Comment on “Anticyclonic Suppression of the North Pacific Transient Eddy Activity in Midwinter” by Okajima et al

Atmospheric energetics is frequently used to diagnose how different atmospheric processes contribute to the development of transient storm track activity. Okajima et al. (2024), https://doi.org/10.1029/2023gl106932 developed an ad hoc method to separate the contributions of cyclones and anticyclones to the energetics using the value of the curvature of the instantaneous local wind. Here, using simple examples in which the physics is exactly known, it is shown that cyclones embedded within a constant zonal flow exhibit large regions with anticyclonic curvature despite the absence of any real anticyclones. Using the method of Okajima et al., a large fraction of the eddy kinetic energy is erroneously attributed to being associated with anticyclones. Furthermore, the fraction that is misattributed varies substantially with changes in the background wind speed. It is concluded that using the curvature to separate energetics contributions from cyclones and anticyclones is not likely to be physically meaningful. Plain Language Summary How different physical processes contribute to the development of cyclones and anticyclones is frequently diagnosed using the energy budget. Previous studies examined the combined contributions of cyclones and anticyclones to the budget terms. A recent study (Okajima et al., 2024, https://doi.org/10.1029/2023gl106932) developed an ad hoc method to use the value of the curvature of the instantaneous local wind to separate the contributions of cyclones from those of anticyclones. In this study, using idealized examples in which the physics is exactly known, it is shown that cyclones embedded within a constant zonal flow exhibit large regions with anticyclonic curvature despite the absence of any real anticyclones within the domain. In these examples, large fractions of the eddy energy can be misattributed as contributions from anticyclones based on the method of Okajima et al. In addition, the fraction that is misattributed varies substantially with changes in the background wind. It is concluded that it may not be physically meaningful to separate the energetics into contributions from cyclones and anticyclones separately. Key Points The use of the curvature of the local wind to identify cyclones and anticyclones can lead to significant misattribution The magnitude of the error varies substantially with changes in the background wind speed