better understand high latitude ocean
turbulence and mixing
Robin D. Muench, Laurence Padman Susan L.
Howard Earth Space Research Seattle, WA USA
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Objectives of the mixing studies
Determine mechanisms responsible for upward
transport of heat, salt, and dissolved materials
from the warm, salty, nutrient-rich Upper
Circumpolar Deep Water to the biologically active
surface mixed layer. Quantify mixing rates for
these mechanisms, in terms of diapycnal
diffusivity Kv calculate vertical turbulent
fluxes associated with this mixing.
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Locations of CTD/microstructure stations winter
2002 (NBP02-04)
Bellingshausen Sea
Palmer Station
ACC
Depth (m)
Weddell Sea
Marguerite Bay
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Potential sources of diapycnal mixing
Shear instabilities, esp. from inertial waves and
baroclinic tides Double diffusion, as the
entire WAP region tends to instability with
respect to double diffusive convection
Intrusions of Upper Circumpolar Deep Water onto
the shelf from the ACC, leading to lateral
interleaving, enhanced property gradients, and
shear. Initial Hypotheses Numerical modeling
suggests that the tidal shear contribution to
mixing will be weak, except perhaps near the
shelf break. We expect, from previous studies in
the Palmer LTER region, that double diffusion may
be significant. The impact of interleaving is
uncertain.
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CTD-Mounted Microstructure Profiling System
(CMiPS)
CMiPS Overview Pmax 1800 m Internal
recording Sample rate512 Hz Resolution 1-10
cm Requires monotonic descent at moderate speed
no large vessel heave on CTD cable.
Pressure Thermistor (x2) Conductivity Cell
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Shear instabilities
CMiPS ADCP
Mixing at SML Base From CTD N2 ? 100 x 10-6
s-2 From CMiPS via Cox Kz ? 8 x 10-6 m2
s-1 FH ? 2 W m-2 From ADCP (?U/?z)2 ? 10 x 10-6
s-2 gt Ri ? 10 ?
This mid-shelf profile is fairly typical of
mixing at SML base. Velocity shear likely related
to wind-forced near-inertial waves. No evidence
of significant tide-induced shear (at least at
mid-shelf).
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Double diffusion
Active double-diffusive convection appears as a
vertical sequence of very sharp diffusing
interfaces separated by homogeneous convecting
layers. Interfaces can be lt10 cm thick, much
thinner than resolution of CTDs. This view is
derived from a microstructure profile obtained in
the eastern Weddell Sea during a
double-diffusive convective event.
ANZFLUX (Maud Rise)
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Double diffusion
GLOBEC IV CTD
ANZFLUX (Maud Rise)
R??3.5
Double- diffusive layers?
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Double diffusion
GLOBEC IV CMiPS
ANZFLUX (Maud Rise)
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Intrusions
CTD 023
WHOI Bathymetry
? Intrusions near Tmax ? Intrusions near base of
SML
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Intrusions
WHOI Bathymetry
Lateral diffusivity (Kh) after Joyce
(1977) Tx?10-5 Km-1 Tz?10-3 Km-1 Kz?10-5
m2s-1 gt KhKz(Tz/Tx)2 ? 0.1 m2s-1
? Intrusions near Tmax ? Intrusions near base of
SML
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Tentative conclusions
During winter 2002, diapycnal mixing at the base
of the surface mixed layer (SML) (100 m) was
driven primarily by near-inertial wave shear,
with some tidal contribution possible near the
shelf-break. Improved assessment of the role of
wind-forced, near-inertial waves requires
knowledge of wind stress at high temporal and
spatial resolution. No evidence of significant
double diffusive instability was found in
microstructure data near Marguerite Bay cf.
Palmer LTER region to the north Smith and
Klinck, 2002. Upward heat and salt fluxes from
UCDW contributed 10 of that required to explain
the observed seasonal variability of the SML. The
remainder presumably results from air-sea
exchange and from lateral advection evidenced by
upper layer interleaving.
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Comparison of T-S
Off-shelf Central WAP shelf SW of Palmer
Stn. Gerlache Strait
Low R?
Med. R?