GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L03308, doi:10.1029/2003GL019210, 2004

Tropical Atlantic surface current variability from 10 years of TOPEX//Pose´ı¨don altimetry Sabine Arnault and Elodie Kestenare IRD LODYC UMR 7617 CNRS/IRD/UPMC/MNHN, UPMC, Paris, France Received 3 December 2003; accepted 14 January 2004; published 12 February 2004.

[1] 10 years of surface geostrophic currents from TOPEX/ se´ı¨don (hereafter T/P) 10-year time series of altimetric data to Pose´ı¨don altimetric data are used to describe the low analyze the tropical Atlantic surface current variability. frequency variability of the tropical Atlantic circulation through Empirical Orthogonal Function analysis. The 2. Data Processing seasonal variability clearly agrees with previous studies based on climatological data. It shows the tropical Atlantic [3] Cycles 5 to 364, November 1992 to August 2002, of response to seasonal fluctuations of the overlying wind T/P geophysical data records were processed over the system. More interesting is the capability, using altimetry, to tropical Atlantic Ocean in order to obtain sea level anoma- reach for the first time on a basin scale the year-to-year lies (SLAs) referenced to a mean profile. This reference to a variability from measurements. Abnormal events occur in mean profile, which is dominated by geoid undulations and 1996–1997 and in 2001 with different spatial scales regarding stationary oceanic circulation, is necessary in order to both large scale zonal distribution and regional variability eliminate unknown marine geoid information. Standard located in the north-western basin. A first attempt to link these corrections were applied [Arnault et al., 1999]. Erroneous events to climatic indexes (El Nin˜o-Southern Oscillation, data with SLAs greater than 50 cm were discarded. North Atlantic Oscillation) is also evocated. INDEX [4] An objective analysis [Bretherton et al., 1976] has TERMS: 4231 Oceanography: General: Equatorial oceanography; been performed on the original data to interpolate them on a 4223 Oceanography: General: Descriptive and regional regular 0.5 Â 0.5 Â 5-day grid. This kind of approach has oceanography; 4572 Oceanography: Physical: Upper ocean already been used successfully in altimetric studies [De Mey processes; 4556 Oceanography: Physical: Sea level variations. and Robinson, 1987; Bonjean and Lagerloef, 2002]. An Citation: Arnault, S., and E. Kestenare (2004), Tropical Atlantic isotropic correlation radius of 200 km (adequate both for surface current variability from 10 years of TOPEX/Pose´ı¨don basinwide zonal circulation and for eddy resolving western altimetry, Geophys. Res. Lett., 31, L03308, doi:10.1029/ boundary) was assumed together with a 15-day temporal 2003GL019210. correlation. The mean accuracy is about 2 cm. This weak error is one of the benefits of the T/P mission for the tropical domain, where the signal amplitudes are weak (about a few 1. Introduction 10 cm). Geostrophic current anomalies were then computed [2] The equatorial ocean system is marked by variability starting 1 off the equator using a finite difference scheme occurring over a wide range of time and space scales, with a and the geostrophic balance. To convert the current anoma- complex area of westward currents and eastward counter- lies to absolute surface currents, we added a mean geostrophic currents. In the tropical Atlantic Ocean, many papers have component referenced to 1200 dbar [Levitus and Boyer, been published, describing locally the oceanic circulation 1994]. In order to calculate currents on the equator,we used either from cruises or from current-meter moorings [e.g., the second derivative of the pressure field on an equatorial b- Bourle`s et al., 1999a; Johns et al., 1998]. However, the plane as sucessfully did by Picaut et al. [1990] and Delcroix description is restricted to a specific current system or a et al. [1991] from Geosat in the tropical Pacific ocean. specific period. Global and synoptic views of the upper- layer oceanic circulation are not easy to obtain. Usually, authors only discuss global analysis on a seasonal cycle 3. Seasonal Variability of the Geostrophic Surface basis [Richardson and McKee, 1984; Stramma and Schott, Circulation in the Tropical Atlantic 1999]. Generalizations can be hypothesized but with the caveat that time-space variations often produce a picture [5] Figure 1 shows the first Empirical Orthogonal Func- differencing from this [Bourle`s et al., 1999b; Richardson tion (hereafter EOF) for the seasonal variability of this T/P and Reverdin, 1987; Schott et al., 1993]. During the last surface geostrophic circulation. It accounts for 60% of the decade, satellite altimetric data have offered a new solution total variance. It represents a simultaneous acceleration to the problem of data paucity. Beside regional investiga- (deceleration) of the major equatorial currents: the westward tions [Arnault et al., 1999; Goni and Johns, 2001], they can South Equatorial Current (SEC), mainly between 2N and lead to global maps of geostrophic currents [Carton and 5S, and the eastward North Equatorial CounterCurrent Katz, 1990; Didden and Schott, 1993; Bonjean and (NECC), between 5 and 7N in the eastern part of the Lagerloef, 2002]. In this paper, we used the TOPEX/Po- basin. In addition to this large scale pattern the EOF presents a meso-scale structure located around 42W, 4N. This EOF principal component has a large major extremum Copyright 2004 by the American Geophysical Union. in June–July when the NECC and SEC flow rapidly. It also 0094-8276/04/2003GL019210$05.00 reveals two opposite extrema, one between January and

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even if an anticyclonic feature is clearly revealed in fall 1997.

