arXiv subjects

Spencer A. Hill

Publications and source records attributed to Spencer A. Hill.

4 recordsLinked to original sources

arXiv2022

Symmetric and antisymmetric components of polar-amplified warming

CO$_2$-forced surface warming in general circulation models (GCMs) is initially polar-amplified in the Arctic but not Antarctic -- a largely hemispherically antisymmetric signal. Nevertheless, we show in CESM1 and eleven LongRunMIP GCMs that the hemispherically symmetric component of global-mean-normalized, zonal-mean warming ($T^*_\mathrm{sym}$) under 4$\times$CO$_2$ changes weakly or becomes moderately more polar-amplified from the first decade to near-equilibrium. Conversely, the antisymmetric warming component ($T^*_\mathrm{asym}$) weakens with time in all models, moderately in some including FAMOUS but effectively vanishing in others including CESM1. We explore mechanisms underlying the robust $T^*_\mathrm{sym}$ behavior with a diffusive moist energy balance model (MEBM), which given radiative feedback parameter ($λ$) and ocean heat uptake ($\mathcal{O}$) fields diagnosed from CESM1 adequately reproduces the CESM1 $T^*_\mathrm{sym}$ and $T^*_\mathrm{asym}$ fields. In further MEBM simulations perturbing $λ$ and $\mathcal{O}$, $T^*_\mathrm{sym}$ is sensitive to their symmetric components only, and more to that of $λ$. A three-box, two-timescale model fitted to FAMOUS and CESM1 reveals a curiously short Antarctic fast-response timescale in FAMOUS. In additional CESM1 simulations spanning a broader range of forcings, $T^*_\mathrm{sym}$ changes modestly across 2-16$\times$CO$_2$, and $T^*_\mathrm{sym}$ in a Pliocene-like simulation is more polar-amplified but likewise approximately time-invariant. Determining the real-world relevance of these behaviors -- which imply that a surprising amount of information about near-equilibrium polar amplification emerges within decades -- merits further study.

physics.ao-ph↗

arXiv2021

Toward a unified theory for the Hadley cell descending and ascending edges

We present theories for the latitudinal extents of both Hadley cells throughout the annual cycle by combining our recent scaling for the ascending edge latitude (Hill et al. 2021) with the uniform Rossby number (Ro), baroclinic instability-based theory for the poleward, descending edge latitudes of Kang and Lu 2012. The resulting analytic expressions for all three Hadley cell edges are predictive except for diagnosed values of Ro and two proportionality constants. The theory captures the climatological annual cycle of the ascending and descending edges in an Earth-like simulation in an idealized aquaplanet general circulation model (GCM), provided the descending edge prediction is lagged by one month. In simulations in this and two other idealized GCMs with varied planetary rotation rate ($Ω$), the winter, descending edge of the solsticial, cross-equatorial Hadley cell scales approximately as $Ω^{-1/2}$ and the summer, ascending edge as $Ω^{-2/3}$, both in accordance with our theory.

physics.ao-ph↗

arXiv2021

Solsticial Hadley Cell ascending edge theory from supercriticality

How far the Hadley circulation's ascending branch extends into the summer hemisphere is a fundamental but incompletely understood characteristic of Earth's climate. Here, we present a predictive, analytical theory for this ascending edge latitude based on the extent of supercritical forcing. Supercriticality sets the minimum extent of a large-scale circulation based on the angular momentum and absolute vorticity distributions of the hypothetical state were the circulation absent. We explicitly simulate this latitude-by-latitude radiative-convective equilibrium (RCE) state. Its depth-averaged temperature profile is suitably captured by a simple analytical approximation that increases linearly with $\sinφ$, where $φ$ is latitude, from the winter to the summer pole. This, in turn, yields a one-third power-law scaling of the supercritical forcing extent with the thermal Rossby number. In moist and dry idealized GCM simulations under solsticial forcing performed with a wide range of planetary rotation rates, the ascending edge latitudes largely behave according to this scaling.

physics.ao-ph↗

arXiv2020

Constraints from invariant subtropical vertical velocities on the scalings of Hadley cell strength and downdraft width with rotation rate

Weak-temperature-gradient influences from the tropics and quasigeostrophic influences from the extratropics plausibly constrain the subtropical-mean static stability in terrestrial atmospheres. Because mean descent acting on this static stability is a leading-order term in the thermodynamic balance, a state-invariant static stability would impose constraints on the Hadley cells, which this paper explores in simulations of varying planetary rotation rate. If downdraft-averaged effective heating (the sum of diabatic heating and eddy heat flux convergence) too is invariant, so must be vertical velocity -- an "omega governor." In that case, the Hadley circulation overturning strength and downdraft width must scale identically -- the cell can strengthen only by widening or weaken only by narrowing. Simulations in two idealized, dry GCMs with a wide range of planetary rotation rates exhibit nearly unchanging downdraft-averaged static stability, effective heating, and vertical velocity, as well as nearly identical scalings of the Hadley cell downdraft width and strength. In one, eddy stresses set this scaling directly (the Rossby number remains small); in the other, eddy stress and bulk Rossby number changes compensate to yield the same, ({\sim}Ω^{-1/3}) scaling. The consistency of this power law for cell width and strength variations may indicate a common driver, and we speculate that Ekman pumping could be the mechanism responsible for this behavior. Extending to moist atmospheres, in an idealized aquaplanet GCM the subtropical static stability is also insensitive to rotation rate but the effective heating and vertical velocity are not.

physics.ao-ph↗