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Abstract The westward-propagating convectively coupled equatorial wave (CCEW) variability produced by an idealized general circulation model (GCM) is investigated. The model is a zonally symmetric aquaplanet with a slab ocean. Water vapor in the model may condense and produce latent heating, but there is no parameterization of cloud processes, only a quasi-equilibrium convection scheme. The CCEWs produced by the model are found to be sensitive to the heat capacity of the slab and the strength of surface friction. In spectral space, the westward-propagating precipitation variability in the model is dominated by sharp peaks in spectral power at zonal wavenumbers 5 and 6. These precipitation peaks are situated along the dispersion curve of the Rossby–Haurwitz waves, suggesting a connection between the global Rossby modes and precipitation variability. Composites of these disturbances reveal global circulation patterns that extend into the midlatitudes. The moisture variance budget of these disturbances shows that moisture advection by the global Rossby modes maintains the accompanying moisture signal. This is interpreted as downgradient advection of the background moisture gradient of the intertropical convergence zone. The locations of the precipitation peaks are sensitive to Doppler shifting by the zonal winds; when this Doppler shift becomes too weak, the frequencies of the global Rossby modes become too high to effectively couple to convection. A linearized primitive equation model shows that the presence of vertical shear in the background zonal winds is vital for producing a forced response that resembles the modes produced by the GCM. The forced response of the linear model is optimally located to enhance the original circulation of the global mode.
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Synthesis of Vortex Rossby Waves. Part III: Rossby Waveguides, Vortex Motion, and the Analogy with Middle-Latitude Cyclones
Abstract Vortex Rossby Waves (VRWs) affect Tropical Cyclones’ (TCs’) motion, structure, and intensity. They propagate within annular waveguides defined by a passband between Ω 1D , the Doppler-shifted frequency of a one-dimensional VRW, and zero. Wavenumber-1 VRWs cause TC motion directly and have wider waveguides than wavenumbers ≥ 2. VRWs forced with fixed rotation frequency propagate away from the forcing. Initially outward-propagating waves are Doppler shifted to zero at a critical radius, where they are absorbed. Initially inward-propagating waves are Doppler-shifted to Ω 1D , reflect from a turning point, propagate outward, and are ultimately absorbed at the critical radius. Between the forcing and the turning radii, the VRWs have standing-wave structure; outward from the forcing they are trailing spirals. They carry angular momentum fluxes that act to accelerate the mean flow at the forcing radius and decelerate it at the critical radius. Mean flow vorticity monopoles are inconsistent with Stokes Theorem on a spherical Earth, because a contour enclosing the monopole’s antipode would have nonzero circulation but would enclose zero vorticity. The Rossby waveguide paradigm also fits synoptic-scale Rossby Waves in a meridionally sheared zonal flow. These waves propagate within a waveguide confined between a poleward turning latitude and an equatorward critical latitude. Forced waves are comma-shaped gyres that resemble observed frontal cyclones, with trailing filaments equatorward of the forcing latitude and standing waves poleward. Even neutral forced Rossby waves converge westerly momentum at the latitude of forcing.
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- Award ID(s):
- 1724198
- PAR ID:
- 10284386
- Date Published:
- Journal Name:
- Journal of the Atmospheric Sciences
- ISSN:
- 0022-4928
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract We consider the relevance of known constraints from each of Hide’s theorem, the angular momentum–conserving (AMC) model, and the equal-area model on the extent of cross-equatorial Hadley cells. These theories respectively posit that a Hadley circulation must span all latitudes where the radiative–convective equilibrium (RCE) absolute angular momentum satisfies or or where the RCE absolute vorticity satisfies ; all latitudes where the RCE zonal wind exceeds the AMC zonal wind; and over a range such that depth-averaged potential temperature is continuous and that energy is conserved. The AMC model requires knowledge of the ascent latitude , which needs not equal the RCE forcing maximum latitude . Whatever the value of , we demonstrate that an AMC cell must extend at least as far into the winter hemisphere as the summer hemisphere. The equal-area model predicts , always placing it poleward of . As is moved poleward (at a given thermal Rossby number), the equal-area-predicted Hadley circulation becomes implausibly large, while both and become increasingly displaced poleward of the minimal cell extent based on Hide’s theorem (i.e., of supercritical forcing). In an idealized dry general circulation model, cross-equatorial Hadley cells are generated, some spanning nearly pole to pole. All homogenize angular momentum imperfectly, are roughly symmetric in extent about the equator, and appear in extent controlled by the span of supercritical forcing.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1175/JAS-D-20-0197.1
