18
Seasonal and interannual variability of surface circulation in the Cape Verde region from 8 years of merged T/P and ERS-2 altimeter data Clara La ´zaro a, * , M. Joana Fernandes a , A. Miguel P. Santos b , Paulo Oliveira b a Faculdade de Cie ˆncias, Universidade do Porto, Porto, Portugal b Instituto Nacional de Investigac ¸a ˜o Agra ´ria e das Pescas (INIAP) IPIMAR, Lisboa, Portugal Received 15 April 2005; received in revised form 27 May 2005; accepted 11 June 2005 Abstract The characterisation of the geostrophic surface flow field around the Cape Verde Archipelago in the northeast Atlantic Ocean with satellite altimeter data is presented. The aim is to analyse the main current systems present in the region 3- –30-N, 40- –10-W and their seasonal and interannual variability. A merged data set of Topex/Poseidon (T/P) and ERS-2 altimeter data for an 8-year period, beginning in June 1995, has been used and corrected sea surface heights were computed by applying a homogeneous set of relevant geophysical corrections. ERS-2 data were crossover adjusted to T/P. Monthly maps of sea level anomalies were created for the whole period and were used in the computation of monthly maps of absolute dynamic topography, geostrophic currents and eddy kinetic energy (EKE). The seasonal signal of the northeast Tropical Atlantic large-scale surface circulation appears as the prevailing cause of the variability in the region, particularly in the southernmost portion of the region being studied. This signal is also present in the flow field along the African coast and in the Guinea Dome. Regions of highest EKE values are clearly associated with the North Equatorial Counter-Current and with the currents along the African coast. The significant interannual variability found for 1998 seems to be associated with the 1997 – 1998 ENSO Pacific event, but other anomalous periods (1996 – 1997 and 2001 – 2002) uncorrelated with ENSO are also evident. D 2005 Elsevier Inc. All rights reserved. Keywords: Satellite altimetry; Sea level anomaly; Absolute dynamic topography; Geostrophic surface currents; Eddy kinetic energy; Seasonal and interannual variations; Northeast Atlantic Ocean; Cape Verde Archipelago 1. Introduction Satellite altimetry, providing a globally and quasi- synoptically description of the geostrophic sea surface currents, can be used to analyse the ocean circulation and its variability. Since 1995, with the simultaneous operation of Topex/Poseidon (T/P) and ERS-2 missions, the estima- tion of the scales of ocean surface circulation processes, in particular the mesoscale, has been made possible with an unprecedented resolution. The advantages of merging data from different altimetry missions, which have complemen- tary temporal and spatial resolutions, have already been discussed by several authors and are described, for instance, in Ducet et al. (2000) and Le Traon and Dibarboure (1999). Several studies have been carried out in the northeast Atlantic Ocean in recent years, allowing the characterisation of the large-scale ocean circulation and its seasonal variability. These studies have been mainly supported by numerical models, historical hydrographic data, and, more recently, in situ observations (e.g., Fratantoni, 2001; Mittelstaedt, 1991; Siedler et al., 1992; Stramma & Isemer, 1988; Stramma & Schott, 1999). Due to the nature of the hydrographic and in situ data used, most of these analyses are restricted both in space and in time. In studies of the north Atlantic surface circulation and eddy kinetic energy with data from satellite-tracked drifters, some authors (e.g., Fratantoni, 2001; Zhurbas & Oh, 2004) have pointed out that, in contrast with other areas such as the western Atlantic and in particular the Gulf Stream region, data are lacking for 0034-4257/$ - see front matter D 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.rse.2005.06.005 * Corresponding author. Faculdade de Cie ˆncias, Universidade do Porto, Departamento de Matema ´tica Aplicada, Rua do Campo Alegre, 687, Porto, 4169-007, Portugal. Tel.: +351 220100879; fax: +351 220100809. E-mail address: [email protected] (C. La ´zaro). Remote Sensing of Environment 98 (2005) 45 – 62 www.elsevier.com/locate/rse

Seasonal and interannual variability of surface circulation in the Cape Verde region from 8 years of merged T/P and ERS-2 altimeter data

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www.elsevier.com/locate/rse

Remote Sensing of Environm

Seasonal and interannual variability of surface circulation in the Cape

Verde region from 8 years of merged T/P and ERS-2 altimeter data

Clara Lazaro a,*, M. Joana Fernandes a, A. Miguel P. Santos b, Paulo Oliveira b

aFaculdade de Ciencias, Universidade do Porto, Porto, PortugalbInstituto Nacional de Investigacao Agraria e das Pescas (INIAP) IPIMAR, Lisboa, Portugal

Received 15 April 2005; received in revised form 27 May 2005; accepted 11 June 2005

Abstract

The characterisation of the geostrophic surface flow field around the Cape Verde Archipelago in the northeast Atlantic Ocean with satellite

altimeter data is presented. The aim is to analyse the main current systems present in the region 3-–30-N, 40-–10-Wand their seasonal and

interannual variability. A merged data set of Topex/Poseidon (T/P) and ERS-2 altimeter data for an 8-year period, beginning in June 1995,

has been used and corrected sea surface heights were computed by applying a homogeneous set of relevant geophysical corrections. ERS-2

data were crossover adjusted to T/P. Monthly maps of sea level anomalies were created for the whole period and were used in the

computation of monthly maps of absolute dynamic topography, geostrophic currents and eddy kinetic energy (EKE). The seasonal signal of

the northeast Tropical Atlantic large-scale surface circulation appears as the prevailing cause of the variability in the region, particularly in the

southernmost portion of the region being studied. This signal is also present in the flow field along the African coast and in the Guinea Dome.

Regions of highest EKE values are clearly associated with the North Equatorial Counter-Current and with the currents along the African

coast. The significant interannual variability found for 1998 seems to be associated with the 1997–1998 ENSO Pacific event, but other

anomalous periods (1996–1997 and 2001–2002) uncorrelated with ENSO are also evident.

D 2005 Elsevier Inc. All rights reserved.

Keywords: Satellite altimetry; Sea level anomaly; Absolute dynamic topography; Geostrophic surface currents; Eddy kinetic energy; Seasonal and interannual

variations; Northeast Atlantic Ocean; Cape Verde Archipelago

1. Introduction

Satellite altimetry, providing a globally and quasi-

synoptically description of the geostrophic sea surface

currents, can be used to analyse the ocean circulation and

its variability. Since 1995, with the simultaneous operation

of Topex/Poseidon (T/P) and ERS-2 missions, the estima-

tion of the scales of ocean surface circulation processes, in

particular the mesoscale, has been made possible with an

unprecedented resolution. The advantages of merging data

from different altimetry missions, which have complemen-

tary temporal and spatial resolutions, have already been

0034-4257/$ - see front matter D 2005 Elsevier Inc. All rights reserved.

doi:10.1016/j.rse.2005.06.005

* Corresponding author. Faculdade de Ciencias, Universidade do Porto,

Departamento de Matematica Aplicada, Rua do Campo Alegre, 687, Porto,

4169-007, Portugal. Tel.: +351 220100879; fax: +351 220100809.

E-mail address: [email protected] (C. Lazaro).

discussed by several authors and are described, for instance,

in Ducet et al. (2000) and Le Traon and Dibarboure (1999).

