We have assembled a new North Atlantic climatology of near-surface velocity and eddy kinetic energy (EKE) based exclusively on 15-m

drogued surface drifter trajectories measured between 1990 and 2000. Our objective is to define the state of the North Atlantic surface

circulation during the 1990’s and provide a reference point for both synoptic circulation studies (e.g. the WOCE Atlantic Circulation and

Climate Experiment) and studies of circulation variability associated with inter-decadal climate variability. In this poster we present

decadal-mean circulation statistics computed on a one-degree grid and compare these results with satellite altimetry measurements

and with previous drifter-based studies in the North Atlantic.

The satellite-tracked surface drifters

used in this study are similar in construc-

tion to the WOCE/Tropical Ocean-Global

Atmosphere (TOGA) Lagrangian drifter

described by Sybrandy and Niiler (1991).

All drifters were tracked using ARGOS and

were fitted with a submerged flexible

drogue which hung suspended at a central

depth of 15 m beneath a buoyant surface

float [Figure 1]. Approximately 1500

individual drifter trajectories totaling

nearly 300,000 drifter-days of position and

velocity information were acquired from

the archives of the Global Drifter Data

Assembly Center at NOAA/AOML in

Miami, Florida, U.S.A. Initial processing of

the data at AOML, including quality

control and temporal interpolation, is

described in detail in Hansen and Herman

(1989) and Hansen and Poulain (1996).

Drifter trajectories were truncated when

drogues detached as indicated by an

onboard submergence sensor or strain

gauge. The final data product consists of 6-

hourly interpolated position, velocity, and

surface temperature measurements.

The drifter trajectories used in this

analysis are shown in Figure 2. The

population of drifters in the North Atlantic

increased during the 1990’s and does not

exhibit any particular seasonal bias

[Figure 3]. The regional distribution of drifter trajectories varies signifi-

cantly throughout the decade [Figure 4]. Note the shift in observational

emphasis from the Gulf Stream region (1990-1992) to the eastern subtropi-

cal gyre (1992-1995) to the subpolar gyre (1995-1998) and finally to the

western tropical Atlantic and Caribbean Sea (1997-1999).

A summary of the fastest- and slowest-moving drifters is shown in Figure

5. The fastest drifter motions were found near the equator, along the

western boundary, in the eastward Gulf Stream extension, on the eastern

and western coasts of Greenland, and in a narrow eastward band corre-

sponding to the Azores Current. The greatest number of slow-moving

drifters were found in the eastern subtropical Atlantic.

1000˚W 900˚W 800˚W 700˚W 600˚W 500˚W 400˚W 300˚W 200˚W 100˚W 100˚E

10˚S

10˚N

20˚N

30˚N

40˚N

50˚N

60˚N

70˚N

All Trajectories 1990-2000

0˚

0˚

Fabric Drogue

Surface Float

Subsurface Float

Pho

togr

aph

cour

tesy

of M

ark

Sw

enso

n, N

OA

A/A

OM

L

FIGURE 1: A WOCE/TOGA

Lagrangian drifter on the surface

shortly after being deployed. The

fabric drogue is weighted and will

quickly sink to a vertical position

beneath the surface float.

FIGURE 2: A

composite dia-

gram of all

drifter trajecto-

ries used in the

present analy-

sis. All trajec-

tory segments

shown were

measured be-

tween January

1990 and June

1999. Note the

very poorly

sampled region

in the south-

eastern sub-

tropical gyre.

1990 1995 2000

Year

0

50

100

150

200

Nu

mb

er o

f O

bse

rvat

ion

s (1

000'

s)

J F M A M J J A S O N D

Month

a b

Speed > 40 cm/s

80W 60W 40W 20W 0

Speed < 10 cm/s

80W 60W 40W 20W 0

0

20N

40N

60N

a b

Overview

FIGURE 3: Temporal distribu-

tion of drifter data presented

in histogram form. The verti-

cal axis indicates the number

of 6-hourly measurements of

position and velocity. (a) Dis-

tribution by year from 1990

onwards (through June 1999). (b) Distribution by month.

