•A seasonal climatology of the NHCS was constructed gathering 50-year of in situ hydrological measurements.•Alongshore and cross-shore thermohaline properties and geostrophic circulation show strong ...variability at a seasonal scale.•Abrupt changes in T/S fields are found in the water column during El Nino and La Nina periods.•Decadal variability also impacts the Peru upwelling system.
Since the 1960s, the Instituto del Mar del Perú (IMARPE) collected tens of thousands of in-situ temperature and salinity profiles in the Northern Humboldt Current System (NHCS). In this study, we blend this unique database with the historical in-situ profiles available from the World Ocean Database for the period 1960–2014 and apply a four-dimensional interpolation scheme to construct a seasonal climatology of temperature and salinity of the NHCS. The resulting interpolated temperature and salinity fields are gridded at a high spatial resolution (0.1° × 0.1° in latitude/longitude) between the surface and 1000 m depth, providing a detailed view of the hydrology and geostrophic circulation of this region. In particular, the mean distribution and characteristics of the main water-masses in the upper ocean of the NHCS are described, as well as their seasonal variations between austral summer and winter. The coastal upwelling region is well documented due to the increased data density along 3 highlighted cross-shore vertical sections off Paita (∼5°S), Chimbote (∼9°S) and San Juan (∼16.5°S). The large and long-term database also allowed us, through a composite analysis, to investigate the impact of the eastern Pacific El Niño and La Niña events on the NHCS hydrology. On average, during these periods, large temperature (±3–4 °C) and salinity (±0.1–0.2) anomalies are observed, impacting the water column of the coastal ocean off Peru down to 100–200 m depth. At 100 km from the coast, these anomalies are associated with a maximum deepening (shoaling, respectively) of the thermocline of 60 m (25 m) during composite El Niño (La Niña) events. At interdecadal scale, a similar approach reveals sea-surface temperature variations of ±0.5°C, associated with a deepening (shoaling) of the thermocline of 5–10 m during warm (cold) periods.
The dynamics of the Peru–Chile upwelling system (PCUS) are primarily driven by alongshore wind stress and curl, like in other eastern boundary upwelling systems. Previous studies have suggested that ...upwelling-favorable winds would increase under climate change, due to an enhancement of the thermally-driven cross-shore pressure gradient. Using an atmospheric model on a stretched grid with increased horizontal resolution in the PCUS, a dynamical downscaling of climate scenarios from a global coupled general circulation model (CGCM) is performed to investigate the processes leading to sea-surface wind changes. Downscaled winds associated with present climate show reasonably good agreement with climatological observations. Downscaled winds under climate change show a strengthening off central Chile south of 35°S (at 30°S–35°S) in austral summer (winter) and a weakening elsewhere. An alongshore momentum balance shows that the wind slowdown (strengthening) off Peru and northern Chile (off central Chile) is associated with a decrease (an increase) in the alongshore pressure gradient. Whereas the strengthening off Chile is likely due to the poleward displacement and intensification of the South Pacific Anticyclone, the slowdown off Peru may be associated with increased precipitation over the tropics and associated convective anomalies, as suggested by a vorticity budget analysis. On the other hand, an increase in the land–sea temperature difference is not found to drive similar changes in the cross-shore pressure gradient. Results from another atmospheric model with distinct CGCM forcing and climate scenarios suggest that projected wind changes off Peru are sensitive to concurrent changes in sea surface temperature and rainfall.
The Humboldt Current System (HCS) sustains the world′s largest small pelagic fishery. While a cooling of this system has been observed during recent decades, there is debate about the potential ...impacts of rising atmospheric CO2 concentrations on upwelling dynamics and productivity. Recent studies suggest that under increased atmospheric CO2 scenarios the oceanic stratification may strongly increase and upwelling‐favorable winds may remain nearly constant off Peru and increase off Chile. Here we investigate the impact of such climatic conditions on egg and larval dispersal phases, a key stage of small pelagic fish reproduction. We used larval retention rate in a predefined nursery area to provide a proxy for the recruitment level. Numerical experiments are based on hydrodynamics downscaled to the HCS from global simulations forced by pre‐industrial (PI), 2 × CO2 and 4 × CO2 scenarios. A biogeochemical model is applied to the PI and 4 × CO2 scenarios to define a time‐variable nursery area where larval survival is optimum. We test two distinct values of the oxycline depth that limits larval vertical distribution: One corresponding to the present‐day situation and the other corresponding to a shallower oxycline potentially produced by climate change. It appeared that larval retention over the continental shelf increases with enhanced stratification due to regional warming. However, this increase in retention is largely compensated for by a decrease of the nursery area and the shoaling of the oxycline. The underlying dynamics are explained by a combination of stratification effects and mesoscale activity changes. Our results therefore show that future climate change may significantly reduce fish capacity in the HCS with strong ecological, economic and social consequences.
