Variability of the Boundary Current systems and AMOC at 11°S
Rebecca Hummels
1, Peter Brandt
1, Marcus Dengler
1, Jürgen Fischer
1,
Moacyr Araujo
2, Doris Veleda
2, Jonathan Durgadoo
1, Josefine Herrford
1and Robert Kopte
1(1) Geomar - Helmholtz Centre for Ocean Research, Kiel, Germany (2) DOCEAN - Department of Oceanography UFPE, Recife, Brazil
*contact: rhummels@geomar.de
REGIONAL ATLANTIC CIRCULATION AND GLOBAL CHANGE
RACE!
References
Biastoch A., C. W. Boning, F. U. Schwarzkopf, and J. R. E. Lutjeharms (2009), Increase in Agulhas leakage due to poleward shift of Southern Hemisphere westerlies, Nature, 462(7272), 495-498, doi:10.1038/nature08519 Dengler M., F. A. Schott, C. Eden, P. Brandt, J. Fischer, and R. J. Zantopp (2004), Break-up of the Atlantic deep western boundary current into eddies at 8 degrees S, Nature, 432(7020), 1018-1020, doi: 10.1038/nature03134
Hummels R., P. Brandt, M. Dengler, J. Fischer, M. Araujo, D. Veldeda, and. J. Durgadoo (2015), Interannual to decadal changes in the western boundary circulation in the Atlantic at 11°S, Geophys. Res. Lett., 42, 7615–7622, doi:10.1002/2015GL065254.
Kolodziejczyk, N., G. Reverdin, F. Gaillard, and A. Lazar (2014), Low-frequency thermohaline variability in the Subtropical South Atlantic pycnocline during 2002–2013, Geophysical Research Letters, 41(18), 2014GL061160.
Schott, F., J. McCreary, and G. Johnson (2004), Shallow overturning circulations of the tropical-subtropical oceans, Earth‘s Climate: The ocean atmosphere interaction, Geopysical Monograph Series 147, AGU, Washington D.C.
Zhang, D., R. Msadek, M. J. McPhaden, and T. Delworth (2011), Multidecadal variability of the North Brazil Current and its connection to the Atlantic meridional overturning circulation, JGR: Oceans, 116(C4), C04012.
WBCS: no significant transport changes between 2000-2004 and 2013-2015; DWBC eddies still present;
interannual to decadal variability agrees among moored observations, numerical simulations and geostrophic estimates
EBCS: weak southward AC transport; 120-day, semian- nual and annual oscillations dominate variability
Outlook: relate assessed variability patterns of the WBCS, EBCS and the AMOC at 11°S to AMOC variabli- ty in remote regions of the Atlantic
Summary
11°S Section 5°S Section
Fig.1: Circulation scheme of the western tropical Atlantic (Dengler et al., 2004).
Warm- (Cold) water routes of the AMOC are indicated in red (blue), hydrographic sections in black and mooring positions as green circ- les.
Introduction
The tropical Atlantic plays an important role for climate variability in the Atlantic region. The western boundary current system serves as a key region, where the variability of the North Brazil Undercurrent (NBUC) and the Deep Western Boundary Current (DWBC) exhibit variations of the AMOC (Fig. 1) and the Subtropical Cells (STCs, Fig. 2).
Measurement program
The boundary current systems at 11°S off the coast of Brazil and Angola are investigated with mooring arrays and ship based observations including direct current as well as hydrographic measurements. Three research cruises in 2013, 2014 and 2015 and the retrieval of the moored observations delivered first insights into the variability of the currents and water mass properties. The observational campaign aims at assessing the vari- ability of the boundary current systems on various time scales from intraseasonal to interannual, whereas at the western boundary even decadal variability is investigated comparing the new data set to similar observa- tions during the period 2000-2004. In addition, bottom pressure sensors are installed on the eastern and western side of the basin at 300m and 500m in order to estimate the interior mid-ocean transport.
Fig.2: Circulation scheme depicting the main current branches of the Subtropical Cells in the Atlantic (Schott et al., 2004). Subduction areas are shaded in blue, upwelling regions in light green. The numbers represent transport estimates.
Brazil Angola
Western Boundary Current (WBC) System
Depth (m)
24.5 24.5
26.8
27.7
28.025
28.085 28.135
alongshore velocity
at 11°S (8 section mean)
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
(cm/s)
−50
−40
−30
−20
−10 0 10 20 30 40 50
36°W 35.5°W 35°W 34.5°W
K1
K2
K3
K4 a)
2014 2015
2014 2015
2000 2001 2002 2003 2004
10 20 30 40
Transport (Sv)
NBUC transport at 11°S
b)
2000 2001 2002 2003 2004
−60
−40
−20 0 20
Transport (Sv)
NADW transport at 11°S
c)
Water mass properties
NBUC
DWBC
Fig. 3: Average ship section of alongshore velocity off the coast of Brazil with mooring array design (a), and NBUC (b) and DWBC (c) trans- port time series obtained from morred observations. Red and black dotted lines in a) mark boxes used for trans- port calculations. Red dots in b) and c) indicate transport estimates from ship sections.
