Alternation of the Atmospheric Teleconnections Associated with the Northeast China Spring Rainfall during a Recent 60-Year Period

Northeast China(NEC)is China’s national grain production base,and the local precipitation is vital for agriculture during the springtime.Therefore,understanding the dynamic origins of the NEC spring rainfall(NECSR)variability is of socioeconomic importance.This study reveals an interdecadal change in the atmospheric teleconnections associated with the NECSR during a recent 60-year period(1961-2020).Before the mid-1980s,NECSR had been related to a Rossby wave train that is coupled with extratropical North Atlantic sea surface temperature(SST),whereas,since the mid-1980s,NECSR has been linked to a quite different Rossby wave train that is coupled with tropical North Atlantic SST.Both Rossby wave trains could lead to enhanced NECSR through anomalous cyclones over East Asia.The weakening of the westerly jet over North America is found to be mainly responsible for the alternation of the atmospheric teleconnections associated with NECSR during two epochs.

A STATISTICAL PREDICTION MODEL FOR SPRING RAINFALL OVER NORTHERN CHINA BASED ON THE INTERANNUAL INCREMENT APPROACH

The spring (March-April-May) rainfall over northern China (SPRNC) is predicted by using the interannual increment approach. DY denotes the difference between the current year and previous years. The seasonal forecast model for the DY of SPRNC is constructed based on the data that are taken from the 1965-2002 period (38 years), in which six predictors are available no later than the current month of February. This is favorable so that the seasonal forecasts can be made one month ahead. Then, SPRNC and the percentage anomaly of SPRNC are obtained by the predicted DY of SPRNC. The model performs well in the prediction of the inter-annual variation of the DY of SPRNC during 1965-2002, with a correlation coefficient between the predicted and observed DY of SPRNC of 0.87. This accounts for 76% of the total variance, with a low value for the average root mean square error (RMSE) of 20%. Both the results of the hindcast for the period of 2003-2010 (eight years) and the cross-validation test for the period of 1965-2009 (45 years) illustrate the good prediction capability of the model, with a small mean relative error of 10%, an RMSE of 17% and a high rate of coherence of 87.5% for the hindcasts of the percentage anomaly of SPRNC.

Importance of orographic gravity waves over the Tibetan Plateau on the spring rainfall in East Asia

The springtime persistent rainfall(SPR)is the major rainy period before the onset of summer monsoon in East Asia,which profoundly affects the regional and even global hydrological cycle.Despite the great importance of the mechanical and thermal effects of the Tibetan Plateau(TP)large-scale orography on the formation of SPR,the impact of small-scale orography over the TP remains poorly understood.Here we show that upward-propagating orographic gravity waves(OGWs),which occur as the subtropical westerlies interact with the TP's small-scale orography,contribute importantly to the SPR.The breaking of OGWs induces a large zonal wave drag in the middle troposphere,which drives a meridional circulation across the TP.The rising branch of the meridional circulation acts to lower the pressure and increase the meridional pressure gradient to the south of the TP by dynamically pumping the lower-tropospheric air upwards.The southwesterly monsoonal flow on the southeastern flank of the TP thus intensifies and transports more water vapor to East Asia,resulting in an enhancement of the SPR.This finding helps more completely understand the impacts of TP's multiscale orography on the SPR and provides a new perspective on the westerly-monsoon synergy in East Asia.

Evaluation of Spring Persistent Rainfall over East Asia in CMIP3/CMIP5 AGCM Simulations

The progress made fi＇om Phase 3 to Phase 5 of the Coupled Model Intercomparison Project （CMIP3 to CMIP5） in simulating spring persistent rainfall （SPR） over East Asia was examined from the outputs of nine atmospheric general circulation models （AGCMs）. The majority of the models overestimated the precipitation over the SPR domain, with the mean latitude of the SPR belt shifting to the north. The overestimation was about 1mm d-1 in the CMIP3 ensemble, and the northward displacement was about 3°, while in the CMIP5 ensemble the overestimation was suppressed to 0.7 mm d-i and the northward shift decreased to 2.5°. The SPR features a northeast-southwest extended rain belt with a slope of 0.4°N/°E. The CMIP5 ensemble yielded a smaller slope （0.2°N/°E）, whereas the CMIP3 ensemble featured an unre- alistic zonally-distributed slope. The CMIP5 models also showed better skill in simulating the interannual variability of SPR. Previous studies have suggested that the zonal land-sea thermal contrast and sensible heat flux over the southeastern Tibetan Plateau are important for the existence of SPR. These two ther- mal factors were captured well in the CMIP5 ensemble, but underestimated in the CMIP3 ensemble. The variability of zonal land-sea thermal contrast is positively correlated with the rainfall amount over the main SPR center, but it was found that an overestimated thermal contrast between East Asia and South China Sea is a common problem in most of the CMIP3 and CMIP5 models. Simulation of the meridional thermal contrast is therefore important for the future improvement of current AGCMs.

