Analysis On Evolution And Causes Of Aridity-Wetness In Northwest China

Applying the data of monthly temperature, precipitation, water surface evaporation in Northwest China(Shanxi, Gansu, Ningxia, Qinghai, Xinjiang and west Inner Mongolia) in 1960～2003 and NCEP/NCAR monthly mean reanalyzing height fields, wind fields, surface pressure fields, humidty fields, aridity-wetness index is defined by precipitation and water surface evaporation, then characteristics which the climate has transited from arid to wet in Northwest China and possible influence mechanism were studied. The main conclusions as follows:(1) In recent 44 years the annualy precipitation has increased in westerly zone climate region and plateau climate region of Northwest China, but it has decreased in monsoon climate region. Water surface evaporation showed marked decreased trend, and this was caused by sunlight, average wind speed, average minimum temperature, average daily change range.(2) Characterstic of aridity and wetness were studied by aridity-wetness index of precipitation and water surface evaporation, it was found that climate showed prominent change from arid to wet in westerly zone climate region and plateau climate region, but climate displayed change from wet to arid in monsoon zone climate region. In addition, when Arctic Oscillation positive(negative) phase is stronger, the East summer Monsoon is generally weaker(stronger), the precipitation of southeast of Northwest China is decreases(increases), and the climate change from wetness(aridity) to aridity(wetness), the precipitation is increases(decreases) in westerly zone climate region and plateau climate region, and the climate change from aridity(wetness) to wetness(aridity).(3) There were clear indication sense that spring SSTA of the Indian Ocean forecast to anaphase summer characterstic of aridity and wetness of north of Northwest China, and key region locate on equator Indian Ocean. If spring SST occurred cold in the Indian Ocean, there was a lack of anomaly warm and wet airflow from the Indian Ocean to Northwest China, climate prone to dry because of lack of water vapor transportation in Northwest China. If spring SST occurred warm in the Indian Ocean, there was clear water vapor transportation zone from Bengal bay to Northwest China, climate prone to wet because of lack of water vapor transportation in Northwest China. In addition, if spring SST occurred cold in the Indian Ocean, South Asia High Pressure tend to weak from spring to summer, and that South Asia High Pressure showed single apices type in anaphase summer. If spring SST occurred warm in the Indian Ocean, South Asia High Pressure prone to strong from spring to summer, and that South Asia High Pressure showed twin apices type in anaphase summer. This is likelihood mechanism of the Indian Ocean SST influence on summer aridity and wetness in Northwest China.(4) There were remarkable relation between prophase winter SSTA of the Pacfic and anaphase summer characterstic of aridity and wetness of east of Northwest China, and key region locate on equator middle-east Pacific Ocean. In anomaly warm winter years of equator middle-east Pacific ocean, the area shows anomaly updraft at corresponding phase and anaphase spring, this cause anomaly strong Hadley cell in middle-east pacific, and compel 500hpa height field to show PNA teleconnection, it displays WP teleconnection at anaphase summer, thus intensity of West Pacific Ocean Subtropical High Pressure boost up, its area augment and west boundary prone to west from winter to summer, North China is effected by warm and wet airflow of Pacific Ocean Subtropical High Pressure border slope, and climate tend to wet. Vice versa. This is likelihood mechanism of equator middle-east Pacific Ocean SST influence on summer drought and wetness in Northwest China.