Introduction

The ocean regulates Earth’s climate through different spatial-temporal scales varying from a few millimetres or seconds to thousands of kilometres or years. Each scale contributes to the control of the carbon cycle, heat balance, global circulation, oceanic biology and energy balance. Ocean processes with spatial scales of 10 to 100 km and temporal scales of days to years are known as mesoscale processes, and include eddies, jets and waves. Mesoscale processes play important roles in energy transfer, biologic dynamics and heat, momentum and tracers transport (Lèvy et al., 2012; Thomas et al., 2008). However, even though mesoscale circulation has been studied since the 1970s (Zhang et al., 2014; Wyrtki et al., 1976; Gill et al., 1974), the global scale interaction and impact of each mesoscale process remains unknown. Mesoscale processes constitute a significant component of the oceanic kinetic and available potential energy. In the case of kinetic energy, a Reynolds decomposition separates the flow into a perturbation, or transient, state and a temporal mean state. These two components are commonly defined as the mean kinetic energy (MKE) and eddy kinetic energy (EKE)

(Kang and Curchitser, 2017; Wyrtki et al., 1976). Analogous to the case of the KE, the available potential energy (APE) can be decomposed into the time mean available potential energy (MAPE) and the eddy available potential energy (EAPE) (Oort et al., 1989). These decompositions provide the mean background and the time-variation energy budget, but do not differentiate between mesoscale and non-mesoscale processes, which are present in both mean and transient fields.

In many parts of the ocean, mesoscale processes contain more EKE and EAPE than the tem- poral mean state (Barthel et al., 2017; Chen et al., 2014). Despite the name EKE and EAPE, both decompositions contain all transient processes, not just eddies. This lack of a process-based definition of EKE and EAPE makes it difficult to understand each process in the oceanic transient adjustment to climate change. Thus, the contribution of eddies and jets in the oceanic energy budget remains unknown.

Additionally, Hogg et al. (2015) found a decadal increase in EKE in the Southern Ocean. The present study will explore the consequences of the increasing trend in EKE in more detail. As the mesoscale variability forms a crucial component of the ocean circulation dynamics, including the large-scale and time-mean circulation, this study will be focused on each mesoscale process to understand it on a global scale.