Insight into anthropogenic forcing on coastal upwelling off south-central Chile

Catalina Aguirre,Laura Farías,Diego Silva,Sebastián García-Loyola,Giovanni Testa,Jody W Deming,Jean-Éric Tremblay

doi:10.1525/elementa.314

Abstract

Coastal upwelling systems off the coasts of Peru and Chile are among the most productive marine ecosystems in the world, sustaining a significant percentage of global primary production and fishery yields. Seasonal and interannual variability in these systems has been relatively well documented; however, an understanding of recent trends and the influence of climate change on marine processes such as surface cooling and primary productivity is limited. This study presents evidence that winds favorable to upwelling have increased within the southern boundary of the Humboldt Current System (35°–42°S) in recent decades. This trend is consistent with a poleward movement of the influence of the Southeast Pacific Anticyclone and resembles the spatial pattern projected by Global Circulation Models for warming scenarios. Chlorophyll a levels (from 2002 to present) determined by satellite and field-based time-series observations show a positive trend, mainly in austral spring–summer (December–January–February), potentially explained by observed increments in nutrient flux towards surface waters and photosynthetically active radiation. Both parameters appear to respond to alongshore wind stress and cloud cover in the latitudinal range of 35°S to 42°S. In addition, net annual deepening of the mixed layer depth is estimated using density and temperature profiles. Changes in this depth are associated with increasing winds and may explain cooler, more saline, and more productive surface waters, with the latter potentially causing fluctuations in dissolved oxygen and other gases, such as nitrous oxide, sensitive to changes in oxygenation. We argue that these recent changes represent, at least in part, a regional manifestation of the Anthropocene along the Chilean coast.

Highlights

Eastern ocean boundaries are some of the most p roductive areas on Earth as a result of wind-driven coastal upwelling systems (CUS), which supply nutrient-rich subsurface waters to the surface layer
Seasonal analysis indicates that in winter (June–July–August) the mixed layer depth (MLD) for temperature criterion (Tcrt) has a negative trend of –3.85 m decade–1 (p = 0.58; Figure 4c), and for density criterion (Dcrt) a trend of –5.06 m decade–1 (p = 0.44; Figure 4f), while during the upwelling season (December–January–February), the MLD trend displays a slight rise of 1.84 m decade–1 (p = 0.25) for Tcrt (Figure 4b), with a slight deepening of –1.44 m decade–1 (p = 0.41) for Dcrt (Figure 4e). These results indicate that the winter MLD represents the greatest contribution to general deepening; they are consistent with results obtained from simulations using a ROMS1D model based on climatological databases corresponding to the geographical location of Station 18 (St18) (García-Loyola S, personal communication), where sensitivity analysis forced by a variation in momentum, heat, and freshwater fluxes shows greater variation in winter as compared to summer
Natural interdecadal variability associated with the Pacific Decadal Oscillation (PDO) likely plays a key role in changes in surface, atmospheric and oceanographic conditions, it is possible that anthropogenic climate change contributes to or amplifies these shifts

Summary

Introduction

Eastern ocean boundaries are some of the most p roductive areas on Earth as a result of wind-driven coastal upwelling systems (CUS), which supply nutrient-rich subsurface waters to the surface layer. Large-scale tropospheric subsidence that maintains the Southeast Pacific Anticyclone (SPA) influences wind intensity and direction, regulating Chilean CUS (Figure 1a). The effects of climate change are likely to manifest in coastal areas in various ways, and changes in CUS represent one important example of these impacts (e.g., Echevin et al, 2012; Sydeman et al, 2014; Wang et al, 2015). Equatorward winds over the coastal ocean are responsible for a majority of CUS by triggering Ekman pumping and offshore Ekman transport (Pickett and Paduan, 2003; Bravo et al, 2016)

Methods

Results

Conclusion