摘要
A most striking summer–winter difference of evolution of the intraseasonal oscillation(ISO)over the equatorial Indian Ocean is a quasi-stationary oscillation in boreal summer but eastward propagation in boreal winter.This feature is consistent with the observational fact that maximum ISO variance appears only in the eastern Indian Ocean in boreal summer while it appears across the entire basin in boreal winter.The cause of the distinctive propagation and initiation characteristics is investigated through the diagnosis of observational and reanalysis data for the period of 1982–2012.It is found that when the ISO convection appears over eastern Indian Ocean,a positive(negative)moisture tendency appears to the east of the convection in boreal winter(summer).It is the moisture tendency difference that is responsible for different propagation behavior in the summer and winter.A further diagnosis of the moisture budget indicates that the major difference lies in anomalous moisture advection by the mean flow.In addition,air–sea interaction also plays a role.While boreal winter ISO starts over western Indian Ocean,boreal summer ISO is initiated over central–eastern equatorial Indian Ocean,due to boundary layer moistening.The moisture increase is caused primarily by the horizontal advection of mean specific humidity by anomalous easterlies induced by preceding suppressed-phase ISO over eastern Indian Ocean.Besides,a delayed SST feedback also plays a role.The overall difference of ISO evolution between the summer and winter is regulated by the seasonal mean state including the mean SST and water vapor content.
A most striking summer–winter difference of evolution of the intraseasonal oscillation(ISO) over the equatorial Indian Ocean is a quasi-stationary oscillation in boreal summer but eastward propagation in boreal winter. This feature is consistent with the observational fact that maximum ISO variance appears only in the eastern Indian Ocean in boreal summer while it appears across the entire basin in boreal winter. The cause of the distinctive propagation and initiation characteristics is investigated through the diagnosis of observational and reanalysis data for the period of 1982–2012. It is found that when the ISO convection appears over eastern Indian Ocean, a positive(negative) moisture tendency appears to the east of the convection in boreal winter(summer). It is the moisture tendency difference that is responsible for different propagation behavior in the summer and winter. A further diagnosis of the moisture budget indicates that the major difference lies in anomalous moisture advection by the mean flow. In addition, air–sea interaction also plays a role. While boreal winter ISO starts over western Indian Ocean, boreal summer ISO is initiated over central–eastern equatorial Indian Ocean, due to boundary layer moistening. The moisture increase is caused primarily by the horizontal advection of mean specific humidity by anomalous easterlies induced by preceding suppressed-phase ISO over eastern Indian Ocean. Besides, a delayed SST feedback also plays a role. The overall difference of ISO evolution between the summer and winter is regulated by the seasonal mean state including the mean SST and water vapor content.
基金
Supported by the National Key Research and Development Program of China(2018YFC1505804 and 2015CB453200)
National Natural Science Foundation of China(41630423 and 41875069)
US Natural Science Foundation(AGS-16-43297)
US National Oceanic and Atmospheric Administration(NA18OAR4310298)
School of Ocean and Earth Science and Technology(SOEST)contribution number 10740
International Pacific Research Center(IPRC)contribution number 1340
Earth System Modeling Center(ESMC)number 271
