Mesoscale eddies and high chlorophyll concentrations off central Chile (29°–39°S)

GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L12604, doi:10.1029/2007GL029541, 2007 Mesoscale eddies and high chlorophyll concentrations off central Chile (29°–39°S) Marco A. Correa-Ramirez,1 Samuel Hormazábal,2 and Gabriel Yuras2 Received 1 February 2007; revised 10 April 2007; accepted 16 May 2007; published 20 June 2007. [ 1 ] The offshore propagation of mesoscale eddies contributes significantly to expanding the area of high chlorophyll concentration beyond the coastal upwelling center as shown by satellite data of chlorophyll, wind stress, sea level anomalies between (1997 – 2003) off central Chile. Mesoscale eddies (6.45  103 km2) formed near the coastal zone in spring-summer have mean seaward speeds of 1.68 km d 1 and reach 500 km offshore in winter. Chlorophyll concentrations along eddy paths are elevated by nutrient pumping and by nutrients and/or chlorophyll transported from the coastal zone. Eddies make up >50% of the winter (weak upwelling) chlorophyll peak in the coastal transition zone. Coupling between eddies and upwelling extends central Chile’s high productivity area offshore and prolongs it towards winter. Citation: Correa-Ramirez, M. A., S. Hormazábal, and G. Yuras (2007), Mesoscale eddies and high chlorophyll concentrations off central Chile (29° – 39°S), Geophys. Res. Lett., 34, L12604, doi:10.1029/2007GL029541. 1. Introduction [2] Coastal upwelling, driven by alongshore (equatorward) wind stress and characteristic of eastern boundary current systems, brings nutrient-rich waters to the surface. The alongshore wind stress in the Peru-Chile Current System (PCCS) shows a strong annual cycle where the upwelling-favorable wind maximum moves southward from spring through summer [Shaffer et al., 1999]. High primary production near the coast is related to wind stress variability and upwelling centers [Daneri et al., 2000]. High (>0.3 mg m3) satellite chlorophyll (CHL) concentrations extend offshore, agreeing with the wind stress mean intensity along the PCCS (Figure 1). However, this relationship is not completely consistent, as the winter CHL peak in the coastal transition zone (CTZ) is not related to upwelling-favorable wind stress [Yuras et al., 2005]. Strong mesoscale eddies and meandering currents characterize the CTZ [Hormazábal et al., 2004] and are known to travel westward like Rossby waves [Hormazábal et al., 2004; Leth and Shaffer, 2001]. These eddies have been observed associated to high CHL concentrations [Thomas, 1999] (S. Hormazábal et al., Mesoscale eddies and jack mackerel (Trachurus murphyi) distribution in the eastern South Pacific, submitted to Progress in Oceanography, 2007) (hereinafter referred to as Hormazábal et al., submitted manuscript, 2007) suggest1 Programa de Doctorado en Oceanografı́a, Departamento de Oceanografı́a, Universidad de Concepción, Concepción, Chile. 2 Departamento de Geofı́sica, Universidad de Concepción, Concepción, Chile. Copyright 2007 by the American Geophysical Union. 0094-8276/07/2007GL029541 ing a link between eddies and the offshore extensions of high CHL in the PCCS. [3] The open ocean surface layers are nutrient limited. Here, Ekman pumping, baroclinic Rossby waves, and mesoscale eddies are the dominant physical processes driving nutrients to the surface and enhancing the phytoplankton productivity. Rossby waves have spatial scales >500 km and yield sea level amplitude signals <10 cm that are mirrored as vertical displacements of the thermocline with the opposite sign and 3 orders of magnitude greater [Chelton and Schlax, 1996]. These waves are associated with 5 –20% of the oceanic satellite CHL variance [Uz et al., 2001]. Killworth et al. [2004] suggest 3 mechanisms that may produce this signal: horizontal passive north-south advection; vertical nutrient upwelling; and vertical CHL upwelling. Surface particles accumulated in Rossby wave convergence zones can also cause sea color anomalies [Dandonneau et al., 2003]. Like Rossby waves, eddies can also cause nutrient pumping and the horizontal advection of chlorophyll/nutrient-rich waters originating near coastal zones [Aristegui et al., 1997]. Numerical models and observations of the Sargasso Sea [McGillicuddy et al., 1998] and the subtropical Pacific [Falkowski et al., 1991] suggest that eddies may supply the missing nutrients for production in the oligotrophic open ocean waters. Evidence from boundary systems is scarce and the role of mesoscale eddies and meanders on the CHL spatial and temporal distribution off central Chile remains unknown. 