Wind-induced upwelling in the Kerguelen Plateau region

S T Gille,R Morrow,M M Carranza,R Cambra

doi:10.5194/bg-11-6389-2014

S T Gille, R Morrow + Show 2 more

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https://doi.org/10.5194/bg-11-6389-2014

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Abstract

Abstract. In contrast to most of the Southern Ocean, the Kerguelen Plateau supports an unusually strong spring chlorophyll (Chl a) bloom, likely because the euphotic zone in the region is supplied with higher iron concentrations. This study uses satellite wind, sea surface temperature (SST), and ocean color data to explore the impact of wind-driven processes on upwelling of cold (presumably iron-rich) water to the euphotic zone. Results show that, in the Kerguelen region, cold SSTs correlate with high wind speeds, implying that wind-mixing leads to enhanced vertical mixing. Cold SSTs also correlate with negative wind-stress curl, implying that Ekman pumping can further enhance upwelling. In the moderate to high eddy kinetic energy (EKE) regions surrounding Kerguelen, we find evidence of coupling between winds and SST gradients associated with mesoscale eddies, which can locally modulate the wind-stress curl. This coupling introduces persistent wind-stress curl patterns and Ekman pumping around these long-lived eddies, which may modulate the evolution of Chl a in the downstream plume far offshore. Close to the plateau, this eddy coupling breaks down. Kerguelen has a significant wind shadow on its downwind side, which changes position depending on the prevailing wind and which generates a wind-stress curl dipole that shifts location depending on wind direction. This leads to locally enhanced Ekman pumping for a few hundred kilometers downstream from the Kerguelen Plateau; Chl a values tend to be more elevated in places where wind-stress curl induces Ekman upwelling than in locations of downwelling, although the estimated upwelling rates are too small for this relationship to derive from direct effects on upward iron supply, and thus other processes, which remain to be determined, must also be involved in the establishment of these correlations. During the October and November (2011) KErguelen Ocean and Plateau compared Study (KEOPS-2) field program, wind conditions were fairly typical for the region, with enhanced Ekman upwelling expected to the north of the Kerguelen Islands.

Highlights

The Southern Ocean is characterized as a region of high macronutrients but low chlorophyll (HNLC), where low concentrations of dissolved iron inhibit growth of phytoplankton, despite the presence of ample quantities of macronutrients (Martin et al, 1990; Boyd, 2002; de Baar et al, 2005; Hopkinson et al, 2007)
Due east of Kerguelen, in the Kerguelen wind shadow region, high winds correlate with cold sea surface temperature (SST) (Fig. 2a), but not necessarily with high chlorophyll a (Chl a), implying that iron and macronutrient sources are somewhat different near the Kerguelen Plateau than they are in the open ocean
SST and satellite wind data are more limited in October and November, 2011, than they have been in earlier years, Chl a distributions in 2011 (Fig. 1c) are typical of climatological Chl a distributions in the region, suggesting that KErguelen Ocean and Plateau compared Study (KEOPS)-2 is a fairly typical year

Summary

Introduction

The Southern Ocean is characterized as a region of high macronutrients but low chlorophyll (HNLC), where low concentrations of dissolved iron inhibit growth of phytoplankton, despite the presence of ample quantities of macronutrients (Martin et al, 1990; Boyd, 2002; de Baar et al, 2005; Hopkinson et al, 2007). The plateau surrounding the Kerguelen Islands supports a strong bloom, which has been the target of two KErguelen Ocean and Plateau compared Study (KEOPS) field campaigns (e.g., Blain et al, 2007, 2008; Park et al, 2008a, b, 2014b; van Beek et al, 2008; Sanial et al, 2014; Zhou et al, 2014; van der Merwe et al, 2014, and other papers in this volume). KEOPS results show evidence that, in comparatively shallow water over the Kerguelen Plateau, bottom waters are enriched in dissolved iron. Enhanced vertical mixing in the upper ocean may help to bring dissolved iron upwards into the euphotic zone (Park et al, 2008a)

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