THE VARIATION OF THE SPRING PRECIPITATION IN GUANGZHOU AND ITS PRECURSORY SIGNALS

Guangzhou spring rainfall mainly exhibits interannual variation of Quasi-biannual and interdecadal variation of 30 yrs, and is in the period of weak rainfall at interdecadal time scale. SST anomalies （SSTA） of Nino3 are the strongest precursor of Guangzhou spring rainfall. They have significant positive correlation from previous November and persist stably to April. Nino3 SSTA in the previous winter affects Guangzhou spring rainfall through North Pacific subtropical high and low wind in spring. When Nino3 SSTA is positive in the previous winter, sprirg subtropical high is intense and westward, South China is located in the area of ascending airflow at the edge of the subtropical high, and water vapor transporting to South China is intensified by anticyclone circulation to the east of the Philippines. So Guangzhou spring rainfall is heavy. When Nino3 SSTA is negative, the subtropical high is weak and eastward, South China is far away from the subtropical high and is located in the area of descending airflow, and water vapor transportirg to South China is weak because low-level cyclonic circulation controls areas to the east of the Philippines and north wind prevails in South China. So Guangzhou spring rainfall is weak ard spring drought is resulted.

Application of Mulching Materials of Rainfall Harvesting System for Improving Soil Water and Corn Growth in Northwest of China

The ridge and furrow rainfall harvesting（RFRH） system is used for dryland crop production in northwest of China.To determine the effects of RFRH using different mulching materials on corn growth and water use efficiency（WUE）,a field experiment was conducted during 2008-2010 at the Heyang Dryland Experimental Station,China.Four treatments were used in the study.Furrows received uncovered mulching in all RFRH treatments whereas ridges were mulched with plastic film（PF）,biodegradable film（BF） or liquid film（LF）.A conventional flat field without mulching was used as the control（CK）.The results indicated that the average soil water storage at depths of 0-200 cm were 8.2 and 7.3%,respectively higher with PF and BF than with CK.However,LF improved soil water storage during the early growth stage of the crop.Compared with CK,the corn yields with PF and BF were increased by 20.4 and 19.4%,respectively,and WUE with each treatment increased by 23.3 and 21.7%,respectively.There were no significant differences in corn yield or WUE with the PF and BF treatments.The net income was the highest with PF,followed by BF,and the 3-yr average net incomes with these treatments were increased by 2 559 and 2 430 CNY ha-1,respectively,compared with CK.BF and PF had similar effects in enhancing the soil water content,crop yield and net income.Therefore,it can be concluded that biodegradable film may be a sustainable ecological alternative to plastic film for use in the RFRH system in northwest of China.

Winter Asia Jetstream and Seasonal Precipitation in East China

The monthly mean geostrophic wind fields for January during 1951 - 1990 period are calculated by using data of500 hpa monthly mean height. The relation between Asia jetstream in winter and the important seasonal precipitationin East China is analysed. The analysis shows that the south branch of jetstream is stronger (weaker) in winter, therainfall will be more (less) than normal in the subsequent spring in South China, and summer rainfall in North Chinawill be more (less). too; these important rainy seasons are related to each other; the indian summer monsoon is notonly related to the summer rainfall in North China, but also related to the spring rainfall in South China and thesouth branch of jetstream in winter.

Inter-decadal variations of springtime rainfall over southern China mainland for 1979-2004 and its relationship with Eurasian snow

This study analyzes the inter-decadal variations of rainfall over southern China in spring (March-April-May) using the observed precipitation data for 1979-2004. The result shows that the variations of spring rainfall over southeastern China are opposite to those over and southwestern China in both inter-annual and inter-decadal time scales. The precipitation over south- ern China exhibits an apparent inter-decadal shift in the late 1980s. The accumulated spring rainfall has reduced 30% over southeastern China after the late 1980s, whereas it has increased twice as much over southwestern China. The atmospheric circulations related to this shift show that an abnormal high at lower and middle troposphere appears over Asian middle and high latitudes, accompanied by stronger-than-normal northerly wind over eastern China. Consequently, the wet air flows from tropical oceans are weakened over southern China, resulting in less rainfall over southeastern China and more rainfall over southwestern China. Furthermore, the anomalous atmospheric circulation over Asian middle and high latitudes is closely related to the inter-decadal downward shift of Eurasian spring snow in the late 1980s, indicating that the inter-decadal shift of Eurasian spring snow in the late 1980s is probably an important factor in the decadal shift of spring rainfall over southern China.