The Madden–Julian Oscillation, Barotropic Dynamics, and North Pacific Tropical Cyclone Formation. Part I: Observations

Low-level barotropic dynamics may help to explain the modulation of eastern and western North Pacific tropical cyclones by the Madden–Julian oscillation (MJO) during Northern Hemisphere summer. The MJO is characterized by alternating periods of westerly and easterly 850-mb zonal wind anomalies across the tropical Pacific Ocean. When MJO 850-mb wind anomalies are westerly, small-scale, slow-moving eddies grow through barotropic eddy kinetic energy (EKE) conversion from the mean flow. These growing eddies, together with strong surface convergence, 850-mb cyclonic shear, and high mean sea surface temperatures, create a favorable environment for tropical cyclone formation. Periods of strong MJO easterlies over the Pacific are characterized by lesser EKE and negligible eddy growth by barotropic conversion. The term 2 ] /]x is a leading contributor to low-level barotropic EKE conversion during MJO westerly 2 u9 u periods across the Pacific, indicating the importance of zonal variations in the westerly jet for producing concentrations of eddy energy. This mechanism can be described as wave accumulation associated with variations of the low-level zonal flow. The conversion term 2 ] /]y contributes a smaller portion of the total conversion u9y9 u over the eastern Pacific, but is of comparable importance to 2 ] /]x during westerly MJO events in the 2 u9 u