Evidence of upwelling events at the northern Patagonian shelf break

Abstract

The Patagonian shelf break marks a transition between relative warm‐fresh shelf waters and relative cold‐salty Subantarctic Water advected northward by the Malvinas Current. From early spring to late autumn, the outer shelf region is characterized by a band of high chlorophyll concentration that sustains higher trophic levels, including significant fisheries. We analyze time series of current and water mass property observations collected at two moorings deployed at the shelf edge at 41°S and 43.8°S to investigate what mechanisms lead to temperature variability at the shelf break, and their role on the nutrient supply to the upper layer. The in situ data are combined with satellite‐derived observations of sea surface temperature and chlorophyll a to analyze a sharp cooling event at the outer shelf that lasted 10 days and extended ∼500 km along the outer shelf. The event is consistent with upwelling of cold waters through the base of the mixed layer. The vertical velocity required to explain the observed cooling is 13–29 m d−1. Satellite‐derived sea surface temperature reveals additional cooling events of similar characteristics. Seventy‐five percent of these events are concurrent with surface chlorophyll increase over a 5 day period suggesting that cooling events observed at the shelf break are associated with nutrient fluxes that promote the growth of phytoplankton.

Wind modulation of upwelling at the shelf‐break front off Patagonia: Observational evidence

“…Valla and Piola [] also carried out an independent assessment based on surface currents and temperature observations from another mooring at the SBF off Patagonia. Their results agree with ours in showing that high‐frequency variability (i.e., subseasonal) surface cooling events, linked to intense upwelling, lead to a significant increase in surface Chl‐a at the SBF.…”

“…Due to the lack of concomitant observations of nutrients, vertical diffusivities and/or vertical velocities, a meaningful assessment of the upwelled nutrient fluxes associated with the along‐front wind induced upwelling [e.g., He et al ., ] is not feasible. Although the existing nutrient data combined with the approach of Valla and Piola [] to estimate vertical velocities from the mooring site may prove useful for an estimate of the upwelled flux of nutrients, quantifying the role of this mechanisms is out of the scope of this study but will be the focus of future work.…”

“…The transition between shelf and open ocean waters is therefore characterized by moderate cross‐front temperature and salinity gradients [ Romero et al ., ; Painter et al ., ; Valla and Piola , ]. Observations show that the shelf‐break salinity front, which closely follows the 33.9 psu isohaline [ Piola et al ., ; Painter et al ., ], is associated with nitrate maxima, located at or inshore of the surface temperature minimum and density maximum [ Romero et al ., ; Painter et al ., ; Poulton et al ., ; Valla and Piola , ]. The seasonal variability in light intensity and nutrient supply to the mixed layer associated with changes in stratification control the strong variability of phytoplankton abundance [ Signorini et al ., ].…”

“…However, BBL detachment cannot fully explain the strong upwelling inferred from observations at the SBF off Patagonia [ Matano and Palma , ; Miller et al ., ]. In situ and remote sensing observations suggest that intense shelf‐break upwelling can reach 13–29 m d −1 [ Valla and Piola , ], while average modeled upwelling rates are 8 m d −1 [ Matano and Palma , ; Combes and Matano , ]. Using numerical experiments based on idealized conditions for the Patagonian shelf, Matano and Palma [] showed that, in the absence of an externally imposed horizontal density gradient and the presence of a strong slope current flowing in the direction of coastally trapped waves (i.e., Malvinas Current), the magnitude of upwelling generated by the detachment of the BBL is very small (i.e., ∼0.2 m d −1 ).…”

Variability and 20-Year Trends in Satellite-Derived Surface Chlorophyll Concentrations in Large Marine Ecosystems around South and Western Central America

“…On the seasonally stratified shelf, phytoplankton growth is limited by nutrient concentrations. As the Malvinas Current flows northward, upwelling along the shelfbreak supplies cold nutrient-rich waters onto the shelf Piola et al, 2010;Valla and Piola, 2015) favoring increased primary production in the vicinity of the shelf break and adjacent shelf waters. Model results suggest that the Malvinas Current controls cross-shelf exchanges at the shelf-break as well as the circulation over the continental shelf (Matano et al, 2010).…”