Several studies have been carried out in the northeast

Atlantic Ocean in recent years, allowing the characterisation

of the large-scale ocean circulation and its seasonal

variability. These studies have been mainly supported by

numerical models, historical hydrographic data, and, more

recently, in situ observations (e.g., Fratantoni, 2001;

Mittelstaedt, 1991; Siedler et al., 1992; Stramma & Isemer,

1988; Stramma & Schott, 1999). Due to the nature of the

hydrographic and in situ data used, most of these analyses

are restricted both in space and in time. In studies of the

north Atlantic surface circulation and eddy kinetic energy

with data from satellite-tracked drifters, some authors (e.g.,

Fratantoni, 2001; Zhurbas & Oh, 2004) have pointed out

that, in contrast with other areas such as the western Atlantic

and in particular the Gulf Stream region, data are lacking for

ent 98 (2005) 45 – 62

Page 2: Seasonal and interannual variability of surface circulation in the Cape Verde region from 8 years of merged T/P and ERS-2 altimeter data

C. Lazaro et al. / Remote Sensing of Environment 98 (2005) 45–6246

a large area in the eastern tropical Atlantic off the western

African coast between 5- and 25-N. Therefore, the knowl-

edge of the ocean dynamics at the sea surface is still far

from being complete in this region.

Due to its global coverage, satellite altimetry is consid-

ered to contribute significantly to improve this knowledge.

Moreover, the length of presently available altimetry data

sets starts to be long enough to allow the retrieval of the

interannual signal with a unique capability. Regional studies

based on altimeter data, either from independent solutions,

data assimilation into models, or data comparison with in

situ observations, are still scarce for the study area.

The ocean variability of this region has been mainly

studied in the scope of global investigations of altimeter

data (Ducet et al., 2000). On the other hand, the northeast

subtropical Atlantic Ocean area, north of 20-N, has been the

focus of several studies using satellite altimetry, but these

are concentrated mainly on the Canary Islands and the

Azores Current regions (e.g., Efthymiadis et al., 2002;

Tejera et al., 2002; Tokmakian & Challenor, 1993; White &

Heywood, 1995).

This study focuses on the description of the seasonal and

interannual variability of the ocean circulation in the

southern region of the northeast Atlantic Ocean surrounding

the Cape Verde Archipelago, from 3- to 30-N in latitude and

from 40- to 10-W in longitude (Fig. 1), using satellite

Fig. 1. Schematic map representing the POCM_4B mean dynamic topography (con

studies, present in the region surrounding the Cape Verde Archipelago: NECC — N

Canary Current; MC — Mauritania Current; GD — Guinea Dome. Locations of th

and sections A and B selected for transport computations are also shown.

altimetry. For this purpose an 8-year time-series of merged

ERS-2 and T/P satellite altimeter data, beginning in June

1995, has been analysed.

The paper is structured as follows. In the next section the

current systems and their variability known from previous

studies are briefly described. In Section 3 the altimeter data

and the processing methodology adopted in the derivation

of the analysed oceanographic variables are explained. The

seasonal patterns of the main current systems and the most

interesting features revealed by the results, as well as the

potential of this time series in the identification of

interannual variability and wave propagation, are high-

lighted in Section 4. Finally, Section 5 presents the

discussion of the results.

2. Study area and background

The Cape Verde Archipelago is located in the northeast

Atlantic Ocean at the eastern boundary of the subtropical

gyre, at about 500 km west of Cape Verde sited in Senegal

at the NW African coast (Fig. 1). A seasonal signal of the

large-scale surface circulation has been documented as the

response to the seasonal variability of the trade winds and

the meridional displacement of the Intertropical Conver-

gence Zone (ITCZ), being also present in the flow field

tours shown every cm) and the main surface current systems, from previous

orth Equatorial Counter-Current; NEC — North Equatorial Current; CC —

e Cape Verde Archipelago, the several capes mentioned throughout the text

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C. Lazaro et al. / Remote Sensing of Environment 98 (2005) 45–62 47

along the northwest African coast and in the associated

upwelling phenomenon (e.g., Stramma & Schott, 1999;

Stramma & Siedler, 1988).

The islands are located at the southern limit of the Canary

Current (CC), responsible for the transport of cool water

southwards off the African coast. This current detaches from

the coast between latitudes 25- and 20-N, near Cape Blanc.At Cape Verde latitude, the CC flows southwestwards and

joins the North Equatorial Current (NEC) (Mittelstaedt,

1991). Stramma and Siedler (1988) found that the CC is

stronger in summer near the African coast while in winter it

is stronger west of the Canary Islands.

Between latitudes 5- and 10-N, the dominant feature is

the eastward flow known as the North Equatorial Counter-

Current (NECC). The NECC shows a pronounced seasonal

variation, being stronger in northern summer and autumn

(from July to December) when the ITCZ reaches its

northernmost position. During this period the NECC is a

continuous zonal flow extending over approximately the

entire Tropical Atlantic (Mittelstaedt, 1991; Stramma &

Siedler, 1988). Some studies also indicate a correlated

northward shift of the NEC during this period due to the

northward shift of the ITCZ (Stramma & Siedler, 1988).

During northern winter and spring the strong trade winds are

responsible for the weakening of the NECC, which at the

same time is pushed back to the Equator and also becomes

irregular (Mittelstaedt, 1991).

As the NECC approaches the African coast, some of its

flow derives towards north resulting in a northward flow

referred by some authors as the Mauritania Current (MC),

responsible for the carriage of warm oligotrophic equatorial

water to the tropical eastern Atlantic. The MC also shows a

seasonal behaviour associated with the NECC. In winter and

early spring the MC only reaches latitudes of about 14-N(Cape Verde, Fig. 1). At this period of the year the wind

field off the African coast between 14- and 20-N is

favourable to coastal upwelling. Cold and nutrient-rich

upwelled water is transported southwards and is responsible

for the high productivity of the region (Mittelstaedt, 1991).

In summer and early autumn, due to the strengthening of the

NECC and the relaxation of northeast trade winds, the MC

reaches latitudes of about 20-N, just south of Cape Blanc

(Fig. 1), at the same time of the occurrence of the non-

upwelling season south of this latitude (Mittelstaedt, 1991).

Siedler et al. (1992) also observed a strong increase in the

northward flow along the African coast during summer.

Another prominent feature in the study region is the

Guinea Dome (GD) a constituent of the large-scale near-

surface flow fields associated with the NEC, the NECC and

the North Equatorial Undercurrent (NEU) (Siedler et al.,

1992). The GD is located southwest of Cape Verde

Archipelago and has an associated cyclonic geostrophic

flow, confirmed by observational data (Mittelstaedt, 1991).

According to Siedler et al. (1992), it exists permanently

throughout the year at NEU levels while seasonal variations

are found above. They also suggested that the associated

flow weakens in winter, probably caused by the similar

seasonal change in the NECC. From their results the upper

thermocline centre of the structure is thought to be found at

about 9-N, 25-W in the summer months, and at 10.5-N,22-W in the winter months. Still these positions were not

accurately determined since an apparent double-cell struc-

ture was noticeable during the summer. The shape of the

dome varies according to season, an elongation of the dome

with a NE–SW direction being seen in summer, while an

approximately circular form is observed in winter.

Fratantoni (2001) found fast drifter motions (larger than

0.4 m s�1) in a relatively continuous range including the

equatorial region. Trajectories in the low latitudes suggested

rotary motions with meridional scales of several hundred

kilometres probably related with meanders of the NECC.

