Interannual to Decadal Changes in the Western Boundary Circulation in the Atlantic at 11°S
Rebecca Hummels
*,1, Peter Brandt
1, Marcus Dengler
1, Jürgen Fischer
1, Moacyr Araujo
2, Doris Veleda
2and Jonathan Durgadoo
1Abstract ID: PO14E-2859
(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.
11°S Section 5°S Section
Fig.1: Circulation sketch of the western tropical Atlantic (from Dengler et al., 2004). Warm and cold water routes of the AMOC are indicated in red and blue. The sections at 5°S and 11°S are marked in black and the mooring array is indicated with green circles.
Introduction
The tropical Atlantic plays an important role for climate variability in the Atlantic region. A key region within the tropical Atlantic is the western boundary current system, where the variability of the North Brazil Under- current (NBUC) and the Deep Western Boundary Cur- rent (DWBC) exhibit variations of the meridional over- turning ciculation (AMOC, Fig. 1) and the subtropical cells (STCs).
Measurement program
The western boundary current system off the coast of Brazil at 11°S (see Fig. 1) is investigated with a moo- ring array and ship based observations including direct current as well as hydrographic measurements. The observational campaign aims at assessing the variabi- lity of the western boundary current system on time scales from intraseasonal to decadal. Three research cruises in 2013, 2014 and 2015 delivered first insights into changes in the currents and water mass properties nowadays compared to similar observations taken during the period of 2000-2004. In addition, the data of two mooring periods was successfully retrieved with an overall instrument performance of over 90%. The next cruises to maintain the moored array are planned for 2016 and 2018.
no significant transport changes between the observational periods; DWBC eddies are still present
interannual to decadal variability agrees among moored observations, numerical simulations and geostrophic estimates
positive (negative) decadal salinity trend within the central water (DWBC layer)
Outlook: relate assessed variability patterns at the western boundary at 11°S to AMOC variablity in remote regions of the Atlantic
Summary Velocity and transport variability
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)
NBUC
DWBC
Fig. 2: Average ship section of alongshore velocity with mooring array design (a), and NBUC (b) and DWBC (c) transport time series obtained from morred observations. Red and black dotted lines in a) mark boxes used for transport calculations. Red dots in b) and c) indicate transport estimates from ship sections.
The high intraseasonal variability below 1500m depth was previously associated with the passage of deep eddies, which accomplish the transport within the DWBC layer instead of a laminar flow (Fig. 1, 2). The cha- racteristics of the intraseasonal variability within the DWBC between the two observational periods (2000/2004, 2013/2015) are similar and the deep eddies are still present (Fig. 2c). On longer timescales the transport variability of both NBUC and DWBC is reduced and on average no significant changes between the two observational periods are yet apparent (NBUC: 25.8±1.2 Sv (2000-2004) vs. 25.5±1.3 Sv (2013-2015);
DWBC: -17±1.6 Sv (2000-2004) vs. -20.5±2.7 Sv (2013-2015) from the moored observations).
Geostrophic velocities
To construct a geostro- phic transport timese- ries overlapping with the moored observa-
tions, geostrophic transports from availa-
ble hydrographic data will be obtainted.
Fig.5 Sections of alongshore ve- locity a) observed with ADCPs and b)-e) geostrophic velocities calcula- ted from CTD profiles for different referencing techniques.
0 0
0
0
0 0
Geo. vel. reference: v=0@27.7 kg/m NBUC 19.93 Sv
DWBC −4.81 Sv
−10 0
0
0 0 0
Geo. vel. reference: vADCP@250m NBUC 19 Sv
DWBC −24.11 Sv
−10
0
0
0
Geo. vel. reference: vADCP@500m NBUC 18.69 Sv
DWBC −21.66 Sv
0
0 0
0
NBUC 17.68 Sv
DWBC −21.44 Sv
ADCP
36°W 35°W 34°W 33°W 32°W36°W 35°W 34°W 33°W 32°W
36°W 35°W 34°W 33°W 32°W36°W 35°W 34°W 33°W 32°W
36°W 35°W 34°W 33°W 32°W
−10
−10 0
0
0
0 0
0 0 0 0
0 0
0 0
0 0
0
8 section average vel. ADCP NBUC 19.94 Sv
DWBC −22.52 Sv 0
500 1000 1500 2000 2500 3000 3500 4000 4500 5000
(cm/s)
−50
−40
−30
−20
−10 0 10 20 30 40 50
depth (m)
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
depth (m)
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
depth (m)
Geo. vel. reference: vADCP@1200m a)
b) c)
d) e)
Longterm NBUC variability
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
Mooring
Fig.4 NBUC transport ano-
malies. 15, 16 and 25 Sv are subtracted for the numerical s i m u l a t i o n INALT01, the g e o s t r o p h i c transports of Zhang et al.
(2011) and the moored obser- vations respec- tively.
Interannual variability agrees among moored estimates and INALT01. Decadal variability (dashed) is similar in INALT01 and the geostrophic estimates.
Fig. 3: Time series of salinity ano- malies on neutral density surfaces (a, b) and inferred salinity and oxygen trends as a function of depth (c, d). For salinity all available profiles from ship cruises, the World Ocean Atlas, Argo and the Brazilian Navy in a box between 40°W and 30°W and 12°S and 8°S are com- bined, for oxygen only data of the ship cruises is used.
The observed decadal salinity increase in the central water range (100-600m) is consistent with previous esti- mates (Biastoch et al. 2009) as well as the interannual variability of the salinity anomalies (Fig. 3a, Kolodziej- czyk et al. 2014).
The inferred vertical structure of salinity and oxygen trends (Fig. 3c, d) can be related to changes in water mass formation regions as well as circulation changes in remote regions of the Atlantic.
Salinity and oxygen changes
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)
−0.05 0 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)
