MJO and Tropical Cyclogenesis in the Gulf of Mexico and Eastern Pacific: Case Study and Idealized Numerical Modeling

Abstract

Abstract The role of the Madden–Julian oscillation (MJO) in modulating the frequency and location of tropical cyclogenesis over the eastern Pacific and the Gulf of Mexico during August–September 1998 is examined. During the nonconvective phase of the MJO, convection and low-level cyclonic vorticity occurred primarily in conjunction with the intertropical convergence zone (ITCZ). During the convective phase, convection, low-level cyclonic vorticity, and convergence expanded into the northeastern Pacific and the Gulf of Mexico. This was accompanied by enhanced eddy kinetic energy and barotropic energy conversions as compared to the nonconvective phase, consistent with previous research. During the nonconvective phase of the MJO, vertical shear was relatively weaker but tropical cyclones tended to form mainly within the ITCZ. On the contrary, during the convective phase, vertical wind shear exceeded 10 m s−1 over much of this region and tropical cyclone development occurred north of the ITCZ, near the Mexican Pacific coast and the Gulf of Mexico. Idealized numerical experiments are conducted using a barotropic model with time-invariant basic states representative of the nonconvective and convective phases of the MJO. The simulations indicate that the propagation paths as well as the amplification of the eddies differ substantially between the two phases. During the nonconvective phase, the waves tend to propagate westward into the eastern Pacific. During the convective phase, stronger southerlies steer the waves into the Gulf of Mexico. The MJO-related modulation of tropical cyclogenesis in the eastern Pacific and Gulf of Mexico thus appears to involve anomalous convergence, cyclonic vorticity, vertical wind shear, and differing tracks of easterly waves.