Brazilian efforts towards a Lagrangean Brazilian efforts towards a Lagrangean description of surface currents and eddies description of surface currents and eddies

in the Southwestern Atlantic Oceanin the Southwestern Atlantic Ocean

Ronald Buss de SouzaRonald Buss de Souza11

João A. LorenzzettiJoão A. Lorenzzetti11

Maurício Magalhães MataMaurício Magalhães Mata22

Carlos A. E. GarciaCarlos A. E. Garcia22

Marcelo Sandin DouradoMarcelo Sandin Dourado11

11Instituto Nacional de Pesquisas Espaciais (INPE)Instituto Nacional de Pesquisas Espaciais (INPE)22Fundação Universidade Federal do Rio Grande (FURG)Fundação Universidade Federal do Rio Grande (FURG)

Overview of the presentationOverview of the presentation

• Currents and frontal systems;• Mesoscale features at the Brazil-Malvinas Confluence region;• The sinergy between buoy and satellite data;• Eddies observed in satellite images;• Eddies observed in buoy trajectories;• Eddy statistics;• Empirical relationships for the eddies;• The history of drifting buoy programmes in Brazil;• GTS drifting buoy data in the SW Atlantic Ocean;• Some results: COROAS; • Some results: PNBoia;• Some results: GOAL;• Conclusions.

Currents and frontal systemsCurrents and frontal systems

Antarctic Circumpolar Current;Antarctic Circumpolar Current;

South Atlantic Current; South Atlantic Current;

Malvinas Current;Malvinas Current;

Brazil Current;Brazil Current;

Brazilian Coastal Current;Brazilian Coastal Current;

Polar Front/Antarctic Convergence;Polar Front/Antarctic Convergence;

Subtropical Convergence.Subtropical Convergence.

Source: Peterson e Stramma (1991)

AVHRR SST image taken in February 1985

(Olson et al., 1988)

• The Brazil-Malvinas Confluence (BMC) is one of the most dynamically active regions of the World Ocean;

• Eddies play an important (but not well understood) role in the across-front changes of properties in the World Ocean;

• Many eddies are formed and shed every year from high amplitude meanders present in the BMC region;

• Olson et al. (1988) presented a classic figure showing a large warm core eddy shed from the Brazil Current towards the Subantarctic environment;

• The authors have also made the first Lagrangean descriptions of an eddy from the BC.

43oS

Mesoscale features at the BMC regionMesoscale features at the BMC region

• Legeckis and Gordon (1982)• Satellite data between 1975, 1976 e 1978;• Warm core eddies formed by the BC retraction at periods of a week;• Elliptical, ~180 km x 120 km;• Translations towards the south at 4 km/day to 35 km/day.

• Garzoli (1993)• Inverted echosounders used to observe eddies;• Diameters between 100 km and 150 km;

• Gordon (1989)• At least 6 eddies could be formed every year• They could be transported towards the eastern side of the South Atlantic gyre.

• Smythe-Wright et al. (1996), Hooker and Brown (1996), Garfield (1990), Olson (1991), Schmid et al. (1995), Podestá (1997), Souza (2000), Lentini et al. (2002), among others

Mesoscale features at the BMC region: eddiesMesoscale features at the BMC region: eddies

• Questions still to be addressed:

• How many eddies are being formed at the BMC region every year?• How long do they last?• Where are they being transported to?• How is their tri-dimensional structure?• What is their impact for the weather and climate of South America?• Many others...

• The search for answers from the Brazilian community:

• Establishment of continuous observing programmes;• Multi-disciplinary teams;• Description of the currents and eddies by using satellite and Lagrangean data.

Mesoscale features at the BMC region: eddiesMesoscale features at the BMC region: eddies

The sinergy between buoy and satellite data

Surface buoy trajectories tend to follow regions of maximum thermal gradients. Comparing the tracks with the images can be helpful to guess past behaviour of the frontal structures.

(Rd = (g Ho)1/2 / f )

Eddies observed in satellite images

Eddies observed in buoy trajectories

Relationship between Ro and the eddies diameter: Strutures ranging from non-linear to quasi-geostrofic or geostrofic ones.