5. Discussion

[9] Richardson and Walsh [1986; hereafter RW86] ana- lyzed shipdrift climatological data in the tropical Atlantic ocean using EOF decomposition. Their second function is very similar to our first seasonal one with a simultaneous speeding up and slowing down of the equatorial NECC and SEC. Due to a finer mesh-size, our study also reveals western recirculation along the Brazilian coast which was not so evident in RW86’s study. Considering the differences in terms of dynamics between altimetric geostrophic cur- rents and shipdrifts (including Ekman drift and ship hull effects), differences in terms of EOF explained variances are not surprising between these two studies. In our analysis, Figure 1. First seasonal EOF of T/P geostrophic current: the equatorial geostrophic convergence is not balanced by spatial structure (up) and principal component (bottom). the Ekman divergence. It implies that this convergence is clearly depicted in the second function of our analysis, contrary to RW86. Therefore, the seasonal features in our analysis seem robust enough to discuss departures (interan- April corresponding to the weakest NECC and SEC, the nual variability) from this cycle. second one in October that could be associated with the [10] In contrast to the strong El Nino events in the Pacific North Brazil Current (NBC) dynamics [Bourle`s et al., Ocean, interannual variability in the tropical Atlantic is 1999a; 1999b]. weaker than the seasonal signal and has only received [6] The second EOF (28% of the variance, not shown) specific attention recently. This interannual variability can mainly reveals convergence/divergence of the geostrophic be separated into 2 different modes [e.g., the review by flow in the equatorial band in response to trade wind Me´lice and Servain, 2003]. The first one can be compared forcing. Divergence occurs in spring, when the winds are to the Pacific El Nin˜o and develops in the equatorial low, convergence occurs during summertime when the region, in the Gulf of Guinea [Greiner et al., 1998]. The winds and thus the equatorial pressure gradient intensify. second mode is characterized by large-scale sea surface temperature (SST) fluctuations [Moura and Shukla, 1981], 4. Interannual Variability of the Geostrophic sometimes referred as a "dipole" index. Recent studies Surface Circulation in the Tropical Atlantic debate the existence of a true anti-symmetrical structure dipole configuration. [7] The same EOF analysis was performed over the [11] However, most of these studies are based on SST interannual cycle 1992–2002 of the geostrophic current data or model results. Recently, Vauclair and Dupenhoat series. Monthly fields were first computed from the 5-day [2001] identify several warm events during 1979–1999 series and the seasonal cycle was removed for the EOF analysis. Results show that most of the variability is located north of the equator, so we will restrict our analysis to the northern part of the domain. Figure 2 shows the first and second EOFs (13% and 9% of the variance respectively). The general pattern of the first function is a large scale recirculation between the NECC and SEC, 2 and 8N, 25 and 50W. East of 25W, only the SEC structure seems to be affected. The principal component associated with this geographical pattern is indicative of a series of events when the SEC and NECC vary simultaneously. The most striking ones occur in 1996–1997, then in 2001–2002. The SEC and NECC increase during the boreal fall in 1996 to reach maximum values in spring-summer 1997. In 2001, the currents start to intensify at the beginning of the year to reach maximum values in December. The same EOF analysis performed separately over each velocity component shows that this interannual variability is only linked to the zonal velocity. [8] The second EOF shows a clear recirculation in the western basin, around 5N and between 40 and 50W but Figure 2. Interannual (1992–2002) EOFs of T/P geos- compared to the previous pattern, this one is of lesser spatial trophic current: spatial structure for EOF 1 (up), spatial scale. It seems to be associated merely to anti-clockwise structure for EOF 2 (middle) and principal components anomalies, for example in fall 1996 and in summer 2001, (EOF 1 solid line, EOF 2 dash-dotted line) (bottom).

