The summertime (Jul-Sep) activity of the Madden Julian Oscillation (MJO) has a local maximum in the tropical eastern Pacific Ocean, just off the west coast of southern Mexico. The influence of this MJO activity is examined in daily precipitation data from 7194 stations across Central and North America for a 20 year period. The related predictive potential is examined out to 21-day lead times in the station data. The dynamics of the MJO influence are examined with respect to changes in moisture transport, thermodynamic balance, and parcel buoyancy.

An index of MJO activity is constructed by averaging the Wheeler et al. estimate of the MJO over (120W-100W,10-15N). The index varies smoothly in time and the phase is estimated from the sign and slope of the index. Compositing the daily station data based on the phase of the eastern Pacific MJO divided into four parts shows a notable influence throughout Central and North America. The largest percentage changes in precipitation are along southwestern Mexico / northwestern Central America as well as along the west coast of the U.S. In both regions, precipitation during one of the quarter-cycles of the MJO differs from the long-term mean by 45% or more. This large difference is realized at individual stations. Precipitation is also modulated in other regions such as the monsoon region of Mexico and the gulf coast of the U.S. As a initial assessment of predictability, a very simple scheme is considered: the phase of the MJO is predicted by assuming it advances at its average rate, and this predicted phase is then used to anticipate whether precipitation at a given lead time will be suppressed or enhanced. Two versions of the MJO estimate are considered: a diagnostic one that is more accurate but not available in real-time, and a real-time estimate. With the simple scheme applied to the diagnostic estimate for a subset of stations in the focus regions previously mentioned, useful predictive information exists out to 21 days; using the real-time estimate, there is predictability out to at least 14 days. More sophisticated MJO forecast products are also considered.

Daily data from the NCEP/NCAR reanalysis are used to examine the changes in moisture transport, the terms of the thermodynamic equation, and Convective Available Potential Energy (CAPE). Central America and southwestern Mexico are directly affected by the convective core of the MJO, while vertical motion over the western U.S. is strongly modulated by changes in the thermodynamic balance due to the interaction of the extratropical MJO winds with the mean flow, while changes in moisture transport appear to be important in the gulf coast region. MJO influence with respect to mean circulation features, especially the subtropical anticyclones, is examined.