Slow trajectories (<0.1 m s�1) were found in the eastern

subtropical latitudes. Unfortunately, the lack of data in the

eastern tropical Atlantic surrounding the Cape Verde

Archipelago, precluded a detailed discussion of this region.

The major current systems are an important source of

eddy kinetic energy through their instabilities and it is

generally accepted that, in areas away from the influence of

topography, high EKE regions correspond to regions of

energetic currents (Reverdin et al., 2003). A conclusion is

usually drawn that EKE may be considered as a proxy for

the paths of the major currents (e.g., Heywood et al., 1994).

The highest velocity values are found for the area where the

NECC is present and consequently higher values of eddy

kinetic energy occur in the same area. At lower latitudes of

the North Atlantic Ocean, south of 5-N, values of EKE as

high as 0.15 m2 s�2 are found west of 35-W and between

longitudes 30-–25-W (Fratantoni, 2001). From the study of

Ducet et al. (2000), values of EKE up to 0.02 m2 s�2 are

found around Cape Verde Islands, the smallest values

(<0.01 m2 s�2) being obtained north of 15-N.Consequently, the area between 10- and 15-N, where the

Cape Verde Archipelago lies, can be viewed as a region of

large-scale interactions between the CC, the NEC and the

NECC. Different water masses meet near the archipelago

forming a large-scale frontal system, a potentially produc-

tive zone favourable for the aggregation of large pelagic fish

species (e.g., tunas).

Studies based on altimeter data series, mainly focusing

on the investigation of interannual variability and long wave

propagation, are hardly found for the study region.

Arnault and Cheney (1994) examined the region

variability based on 4 years of Geosat data (April 1985 to

September of 1989) in spite of the difficulty in obtaining

relevant oceanographic information from this data set. Apart

from the expected strong seasonal signal, the authors found

significant interannual variations occurring 1 year after the

Pacific El Nino event of 1986–1987. Arnault and Kestenare

(2004) analysed the variability of the surface currents in the

tropical Atlantic from 10 years (November 1992 to August

of 2002) of T/P data. From their results, anomalous

interannual events were found for 1996–1997 and 2001–

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C. Lazaro et al. / Remote Sensing of Environment 98 (2005) 45–6248

2002 and a simple relation between these events and the

Pacific El Nino events was difficult to establish.

The propagation of Rossby waves in the ocean plays a

significant role in its adjustment on annual to decadal time

scales (Killworth et al., 1997). On the other hand, these long

waves are fast enough in the tropics to allow the adjustment

of the ocean on seasonal time scales and are important in

regions where the seasonal variability is strongest (Doos,

1999; Jochum et al., 2004). Evidence of their westward

motion has been found in satellite sea surface temperature

data and, more recently, in sea surface height anomalies as

the time series of satellite altimetry became long enough to

allow these studies (Cipollini et al., 2000; Doos, 1999;

Jochum et al., 2004; Katz, 1997; Killworth et al., 1997).

An analysis concerning the wave propagation along the

equatorial Atlantic was performed by Katz (1997) using 2

years (1993–1995) of T/P and inverted echo sounder (IES)

data. The author found a westward propagating wave with

phase speeds, inferred from T/P and IES data respectively,

of �0.23 m s�1 and �0.30 m s�1, the differences being

explained to some extent by natural variations between the

measurements.

Jochum et al. (2004) and Doos (1999) compared their

results, obtained from general circulation model simula-

tions, to altimeter and in situ data. The former authors found

that there is evidence of westward propagating tropical

Scheme 1. Summary of the main steps used in the altimeter data processing. Boxe

text represent mean monthly products (computed by averaging, for the whole 8-y

instability waves on both sides of the Equator, generated

from May to January. Changes in the phase speed, as the

waves propagate from east to west, are pointed out as one of

the causes for the discrepancies in the range of the phase

speed values found by various authors. Apart from direct

meteorological forcing, unstable flow fields can also be

responsible for wave generation. Therefore, a range of

possible phase speed values can be expected caused by the

variability of current properties.

Doos (1999) found that the latitudinal change in the

westward phase velocity of the waves is confirmed by

observational and model data, being however smaller than

predicted by theory. The phase speeds computed were lower

than predicted for latitudes south of 10-N and the opposite

was verified for latitudes north of 10-N.

3. Data and methodology

The altimeter data used in this study include ERS-2 and

T/P for the 8-year period from June 1995 to May 2003. The

ERS-2 data are the OPR02 Version 6 precise Ocean

Products (OPR) provided by the European Space Agency

(ESA, 1996), from cycles 2 to 85. The spatial resolution of

these data in the study region varies from 80 km at latitude

3-N to about 69 km at latitude 30-N. The temporal

s with text in italic represent final monthly products, while boxes with bold

ear period, all the grids corresponding to the same month of the year).

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C. Lazaro et al. / Remote Sensing of Environment 98 (2005) 45–62 49

resolution is 35 days. The T/P data are the Merged

Geophysical Data Records (MGDR) provided by the

Archivage, Validation et Interpretation des donnees des

Satellites Oceanographiques (AVISO, 1996), from cycles

103 to 394, with a spatial resolution of 315 km at latitude

3-N and approximately 273 km at latitude 30-N and a

temporal resolution of 10 days.

These data have been used to derive a set of oceano-

graphic products, adopting a methodology described in the

following two subsections and summarised in Scheme 1.

3.1. Altimeter data processing description

All geophysical corrections have been applied to data to

compute the corrected sea surface height: ionospheric

correction, dry and wet tropospheric correction, sea state

bias (SSB), earth and ocean tides, tidal loading, pole tide

and the inverse barometric effect. When necessary, correc-

tions have been updated to introduce state-of-the-art models.

In order to obtain homogenous data sets the same models

were used for the following corrections: dry troposphere,

earth tide, ocean tide, pole tide and inverse barometer. The

criteria used in the choice of the remaining geophysical

corrections and orbit solutions were twofold: the best

models available for each mission should be used and the

T/P data, used as reference, should be kept at its highest

level of accuracy. Table 1 summarises the corrections

applied to each data set.

The ERS-2 satellite ephemeris used were the precise

DGM-E04 orbits available from Delft University of

Table 1

Geophysical corrections and models used in the processing of ERS-2 and T/

P altimeter data

Corrections/models ERS-2 T/P

Mean Sea

Surface

GSFC00.1 (Wang,

2001)

GSFC00.1 (Wang, 2001)

Orbit TuDelft DGM-E04

(Scharroo & Visser,

1998)

NASA JGM3 (AVISO,

1996)

Dry troposphere ECMWF (ESA, 1996) ECMWF (AVISO, 1996)

Wet troposphere ATSR-M model,

improved in coastal

regions (Fernandes

et al., 2002)

TMR model (yaw state

and drift effect on TMR

TBs applied, (Scharroo

et al., 2004))

Ionosphere Bent’s model (ESA,

1996)

Topex: double frequency

(filtered, (Fernandes

et al., 2002))

Poseidon: DORIS model

Sea state bias BM3 formulae (Gaspar

& Ogor, 1996)

(Chambers et al., 2003)

Earth tide Applied (ESA, 1996) Applied (AVISO, 1996)

Ocean tide NAO99 (Matsumoto

et al., 2000)

NAO99 (Matsumoto

et al., 2000)

Pole tide Applied (AVISO, 1996) Applied (AVISO, 1996)

Inverse

barometer

(Dorandeu & Le Traon,

1999)

(Dorandeu & Le Traon,

1999)

SPRT and

USO drift

Applied –

Technology (Scharroo & Visser, 1998). For ERS-2 all

corrections recommended in the CLS ERS-2 cycle valida-

tion reports have been introduced. These include the effect

on the wet tropospheric correction due to the gain fall and

drift occurred on the 23.8 GHz channel of the microwave

radiometer and the BM3 formulae for the sea state bias

correction (Gaspar & Ogor, 1996). With the aim of reducing

land effects on data near coastal zones the ERS-2 radiometer

wet tropospheric correction has been improved using

appropriate methodologies (Fernandes et al., 2002). ERS-2

data have also been corrected for the SPTR (Scanning Point

Target Response) and USO (Ultra Stable Oscillator) drift

effects and for the pole tide.

