Transcript Brazil Current transport

Intraseasonal to interannual variability of the
Brazil Current transport measured at 34.5°S
M. P. Chidichimo*1, A. R. Piola1, C. S. Meinen3,
E. Campos4, S. Garzoli3,5, S. Speich6, R. Perez3,5, S.
Dong3,5, R. Matano7, V. Combes7
1Consejo
Nacional de Investigaciones Científicas y Técnicas (CONICET),
Servicio de Hidrografía Naval, Universidad de Buenos Aires, Argentina
3NOAA/AOML, Physical Oceanography Division, Miami, Florida, USA
4Oceanographic Institute, University of Sao Paulo, Brazil
5CIMAS, University of Miami, Miami, Florida, USA
6ENS-Laboratoire de Météorologie Dynamique, Paris, France.
7Oregon State University, Corvallis, Oregon, USA
*[email protected]
Nov. 2015, Aquarius Meeting, Buenos Aires, Argentina
Introduction & Motivation
●The Brazil Current is a key element of the South Atlantic
circulation, as it advects warm water from subtropical to subpolar
regions carrying components of Atlantic Meridional Overturning
Circulation variability.
●In the South Atlantic, IES have been previously utilized to analize
the variability of the Brazil Malvinas Confluence
(Garzoli & Bianchi 1987; Garzoli & Garrafo 1989 ).
Mean Brazil Current transport above 800 m of ~10 Sv at 37.5S.
●Here: Continuous observations of the Brazil Current transport at
34.5°S since 2009.
SAMBA ARRAY
PIES/CPIES (NOAA, USP, UCP, UBR)
ADCP/BPR (USP)
Short Moorings (UCP)
Tall Moorings (UBO)
Bottom ADCP (UCP)
Thermistor mooring (UCP)
Figure: Courtesy Chris Meinen
Main Objective: Observe and understand the mechanisms that
control the mean and time-varying MOC in the South Atlantic
and the interocean exchanges.
SAM, Southwest Atlantic MOC project
Study western boundary
components of the MOC in
the South Atlantic.
Collaboration between US,
Brazil and Argentina.
Depth [km]
4 PIES (2009 – present)
PIs: C. Meinen, S. Garzoli,
R. Perez and S. Dong.
Funded by NOAA, US.
3 CPIES (2012 – present)
PI: E. Campos. Funded by
FAPESP, Brazil.
CPIES: Current and Pressure Equipped Inverted Echo Sounder
• tau -- round trip travel times of acoustic
pulses to sea surface and back.
• bottom pressure.
• bottom temperature.
• currents 50 m
above the seafloor.
tau combined with hydrography: estimates of full water column
T, S, density and geopotential anomaly profiles using GEM look
up tables (Meinen et al. 2012 and 2013).
• Baroclinic component: geostrophic velocities rel. to the bottom.
• Barotropic component: Time varying geostrophic velocities from
bottom pressure measurements.
Add time-mean bottom velocity from model.
Time-mean absolute velocity section from PIES
Southward flow of the
Brazil Current at the western
side of the array between sites
A (51.5°W) and B (49.5°W).
Array misses a portion of the
Brazil Current inshore of site
A.
Strongest velocity variability
in the upper 800-1000 m.
Time-mean bottom velocities
from high-resolution model
(Combes and Matano, 2014)
Adapted from Meinen et al. 2012
Salinity section at 34.5°S – SACW/AAIW interface?
Water masses
(Preu et al. 2012):
SACW: S > 35 psu
AAIW: S < 34.25 psu
UCDW:
27.75 < γn < 27.90
O2 < 4.5 mL L-1
NADW:
27.90 < γn < 28.10
S > 34.8 psu.
Brazil Current: from the surface to
SACW/AAIW interface at γn = 26.82 kg m-3
(same as Biló et al. 2014)
LCDW:
28.06 < γn < 28.20
S < 34.8 psu
AABW: θ < 0°C.
Brazil Current transport between 51.5°W (A)
and 49.5°W (B)
o
o
o
o
o
o
o
o
o
o
60 W 58 W 56 W 54 W 52 W 50 W 48 W 46 W 44 W 42 W
o
31 S
33oS
AA
o
35 S
A
B
o
37 S
o
39 S
Absolute (including AA): -9.5 ± 7.5 Sv
Baroclinic (including AA): -3.7 ± 5.8 Sv
Barotropic (including AA): -5.6 ± 4.2 Sv
 Absolute
transport is
mostly southward;
occasionally
northward.
 Large fluctuations
of 30 Sv in short
periods of 3 to 5
weeks.
Transport inshore 51.5°W (A)? – CTD
sections
o
o
o
o
o
o
o
o
o
o
60 W 58 W 56 W 54 W 52 W 50 W 48 W 46 W 44 W 42 W
o
31 S
33oS
o
35 S
A
B
C
D
o
37 S
o
39 S
 Transport inshore A:
-4.9 ± 2.4 Sv (baroclinic from
3 hydrographic sections).
Transport inshore 51.5°W (A)? – Models
Model time-mean absolute meridional velocity along 34.5°S
between the coast and site B.
BB
o
o
o
o
o
o
o
o
o
o
60 W 58 W 56 W 54 W 52 W 50 W 48 W 46 W 44 W 42 W
o
31 S
Inshore A
-3.8 ± 4.6 Sv
33oS
o
35 S
ROMS_AGRIF
(Combes and
Matano, 2014)
2000-2012
Inshore A
-3.0 ± 1.6 Sv
OFES
1980-2010
A
B
o
37 S
o
39 S
 Absolute transport
inshore A represents ~30%
of the mean transport.