2. Methods [4] Satellite CHL, sea level height anomalies, and wind speeds in the PCCS from October 1997 to December 2003 were used. Daily SeaWiFS L3 CHL data (resolution of 9 km at nadir) was obtained from the Goddard Earth Sciences Distributed Active Archive Center (http:// daac.gsfc.nasa.gov). Several gaps in this data set are mainly caused by stratocumulus interference with the satellite sensors. Lesser stratocumulus occurrence off central than north Chile and Perú allowed a detailed study of local mesoscale characteristics. SeaWiFS gaps were filled with Kriging 3D (longitude-latitude-time) interpolation with a search radius of 63 km and 7 days, since the clouds interference in the region allows 3 images with over 50% area clouds free per week. The Kriging worked point by point when more than 35% of the valid data were evenly present. Sea level anomalies (merged TOPEX-Poseidon and ERS data distributed by AVISO; www.aviso.oceanobs.com) were used to obtain the geostrophic velocity field. Time mean eddy kinetic energy (EKE) per unit of mass was calculated following Hormazábal et al. [2004]. Zonal and meridional geostrophic current components were linear- L12604 1 of 5 L12604 CORREA-RAMIREZ ET AL.: EDDIES AND CHLOROPHYLL OFF CENTRAL CHILE L12604 Figure 1. (a) Time mean, SeaWiFS CHL calculated from 6 years daily observations. (b) Time mean, eddy kinetic energy calculated from 7.5 years of geostrophic velocity estimated from combined TOPEX/Poseidon and ERS-1/2 altimeter measurements. Dashed red line represents a 800 km offshore distance. (c) Cross-shore Ekman transport mean (solid line) and variance (dashed line) obtained from 6 years of daily QuikScat satellite wind data for the nearshore 1°  1° grid. interpolated to CHL data resolution (daily, 9 km) and eddies were identified with the Okubo-Weiss parameter (W), which allows separating vorticity-dominated regions where W < 0.2sw, here sw the spatial standard deviation of W [Isern-Fontanet et al., 2004]. Eddy sizes were calculated through enclosed isolines of W. Surface transport associated with eddies was estimated using a conservative depth of 200 m, although recent studies suggest eddies may have a vertical coherence >600 m depth (Hormazábal et al., submitted manuscript, 2007). Eddy track and speed were determined following the eddy rotational centers (the absolute vorticity maximum in the eddy). Cyclonic (CE) and anticyclonic eddies (AE) were distinguished by the vorticity sign ( or +, respectively) in their centers. Contour lines of W were used on CHL fields to determine the CHL concentration in CEs and AEs. [5] Daily sea surface wind stress is based on QuikScat L3 daily gridded ocean wind vectors (Jet Propulsion Laboratory SeaWinds Project, available at http://podaac.jpl.nasa. gov/quikscat). QuikScat data within 1° degree of the coast were averaged to get wind stress along the PCCS and lowpass filtered with a 91-day half amplitude and 200 day weights Cosine-Lanczos filter. 3. Satellite Chlorophyll and EKE in the PCCS [6] Satellite CHL in the PCCS can be separated into 3 latitudinal regions (Figure 1). The regions off Peru (10° – 15°S) and south of 30°S have wide extensions of higher CHL reaching 800 km offshore and are separated by the third region (18°– 30°S) with high CHL close to the coast (Figure 1a). Mean equatorward (upwelling-favorable) wind stress along the west coast of South America is similar, with prominent maxima at 15°S and 30°S and minimum equatorward wind stress at 20°S (Figure 1c). Coastal (0 – 100 km) wind forced upwelling and CHL oscillate in phase; however, >200 km offshore, in the CTZ, the annual chlorophyll peak is in winter, 6 months out of phase with the summer wind stress maxima [Yuras et al., 2005], suggesting that a mechanism other than alongshore wind forcing is responsible for the offshore extension and seasonality of CHL. In the CTZ along the PCCS, mesoscale eddies and meanders generate a broad band of high EKE, which extends from the coastal limit (100 km) to 600 – 800 km offshore [Hormazábal et al., 2004]. The EKE spatial distribution resembles the CHL distribution: two wide regions of high EKE values (off central Chile and Peru) are separated by a region of relatively low EKE (off northern Chile) (Figure 1b). 