Poleward migration of eddy‐driven jets

Abstract

Poleward migration of eddy-driven jets is found to occur in the extratropics when the subtropical and eddy-driven jets are clearly separated, as achieved by simulations at high-rotation rates. The poleward migration of these eddy-driven baroclinic jets over time is consistent with variation of eddy momentum flux convergence and baroclinicity across the width of the jet. We demonstrate this using a high-resolution idealized GCM where we systematically examine the eddy-driven jets over a wide range of rotation rates (up to 16 times the rotation rate of Earth). At the flanks of the jets, the poleward migration is caused by a poleward bias in baroclinicity across the width of the jet, estimated through measures such as Eady growth rate and supercriticality. The poleward biased baroclinicity is due to the meridional variation of the Coriolis parameter, which causes a poleward bias of the eddy momentum flux convergence. At the core of the jets, the poleward biased eddy momentum flux convergence relative to the mean jet deflects over time the baroclinicity and the jets poleward. As the rotation rate is increased, and more (narrower) jets emerge the migration rate becomes smaller due to less eddy momentum flux convergence over the narrower baroclinic zones. We find a linear relation between the migration rate of the jets and the net eddy momentum flux convergence across the jets. This poleward migration might be related to the slow poleward propagation of temporal anomalies of zonal winds observed in the upper troposphere.

Comparative terrestrial atmospheric circulation regimes in simplified global circulation models. Part I: From cyclostrophic super‐rotation to geostrophic turbulence

“…This is broadly consistent with the zonal wind map shown in Figure , where at least eight distinct eastward jets can be located in each hemisphere. The multiple jets for Ω ∗ = 2 and 4, however, were seen to drift slowly polewards, much as found by Chemke and Kaspi (), although this was less evident for the Ω ∗ = 8 case, for which the spatial resolution may have been insufficient to reveal this behaviour clearly.…”

“…This is broadly consistent with the zonal wind map shown in Figure 4, where at least eight distinct eastward jets can be located in each hemisphere. The multiple jets for Ω * = 2 and 4, however, were seen to drift slowly polewards, much as found by Chemke and Kaspi (2015b), although this was less evident for the Ω * = 8 case, for which the spatial resolution may have been insufficient to reveal this behaviour clearly. As we can see from Figure 4, the subtropical jet stream moves to higher latitude as the rotation rate decreases, which is consistent with the prediction from quasi-inviscid axisymmetric Hadley cell theory (see Held and Hou, 1980;Caballero et al, 2008).…”

The Atmospheric Circulation and Climate of Terrestrial Planets Orbiting Sun-like and M Dwarf Stars over a Broad Range of Planetary Parameters

“…For example, the rotation period affects the width of the tropical circulation (Held 2000, Kaspi & Showman 2015, the size and distribution of extratropical eddies (Eady 1949, Charney 1967, Kaspi & Showman 2015, and the number of extratropical jets (Williams 1978, Cho & Polvani 1996, Chemke & Kaspi 2015b. Meanwhile, the atmospheric mass affects the strength of the atmospheric circulation and the surface temperature distribution (Chemke et al 2016, Chemke & Kaspi 2017. Additionally, the spin state of terrestrial exoplanets orbiting M-dwarf stars varies substantially from that of planets orbiting Sun-like stars, as the spins of planets orbiting M-dwarfs are affected by tidal dissipation due to their close-in orbits (Leconte et al 2015).…”

“…How the atmospheric dynamics and climate of planets orbiting Sun-like stars vary with planetary parameters has been explored in a wide variety of circulation models. As a first step, many modeling studies utilize idealized general circulation models (GCMs) which have reduced complexity in radiative transfer and cloud treatment relative to full physics simulations in order to understand the basic physics controlling atmospheric properties (Schneider 2006, Kaspi & Showman 2015, Chemke & Kaspi 2017. Notably, Kaspi & Showman (2015) varied a wide swath of planetary parameters (rotation period, incident stellar flux, surface pressure, surface gravity, and radius) and found that each parameter can significantly alter the atmospheric circulation of exoplanets.…”

An Axisymmetric Limit for the Width of the Hadley Cell on Planets With Large Obliquity and Long Seasonality

“…Previous studies showed that as a planet rotates faster, the Hadley circulation contracts, the streamfunction becomes multicellular, and the number of jets increases (Chemke & Kaspi, 2015a, 2015bKaspi & Showman, 2015;Navarra & Boccaletti, 2002;Walker & Schneider, 2006). Faulk et al (2017), studying the effect of the rotation rate in a seasonal cycle, showed that for a planet with an Earth-like rotation rate the Hadley cell ascending branch and the latitude of the ITCZ do not reach the pole, even when the maximum surface temperature is at the pole and the seasonal cycle is very long.…”