(Ro = U / fL)

Relationship between Rd and the eddies diameter:D/Rd < 1: turbulent flowD/Rd >> 1: well defined eddy field

(Rd = (g Ho)1/2 / f )

+ BC +BCC +SAC

+ BC +BCC +SAC

Size and period statistics for the eddies found in the buoys’ trajectories class 1 eddies class 2 eddies min. max. mean std. min. max. mean std.

period (day) 0.1 4.9 2.1 1.5 5.2 42.2 16.7 9.4

perimeter (km) 1.3 99.9 36.3 31.7 103.8 1087.7 442.2 271.8

diameter (km) 0.4 31.8 11.5 10.1 33.0 346.2 140.7 86.5

Size statistics for the eddies found in the AVHRR images cold core eddies warm core eddies

size (km) min. max. mean std. min. max. mean std. major axis 20 284 101 61 16 244 83 64

minor axis 12 252 63 46 8 144 48 43

diameter 18 262 82 51 16 182 65 51

perimeter 57 824 269 163 50 604 217 165

Eddy statistics

Linear regression (Y = ax + b) between the rotational period (TR), tangential velocity (VT), perimeter (P) and diameter (D) for the eddies in class 2.

dependent variable (Y)

independent variable

(x)

slope (a)

intercept (b)

N

r

TR (day) P (km) 0.0233 6.717 41 0.65 VT (cm/s) P (km) 0.0219 23.24 41 0.39 TR (day) D (km) 0.0732 6.717 41 0.65

VT (cm/s) D (km) 0.0688 23.24 41 0.39

Comparison between measured (meas.) and estimated (est.) VT and TR of the surface eddies in the Southwestern Atlantic Ocean. Authors: (1) Chassignet et al. (1990); (2)

Lorenzzetti et al. (1994); (3) Lorenzzetti and Kampel (1998)

author region / current

D (km)

VT (meas.)

(cm/s) VT (est.) (cm/s)

% error

TR (meas.) (day)

TR (est.) (day)

% error

1 BMC 270 53 42 21.1 ----- 26.5 ----- 1 BMC 110 36 31 14.4 ----- 14.8 ----- 1 BMC 130 77 32 58.2 ----- 16.2 ----- 2 BC 70 24 28 17.1 6 11.8 96.7 2 BC 275 71 42 40.6 14 26.8 91.4 3 BC 50 40 27 33.2 ----- 10.3 -----

Empirical relationships for the eddies

• The development of an in-house technology came together with the COROAS project in 1993;

• Projects PNBoia and GOAL came later...

• Project MEDICA had used past technologies but launched a sole LCD in 1994 at the Polar Front.

The history of drifting buoy programmes in BrazilThe history of drifting buoy programmes in Brazil

A Low Cost Drifter being launched at the Brazil-Malvinas Confluence region from the N.Ap.Oc. Ary Rongel in November 2003. Four of such instruments were deployed as part of GOAL project.

The history of drifting buoy programmes in BrazilThe history of drifting buoy programmes in Brazil

The history of drifting buoy programmes in BrazilThe history of drifting buoy programmes in Brazil

The history of drifting buoy programmes in BrazilThe history of drifting buoy programmes in Brazil

GTS data from the Atlantic Oceanographic and Meteorological Laboratory (AOML), Drifting Buoy Data Assembly Center, NOAA.http://www.aoml.noaa.gov/phod/dac/dac.html

GTS drifting buoy data in the SW Atlantic OceanGTS drifting buoy data in the SW Atlantic Ocean

Source: Souza (2000)

Some results: COROASSome results: COROAS

Some results: COROASSome results: COROAS

BC and SAC velocity, kinetic energy and temperature statistics (a) Brazil Current

buoy velocity kinetic energy temperature

speed (cm/s)

direction (degrees)

MKE (cm2/s2)

EKE (cm2/s2)

TKE (cm2/s2)

%EKE/TKE mean (oC) std.dev (oC)

3179 12.2 252.0 75 1080 1155 93.5 24.17 0.96 3181 53.6 212.1 1436 3576 5012 71.3 24.99 0.73 3182 34.0 181.7 577 371 948 39.1 26.27 0.13 3185 12.8 237.1 82 901 983 91.6 19.17 0.82 3187 47.4 214.4 1121 899 2021 44.5 23.52 0.76 3188 36.7 212.1 674 705 1379 51.1 24.52 0.43 3189 31.6 205.3 499 1009 1508 66.9 23.85 0.72 3190 49.2 212.7 1209 1530 2739 55.8 23.97 0.86 3191 46.3 214.4 1072 2132 3204 66.5 24.56 0.66 3192 50.3 215.3 1265 743 2008 37.0 24.00 0.99 mean 37.4 215.7 801 1294 2097 61.7 23.90 0.71

std.dev. 15.0 18.5 490 937 1266 20.0 1.83 0.26

(b) South Atlantic Current

buoy velocity kinetic energy temperature speed

(cm/s) direction (degrees)

MKE (cm2/s2)

EKE (cm2/s2)

TKE (cm2/s2 )

%EKE/TKE mean (oC) std.dev. (oC)