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conclude on the mechanisms involved in these teleconnec- tions. The results obtained differ from one study to another although they all are done with SST analysis either from numerical models or observations. Our study is the first one based on observed oceanic circulation, and also the first one, to our knowledge, suggesting such connection with a 14 to 17-month delay. However this diagnosis needs to be reinforced by further studies, for instance with numerical experiments, which are beyond the scope of this present short descriptive paper. Figure 3. Correlations between the principal component of the first EOF and NAO (DJF, thick line) or ENSO (DJF, 6. Conclusion thin line). NAO and ENSO lead at positive lags. Dashed lines indicate the 5% level for zero correlation. [14] This study based on altimetric T/P data in the tropical Atlantic Ocean reveals how powerful satellite altimetric data can be for ocean circulation understanding. We were able to describe seasonal variability of the tropical from a global temperature profile data set. Their results Atlantic geostrophic circulation, and what is more interest- reveal that considering either SST, thermocline depth or ing, to identify specific patterns (temporal and geographi- heat content leads to different patterns of variability due to cal) of interannual variability which can only be obtained different contributions of dynamics and thermodynamics. using this altimetric series. The success of T/P, now pursued However the 2-layer system which prevails in the tropics by JASON mission, will contribute in answering climate implies rather similar patterns for thermocline depth, heat questions still pending. Future work will be to understand content and dynamic height, thus geostrophic current. the dynamics and thermodynamics responsible for these [12] The absence of any low frequency signal south of variations using both numerical models and altimetry. In 5S is the first significant result of our geostrophic circula- particular, it will be necessary to investigate how the tion analysis. The northern hemisphere is driving most of interannual oceanic circulation anomalies depicted in this low frequency variability between 1992 and 2002. 1996–1997 then in 2001–2002 can be related to extra- Notice that the area of the Benguela upwelling, where a tropical climate indexes such as North Atlantic Oscillation significant event occurred in 1995 [Gammelsrod et al., (NAO) or even Antarctic Oscillation (AAO) indexes. 1998], was southward of our domain. Anomalies in oceanic circulation are all the more though [13] As a second result, our EOF analysis splits this important than they imply anomalies in heat and mass interannual variability in both large-scale and regional transports, and thus this is an important step towards global recirculation schemes, with similar explained variances. climate variability. Important year-to-year variations have been noticed during the North Brazil Ring Experiment in the NECC retroflection [15] Acknowledgments. This work was funded by the french IRD area, in agreement with our results concerning the western (Institut de Recherche en De´veloppement) and by CNES (Centre National meso-scale activity [S. Garzoli, personal communication, d’Etudes Spatiales). S. Arnault and E. Kestenare are supported by IRD. 2002]. For the large scale one, the most striking events Special thanks to Y. Me´nard (CNES) for helpful comments on altimetric data processing, and to G. Reverdin and N. Lefevre. The climatic NAO and occur in 1996–1997, then in 2001–2002. They cannot ENSO indexes have been provided by the UK Climatic Research Unit simply be related to El Nino events, one of the best (www.cru.uea.ac.uk) and by the US Climatic Predication Center candidates for external connection. Indeed, following usual (www.cpc.noaa.gov). Walker circulation displacement schemes [Enfield and References Mayer, 1997], warm Pacific events should lead by 3 to Arnault, S., B. Bourle`s, Y. Gouriou, and R. Chuchla (1999), Intercompar- 5 months a warm signature in the northern tropical Atlantic. ison of the upper layer circulation of the western equatorial Atlantic: Preliminary results establishing correlations between the In-situ and satellite data, J. Geophys. Res., 104(C9), 21,171–21,194. time series of our first EOF principal component and the Bonjean, F., and G. Lagerloef (2002), Diagnostic model and analysis of the surface currents in the tropical Pacific Ocean, J. Phys. Oceanogr., 32, ENSO (El Nin˜o-Southern Oscillation) winter (December– 2938–2954. January–February) index do not evidence any significant Bourle`s, B., Y. Gouriou, and R. Chuchla (1999a), On the circulation in the coefficient when ENSO leads (Figure 3) On the contrary, a upper layer of the western equatorial Atlantic, J. Geophys. Res., 104(C9), slight correlation can be noticed around 6–7 months when 21,151–21,170. Bourle`s, B., R. L. Molinari, W. E. Johns, W. D. Wilson, and K. D. Leaman ENSO lags. The 1997 tropical Atlantic event occurs 7 months (1999b), Upper layer currents in the western tropical north Atlantic, before the 1997–98 El Nino. Maybe the correlation we J. Geophys. Res., 104(C1), 1361–1375. found when ENSO lags the tropical Atlantic is only the Bretherton, F., R. Davis, and C. Fandry (1976), A technique for objective analysis and design of oceanographic experiments applied to MODE-73, signature of these successive features. On the other hand, Deep Sea Res., 23, 559–582. similar correlations with NAO (North Atlantic Oscillation) Carton, J. A., and E. J. Katz (1990), Estimates of the zonal slope and winter index reveal significant peaks at 14 to 17 months seasonal transport of the Atlantic North Equatorial Countercurrent, J. Geophys. Res., 95(C3), 3091–3100. when NAO leads. 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