Precipitation Change And Its Related Mechanism In Semi-arid Region Of Middle Latitude In Northern Hemisphere

The semi-arid region is one of the regions with the largest contribution to global warming,the fastest expanding,and the most ecologically fragile.As one of the key variables in semi-arid regions,the changes of precipitation has been widely concerned and many research achievements have been obtained.However,due to the unique of the climate in semi-arid regions,precipitation in semi-arid regions is crucial for regional ecological recovery and economic development.We studied the four typical semi-arid regions in the Northern Hemisphere separately,and analyzed the characteristics of precipitation changes in each semi-arid region,explained the dynamic mechanism of precipitation changes in each semi-arid region one by one.The future precipitation changes in some semi-arid regions are predicted by some conclusions derived from our study combined with CMIP6 model,and the main points show that:(1)In the past 30 years,Atlantic Multidecadal Oscillation(AMO)has changed from negative phase to positive phase,which led to a decrease of precipitation in the regions with annual precipitation below 400 mm and an increase of precipitation in the regions with annual precipitation above 400 mm in the semi-arid regions of the Northern Hemisphere.Therefore,AMO can have an important impact on decadal precipitation variability in most semi-arid regions of the Northern Hemisphere.Precipitation in most of the semi-arid regions of the Northern Hemisphere showed the same variability characteristics during two different AMO cycles,1901-1963 and 1964-2016.This indicates that AMO maintains a long-term and stable relationship with precipitation in these parts of the semi-arid regions.Besides,the closer to the North Atlantic Ocean,the faster the response of precipitation and the stronger the relationship between precipitation and AMO.(2)The influence of AMO on precipitation in semi-arid regions is through atmospheric circulation system.In summer,during the positive phases of AMO,the northerly winds from the western part of the North Atlantic cyclonic circulation anomaly meet the southerly winds from the eastern part of the North American continental cyclonic circulation anomaly,causing anomalous low-level divergence and making the precipitation in the semi-arid region of North America decrease.Meanwhile,the decrease of sea level pressure in the North Atlantic extends to the African continent,which will enhance the Saharan thermal low pressure,thus enhancing the North African summer monsoon and causing the precipitation in the semi-arid region of North Africa to increase.In contrast,a roughly opposite flow field occurs during the negative phases of the AMO,leading to opposite changes in precipitation in the semi-arid regions of North America and North Africa.It is predicted by the E3SM-1-1 model that the AMO will be in negative phase in the next decade,and the precipitation in the southern part of semi-arid region in North America will increase by more than 20 mm,and the precipitation in the whole semi-arid region of North Africa will decrease by more than20 mm.(3)Precipitation in Central Asia is mainly concentrated in the cold season,and the dynamically induced precipitation in the cold season is extracted by the dynamical adjustment methodology in this study.From 1950 to 2015,the dynamically induced precipitation in the region north of 45°N experienced anomalous changes of positivenegative-positive-positive-negative-negative patterns on the decadal scale;the dynamically induced precipitation in the region south of 45°N experienced anomalous changes of positive-negative-positive-negative-positive patterns on the decadal scale.the dynamically induced precipitation changes in the north and south of 45°N were not synchronized and their respective cycles were not fixed,with the cycles varying around10-20 years.It revealed that precipitation in the region north of 45°N is dominated by westerly winds only,and in the region south of 45°N,the variation of westerly winds and precipitation at high altitudes above 700 h Pa is basically in the same steps,while the westerly winds below 700 h Pa are blocked by the topography,which causes the difference of precipitation variation between the two regions.(4)Annual precipitation in the semi-arid region of East Asia is mainly dominated by summer precipitation,which accounts for 63% of annual precipitation and is highly correlated with annual precipitation with a correlation coefficient as high as 0.874.Based on summer precipitation in the semi-arid region of East Asia,the past 70 years were divided into wet years and dry years.Our study finds that,in wet years,there was a negative geopotential height anomaly of 850 h Pa at the Mongolian low,which would enhance the Mongolian low and the westerly wind.At the same time,the negative geopotential height anomaly over the land combined with the positive geopotential height would enhance the summer monsoon.The simultaneous enhancement of monsoon and westerly wind will lead to the increase of precipitation.In dry years,the monsoon and westerly winds weakened simultaneously.Also,the monsoon edge extended westward into the interior of semi-arid region in wet years,and in dry years,the monsoon edge retreated eastward.When the monsoon edge extended deep into the semi-arid region of East Asia,El Nino-Southern Oscillation(ENSO)would have a negative effect on precipitation.In the future,the monsoon edge will expand westward,and the higher the emission scenario,the more obvious the expansion will be.It will lead to the enhanced regulation of the climate in semi-arid region of East Asia by ENSO,the larger area affected by the monsoon,wetter soil land,and more vegetation.