(Sv) 2000-2004 2013-2015 NBUC 25.8±1.2 25.5±1.3 DWBC -17±1.6 -20.5±2.7
1960 1965 1970 1975 1980 1985 1990 1995 2000 2005
−8
−6
−4
−2 0 2 4 6 8
Transport anomalies (Sv)
Zhang et al. 2011 INALT01 until 33°W Mooring NBUC Box
Fig. 4: NBUC transport anomalies, where 15, 16 and 25 Sv were subtracted (INALT01, geostrophic transports and moored obser- vations).
On intraseasonal timescales the Deep Eddies, which have been shown to accomp- lish the transport of NADW instead of a laminar flow and were predicted to disap- pear with a weakening AMOC (Dengler et al. 2004), are still present with similar characteristics (Fig. 1,3).
On longer timescales the variability of the NBUC and DWBC is reduced compared to intraseasonal timescales.
On average moored observations do not show significant changes between the two observational periods (see table).
Interannual NBUC variability as assessed from moored observations between
2000-2004 is consistently found in the output of a forced ocean model (INALT01).
Decadal variability is similar in INALT01 and geostrophic transport estimates from Zhang et al. (2011).
The observed decadal salinity increase in the central water range (100-600m) is consistent with previous estimates (Biastoch et al. 2009) as well as the interan- nual variability of salinity anomalies (Fig. 5a, Kolodzie- jczyk et al. 2014).
The inferred vertical structure of salinity and oxygen trends (Fig.5c,d) can be related to changes in water mass formation regions as well as circulation changes in remote regions of the Altanic.
−0.050 0.05 0.15 0
500
1000
1500
2000
2500
3000
3500
1/decade
Depth (m)
23.82
27.24
27.61
27.86
27.98
28.03
28.06
28.09 γ n (kg/m³) c)
−10 0 10
0
500
1000
1500
2000
2500
3000
3500
Depth (m)
μmol kg−1/decade
23.82
27.24
27.61
27.86
27.98
28.03
28.06
28.09 γ n (kg/m³) d)
2000 2002 2004 2006 2008 2010 2012 2014
−0.2 0 0.2
γ = 26.3 (kg/m³) n
2000 2002 2004 2006 2008 2010 2012 2014
−0.02 0
0.02 γ = 28.08 (kg/m³) n a)
b)
Fig. 5: Timeseries of salinity anomalies on neutral density surfaces (a,b) and the resulting salinity as well as oxygen trends (c,d).
Future work: AMOC estimate at 11°S
Construction of a transport time
series of the AMOC at 11°S in order to analyze the mean, and seasonal as well as interannual variability:
TAMOC(t)=TMO(t)+TEK(t)+TWBC(t)+TEBC(t)
Ekmantransport TEK:
Comparison of different wind pro- ducts and observations
WBC Transport TWBC:
Transport estimates using the
moored and shipboard observations as well as a geostrophic approach
EBC Transport TEBC:
Transport estimates using moored and shipboard observations
Mid Ocean transport TMO:
Determination of an interior geostro- phic transport time series for the
upper 1000m at 11°S using
Fig. 8: Section of bathymetry at 11°S with colors depicting the different components of the AMOC
Sea surface height from satellite altimetry or PIES installed on
both sides of the Atlantic basin
Geostrophic transports from Argo and ship based hydrography
Meridional velocities at 1000m depth from Argo floats
Eastern Boundary Current (EBC) System
12.8°E 13°E 13.2°E 13.4°E
−5 0
0 5
5 5
0m 100
200 300 400
500
600 12.8°E 13°E 13.2°E 13.4°E
−25 −15 −5
0
0 5
15 25
15 25
−25
−20
−15
−10
−5 0 cm/s 5
10 15 20 25
ADCP shield Flotec ADCP
Fig. 6: Alongshore velocity off the coast of Angola obtained during two ship sections with moorings, black dashed box used for transports (a,b). Time series (c) and average (d) of moored alongshore velocity.
Alongshore velocities are highly variable at the eastern boundary and dominated by alternating periods of south- ward/northward flow with a duration of several months (Fig. 6a,b,c).
On average a weak mean southward flow is observed ex- tending to about 200m depth with maximum southward flow of 5-8 cm/s at 50m depth (Fig. 6d).
Seasonal variability is dominated by 120-day, semi-annual and annual oscillations.
The Angola Current (AC) transport derived from the combi- nation of moored and shipboard observations shows a
mean of -0.32±0.05 Sv (Fig. 7) and its semi-annual cycle is quasi-synchronized with semi-annual coastal Kelvin waves.
2013 2014 2015
A S O N D J F M A M J J A S O N D J F M A M J J A S O 0m
100 200 300 400
(cm/s)
−40
−20 0 20 40
−10 0 10(cm/s) 0m 100 200 300 400
A S O N D J F M A M J J A S O N D J F M A M J J A S O
−2
−1 0 (Sv)
2013 2014 2015
AC transport Semiannual+annual harm. Ship sections Fig. 7: AC Trans- port time series.
Superposition of se- miannual and annual harmonic in red, esti- mates from ship sec- tions as blue circles.
a) b)
c) d)
November 2015 July 2013
