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Chlorophyll Variability and Eddies in the Brazil–Malvinas Confluence

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Chlorophyll Variability and Eddies in the Brazil–Malvinas Confluence ARTICLE IN PRESS Deep-Sea Research II 51 (2004) 159–172 Chlorophyll variability and eddies in the Brazil–Malvinas Confluence region Carlos A.E. Garciaa,*, Y.V.B. Sarmaa, Mauricio M. Mataa, Virginia M.T. Garciab a Department of Physics, Funda@ao* Universidade Federal do Rio Grande, Av. Italia,! km 8, Rio Grande RS 96201-900, Brazil b Department of Oceanography, Funda@ao* Universidade Federal do Rio Grande, Av. Italia,! km 8, Rio Grande RS 96201-900 Brazil Received27 January 2003; receivedin revisedform 21 July 2003; accepted30 July 2003 Abstract Ocean-color data from Sea-viewing Wide Field-of-view Sensor (SeaWiFS) have been used to investigate temporal andspatial variability of chlorophyll- a concentration in the Brazil–Malvinas Confluence (BMC) region (30–50S and 30–70W). Our analysis is basedon 60 monthly averagedand230 weekly averagedimages (9 Â 9km2 resolution) that span from October 1997 to September 2002 in the Southwestern Atlantic Ocean. A nonlinear model was used to fit the annual harmonic of the chlorophyll concentration anomalies in the region. Analysis of the spatial variability of model parameters has shown the existence of a markedannual cycle at several locations in the studyregion. We also have observedshorter-periodoscillating features on smaller spatial scale, associatedwith the BMC dynamics.These mesoscale features have periods of about 9–12 weeks and have a marked northward propagation with higher chlorophyll values relative to the surrounding waters. Further investigation of these mesoscale features with an advanced very high-resolution radiometer thermal infrared and TOPEX/POSEIDON (T/P) altimeter data has unveiled interesting eddy-like surface structures in the BMC region. These multiple eddies are clearly visible on both thermal and color imagery off southern Brazil. Furthermore, the SeaWiFS images have shown a decrease in chlorophyll-a concentration as these eddies propagate northward, due to mixing with chlorophyll-poor tropical waters carriedsouthwardby the Brazil Current. Several rings along the shelf region, which are probably generatedby shear instabilities, also have been detectedby the ocean-color images. r 2004 Elsevier Ltd. All rights reserved. 1. Introduction eddies (e.g., Olson et al., 1988). This frontal zone is known as the Brazil–Malvinas Confluence (BMC) The Brazil Current andthe Malvinas (also andis recognizedas one of the most energetic known as Falkland) Current converge along the regions of the worldoceans ( Gordon, 1989). The continental margin of South America within the strong mixing causes rapidcooling of subtropical latitude band 35–45S, where both currents flow waters brought by the Brazil Current, making seawardin a complex pattern of meandersand this area very important for understanding circu- lation andheat transport processes ( Wainer et al., *Corresponding author. Tel.: +55-53-233-6643; fax: +55- 2000). 53-233-6652. The typical position of BMC is at approximately E-mail address: [email protected] (C.A.E. Garcia). 38S(Gordon and Greengrove, 1986), but several 0967-0645/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2003.07.016 ARTICLE IN PRESS 160 C.A.E. Garcia et al. / Deep-Sea Research II 51 (2004) 159–172 studies have demonstrated remarkable variability Surface chlorophyll-a concentration over the of this position. For instance, Provost et al. (1992) shelf in the northern part of the area (31–34S) has studied the dominant periodic variations of been reportedto be high in coastal waters south of sea-surface temperature (SST) in the BMC region Patos Lagoon (>5 mglÀ1) andmuch lower (32–52S and50–70 W) from 202 advanced very (o0.5 mglÀ1) near the shelf break in early spring high-resolution radiometer (AVHRR) SST images (Ciotti et al., 1995). Farther south (approximately with 4 Â 4km2 resolution. Although the annual 38–39S), between the coast andthe shelf signal dominates the SST time series, strong break, chlorophyll-a is highest in early spring departures from this signal also are found in the (B4–10 mglÀ1), associatedwith the coastal front BMC frontal region. The authors further showed andthe development of water-column stability that a semi-annual frequency is present in the (Carreto et al., 1995). This nucleus moves toward region, associatedwith Southern Hemisphere the shelf break during spring, where SAW provide atmospheric forcing. Garzoli andGiulivi (1994) a rich nutrient source. Lowest values have been studied the BMC frontal position and its varia- measuredin winter ( o1 mglÀ1), while in summer bility through modeling experiments forced by chlorophyll is generally low in the shelf, except for climatological winds. They concluded that the a few sites under influence of SAW (Carreto et al., main source for the BMC frontal variability is a 1995). The region between approximately local windforcing associatedwith the seasonal 39–46S, along approximately 56W was sampled cycle in the Southern Hemisphere. No apparent during four cruises of the Atlantic Meridional correlation was foundbetween wind-forcing Transect (AMT, April 1996–October 1997), and events in the Antarctic Circumpolar Current and total chlorophyll (chlorophyll-a+divinyl chloro- observedanomalous northwardpenetration of the phyll-a) was in the range 0.2–1.2 mglÀ1 (Gibb et al., Malvinas Current. 