Intrathermocline eddies at the Juan Fernandez Archipelago, southeastern Pacific Ocean

Isabel Andrade,Vincent Combes,Samuel Hormazabal

doi:10.3856/vol42-issue4-fulltext-14

Abstract

Regional Ocean Modeling System (ROMS) results, combined with chlorophyll-a (Chl-a) and satellite altimetry information as well as information from oceanographic cruises were analyzed to identify interactions between intrathermocline eddies (ITEs) and the Juan Fernandez Archipelago (JFA), and discuss their potential impact on surface Chl-a concentrations. The JFA is located off the coast of central Chile (33°S), and is composed of three main islands: Robinson Crusoe (RC), Alejandro Selkirk (AS) and Santa Clara (SC). Results indicate that the surface and subsurface anticyclonic eddies that interact with the JFA are formed primarily within the coastal transition zone between 33° and 39°S. ITEs are present within the JFA region with a semiannual frequency, mainly during the austral autumn, and have a weak surface expression in relation to the adjacent surface eddies, with a slow displacement (1.16 to 1.4 km d-1 ) in a northwest direction and a coherent structure for periods of ≥1 year. During the ITEs’ interaction with RC-SC islands and an adjacent seamount, a slight (prominent) thermocline deflection of the upper limit (lower) was observed. The horizontal extent (~70- 100 km) was greater than the internal Rossby deformation radius and the average vertical extent was ~400 m. The interaction between the weak surface expression of ITEs, identified with satellite altimetry, and the JFA persisted during autumn for nine weeks until reaching the winter period. Approximately one month after the beginning of the interaction between ITEs and the islands, increases in surface Chl-a associated with the eddy were observed, with values up to three times higher than adjacent oceanic waters.

Highlights

Intrathermocline eddies (ITEs) are mesoscale vortices characterized by a lens-like shape with maximum velocity 200-300 m below the thermocline (Dugan et al, 1982; Gordon et al, 2002; Colas et al, 2011)
We argue that intrathermocline eddies (ITEs) generated off central Chile affect the water properties within the Juan Fernández Archipelago, favoring the horizontal and vertical transport of nutrients/heat known as eddy pumping, that corresponds to one of the mechanisms associated with the biological production increase at these oceanic zones (Falkowski et al, 1991; Aristegui et al, 1997; McGillicuddy et al, 2007)
The results of this study revealed the presence of longlife (≥1 year) and weak surface expression ITEs interacting with the topography of the Juan Fernández Archipelago (JFA) with a semiannual frequency, mainly in the austral autumn period

Summary

Introduction

Intrathermocline eddies (ITEs) are mesoscale vortices characterized by a lens-like shape with maximum velocity 200-300 m below the thermocline (Dugan et al, 1982; Gordon et al, 2002; Colas et al, 2011). When the ITEs detach from the PCUC, they transport high-nutrient and low-oxygen concentrations from the Equatorial Subsurface Water (ESSW) into the open ocean (Johnson & McTaggart, 2010; Hormazábal et al, 2013). ITEs can modify productivity offshore through the transport of nutrient-rich ESSW waters. We argue that ITEs generated off central Chile affect the water properties within the Juan Fernández Archipelago, favoring the horizontal and vertical transport of nutrients/heat known as eddy pumping, that corresponds to one of the mechanisms associated with the biological production increase at these oceanic zones (Falkowski et al, 1991; Aristegui et al, 1997; McGillicuddy et al, 2007)

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Conclusion