54oW 36oW 18oW 0o 18oE 24oS
12oS 0o 12oN 24oN 36oN
Model domain
Damping of sponge layer [s−1 ]
0.5 1 1.5 x 102 −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 1
stbaroclinic mode
5) Summary and outlook
0 50 100 150 200 250 300 350 400
0 1 2 3 4
Spatially averaged amplitude of SLA [cm]
Period [days]
0 50 100 150 200 250 300 350 4000
1 2 3 4 5
Depth−averaged amplitude of zonal vel. 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 f v = gn0 ⌘x+⌧sx
⇢0 , vt+f u = gn0 ⌘y+⌧sy
⇢0 , ⌘t+H(ux+vy) = 0, gn0 = 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°
−20[cm]
−15
−10
−5 0 5 10 15 20
Aviso J
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°E 10°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 Frequency [days−1 ]
Wavenumber [10−3 km−1]
−1.5 −1 −0.5 0 0.5 1 1.5
0 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
Period [days]
Wavenumber [10−3 km−1]
−1.5 −1 −0.5 0 0.5 1 1.5
365 100 50 30
Log
10 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 −−4N−−> WEST −−EQ−−> EAST −−4N−−>WEST
Month
0E 20W 40W40W 20W 0E 0E 20W 40W
Jan Apr Jul Oct
AVISO
Month
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
40J°W 20°W 0° F
M A M J J A S O N D
RGM M1−5
−40 −20 0 [cm]−2
−1.5
−1
−0.5 0 0.5 1 1.5 2
