(1) IFM-GEOMAR, Leibniz-Institut für Meereswissenschaften, 24105 Kiel, Germany; (2) Corresponding author: pbrandt@ifm-geomar.de;
(3) Present address: CIMAS, University of Miami, and NOAA/AOML, Miami, Florida 33149, USA; (4) Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA.
Peter Brandt
1,2, Andreas Funk
1, Verena Hormann
1,3, Marcus Dengler
1, Richard J. Greatbatch
1& John M. Toole
4Interannual atmospheric variability forced by the deep equatorial Atlantic Ocean
25 25.5 26 26.5 27 27.5 28 28.5
−0.05 0 0.05
0.1 0.15 0.2 0.25 0.3
Jan/Feb
Mar/Apr May/Jun
Jul/Aug Sep/Oct
Nov/Dec
U (m/s)∧
b)
〈 TATL3〉 (°C) 24 24.5 25 25.5 26 26.5 27 27.5 28 28.5 29 29.5
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Jan/Feb
Mar/Apr May/Jun
Jul/Aug
Sep/Oct
Nov/Dec
T (°C)∧
a)
〈 TATL3〉 (°C)
The amplitude of the 1,670-d cycle of zonal velocity is seasonally independent whereas the corresponding amplitudes of the ATL3 SST anomalies at this period are instead strongest during boreal summer and
November/December. Such behaviour is consistent with the equatorial zonal surface flow forced by interior ocean dynamics, whereas associated SST variations are seasonally modulated.
Seasonality of climate cycle
Fig.6: 1,670-d harmonic amplitude of bimonthly averaged (a) ATL3 SST indices and (b) geostrophic zonal velocity (Equator, 35°W-15°W) indices (b).
2002 2003 2004 2005 2006 2007 2008 2009
0m
200m
400m
600m
u [cm/s]
−100
−50 0 50 100 Jul Sep Nov Jan Mar May Jul Sep Nov Jan Mar
0m 500m 1000m 1500m 2000m 2500m 3000m 3500m
u [cm/s]
vertical phase velocity
−90 m/yr at [380 − 530 m]
−51 m/yr at [1260 − 1350 m]
−117 m/yr at [1470 − 1670 m]
2008 2007
2006 | |
<−20
−15
−10
−5 0 5 10 15
>20
Linearized phase lines with vertical phase velocities of EDJs are given in the figure.
Fig.3: Zonal velocity at the Equa- tor, 23°W, acquired by a mooring equipped with an ADCP, single-point current meters and a moored profiler.
Moored observations reveal the existence of EDJs oscillating at a period of about 4.5 yr. EDJs are associated with downward phase propagation from below the Equatorial Undercurrent (EUC) at about 200-m depth to about 2,000-m depth that corresponds, ac- cording to linear internal wave theory, to upward energy propagation.
Atlantic equatorial deep jets (EDJs)
Fig.1: Mean June/July/August SSTs in the tropical Atlantic (a). Also included are the main surface (solid) and thermocline (dashed lines) current bands. The white box in (a) defines the region for the ATL3 SST index shown in (b).
1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010
−1 0 1
SST index [°C]
b)
SST [°C]
60°W 50°W 40°W 30°W 20°W 10°W 0° 10°E 10°S
0°
10°N
20°N Mean June/July/August SST
and ATL3 box a)
20 22 24 26 28 30
NECC/NEUC
SEUC
EUC NEC
nSEC NBC cSEC
nNECC GD
NBUC
SEC
AD
Climate variability in the tropical At- lantic Ocean is sensitive to changes in SST particularly affecting deep at- mospheric convection over the ocean and surrounding continents.
During boreal summer the seasonal strengthening of easterlies along the equator leads to the development of the eastern Atlantic SST cold tongue centered slightly south of the equa- tor at about 10°W. The interannual variability of the onset of the cold tongue is found to be strongly linked to the onset of the West African Monsoon. Traditionally, climate vari- ability in the tropical Atlantic has fo- cused on the so-called zonal and meridional modes. Here, new obser- vations acquired during the Tropical Atlantic Climate Experiment (TACE, 2006-2011) are analyzed to identify a previously overlooked mode of tropical Atlantic climate variability that originates in the deep equato- rial Atlantic Ocean.
