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54oW 36oW 18oW 0o 18oE 24oS
12oS
0o
12oN
24oN
36oN
Model domain
Dam
pin
g o
f s
ponge la
yer
[s−
1]
0.5
1
1.5
2x 10
−5
Intraseasonal variability in the tropical Atlantic: Observations vs. reduced gravity simulations
Robert Kopte, Peter Brandt, Richard J. Greatbatch, Mar7n Claus
1) Variability in the tropical Atlan7c: 2) Reduced gravity simula7ons of the tropical Atlan7c
3) Comparison of reduced gravity simula7ons with AVISO SLA:
4b) Equatorial wave analysis 4a) Basin mode of the 1st baroclinic mode
5) Summary and outlook
0 50 100 150 200 250 300 350 4000
1
2
3
4
Sp
atia
lly a
vera
ge
d a
mp
litu
de
of
SL
A [
cm]
Period [days]
0 50 100 150 200 250 300 350 4000
1
2
3
4
5
De
pth
−a
vera
ge
d a
mp
litu
de
of
zon
al v
el.
at
23°
W [
cms−
1]
SLA [42W−15E, 10S−10N]Zonal velocity [23W]
Fig. 1: Periodogram of SLA in the tropical Atlan7c and zonal velocity at 23°W, 0°N.
120d
182d
365d ▶ Spectral peaks at annual and
semi-‐annual, and 120-‐day periods associated with 4th, 2nd and 1st baroclinic modes
▶ Peaks correspond to resonant basin modes, composed of equatorial Kelvin and Rossby waves, as well as coastally trapped waves[1]
Fig. 2: Model domain with sponge layers at northern/southern boundaries, and area/sec7ons of interest.
▶ To study the intra-‐seasonal variability in par7cular, reduced gravity model (RGM) simula7ons are used:
▶ The model is run separately for the first five baroclinic modes (c1=2.47 m/s, c2=1.32 m/s, c3=0.94 m/s, c4=0.74 m/s, c5=0.57 m/s), forced with interannually varying wind stress from NCEP (1990-‐2014)
▶ To allow for comparison of model and observa7ons, the model output is ficed to AVISO sea level anomaly (SLA)[2]
ut
�fv = �g0n
⌘x
+⌧xs
⇢0, v
t
+fu = �g0n
⌘y
+⌧ys
⇢0, ⌘
t
+H(ux
+vy
) = 0, g0n
=c2n
H
0.25° x 0.25°
Aviso
40°W 20°W 0°
1997
2000
2003
2006
2009
2012
RGM M1−5
40°W 20°W 0°
[cm]−20
−15
−10
−5
0
5
10
15
20
AvisoJ
F
M
A
M
J
J
A
S
O
N
D
RGM M1−5
[cm]−8
−6
−4
−2
0
2
4
6
8
Fig. 3: Hovmöller plot of SLA along equatorial wave guide (green line in Fig. 2). Le#: AVISO, right: Reconstruc7on from RGM modes 1-‐5.
Fig. 4: Climatologic (1995-‐ 2014) seasonal cycle of SLA along equatorial wave guide.
Fig. 5: Climatologic (1995-‐2014) seasonal cycle of band-‐passed (25-‐130d) SLA along equatorial wave guide.
▶ Dominance of the annual and semi-‐annual cycle, well reproduced by the RGM (Fig. 3 and Fig.4)
▶ In AVISO, presence of con7nuous and recurrent eastward propaga7ons[3], with the intra-‐seasonal climatology represen7ng ~25% of the seasonal cycle amplitude (Fig. 5)
RGM M1
120−day
10°S
5°S
Eq.
5°N
10°N
50°W 40°W 30°W 20°W 10°W 0° 10°E
RGM M1−5
120−day
10°S
5°S
Eq.
5°N
10°N
[cm]
0
1
2
3
4
AVISO
120−day
10°S
5°S
Eq.
5°N
10°N
AVISO
120−day
10°S
5°S
Eq.
5°N
10°N
RGM M1−5
120−day
10°S
5°S
Eq.
