Climate impacts of forced equatorial superrotation in an idealized GCM

TL;DR

The paper investigates the climate consequences of forced equatorial superrotation using an idealized moist GCM in an aquaplanet configuration, imposing a tripolar equatorial torque to drive a strong equatorial jet. By combining a forcing/feedback framework with radiative kernels and a moist energy balance model, the study shows that the emergent superrotating state reconfigures global energy transport—collapsing the tropical MMC while enhancing equatorial eddy fluxes—leading to a global surface warming of about $3.5$ K and a dramatic redistribution of precipitation toward higher latitudes and subtropics. Tropics experience radiative forcing dominated by relative-humidity changes, whereas extratropics are governed by Planck feedback, highlighting region-dependent climate responses to circulation changes. The work demonstrates a practical method for diagnosing circulation-driven climate shifts and provides insights relevant to paleoclimate interpretations and potential future warm climates, while noting that clouds and land processes could modify these results in more complex models.

Abstract

While it is expected that the large-scale tropical circulation should undergo some changes in a warmer climate, it remains an open question whether its characteristic features, such as the Hadley cell, the intertropical convergence zone, or the weak surface easterlies, could take a completely different shape. As an example, it has been hypothesized that the Earth's atmosphere may have experienced equatorial superrotation -- i.e. westerly winds at the equator -- during its history. The possibility of equatorial superrotation has been studied in a range of planetary atmospheres, including Earth-like ones, with the objective of understanding the underlying dynamical processes. However, the broader impact that this dramatic circulation change would have on the climate system is practically unexplored. This is the question we address here. We perform idealized GCM simulations with an imposed equatorial torque to investigate how a forced superrotating atmosphere affects surface temperature and the water cycle. We show that these effects are quite large and directly related to the global circulation changes, which extend beyond the tropical atmosphere. Using tools including a forcing/feedback analysis and a moist energy balance model, we argue that the dominant mechanism is changes in atmospheric energy transport, driven in particular by the collapse of the meridional overturning circulation, and to a smaller extent by the appearance of an equatorial jet, and the concomitant redistribution of moisture in the tropics, leading to a much weaker relative humidity gradient which has strong radiative effects.

Figures (13)

• Figure 1: Zonally- and time-averaged zonal wind (colors) and meridional mass streamfunction (contours with an interval of $10^{10}~\mathrm{kg.s}^{-1}$ and dashed lines indicating negative values), for experiments with $F_0 = 0$ m.s$-1$.day$-1$ (control run, panel a) and $F_0 = 4$ m.s$-1$.day$-1$ (panel b) and interactive SSTs.
• Figure 2: Zonally- and time-averaged zonal wind (colors) and meridional mass streamfunction (black contours, in units of $10^{10}~\mathrm{kg.s}^{-1}$), for $F_0 = 0$ m.s$-1$.day$-1$ (control run, panel a) and $F_0 = 4$ m.s$-1$.day$-1$ (panel b), for the fixed SST experiments.
• Figure 3: Zonally- and time-averaged surface temperature profiles for two different values of $F_0$: $F_0=0$ m.s-1.day-1 (control run, blue curve) and $F_0=4$ m.s-1.day-1 (superrotating state, orange curve). The value of the globally-averaged surface temperature in each case is given in the legend.
• Figure 4: Zonally- and time-averaged precipitation rates for two different values of $F_0$: $F_0=0$ m.s-1.day-1 (control run, blue curve) and $F_0=4$ m.s-1.day-1 (superrotating state, orange curve), in the interactive-SST (a) and fixed-SST experiments (b). The value of the globally-averaged precipitation rate in each case is given in the legend.
• Figure 5: a) total atmospheric meridional energy transport, positive northward, along with its mean meridional circulation and eddy components in the control run with interactive SST. Bottom: Transport changes between the superrotating ($F_0 = 4$ m.s$-1$.day$-1$) and control ($F_0=0$ m.s$-1$.day$-1$) runs, for interactive (b) and fixed SST (c).