 Transport A to B
overestimates
variability by ~ 15-20%
(compared with transport
from the coast to B)
Spectral analysis – Brazil Current transport
40 d
20 d
10 d
Absolute transport:
60% of the variance associated
with periods shorter than 80 days.
Peaks at 40, 20, and 10 days.
Baroclinic transport:
75% of the variance associated with
periods less than 120 days.
20,10 d
Barotropic transport:
65% of the variance associated
with periods shorter than 50 days.
Spectral analysis – geopotential anomaly Φ and
bottom pressure at 51.5°W (A) and 49.5°W (B)
(endpoints)
o
o
o
o
o
o
o
o
o
o
60 W 58 W 56 W 54 W 52 W 50 W 48 W 46 W 44 W 42 W
o
31 S
33oS
o
35 S
A
B
o
37 S
o
39 S
 Φ at B: more
variability at
longer periods (50
to 80 days)
compared to
Φ at A.
 Bottom pressure
at A and B: peaks
at 30, 20, 10
days.
Seasonal and interannual variability Brazil Current transport
Brazil Current:
strengthens on May and Dec;
weakens on Feb and July
No significant seasonal cycle
is found during 2009-2014.
Seasonal and interannual variability Brazil Current transport
Brazil Current:
strengthens on May and Dec;
weakens on Feb and July
No significant seasonal cycle
is found during 2009-2014.
Mean annual absolute
transport is remarkably
steady.
Observed basin-wide MOC and Brazil Current
transport at 34.5°S
Observed basin-wide MOC and Brazil Current
transport at 34.5°S
r = 0.22
Strength of basin-wide MOC and Brazil
Current not significantly correlated during
first year of continuous measurements.
Conclusions
● Absolute Brazil Current transport at 34.5°S is -13.5  0.8 Sv
with a temporal standard deviation of 6.3 Sv.
● Fluctuations with periods shorter than 80 days account for 60%
of the absolute transport variance.
● Baroclinic transport mostly contributes variability at periods less
than 100 days; barotropic transport contributes shorter-term
fluctuations with periods less than 30 days.
● Baroclinic transport variability accounts for the largest fraction
of the absolute transport variability (80%).
● No significant seasonal cycle is found during 2009-2014.
● Mean annual absolute transport is remarkably steady.
Conclusions
THANK YOU!
● Absolute Brazil Current transport at 34.5°S is -13.5  0.8 Sv
with a temporal standard deviation of 6.3 Sv.
● Fluctuations with periods shorter than 80 days account for 60%
of the absolute transport variance.
● Baroclinic transport mostly contributes variability at periods less
than 100 days; barotropic transport contributes shorter-term
fluctuations with periods less than 30 days.
● Baroclinic transport variability accounts for the largest fraction
of the absolute transport variability (80%).
● No significant seasonal cycle is found during 2009-2014.
● Mean annual absolute transport is remarkably steady.
ROMS AGRIF
o
o
o
o
o
o
o
o
o
o
60 W 58 W 56 W 54 W 52 W 50 W 48 W 46 W 44 W 42 W
o
31 S
33oS
o
35 S
o
37 S
o
39 S
A
B
o
o
o
o
o
o
o
o
o
o
60 W 58 W 56 W 54 W 52 W 50 W 48 W 46 W 44 W 42 W
o
31 S
33oS
o
35 S
o
37 S
o
39 S
A
B
Transport inshore 51.5°W? – high-res. model
ROMS_AGRIF (Combes and
Matano, 2014)
Parent grid: Spatial resolution
1/4˚ (~23 km near 34˚S).
Child grid: 82˚W a 41˚W; 64˚S20˚S, spatial resolution 1/12˚.
40 sigma levels in the vertical;
increased resolution near the
surface.
Transport inshore 51.5°W ? - Summary
Source
Meinen et al. (2013)
18 XBT sections -2.1/2.5
Baroclinic
OFES
-3.0/1.6
Absolute
NEMO
-4.6/3.3
Absolute
CTD SAMOC07
(Jul/12)
-3.4
Baroclinic
CTD SAMOC08
(Nov-Dec/12)
-4.1
CTD STSF2013
(Oct/13)
-7.6
Baroclinic
ROMS_AGRIF
(Combes and
Matano, 2014)
-3.8/4.6
Absolute
This study
Mean/st
d dev
(Sv)
Average
Mean/std
dev (Sv)
Author
-4.9/2.4
Notes
Baroclinic
o65
20 S
o
W
o
60 W
o
55 W
o
50 W
o
45 W
o
40 W
o
25 S
A095
30oS
o
35 S
A B C
A095
40oS
D
o
35 W
SSH from PIES and SSH from altimetry –Site A
Gridded SHH
Product
Low orrelations
(0.2-0.3) with
PIES at A
Transport inshore A? - SSH from PIES and
SSH from altimetry
60oW
58oW
56oW
54oW
52oW
50oW
A
B
o
31 S
33oS
o
35 S
o
37 S
o
39 S
48oW
Water masses - neutral density layers (γn, kg m-3 ) from PIES
Transport calculation
• Baroclinic component (relative geostrophic velocities)
Φ: geopotential anomaly
f: Coriolis parameter
L: distance between sites
• Barotropic component:
- Time varying absolute geostrophic velocities from bottom
pressure gauges.
- Add time-mean bottom velocity from model.
Mean absolute
meridional velocity
ROMS_AGRIF
ROMS AGRIF