4. Eddies and Chlorophyll in the CTZ Off Central Chile [7] On average, 9.4 CEs (6.41  103 km2) and 10.0 AEs (6.50  103 km2) were observed daily during this study, traveling mainly westward at mean speeds of 1.68 km d 1 (Table 1). These eddies are 2.5 times bigger and 0.5 times slower than a first barocline Rossby wave, and show residence times from 2 months to >1 year in the CTZ. According to their size, speed, and considering a 200 m vertical length, each eddy produces a mean offshore transport of 0.32 Sv (Table 1), and all eddies account for a 6 Sv transport in the study region. Mean CHL in CEs (0.43 mg m 3) 2 of 5 L12604 CORREA-RAMIREZ ET AL.: EDDIES AND CHLOROPHYLL OFF CENTRAL CHILE Table 1. Basic Statistics for Eddies in the CTZ Off Central Chile (1997 2003)a Parameter Cyclonic eddies Number Vorticity (10 6 s 1) Area (103 km2) u speed (km d 1) v speed (km d 1) Transport (Sv) Persistency (days) Chlorophyll (mg m 3) Anticyclonic eddies Number Vorticity (10 6 s 1) Area (103 km2) u speed (km d 1) v speed (km d 1) Transport (Sv) Persistency (days) Chlorophyll (mg m 3) Coastal Transition Zone Chlorophyll (mg m 3) a Mean Min Max Std 13.50 6.00 6.50 0.71 0.19 0.31 59.5 0.43 8.00 1.94 2.83 0.00 0.00 0.00 15.0 0.17 20.00 20.50 26.10 4.91 7.22 1.62 430.0 0.82 2.09 2.13 3.10 1.21 1.28 0.20 61.7 0.11 13.82 5.88 6.41 0.78 0.54 0.34 64.7 0.33 8.00 2.10 2.83 0.00 0.00 0.00 15.0 0.19 21.00 21.00 24.00 4.44 6.77 1.71 339.0 0.52 1.83 1.97 2.85 1.22 1.30 0.20 62.5 0.08 0.22 0.61 0.08 0.37 Transport is in Sv units (1 Sv = 106 m (conservative depth). 3 s 1) and was obtained at 200 m was higher than that in AEs (0.33 mg m 3) and the regional CHL mean (0.37 mg m 3). [8] CHL values and CHL offshore extension in the CTZ are greater in austral winter (Figure 2b), opposite in phase to the alongshore wind stress which peaks in summer when the CTZ has a CHL minimum, as observed by Yuras et al. [2005]. The CHL inside an eddy follows its offshore path and can be seen in the vorticity contour lines (Figures 2c and 2d). Main eddy paths start near the coast in summer, reaching a maximum offshore distance in winter. Some L12604 structures are relatively stationary, remaining near the coast from winter until spring. High CHL in CE paths are observed often far from the coastal upwelling zone and could be associated to eddy CHL advection and eddy nutrient pumping. Conversely, high CHL values in AE paths was found near the coast and seemed to be more linked to eddy advection of CHL-rich waters from coastal upwelling. [9] The CHL winter maximum in the CTZ off central Chile is weaker at 30°S (Figure 3a) and stronger at 36°S (Figure 3e), agreeing with the latitudinal gradient of upwelling-favorable wind stress. The CHL winter maximum has 2 main peaks that occur first (later) in May– August (June – September) at 36°S (30°S). The annual cycle of CHL in the eddies shows about 4 paths per year: main two start near the coast (January – February at 36°S and the previous November – December at 30°S) (Figures 3b, 3c, 3f, and 3g) and reach their maximum offshore distances in winter, matching the observed CHL winter peak. The CHL in the eddies at both latitudes accounts for >50% of the wintertime CHL in the CTZ. The latitudinal phase difference of the winter CHL maximum may be produced by the southward propagation of the alongshore wind stress maximum, which delays the eddy generation southward. This phase difference can be increased by eddy equatorward acceleration, since eddies move in the b-plane like first barocline mode Rossby waves. Hence, mechanisms associated with eddies (pumping, transport) may be mainly responsible for extending the high productivity area offshore. 