Introduction
Fig.4: Zonal velocity at the Equator, 23°W, acquired by different moored ADCPs.
Phase lines are obtained from harmonic analysis using a period of 1,670 d.
Reference
Brandt, P., A. Funk, V. Hormann, M. Dengler, R. J. Greatbatch, J. M. Toole, Interannual atmospheric variability forced by the deep equatorial Atlantic Ocean, Nature, 473, 497-500, doi: 10.1038/nature10013, 2011.
Acknowledgement
This study was supported by the Deutsche Forschungs- gemeinschaft as part of the Sonderforschungsbereich 754
“Climate - Biogeochemistry Interactions in the Tropical Ocean” and by the German Federal Ministry of Education and Research as part of the co-operative project
“Nordatlantik”.
Fig.5: E q u a t o r i a l zonal velocities from 1,000-m Argo float drift data (1°S-1°N).
The dominant in- terannual variability in the Atlantic and Pa- cific oceans obtained by maximizing ex- plained variance using a plane wave fit is visualized by colour shadings.
−25 −20 −15 −10 −5 0 5 10 15 20 25
u [cm/s]
1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010
p = 1580 d λ = 11*103 km expl. var.: 0.28 baro. mode: 13
Atlantic Ocean Indian Ocean
p = 740 d λ = 56*103 km expl. var.: 0.08 baro. mode: 1 Pacific Ocean
5°W
25°W 60°E 80°E 180°W 160°W 140°W 120°W
On interannual timescales, high-baroclinic mode waves, i.e. the EDJs, dominate in the Atlantic while in the Pacific the dominant signal is associated with low-baroclinic-
mode variability likely wind-generated. The Indian Ocean Argo float velocities are cha- racterized by merely incoherent signals.
Global deep equatorial flow
Fig.2: (a) Regression of SST, surface wind and rainfall (white contours, mm/d) on the harmonic fit of the ATL3 SST anoma- lies. (b) ATL3 SST anomaly (microwave op- timally interpolated SST, red dashed;
HadISST, red thin solid) with 1,670-d har- monic fit (red thick solid), surface geostro-
phic zonal velocity anomaly (Equator, 35°W–15°W; black thin solid) with 1,670-d harmonic fit (black thick solid), and 1,000-m zonal velocity (1°S–1°N, 35°W- 15°W; black dots with standard errors) with 1,670-d harmonic fit (black thick dashed).
93 94 95 96 97 98 99 00 01 02 03 04 05 06 07 08 09 10
−0.1 0 0.1
Zonal velocity (m/s)
b)
−0.5 0 0.5
SST anomaly (°C)
50°W 40°W 30°W 20°W 10°W 0° 10°E
10°S 0°
10°N
0.15
0.15 0.15 0.15
0.15 0.15 0.15
0.3
−0.15
−0.15
−0.15
− 0.15
−0.15 0
0
0
0
0
0
0
0.2 m/s a)
−0.4
−0.3
−0.2
−0.1 0 0.1 0.2 0.3 0.4
Anomalous westerlies along the Equator, convergent meridional wind anomalies par- ticularly in the western tropical Atlantic, and positive rainfall anomalies in a wide belt around the Equator are associated with positive SST anomalies. A 4.5-yr cycle is
also found in the surface geostrophic zonal velocity anomaly at the Equator as well as in the 1,000-m zonal velocity from Argo floats. Phases of eastward surface flow coincide with SST warm phases in the eas- tern equatorial Atlantic.
4.5-yr climate cycle
Vertically alternating deep zonal jets of short vertical wavelength with a period of about 4.5 yr and amplitudes of more than 10 cm/s are observed, in the deep Atlantic, to propagate their energy upwards, to- wards the surface. They are linked, at the sea surface, to equatorial zonal current an- omalies and eastern Atlantic temperature anomalies that have amplitudes of about 6
cm/s and 0.4°C, respectively, and are asso- ciated with distinct wind and rainfall pat- terns. Although deep jets are also observed in the Pacific and Indian oceans, only the Atlantic deep jets seem to oscillate on in- terannual timescales. The oscillatory beha- viour can be used to improve predictions of sea surface temperature (SST) in the tropi- cal Atlantic.
