Westerly wind bursts and their relationship with intraseasonal variations and ENSO. Part I: Statistics

Statistical features of the relationship among westerly wind bursts (WWBs), the El Nino-Southern Oscillation (ENSO), and intraseasonal variations (ISVs) were examined using 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis data (ERA-40) for the period of January 1979-August 2002. WWBs were detected over the Indian Ocean and the Pacific Ocean, but not over the Atlantic Ocean. WWB frequencies for each region were lag correlated with a sea surface temperature anomaly over the Nino-3 region. WWBs tended to occur in sequence, from the western to eastern Pacific, leading the El Nino peak by 9 months to 1 month, respectively, and after around 11 months, over the Indian Ocean. These results suggest that WWB; occurrences are not random, but interactive with ENSO. Composite analysis revealed that most WWBs were associated with slowdowns of eastward-propagating convective regions like the Madden-Julian oscillation (MJO), with the intensified Rossby wave response. However, seasonal and interannual variations in MJO amplitude were not correlated with WWB frequency, while a strong MJO event tended to bear WWBs. It is suggested that the strong MJO amplitude promotes favorable conditions, but it is not the only factor influencing WWB frequency. An environment common to WW`B generation in all regions was the existence of background westerlies around the WWB center near the equator. It is inferred that ENSO prepares a favorable environment for the structural transformation of an MJO, that is, the intensified Rossby wave response, that results in WWB generations. The role of the background wind fields on WWB; generations will be discussed in a companion paper from the perspective of energetics.