5. Discussion and Summary [10] Mesoscale eddies play an important role in expanding the zone of high satellite CHL concentrations beyond Figure 2. (a) Low-pass filtered alongshore wind stress obtained from daily QuikScat satellite wind data for a nearshore 1°  1° grid centered at 36°S and sea surface temperature anomalies for the El Niño 3.4 region. Distance from the coast – time plots of SeaWiFS chlorophyll data: (b) total chlorophyll, chlorophyll in (c) cyclonic and (d) anticyclonic eddies. Black contour lines represent vorticity values of 4  10 6. The phase speed of a linear first baroclinic Rossby wave is shown by a red dashed line and year boundaries are drawn as continuous red vertical bars. The figure spatial coverage is 230.000 km2. 3 of 5 L12604 CORREA-RAMIREZ ET AL.: EDDIES AND CHLOROPHYLL OFF CENTRAL CHILE L12604 Figure 3. Annual cycle of: (a) total chlorophyll, chlorophyll in (b) cyclonic and (c) anticyclonic eddies; (d) eddy (cyclonic + anticyclonic) contribution to observed chlorophyll in the CTZ off 30°S. (e – h) Same annual cycles off 36°S. The phase speed of a first baroclinic Rossby wave is shown by a yellow dashed line. the PCCS coastal zone. The EKE, used to define the spatial characteristics of eddy activity in the CTZ, resembles the CHL distribution and is related to the latitudinal gradient of upwelling-favorable wind stress. High resolution numerical models show that density fronts set by upwelling and alongshore wind stress are the main energy source for eddy generation in this region [Leth and Shaffer, 2001]. We found a 6-month lag between maximum upwelling-favorable winds and maximum EKE in the CTZ, that is, the time between an eddy’s generation and its seaward propagation. This lag may explain the oceanic CHL maximum observed in winter by Yuras et al. [2005] off Chile by way the eddyinduced CHL enhancement. A seasonal delayed (winter time) maximum EKE has also been observed offshore the California Current System from satellite data [Strub and James, 2000]. Numerical model results have associated this delay with eddy propagation [Di Lorenzo, 2003], and such propagation could be also responsible for the low coherence between CHL and winds observed by Thomas et al. [2004]. [11] Seaward eddy propagation may drive the large offshore transport of coastal waters [Leth and Shaffer, 2001]. We found that each eddy produces 0.3 SV offshore transport, less than half that reported by Hormazábal et al. [2004] using mooring observations, maybe because of our more conservative eddy depth scale. However, regional offshore transport produced by eddies (6 Sv) is about twice the equatorward surface transport estimated off central Chile [Shaffer et al., 2004], highlighting the importance of eddies in coastal-ocean fluxes of salt, nutrients, and heat. Eddies in the Alaska basin carry 35– 60% of the heat [Crawford, 2005] and are a major source of iron in this high nitrate-low chlorophyll region [Johnson et al., 2005]. High nutrient-CHL offshore transport by eddies was reported off northwestern Africa, showing these to be important mechanisms in the formation of oceanic organic matter and CHL distribution [Aristegui et al., 1997]. Eddy transport could explain the similarity between the high offshore distributions of EKE and CHL along the PCCS. Reductions in the CHL extension associated with offshore high EKE values, as observed off 33°S, could represent a local decoupling between eddy transport and coastal upwelling. [12] In the CTZ, CHL associated with CEs was higher than with AEs. High CHL values found far from the coastal upwelling zone can be associated with vertical eddy nutrient pumping, as observed in oceanic waters [Aristegui et al., 1997; McGillicuddy et al., 1998]. An important part of the CHL observed in CEs may not be associated with new primary production since eddy pumping also raises more phytoplankton cells to the surface and increases the satelliteobserved CHL without changing the total (vertical integrated) phytoplankton [Cipollini et al., 2001]. Dandonneau et al. [2003] proposed that a large part of the oceanic satellite CHL signal is caused by floating, non-CHL particles evolved from the ecosystem. Determining the nature of the CHL and eddy pumping strength in a CE is a complex task due