The Optimal Synoptic Scale for Weather Analysis:
Many schools of meteorology teach quasi-geostrophic (QG) synoptic analysis principles to help operational forecasters interpret the evolution of extratropical cyclones. Further, the QG framework provides a well-developed diagnostic for understanding the dynamic and thermodynamic forcing and its changes with time, not just seasonally but also over decades. Unfortunately, no simple criteria existed for the best spatial scales to calculate these diagnostics. In Battalio and Dyer (2017) [PDF], I used the QG omega equation to evaluate 28 cases of baroclinic cyclone development in a mesoscale model. I tested two methods to upscale the high-resolution grid to a synoptic scale. I evaluated the optimum length scale by correlating the model-native vertical velocity to the QG value. Both methods identified a length scale of 140 km as the most representative of the “true” vertical motion. These analyses indicate to forecasters when diagnosis from QG theory is most applicable. This method can be applied to CMIP simulations to diagnose changes in synoptic forcing.
Extratropical Storm Track Intensification on Earth:
Changes in the extratropical storm track impact the distribution of precipitation and atmospheric rivers, the transport of momentum and heat in the mid-latitudes, and the outbreak of extreme temper- atures. Quantifying the amount of change that has already occurred is necessary to provide context for evaluations of future climate simulations and to diagnose the underlying dynamical causes. In Battalio and Lora (2024) [PDF], I applied the the eddy energetics paradigm is applied to synoptic-scale waves in the ERA5 dataset to quantify the rates of change and locations of greatest changes to the transient-wave storm tracks. Over the last several decades, the eddy kinetic energy (EKE) in the southern storm track and the Atlantic and Pacific storm tracks has steadily increased, and the rates of baroclinic energy conversion increase rise in kind. Despite increases in EKE and baroclinic con- version, barotropic energy conversion exhibits smaller changes, suggesting that internal dissipation and frictional effects compensate for increases in EKE. As expected, EKE advection and geostrophic flux convergence experience little change globally but exhibit local changes. The global increase in the average EKE suggests more frequent extreme weather, and changes in the variance of the global EKE hint at changes in the amplitude of extreme events. Strengthening in the Northern Hemisphere is limited to the winter in a narrow latitudinal band, because of warming in the Arctic, reducing the primary instability that drives eddies. The locations of northern warming and storm track strengthening suggest a role for tropical dynamics.
Midlatitude, Near-surface Warming on Earth due to Hadley Cell:
Anthropogenic emission of CO2 primarily forces two areas of warming in the zonal-mean northern hemisphere temperatures . Arctic amplification near the surface at the northern pole and tropical, upper-level warming compete to influence the amplitude and location of midlatitude disturbances. The larger temperature increases in the Arctic cause weakening temperature gradients that are predicted to reduce the intensity of the northern storm track. In my previous work (Battalio and Lora, 2024) [PDF], I found that observations find continued intensification of the northern storm track despite Arctic am- plification occurring as predicted, combined with less robust tropical warming. In a work submitted to Nature Climate Change (Battalio 2025, submitted), I identify, using the ERA5 and MERRA-2 reanalyses, an under-appreciated third region of increasing temperatures in the northern midlatitudes from the surface to 10 km in height forced by changes of the tropical overturning circulation, the Hadley cell. The Hadley circulation is expanding with time, and the expansion of the tropics and downward vertical motion with associated adiabatic heating induce a column of increasing temperatures within 30–60°N. This warming enhances baroclinic instability that drives midlatitude storms, and in turn, midlatitude storms interact with the Hadley circulation by changing its latitudinal extent. Warming throughout the troposphere shapes the response of extratropical waves in the northern hemisphere, combatting the predicted reduction in storm tracks, but continued anthropogenic forcing may eventually overwhelm the natural variability driving Hadley cell strengthening. Then, midlatitude warming should slow, and with it, storm track intensification.