High-Frequency Variability of the Surface Ocean Properties Off Central Chile During the Upwelling Season

Catalina Aguirre,Lucy Belmar,René Garreaud,Laura Farías,Laura Ramajo,Facundo Barrera

doi:10.3389/fmars.2021.702051

Abstract

The ocean off south-central Chile is subject to seasonal upwelling whose intensity is mainly controlled by the latitudinal migration of the southeast Pacific subtropical anticyclone. During austral spring and summer, the mean flow is equatorward favoring coastal upwelling, but periods of strong southerly winds are intermixed with periods of relaxed southerlies or weak northerly winds (downwelling favorable). This sub-seasonal, high-frequency variability of the coastal winds results in pronounced changes in oceanographic conditions and air-sea heat and gas exchanges, whose quantitative description has been limited by the lack of in-situ monitoring. In this study, high frequency fluctuations of meteorological, oceanographic and biogeochemical near surface variables were analyzed during two consecutive upwelling seasons (2016–17 and 2017–18) using observations from a coastal buoy located in the continental shelf off south-central Chile (36.4°S, 73°W), ∼10 km off the coast. The radiative-driven diel cycle is noticeable in meteorological variables but less pronounced for oceanographic and biogeochemical variables [ocean temperature, nitrate (NO3−), partial pressure of carbon dioxide (pCO2sea), pH, dissolved oxygen (DO)]. Fluorescence, as a proxy of chlorophyll-a, showed diel variations more controlled by biological processes. In the synoptic scale, 23 active upwelling events (strong southerlies, lasting between 2 and 15 days, 6 days in average) were identified, alternated with periods of relaxed southerlies of shorter duration (4.5 days in average). Upwelling events were related to the development of an atmospheric low-level coastal jet in response to an intense along-shore pressure gradient. Physical and biogeochemical surface seawater properties responded to upwelling favorable wind stress with approximately a 12-h lag. During upwelling events, SST, DO and pH decrease, while NO3−, pCO2sea, and air-sea fluxes increases. During the relaxed southerly wind periods, opposite tendencies were observed. The fluorescence response to wind variations is complex and diverse, but in many cases there was a reduction in the phytoplankton biomass during the upwelling events followed by higher values during wind relaxations. The sub-seasonal variability of the coastal ocean characterized here is important for biogeochemical and productivity studies.

Highlights

Eastern Boundary Upwelling Systems (EBUS) are the world’s most biologically productive marine regions covering less than 1% of the ocean area but providing up to 20% of the world’s capture fisheries (Pauly and Christensen, 1995)
El Niño – Southern Oscillation (ENSO), the main driver of interannual sea surface temperatures (SST) anomalies in central Chile (Montecinos et al, 2003), was in its neutral phase during spring and summer of 2016/2017 and 2017/2018, Niño 3.4 was slightly negative during these periods and a major coastal El Niño event developed off Peru in January 2017 (e.g., Garreaud, 2018)
We have characterized the response of surface physical and biogeochemical properties to the sub-seasonal, high frequency variability of the alongshore wind stress off southcentral Chile (36.4◦S)

Summary

Introduction

Eastern Boundary Upwelling Systems (EBUS) are the world’s most biologically productive marine regions covering less than 1% of the ocean area but providing up to 20% of the world’s capture fisheries (Pauly and Christensen, 1995). During austral spring and summer (September to March), the southward displacement of the SPA results in prevailing equatorward alongshore winds (Figure 1A) that favor coastal upwelling of Equatorial Subsurface Water (ESSW), which tilts isotherms upward toward the east (Figure 1B) and fostering high primary productivity rates (Testa et al, 2018) During this part of the year, the coastal near-surface wind field often exhibits an intense southerly low-level jet driven by transient strengthening of in the alongshore sea level pressure gradient (Muñoz and Garreaud, 2005) between a coastal low in central Chile and a migratory anticyclone farther south (Garreaud et al, 2002; Rahn and Garreaud, 2014). Intense upwelling-favorable conditions tend to last 3–7 days, interrupted by a southerly wind relaxation or weak northerly flow (downwelling-favorable) lasting a few days (Garreaud et al, 2002)

Methods

Results

Discussion

Conclusion