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The North Pacific gyre boundaries are characterized by stark contrasts in physical and biogeochemical properties. Meridional movement of gyre boundaries, influenced by climate change, can therefore exert a large influence not only on marine ecosystems but also on climate. We examine the evidence for wind-driven southward shifts in subsurface temperature, salinity, PO4, and O2 within the northwest Pacific from the 1950s to the 2000s. Gyre boundary shifts can explain 30-60% of temperature and salinity trends zonally averaged in the northwest Pacific and observed PO4 and O2 trends along the 137°E and 144°E meridians. The close tie between the wind-driven shifts in gyre boundaries and the tracer distributions is further supported by results from an eddy-resolving (0.1° × 0.1°) Geophysical Fluid Dynamics Laboratory climate model, suggesting that the physical and biogeochemical properties averaged within the northwest Pacific gyre boundaries closely follow the latitude changes of the zero Sverdrup stream function with lags of 0 to 3 years. The gyre shift effect on tracer distribution is poorly represented in a coarse resolution (1° × 1°) model due partly to poor representations of fronts and eddies. This study suggests that future changes in northwest Pacific PO4 and O2 content may depend not only on ocean temperature and stratification but also on the ocean gyre response to winds. [ABSTRACT FROM AUTHOR]

A strong El Niño developed in early 2015. Measurements from a research cruise on the R/V Sonne in October 2015 near the Equator east of the Galapagos Islands and off the shelf of Peru are used to investigate changes related to El Niño in the upper ocean in comparison with earlier cruises in this region. At the Equator at 85° 30' W, a clear temperature increase leading to lower densities in the upper 350 m had developed in October 2015, despite a concurrent salinity increase from 40 to 350 m. Lower nutrient concentrations were also present in the upper 200 m, and higher oxygen concentrations were observed between 40 and 130 m. In the equatorial current field, the Equatorial Undercurrent (EUC) east of the Galapagos Islands almost disappeared in October 2015, with a transport of only 0.02 Sv in the equatorial channel between 1° S and 1°N, and a weak current band of 0.78 Sv located between 1 and 2° 30' S. Such near-disappearances of the EUC in the eastern Pacific seem to occur only during strong El Niño events. Off the Peruvian shelf at ~ 9° S, characteristics of upwelling were different as warm, saline, and oxygen-rich water was upwelled. At ~ 12, ~ 14, and ~ 16° S, the upwelling of cold, low-salinity, and oxygen-poor water was still active at the easternmost stations of these three sections, while further west on these sections a transition to El Niño conditions appeared. Although from early 2015 the El Niño was strong, the October measurements in the eastern tropical Pacific only showed developing El Niño water mass distributions. In particular, the oxygen distribution indicated the ongoing transition from "typical" to El Niño conditions progressing southward along the Peruvian shelf. [ABSTRACT FROM AUTHOR]

As anthropogenic activities, notably the burning of fossil fuels, increase carbon dioxide (CO2) and result in a decrease in oxygen (O2) concentrations in the ocean system, it becomes important to understand how different populations of marine animals will respond. Water that is naturally lower in pH, with a high concentration of carbon dioxide (hypercapnia) and a low concentration of oxygen, occurs at shallow depths (200-500 m) in the North Pacific Ocean, whereas similar conditions are absent throughout the upper water column in the North Atlantic. This contrasting hydrography provides a natural experiment to explore whether differences in environment cause populations of cosmopolitan pelagic calcifiers, specifically the aragonitic-shelled pteropods, to have a different physiological response when exposed to hypercapnia and low O2. Using closed-chamber end-point respiration experiments, eight species of pteropods from the two ocean basins were exposed to high CO2 (~ 800 µatm) while six species were also exposed to moderately low O2 (10%, or ~ 130 µmol kg-1) and a combined treatment of low O2/high CO2. None of the species tested showed a change in metabolic rate in response to high CO2 alone. Of those species tested for an effect of O2, only Limacina retroversa from the Atlantic showed a response to the combined treatment, resulting in a reduction in metabolic rate. Our results suggest that pteropods have mechanisms for coping with short-term CO2 exposure and suggest that there can be interactive effects between stressors on the physiology of these open ocean organisms that correlate with natural exposure to low O2 and high CO2; these are considerations that should be taken into account in projections of organismal sensitivity to future ocean conditions. [ABSTRACT FROM AUTHOR]

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