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The Eocene‐Oligocene transition (EOT) (∼34 Ma) is marked by the rapid development of a semi‐permanent Antarctic ice‐sheet, as indicated by ice‐rafted debris and a 1–1.5‰ increase in deep sea δ18O. Proxy reconstructions indicate a drop in atmospheric CO2 and global cooling. How these changes affected surface ocean temperatures in the North Atlantic and ocean water stratification remains poorly constrained. In this study, we apply clumped‐isotope thermometry to well‐preserved planktonic foraminifera, that are associated with lower mixed‐layer to subthermocline dwelling depths from the drift sediments at international ocean discovery program Site 1411, Newfoundland, across four intervals bracketing the EOT. The thermocline/lower mixed‐layer dwelling foraminifera record a cooling of 1.9 ± 3.5 K (mean ± 95% CI) across the EOT. While the cooling amplitude is similar to previous sea surface temperature (SST) reconstructions, absolute temperatures (Eocene 20.0 ± 2.9°C, Oligocene 18.0 ± 2.2°C) appear colder than previous organic proxy reconstructions for the northernmost Atlantic extrapolated to this location. We discuss seasonal bias, recording depth, and appropriate consideration of paleolatitudes, all of which complicate the comparison between SST reconstructions and model output. Our subthermocline dwelling foraminifera record a larger cooling across the EOT (Eocene 19.0 ± 3.5°C, Oligocene 13.0 ± 3.2°C, cooling of 5.5 ± 4.6 K) than foraminifera from the thermocline/lower mixed‐layer, consistent with global cooling and an increase in ocean stratification which may be related to the onset or intensification of the Atlantic meridional overturning circulation. Plain Language Summary: During the Eocene temperatures on Earth were much warmer than today. It is generally believed that the Antarctic ice‐sheet first developed around 34 million years ago, during the Eocene‐Oligocene transition (EOT). How this change occurred is still widely debated, but it was probably caused by decreased CO2 levels and changes to heat distribution through ocean currents. Here, we study how water temperatures at the dwelling depths of planktonic microfossils in the Atlantic Ocean changed across this event. We use clumped isotopes—a way of reconstructing the temperature from fossil shells. We measured shells of planktonic foraminifera, single‐celled microscopic plankton, that lived at two different depth levels in the upper ocean. We find that the foraminifera with a shallower (thermocline/lower mixed‐layer) water dwelling depth in the North Atlantic Ocean cooled by about ∼1.9 K, while those that dwelled deeper cooled by ∼5.5 K across the EOT. The thermocline/lower mixed‐layer cooling is similar in magnitude to reconstructions from organic biomarkers. However, our reconstructed absolute temperatures are colder than previous estimates. We think that our deeper water temperature reconstructions reflect global cooling, while thermocline/lower mixed‐layer temperatures did not cool as much because a warm water current developed, similar to the Gulf Stream. Key Points: Clumped isotopes from well‐preserved planktonic foraminifera imply 1.9 K lower mixed‐layer/thermocline cooling across Eocene‐Oligocene transition (EOT) in North AtlanticOur thermocline temperatures for Eocene (20°C) and Oligocene (18°C) are cooler than organic proxy sea surface temperature estimatesIncreased subthermocline cooling compared to the thermocline indicates increased stratification across the EOT, hinting at intensified Atlantic meridional overturning circulation [ABSTRACT FROM AUTHOR]

The Tropical Pacific Ocean is characterized by its dominant zonal flow, strong climate dependence on the El Niño Southern Oscillation (ENSO) and abundant tuna stocks. Tuna fisheries in the West and Central Pacific Ocean accounted for 55% of world-wide tuna catch in 2019 and are one of the main sources of income in many Pacific island nations. One of the dominant fishing methods in this region relies on the use of drifting Fish Aggregating Devices (dFADs): rafts with long underwater appendages (on average 50m deep) that attract and aggregate fish. Although currents such as the North Equatorial Countercurrent (NECC) and South Equatorial Current (SEC) in the tropical Pacific Ocean vary strongly with ENSO, little is known about the impact of this variability in flow on dFAD dispersion. In this study, virtual Lagrangian particles are tracked for the period 2006 to 2021 over the domain in a 3D hydrodynamic model and are advected in simulations with only surface flow as well as simulations using a depth-averaged horizontal flow over the upper 50 meters. The particle trajectories are used to determine zonal displacements, eddy-like behaviour and ENSO variability for drifters that are subjected to either surface or depth-averaged currents. It was found that virtual particles that are advected by only surface flow are displaced up to 35% farther than those subjected to a depth-averaged flow, but no other major differences are found in dispersion patterns. Strongest correlations between ENSO and dFAD dispersion for the assessed variables were found in the West Pacific Ocean, with Pearson correlation coefficients up to 0.59 for dFAD displacement. Connections between ENSO and eddy-like behaviour were found in the western part of the SEC, indicating more circulation and meandering during el Niño. These findings may be useful for improving sustainable deployment strategies during ENSO events, and understanding the ocean processes driving the distribution of dFADs. [ABSTRACT FROM AUTHOR]