热带印度洋偶极子的季节性位相锁定可能原因

利用1960年1月～1999年12月Hadley气候预测和研究中心的全球海表温度资料和改进的中国科学院大气物理研究所大气科学和地球流体力学数值模拟国家重点实验室的第三代海洋环流模式，以及1949年1月～1999年12月NCEP/NCAR月平均海表面大气距平资料，采用资料合成分析和数值试验相结合的方法，研究了热带印度洋偶极子从发展到成熟的特征。海表温度异常的分析结果表明，正、负热带印度洋偶极子的强度和发生强偶极子事件的次数都在北半球秋季达到最强、最多，与季节循环存在位相锁定。偶极子的数值模拟结果与此分析结果一致，表明海洋表面的大气强迫对激发印度洋偶极子有重要作用。对比试验的结果表明，赤道印度洋上空风应力异常是偶极子形成的主要原因。文中还设计了12个敏感性试验研究在相同大气异常强迫下1～12月大气气候基本态对印度洋偶极子的作用，结果发现大气气候态对偶极子的强度有很大影响，其中9月的大气气候态最有利于印度洋偶极子达到最强。这是由赤道东南印度洋地区东南风和海洋之间的正反馈过程决定的，因此大气基本态是偶极子成熟位相锁定在秋季的一个重要原因。

Possible Causes for Seasonal Phase Locking of the Tropical Indian Ocean Dipole

The possible causes for the seasonal phase locking of the tropical Indian Ocean dipole events are investigated based on observational analyses and numerical experiments.The data used in the context include the global ice and sea surface temperature(GISST) data from Jan 1960 to Dec 1999 supported by Hadley center for climate prediction and research,and the NCEP/NCAR sea surface atmosphere data during Jan 1949-Dec 1999.To carry out the numerical experiments,the revised oceanic general circulation model developed by the State Key Laboratory of Numerical Atmospheric Sciences and Geophysical Fluid Dynamics,Institute of Atmospheric Physics,Chinese Academy of Sciences(IAP/LASG L30T63 OGCM) is also explored.The diagnosis results of sea surface temperature anomalies show that both the composite strength and the accumulated amount of the positive and negative dipole events peak in the boreal fall,which is in phase to the seasonal cycle.The numerical simulations are in good agreement with the analyses,indicating that the dipole events can be motivated by the atmospheric anomalies.The contrast between the experiments forced only by wind stress anomalies and those only by sea surface heat flux anomalies reveals that the wind stress anomalous forcing is much more important than the other to the occurrence of dipole event.In order to examine the effects of atmospheric state in the context of climatology on the Indian Ocean dipole,12 sensitive experiments,which represent 12 independent basic conditions for atmosphere from January to December,are carried out with the same atmospheric anomalies.The model output results present that the basic state in September are most favorable for the mature of dipole.Such phenomena can be explained by the positive thermodynamic air-sea feedback in the southeast Indian Ocean where the southeast wind is pronounced in the episode from May to October.But during the period from November to April,there exists a negative feedback between the dominated northwest wind and the ocean off Sumatra,which is not favorable to the development of dipole.On account of its dependence on the basic state wind,the nature of air-sea interactions is dependent on the season.Since September is the month that the southeast wind stress off Sumatra is most robust with the shallowest thermocline in the equatorial Indian Ocean, the positive feedback is strongest,leading to a consequence that the dipole mode strength is most powerful in September.On this ground,the basic state of atmosphere is one of the most important causes for the dipole seasonal phase locking.