The ocean is an important source of nitrous oxide (N2O) to the atmosphere, yet the factors controlling N2O production and consumption in oceanic environments are still not understood nor constrained. ...We measured N2O concentrations and isotopomer ratios, as well as O2, nutrient and biogenic N2 concentrations, and the isotopic compositions of nitrate and nitrite at several coastal stations during two cruises off the Peru coast (~5–16°S, 75–81°W) in December 2012 and January 2013. N2O concentrations varied from below equilibrium values in the oxygen deficient zone (ODZ) to up to 190 nmol L−1 in surface waters. We used a 3‐D‐reaction‐advection‐diffusion model to evaluate the rates and modes of N2O production in oxic waters and rates of N2O consumption versus production by denitrification in the ODZ. Intramolecular site preference in N2O isotopomer was relatively low in surface waters (generally −3 to 14‰) and together with modeling results, confirmed the dominance of nitrifier‐denitrification or incomplete denitrifier‐denitrification, corresponding to an efflux of up to 0.6 Tg N yr−1 off the Peru coast. Other evidence, e.g., the absence of a relationship between ΔN2O and apparent O2 utilization and significant relationships between nitrate, a substrate during denitrification, and N2O isotopes, suggest that N2O production by incomplete denitrification or nitrifier‐denitrification decoupled from aerobic organic matter remineralization are likely pathways for extreme N2O accumulation in newly upwelled surface waters. We observed imbalances between N2O production and consumption in the ODZ, with the modeled proportion of N2O consumption relative to production generally increasing with biogenic N2. However, N2O production appeared to occur even where there was high N loss at the shallowest stations.
Key Points
High N2O concentrations were observed in coastal waters off Peru
Incomplete denitrifier‐denitrification was an important N2O production pathway
N2O production occurred at high extent of N loss in the shallow ODZ
The Peruvian upwelling system (PUS) is the most productive Eastern Boundary Upwelling System (EBUS) of the world ocean. Contrarily to higher latitude EBUSs, there is no consensus yet on the response ...of upwelling-favorable winds to regional climate change in this region. Global climate models are not able to reproduce the nearshore surface winds, and only a few downscaling studies have been performed by using relatively coarse-grid atmospheric models forced by idealized climate change scenarios. In the present study, the impact of climate change on the PUS upwelling-favorable winds was assessed using a high resolution regional atmospheric model to dynamically downscale the multi-model mean projection of an ensemble of 31 CMIP5 global models under the RCP8.5 worst-case climate scenario. We performed a 10-year retrospective simulation (1994–2003) forced by NCEP2 reanalysis data and a 10-year climate change simulation forced by a climate change forcing (i.e. differences between monthly-mean climatologies for 2080–2100 and 1989–2009) from CMIP5 ensemble added to NCEP2 data. We found that changes in the mean upwelling-favorable winds are weak (less than 0.2 m s
−1
). Seasonally, summer winds weakly decrease (by 0–5%) whereas winter winds weakly increase (by 0–10%), thus slightly reinforcing the seasonal cycle. A momentum balance shows that the wind changes are mainly driven by the alongshore pressure gradient, except in a local area north of the Paracas peninsula, downstream the main upwelling center, where wind increase in winter is driven by the shoreward advection of offshore momentum. Sensitivity experiments show that the north–south sea surface temperature gradient plays an important role in the wind response along the north and central coasts, superimposed onto the South Pacific Anticyclone large-scale forcing. A reduction (increase) of the gradient induces a wind weakening (strengthening) up to 15% (25%) off the northern coast during summer. This local mechanism is not well represented in global climate models projections, which underlines the strong need for dynamical downscaling of coastal wind in order to study the impact of climate change on the Peruvian upwelling ecosystem.
El Niño events, in particular the Eastern Pacific type, have a tremendous impact on the marine ecosystem and climate conditions in the Eastern South Pacific. During such events, the accumulation of ...anomalously warm waters along the coast favors intense rainfall. The upwelling of nutrient-replete waters is stopped and the marine ecosystem is strongly impacted. These events are generally associated with positive surface temperature anomalies in the Central and Eastern equatorial Pacific. During austral summer 2017, a strong surface temperature anomaly reaching ~3-4°C off Northern Peru and Ecuador led to intense coastal precipitations. However, neutral temperature anomalies were recorded in the equatorial Pacific. Using in situ measurements, satellite observations, and simulations from a eddy-resolving regional ocean circulation model, we investigate the physical processes triggering this peculiar 'coastal El Niño'. Its impact on the regional ocean circulation and heat budget off northern Peru and Ecuador is assessed. Using model sensitivity experiments, we investigate the respective roles of the equatorial Kelvin waves and local wind anomalies in driving the anomalously high nearshore SST. The atmospheric teleconnections which triggered the event are investigated using reanalysis data.
