Intraseasonal Variability of the Equatorial Atlantic Ocean
Franz Philip Tuchen 1 , Peter Brandt 1,2 , Martin Claus 1,2
1 GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
2 Christian-Albrechts-Universität zu Kiel, Kiel, Germany
Motivation
• The Equatorial Atlantic circulation is characterized by zonal currents varying on seasonal to interannual timescales;
enhanced energy in the intraseasonal frequency range (20- 70 days) is dominated by meridional velocity variability.
• Tropical Instability Waves (TIWs) are the dominant intraseasonal variability near the surface with intraseasonal energy observed to penetrate into the deep ocean.
• It is suggested, that part of the TIW energy radiates down- and eastward as beams of monthly Yanai waves to supply energy for the maintenance of zonal circulation variability (Ascani et al., 2015), possibly affecting tropical Atlantic variability (Brandt et al., 2011).
• Comprehensive observational evidence of deep equatorial intraseasonal variability (DEIV) including the downward energy propagation of intraseasonal waves derived from long-term mooring data is presented.
• Intraseasonal energy undergoes an annual intensification in boreal summer close to the surface and also exhibits a pronounced seasonal cycle at depth.
• A vertical mode decomposition of both horizontal velocity components suggests, that the intraseasonal frequency range is dominated by Yanai waves of baroclinic modes 2-10.
Results
Vertical Energy Distribution
References:
• Ascani, F., Firing, E., McCreary, J.P., Brandt, P., Greatbatch,R.J. (2015), The Deep Equatorial Ocean Circulation in Wind-Forced Numerical Solutions, J. Phys. Oc.,45(6), 1709-1734.
• Brandt, P., Funk, A., Hormann, V., Dengler, M., Greatbatch, R.J., Toole, J.M. (2011), Interannual atmospheric variability forced by the deep equatorial Atlantic Ocean,Nature, 473, 497-500.
• Cane, M.A., Sarachik, E.S. (1976), Forced baroclinic motions. I. The linear equatorial unbounded case,J. Mar. Res., 34(4), 629-665.
• Claus, M., Greatbatch, R.J.,Brandt, P., Toole, J.M. (2016), Forcing of the Atlantic Equatorial Deep Jets Derived from Observations, J. Phys. Oc., 46(12), 3549-3562.
Mode Analysis
§
Based on almost 15 years of equatorial moored velocity data at 23°W, intraseasonal variability is analyzed.§
Near the surface, intraseasonal energy is at its maximum and dominated by TIWs.§
Below the near-surface layer, downward energy propagation is implied, according to the linear wave theory, by upward phase propagation.Moored meridional velocity at 23°W (July 2006 to January 2008). Positive (negative) values represent northward (southward) velocities, whereas grey areas show missing data .
§
Close to the surface, meridional velocity variability occurs in a wide range of frequencies.§
Below 50-100m and down to about 3000m, the variability sharpens towards the frequency range from 5-15 cycles per year.Periodogram of meridional velocity at 23°W from moored observations (2001-2016). The orange box marks the intraseasonal frequency range (approx. 5-15 cycles per year).
Seasonal Cycle
Daily and monthly climatology of intraseasonal specific kinetic energy of the meridional flow averaged over the near-surface layer (20-50m).
§
The intraseasonal variability of the near-surface layer meridional velocity (20-50m) is dominated by an annual intensification of specific kinetic energy (𝟏⁄ 𝒗′𝟐 𝟐) from July to September.§
A linear combination of a semiannual and an annual harmonic cycle is able to explain a large fraction of the observed seasonal variability.Monthly climatology of anomalous (with respect to the mean) intraseasonal kinetic energy over depth (fitted with an annual and a semiannual harmonic cycle at each depth).
Months and depths with insufficient data coverage are marked grey.
§
In boreal summer, positive anomalies of intraseasonal energy start to propagate downward and reach a depth of about 800m during boreal winter.§
At larger depths, observations indicate interference of downward and upward energy propagation.Dispersion relation (𝜔 −k) for waves on an equatorial 𝛽– plane. The dashed line connects the extrema and n represents the meridional mode. Taken from Cane &
Sarachik (1976).
Vertical mode structure function of the barotropic and chosen baroclinic modes for H=4500m.
They are normalized as follows (with 𝑝̂+ representing the nth vertical mode structure function):
1
𝐻 . 𝑝̂3 +/𝑑𝑧 = 1
45
§
At the equator, the dispersion relation allows for three different wave types within the intraseasonal frequency band: Kelvin, Rossby and Yanai waves.§
A set of vertical mode structure functions is used to decompose both moored velocity components (e.g. Claus et al., 2016).§
Zonal velocity amplitudes are largest at low frequencies.§
The meridional velocity spectrum peaks in the intraseasonal band and above the cut- off frequency of equatorial Rossby waves suggesting that Yanai waves dominate the DEIV.Amplitude spectrum [ms-1] of zonal (a) and meridional (b) velocity with the cut-off frequency of equatorial Rossby waves (grey lines) superimposed; variance spectra (c,d) derived by summing the squared amplitude spectra (a,b) over all baroclinic modes.
Possible impact on tropical Atlantic variability
Oct2006 Jan2007 Apr2007 Jul2007 Oct2007 Jan2008 0m
500m
1000m
1500m
2000m
2500m <-20
-15 -10 -5 0 5 10 15
>20
Upward phase propagation
Downward energy propagation
0m
500m
1000m
1500m
2000m
2500m
3000m
3500m
4000m
4500m
-0.1 -0.05 0 0.05 0.1
Structure Function Barotropic Mode
2nd Baroclinic Mode 4th Baroclinic Mode 16th Baroclinic Mode
0.15
0.15
0.15 0.15
0.15 0.15
0.15
0.3
.1–0
5 –0.15
–0.15 –0.15
–0.15 0
0
0
0
0 0 0
5 10 15 20 25 30
Frequency [cycles per year]
0m
500m
1000m
1500m
2000m
2500m
3000m
3500m <10-8
10-7 10-6 10-5 10-4 10-3 10-2 10-1
>1
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07
Kinetic Energy [m2 *s-2 ]
Monthly 25-75 Percentile Range Daily Kinetic Energy
Smoothed Climatology Semiannual + Annual Harmonic Monthly Means
Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun 0m
500m
1000m
1500m
2000m
2500m
3000m
3500m <-10-15
-10-12 -10-9 -10-6 -10-3 0 10-3 10-6 10-9 10-12
<10-15
0 5 10 15 20 25
Baroclinic Mode
0 0.05 0.1 0.15
0 5 10 15 20 25
0 0.05 0.1 0.15
5 10 15 20
Frequency [cycles per year]
0 0.1 0.2 0.3 0.4 0.5
Variance [m2 *s-2 ]
5 10 15 20
Frequency [cycles per year]
0 0.1 0.2 0.3 0.4 0.5
a)
c) d)
b)