For T/P the cycle dependent drift effect in the TMR wet

tropospheric correction, including the yaw state correction

to the TMR measured brightness temperatures, has been

modelled according to Scharroo et al. (2004). The SSB

correction was derived from the Chambers et al. (2003)

model; a residual SSB correction of �3 mm was applied to

cycles 236 and greater (Topex B) (Berwin, 2003).

The Topex dual frequency ionospheric correction has

been smoothed using a second order Butterworth filter

(Kulhanek, 1976), with a cut-off period of 20 seconds

(k�120 km). The result of this filtering is similar to the

procedure referred by Imel (1994) who used a 20-s

averaging window, with the advantage that the effects of

discontinuities introduced by data gaps or land regions are

minimized.

For both T/P and ERS-2 the same ocean tide and inverse

barometer models have been used. The ocean tide model

used was the NAO99b model (Matsumoto et al., 2000),

which has been found to give smaller residuals in coastal

regions (Fernandes & Antunes, 2003). The adopted inverse

barometer model makes use of a non-constant surface

atmospheric pressure interpolated from global average

values obtained from CNES/CLS (Dorandeu & Le Traon,

1999).

Sea level anomalies (SLA) have been derived from the

corrected sea surface height values by removing the mean

sea surface from the GSFC00.1 model (Wang, 2001).

3.2. Computation and mapping of geostrophic currents

For the analysed period of 8 years (June 1995 to May

2003) monthly maps of absolute dynamic topography

(ADT) and geostrophic currents were derived using the

procedure described as follows.

ERS-2 35-day and T/P 10-day along track data (SLA

values) were split into monthly files, containing data centred

on each month of the year, including 35 days of ERS-2 and

365.25/12 days of T/P data. This procedure was adopted to

get full ERS-2 coverage and consequently to improve the

spatial resolution.

Each ERS-2 file was crossover adjusted to the corre-

sponding T/P data file, considered as the reference data set,

using tilt and bias parameters. This methodology allows the

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C. Lazaro et al. / Remote Sensing of Environment 98 (2005) 45–6250

reduction of ERS-2 orbital error (Rummel, 1993) and more

generally any inconsistencies between the two missions (Le

Traon et al., 2003). Before adjustment the mean and the root

mean square (rms) error of the differences at the crossover

points are of the order of 4–10 cm and 11–15 cm,

respectively. After adjustment the mean is zero and the

rms is reduced to 7–8 cm. These results are in agreement

with those obtained by Le Traon et al. (2003).

The ERS-2 adjusted and T/P data were joined together,

creating monthly files of merged along-track data.

Monthly grids of sea level anomalies were computed

using kriging. A data spacing of three arc minutes has been

used (0.05-).To reduce measurement noise, the SLA grids were

filtered using a low-pass Butterworth filter of second order

with a cut-off half-wavelength of 100 km. The adopted

small data spacing was selected to minimize the sampling

effects of the frequency domain filter.

ADT grids were generated by adding to each SLA grid a

20-year mean dynamic topography modelled by the global

ocean circulation model Parallel Ocean Climate Model 4B

(POCM_4B) (Tokmakian, personal communication) (Fig.

1). The model is a version of the Semtner and Chervin

(1992) global ocean model derived from Bryan’s multilevel,

primitive equation formulation and with an average reso-

lution of one-quarter degree (Stammer et al., 1996). The

methodology adopted to derive the ADT maps is one of the

possible methods that can be used while a sufficiently

accurate global geoid model is not available (Le Traon &

Morrow, 2001).

From the ADT grids, sea surface currents along the zonal

(u) and meridional (v) directions were computed, using the

known geostrophic equations given by the following

formulae (Knudsen, 1993):

u ¼ � cf R

flfflu

ð1Þ

v ¼ cf Rcosu

flfflk

ð2Þ

where f is the absolute dynamic topography, u and k are the

latitude and longitude respectively, A =2xesinu is the

Coriolis parameter, c is the normal gravity, R is the Earth

mean radius and xe is the Earth’s angular velocity.

Based on u and v grids, monthly maps representing the

magnitude of the geostrophic currents were also computed.

These will be referred as magnitude maps. Both geostrophic

surface current components were computed for each ADT

grid and the resultant geostrophic current was represented as

a vector overlaid on the ADT and magnitude maps. Due to

the different magnitudes of the geostrophic velocities in the

northernmost and southernmost regions of the study area,

the geostrophic velocities were scaled by a linear function of

latitude. In this way, the velocities for latitude 30-N are

represented by arrows four times greater than they would be

if the velocity scale was maintained in the meridional

direction. This procedure allows a better analysis of the

ocean circulation patterns in the northernmost region of the

study area.

3.3. Computation of transport and eddy kinetic energy

Transport computations for the upper 100 m layer were

carried out along specific sections, perpendicular to the

mean current direction. A meridional section of 500 km

length at 25-W and a zonal section of 150 km length at

16-N were selected as representative of the NECC and MC

flows respectively, represented by symbols A and B in Fig.

1. Transport computations were performed by integrating

the current velocity over the width and depth of each section

(Tomczak & Godfrey, 2003), assuming no velocity change

with depth. In the computations along the meridional

section A only the zonal (u) velocity grids were used;

along the zonal section B only the meridional (v) velocity

grids were used. Positive values represent eastward and

northward transport, respectively.

Assuming geostrophy only, monthly maps describing

EKE per unit mass were computed from surface time-

varying geostrophic velocity components (Ducet et al.,

2000):

EKE ¼ 1

2uV2 þ vV2

��ð3Þ

where uV and vV are the geostrophic velocity components,

given by formulae (1) and (2) replacing f by fV, the latter

representing the sea level anomaly variable.

3.4. Computation of mean monthly maps and Hovmoller

diagrams

For the purpose of studying the seasonal signal in the

region, mean monthly maps were computed by averaging,

for the whole 8-year period, all the grids corresponding to

the same month of the year. In this way, twelve maps

were generated for each analysed oceanographic variable

(SLA, ADT, EKE, geostrophic velocity components (u,

v), and magnitude of geostrophic velocity), representing

the mean conditions for each month, from January to

December.

Longitude-time plots (Hovmoller diagrams) were com-

puted along selected latitude profiles and for the entire study

area width. In spite of the irrelevance of the propagation

angle for linear planetary waves, data were analysed just for

zonal profiles, as evidence of different orientations were

found only rarely (Killworth et al., 1997). For each variable

three types of Hovmoller diagrams were generated, repre-

senting the total, the seasonal and the deseasonalized signals

respectively. The seasonal signal was computed considering

the 8-year average of each month, as referred above; the

difference between the total and the seasonal signal is

considered the deseasonalized signal.

