第二届未来大气科学论坛

Traditionally, the large-scale variability of the Southern Hemisphere (SH) extratropical circulation has been partitioned into two largely independent phenomena, that is, the Southern (barotropic) annular mode (SAM) and the baroclinic annular mode (BAM), with which the barotropic and baroclinic processes are considered decoupled from each other. The SAM is characterized by red-noise latitudinal shifts of the jet stream driven by eddy momentum flux, while the BAM is characterized by a 20-to-30-day pulsing of storm track amplitude driven by eddy heat flux. However, regional analyses suggest these modes are dynamically linked through the life cycle of baroclinic eddies. Here, we unify the SAM and BAM into a single framework, the coupled Southern annular mode-baroclinic annular mode (SAMBA mode) using variate extended empirical orthogonal function (MEEOF) analysis on the coupled fields of zonal-mean zonal wind and eddy kinetic energy (EKE) in ERA5 data.

Our analysis shows that the leading MEEOF (MEEOF 1) captures the low-frequency "shifting" SAMBA mode, representing a meridional displacement of the eddy-driven jet followed by the shift of the storm track. Regression analyses show this mode is initiated by momentum forcing but is fundamentally coupled to thermodynamic fields through adiabatic heating and baroclinicity shifts. Meanwhile, cross-spectral and phase space analyses demonstrate that MEEOF 2 and 3 emerge as a degenerate pair capturing the "pulsing" SAMBA mode. This pair isolates a robust 20-to-30-day baroclinic oscillator in jet and storm track strength wherein enhanced temperature gradient triggers stronger poleward heat flux and explosive EKE growth, which subsequently weakens baroclinicity and decelerate the background jet.

We demonstrate that the barotropic and baroclinic processes associated the annular modes are not physically independent, but rather coupled phases of a unified SAMBA lifecycle. Integrating these modes yields profound implications for our understanding of midlatitude atmospheric dynamics. By demonstrating that eddy momentum forcing and eddy heat forcing are fundamentally coupled at different time scales, this framework links the long-term memory and predictability of extratropical jet shifts to the 20-to-30-day thermodynamic oscillator. Furthermore, because the variability of these modes depends sensitively on the background climate, capturing the coupled SAMBA processes is essential for accurately projecting the sensitivity and response of the large-scale circulation variability to anthropogenic climate change.

eddy-jet,westerly jet,storm track,southern annular mode,baroclinic annular mode