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Continuous measurements of the Atlantic meridional overturning circulation (AMOC) and meridional ocean heat transport at 26.5° N began in April 2004 and are currently available through December 2020. Approximately 90% of the total meridional heat transport (MHT) at 26.5° N is carried by the zonally averaged overturning circulation, and an even larger fraction of the heat transport variability (approx. 95%) is explained by the variability of the zonally averaged overturning. A physically based separation of the heat transport into large-scale AMOC, gyre and shallow wind-driven overturning components remains challenging and requires new investigations and approaches. We review the major interannual changes in the AMOC and MHT that have occurred over the nearly two decades of available observations and their documented impacts on North Atlantic heat content. Changes in the flow-weighted temperature of the Florida Current (Gulf Stream) over the past two decades are now taken into account in the estimates of MHT, and have led to an increased heat transport relative to the AMOC strength in recent years. Estimates of the MHT at 26.5° N from coupled models and various surface flux datasets still tend to show low biases relative to the observations, but indirect estimates based on residual methods (top of atmosphere net radiative flux minus atmospheric energy divergence) have shown recent promise in reproducing the heat transport and its interannual variability. This article is part of a discussion meeting issue 'Atlantic overturning: new observations and challenges'. [ABSTRACT FROM AUTHOR]

The ocean is filled with mesoscale eddies that account for most of the oceanic kinetic energy. The importance of eddies in transporting properties and energy across the ocean basins has led to numerous efforts to track their motion. Here, we implement a computer vision technique—the optical flow—to map the pathways of mesoscale eddies in the South Atlantic Ocean. The optical flow is applied to the pairs of consecutive sea surface height maps produced from a nearly 30-year-long satellite altimetry record. In contrast to other methods to estimate the eddy propagation velocity, the optical flow can reveal the temporal evolution of eddy motion, which is particularly useful in the regions of strong currents. We present the time-dependent estimates of the speed and direction of eddy propagation in the Eulerian frame of reference. In an excellent agreement with earlier studies, the obtained pattern of eddy propagation reveals the interaction of eddies with the background flow and the bottom topography. We show that in the Antarctic Circumpolar Current, the variability of the eddy propagation velocity is correlated with the variability of the surface geostrophic velocity, demonstrating the robustness of the optical flow to detect the time-variable part of eddy motion. [ABSTRACT FROM AUTHOR]

The subtropical southern Indian Ocean (SIO) has been described as one of the world's largest heat accumulators due to its remarkable warming during the past 2 decades. However, the relative contributions of remote (of Pacific origin) forcing and local wind forcing to the variability of heat content and sea level in the SIO have not been fully attributed. Here, we combine a general circulation model, an analytic linear reduced-gravity model, and observations to disentangle the spatial and temporal inputs of each forcing component on interannual to decadal timescales. A sensitivity experiment is conducted with artificially closed Indonesian straits to physically isolate the Indian Ocean and Pacific Ocean, intentionally removing the Indonesian Throughflow (ITF) influence on the Indian Ocean heat content and sea level variability. We show that the relative contribution of the signals originating in the equatorial Pacific vs. signals caused by local wind forcing to the interannual variability of sea level and heat content in the SIO is dependent on location within the basin (low latitude vs. midlatitude and western side vs. eastern side of the basin). The closure of the ITF in the numerical experiment reduces the amplitude of interannual-to-decadal sea level changes compared to the simulation with a realistic ITF. However, the spatial and temporal evolution of sea level patterns in the two simulations remain similar and correlated with El Niño–Southern Oscillation (ENSO). This suggests that these patterns are mostly determined by local wind forcing and oceanic processes, linked to ENSO via the "atmospheric bridge" effect. We conclude that local wind forcing is an important driver for the interannual changes of sea level, heat content, and meridional transports in the SIO subtropical gyre, while oceanic signals originating in the Pacific amplify locally forced signals. [ABSTRACT FROM AUTHOR]

The article discusses the onset of the strong 2014–2016 El Niño, a decade-long increase of the basin-wide sea level and heat content in the subtropical southern Indian Ocean. Topics include the ensuing recovery has forced by an anticyclonic wind anomaly associated with stronger trade winds that caused deepening of isotherms; and the ocean absorbs about 93% of the excess heat, accumulated in the Earth's climate system due to the net radiation imbalance at the top of the atmosphere.