2000). Relevant biological processes take place in the Ocean-color data have seldom been used to BMC region, where photosynthetic uptake of investigate the temporal andspatial scales of carbon is enhancedby the nutrient-rich subantarc- dynamical processes in the South Atlantic Ocean. tic waters (SAW) of the Malvinas Current, However, previous works from other areas have contributing significantly with the global carbon shown that chlorophyll from ocean-color sensors budget (Moore andAbbott, 2000 ). This area is can be usedas a goodtracer of water masses (e.g., considered an important region of ocean CO2 Ginzburg et al., 2002) andsurface currents (e.g., uptake, due to high biological utilization of carbon Garcia andRobinson, 1989 ). Spatial andtime in spring andsummer ( Feely et al., 2001). In coverage by previous ocean-color sensors (e.g., addition to the SAW, other sources of nutrients in Coastal Zone Color Scanner) did not allow such the region are the freshwater runoffs from both La analysis to be made in the past. Signorini et al. Plata River (approximately 36S) andPatos (1999) investigatedthe variability of sea-surface Lagoon (approximately 32S), which also produce chlorophyll using first-year Sea-viewing Wide water-column stratification over the continental Field-of-view Sensor (SeaWiFS) data, in conjunc- shelf (Brandhorst and Castello, 1971). In general, tion with an ocean circulation model, TOPEX/ phosphate levels in the low-salinity coastal waters POSEIDON (T/P) altimetry andwind-stress are similar to SAW ranges (0.4–1.0 mM), while data sets, in the tropical and subtropical Atlantic silicate is usually higher, reaching 45 mM(Carreto Ocean during 1997–1998 El Nin˜ o event. They et al., 1986; Niencheski andFillmann, 1997 ; Ciotti concluded that the regions of high-chlorophyll et al., 1995). On the other hand, nitrate levels can concentration are correlatedwith mesoscale and be very low in the freshwater plumes (o1 mM), large-scale physical phenomena. In the eastern contrasting with variable andhigh values tropical Atlantic, for instance, the absence of fall (o1–20 mM) in the shelf break or deeper waters blooms was correlatedwith SST anomalies andEl under SAW influence (Carreto et al., 1995; Ciotti Nin˜ o Southern Oscillation (ENSO) whereas the et al., 1995). Ekman pumping was responsible for spreading ARTICLE IN PRESS C.A.E. Garcia et al. / Deep-Sea Research II 51 (2004) 159–172 161 surface blooms over the region (Signorini et al., other instabilities observedoff southern Brazil 1999). coast. Raw images (HRPT) were processedto level In this study, we first investigate the spatial and L1A, andsubscenes of 900 pixels by 2100 lines temporal variability of chlorophyll-a concentra- from these full-resolution (1 Â 1km2) images were tion in the BMC region. Then, we explore the selectedfor further processing. Each subscene possibility of using the pigment concentration as a covers the geographical region between 27–50S passive tracer to study surface dynamics in the and37–58 W in the South Atlantic Ocean. The Southwestern Atlantic Ocean. We also present a subscenes were then processedto Level L2 using case study of the evolution of eddies in the area OC2 algorithm provided in SEADAS (version 4.1) during January 2001. The importance of ocean- software. The Level-2 images have a spatial color data in this context is highlighted by close resolution of 1 km. examination of temporally coincident SeaWiFS, AVHRR andT/P satellite data. 2.1.2. Advanced very high-resolution radiometer images AVHRR images, also receivedat Rio Grande 2. Data and methods station, were processedto generate SST fields (Kidwell, 1995). Both the AVHRR andSeaWiFS 2.1. Satellite data images from January 12, 2001 were analyzedin combination to study the mesoscale eddy-structure 2.1.1. Sea-viewing Wide Field-of-view Sensor fieldin the BMC. images We used60 monthly averagedand230 weekly 2.1.3. Altimetry data averagedSeaWiFS Level-3 StandardMapped Along-track sea-level anomalies (SLA) from the Images (SMI) with 9 Â 9km2 resolution to study T/P MergedGeophysical Data records( AVISO, the variability of chlorophyll-a concentration (in 1998) were usedto estimate the dynamic height logarithm scale)—log½C—in the BMC region. The signal associatedto the mesoscale features ob- data set corresponds to the period from October servedby the SeaWiFS sensor. The along-track 1997 to September 2002 (5 years) that were SLA usedhere refers to ascending track 239 on the available by NASA after reprocessing #4. The January 12, 2001 (cycle 306). The corrections original SMI, which have dimensions of 4096 needed to minimize the environmental effects on (longitude) Â 2048 (latitude) pixels, were converted the altimeter signal (tides, water vapor, iono- into subsamples of an area corresponding to sphere, etc.) were appliedaccording to the AVISO 30–50S and30–70 W.
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