Lack of Westerly Wind Bursts in Unmaterialized El Nino Years

摘要

The lack of westerly wind bursts (WWBs) when atmospheric intraseasonal variability (ISV) events occur from boreal spring to autumn is investigated by comparing two types of El Nino years with unmaterialized El Nino (UEN) years. Although high ocean heat content buildup and several ISV events propagating eastward are observed in all three types of years, few WWBs accompany these in the UEN years. The eddy kinetic energy budget analysis based on ISV shows that mean westerly winds in the lower troposphere facilitate the development of eddy disturbances, including WWBs, through convergence and meridional shear of zonal winds. In the UEN years, these westerly winds are retracted westward and do not reach the equatorial central Pacific mainly as a result of interannual components. In addition, positive sea surface temperature anomalies in the western Pacific, which are conducive to active convection, spread widely in a meridional direction centered on 15 degrees N. Both westward-retracted mean westerlies and off-equatorial warming enhance off-equatorial eddies, which result in a reduction in equatorial eddies such as WWBs. The characteristics of the UEN years are significantly different from those observed during the eastern Pacific El Nino (EP-EN) years, which are characterized by anomalous cooling (warming) and suppressed (enhanced) convective eddies in the off-equatorial (equatorial) western Pacific. The central Pacific El Nino years show mixed features during both EP-EN and UEN years. Different background states not only in the equatorial region but also in the off-equatorial region can be a reason for the lack of WWBs in the UEN years.