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C. Lazaro et al. / Remote Sensing of Environment 98 (2005) 45–62 51

4. Results

4.1. Seasonal variability

This subsection describes the mean and seasonal

characteristics of the surface circulation in the study region.

The interannual variability will be discussed in the next

subsection. In order to simplify the analysis, the year is

divided in four boreal seasons: January to March (winter),

April to June (spring), July to September (summer) and

October to December (autumn).

In the analysis below various levels of information have

been used. First, the whole sequence of monthly maps of

SLA, ADT, EKE and magnitude of the currents were used in

animated sequences in order to observe the features

described as a function of time. Second, the mean maps

computed for each month of the year, representing the

seasonal signal, were analysed.

From these, maps representative of each season were

selected. Mean ADT maps for March, June, August, and

December are presented in Fig. 2; the corresponding maps

of mean SLA are shown in Fig. 4. Mean geostrophic surface

velocities overlaid in the corresponding mean magnitude

Fig. 2. Mean monthly ADT maps for the following months: (a) March (winter); (b

shown every 1 cm (solid lines represent ADT�0 m; dashed lines represent ADT

maps are shown in Fig. 3 for some representative months

when the most interesting features occur (February, June,

October and December).

4.1.1. The North Equatorial Counter-Current and Maur-

itania Current

The results show that the surface circulation has a strong

seasonal pattern particularly evident in the southernmost

region of the study area.

In early spring, starting in April, the North Equatorial

Counter-Current (NECC) appears as a zonal flow in the

southeastern part of the study region (east of 25-W and

south of 5-N). During this season, the area occupied by

this zonal flow gradually expands westwards and north-

wards, reaching 35-W in June, being generally centred on

latitudes 5- to 6-N (Figs. 2b and 3b). In June, it is a

narrow flow, 300–400 km wide, with an almost zonal

direction covering the whole range of longitudes from 35-to 15-W, with mean velocity of 0.30 m s�1, although its

core velocity can be as high as 0.60 m s�1, particularly

between longitudes 32-–22-W (Fig. 3b). East of 20-Wtwo main branches of the NECC can be observed (Fig.

3b). One branch turns in the northeast direction and the

) June (spring); (c) August (summer); and (d) December (fall). Contours are

<0 m).

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Fig. 3. Vectors representing surface geostrophic currents derived from mean monthly ADT grids, superimposed on the corresponding magnitude maps for (a)

February, (b) June, (c) October, and (d) December. Note that the vectors were scaled by a linear function of latitude accordingly to the scale on the right.

C. Lazaro et al. / Remote Sensing of Environment 98 (2005) 45–6252

other extends eastwards until 12-–13-W where part of the

flow turns northwards, along the African coast and joins

the previous branch. The result is a poleward current,

commonly known as the Mauritania Current. This current

reaches velocities of about 0.25 m s�1, in particular

between latitudes 14-–17-N, north of Cape Verde. In June

this flow can reach the latitude of Cape Timiris (20-N), butwith smaller velocities, of about 0.10 m s�1.

In July, the NECC occupies the whole study area, south

of 8-N, and is wider. It is during the period June–July when

the NECC is fastest. The Mauritania Current is found along

the African coast, up to Cape Blanc, with mean velocity

values similar to those observed in June. It is during July

and August that the Mauritania Current reaches its north-

ernmost position.

During August and September the NECC reaches its

northernmost position, approximately 11-N (Fig. 2c). Its core

velocity is then smaller, with values lesser than 0.45 m s�1.

In October, the NECC clearly becomes irregular and

typically forms two narrow branches west of 30-W, a

stronger flow centred on approximately 4-N, with

velocities reaching 0.80 m s�1 in some years and a

weaker flow further north, at about 8-N (Fig. 3c). A

mean zonal flow with velocities of 0.25 m s�1 is still

visible at longitudes between 30- and 20-W. In November

the NECC becomes narrower and velocities reaching 0.80

m s�1 are common west of 30-W. These high velocity

values are associated with large meanders that start to

develop between latitudes 5- and 10-N in late autumn.

There is still evidence of an eastward transport between

30-–20-W (Fig. 3d). In early winter, the meanders can

have wavelengths of about 700 to 800 km and amplitudes

of 300 to 400 km. Due to their shape a southward

meridional transport can be found along some profiles

west of 30-W and the highest velocities are found in

these meanders (Fig. 3a). During this season the

continuous eastward flow between longitudes 30-–20-Wis less pronounced but the NECC is already visible as a

zonal flow east of 20-W. The intensity of the Mauritania

Current decreases during autumn, reaching lowest lati-

tudes, in general south of Cape Verde, by late winter

(Figs. 2a and 3a).

The transport values found for sections A and B are in

agreement with the described seasonal behaviour of the

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C. Lazaro et al. / Remote Sensing of Environment 98 (2005) 45–62 53

NECC and the MC. The mean transport along section A is

maximum in June, reaching 12.9T2.4 Sv (1 Sv=106 m3

s�1). From this point onwards it decreases until November

(5.4T2.2 Sv), raising again and reaching a relative

maximum in January (8.9T1.9 Sv). This transport in early

winter is related to the large meanders associated with the

instability of the NECC at this time of the year. From now

onwards the current transport decreases being minimum in

March (2.3T2.2 Sv). The transport along section B reveals a

similar seasonal behaviour with maximums in June

(1.9T1.1 Sv) and early autumn (1.3T0.6 Sv). The minimum

occurs in December (�0.1T0.8 Sv) associated with the

weakening of the Mauritania Current.

4.1.2. The Canary Current and the North Equatorial

Current

In what concerns the Canary Current (CC), results show

that during the period from December to June (winter and

spring), it appears as a strengthened southward flow and is

wider, presenting flows along and off the African coast,

west of Canary Islands. It is located, in general, north of

20-N, with mean velocities less than 0.10 m s�1. In winter it

reaches the lowest latitudes, as a southward flow along the

Fig. 4. Mean monthly SLA maps for the following months: (a) March (winter); (b

shown every 1 cm (solid lines represent SLA�0 m; dashed lines represent SLA

African coast between capes Blanc and Verde, with

velocities up to 0.20 m s�1 (Fig. 3a). The weakening of

the CC is verified after July, when the NECC and its related

northward flow are seasonally intensified and confined to

the African coast, being weakest in autumn.

Several cyclonic and anticyclonic eddies are shown in

the southern vicinity of the Canary Islands. Both kinds of

features are likely to occur independently of the season, but

an anticyclonic eddy is commonly seen southeast of the

archipelago from January to May. Cyclonic eddies are more

common in summer and autumn.

In the winter the NEC is less well defined in ADT maps.

Although it appears in the POCM4B mean dynamic

topography (Fig. 1), the positive anomaly present in the

SLA maps at the latitude range 5-–20-N west of the Cape

Verde Archipelago almost masks the mean signal of the

current (Figs. 2a and 4a). In spring and summer it is well

defined as a southwest flow east of the Archipelago (Figs.

2b, c, 4b and c). The magnitude of the current does not

suffer significant variations throughout the year, with mean

values lower than 0.1 m s�1. Associated with the NEC,

there is evidence of a southwestward transport between

latitudes 10- and 20-N during the whole year, although

) June (spring); (c) August (summer); and (d) December (fall). Contours are

<0 m).

