We report a discovery of steady long-living slowly eastward moving large-scale coherent twin cyclones, the equatorial modons, in the shallow water model in the equatorial beta-plane, the archetype model of the ocean and atmosphere dynamics in tropics. We start by constructing analytical asymptotic modon solutions in the non-divergent velocity approximation, and then show by simulations with a high-resolution numerical scheme that such configurations evolve into steady dipolar solutions of the full model. In the atmospheric context, the modons persist in the presence of moist convection, being accompanied and enhanced by specific patterns of water-vapour condensation.
We report a discovery of steady long-living slowly eastward moving large-scale coherent twin cyclones, the equatorial modons, in the shallow water model in the equatorial beta-plane, the archetype model of the ocean and atmosphere dynamics in tropics. We start by constructing analytical asymptotic modon solutions in the non-divergent velocity approximation and then show by simulations with a high-resolution numerical scheme that such configurations evolve into steady dipolar solutions of the full model. In the atmospheric context, the modons persist in the presence of moist convection, being accompanied and enhanced by specific patterns of water-vapour condensation.
21The process of geostrophic adjustment of localized large-scale pressure anomalies in the standard adia-22 batic shallow-water model on the equatorial beta-plane is revisited, and it is shown that the standard scenario 23 of generation of westward-moving Rossby and eastward-moving Kelvin waves, which underlies the classi-24 cal Gill theory of tropical circulation due to a localized heating, is not unique. Depending on strength and 25 aspect ratio of the initial perturbation, the response to the initial perturbation in the western sector can be 26 dominated by inertia-gravity waves. The adjustment in the diabatic moist-convective shallow water model 27 can be totally different , and produces, depending on parameters, either Gill-like response, or eastward-28 moving coherent dipolar structures of the type of equatorial modons, which do not appear in the "dry" 29 adjustment, or vortices traveling, respectively, North-West in the Northern, and South-West, in the Southern 30 hemispheres. 108 to saturation: Q i = Q s − 0.01 with Q s = 0.9, a short relaxation time τ equal to several time-steps 109 of the numerical scheme, γ = 1, and α = O(10 −1 ). 110The main result of experiments on the "dry" adjustment is a discovery of a significant difference 111 between the evolution of pressure anomalies with large and order-one aspect ratios. A comparison 112 of adjustments of circular and elongated negative pressure anomalies with s = 4 and ∆H/H = 113 0.1, as seen in relative vorticity and velocity fields, is presented in Fig. 1. As follows from the 114 left column, the adjustment of a circular pressure anomaly in the eastern sector is similar to the 115 standard Gill-like scenario, cf. [8], and is dominated by a Kelvin wave (KW), with characteristic 116 velocity pattern although a signal corresponding to the eastward-propagating inertia-gravity wave 117 (EIGW) is also visible. However, it is totally different in the western sector, where a typical signal 118 5 FIG. 1. Upper left panel: "Dry" adjustment of a circular pressure anomaly. Relative vorticity (colours), and velocity (arrows): t = 0, 1, 3, 5, 9 [1/β L d ] from top to bottom. Upper right panel: Same as in the left panel, but for a zonally elongated initial anomaly with aspect ratio a = 2.5. Bottom panel: Snapshot of adjustment of the circular anomaly at a later time t = 12 [1/β L d ]. corresponding to the faster westward-propagating inertia-gravity wave (WIGW) with the structure 119 of lowest meridional mode is clearly seen in the velocity field, in front of a slower propagating 120 Rossby wave (RW) signal. This signal is even clearer identified at a later time t = 12 [1/β L d ], as 121 follows from the lowermost panel and also Fig. 2. In comparison, the evolution of the elongated 122 pressure anomaly is closer to the standard scenario with a well-pronounced RW structure, and less 123 pronounced, although still present, WIGW structure (right panels). At large aspect ratios a ≥ 5 124 the standard scenario of adjustment described in [8] is reproduced. The evolution of pr...
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