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Wind-Induced Upwelling in the Kerguelen Plateau Region

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Wind-Induced Upwelling in the Kerguelen Plateau Region Biogeosciences, 11, 6389–6400, 2014 www.biogeosciences.net/11/6389/2014/ doi:10.5194/bg-11-6389-2014 © Author(s) 2014. CC Attribution 3.0 License. Wind-induced upwelling in the Kerguelen Plateau region S. T. Gille1, M. M. Carranza1, R. Cambra2,*, and R. Morrow2 1Scripps Institution of Oceanography, University of California, San Diego, USA 2Laboratoire d’Etudes en Géophysique et Océanographie Spatiale, Observatoire Midi-Pyrénées, 31400 Toulouse, France *now at: Laboratoire Atmosphères, Milieux, Observations Spatiales, Institute Pierre-Simon Laplace, Paris, France Correspondence to: S. T. Gille ([email protected]) Received: 14 May 2014 – Published in Biogeosciences Discuss.: 5 June 2014 Revised: 11 October 2014 – Accepted: 20 October 2014 – Published: 26 November 2014 Abstract. In contrast to most of the Southern Ocean, the Ker- During the October and November (2011) KErguelen Ocean guelen Plateau supports an unusually strong spring chloro- and Plateau compared Study (KEOPS-2) field program, wind phyll (Chl a) bloom, likely because the euphotic zone in the conditions were fairly typical for the region, with enhanced region is supplied with higher iron concentrations. This study Ekman upwelling expected to the north of the Kerguelen Is- uses satellite wind, sea surface temperature (SST), and ocean lands. color data to explore the impact of wind-driven processes on upwelling of cold (presumably iron-rich) water to the eu- photic zone. Results show that, in the Kerguelen region, cold 1 Introduction SSTs correlate with high wind speeds, implying that wind- mixing leads to enhanced vertical mixing. Cold SSTs also The Southern Ocean is characterized as a region of high correlate with negative wind-stress curl, implying that Ek- macronutrients but low chlorophyll (HNLC), where low con- man pumping can further enhance upwelling. In the moder- centrations of dissolved iron inhibit growth of phytoplank- ate to high eddy kinetic energy (EKE) regions surrounding ton, despite the presence of ample quantities of macronutri- Kerguelen, we find evidence of coupling between winds and ents (Martin et al., 1990; Boyd, 2002; de Baar et al., 2005; SST gradients associated with mesoscale eddies, which can Hopkinson et al., 2007). Exceptions to the HNLC regime oc- locally modulate the wind-stress curl. This coupling intro- cur in a few limited areas, primarily near the Antarctic con- duces persistent wind-stress curl patterns and Ekman pump- tinental shelf (e.g., Moore and Abbott, 2000; Holm-Hansen ing around these long-lived eddies, which may modulate the et al., 2004; Kahru et al., 2007; Charette et al., 2007; Ar- evolution of Chl a in the downstream plume far offshore. rigo et al., 2008; Dulaiova et al., 2009; Frants et al., 2013) or Close to the plateau, this eddy coupling breaks down. Ker- near specific islands (e.g., Blain et al., 2007; Pollard et al., guelen has a significant wind shadow on its downwind side, 2007; Planquette et al., 2007; Venables et al., 2007; Korb which changes position depending on the prevailing wind et al., 2008), where iron can be supplied to surface waters and which generates a wind-stress curl dipole that shifts lo- from lithogenic sources or from ocean sediments (e.g., Mea- cation depending on wind direction. This leads to locally en- sures et al., 2012). The plateau surrounding the Kerguelen hanced Ekman pumping for a few hundred kilometers down- Islands supports a particularly strong bloom, which has been stream from the Kerguelen Plateau; Chl a values tend to be the target of two KErguelen Ocean and Plateau compared more elevated in places where wind-stress curl induces Ek- Study (KEOPS) field campaigns (e.g., Blain et al., 2007, man upwelling than in locations of downwelling, although 2008; Park et al., 2008a,b, 2014b; van Beek et al., 2008; the estimated upwelling rates are too small for this relation- Sanial et al., 2014; Zhou et al., 2014; van der Merwe et al., ship to derive from direct effects on upward iron supply, and 2014, and other papers in this volume). KEOPS results show thus other processes, which remain to be determined, must evidence that, in comparatively shallow water over the Ker- also be involved in the establishment of these correlations. guelen Plateau, bottom waters are enriched in dissolved iron Published by Copernicus Publications on behalf of the European Geosciences Union. 