Climatic Characteristics Of East Asian Tropical Monsoon Depressions

Based on a tracking dataset of global monsoon low-pressure systems and the ERA-Interim reanalysis dataset at six-hour interval and 0.7°×0.7° horizontal resolution,the global climatology of monsoon low-pressure systems is presented here,including spatial distribution,lifetime as well as structural characteristics.We focused on the climatic characteristics of East Asian tropical monsoon depressions(EAMDs),and further compared them with Indian monsoon depressions(IMDs)and northwest Pacific tropical cyclones,respectively.A study of 379 monsoon lows,313 monsoon depressions and 261 deep depressions occurring over East Asian tropical region(100°–160°E,0–20°N)during May–October in the time period 1979–2012 showed that the average lifetime of monsoon lows are about 7 days,which is the weakest low-pressure system among three intensity categories.Monsoon lows meanly occurred over Indochina Peninsula and northern South China Sea(SCS),while others low-pressure system hardly occurred on the land.Most monsoon depressions occurred over the SCS and western tropical Pacific Ocean with the average lifetime of 10 days.Deep depressions meanly occurred over western tropical Pacific Ocean with the average lifetime of 14 days.The warm-over-cold thermal structure is a distinctive characteristic of monsoon lows and monsoon depressions,and the cold core restrict the development of them.While deep depressions are warm throughout all the troposphere without cold core,which contributes to their development.Then,the research focused on 260 EAMDs which are all generated in the monsoon trough.Most EAMDs formed over the northern SCS and near the Philippine Islands in July–October and have an average lifetime of 10 days.The vertical structures of EAMDs are usually upright or tilt slightly westward with height.The typical horizontal scale of EAMDs is about 1000 km,and the warm-over-cold thermal structure is distinctly.We divided the EAMDs into four groups: eastward-moving,westward-moving,turning and northwestward-moving EAMDs.Most of the eastward-moving EAMDs form over the SCS in May and June,whereas the westward-moving EAMDs form over both the SCS and the western Pacific Ocean in July–October.The turning and northwestward-moving EAMDs are mainly generated over the western Pacific Ocean and have longer lifetimes.The structures of the eastward-moving and turning EAMDs show common characteristics in each stage.Their vertical structures change from upright in the developing and peak stages to northeast tilting with height in the attenuating stage,especially for the specific humidity.By contrast,the structures of westward-and northwestward-moving EAMDs show little change during their lifetime.They are symmetrical relative to the vertical axis of the EAMDs over their whole lifetime and only vary in strength.In addition,the surface sea temperature anomaly of turning EAMDs displays a significant El Ni?o pattern,while the surface sea temperature anomaly of westward-and northwestward shows a slight La Ni?a pattern.Finally,we compared EAMDs with IMDs and northwest Pacific tropical cyclones,respectively.The monthly frequency of EAMDs genesis displayed double-peak mode,corresponding to South China precipitation,and the two peaks in May and August.However,the monthly frequency of IMDs genesis displayed a single-peak mode.They meanly generated over the North Bay of Bengal in June–July and have an average lifetime of 9 days.The typical horizontal scale of IMDs is about 1500 km,and their structure tilting toward south and west with height.So compared with IMDs,EAMDs are more compact and symmetrical and are almost upright in troposphere.Both the EAMDs and the IMDs have a cold core near the surface,but the cold layer of the EAMDs is much thinner than the cold layer of the IMDs,and the cold core of IMDs are located in the northeast of center.Northwest Pacific tropical cyclones only have a warm core throughout almost all the troposphere,and this thermal structure contributes to their development.Besides,the environmental vertical wind shear of tropical cyclones is about 9 m/s,which is weaker than EAMDs with the average environmental vertical wind shear is about 13 m/s.Therefore,the strong environmental vertical wind shear of EAMDs prevents the development of their warm core,and make the system couldn't further develop.