J Hopkins

FASTNEt annual science meeting
Jo Hopkins, NOC
1. WP1/4 – Cross-shelf exchange estimates from
D376 moorings
2. WP3 – Do storms modify baroclinic energy
fluxes in the Celtic Sea?
3. WP3 - JC88 on-shelf moorings summary
Plymouth, 13th-14th November 2013
WP4: D376 short term mooring flux estimates
Across shelf flux
where
is the time mean
across-shelf current profile
Deployment mean flux (m2 s-1)
16-27 Jun 2012
Tidal residual fluxes over integral M2 tidal periods
Per
BT tide – large exchange, zero flux
100 km
-1.7 Sv
-0.4 Sv
-0.1 Sv
-0.4 Sv
Long term Celtic Sea mooring
8 Sv 100 km-1
2 Sv 100 km-1
Deployment mean flux (m2 s-1)
Off-shelf flux: Aug, Nov-Jan
On-shelf flux: Jul, Oct, Feb-Apr
1.2 Sv per 100 km shelf section over deployment
When were slope current reversals?
Do storms modify baroclinic energy fluxes in
the Celtic Sea?
Jo Hopkins, Gordon Stephenson, Mattias Green
• PART A: Inertial and tidal energy interaction
• PART B: Modification of IT propagation and energy in response to changes
in slope criticality as N2 is modified during storms
• Magnitude, direction and vertical structure of baroclinic IT energy fluxes
• What role did the storm and strong wind events play in determining these
fluxes? Consider…
•
•
Partitioning of potential and kinetic baroclinic energy
Bulk shear
• Differences between two locations (separated by 26 km)
D376 Celtic Sea – June 2012
26 km
43 km
Air Pressure
The “Thursday Lows”
Photo: Marie Porter
ECMWF winds
SPRINGS
1. Main storm breaking
down stratification
ST4
2. Re-stratification
(~7 days)
3. Short, intense wind
episode
4. Less pronounced M2
IT at ST5
ST5
Tidal residual PSD
ST4
f
M2
M4
ST4
ST5
Significant clockwise inertial peak (f>M2)
f M2
Energy flux calculations (following Nash et al. (2005)
< > = average over integral no. M2 wave periods
Perturbations from vertical mean profiles formed over two IT periods
(account for evolution of h20 column structure + minimise contamination by advection/low frequency variability)
Unknown depth average u0(t) calculated
by requiring
p’ from hydrostatic eqn.
Unknown surface pressure p0(t)
calculated by requiring
Perturbations band pass filtered
(10.5 - 17) hrs to retain f-M2 only
This does not capture the contribution from non-linear, high-freq. , non-hydrostatic waves
Average energy fluxes in f-M2 band
Deployment mean depth
integrated energy fluxes
Across
W m-1
Along
W m-1
ST4
16
-45
ST5
-20
-14
ST1
-158
189
Range of flux magnitudes at ST4 and ST5 of 10-120 W m-1
ST4 and ST5 vectors scattered through full 360° - complex 3D wave field, chaotic
generation and propagation
Agreement with Green et al. (2008): 8 Wm-1 across-shelf with up to 200 Wm-1 in
individual waves.
Vertical structure of f-M2 energy fluxes
Maximum fluxes
concentrated in
surface layer
ST4 - across
ST4 - along
ST5 - across
ST5 - along
Little evidence of SN cycle except
perhaps at ST4
Depth integrated f-M2 energy fluxes
2.3 day pulsing of
energy corresponding
to expected f-M2
interference frequency
Can we explain this
interaction by
exploring....
1) M2 and f potential
and kinetic
baroclinic energy
2) Bulk Shear
Evolution of KE and PE at ST5
Harmonic fits for M2 and inertial
amplitudes in 54 hr windows
Injections of KEf at
surface following wind
events
Increase in eta2 and
increase in PEM2 within
pycnocline and near
surface following
prolonged storm
Increase in KEM2 beneath
pycnocline
PEf increase initially near
surface then deepening
with pycnocline. Pulses
every 1.5-2.5 days
Evolution of KE and PE at ST4
Increase in KEM2 and
KEf following second
wind event
Increase in PEM2
following first AND
second wind event
Bulk shear 2
Main S2 peaks occur when
surface wind stress aligns with
bulk shear direction
ST4
ST5
B
C
D
A
Peaks in S2 every 2 days
following re-stratification
A
B
C
D
Bulk shear vs. energy flux magnitude
Is shear peaking in
response to
increased flux (high
baroclinic energy)
ST4
OR
Is increased IT flux
a response to
increased wind
driven shear?
ST5
JC88 moorings - Wirewalker
SD: 75 kHz ADCP
SE: t-chain and 2 x 300 kHz ADCPS
SG: t-chain and 150 kHz ADCP
SF: Wirewalker and 150 kHz ADCP
RBR CTD – CTD at 6 Hz
Wetlabs fluorometer at 1 Hz
Full 15 day pressure record
Average rising rate
0.45 to 0.55 m s-1
Average sinking rate
0.1-0.2 m s-1
12 hour pressure record
Time between upcasts
12-30 minutes
431 upcasts collected
“Stepped”
downcast
30 minute pressure record
Smooth freefloating up-cast
Salinity
Temperature
2 to 3 layer structure
Change in salinity + finer scale structure
Salinity (I)
Temperature (I)
CHL
SF
SE
Power spectra of depth mean currents – strong K1 & M2 components
Salinity (II)
Temperature (II)