Coastal upwelling off Peru and Mauritania inferred from helium isotope disequilibrium

Abstract

Abstract. Oceanic upwelling velocities are too small to be measured directly. The surface disequilibrium of the 3He/4He ratio provides an indirect method to infer vertical velocities at the base of the mixed layer. Samples of helium isotopes were taken from two coastal upwelling regions, off Peru on cruise M91, and off Mauritania on 3 cruises. The helium-3 flux into the mixed layer also depends on the diapycnal mixing. Direct observations of the vertical diffusivity have been performed on all 4 cruises and are also used in this study. The resulting upwelling velocities in the coastal regions vary between 1.1 × 10−5 and 2.8 × 10−5 m s−1 for all cruises. Vertical velocities off the equator can also be inferred from the divergence of the wind driven Ekman transport. In the coastal regimes, the agreement between wind and helium derived upwelling is fairly good at least for the mean values. Further offshore, the helium derived upwelling still reaches 1 × 10−5 m s−1, whereas the wind driven upwelling from Ekman suction is smaller by at least one order of magnitude. One reason for this difference might be eddy induced upwelling. Both advective and diffusive nutrient fluxes into the mixed layer are calculated based on the helium derived vertical velocities and the measured vertical diffusivities. The advective part of these fluxes makes up at least 50 % of the total. The nutrient flux into the mixed layer in the coastal upwelling regimes is equivalent to a net community production (NCP) of 1.3 g C m2 d−1 off Peru and 1.6–1.9 g C m2 d−1 off Mauritania.

Controls on redox-sensitive trace metals in the Mauritanian oxygen minimum zone

“…The vertical advective flux by Ekman divergence requires determination of vertical velocity in the water column that varies with depth and distance from the coastline. Convincing agreement between vertical velocities derived from Ekman divergence following Gill (1982) determined from scatterometer winds and from helium isotope disequilibrium I. Rapp et al: Controls on redox-sensitive trace metals in the Mauritanian oxygen minimum zone within the Mauritanian and Peruvian coastal upwelling regions was found by Steinfeldt et al (2015) (see their Fig. 4).…”

“…1: right term, right-hand side) were estimated. On the continental margin below the surface mixed layer, solutes are transferred vertically toward the near-surface layers by turbulent mixing processes and by vertical advection forced by Ekman divergence (e.g., Kock et al, 2012;Milne et al, 2017;Rhein et al, 2010;Steinfeldt et al, 2015;Tanhua and Liu, 2015):…”

“…However, in our study TM concentration time series data are not available. Previous studies have used a vertical length scale of 20 m to calculate the concentration differences between the target depth and the water below (e.g., Hayes et al, 1991;Steinfeldt et al, 2015;Tanhua and Liu, 2015). For our calculations, we chose to use a smaller length scale of 10 m following Hayes et al (1991), which results in vertical advective TM flux presumably on the lower side of possible values.…”

The Influence of Diapycnal Nutrient Fluxes on Phytoplankton Size Distribution in an Area of Intense Mesoscale and Submesoscale Activity off Concepción, Chile

“…In EBUSs, besides horizontal advection of nutrients from the CZ to the CTZ by mesoscale and submesoscale features, diapycnal mixing has been identified as an important mechanism of vertical mixing in the CTZ. This is especially so at the continental margin, contributing to vertical injections of gases, solutes, and organic matter toward the upper layer (Callbeck et al, ; Li et al, ; Loginova et al, ; Steinfeldt et al, ). The effects of diapycnal mixing on phytoplankton community size distribution in EBUSs based on observational data remain to be investigated.…”

Variation and Change of Upwelling Dynamics Detected in the World’s Eastern Boundary Upwelling Systems

“…Recent studies have shown ecological changes in EBUS ecosystem structure (Fréon et al, 2009;Wang et al, 2015), hence monitoring these systems is becoming increasingly important. EBUS are formed in part by wind-driven ocean circulation, and their upwelling is dependent on wind direction and strength (Capet et al, 2004;Messié and Chavez, 2015;Steinfeldt et al, 2015). Recognizing that alongshore winds that drive upwelling are initiated by changes in atmospheric pressure gradients at the cross-shore, Bakun (1990) hypothesized that long-term changes in climatic conditions would likely intensify continental oceanic pressure gradients (Bakun, 1990;García-Reyes et al, 2015) and subsequently result in an increase in the frequencies and intensities of upwelling-favorable winds.…”