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C. Lazaro et al. / Remote Sensing of Environment 98 (2005) 45–6254

more difficult to infer during winter. In spring the NEC

reaches its southernmost position. During these months a

southwest transport is also visible south of 10-N, whichmeans that in summer and autumn the NEC migrates

northwards following the similar displacement of the

NECC.

At the Cape Blanc latitude, 250 to 300 km off the African

coast, the NEC seems to split in two branches, both flowing

southwest, one north of Cape Verde islands and another

flowing south just east of the islands. This is more evident

during the first half of the year. South of the archipelago,

part of the latter flow recirculates northwards strengthening

the Mauritania Current, particularly during the spring and

summer months (e.g., Fig. 3b). A nearly permanent

anticyclonic eddy, with diameter as large as 300–400 km

is found south or southwest of the Cape Verde islands, with

Fig. 5. Hovmoller diagrams representing the seasonal EKE signal for latitudes: (a) 4

the African coast. Please note the change in the range of the colour scale for lati

mean velocity of 0.10–0.15 m s�1 reaching maximum

values in spring (Fig. 3b).

There is a large-scale cyclonic circulation in the area

between 10- and 15-N that in the most intense months

(summer until late autumn) can reach longitudes up to

35-W. The results show that this circulation seems to be the

interaction of the NEC, mainly the branch that flows south

of Cape Verde Archipelago, the NECC and the northward

flow along the African coast, the Mauritania Current.

4.1.3. The Guinea Dome

Seasonal variations of the Guinea Dome (GD) are also

found. The cyclonic circulation associated with the GD is

clearly seen in ADT maps from May onwards until

December. In late spring the GD centre is located at

approximately 9.5-N and its centre moves westwards

-N, (b) 8-N and (c) 27-N. Blank values, represented in white, correspond totude 27-N.

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C. Lazaro et al. / Remote Sensing of Environment 98 (2005) 45–62 55

between longitudes 22- and 23.5-W. During this season, the

GD exhibits a more circular shape (Fig. 2b). A significant

westward displacement of the GD centre is clear during

summer and autumn, being accompanied by a slight

northward shift during this period. In August, the develop-

ment of a double-cell structure is already evident (Fig. 2c).

During this period it is centred on approximately 10.5-N,27-W and it reaches the northwesternmost position 11-N,29-W in October, probably due to the similar northward

migration of the NECC. The GD clearly weakens from

November onwards, being inconspicuous during winter.

4.1.4. Eddy kinetic energy

Also inspected was the seasonal variation of eddy kinetic

energy (EKE) for each season. Geostrophic time-varying

vectors were overlaid in EKE grids to distinguish the sense

of movement, cyclonic or anticyclonic, and not only the

maxima or minima of energy.

Higher energies are found south of 10-N corresponding

to the area where the mesoscale variability is highest, where

values up to 0.30 m2 s�2 can be found mostly in autumn,

while the rest of the area does not show EKE values larger

than 0.010 m2 s�2. During the winter, EKE values are

smaller than 0.020 m2 s�2 in the region south of 10-N and

east of 30-W. Values up to 0.050 m2 s�2 can be found west

of 30-W, associated with the previously mentioned eddies

and meanders which reach their maxima in late autumn, but

are still present although weaker at this time of the year

(Figs. 2a and 5a). During spring, EKE values in the range

0.20–0.35 m2 s�2 can be found east of 24-W at latitude

4-N. These values increase westwards as the season

progresses, in a similar fashion with the behaviour of the

NECC (Fig. 5a). It is interesting to note that at latitude 8-Nthe corresponding EKE values for this season are consid-

erably lower, below 0.012 m2 s�2. In summer, EKE values

present an almost zonal pattern at 4-N. The highest EKE

values found at latitude 8-N in late summer and early

autumn are representative of the northward displacement of

Fig. 6. Hovmoller diagrams representing the seasonal SLA signal for latitudes: (a

latitude and every 1 cm for the last two (solid lines represent SLA�0 m; dashed

the NECC during this period. This is well depicted in Fig.

5b. The most energetic features in the study region are found

west of 30-W in autumn. These are clearly associated with

the large meanders and eddies which are associated with the

NECC instability.

The region north of 10-N is generally low energetic (Fig.

5c), apart from the region close to the African coast, where

EKE values in the range of 0.010–0.020 m2 s�2 are

observed in some winter months, associated with the

strengthening of the Canary Current.

In spring the Mauritania Current has generally EKE

values of 0.010 m2 s�2, which can be higher (0.025 m2 s�2)

west of Cape Verde and in the southwest African coast. In

summer it is clear that EKE values similar to those found for

the Mauritania Current in spring are now seen at higher

latitudes up to Cape Blanc.

As referred above, eddies south of Cape Verde Archipe-

lago are strongest in spring when they can reach EKE values

up to 0.020 m2 s�2, whether they are cyclonic or anti-

cyclonic.

4.2. Interannual variability

In the analysis of the interannual variability various

levels of information have been used. Apart from the whole

series of monthly maps of SLA, ADT, EKE and magnitude

of the currents, used in animated sequences, transport

anomalies along sections A and B and Hovmoller (time–

longitude) diagrams were analysed.

The inspection of the referred animated sequences

revealed anomalous events mainly in the years 1996–

1998 and 2001–2002.

Hovmoller diagrams representing the variation of the

described oceanographic variables for the 8-year period

were created for various latitudes. Here only some examples

of SLA diagrams are shown for representative latitudes.

Hovmoller diagrams were created for the full and

residual deseasonalized signal. As mentioned before, it

) 4-N, (b) 14-N and (c) 27-N. Contours are shown every 2 cm for the first

lines represent SLA<0 m).

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C. Lazaro et al. / Remote Sensing of Environment 98 (2005) 45–6256

was found that the seasonal signal is dominant in the study

region. This is evident by comparing the Hovmoller

diagrams for example, for latitude 4-N (Figs. 6a and 7).

For the analysis of these diagrams, an extended region from

40-W towards 7-E is considered for the latitude 4-N and

from 40-W towards east, until the African coast is found, for

the remaining latitudes.

At 4-N two seasonal positive anomalies can be seen,

moving westwards (Fig. 6a). The first one crosses the study

area from February until September and seems to be

associated with a planetary wave with a phase speed of

approximately �0.3 m s�1. In this wave a change in the

phase velocity can be observed around longitude 30-W. The

Fig. 7. Hovmoller diagrams representing (a) the full SLA signal and (b) the deseas

lines represent SLA�0 m; dashed lines represent SLA<0 m).

second one crosses the same region from late autumn to

early winter, in approximately two and a half months, with a

mean speed of �0.8 m s�1 (Figs. 6a and 7a). In the residual

anomaly map (Fig. 7b) it can be seen that the first Rossby

wave of the year (February–September) is weaker in the

years 1997 and 1998. The behaviour of the 1998 wave is

associated with 1997–1998 El Nino Southern Oscillation

(ENSO) condition, while the 1997 one is related to an

anomalous condition not associated with ENSO, as it will be

further discussed in Section 5.

During 1998 another positive anomaly propagates west-

wards with similar phase speed but at a different time of the

year (from December 1997 to June 1998). During these 2

onalized SLA signal for latitude 4-N. Contours are shown every 4 cm (solid

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C. Lazaro et al. / Remote Sensing of Environment 98 (2005) 45–62 57

years (1997 and 1998) the positive anomaly in the

westernmost side of the study region is stronger in late

spring and early summer.

