Based on NCEP/NCAR and ERA-40 reanalysis data,interannual and interdecadal variations of the Mascarene high are analyzed,with specific focus on exchanges in the factors affecting the interannual variability of the Mascarene high against different interdecadal backgrounds.In order to investigate the intensity of the Mascarene high on different timescales,a Mascarene high area index(MHI) is defined to represent the intensity of the Mascarene high.Results show that significant changes of the Mascarene high are apparent on different timescales.On the seasonal timescale,the intensity of the Mascarene high is largest in summer,with its position in the west;whereas,the intensity is weakest in winter,with its position in the east.In the long-term trend,the intensity of the Mascarene high in the four seasons presents a significant linear increasing trend,with its trend strongest in spring and weakest in winter.On the interdecadal timescale,the four seasons experienced a marked interdecadal shift around 1976,characterized by a weaker Mascarene high before 1976 and a significantly strengthened one after 1976.The latest research results,both here and abroad,show that there are three possible factors affecting the Mascarene high:the local sea surface temperature(SST) over the southern Indian Ocean,the Antarctic Oscillation(AAO),and the El Niño-Southern Oscillation(ENSO).On the basis of these achievements,we put out two questions:What are the main factors affecting the Mascarene High? And has the relationship between the Mascarene high and its affecting factors changed since 1976? To answer these questions,regression analysis and correlation analysis are adopted.The results show that the factors affecting the interannual variability of the Mascarene high are different during these two periods.The interannual variability of the Mascarene high is closely related with SST over the southern Indian Ocean and the AAO before 1976,whereas it is closely related with ENSO and the AAO after 1976.In the latter period,the correlation of local SST over the southern Indian Ocean with the Mascarene high is weakened because of the weakening of upward net surface heat flux,while that of ENSO is remarkably strengthened,which is possibly because of the strengthening of ENSO's amplitude after 1976.With the warming of the eastern Pacific Ocean SST,anomalous upward movement in the South Pacific and anomalous downward movement in the Indian Ocean are triggered,thus promoting the strengthening of the Mascarene high.In every month of winter before 1976,local SST over the mid-and low-latitude Indian Ocean is closely related with the Mascarene high from June to August,with the strongest forcing in July.There is no significant relationship between ENSO and the Mascarene high from June to August.After 1976,local SST over the low-latitude Indian Ocean shows no closely relationship with the Mascarene high from June to August,while the relationship between ENSO and the Mascarene high is gradually strengthened from June to August.The relationship between the AAO and the Mascarene high is strongest in June and weakest in July,both before and after 1976.Besides,compared with that before 1976,the relationship between the AAO and the Mascarene high is significantly strengthened in every month in winter.The strengthening of the AAO triggers the anomalous easterly wind along 60°S and the anomalous westerly wind along 20°S,and then promotes the intensity of the Mascarene high.