Seasonal variability of the oxygen minimum zone off Peru in a high-resolution regional coupled model

Abstract

Abstract. In addition to being one of the most productive upwelling systems, the oceanic region off Peru is embedded in one of the most extensive oxygen minimum zones (OMZs) of the world ocean. The dynamics of the OMZ off Peru remain uncertain, partly due to the scarcity of data and to the ubiquitous role of mesoscale activity on the circulation and biogeochemistry. Here we use a high-resolution coupled physical/biogeochemical model simulation to investigate the seasonal variability of the OMZ off Peru. The focus is on characterizing the seasonal cycle in dissolved O 2 (DO) eddy flux at the OMZ boundaries, including the coastal domain, viewed here as the eastern boundary of the OMZ, considering that the mean DO eddy flux in these zones has a significant contribution to the total DO flux. The results indicate that the seasonal variations of the OMZ can be interpreted as resulting from the seasonal modulation of the mesoscale activity. Along the coast, despite the increased seasonal low DO water upwelling, the DO peaks homogeneously over the water column and within the Peru Undercurrent (PUC) in austral winter, which results from mixing associated with the increase in both the intraseasonal wind variability and baroclinic instability of the PUC. The coastal ocean acts therefore as a source of DO in austral winter for the OMZ core, through eddy-induced offshore transport that is also shown to peak in austral winter. In the open ocean, the OMZ can be divided vertically into two zones: an upper zone above 400 m, where the mean DO eddy flux is larger on average than the mean seasonal DO flux and varies seasonally, and a lower part, where the mean seasonal DO flux exhibits verticalzonal propagating features that share similar characteristics than those of the energy flux associated with the annual extratropical Rossby waves. At the OMZ meridional boundaries where the mean DO eddy flux is large, the DO eddy flux has also a marked seasonal cycle that peaks in austral winter (spring) at the northern (southern) boundary. In the model, the amplitude of the seasonal cycle is 70 % larger at the southern boundary than at the northern boundary. Our results suggest the existence of distinct seasonal regimes for the ventilation of the OMZ by eddies at its boundaries. Implications for understanding the OMZ variability at longer timescales are discussed.

Positive Indian Ocean Dipole events prevent anoxia off the west coast of India

“…For the case of the narrow continental shelf along the west coast of North America, several studies have shown that a minimum horizontal resolution of 10 km is required (Marchesiello et al, 2003;Veneziani et al, 2009). For example, at 1/4 • resolution our model is eddy-permitting but not eddy-resolving, and hence does not fully capture oceanic mesoscale eddies, which play an important role in the exchanges between the shelf and the open ocean in upwelling regions (e.g., Marchesiello et al, 2003;Bettencourt et al, 2015;Vergara et al, 2016). Another limitation arises from the absence of tidal forcing in the model, which may play an important role, as strong internal tides can be generated on the shelf break and contribute to enhancing the thermocline vertical excursion and mixing, which can both contribute to bringing more deep-ocean oxygen-deficient water to the shelf (Monteiro et al, 2008).…”

The OMZ and nutrient features as a signature of interannual and low-frequency variability in the Peruvian upwelling system

“…The OMZ off Peru is associated with the presence of nutrient-rich equatorial subsurface water (ESSW) transported poleward by the Peru-Chile Undercurrent (PCU; Strub et al, 1998;Fuenzalida et al, 2009;Silva et al, 2009) and the transport of low-oxygenated waters by the narrow primary and secondary Southern Subsurface Countercurrents near 4 and 7 • S, respectively, also known as Tsuchiya jets (Furue et al, 2007;Montes et al, 2010). Recent modeling studies also highlight the important role of submesoscale to mesoscale dynamics in constraining the upper OMZ meridional boundaries (Bettencourt et al, 2015;Vergara et al, 2016).…”

“…The region of Callao has been identified as one of the major upwelling cells off central Peru (Rojas de Mendiola, 1981) with a well-developed OMZ at the subsurface (Wooster and Gilmartin, 1961;Zuta and Guillén, 1970). The presence of nitrate-rich ESSW (Zuta and Guillén, 1970;Strub et al, 1998;Silva et al, 2009) triggers the high primary production of the region, with maximum values in spring and summer, out of phase with the winter upwelling maximum (Echevin et al, 2008;Chavez and Messié, 2009;Gutiérrez et al, 2011;Pennington et al, 2006;Vergara et al, 2016).…”

Impact of mesoscale eddies on water mass and oxygen distribution in the eastern tropical South Pacific

“…(northern) boundary of the OMZ (Vergara et al, 2016). As the PCUC also shows strong fluctuations lasting a few days up to a few weeks (Huyer et al, 1991) it might be difficult to determine a seasonal dimension between the PCUC and the rate of eddy formation.…”

“…and might explain the higher number of eddy generation during this season. Other model simulations have revealed a seasonal cycle in eddy flux that peaks in austral winter at the northern boundary of the OMZ, while it peaks a season later at the southern boundary (Vergara et al, 2016). The PCUC also experiences relatively strong fluctuations with periods of a few days to a few weeks (Huyer et al, 1991).…”