Abstract
The upwelling system off southern Peru has been observed using an autonomous underwater vehicle (a Slocum glider) during October–November 2008. Nine cross-front sections have been carried ...out across an intense upwelling cell near 14°S. During almost two months, profiles of temperature, salinity, and fluorescence were collected at less than 1-km resolution, between the surface and 200-m depth. Estimates of alongshore absolute geostrophic velocities were inferred from the density field and the glider drift between two surfacings. In the frontal region, salinity and biogeochemical fields displayed cross-shore submesoscale filamentary structures throughout the mission. Those features presented a width of 10–20 km, a vertical extent of ~150 m, and appeared to propagate toward the shore. They were steeper than isopycnals and kept an aspect ratio close to f/N, the inverse of the Prandtl ratio. These filamentary structures may be interpreted mainly as a manifestation of submesoscale turbulence through stirring of the salinity gradients by the mesoscale eddy field. However, meandering of the front or cross-frontal wind-driven instabilities could also play a role in inducing vertical velocities.
The influence of the eastern Pacific equatorial circulation on the dynamics of the Northern Humboldt Current System is studied using an eddy‐resolving regional circulation model forced by boundary ...conditions from three distinct ocean general circulation models. The seasonal variability of the modeled nearshore circulation and the mesoscale activity are contrasted in order to evaluate the role of the density forcing. The seasonal variability of the surface and subsurface alongshore currents strongly depends on the amplitude and timing of the seasonal eastward propagating equatorial waves. The equatorward flow and upwelling intensity are also impacted by nonlinear processes, such as the seasonal generation of nearshore mesoscale eddies, which create alongshore pressure gradients modulating the surface current. Boundary conditions affect differently the intensity and phase of the eddy kinetic energy, as baroclinic instability is triggered by coastal waves during austral summer and fall, whereas it is sustained by the wind‐driven upwelling during austral winter.
Key Points
Impact of remotely forced coastal waves on upwelling current system
Nonlinear response of the current system to the wave forcing
The coast of central Chile is characterized by intermittent low‐level along‐shore southerly wind periods, called coastal jets (CJs). In this study, we take advantage of long‐term satellite data to ...document the CJs characteristics over 2000–2007 and investigate its impact on upwelling. The CJ structure has a core some 100 km from the shore and a cross‐shore scale of ∼160 km, and it usually lasts for several days (3–10). Its period of occurrence ranges from weekly to a few months. On the basis of covariance analyses between wind stress and sea surface temperature (SST) anomalies, it is found that CJ activity is seasonally phase locked with SST, with a peak season in August–October. The statistically dominant forcing mechanisms of the SST cooling during CJ event is a combination of seaward advection of temperature resulting from Ekman transport, air‐sea heat exchange, and Ekman‐driven coastal divergence. However, case studies of two events suggest a significant sensitivity of the dominant upwelling forcing mechanisms to the background conditions. For instance, the upward Ekman pumping associated with cyclonic wind stress curl is enhanced for the event with the CJ located more to the south. Although there are limitations associated with both the formulation of the heat budget and the data sets, the results illustrate the complexity of the upwelling forcing mechanisms in this region and the need for realistic high‐resolution forcing fluxes. A CJ activity index is also proposed that takes into account the coastal upwelling variability, which can be used for teleconnection studies.
The spatial and seasonal variability of nutrients and chlorophyll in the southern Humboldt Current System were assessed using a high-resolution regional ocean circulation model (ROMS) coupled to a ...biogeochemical model (Pelagic-Interactions Scheme for carbon and Ecosystem Studies; PISCES). The simulated nutrients and chlorophyll fields were validated using satellite and in situ observations at a continental shelf time-series station. The annual cycles of modeled chlorophyll and nutrients were consistent with the highest values observed in spring and summer, which is in agreement with enhanced upwelling observations. Co-limitation of phytoplankton growth by nutrients and light was analyzed for diatoms, the dominant phytoplankton group in the simulations. The results showed that co-limitation, near the coast, was governed in autumn and winter by light, and by silicate in spring and summer, whereas other nutrients were limiting offshore between January and April. Nutrient transport in the surface layer was analyzed. Vertical advection reflected areas with higher coastal upwelling, and was partly offset by horizontal processes related to eddy-induced transport from the nearshore to the open ocean. Vertical mixing was shown to play a key role in replenishing the surface layer with nutrients.
•Nutrients and chlorophyll variability in the southern Humboldt Current System was assessed using a ROMS-PISCES model.•The results were validated using satellite and in situ observations from the Eastern South Pacific Region.•Co-limitation near the coast was governed by light in autumn/winter and by silicate in spring/summer.•Enhanced horizontal advection could be related to eddy-induced transport of nutrients from the nearshore to the open ocean.•Vertical mixing play a key role in replenishing the surface layer with nutrients.
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