Oceanic eddies play a profound role in mixing tracers such as heat, carbon, and nutrients, thereby regulating regional and global climate. Yet, it remains unclear how global oceanic eddy kinetic energy has evolved over the past few decades. Furthermore, coupled climate model predictions generally fail to resolve oceanic mesoscale dynamics, which could limit their accuracy in simulating future climate change. Here we show a global statistically significant increase of the eddy activity using two independent observational datasets of mesoscale variability, one directly measuring currents and the other from sea surface temperature.Regions characterized by different dynamical processes show distinct evolution in the eddy field. For example, eddy-rich regions such as boundary current extensions and the Antarctic Circumpolar Current show a significant increase of 2% and 5% per decade in eddy activity, respectively. In contrast, most of the regions of observed decrease are found in the tropical oceans. Because eddies play a fundamental role in the ocean transport of heat, momentum, 1 and carbon, our results have far-reaching implications for ocean circulation and climate, and the modelling platforms we use to study future climate change.Changes in the climate system over recent decades have warmed the upper ocean and modified the wind stress, heat and freshwater fluxes that drive ocean circulation 1, 2 . These changes have the capacity to modify the ocean circulation at all scales, including the overturning circulation 3, 4 , basin-scale gyres 5,6 , boundary currents 7,8 , and the mesoscale 9 . The ocean's mesoscale incorporates motions that occur at spatial scales from ∼10 to ∼100 km. These motions include both steady flows, such as jets and re-circulations, and time-varying flows, generally referred to as eddies. Mesoscale eddies are ubiquitous in the global ocean and feed back onto all scales, from regional processes 10 up to the meridional overturning circulation 3 . Moreover, these eddies act to transport and mix tracers such as heat, salt, and nutrients 11,12 . Thus, understanding the evolution of the mesoscale circulation is crucial to better predict our changing oceans. Kinetic energy (KE) quantifies the magnitude of ocean currents 9,[13][14][15] . Kinetic energy is proportional to the square of the velocity, and is commonly separated into the mean KE (MKE; computed from the time-mean velocity field) and the KE of the time-varying velocity (known as the Eddy Kinetic Energy; EKE). The EKE is dominated by mesoscale variability and is a significant fraction of the total KE 16,17 . A recent study has inferred a global increase of KE anomaly from ocean reanalyses and ARGO floats 15 . However, these reanalyses and observations do not have the spatial resolution required to resolve the mesoscale field. Satellite observations, which can resolve the mesoscale, suggest that EKE in the Southern Ocean has a robust increasing trend 9,18,19 . How-

The mesoscale eddy field plays a key role in the mixing and transport of physical and biological properties and redistribution of energy in the ocean. Eddy kinetic energy is commonly defined as the kinetic energy of the time‐varying component of the velocity field. However, this definition contains all processes that vary in time, including coherent mesoscale eddies, jets, waves, and large‐scale motions. The focus of this paper is on the eddy kinetic energy contained in coherent mesoscale eddies. We present a new method to decompose eddy kinetic energy into oceanic processes. The proposed method uses a new eddy identification algorithm (TrackEddy). This algorithm is based on the premise that the sea level signature of a coherent eddy can be approximated as a Gaussian feature. The eddy Gaussian signature then allows for the calculation of kinetic energy of the eddy field through the geostrophic approximation. TrackEddy has been validated using synthetic sea surface height data and then used to investigate trends of eddy kinetic energy in the Southern Ocean using satellite sea surface height anomaly (AVISO+). We detect an increasing trend of eddy kinetic energy associated with mesoscale eddies in the Southern Ocean. This trend is correlated with an increase in the coherent eddy amplitude and the strengthening of wind stress over the last two decades.

Oceanic eddies play a profound role in mixing tracers such as heat, carbon, and nutrients, thereby regulating regional and global climate. Yet, it remains unclear how global oceanic eddy kinetic energy has evolved over the past few decades. Furthermore, coupled climate model predictions generally fail to resolve oceanic mesoscale dynamics, which could limit their accuracy in simulating future climate change. Here we show a global statistically significant increase of the eddy activity using two independent observational datasets of mesoscale variability, one directly measuring currents and the other from sea surface temperature. Regions characterized by different dynamical processes show distinct evolution in the eddy field. For example, eddy-rich regions such as boundary current extensions and the Antarctic Circumpolar Current show a significant increase of 2% and 5% per decade in eddy activity, respectively. In contrast, most of the regions of observed decrease are found in the tropical oceans. Because eddies play a fundamental role in the ocean transport of heat, momentum, and carbon, our results have far-reaching implications for ocean circulation and climate, and the modelling platforms we use to study future climate change.