In Fig. 8 the full and the deseasonalized signal

Hovmoller diagrams for latitude 14-N are shown. The

seasonal signal present at this latitude is depicted in Fig. 6b,

where an annual Rossby wave crossing the study area from

November until May, with a change in the phase velocity

around longitude 25-W, is well represented. Another

interesting feature which can be seen in Fig. 6b is the

variation of the SLA near the African coast, with minimum

values in February–March and maximum values in

November, associated with the seasonal behaviour of the

Mauritania Current. The most relevant features shown in the

deseasonalized SLA diagram (Fig. 8b) are the anomalous

positive and negative anomalies occurring respectively from

Fig. 8. Hovmoller diagrams representing (a) the full SLA signal and (b) the deseaso

lines represent SLA�0 m; dashed lines represent SLA<0 m).

late 2001 to the middle of 2002 mainly east of 35-W and

during autumn 1999 and winter 2000 in the half eastern side

of the region.

The seasonal SLA signal for latitude 27-N is represented

in Fig. 6c. The full and deseasonalized SLA diagrams are

shown in Fig. 9a and b, respectively. At this latitude there is

no evidence of seasonal propagation of planetary waves.

This can be explained by the relatively weak and low

energetic flows present in this region of small variability in

the wind field. Instead there is a seasonal pattern of higher

and lower SLA during the periods September to January and

February to August, respectively. These are related to

anticyclonic and cyclonic eddies present at this latitude.

Fig. 9b shows that there is clear evidence in some years of a

westward movement of both kinds of eddies. For example in

the years 2000 and 2002 there is a cyclonic eddy in the

nalized SLA signal for latitude 14-N. Contours are shown every 4 cm (solid

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Fig. 9. Hovmoller diagrams representing (a) the full SLA signal and (b) the deseasonalized SLA signal for latitude 27-N. Contours are shown every 4 cm (solid

lines represent SLA�0 m; dashed lines represent SLA<0 m).

C. Lazaro et al. / Remote Sensing of Environment 98 (2005) 45–6258

eastern part of the region, propagating westwards from the

beginning of the year until September. Roughly in this

period of the year, in the same region, there is an

anticyclonic eddy with the same phase propagation. The

westward movement of these eddies is clearly seen in the

animated sequence of ADT maps.

5. Discussion

Eight years of merged T/P and ERS-2 data have been

used to characterise the seasonal and interannual variability

of the northeast Tropical Atlantic. The period of analysis is

believed to be long enough to contain the most important

ocean–atmosphere coupled events inducing anomalous

oceanographic variations, such as the strong El Nino, which

occurred in 1997–1998, and La Nina in 1998–1999.

Therefore the analysed seasonal signal is influenced by

both normal and extreme conditions. The use of merged

data from T/P and ERS-2 significantly increases the

capability of mapping mesoscale features compared to the

use of T/P only. In this study, monthly maps were created in

a compromise between keeping the full spatial sampling

capability provided by ERS-2 rather than the higher

temporal sampling of T/P.

The results show that the dominant signal in the study

area has a seasonal pattern, accompanying the seasonal wind

field variability known for the region. The interannual

signal, although typically weaker than the seasonal one, is

not negligible in some years, being strongest at lower

latitudes, where maximum values of the seasonal cycle are

also found.

In what concerns the geostrophic circulation, pronounced

seasonal variations are found for the North Equatorial

Counter-Current and the Mauritania Current as a result of

the north–south migration of the Intertropical Convergence

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C. Lazaro et al. / Remote Sensing of Environment 98 (2005) 45–62 59

Zone (ITCZ) and corresponding changes in wind-stress curl.

The NECC is stronger in summer and autumn (from July to

December) and its zonal pattern is most evident in late

spring and summer. From this study, the NECC is fastest

during June and July. In late summer the continuous zonal

flow reaches its northernmost position. In October large

meanders start to develop west of 30-W and the establish-

ment of the mean flow position is difficult, a result also

reported by Verdy and Jochum (2005). From the analysis of

the results of an ocean general circulation model of the

Tropical Atlantic Ocean, these authors found that west of

32-W the advection of relative vorticity by the mean flow

and by eddies are not negligible in the vorticity budget,

pointing it as the main difficulty in the development of a

theoretical framework for the NECC and a challenge to

observational studies. These energetic meanders can be

responsible for a southward heat transport near the equator

reported by, e.g., Lamb (1981), Carissimo et al. (1985), and

Oort and Vonder Harr (1976). According to some authors

(Stramma & Isemer, 1988) this transport is still controver-

sial since is not present in their results. These large

anticyclonic meanders are still common in the westernmost

part of the study region during January and February. In late

winter and early spring, the NECC is either not present or

weakened west of 20-W. This latter condition is reached in

some years, when an eastward flow, although weaker during

this period, is present at these longitudes throughout the

year. These results confirm those obtained by studies based

on historical hydrographic data (e.g., Mittelstaedt, 1991;

Stramma & Siedler, 1988), and altimetry data (Arnault &

Kestenare, 2004).

The seasonal behaviour of the NECC can be interpreted

by the analysis of the mean SLA maps shown in Fig. 4. In

March and April a clear zonal negative anomaly is present

in the NECC region, responsible for a time-variable

westward flow, thus reducing the eastward NECC pre-

dominant flow (Fig. 4a). During these two months the

NECC is either weak or not present west of 20-W (Figs.

2a and 4a). On the other hand the positive zonal anomaly

found in the southeastern part of the region during late

spring contributes to the strengthening of the NECC during

this period (Fig. 4b). These positive anomalies intensify

and progress westwards and northwards during summer, in

a similar fashion as the described NECC behaviour during

this period (Fig. 4c). During late summer and autumn the

positive anomalies lose their zonal characteristic and so

does the NECC (Fig. 4d).

The strengthening of the Mauritania Current, which is

evident in summer, follows the behaviour of the NECC.

Cross-correlation between monthly transport values for

sections A and B were computed. A maximum value of

0.72 was found for lag of 1 month. During this period the

MC, which is originated from NECC branches, reaches the

northernmost latitude up to Cape Blanc. It is responsible for

the transport of warm oligotrophic equatorial waters to the

tropical Atlantic and occurs simultaneously to the suppres-

sion of coastal upwelling south of the mentioned cape. The

intensity of the MC decreases during autumn reaching the

lowest latitudes, south of Cape Verde, by late winter.

Also directly related with the seasonal variability of the

NECC is the variability of the position, shape, and intensity

of the Guinea Dome. The GD and its associated cyclonic

circulation are clearly depicted in ADT maps from May to

December. During this period, the GD shape varies from a

nearly circular to a double-cell structure with its centre

showing a northwestward migration. These results differ

from those obtained by Siedler et al. (1992). According to

these authors the circular-shaped GD centre in winter was

located north of its corresponding position in summer. From

our results, the GD is inconspicuous during winter and

migrates northwards during summer, following the analo-

gous migration of the NECC. Also, our results indicate that

the GD can be found as west as 29-W, in summer,

considerably west of the position given by the mentioned

authors (25-W).

The Canary Current does not show a strong seasonal

variability, although it is more intense and wider during

winter and spring and weaker and closer to the African coast

from July onwards. The southwestward transport related to

the North Equatorial Current is visible throughout the year

between latitudes 10- and 20-N, while less defined in ADT

winter maps. At Cape Verde latitude results indicate the

existence of two branches of the NEC during the first half of

the year, one turning southwestwards north of Cape Verde

Archipelago and the other becoming a southwest flow south

of the archipelago, both appearing to join together again at

lower latitudes.

