Ocean eddies influence regional and global climate through mixing and transport of heat and properties. One of the most recognizable and ubiquitous feature of oceanic eddies are coherent vortices with spatial scales of tens to hundreds of kilometers, frequently referred as “mesoscale eddies”. Coherent mesoscale eddies are known to transport properties across the ocean and to locally affect near-surface wind, cloud properties, and rainfall patterns. Although coherent eddies are ubiquitous, their climatology, seasonality, and long-term temporal evolution remains poorly understood. Here, we examine the kinetic energy contained by coherent eddies and present the seasonal, interannual and long-term variability using satellite observations between 1993 to 2019. A total of $\sim$37 million coherent eddies are detected in this analysis. Around 50% of the kinetic energy contained by ocean eddies corresponds to coherent eddies. Additionally, a strong seasonal cycle is observed, with a 3-6 months lag between the wind forcing and the response of the coherent eddy field. The seasonality of the number of coherent eddies and their amplitude reveals that the number of coherent eddies responds faster to the forcing ($\sim$3 months), than the coherent eddy amplitude (which lags by $\sim$6 months). This seasonal cycle is spatially variable, so we also analyze their climatology in key oceanic regions. Our analysis highlights the relative importance of the coherent eddy field in the ocean kinetic energy budget, implies a strong response of the eddy number and eddy amplitude to forcing at different time-scales, and showcases the seasonality, and multidecadal trends of coherent eddy properties.

Changes to the Southern Hemisphere (SH) surface westerlies not only affect air temperature, storm tracks and precipitation; they are also pivotal in controlling global ocean circulation, ocean heat transport, and ocean carbon uptake. Wind-forced ocean perturbation experiments have commonly applied idealized poleward wind shifts ranging between 0.5 and 10 degrees of latitude, and wind intensification factors of between 10 and 300%. In addition, changes in winds are often prescribed ad-hoc without consistently accounting for physical constraints and can neglect important regional and seasonal differences. Here we quantify historical and future projected SH westerly wind changes based on examination of CMIP5, CMIP6 and reanalysis data. Under a high emission scenario, we find a projected end of 21st Century annual mean westerly wind increase of ~10% and a poleward shift of ~0.8° latitude, although there are also significant seasonal and regional variations.

The meridional variability of the Subtropical Front (STF) and the drivers of variability on interannual time scales in the New Zealand region are analysed using a multi-decadal eddy-resolving ocean hindcast model, in comparison with Argo data. The STF marks the water mass boundary between subtropical waters and subantarctic waters, and is defined as the southern-most location of the 11 degree C isotherm and 34.8 isohaline between 100 m and 500 m. The STF shifts up to 650 km (6 degree) meridionally on seasonal timescales. In addition to seasonal variability, shifts of around 200 km (2 degree) occur on interannual time scales. These shifts are connected to local wind stress curl anomalies in the eastern Tasman Sea, which trigger Ekman convergence/divergence and result in meridional transport of heat and salt into/out of the Tasman Sea. The net transports across the northern boundary of the Tasman Sea show the largest sensitivity to these wind stress curl anomalies. During periods of positive wind stress curl anomalies and Ekman convergence, the heat and salt content increases shifting the position of the STF southward. The opposite tendency occurs during periods of negative wind stress curl anomalies. The migration of the STF does not appear to be directly linked to regional climate oscillations.

Climate models exhibit a broad range in the simulated properties of the global climate. In the early historical period, the absolute global mean surface air temperature of models contributing to the fifth phase of the Coupled Model Intercomparison Project (CMIP5) spans a range of ~12-15 °C. Other climate parameters are linked to the global mean temperature, such as sea ice area, atmospheric circulation patterns, and by extension cloudiness, precipitation and albedo. Accurate representation of the baseline climate state is crucial for meaningful future climate projections, since the baseline conditions may dictate the capacity for change. For example, a model with initially smaller sea ice area has less potential to lose sea ice as the planet warms. Amongst the CMIP5 models, it is found that in the baseline climate state there are coherences between Southern Ocean temperature, outgoing shortwave radiation, cloudiness, the position of the mid-latitude eddy-driven jet, and Antarctic sea ice area. The baseline temperature relationship extends to projected future changes in the same set of variables. The tendency for models with initially cooler Southern Ocean surface temperature to exhibit more global warming, and vice versa for initially warmer models, can therefore be linked to baseline Southern Ocean climate system biases. A first look at emerging data from CMIP6 reveals a shift of the tendency towards the Antarctic region, potentially linked to a reduction in biases over the Southern Ocean, which prompts an examination of biases in the Antarctic region as more CMIP6 model data becomes available.