FIGURE 5: A summary of the fastest and slowest drifters in the present database. Only

trajectory segments meeting the specified speed criterion for a contiguous 36-hour period

are plotted. (a) Fast drifters, with speeds exceeding 40 cm/s. The fastest drifters were found

near the equator, along the western boundary, in the eastward Gulf Stream extension, on

the eastern and western coasts of Greenland, and in a narrow eastward band correspond-

ing to the Azores Current. (b) Slow drifters, with speeds less than 10 cm/s. The greatest

number of slow-moving drifters were found in the eastern subtropical Atlantic.

Using historical hydrographic observations Curry and McCartney (2000)

describe a relationship between baroclinic ocean transport in the

subtropical gyre and the phase of the atmospheric North Atlantic Oscilla-

tion (NAO). We now consider whether the present drifter data, in combina-

tion with previous drifter observations, can be used to directly measure

interdecadal changes in surface circulation strength or character.

Richardson (1983) synthesized North Atlantic surface drifter measure-

ments during the period 1977-1980. This time interval coincides with a

period during which the NAO index (Hurrell, 1995) was relatively low

[Figure 10]. In contrast, the first half of the 1990’s were characterized by

particularly large values of the NAO index. Large values of the NAO index

correspond to stronger westerly winds and tend to result in a more

northerly Gulf Stream position (Taylor and Stephens, 1998). We hypoth-

esize that variations in the baroclinic transport of the subtropical gyre/Gulf

Stream system associated with these NAO index extrema should result in

observable changes in the near surface circulation.

1998

Gulf Stream

1Newfoundland

Basin

3

Gulf StreamExtension

2

North AtlanticCurrent

4

70W 50W 30W

40N

50N

0

500

1000

1500

2000

2500

Gulf Stream Gulf StreamExtension

NewfoundlandBasin

North AtlanticCurrent

1990-2000 (This study)1977-1980 (Richardson, 1983)

EKE

(cm

2/s

2)

1 32 4

AcknowledgementsEddy kinetic energy derived from TOPEX and ERS altimeters was provided by Nicolas Ducet of

CLS Space Oceanography Division, France. Processing of the raw drifter data was performed at the

Global Drifter Data Assembly Center at NOAA/AOML under the direction of Mark Swenson and

Mayra Pazos. This work was supported by the National Oceanic and Atmospheric Administration as

part of a cooperative project with Dr. Robert Cheney of the NOAA Laboratory for Satellite Altimetry.

ReferencesHansen, D.V. and A. Herman, Temporal sampling requirements surface drifter buoys in the tropical Pacific, J. Atmos.

Ocean. Tech., 6, 599-607, 1989.Hansen, D.V. and P.-M. Poulain, Quality control and interpolations of WOCE/TOGA drifter data, J. Atmos. Ocean.

Tech., 13, 900-909, 1996.Hurrell, J. W., Decadal trends in the North Atlantic Oscillation: Regional temperatures and precipitation. Science,

269, 676-679, 1995.Curry, R.G. and M.S. McCartney, Ocean Gyre Circulation Changes Associated with the North Atlantic Oscillation,

submitted to J. Phys. Oceanogr., 2000.Richardson, P. L., Eddy kinetic energy in the North Atlantic from surface drifters, J. Geophys. Res., 88, 4355-4367, 1983.Sybrandy, A. L. and P. P. Niiler, WOCE/TOGA Lagrangian drifter construction manual, SIO Ref. 91/6, WOCE Rep. 63,

58 pp., Scripps Institution of Oceanography, La Jolla, Calif., 1991.Taylor, A. H., and J. A. Stephens, The North Atlantic Oscillation and the latitude of the Gulf Stream, Tellus, 50A, 134-142, 1998.

As additional drifter data from the 1990’s become available (or are made

known to us by others) we will continue to update and improve this

climatology. We are presently extending our study of the drifter-derived

quasi-Eulerian velocity and EKE fields to include joint analysis of contem-

porary hydrographic data, TOPEX and ERS altimetry, and surface wind

products. Drifter-derived surface velocity and EKE data on one- and two-

degree grids are available for general use by the oceanographic commu-

nity. Please contact the lead author for details (dfratantoni@whoi.edu).

FIGURE 4: Temporal and spatial distribution of drifter

trajectories for each year from 1990-1999. Note the shift

in observational emphasis from the Gulf Stream region

(1990-1992) to the eastern subtropical gyre (1992-1995)

to the subpolar gyre (1995-1998) and finally to the west-

ern tropical Atlantic and Caribbean Sea (1997-1999).