Consequently, the area between 10- and 20-N, where theCape Verde Archipelago is located, can be regarded as a

region of interactions between the main current systems

described. Since these transport different water masses and

meet in the surroundings of the archipelago, a large-scale

frontal system is formed, and a potentially productive zone

can be expected near the islands. Favourable conditions for

the aggregation of large pelagic migratory fish species can

arise, sustaining the fisheries activity of the country.

Higher EKE values are found south of 10-N, clearly

related with areas where both mesoscale variability is higher

and the strongest currents are found, namely the NECC and

the MC. The highest values are found during autumn, west

of 30-W, when the instability of the NECC is evident in the

westernmost part of the study area. The comparison between

the EKE, ADT and magnitude monthly maps allow to

conclude that EKE can be used as a useful indicator of the

paths and intensity of these currents.

The SLA deseasonalized Hovmoller diagrams created for

different latitude profiles clearly reveal the existence of

years showing anomalous conditions.

In deseasonalized SLA Hovmoller diagram for latitude

4-N a sequence of anomalous positive and negative SLA

values can be identified, from the beginning of 1996 to the

summer 1999 with apparent westward propagation.

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C. Lazaro et al. / Remote Sensing of Environment 98 (2005) 45–6260

The sequence of positive and negative SLA in the

equatorial Atlantic, prior to the 1997–1998 Pacific ENSO

event, have been reported as the result of an anomalous

event which occurred in the tropical Atlantic in the period

1995–1997. This Equatorial Atlantic Oscillation (EAO)

was analysed by Handoh and Bigg (2000) using altimetry

amongst other data. During this event, which started in the

autumn of 1995 and continued until the autumn of 1997,

time sequences of distinct anomalies in sea surface temper-

ature (SST) and sea surface height (SSH) were observed in

the equatorial band. This event seems to be the long-lived

oceanic response to a westerly wind anomaly in the western

Atlantic during the autumn (October–December) of 1995.

Warm (early 1996 to summer of 1996) and cold (early 1997

to summer of 1997) phases were reported (Handoh & Bigg,

2000). The authors also identified a sequence of Kelvin

waves during the period of this event, followed by the

propagation of slow westward waves. Since there was also

evidence of westward propagation with similar speed in the

SST field and in atmospheric convection data, the authors

called these westwards propagating coupled-mode waves

instead of Rossby waves. Handoh and Bigg (2000) thus

found that the climate of the tropical Atlantic during 1996

and 1997 was dominated by a coupled atmosphere–ocean

mode of behaviour and suggested that this climate

oscillation in the Atlantic was independent of the ENSO.

In the deseasonalized Hovmoller diagram for the equator

(not shown here), both positive (warm) and negative (cold)

phases reported by Handoh and Bigg (2000) were identified.

In addition, as referred by the authors that it should be

expected but not observed in their results, another sequence

of positive (after summer 1997 until the end of the year) and

negative (since the start of 1998 until late spring) SLA are

identified. This could be a consequence of the higher spatial

resolution present in our merged T/P and ERS-2 data set

compared to the T/P data used by those authors.

The anomalous Atlantic event referred above explains

the described interannual variability between 1995 and mid-

1998. Assuming that the North Atlantic oceanic response to

the Pacific ENSO events has an approximate lag of 4–5

months, as reported by Enfield and Mayer (1997), the 1997

ENSO event should be the responsible for the positive SLA

observed between June and September 1998, clearly visible

at the equator and still present, although less well defined at

the latitudes of our study region. On the contrary, the

response of the tropical Atlantic to the 1998 La Nina event

is well depicted in the Hovmoller diagram of 4-N, betweenspring and early summer of 1999 (Fig. 7).

In the time-sequence of ADT maps, an intensified NECC

is seen in the spring of 1997 and 1998, resulting, in June, in

a continuous zonal flow covering the width of the study

region. In regular years this situation is only reached in July

while in June it is confined at longitudes east of 35-Wyears.

This seems to be associated with the earlier northward

displacement of the ITCZ, reported by Enfield and Mayer

(1997), causing an early spring transition.

This behaviour of the NECC is related with the

anomalous positive anomalies found at the NECC band of

latitudes during the warm phase of the EAO event and the

1997–1998 El Nino. Also intensified is the GD in the

summer of 1997 due to the strong SLA negative anomalies

seen in the GD region for the 10-N profile (not shown). The

negative anomalies found west of 30-W, during late autumn

of 1997 and early winter of 1998, are responsible for the

weakening, visible in ADT maps, of the large anticyclonic

eddies that in general are seen west of this longitude. This

seems to be related to the cold phase observed in the

summer of 1997.

The anomalous negative anomalies found west of 20-Win the spring of 1996 and 1999 can be associated with the La

Nina event. This corresponds to an eastward shift of the

NECC zonal flow at this time of the year.

During October 1997, following the cold EAO phase,

extensive phytoplankton blooms (¨2�105 km2), charac-

terised by chlorophyll a concentrations ¨40 times greater

than the adjacent waters were observed in offshore

oligotrophic waters approximately 1200 km from the coast

(Fernandes, submitted for publication).

Hovmoller diagrams of EKE for 4-N reveal an anom-

alous higher energetic period during the year 1996 (March

to October) extending over the whole range of longitude of

the study area with highest values in the western part.

Although not evident in the corresponding deseasonalized

SLA Hovmoller diagram this anomaly corresponds to the

period of occurrence of the first Rossby wave of the year.

No increase in EKE values was found in 1997.

In the period 2001–2002 another anomalous event is

depicted in the deseasonalized SLA diagrams at all latitudes.

Anomalous positive anomalies can also be seen in the

westernmost side of the study region from December 2001

to March 2002, associated with the eddy intensification. In

particular, in the region north of 10-N, this seems to be the

most remarkable anomalous period (Figs. 8 and 9).

In this study the mean dynamic topography (MDT) has

been modelled by a 20-year mean run of the POCM_4B

model. Comparing the magnitude of the model MDT

values and the altimetry-derived SLA it is clear that the

magnitude of the former is at least of the same order of

magnitude of the SLA values found for most of the

periods. In spite of being a region of weak oceanic

variability, and has not been the subject of attention such

as the western tropical Atlantic, it is a challenge for

techniques such as satellite altimetry which proved to be

effective in detecting the seasonal and interannual varia-

bility of this small magnitude. With the recent develop-

ments in geopotential modelling from the satellite gravity

missions, with particular emphasis to GRACE (and GOCE

in the future) it is expected that independent models of

MDT will be derived. In parallel with the advances in SLA

variability determined from satellite altimetry, these models

will certainly improve the knowledge of the absolute

dynamic topography.

Page 17: Seasonal and interannual variability of surface circulation in the Cape Verde region from 8 years of merged T/P and ERS-2 altimeter data

C. Lazaro et al. / Remote Sensing of Environment 98 (2005) 45–62 61

Acknowledgments

This study has been funded by FCT project POCTI/

36095/CTA/2000. The ERS-2 altimeter data have been

provided by ESA under the scope of the project ‘‘Remote

Sensing Applied to Fisheries in the Cabo Verde Region’’,

AO3-265. The T/P data have been provided by AVISO. The

authors also want to acknowledge Robin Tokmakian for

providing the 20-year mean dynamic topography from

POCM_4B model used in the study.

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