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The Atlantic Meridional Overturning Circulation (AMOC) is responsible for a variable and climatically important northward transport of heat. Using data from an array of instruments that span the Atlantic at 26°N, we show that the AMOC has been in a state of reduced overturning since 2008 as compared to 2004–2008. This change of AMOC state is concurrent with other changes in the North Atlantic such as a northward shift and broadening of the Gulf Stream and altered patterns of heat content and sea surface temperature. These changes resemble the response to a declining AMOC predicted by coupled climate models. Concurrent changes in air‐sea fluxes close to the western boundary reveal that the changes in ocean heat transport and sea surface temperature have altered the pattern of ocean‐atmosphere heat exchange over the North Atlantic. These results provide strong observational evidence that the AMOC is a major factor in decadal‐scale variability of North Atlantic climate.

The current Earth's Energy Imbalance (EEI) is mostly the result of human activities and is driving global warming. The absolute value of EEI represents the most fundamental metric defining the status of global climate change and will be more useful than using global surface temperature. EEI can best be estimated from Ocean Heat Content changes, complemented by radiation measurements from space. Sustained observations from the Argo array of autonomous profiling floats and further development of the ocean observing system to sample the deep ocean, marginal seas, and the sea ice regions are crucial to refining future estimates of EEI. Combining multiple measurements in an optimal way holds considerable promise for estimating EEI and thus assessing the status of global climate change, improving climate syntheses and models, and testing the effectiveness of mitigation actions. Progress has been and can be achieved with a concerted international effort. Earth's energy imbalanceWeather and climate on planet Earth arise primarily from differential radiative heating and resulting movement of energy by the dynamic components of the climate system: the atmosphere and the oceans. Both of these fluids can move heat and moisture through advective processes by atmospheric winds and ocean currents, as well as through eddies, large-scale atmospheric jet streams and convection. Other major components of the climate system include sea ice, the land and its features (including albedo, vegetation, other biomass, and ecosystems), snow cover, land ice (including the ice sheets of Antarctica and Greenland, and mountain glaciers), rivers, lakes, and surface and ground water. About 30% of the incoming solar radiation is reflected and scattered from clouds and the Earth's surface back to space. The remaining absorbed solar radiation (ASR) in the climate system is transformed into various forms (internal heat, potential energy, latent energy, kinetic energy, and chemical forms), moved, stored and sequestered primarily in the ocean, but also in the atmosphere, land and ice components of the climate system. Ultimately it is radiated back to space as outgoing longwave radiation (OLR) [1][2][3] . In an equilibrium climate there is a global balance 2 between the ASR and OLR, which determines the Earth's radiation budget 1-2 . Perturbations of this budget from internal or external climate variations create EEI 4 , manifested as a radiative flux imbalance at the top of the atmosphere (TOA).The EEI is shaped by a number of climate forcings, some of which occur naturally and others that are anthropogenic in origin. A sense of the relative importance of these factors for a given timescale is obtained through estimates of their "Effective Radiative Forcing" (ERF, Fig. 1). The phenomena giving rise to changes in ERF vary regionally and over time. Internal climate variability occurs from day-to-day and month-to-month associated with weather systems and phenomena like the MaddenJulian Oscillation (MJO) that cause short-term changes in cloudiness 5 . On ...

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[1] The exchange between the Persian (Arabian) Gulf and the Indian Ocean is investigated using hydrographic and moored acoustic Doppler current profiler data from the Straits of Hormuz during the period December 1996 to March 1998. The moored time series records show a relatively steady deep outflow through the strait from 40 m to the bottom with a mean speed of approximately 20 cm/s. A variable flow is found in the upper layer with frequent reversals on timescales of several days to weeks. The annual mean flow in the near-surface layer is found to be northeastward (out of the Persian Gulf) in the southern part of the strait, suggesting a mean horizontal exchange with the Indian Ocean that is superimposed on the vertical overturning exchange driven by evaporation over the gulf. The salinity of the deep outflow varies from 39.3 to 40.8 psu with highest outflow salinities occurring in the winter months (December-March). The annual mean deep outflow through the strait is estimated to be 0.15 ± 0.03 Sv. Calculation of the associated heat and freshwater fluxes through the strait yields estimates for the annual heat loss over the surface of the gulf of À7 ± 4 W/m 2 and an annual water loss (E-P-R) of 1.68 ± 0.39 m/yr. These values are shown to be in relatively good agreement with climatological surface fluxes derived from the Southampton Oceanography Centre global flux climatology after known regional biases in the radiative budget are taken into account.

The Atlantic Ocean overturning circulation is important to the climate system because it carries heat and carbon northward, and from the surface to the deep ocean. The high salinity of the subpolar North Atlantic is a prerequisite for overturning circulation, and strong freshening could herald a slowdown. We show that the eastern subpolar North Atlantic underwent extreme freshening during 2012 to 2016, with a magnitude never seen before in 120 years of measurements. The cause was unusual winter wind patterns driving major changes in ocean circulation, including slowing of the North Atlantic Current and diversion of Arctic freshwater from the western boundary into the eastern basins. We find that winddriven routing of Arctic-origin freshwater intimately links conditions on the North West Atlantic shelf and slope region with the eastern subpolar basins. This reveals the importance of atmospheric forcing of intra-basin circulation in determining the salinity of the subpolar North Atlantic.

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The North Atlantic and Europe experienced two extreme climate events in 2015: exceptionally cold ocean surface temperatures and a summer heat wave ranked in the top ten over the past 65 years. Here, we show that the cold ocean temperatures were the most extreme in the modern record over much of the mid-high latitude North-East Atlantic. Further, by considering surface heat loss, ocean heat content and wind driven upwelling we explain for the first time the genesis of this cold ocean anomaly. We find that it is primarily due to extreme ocean heat loss driven by atmospheric circulation changes in the preceding two winters combined with the re-emergence of cold ocean water masses. Furthermore, we reveal that a similar cold Atlantic anomaly was also present prior to the most extreme European heat waves since the 1980s indicating that it is a common factor in the development of these events. For the specific case of 2015, we show that the ocean anomaly is linked to a stationary position of the Jet Stream that favours the development of high surface temperatures over Central Europe during the heat wave. Our study calls for an urgent assessment of the impact of ocean drivers on major European summer temperature extremes in order to provide better advance warning measures of these high societal impact events.