5°N
10°N
Nov/Mar/Jul
Dec/Apr/Aug
Jan/May/Sep
Feb/Jun/Oct
RGM M1
120−day
50°W 40°W 30°W 20°W 10°W 0° 10°E10°S
5°S
Eq.
5°N
10°N
Fig. 6: Maps of amplitude (le#) and phase (right) of 120-‐day harmonics ficed to SLA data. Top: Aviso, middle: RGM modes 1-‐5, bo0om: RGM mode 1 only.
AVISO
Fre
qu
en
cy [
da
ys−
1]
Wavenumber [10−3 km−1]
−1.5 −1 −0.5 0 0.5 1 1.50
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
0.05−10 −20 20 10
Wavelength [degrees longitude]
RGM
Pe
rio
d [
da
ys]
Wavenumber [10−3 km−1]
−1.5 −1 −0.5 0 0.5 1 1.5
365
100
50
30
Log10
of spectral density [cm2/cpd/km−1]6 7 8 9 10
−10 −20 20 10
Wavelength [degrees longitude]
Fig. 7: Mean (5°S-‐5°N, see horiz. black lines in Fig. 2) wave-‐number-‐frequency diagram. Le#: Aviso, right: RGM (modes 1-‐5). The white lines represent the theore7cal dispersion curves of the Kelvin wave and the first three meridional Rossby waves.
▶ Basin-‐wide structure of SLA associated with the 1st baroclinic mode with variability detectable as far as 10°S
▶ Consistent pacern in the RGM, although with considerable lower amplitude
▶ Mode 1 in the RGM simula7ons explains most of the structure seen in the “full” model
▶ Averaged over the equatorial belt (5°S-‐5°N), zonal wavenumber-‐frequency diagrams of SLA exhibit spectral peaks near the theore7cal dispersion curves of the first baroclinic mode equatorial Kelvin and Rossby waves
▶ In the RGM, total energy is lower, however there is considerable energy in the mixed Rossby-‐gravity wave range
RGM M1−5
EAST WEST EAST WEST−−4N−−> −−EQ−−> −−4N−−>
Mo
nth
0E 20W 40W40W 20W 0E 0E 20W 40WJan
Apr
Jul
Oct
AVISO
Mo
nth
Jan
Apr
Jul
Oct
[cm]−4
−3
−2
−1
0
1
2
3
4
Fig. 8: Climatologic (1995-‐2014), band-‐passed (25-‐130d) SLA along 4°N-‐Equ.-‐4°N (see Fig. 2). Top: Aviso, bo0om: RGM modes 1-‐5.
▶ Only ~50% of the intra-‐seasonal signal amplitude is reproduced by the RGM, however the phase-‐lock of the propaga7ons appears to be consistent
▶ Intra-‐seasonal SLA variability in the tropical Atlan7c is essen7ally wind-‐driven, as it can be reproduced by reduced gravity simula7ons, although with weaker amplitudes
▶ Possible reasons for discrepancies to be tested: ▶ Bad choice / spa7al variability of phase
speeds, which leads to the missing of resonance to a periodic forcing
▶ Uncaptured (i.e. non-‐linear) effects of the North Equatorial Counter Current (NECC) on westwards propaga7ng Rossby waves at ~4°N (Fig. 8)
References [1] Cane MA, Moore DW (1981): A Note on Low-‐Frequency Equatorial Basin Modes. J Phys Oceanogr 11:1578-‐1584 doi:10.1175/1520-‐0485 [2] The al7meter products were produced by Ssalto/Duacs and distributed by Aviso, with support from Cnes (hcp://www.aviso.al7metry.fr/duacs/) [3] Polo I, Lazar A, Rodriguez-‐Fonseca B, Arnault S (2008): Oceanic Kelvin waves and tropical Atlan7c intraseasonal variability: 1. Kelvin wave characteriza7on. J Geophys Res-‐Oceans 113:18 doi:10.1029/2007jc004495
Aviso
40°W 20°W 0°J
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J
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A
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RGM M1−5
−40 −20 0 [cm]−2
−1.5
−1
−0.5
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