We compared the Richardson (1983) quasi-Eulerian circulation statistics

with those resulting from our 1990-2000 analysis. There are significant

differences in both data volume and spatial distribution between the two

climatologies (the 1990-2000 climatology contains almost an order of

magnitude more data than the 1977-1980 analysis). We tried to minimize

the importance of these differences by concentrating our comparisons in a

region encompassing the Gulf Stream and the North Atlantic Current,

features that are reasonable well sampled in both analyses. In addition, we

focused our comparisons on EKE rather than on the mean velocity field. As

found by Richardson (1983), EKE exceeds the energy of the mean circula-

tion over most of the North Atlantic by a factor of about 10-20. This strong

variability makes it difficult to accurately resolve the mean circulation

without enormous quantities of data. The present 1990-2000 climatology is

sufficiently data-dense to compute meaningful means over large areas —

the 1977-1980 dataset is not.

-10

0

10

20

30

40

50

U (

cm/s

)

0

500

1000

1500

2000

2500

3000

EKE

(cm

2/s

2)

30 35 40 45 Latitude

30 35 40 45 Latitude

1990-2000

1977-1980

55W65W

55W65W

1990-2000

1977-1980

1990-2000

1977-1980

Gulf Stream Sections at 65W and 55W

We computed EKE within four spatial subregions [Figure 11] using both

climatologies. In the Gulf Stream, Gulf Stream Extension, and Newfound-

land Basin subregions we find an increase in EKE in the 1990-2000

climatology relative to the 1977-1980 measurements [Figure 12]. However,

within the computed 90% confidence limits there has been no significant

change in the regionally-averaged EKE over the last 20 years. Note that the

Richardson (1983) climatology includes both drogued and undrogued

drifter trajectories. This suggests that the 1977-1980 EKE values could be

overestimates of the actual EKE, particularly in the Gulf Stream region

where large wind stress and strong synoptic atmospheric variability may

account for much of the motion of an undrogued drifter.

We compared sections of zonal velocity and EKE across the Gulf Stream at

55W and 65W [Figure 13] and found little difference in the structure of the

zonal Gulf Stream jet. There is a slight northward shift in the location of the

mean jet and the associated EKE maximum at 55W in the 1990-2000

climatology relative to the 1977-1980 realization. While the sense of this shift is

consistent with our expectations based on the phase of the NAO (e.g. Taylor

and Stephens, 1998), the magnitude of the shift is not statistically significant.

To summarize, we find that Lagrangian observations of surface velocity

and EKE in the vicinity of the Gulf Stream do not show a significant change

in circulation strength or character from the late 1970’s to the 1990’s.

Although the differences we observed in regional eddy variability and in

the structure of the Gulf Stream jet are suggestive, the available data are

insufficient to prove or disprove our initial hypothesis.

-6

-4

-2

0

2

4

6

1960 1970 1980 1990 2000

Year

NA

O In

dex

North Atlantic Oscillation Index

This study

Richardson (1983)

5-year mean

FIGURE 10: The North At-

lantic Oscillation (NAO)

index for the past 40 years.

The black curve corre-

sponds to a 5-year running

mean. Intervals corre-

sponding to the Richardson

(1983) surface drifter

analysis (1977-1980) and

the present analysis (1990-

2000) are shaded.

FIGURE 13: A comparison of the meridional structure of zonal velocity (upper panels)

and eddy kinetic energy (EKE; lower panels) between the present surface drifter clima-

tology (1990-2000; red) and the previous 1977-1980 (blue) climatology of Richardson

(1983). There is a slight (but statistically insignificant) northward shift of the Gulf

Stream and its associated EKE maximum at 55W.

FIGURE 11: Eddy kinetic energy (EKE)

was computed within four rectangular

subregions corresponding

to areas of relatively dis-

tinct circulation character.

(1) the Gulf Stream; (2)

the eastward

Gulf Stream

Extension; (3)

the Newfound-

land Basin, and (4) the

North Atlantic Current. Tra-

jectories from a single year

of the 1990-2000 drifter cli-

matology are shown.

FIGURE 12: A compari-

son of eddy kinetic en-

ergy (EKE) between the

present surface drifter

climatology (1990-

2000; red) and the pre-

vious 1977-1980 (blue)

climatology of

Richardson (1983). EKE

was computed in four

subregions (see Figure

11). The vertical black

bars denote the 90%

confidence interval.