to scarce in situ data. Recent hydrographic profiles of a CE in the CTZ off central Chile show high CHL concentrations are mainly associated with eddy-upwelled Equatorial Subsurface Waters, the main source of nutrients in the coastal upwelling zone of the PCCS (Hormázabal et al., submitted manuscript, 2007). Thus, the offshore extension of high CHL in the CTZ could be partially sustained by new production and eddy nutrient pumping. The relative importance of eddy pumping as a nutrient supply in the study area has not been addressed before. The strength of eddy nutrient pumping in the PCCS must be studied in order to fix regional ocean biogeochemical budgets. [13] The effect of mesoscale eddies on CHL could affect the distribution of pelagic fish such as jack mackerel (Trachurus murphyi), whose fishing ground is associated with 4 of 5 L12604 CORREA-RAMIREZ ET AL.: EDDIES AND CHLOROPHYLL OFF CENTRAL CHILE cyclonic eddies and coastal meander currents (Hormazábal et al., submitted manuscript, 2007). Its main spawning zone is located in the central Chile CTZ [Nuñez et al., 2004], where winter eddy CHL enhancement could be important to early life stages of the specie and is likely to affect recruitment. The relationships between early fish life stages and mesoscale structures in this zone remain unknown. [14] Eddies and Rossby waves have similar surface structures and propagation features, but their relationships are intricate and it may be difficult to distinguish between them [Hormazábal et al., 2004]. The squeezing of upper layers caused by a Rossby wave is similar to that caused by a small amplitude eddy and the same mechanism of nutrient enhancement should apply [Uz et al., 2001]. In subtropical gyres, Rossby waves have been associated with 5 – 20% of the observed CHL variance [Uz et al., 2001]. However, the larger size and slower mean speed of the eddies, along with their non-linear paths, suggest they could have a stronger influence on the CHL distribution in the central Chile CTZ than do Rossby waves. [15] Eddies seem to account for 50% of winter CHL in the CTZ, mainly due to increased CHL produced by eddy pumping and eddy offshore transport. This is a conservative estimate since the criteria used here (W) only considers CHL in the vorticity-dominated eddy core and not in the surrounding regions. The regions between eddies are important to the stirring and mixing of water properties [Isern-Fontanet et al., 2004]. Eddies may entrain CHL-rich waters in their outer rings through interactions with upwelling filaments [Aristegui et al., 1997] or adjacent eddies, forming a conveyor belt transport that injects CHL into the open sea [Crawford et al., 2005]. Numerical experiments have shown how eddy induced horizontal mixing along upwelling centers strongly increases regional primary production and CHL [Martin et al., 2002]. Thus, eddies could have a stronger influence on CHL than reported herein. Further studies must be done to establish the relative importance of pumping, transport, and mixing caused by eddies in the CTZ. [16] Eddies contribute significantly to increased CHL in the CTZ off central Chile. They are linked to the upwelling system, being generated from density fronts produced by upwelled waters. Their offshore propagation results in a seasonal CHL peak in the CTZ 6 months after the coastal summer CHL peak. Eddies can be considered to be a delayed effect of upwelling that increase the CHL concentration beyond the coastal zone in winter, when coastal upwelling is weaker. The coupling between upwelling and eddy activity, therefore, enables extended spatial and temporal high CHL biomass in the PCCS. [17] Acknowledgments. 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Hormazábal (2005), On the annual cycle of coastal and open ocean satellite chlorophyll off Chile (18° – 40°S), Geophys. Res. Lett., 32, L23604, doi:10.1029/2005GL023946. M. A. Correa-Ramirez, Programa de Doctorado en Oceanografı́a, Departamento de Oceanografı́a, Universidad de Concepción, Concepción, Chile. (mcorrea@udec.cl) S. Hormazábal and G. Yuras, Departamento de Geofı́sica, Universidad de Concepción, Concepción, Chile. (sam@dgeo.udec.cl; gabriel@dgeo.udec.cl) 5 of 5