While the Kuroshio is known to be a nutrient stream, as these nutrients are in dark subsurface layers, they are not immediately available for photosynthesis unless they are supplied to the sunlit surface layers. Recent microstructure observations have revealed that strong diapycnal mixing caused by the Kuroshio flowing over topographic features and double diffusion in the subsurface layers of the Kuroshio. However, it is still unclear how much nutrient flux can be provided by these microscale mixing processes. In this study, using an autonomous microstructure float and nutrient samplings, nutrient flux caused by the Kuroshio over the Izu Ridge, and that caused by double diffusion in the Kuroshio Extension are quantified. The nitrate diffusive flux is estimated to be $$>1 \,\hbox {mmol} \,\hbox {N}\,\hbox {m}^{-2}\hbox {day}^{-1}$$ > 1 mmol N m - 2 day - 1 over a distance, 20–30 km near the Izu Ridge and $$>0.1 \,\hbox {mmol} \,\hbox {N}\, \hbox {m}^{-2}\hbox {day}^{-1}$$ > 0.1 mmol N m - 2 day - 1 , which persists further downstream direction over 100 km along the Kuroshio, increasing the subsurface chlorophyll-a concentration in the region 200 km downstream. The double-diffusion-induced nitrate flux is estimated to be 1-$$10 \,\hbox {mmol} \,\hbox {N} \,\hbox {m}^{-2}\hbox {day}^{-1}$$ 10 mmol N m - 2 day - 1 in the pycnostad 26–$$26.5\,\hbox {kgm}^{-3}$$ 26.5 kgm - 3 of the Kuroshio Extension, suggesting that whether this double-diffusion-induced nutrient flux in the subsurface layers can ultimately contribute to surface primary production depends on additional eddy up- and northward fluxes.

Abstract. Time-varying sources of upwelling waters off the coast of northern Peru are analyzed in a Lagrangian framework, tracking virtual particles backwards in time for 12 months. Particle trajectories are calculated with temperature, salinity and velocity fields from a hindcast spanning 1988–2007, obtained with an eddy-resolving (1/12∘) global configuration of the Nucleus for European Modelling of the Ocean (NEMO) ocean model. At 30 and 100 m, where coastal upwelling rates exceed 50 m month−1, particles are seeded at monthly intervals in proportion to the upwelling rate. Ensemble maps of particle concentration, age, depth, temperature, salinity and density reveal that a substantial but variable fraction of the particles upwelling off Peru arrives via the Equatorial Undercurrent (EUC). Particles follow the EUC core within the depth range 125–175 m, characterized by temperatures <17 ∘C, salinities in the range 34.9–35.2 and densities of σθ=25.5–26.5. Additional inflows are via two slightly deeper branches further south from the main system, at around ≈3 and ≈8∘ S. Averaged across the hindcast, annual-mean percentages of particles upwelling at 30 m (100 m) associated with the EUC vary from 57.4 % (52 %) at 92∘ W to 19.2 % (17.9 %) at 165∘ W. Considerable interannual variability in these percentages reveals that more of the Peruvian upwelling can be traced back to the EUC during warm events, such as El Niño. In contrast, upwelling waters are of more local origin during cold events such as La Niña. Despite weaker EUC transport during El Niño, relative flattening of the equatorial thermocline brings the EUC upwelling waters much closer to the Peruvian coast than under neutral or La Niña conditions. Annually averaging EUC transport at specific longitudes, a notable negative-to-positive transition is evident during the major El Niño/La Niña events of 1997/99. On short timescales, a degree of longitudinal coherence is evident in EUC transport, with transport anomalies at 160∘ W evident at the Galápagos Islands (92∘ W) around 30–35 d later. It is concluded that the Peruvian upwelling system is subject to a variable EUC influence, on a wide range of timescales, most notably the interannual timescale of El Niño–Southern Oscillation (ENSO). Identifying this variability as a driver of shifts in population and catch data for several key species, during the study period, these new findings might inform sustainable management of commercially important fisheries off northern Peru.

The link between the coastal ocean and the open sea varies greatly by region. This factor must be elucidated to consider climate projections for a particular coastal region. This study examines this link between coastal areas in Peru and open seas. Water temperature loggers were installed at six sites along the Peruvian coast between 2017 and 2020. The obtained data and that from the EN4 dataset of the open ocean were compared. The coastal water temperature changes in Peru correlates well with the open ocean sea surface temperature in terms of monthly mean anomalies. The correlation coefficient values range from 0.80 to 0.92 with a > 99% confidence level except for one observation point (0.26). This suggests a global ocean observation system underlying EN4 can directly contribute to a reliable climate (water temperature) prediction of coastal areas in Peru. On shorter timescales, the logger-derived temperature changes show spectral peaks at 80 and 120 days north of 5°S, suggesting the coastal monitoring can capture the sub-seasonal dynamical aspects of equatorial Kelvin waves. In contrast, clear peaks are not detected south of 5ºS, implying that a practical leak of equatorial wave energy into off-equatorial regions is not anticipated along Peruvian coast on this time scale.

The linkage between environmental conditions in the coastal ocean and the open sea varies greatly by region. It is important to clarify, on an area-by-area basis, what coastal monitoring information reveals about the open ocean and how much predictive information for the open ocean may be applicable to the coastal ocean. The Pacific Ocean off the coast of Peru is a monitoring area for the El Niño/La Niña, an oceanic–atmospheric phenomenon of global importance. However, there are not many reliable data along the Peruvian coast. We deployed a network of 6 logger sites along the Peruvian coast during 2017–2020 and compiled a useful, high-resolution dataset of water temperatures. We examined a possible link between temperatures in the coastal waters of Peru and the open sea by comparing the new dataset with historical temperatures in the open ocean. We confirmed that monthly mean anomalies of seashore water temperatures in coastal Peru were strongly correlated with those of open ocean sea surface temperatures. With one exception, the correlation coefficients ranged from 0.80 to 0.92 and were significant at p < 0.01. This result suggested that data obtained from monitoring along the Pacific coast of Peru could be used to indicate the state of the open ocean and that El Niño forecasts for the open ocean could be applied to coastal forecasting as well. Spectral analysis revealed that the periods of changes of seashore water temperature peaked at 80 and 120 days in the region north of 5° S. This result suggested that coastal monitoring might capture intraseasonal dynamics of equatorial Kelvin waves. The absence of clear peaks south of 5° S implied that equatorial wave energy did not penetrate far into off-equatorial regions along the Peruvian coast on intraseasonal timescales.