3182 4.2 123.0 9 1769 1778 99.5 22.56 1.61 3185 6.7 112.6 23 2509 2532 99.1 21.69 1.21 3187 16.6 74.6 138 3413 3551 96.1 17.66 1.32 3189 16.3 69.6 132 5769 5901 97.8 18.61 1.60 3190 4.2 111.2 9 2312 2321 99.6 19.08 3.19 3191 7.1 88.1 25 3876 3901 99.3 18.75 2.51 3192 12.5 83.5 78 3225 3304 97.6 18.61 2.13 mean 9.7 94.7 59 3268 3327 98.4 19.57 1.94

std.dev. 5.4 20.8 57 1315 1357 1.3 1.82 0.71

Some results: COROASSome results: COROAS

BCC velocity, kinetic energy and temperature statistics

LCD velocity kinetic energy Temperature speed

(cm/s) direction (degrees)

MKE (cm2/s2)

EKE (cm2/s2)

TKE (cm2/s2)

%EKE/TKE average (oC)

std.dev. (oC)

3178 10 35 50 2384 2434 97.9 23.34 1.08 3179 6 15 17 771 788 97.8 19.82 1.43 3180 17 31 145 2089 2234 93.5 17.63 0.82

average 11 27 71 1748 1819 96.4 20.26 1.11 std.dev. 6 11 66 859 898 2.5 2.88 0.31

The Brazilian Coastal CurrentThe Brazilian Coastal Current

BCC (a,b) and BC (c,d) extreme positions between 1982 and 1995 from MCSST images. Source: Souza e Robinson (2004).

Sardinella brasiliensis total catch at the southern coast of Brazil between 1980 and 1990. Source: Sunyé and Servain (1998).

The Brazilian Coastal CurrentThe Brazilian Coastal Current

Source: Assireu (2003)

Some results: PNBoiaSome results: PNBoia

AMSR-E SST image of 15 November 2002.

Some results: GOAL, November 2002 eddy ED1Some results: GOAL, November 2002 eddy ED1

XBT profiles inside ED1 in November 2002.

AB1,B2

CD

Temporal evolution of ED1 between September and November 2003 at the BMC region recovered from AMSR-E images.

Some results: GOAL, November 2002 eddy ED1Some results: GOAL, November 2002 eddy ED1

AVHRR SST image in 5 November 2003.

Some results: GOAL, November 2003 eddiesSome results: GOAL, November 2003 eddies

Buoy trajectories and positional time series from November 2003 to August 2004.

AVHRR SST image of 5 November 2003.

Buoy trajectories and positional time series between 11 and 21 November 2003.

Some results: GOAL, Brazil Current return flowSome results: GOAL, Brazil Current return flow

buoy

Juliano day

start

Julian day

end

zonal vel.

(cm/s)

meridional vel.

(cm/s)

mean vel.

(cm/s)

direction (degrees)

09002 315.7 326.8 -5 -58 58 265 09003 315.7 324.8 -29 -66 72 262 32453 315.1 325.8 -16 -51 53 253

32456 315.1 324.8 -6 -43 44 261 mean 315.4 325.6 -14 -55 57 260

std.dev. 0.35 0.96 11.2 9.8 11.7 5.1

Comparison of the monthly mean translational velocities found for ED1 from the visual interpretation and the MCC method.

visual interpretation

MCC method

vel dir vel dir vel dir

month mean (km/day)

(cm/s)

mean (degrees)

mean (km/day)

(cm/s)

mean (degrees)

std.dev. (km/day)

std.dev. (degrees)

difference (km/day)

September 13.0 226 5.2 243 1.6 31 7.8 October 6.4 258 6.0 222 2.8 68 0.4

November 3.9 260 6.2 273 2.4 52 2.3 December 7.0 347 5.6 328 3.0 19 1.4

mean 7.6 8.8 273 5.7 6.6 266 2.4 42 1.9 std.dev. 3.9 52 0.4 46 0.6 22 -

Some results: GOAL, Brazil Current return flowSome results: GOAL, Brazil Current return flow

XBT profile along the N.Ap.Oc. Ary Rongel route in November 2003.

Some results: GOAL, XBT profilesSome results: GOAL, XBT profiles

ConclusionsConclusions

• Brazil is continuously launching drifting buoys into the ocean since the COROAS project as part of WOCE;

• Recent programmes such as the PNBoia and ProAntar/GOAL hae taken the opportunity to widening the sampling of waters in the South Atlantic Oean;

• MMultidisciplinary projects using Lagrangian data in conjuntion with satellite data are contributing to a better understanding of the currents and associated features in the Southwestern Atlantic Ocean;

• Some areas such as the continental shelf off South America are still very poorly sampled.