IL NUOVO CIMENTO Novembre-Dicembre 1999 VOL. 22 C, N. 6 High-resolution simulations of freely decaying shallow-water turbulence on a rotating sphere (*)(**) R. IACONO ( 1 )(***), M. V. STRUGLIA ( 1 ), C. RONCHI ( 2 ) and S. NICASTRO ( 1 ) ( 1 ) ENEA - C.R. Casaccia, Gruppo di Dinamica Atmosferica e Oceanica V. Anguillarese 301, 00060 S. Maria di Galeria, Roma, Italy ( 2 ) Via Flaminia 357, 00196 Roma, Italy (ricevuto il 18 Novembre 1998; approvato il 6 Maggio 1999) Summary. — Results of high-resolution, long-time numerical integrations of the unforced shallow-water equations on a rotating sphere are presented. A new accurate and efficient grid point method is used for these simulations, that allows to easily reach very high spatial resolutions (up to an equivalent T680 spectral truncation). It is found that, for small values of the Rossby deformation radius L D , the final quasi-steady states of the free evolution are characterized by the formation of robust westward (retrograde) equatorial jets, whose strengths and widths depend on L D and on the rotation speed. It is also shown that the presence of a westward equatorial jet is related to the global prevalence of anticyclonic vorticity. PACS 92.10.Lq – Turbulence and diffusion. PACS 92.60.Bh – General circulation. PACS 47.27.Eq – Turbulence simulation and modeling. PACS 01.30.Cc – Conference proceedings. 1. – Introduction A large body of literature, steadily growing in the last twenty years, has been devoted to the theoretical and computational investigation of incompressible two-dimensional (2D) turbulence in planar geometry. Many of these works have dealt with the emergence of coherent structures, such as vortices and jets, both in forced and in freely decaying situations. More recently, in the attempt of increasing the relevance to the dynamics of planetary atmospheres, more complex physical models have been considered, and, in few cases, spherical geometry has been used, instead of local projections on tangent planes (the so-called f and b planes). Results of high-resolution numerical simulations of incompressible 2D turbulence on a rotating sphere have been presented in ref. [1], where the attention was focused on the role of rotation in the (*) Paper presented at the International Workshop on “Vortex Dynamics in Geophysical Flows”, Castro Marina (LE), Italy, 22-26 June 1998. (**) The authors of this paper have agreed to not receive the proofs for correction. (***) E-mail: robertoHmantegna.casaccia.enea.it G Società Italiana di Fisica 813