Drifter velocity was computed using a cubic spline

function at each 6-hourly interpolated position. The

resulting velocities were grouped into spatial and temporal

bins to construct quasi-Eulerian fields of velocity and eddy

kinetic energy (EKE). EKE was defined as one-half the sum

of the zonal and meridional velocity variances within each

grid box. The number of individual velocity observations in

each one-degree square is shown in Figure 6.

The decadal-mean surface velocity field for the North

Atlantic constructed at one-degree resolution for the period

January 1990 – June 1999 is shown in Figure 7a. The

corresponding EKE field is shown in Figure 7b. Note the

enhanced EKE in the vicinity of the Gulf Stream down-

stream of the New England Seamounts (40N), and in the

Labrador Sea near the West Greenland Current (60N). There

is also a zonal band of elevated EKE near 34N associated

with the Azores Current. More detailed views of the surface

circulation in the subpolar gyre, the Gulf Stream region,

and the Caribbean Sea are shown in Figure 8.

In Figure 9 we compare EKE computed from our surface

drifter climatology with EKE derived from TOPEX and ERS

altimetry during the period 1992-1998. The general spatial

distributions of EKE are similar with highest values

located within the Gulf Stream downstream of the New

England Seamounts. The enhanced EKE in the Labrador

Sea seen in Figure 9a is absent in the altimeter-derived

EKE field. This is probably due to the choice of a constant

(rather than latitude-dependent) horizontal lengthscale in

the altimeter EKE calculations. RMS sea level anomalies

from ERS and TOPEX confirm the region of enhanced

variability in the vicinity of the West Greenland Current as

seen in the drifter-based EKE field.

70W 60W 50W 40W 30W 20W 10W30N

40N

50N

60N

Surface Drifter Climatology1990-2000

Eddy Kinetic Energy (cm2/s2)

TOPEX and ERS Altimetry1992-1997

Variance Axes1000 cm2/s2

a

Data Source: Saskia Esselborn, Institut fur Meereskunde, Hamburg, Germany

300 400 500 750 1000 1500 2000 2500 3000

70W 60W 50W 40W 30W 20W 10W30N

40N

50N

60N

b

FIGURE 8: Detailed view of mean velocity field in three regions. (a) The Labrador Sea and Subpolar

Gyre. (b) The Gulf Stream and its eastward extension. (c) The Caribbean Sea.

60W 50W 40W 30W 20W 10W 0 10E

50N

60N

70N

50 cm/s

10 cm/s

80W

30N

40N

50N

70W 60W 50W 40W 30W

50 cm/s

10 cm/s

90W 80W 70W 60W

10N

20N

30N

0 15 30 100

50 cm/s

10 cm/s

FIGURE 6: Number of individual 6-hourly velocity observations per one-degree square.

FIGURE 7: (a) A decadal-mean surface velocity field for the North Atlantic computed by aver-aging northward and eastward drifter velocities into one-degree square bins over the periodJanuary 1990 – June 1999. Vectors are only shown for bins containing more than 100 individual

observations. Error ellipses correspond to one standard error in the zonal and meridional di-rections. We assumed a 10-day Lagrangian integral timescale in the error computations.

FIGURE 7: (b) The corresponding field of eddy kinetic energy (EKE). Regions with insufficientdata (less than 100 observations per one-degree square) are shaded gray. Note the enhanced

EKE in the vicinity of the Gulf Stream downstream of the New England Seamounts (40N), andin the Labrador Sea near the West Greenland Current (60N).

90W 80W 70W 60W 50W 40W 30W 20W 10W 0

0

10N

20N

30N

40N

50N

60N

70N

<100

100-200200-500

>500

80W 70W 60W 50W 40W 30W 20W 10W 0 10E

20N

30N

40N

50N

60N

70N

0 15 30 100

50 cm/s

10 cm/s

Mean Surface Velocity (cm/s)

80W 70W 60W 50W 40W 30W 20W 10W 0 10E

20N

30N

40N

50N

60N

70N

0 100 200 300 400 500 750 1000 1500 2000 2500 3000

Eddy Kinetic Energy (cm2/s2) FIGURE 9: Eddy kinetic energy (EKE) in the subtropical and

subpolar gyres computed from (a) the present surface drifter

climatology, and (b) a blend of ERS and TOPEX satellite altim-

etry for an overlapping time period. The contour intervals and

shading are identical.