Storm tracks, the preferred highways along which extratropical cyclones and their attendant wind, rain and temperature swings travel, are among the most consequential features of the climate system. They regulate weather variability and extremes throughout the midlatitudes of both hemispheres, yet climate models still struggle with stubborn biases in their strength and location, and emerging discrepancies have appeared between simulated and observed storm track trends in some regions. That makes a return to first principles, asking which parts of Earth’s uneven surface actually build the storm tracks we observe, both timely and practical.

The team attacked the problem using MiMA, the Model of an idealized Moist Atmosphere, an intermediate-complexity general circulation model that bridges the gap between stripped-down aquaplanet experiments and fully comprehensive climate models. Crucially, MiMA carries full radiative transfer through the Rapid Radiative Transfer Model, a physically consistent treatment of moisture transport and latent heat release, and a slab ocean with prescribed ocean heat fluxes. That combination allowed the researchers to switch Earth-like land–sea contrast, topography and ocean heat transport on and off in any combination, an experiment that no single comprehensive model could perform cleanly, since in the real world it is difficult to imagine mountains without land or ocean circulation without continents.

The authors ran a suite of eight core configurations, each lasting 38 years after spin-up, beginning with a perfectly zonally symmetric aquaplanet and building up to a realistic ALL3 configuration containing all three surface inhomogeneities. By comparing each partial configuration with the full one, they could isolate both the isolated nonlinear response of a single ingredient, added first onto an aquaplanet, and its full nonlinear response, added last to a world that already had the other two. In parallel, they updated the model’s prescribed albedo profile to match satellite observations from CERES and revised the implied ocean heat transport so that the net flow of heat from the Southern Hemisphere to the Northern Hemisphere reached 0.54 petawatts, in line with observationally derived estimates, up from just 0.24 petawatts in the earlier configuration.

The realism check was encouraging. When all three ingredients were switched on, the model reproduced the observed pattern of storm tracks, with peaks over the North Pacific, the North Atlantic and the Indian-Ocean sector of the Southern Ocean, although the North Pacific came out somewhat too strong, the North Atlantic somewhat too weak, and the finer zonal structure of the Southern Hemisphere storm track was only fully captured at the higher T85 resolution. With the earlier albedo and ocean-flux settings, storm tracks had been 17 percent too weak at T85 and 29 percent too weak at T42; the updated configuration brought that bias down to 7 percent at T42 and essentially eliminated it at T85.

The central result concerns additivity. If the climate system were linear, the storm tracks in the full realistic configuration would equal the sum of the responses to land, mountains and ocean heat transport imposed individually. They do not. The team found substantial non-additivity both in the regional structure of the storm tracks and in the hemispheric asymmetry. Adding all three inhomogeneities reduces transient kinetic energy everywhere, partly because more energy is handed off to stationary waves forced by the uneven surface, and partly because realistic ocean heat transport carries some of the poleward energy load that transient eddies would otherwise shoulder. But the size of that effect depends dramatically on the order of assembly. In most sectors, the impact of the surface inhomogeneities is magnified when they are imposed on an aquaplanet compared with when they are removed from the full configuration, and the regional signature of ocean heat transport flips qualitatively between the two, weakening storm tracks most strongly over Eurasia when added first but over the North Pacific and North Atlantic when added last.

The same story holds for the headline asymmetry: in the full configuration the model’s Southern Hemisphere storm tracks are roughly 48 percent stronger than the north’s when measured with 2-to-8-day filtered transient kinetic energy, and about 25 percent stronger with a sub-monthly filter, broadly consistent with the observed excess of about 23 to 24 percent in the moist static energy framework applied to ERA5 reanalysis. Yet the sum of the isolated responses of the three ingredients accounts for only half of that asymmetry, while the sum of the full nonlinear responses overshoots it by 30 percent. Among the individual ingredients, land–sea contrast emerges as the most important driver of the hemispheric asymmetry, followed by ocean heat transport, with topography a distant third, a ranking that challenges earlier conclusions from comprehensive models in which topography and ocean circulation dominated.

To explain the mechanisms, the authors turn to the atmospheric moist static energy budget, decomposing poleward heat transport into contributions from the ocean, the mean meridional circulation, stationary eddies and transient eddies. In the realistic configuration, the surface energy input term, reflecting the ocean’s preferential delivery of heat to the Northern Hemisphere extratropics, contributes about 15.6 percentage points to the hemispheric asymmetry, while the stationary eddy term contributes 22.4 points, partially offset by a top-of-atmosphere term of minus 5.2 points. A striking simplification emerges when the researchers examined outgoing longwave radiation: across all 21 simulations, the correlation between the hemispheric asymmetry in longwave emission and the asymmetry in surface temperature was 0.99, meaning surface temperature essentially dictates how much energy the planet radiates to space in each hemisphere. Ocean heat transport and topography both preferentially cool the Southern Hemisphere, warming the north and increasing its longwave loss, which partially cancels their own intensifying effect on the asymmetry; land–sea contrast, by contrast, preferentially cools the north, deepening the asymmetry.

Two sensitivity experiments underline how delicate these balances are. Boosting the implied Southern-Ocean-to-North-Atlantic heat transport from 0.54 to 0.74 petawatts, a shift well within observational uncertainty, produced dramatic cancellations: transient eddy heat transport in the north actually increased by 0.34 petawatts rather than decreasing, because the stationary eddy term weakened by nearly the same amount, even as the North Atlantic storm track intensified locally. Likewise, tweaking midlatitude albedo over Northern Hemisphere land and ocean produced compensating changes in absorbed shortwave and emitted longwave radiation, leaving the hemispheric asymmetry nearly untouched despite pronounced regional shifts. The authors caution that MiMA lacks radiatively active clouds, whose treatment differs widely across climate models, and that its overly strong stationary eddy asymmetry may inflate the apparent importance of land–sea contrast. Even so, the message is clear: storm tracks cannot be understood ingredient by ingredient. Each piece of Earth’s patchwork surface sets the background state on which the others act, and that entanglement may be precisely why climate models, with their differing ocean heat transports and cloud biases, still disagree about where and how strongly the planet’s storms will rage in a warming world.

Subject of Research: How land–sea contrast, ocean heat transport and topography shape the localization and hemispheric asymmetry of midlatitude storm tracks

Article Title: Building blocks of localized storm tracks: revisiting asymmetries between the NH and SH in storm track strength

Article References: Building blocks of localized storm tracks: revisiting asymmetries between the NH and SH in storm track strength. (n.d.). https://doi.org/10.5194/wcd-7-1951-2026

Image Credits: AI Generated

DOI: 10.5194/wcd-7-1951-2026

Keywords: storm tracks, midlatitude cyclones, land-sea contrast, ocean heat transport, topography, stationary waves, transient eddies, moist static energy budget, general circulation model, hemispheric asymmetry, albedo, climate modeling