Modulation of the vertical particle transfer efficiency in the oxygen minimum zone off Peru

The fate of the organic matter (OM) produced by marine life controls the major biogeochemical cycles of the Earth's system. The OM produced through photosynthesis is either preserved, exported towards sediments or degraded through remineralisation in the water column. The productive eastern boundary upwelling systems (EBUSs) associated with oxygen minimum zones (OMZs) would be expected to foster OM preservation due to low O2 conditions. But their intense and diverse microbial activity should enhance OM degradation. To investigate this contradiction, sediment traps were deployed near the oxycline and in the OMZ core on an instrumented moored line off Peru. Data provided high-temporal-resolution O2 series characterising two seasonal steady states at the upper trap: suboxic ([O2]  50%) and remineralisation (intermediate Teff 20  50%) has been reported in summer and winter associated with extreme limitation in O2 concentrations or OM quantity for OM degradation. However, higher levels of O2 or OM, or less refractory OM, at the oxycline, even in a co-limitation context, can decrease the OMZ transfer efficiency to below 50%. This is especially true in summer during intraseasonal wind-driven oxygenation events. In late winter and early spring, high oxygenation conditions together with high fluxes of sinking particles trigger a shutdown of the OMZ transfer (Teff < 6%). Transfer efficiency of chemical elements composing the majority of the flux (nitrogen, phosphorus, silica, calcium carbonate) follows the same trend as for carbon, with the lowest transfer level being in late winter and early spring. Regarding particulate isotopes, vertical transfer of δ15N suggests a complex pattern of 15N impoverishment or enrichment according to Teff modulation. This sensitivity of OM to O2 fluctuations and particle concentration calls for further investigation into OM and O2-driven remineralisation processes. This should include consideration of the intermittent behaviour of OMZ towards OM demonstrated in past studies and climate projections

SST subseasonal variability in the central Benguela upwelling system as inferred from satellite observations (1999–2009)

International audienceSubseasonal variability of Sea Surface Temperature (SST) in the central Benguela upwelling system is investigated using TMI satellite-derived data over the period 1999–2009. Spatial patterns and time-frequency characteristics of subseasonal variability are documented based on Empirical Orthogonal Functions (EOF) decomposition and wavelet analysis. Despite the land contamination of the TMI satellite data within approximately 100 km off the coast the first EOF of SST anomalies allows characterizing the coastal upwelling variability at the subseasonal scale. Two regimes of variability are evidenced: a submonthly (2–30 days) regime with a dominant 11 days oscillation and a lower frequency intraseasonal (30–90 days) regime with a dominant 61 days oscillation. Both regimes are modulated, to a large extent, by the local surface wind stress and are consistent with Ekman dynamics. The seasonality of the relationship between wind stress and SST for submonthly (intraseasonal) regime is characterized by a marked semiannual (annual) cycle, which is explained in terms of the impact of seasonal change of the upper ocean stratification on the vertical advection process. The wind-driven SST subseasonal variability is shown to be associated with eastward propagating disturbances in the midlatitudes corresponding to a wave number 4. The results also suggest an important role of the Antarctic Oscillation in modulating the intraseasonal wind-driven SST variability. The characteristics of the equatorial intraseasonal Kelvin waves are documented in order to discuss possible impact of remote oceanic forcing on SST variability along the coast in the Benguela upwelling system

Assessing the impact of downscaled winds on a regional ocean model simulation of the Humboldt system

International audienceSimulating the oceanic circulation in Eastern Boundary Upwelling Systems (EBUS) is a challenging issue due to the paucity of wind stress products of a sufficiently high spatial resolution to simulate the observed upwelling dynamics. In this study, we present the results of regional simulations of the Humboldt current system (Peru and Chile coasts) to assess the value of a statistical downscaling model of surface forcing. Twin experiments that differ only from the momentum flux forcing are carried out over the 1992-2000 period that encompasses the major 1997/98 El Niño/La Niña event. It is shown that the mean biases of the oceanic circulation can be drastically reduced simply substituting the mean wind field of NCEP reanalysis by a higher resolution mean product (QuikSCAT). The statistical downscaling model improves further the simulations allowing more realistic intraseasonal and interannual coastal undercurrent variability, which is notoriously strong off Central Peru and Central Chile. Despite some limitations, our results suggest that the statistical approach may be useful to regional oceanic studies of present and future climates

Change in El Niño flavours over 1958-2008: Implications for the long-term trend of the upwelling off Peru

International audienceThe tropical Pacific variability has experienced changes in its characteristics over the last decades. In particular, there is some evidence of an increased occurrence of El Niño events in the central Pacific (a.k.a. 'Central Pacific El Niño' (CP El Niño) or 'El Niño Modoki'), in contrast with the cold tongue or Eastern Pacific (EP) El Niño which develops in the eastern Pacific. Here we show that the different flavours of El Niño imply a contrasted Equatorial Kelvin Wave (EKW) characteristic and that their rectification on the mean upwelling condition off Peru through oceanic teleconnection is changed when the CP El Niño frequency of occurrence increases. The Simple Ocean Data Assimilation (SODA) reanalysis product is first used to document the seasonal evolution of the EKW during CP and EP El Niño. It is shown that the strong positive asymmetry of ENSO (El Niño Southern Oscillation) is mostly reflected into the EKW activity of the EP El Niño whereas during CP El Niño, the EKW is negatively skewed in the eastern Pacific. Along with slightly cooler conditions off Peru (shallow thermocline) during CP El Niño, this is favourable for the accumulation of cooler SST anomalies along the coast by the remotely forced coastal Kelvin wave. Such a process is observed in a high-resolution regional model of the Humboldt Current system using the SODA outputs as boundary conditions. In particular the model simulates a cooling trend of the SST off Peru although the wind stress forcing has no trend. The model is further used to document the vertical structure along the coast during the two types of El Niño. It is suggested that the increased occurrence of the CP El Niño may also lead to a reduction of mesoscale activity off Peru

Upwelling response to atmospheric coastal jets off central Chile : a modeling study of the October 2000 event

The spatial and temporal variability of nearshore winds in eastern boundary current systems affect the oceanic heat balance that drives sea surface temperature changes. In this study, regional atmospheric and oceanic simulations are used to document such processes during an atmospheric coastal jet event off central Chile. The event is well reproduced by the atmospheric model and is associated with the migration of an anomalous anticyclone in the southeastern Pacific region during October 2000. A robust feature of the simulation is a sharp coastal wind dropoff, which is insensitive to model resolution. As expected, the simulated oceanic response is a significant sea surface cooling. A surface heat budget analysis shows that vertical mixing is a major contributor to the cooling tendency both in the jet core area and in the nearshore zone where the magnitude of this term is comparable to the magnitude of vertical advection. Sensitivity experiments show that the oceanic response in the coastal area is sensitive to wind dropoff representation. This is because total upwelling, i.e., the sum of coastal upwelling and Ekman pumping, depends on the scale of wind dropoff. Because the latter is much larger than the upwelling scale, coastal wind dropoff has only a weak positive effect on vertical velocities driven by Ekman pumping but has a strong negative effect on coastal upwelling. Interestingly though, the weakening of coastal winds in the dropoff zone has a larger effect on vertical mixing than on vertical advection, with both effects contributing to a reduction of cooling