季风区降水δ18O与云顶气压关系

THE RELATIONSHIP BETWEEN δ18O IN PRECIPITATION AND CLOUD TOP PRESSURE IN MONSOON REGIONS

不同于高纬地区,中低纬度季风区降水稳定同位素变化较复杂,降水稳定同位素＂温度效应＂不明显,而出现显著的＂降水量效应＂.而通过地表的数据分析很难解释影响季风区降水稳定同位素变化的过程与机制,这使得对于季风区稳定同位素气候记录的解释存在不同认识.本文利用GNIP降水稳定同位素数据和ISCCP D2云气候资料,通过分析降水δ18O与云顶气压的关系,试图从降水过程中的水汽传输来解释季风区降水稳定同位素的变化特征.研究发现季风区降水δ18O与云顶气压在局地和区域尺度上均有显著正相关关系,夏季风爆发时云顶气压和降水δ18O都显著降低,表明云抬升高度对降水δ18O的重要影响.这种关系表明季风区降水δ18O除受局地过程的影响外,还受大尺度对流活动的影响.本研究从不同角度证明了对流降水是季风区降水稳定同位素出现＂降水量效应＂的重要因素.季风活动越强,云顶高度越高,凝结温度越低,从而导致降水中δ18O越低,是出现同位素＂降水量效应＂的机制.

Besides temperature effect, precipitation isotope in low latitude or monsoon regions has an inverse relationship with precipitation amount, the so called ＂amount effect＂. While the ＂amount effect＂ is based on the empirical relationship, and the underlying mechanism is still not fully addressed. Moreover, this relationship does not exist in all monsoon regions, and may change through time. However, it is difficult to explain the processes and mechanism controlling the isotopic variation in monsoon precipitation by using only the ground based observation data. These uncertainties hinder the explanation of isotope signal in paleoclimate studies. To investigate the mechanism controlling the precipitation isotope variability in monsoon regions, we analyzed the relationship between precipitation δ18O and cloud top pressure （CTP） by using monthly precipitation isotope data obtained from GNIP and monthly cloud data obtained from ISCCP D2 dataset. The criteria for selecting the GNIP stations is that the number of monthly δ18O records during 1984-2009 should be over 80. CTP in grid box covered each station is used to represent local CTP. Our analysis work shows that there exists a significant positive correlation between δ18O and local CTP in low latitudes, especially in monsoon regions. While in middle and high latitudes, δ18O and local CTP do not have significant correlation or have a negative correlation. Then, we choose Hong Kong to represent stations in East Asian monsoon region and Bangkok to represent stations in India monsoon region. The spatial distribution of correlation coefficients between monthly δ18O and CTP show a good correlation between δ18O and CTP in a regional scale for both stations. The zones with strong positive correlation between δ18O in Hong Kong precipitation and CTP span from Bay of Bengal in the west to central North Pacific in the east, while the strong positive correlation between δ18O in Bangkok precipitation and CTP is restricted in a smaller region spanning from east Bay of Bengal to west Pacific. These results indicate that cloud top height is closely linked with the isotopic composition of precipitation in monsoon regions. The mechanism involved in these phenomenon is that the more intense the monsoon convective activity, the higher the cloud top height and the lower the condensation temperature which controls the precipitation δ18O, and consequently, the more depleted the heavy isotope in precipitation. In these processes, the temperature effect is also involved, but with the air temperature in the cloud top. While in monsoon regions, heavier precipitation in convection is generally associated with higher cloud top, e.g., the precipitation amount in Hong Kong has a strong negative correlation with local CTP. Therefore, the precipitation δ18O is ultimately negatively correlated with precipitation amount. This study demonstrates that monsoon convective precipitation is a main factor controlling the isotopic composition of precipitation in monsoon regions. This finding is important for understanding the mechanism responsible for the isotopic variability in monsoon regions as well as the interpretation of paleoclimate in these regions.