平流层极涡偏移对我国冬季降水的影响

利用1970—2010年NCEP/NCAR再分析资料、我国160站月平均降水资料分析了平流层极涡向欧亚大陆偏移与我国冬季降水的关系。结果表明:1月极涡偏欧亚大陆强度指数与同期1月降水的显著正相关区域主要分布在我国中部大面积地区及新疆西南部的少数地区,显著负相关区域主要分布在新疆中部;相对1月而言,与后期2月显著正相关区域仍然主要分布在我国的中部地区但向西北方向延伸,使得华中北部、华北南部相对减少,而华北西部、西北东部等地区增大。对流层环流形势显示出在欧亚型强极涡年的1月,东亚冬季风和东亚大槽异常减弱,我国内陆中东部东南风距平显著,而贝加尔湖北部北风距平显著,南下的冷空气与暖湿气流交汇地区较常年偏北,同时我国中部地区低层水汽向上传播也明显增强,存在显著的水汽强辐合中心。     

MODULATION OF TC GENESIS OVER THE NORTHWESTERN PACIFIC BY ATMOSPHERIC INTRASEASONAL OSCILLATION

The influence of intraseasonal oscillation (ISO) on TC genesis over the northwestern Pacific is studied through comparing analyses of the more and less TC years from 1979 to 2006. It is indicated that the ISO strongly affects the TC genesis. In the years for more TC genesis, the ISO is weak and propagates insignificantly in the area to the west of the Philippines, but the ISO is strong in the area to the east of the Philippines and propagates significantly northwestward. In this situation, the Walker cell shifts gradually westward from the tropical western Pacific to the tropical eastern Indian Ocean. Convergent winds appear in the lower atmosphere while divergent winds in the upper atmosphere, suggesting the presence of enhanced ascending flow over the 140-160°E region and a favorable condition for TC genesis. Moreover, in the years for less TC genesis, the ISO gradually becomes stronger in the area to the west of the Philippines and significant eastward propagation prevails from the eastern Indian Ocean to the area around 120°E; the ISO is weak in the area to the east of the Philippines. During these years, the Walker circulation gradually moved eastward, with convergent winds in the upper troposphere and divergent winds in the lower troposphere. Sinking motion was significant, unfavorable for the TC genesis over the Northwestern Pacific.     

ENSO phase-locking in an ocean-atmosphere coupled model FGCM-1.0

The mean climatology and the basic characteristics of the ENSO cycle simulated by a coupled model FGCM-1.0 are investigated in this study. Although with some common model biases as in other directly coupled models, FGCM-1.0 is capable of producing the interannual variability of the tropical Pacific, such as the ENSO phenomenon. The mechanism of the ENSO events in the coupled model can be explained by “delayed oscillator” and “recharge-discharge” hypotheses. Compared to the observations, the simulated ENSO events show larger amplitude with two distinctive types of phase-locking： one with its peak phase-locked to boreal winter and the other to boreal summer. These two types of events have a similar frequency of occurrence, but since the second type of event is seldom observed, it may be related to the biases of the coupled model. Analysis show that the heat content anomalies originate from the central south Pacific in the type of events peaking in boreal summer, which can be attributed to a different background climatology from the normal events. The mechanisms of their evolutions are also discussed.     

Improved ENSO forecasts by assimilating sea surface temperature observations into an intermediate coupled model

A simple method for initializing intermediate coupled models (ICMs) using only sea surface temperature (SST) anomaly data is comprehensively tested in two sets of hindcasts with a new ICM. In the initialization scheme, both the magnitude of the nudging parameter and the duration of the assimilation are considered, and initial conditions for both atmosphere and ocean are generated by running the coupled model with SST anomalies nudged to the observations. A comparison with the observations indicates that the scheme can generate realistic thermal fields and surface dynamic fields in the equatorial Pacific through hindcast experiments. An ideal experiment is performed to get the optimal nudging parameters which include the nudging intensity and nudging time length. Twelve-month-long hindcast experiments are performed with the model over the period 1984–2003 and the period 1997–2003. Compared with the original prediction results, the model prediction skills are significantly improved by the nudging method especially beyond a 6-month lead time during the two different periods. Potential problems and further improvements are discussed regarding the new coupled assimilation system.     

