Effects Of Multi-scale Climate Modes On The Diurnal Variation Of Oceanic Continental Precipitation

The rainfall variability in the Maritime Continent(MC)exhibits a wide range of temporal scales from diurnal to interdecadal.The diurnal cycle is one of the most important components of natural climate variability in the MC region.Understanding the multi-scale interactions associated with the diurnal cycle of precipitation over the MC,especially the influence of multiscale climate modes on the precipitation diurnal cycle over the MC,is of great significance for predicting the occurrence of high-impact events such as flood and drought in this region and improving the simulation of regional and global climate models.Therefore,the influence of the climatological annual cycle,El Ni(?)o-Southern Oscillation(ENSO)and the Madden-Julian Oscillation(MJO)on the precipitation diurnal cycle over the MC was studied for the period of1998-2016 through observational analyses using Tropical Rainfall Measuring Mission(TRMM)rainfall and Modern-Era Retrospective Analysis for Research and Applications(MERRA)reanalysis data and high-resolution simulations using the Weather Research and Forecasting(WRF)model.The main conclusions are as follows:(1)The observational analyses reveal that there are statistically significant differences in the diurnal cycle amplitude between boreal winter and summer.The amplitude of precipitation diurnal cycle reduces by about 35% during boreal summer compared to boreal winter,especially over the MC major islands and adjacent oceans.A precipitation budget analysis indicates that the diurnal cycle amplitude difference is primarily attributed to vertically integrated convergence of the seasonal mean moisture by diurnal winds.The relative roles of the background dynamic and thermodynamic states in causing the enhanced diurnal wind activity in boreal winter are further investigated through idealized WRF simulations.The results show that the seasonal mean background moisture condition is most critical in inducing the winter-summer difference of the precipitation diurnal cycle over the MC,followed by atmospheric static stability(i.e.,vertical temperature gradient)and circulation conditions.(2)There are statistically significant differences in the diurnal cycle amplitude between El Ni(?)o and La Ni(?)a mature winters,with a non-uniform spatial pattern over the MC.The amplitude of precipitation diurnal cycle is larger over the western MC but smaller over the eastern MC during El Ni(?)o winter,and vice versa for La Ni(?)a winter.A diagnosis of the moisture budget shows that the difference in the diurnal cycle amplitude is primarily attributed to the vertically integrated convergence of the interannual background moisture by diurnal winds.A further analysis indicates that the ENSO-related background moisture plays a key role in causing the difference in diurnal wind activity between El Ni(?)o and La Ni(?)a winters,which leads to the difference in the precipitation diurnal cycle through different positive circulation–convection–moisture feedback processes on the diurnal time scale.(3)There are statistically significant differences in the diurnal cycle amplitude between different MJO phases during boreal winter.The amplitude of precipitation diurnal cycle is largest(smallest)during phases 2-3(6-7),especially for the diurnal cycle over the western MC.Analysis of the local precipitation budget indicates that vertically integrated horizontal moisture convergence plays an important role in causing the amplitude difference.A further diagnosis indicates that the difference is primarily attributed to the convergence of the MJO-scale moisture by diurnal wind.The enhanced diurnal wind activity during phases 2-3 results from enhanced condensational heating on the diurnal scale,owing to the increase of surface moisture caused by MJO induced boundary layer convergence in front of convection.The increase of the background low-level moisture associated with MJO further promotes a positive circulation–convection–moisture feedback to enhance the diurnal cycle activity.Opposite processes operate at phases 6-7.(4)To compare the relative role for the influence of the climatological annual cycle,ENSO and the MJO on the precipitation diurnal cycle over the MC.It is found that the MJO acts as a key role,followed by annual cycle and ENSO.The ratios of contribution are 56%,30%,and14%,respectively.As the most prominent intraseasonal climate variation in the tropics,the MJO passes through the MC during its eastward propagation.The influence of the MJO on the precipitation diurnal cycle over the MC presents a regional uniform characteristic,which impacts most directly and significantly.As a dominant mode of interannual variability in the tropical Pacific,ENSO affects the precipitation diurnal cycle over the MC by influencing the ascending branch of the Walker circulation.The influence of ENSO on the diurnal cycle of precipitation is non-uniform over the eastern and western MC,which impacts most weakly.