6390 S. T. Gille et al.: Wind-induced upwelling near Kerguelen and then advected horizontally by the Polar Front (Blain ences on winds. For example, Kerguelen is 1850 m tall and et al., 2008; van Beek et al., 2008; Sanial et al., 2014). En- produces a substantial wind shadow, which, in turn, modifies hanced vertical mixing in the upper ocean may help to bring the wind-stress curl (Chelton et al., 2004). The present study dissolved iron upwards into the euphotic zone (Park et al., was thus carried out as a complement to the second KEOPS 2008a). field campaign (KEOPS-2), with the aim to assess the ex- In most of the Southern Ocean, wind forcing is thought tent to which wind-driven upwelling mechanisms might con- to influence seasonal and interannual variations in biological tribute to the iron availability and biological productivity in productivity through several competing mechanisms. Strong the region around Kerguelen. winds can deepen the mixed layer, bringing up to the eu- In this study, we focus on two key issues. First, for the re- photic zone cold water that is rich in iron and macronutri- gion around Kerguelen, we evaluate the extent to which the ents, thus enhancing phytoplankton growth. In some places, wind stress and wind-stress curl driven mechanisms control and particularly in low-stratification environments, such as SST (and, by extension, iron, macronutrients, and Chl a, as those that commonly occur in the Southern Ocean, strong shown in Fig. 1). This is examined both on the regional scale winds can also deepen the mixed layer enough to mix phy- and also for wind-eddy coupling associated with mesoscale toplankton over large vertical distances, thus reducing aver- motions. As part of this analysis, we specifically consider the age light levels and possibly limiting phytoplankton growth role that the Kerguelen wind shadow could play in modifying (e.g., Kahru et al., 2010; Fauchereau et al., 2011). Typically wind-driven upwelling. Second, we evaluate whether wind- along the primary axis of the Antarctic Circumpolar Current driven processes play a significant role in the region around (ACC), around 50◦ S in the Kerguelen region, mean wind Kerguelen, where a large bloom is present. SST will serve as stress is a maximum, and wind-stress curl is zero. Depend- a proxy for iron and macronutrients for much of this anal- ing on the sign of the wind-stress curl, this may play a further ysis. Chl a is more difficult to interpret than SST, because role in biological productivity: south of the zero wind-stress satellite-derived Chl a estimates are only available in cloud- curl line, Ekman pumping drives upwelling, which can bring free conditions, which are comparatively rare in the Southern iron and macronutrients upward from below the mixed layer, Ocean, and because phytoplankton require time to grow af- while, to the north, Ekman pumping results in downwelling ter they are first exposed to optimal iron, macronutrient, and (Carranza and Gille, 2014). While wind mixing and Ekman light levels, and thus Chl a concentrations can lag physical pumping provide a basic framework for evaluating large- parameters by an increment of time that has not been deter- scale processes, at any given location, local effects can sig- mined precisely. nificantly modify large-scale wind processes. At eddy-length SST and Chl a can also differ in fundamental ways: SST scales, sea surface temperature (SST) anomalies can modify anomalies can influence wind, inducing a coupled air–sea surface winds, which tend to accelerate over warm water and interaction, whereas Chl a concentrations do not influence slow over cool water, resulting in local modifications to the wind. Section 2 discusses the range of satellite data and re- wind-stress curl and divergence (e.g., O’Neill et al., 2003, analysis products that we consider for this analysis; Sect. 3 2005) and correspondingly to the local Ekman pumping ve- briefly summarizes the computational methodology; Sect. 4 locity. In addition, eddies themselves influence local chloro- presents the results; Sect. 5 provides a summary and conclu- phyll a (Chl a) concentrations, and this may be associated sions. both with horizontal advection induced by the eddies (e.g., Chelton et al., 2011), with eddy influences on upper ocean stratification (e.g., Mahadevan et al., 2012), or from strong vertical velocities induced within the filaments surrounding 2 Data the eddies (e.g., Lévy et al., 2012). While wind-driven upwelling appears to help determine Vector wind data were derived from scatterometer measure- variability in phytoplankton growth in regions of the South- ments obtained by the Quick Scatterometer (QuikSCAT) ern Ocean that are far from land (e.g., Kahru et al., 2010; satellite, which operated from June 1999 through November Fauchereau et al., 2011; Carranza and Gille, 2014), the role 2009 (Freilich et al., 1994). We made use of daily and weekly of wind is less clear in areas near islands where phytoplank- averages of the QuikSCAT level 4 gridded mean wind ton blooms are stronger and more persistent, and where a fields produced by L’Institut Français de Recherche pour broader range of processes might be expected to control iron l’Exploitation de la Mer’s Center for Satellite Exploitation availability. These processes include oceanic transport pro- and Research (Ifremer/CERSAT). Data are released on a 0.5◦ cesses, which might vary the sediment content of water ad- regular grid (cersat.ifremer.fr).
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