N li
Nonlinear internal wave gener
i t
l ration in the South China Sea
ti i th S th Chi S
PO45M-08
PO45M
08
c
c
Qiang Li and David Farmer
Graduate School of Oceanography Univversity of Rhode Island, [email protected]
Graduate School of Oceanography, Univ
versity of Rhode Island qli@gso uri edu
D th (m)
Depth
St
Steepening
i
i t
internal
l tides
tid in
i the
th South
S th Chin
Chi Sea
S
generate
t large
l e amplitude
lit d nonlinear
li
i t
internal
l waves (NLIW).
(NLIW) Six
Si months
th off inverted
i
t d echo
h sounder
d meaasurements
t reveall their
th i
properties
ti and
d relationship
l ti
hi to
t barotropic
b t i forcing
f i in
i Luzon
L
St it
Strait
Strait.
I NLIW pattern generation
I. NLIW pattern generation
NLIW p
g
i
03
0.3
m)
η (m
For dominant semidiurnal (M2+S2+N2+K2) tidal forcing (BLUE)), the internal tide evolves to a ((BLUE), the internal tide evolves to a cusp shape with single large NLIW. p h p ith i gl l g NLIW A large diurnal tidal inequality (RED) l
d
l d l
l (
)
leads to alternation between rank‐
ordered wave packets and a single p
g
large wave
large wave. g
Total
Diurnal
Semidiurnal
TPXO:
M
Model
Harmonic
i tidal
id l forcing
f i
01
0.1
0
L
Li
Linear internal tide generation i
l id i ( b
(Hibiya 1986) coupled to a fully )
l d
f ll
n
nonlinear evolution model with r
rotation
((Helfrich 2007)
(Helfrich 2007), simulates 7) simulates t BLUE, RED and GREEN
th
the BLUE, RED
d GREEN p
patterns.
tt
-0
0.1
1
-0.2
16
18
20
22
24
26
28
50
0
-50
50
-100
15
16
17
0
April 2007 UTC
M2+K1
K1
Linear internal tide response (Hibiya 1986)
Nonlinear evolution ((Helfrich 2007))
50
-50
-100
100
M2
18
20
21
April 2007 UTC
Primarily diurnal (K1+O1+P1+Q1) forcing (GREEN) generates semidiurnal internal tides without NLIW
generates semidiurnal internal tides without NLIW.
g
m)
η (m
-1
u (m
ms )
0.2
m)
η (m
Ob
Observations
ti
0
22
-50
-100
26
27
28
29
April 2007 UTC
II Asymmetric radiation of NLIW away from Luzon Strait
II. Asymmetric radiation of NLIW away from Luzon Strait
y
d
f
yf
u ((m s-11)
MITgcm
g
()
(1) The thermocline tilt resulting from g
th K
the Kuroshio enhances nonlinearity of hi h
li
ity f waves propagating westwards into the i d i h S h Chi S but reduces waves South China Sea
South China Sea, but reduces waves b d
propagating eastwards into the Pacific
propagating eastwards into the Pacific.
South China Sea Pacific Ocean
III Seasonal variability and the Kuroshio intrusion
III. Seasonal variability and the Kuroshio intrusion
y
( ) Intensification of M2 internal tides (2) In
ifi i f M i
l id propaagating westwards into the South China Sea: d
h
h h
Sepa
aration of the two ridges in Luzon Strait is close e to the M2 wavelength
e to the M2 wavelength. The internal M2 tide g The internal M2 tide generrated at the western ridge is in phase with g
g
p
tthe internal tide generated at the eastern ridge, the in
t al tid
te
t de ge e ated
t d att th
t e easte
t id
dge, enha
h ncing westwards propagation and i d i d stren
ngthening the nonlinear transformation ngthening the nonlinear transformation. Correesponding eastwards propagation from the westeern ridge is blocked by a supercritical slope g
y
p
p
on th
th
h t ill
he eastern sill.
M2 forcing
f i g
MITgcm
g
u (m
( s-11)
SSH & surface velocity
y
Summer
NLIW are reduced in winter. Model (MITgcm) calculations show that this NLIW are reduced in winter
d d
Model (MITgcm) calculations show that this d l(
) l l
h
h h
is primarily caused by the Kuroshio intrusion not stratification changes. is primarily caused by the Kuroshio intrusion, not stratification
changes Doppler shift of internal tide generation reduces amplitude of waves pp
g
p
tra eling traveling west.
g est
Kuroshio
|vv| (m
m s-1)
Winter
Wi
MITgcm
W k suppo
Work
Wo
supported
edd by
b O
ONR NLIWI
W & IWISE
W S pprograms.
og
s.