Study On The Causes And Physical Processes Of Multi-timescale Variation Of Atmospheric Precipitable Water In Summer In Bengal Region

Located midway between the Tibetan Plateau and the Bay of Bengal and the Indian Peninsula and the Indo-China Peninsula,the Bengal region is influenced by both the Indian Monsoon and the Western Pacific Monsoon,and is the first region to be affected by the outbreak of the Asian monsoon.The changes of water vapor in the Bengal region are important indicators of climate in southern Asia as well as in East Asia.In this study,the causes of intra-seasonal,seasonal,inter-annual and inter-decadal multi-temporal scale variation of atmospheric precipitable water(APW)in summer(June?September)in Bengal region and its possible physical processes were analyzed by using data such as ERA5 reanalysis data and the monsoon intensity index for 1979?2020.The main conclusions were obtained as follows:(1)Seasonal,interannual and interdecadal variability characteristics of APW in Bengal region and its accompanying contribution to water vapor balance.The summer APW in Bengal region is the largest area at the same latitude,and the summer APW accounts for half of the year.The average summer APW shows not only a significant interannual variation but also a significant interdecadal increasing trend.According to the whole layer water vapor transport and vertical profile of the four borders in Bengal region,the trend of whole layer water vapor transport of the eastern and southern borders is favorable to the increase of APW in the region,and the trend of whole layer water vapor transport of the western and northern border is unfavorable to the increase of APW.The total whole layer water vapor transport of the four borders in Bengal region shows an increasing trend,which is favorable to the increase of APW in the region.(2)Linkage between the Interdecadal Pacific Oscillation and the summer APW in the Bengal region on interannual and interdecadal scales and their physical processes.The summer APW in Bengal region shows a significant negative correlation with the Interdecadal Pacific Oscillation(IPO)in both interannual and interdecadal scales.When the IPO is in its positive phase,in the lower troposphere,the westerly(easterly)wind anomaly prevails in the equatorial Pacific(equatorial Indian Ocean),and the opposite in the upper troposphere,indicating a weakening of the Walker circulation in the Indian and Pacific Oceans.The Gill-type anticyclonic circulation anomaly responded Rossby waves is observed in the lower troposphere in the north and south sides of the equatorial Indian Ocean,and a northwest wind anomaly prevails from the Arabian Peninsula to Bengal.The strong westerly airflow is not conducive to the transport of water vapor from the Bay of Bengal and the western Pacific Ocean to the Bengal region,while the sinking airflow accompanying the anticyclonic circulation is not conducive to the convergence of water vapor in the region,resulting in the reduction of APW in Bengal region.On the contrary,when the IPO is in a negative phase,it is favorable for the increase of APW in Bengal region in summer.(3)Evolutionary characteristics of summer APW and contribution of whole layer water vapor balance in Bengal during 10–30 day and 30–60 day.The summer APW in Bengal region shows not only obvious interannual and interdecadal variations,but also significant 10–30 and 30–60 day intra-seasonal cycle oscillations.In the eight phases of the 10–30 and 30–60 day oscillations,when the APW anomaly negative is in Bengal region,the APW anomaly positive is in northwest India,showing a latitudinal dipole mode with a continuous eastward shift,weakening,and alternating positive and negative processes.Among the eight phases of these two cycles,the most significant contribution to the water vapor income in the Bengal region is the4 th phase.In the 4th phase of the 10–30 day scale,the west Pacific summer monsoon is weak and the Indian monsoon has no obvious trend,which is unfavorable to the water vapor transport from the Bay of Bengal to the Bengal region;in the 4th phase of the30–60 day scale,the Indian summer monsoon is weak and the west Pacific monsoon has no obvious trend,which is favorable to the water vapor transport from the southern border into the Bengal region,favoring the water vapor transport from Bay of Bengal to Bengal region,and favoring the increase of APW in Bengal region.(4)The evolutionary characteristics of the circulation accompanying the 10–30 and 30–60 day summer APW in Bengal region and its physical processes.In the 3rd phase of 10–30 day and 30–60 day,the wind at 850 h Pa shows a weak westerly trough anomaly in the Arabian Sea-India-Bengal region,which shows a weak Indian monsoon,and an anomalous anticyclonic circulation in the southern Bay of Bengal-Indo-China Peninsula-South China Sea,which shows a weak western Pacific monsoon.Meanwhile,OLR in the southwestern Bay of Bengal is positive anomaly with weak convective activity,while the negative anomaly is in the Bengal region with strong convective activity.The meridional circulation,the anomalous upwelling in Bengal region and sinking in southwestern Bay of Bengal,indicates a weakening Hadley circulation.There is anomalous updraft over Bengal,there is also a sinking airflow in northwestern India in latitudinal direction.The Bengal region is located in the updraft branch,which is favorable for the convergence of water vapor,thus the APW is large,while there is sinking branch over the northwest Indian,which is not favorable for the convergence of water vapor,thus the APW is small.In the 7th phase of the 10–30 day and 30–60 day APW in Bengal region,the situation is opposite to the3 rd phase.