An array of overturning data: The Atlantic meridional overturning circulation (AMOC) has a strong influence on climate, so it is important to understand how global warming may affect it. Lozier et al. report initial results from the Overturning in the Subpolar North Atlantic Program (OSNAP) (see the Perspective by Rhein). OSNAP has been measuring the flux of water transported by overturning in the high latitudes in the North Atlantic. The measurements reveal the strong variability of transport in the region and show that deep water formation in the Labrador Sea may not, as previously believed, be the major determinant of AMOC variability. Science , this issue p. 516 ; see also p. 456

The ocean’s role in global climate change largely depends on its heat transport. Therefore, understanding the oceanic meridional heat transport (MHT) variability is a fundamental issue. Prevailing observational and modeling evidence suggests that MHT variability is primarily determined by the large-scale ocean circulation. Here, using new in situ observations in the eastern subpolar North Atlantic Ocean and an eddy-resolving numerical model, we show that energetic mesoscale eddies with horizontal scales of about 10–100 km profoundly modulate MHT variability on time scales from intra-seasonal to interannual. Our results reveal that the velocity changes due to mesoscale processes produce substantial variability for the MHT regionally (within sub-basins) and the subpolar North Atlantic as a whole. The findings have important implications for understanding the mechanisms for poleward heat transport variability in the subpolar North Atlantic Ocean, a key region for heat and carbon sequestration, ice–ocean interaction, and biological productivity.

Subpolar North Atlantic is a vital region to modulate the variation of Atlantic Meridional Overturning Circulation (MOC). Recently, trans‐basin mooring arrays were deployed to monitor the meridional volume and heat transport in this region. Motivated by these new observations, this study investigates the structure and variability of MOC using a high resolution data assimilative reanalysis product. Our results indicate that the reanalysis product successfully captures the vertical structure and variability for the basinwide integrated stream function. In accord with observations, the meridional volume transport across the section from Greenland to Scotland (i.e., the East Section) is much stronger than that in the Labrador Sea (i.e., the West Section). While the boundary currents in the Labrador Sea display dramatic seasonal variations, their seasonal cycles are largely compensated, leading to very weak seasonality in the MOC. On the other hand, the MOC across the East section has an apparent seasonal cycle with the density changes in the Irminger Basin playing a dominant role. On the interannual time scale, MOC variations across both the West and East sections have comparable amplitudes. In addition, we further split the MOC fluctuations into components induced by barotropic and baroclinic processes. The decomposed fields point out that the isopycnal layer thickness at the west of Greenland is the primary factor to modulate the MOC interannual variability across the West section. MOC interannual variability across the East section is mostly determined by barotropic velocity changes near the pathway of the North Atlantic Current.

The Iceland Basin has the most energetic eddy activities in the subpolar North Atlantic. This study documents the structure for an anticyclonic eddy in the Iceland Basin using high‐resolution hydrographic and velocity observations. The eddy core waters have lens‐like structure with warm and salty features in the upper 1,000 m. The eddy distorts the density surface by doming the upper isopycnals and deepening the ones near the permanent pycnocline. The eddy has a diameter of about 120 km with substantial barotropic component in the velocity profiles. One branch of the North Atlantic Current in the central Iceland Basin is superimposed onto the eddy, leading to asymmetric velocity structure. Satellite maps show that eddy first shows up over the western slope of the Hatton Bank and moves westward to the central Iceland Basin. The waters enclosed in the eddy core share the same properties with Subpolar Mode Waters. Similar anticyclonic eddies are also found in high‐resolution numerical model simulations, which is used to explore eddy formation. The model results reveal that the potential vorticity gradient prior to the eddy event change signs in both horizontal and vertical directions. This potential vorticity gradient structure meets the necessary condition for the barotropic and baroclinic instabilities. Further calculation of the energy conversions suggests that eddies extract mean potential energy from the large‐scale isopycnal slope and gain the mean kinetic energy in the upper ocean. Therefore, both barotropic and baroclinic instabilities are involved to support the eddy growth.

The original version of this Article omitted the author N. Penny Holliday from the National Oceanography Centre, European Way, Southampton SO14 3ZH, UK. Consequently, the following was originally omitted from the Acknowledgements: ‘N.P.H. and the JR302 cruise were funded through the UK Natural Environment Research Council programmes UK OSNAP (NE/K010875/1), RAGNARRoCC (NE/K002511/1) and the Extended Ellett Line (National Capability)’. The corrected version of the Acknowledgements also removes the following from the original version: ‘The Zonally Accumulated Heat Transport in observation was calculated by N.P.H. from the National Ocean Center, United Kingdom’.Additionally, the following was originally omitted from the Author Contributions: ‘N.P.H. calculated the observed Zonally Accumulated Heat Transport along the OSNAP east section’.This has been corrected in both the PDF and HTML versions of the Article.

Using tide gauge (TG) observations, we identify pronounced multidecadal fluctuations in sea level along the US Northeast Coast (USNEC) superimposed on a long-term increasing trend. This multidecadal sea level variability, largely arising from fluctuations in the buoyancy-driven Atlantic meridional overturning circulation (AMOC), substantially modulates the frequency of flood occurrences along the USNEC and serves as a source of multiyear predictability. Using an initialized dynamical downscaling decadal prediction system with a ¹ / ₁₂ ° ocean resolution, we demonstrate that flood frequency along the USNEC can be predicted on multiyear to decadal timescales. The long-term increasing trend in flood frequency, mainly driven by increasing greenhouse gases and associated radiative forcing changes, can be predicted a decade ahead. Furthermore, detrended flood frequency along the USNEC exhibits prediction skill for up to 3 years, as verified by TG observation. This multiyear prediction skill is achieved using prediction models that are initialized from our best estimate of observed AMOC.