Gal\xe1pagos upwelling driven by localized wind\u2013front interactions

Alexander Forryan,Alberto C Naveira Garabato,A J George Nurser,Alexander R Hearn,Clément Vic

doi:10.1038/s41598-020-80609-2

Alexander Forryan, Alberto C Naveira Garabato + Show 3 more

Open Access

https://doi.org/10.1038/s41598-020-80609-2

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Abstract

The Galápagos archipelago, rising from the eastern equatorial Pacific Ocean some 900 km off the South American mainland, hosts an iconic and globally significant biological hotspot. The islands are renowned for their unique wealth of endemic species, which inspired Charles Darwin’s theory of evolution and today underpins one of the largest UNESCO World Heritage Sites and Marine Reserves on Earth. The regional ecosystem is sustained by strongly seasonal oceanic upwelling events—upward surges of cool, nutrient-rich deep waters that fuel the growth of the phytoplankton upon which the entire ecosystem thrives. Yet despite its critical life-supporting role, the upwelling’s controlling factors remain undetermined. Here, we use a realistic model of the regional ocean circulation to show that the intensity of upwelling is governed by local northward winds, which generate vigorous submesoscale circulations at upper-ocean fronts to the west of the islands. These submesoscale flows drive upwelling of interior waters into the surface mixed layer. Our findings thus demonstrate that Galápagos upwelling is controlled by highly localized atmosphere–ocean interactions, and call for a focus on these processes in assessing and mitigating the regional ecosystem’s vulnerability to 21st-century climate change.

Highlights

The Galápagos archipelago, rising from the eastern equatorial Pacific Ocean some 900 km off the South American mainland, hosts an iconic and globally significant biological hotspot
The islands are renowned for their unique wealth of endemic species, which inspired Charles Darwin’s theory of evolution and today underpins one of the largest UNESCO World Heritage Sites and Marine Reserves on Earth
Crosscorrelation of the amplitude of the sea surface temperature (SST) pattern with the zonal and meridional components of the wind stress reveals a significant relationship between SST in the upwelling-prone area to the west of the archipelago and the strength of meridional winds (Fig. 2c)

Summary

Introduction

The Galápagos archipelago, rising from the eastern equatorial Pacific Ocean some 900 km off the South American mainland, hosts an iconic and globally significant biological hotspot. Establishing the causes of the phytoplankton’s marked seasonality and climate sensitivity is key to assess the resilience of the regional ecosystem against the mounting pressures of contemporary climatic change This is illustrated by the observed correspondence of the most severe perturbations to the islands’ ecosystem in recent decades with major reductions in phytoplankton levels, coinciding at times with strong El Niño e vents[3]. Present understanding of the upwelling’s causes, dating back to the 1960s, often attributes a pivotal regulatory role to the sub-surface Equatorial Undercurrent (EUC)[1,6,7,8] (Fig. 1a) In this view, waters conveyed eastward by the EUC at depths of up to 100 m are thought to rise upon colliding with the islands, such that fluctuations in upwelling are presumed to result from remotelyforced or stochastic changes in the current’s intensity and path. Observations of the EUC’s impingement on the archipelago are scarce[9,10], while evidence of the upwelling’s responsiveness to fast-evolving, transitory phenomena such as tropical instability waves[5,11] and, possibly, wind forcing[12] suggests that other, more geographically focussed processes may be at play

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