MJO modulation of surface variables over South America in the CFS reanalysis

This study examines the responses in temperature and precipitation over South America due to variations in the Madden-Julian Oscillation (MJO). The data analyzed was acquired from the Climate Forecast System Reanalysis (CFSR), covering every day from January 1979 to June 26, 2010 for precipitation data, and through December 2010 for temperature data. To understand how the MJO affects the characteristics of daily precipitation, changes in the frequency and intensity of precipitation events were examined. Global composites were created for each of the eight MJO phases as the oscillation progresses eastward across the planet. Focusing on South America, anomalies in temperature and precipitation are studied for two seasons, May-September and November-March. The use of two extended seasons was decided in order to highlight responses that may only occur during wet or dry seasons and still include enough active MJO events going into the composites to get statistically significant results.

Over South America in both seasons the results tend to blend into two different states each made up of three consecutive phases of the MJO with one transition phase in between. In other words, phases 8, 1, and 2 tend to show similar patterns in temperature and precipitation responses while phases 4, 5, and 6 show similar patterns, but opposite to those of phases 8, 1, and 2. Finally, phases 3 and 7 appear to be transition phases, showing intermediate results with weaker signals over. The same transition patterns are seen to an extent in both seasons and for both variables. Additionally, the data for November-March corresponds well to a strong response in the South American summer monsoon. Potential causes of these anomalies are also speculated by looking at deviations in zonal wind, velocity potential, and stream function for each of the eight phases.