Deepening mechanisms of cut-off lows in the Southern Hemisphere and the role of jet streams: insights from eddy kinetic energy analysis

. Cut-off lows (COLs) exhibit diverse structures and lifecycles, ranging from conﬁned upper-tropospheric systems to deep, multi-level vortex structures. While COL cli-matologies are well documented, the mechanisms driving their deepening remain unclear. To bridge this gap, a novel track matching algorithm applied to ERA-Interim reanalysis investigates the vertical extent of Southern Hemisphere COLs. Composite analysis based on structure and eddy kinetic energy budget differentiates four COL categories: shallow, deep, weak, and strong, revealing similarities and disparities. Deep, strong COLs concentrate around Australia and the southwestern Paciﬁc, peaking in autumn and spring, while shallow, weak COLs are more common in summer and closer to the Equator. Despite their differences, both contrasting types evolve energetically via anticyclonic Rossby wave breaking. The distinct roles of jet streams in affecting COL types are addressed: intense polar front jets correlate with more deep COLs, whereas stronger subtropical jets relate to fewer shallow COLs. The COL deepening typically occurs in the presence of a robust upstream polar front jet, which enhances ageostrophic ﬂux convergence and baroclinic processes. The subtropical jet positively correlates with COL intensity but weakens when considering the seasonality, suggesting uncertainties in this relationship. Additionally, we highlight the signiﬁcance of diabatic processes in COL deep-ening, addressing their misrepresentation in reanalysis and emphasizing the need for more observational and modelling studies to reﬁne the energetic framework.

Unit is 10 -1 s -1 for mean intensity and number per season per unit area for the other densities, where the unit area is equivalent to a 5 o spherical cap (≅10 6 km 2 ).Mean intensity is suppressed for track density below 1.0.Feature density is calculated using all track points, implying a concentrated density contribution in a small region for slow-moving systems due to the higher point density.

Figure S1 :
Figure S1: (a) Track density (shaded) with genesis density (contour) and (b) feature density (shaded) with mean intensity (contour) for all identified 300-hPa vorticity COLs.Regions of maximum genesis are denoted by A(32 o S 10 o E), B(29 o S 39 o E), C(33 o S 105 o E), D(34 o S 142 o E), E(33 o S 161 o E), F(34 o S 166 o E), G(34.5 o S 80 o W) and H(35 o S 57 o W).Unit is 10 -1 s -1 for mean intensity and number per season per unit

Figure S3 :
Figure S3: Scatter plots indicating the relationships between monthly mean COL intensity and jet intensity for (a, c) polar front jet and (b, d) subtropical jet using (a, b) raw values and (c, d) anomaly values.Anomalies are calculated by subtracting the monthly climatological mean from the observed value.Deep and shallow COLs are depicted by blue and red colors, respectively.Deep and shallow COLs are depicted by blue and red colors, respectively.Unit is in m.s -1 for jet intensity and 10 -5 s -1 for COL intensity (scaled by -1) as measured by the 300-hPa vorticity.

Figure S4 :
Figure S4: Monthly variations of mean intensity (black lines) and mean anomaly intensity (red line) of subtropical jet for the period from 1979 to 2014.Unit is m.s -1 .

Figure S5 :
Figure S5: Temporal evolution of shallow COLs in the Southern Hemisphere relative to the time and space of maximum intensity in  300 .The panels depict: (a) vertical cross-sections of total EKE (shaded) with baroclinic conversion (contour); (b) vertically integrated ageostrophic flux convergence (blue and red contours) with EKE (shaded), geopotential height (orange line), zonal wind mean (green line) and ageostrophic fluxes (vectors) at 300 hPa; (c) vertically integrated baroclinic conversion (red contour) with EKE (shaded), geopotential height (orange line) and ageostrophic fluxes (vectors) at 500 hPa; and (d) EKE, geopotential height (orange line) and ageostrophic fluxes (vectors) at 1000 hPa.Contours represent 0.003 × 10 10 Joule.s - for integrated quantities, 50 gpm for geopotential height at 300 and 500 hPa, and 20 gpm for geopotential height at 1000 hPa, while total EKE is indicated by 10 9 Joule.