1998年夏季青藏高原东南部降水30~60 d低频振荡特征

根据地面观测站的逐日降水资料及NCEP/NCAR逐日再分析资料,分析了1998年夏季青藏高原东南部地区的低频降水特征,并重点讨论了30～60 d低频降水正、负位相期间相关要素场低频分量的异常分布及传播特征。结果表明:(1)1998年高原东南部降水存在10～20、20～30以及30～60 d周期的低频振荡,其中30～60 d振荡的正(负)位相基本对应着降水的盛(间歇)期。(2)在降水正(负)位相期间,高原南侧存在一个低频气旋(反气旋),而日本海上空维持着一个异常低频反气旋(气旋),受高原南侧低频气旋(反气旋)东北侧的偏南(北)气流以及日本海上空低频反气旋(气旋)西南侧的偏南(北)气流的共同影响,高原东南部为明显的低频水汽辐合(散)区。(3)在低频降水正位相期间,高原地区经孟加拉湾至中南半岛到南海均为低频热源区;负位相,热源、汇低频分量的分布与正位相基本相反。(4)1998年夏季存在从西太平洋经长江中下游西传至高原地区的30～60 d整层积分水汽通量辐合(散)和100 hPa低频辐散(合),且西传至高原东南部时基本与高原东南部30～60 d低频降水的正(负)位相对应。     

大气季节内振荡研究进展

在简单回顾大气季节内振荡(MJO)的特征,热带和中高纬MJO的联系及其在半球间相互作用的基础上,较系统地总结了近年来关于MJO的研究进展,涉及MJO和El Nino的关系,MJO的动力学机制及其季节变化和年际异常,并简单讨论了MJO研究中存在的问题及未来的研究前景.     

近45a冬季北大西洋涛动异常与我国气候的关系

利用1873－1995年的北半球海平面气压月平均资料，定义了北大西洋涛动指数。用近45a资料研究了北大西洋涛动与我国冬、夏季气候变化的关系。指出，北大西洋涛动异常变化与我国冬、夏季天气气候关系密切。强涛动年，冬季我国是偏暖、多雨的气候特征；夏季我国江淮之间地区气温明显偏低。还表明，强涛动年冬季，西太平洋副热带高压强度与西伯利亚高压及高空经向环流都明显偏弱，大气环流具有弱WA遥相关型、弱的东亚冬季风特征，对应的夏季环流特征与强东亚夏季风特征较接近。     

2009年冬季至2010年春季黑龙江省气温持续偏低成因

利用常规观测温度资料和中国国家气候中心提供的环流特征量、NCEP／NCAR再分析资料及美国气候预测中心（CPC）提供的AO指数等,分析了2009年11月至2010年4月中高纬大气环流异常特征,探讨了AO与同期气温的关系。结果表明：黑龙江省冬春气候异常与500hPa大尺度环流背景有关。冬春持续偏冷,对应北半球欧亚中高纬地区呈“-＋-”的波列分布,90°-180°E呈现出“北正南负”的环流形势;北半球极涡面积偏大,冬季东亚大槽位置偏西,春季东亚大槽强度偏强,冬春AO指数持续异常偏强,显著负位相。     

Evaluation of Haney-Type Surface ThermalBoundary Conditions Using a CoupledAtmosphere and Ocean Model

A coupled atmosphere-ocean model developed at the Institute for Space Studies at NASA Goddard Space Flight Center (Russell et al., 1995) was used to verify the validity of Haney-type surface thermal boundary condition, which linearly connects net downward surface heat flux Q to air / sea temperature difference △T by a relaxation coefficient k. The model was initiated from the National Centers for Environmental Prediction (NCEP) atmospheric observations for 1 December 1977, and from the National Ocean Data Center (NODC) global climatological mean December temperature and salinity fields at 1° ×1° resolution. The time step is 7.5 minutes. We integrated the model for 450 days and obtained a complete model-generated global data set of daily mean downward net surface flux Q, surface air temperature TA,and sea surface temperature To. Then, we calculated the cross-correlation coefficients (CCC) between Q and △T. The ensemble mean CCC fields show (a) no correlation between Q and △T in the equatorial regions, and (b) evident correlation (CCC≥ 0.7) between Q and △T in the middle and high latitudes.Additionally, we did the variance analysis and found that when k= 120 W m-2K-1, the two standard deviations, σQ and σk△T, are quite close in the middle and high latitudes. These results agree quite well with a previous research (Chu et al., 1998) on analyzing the NCEP re-analyzed surface data, except that a smaller value of k (80 W m-2K-1) was found in the previous study.     

Seasonal to interannual climate predictability in mid and high northern latitudes in a global coupled model

The upper limit of climate predictability in mid and high northern latitudes on seasonal to interannual time scales is investigated by performing two perfect ensemble experiments with the global coupled atmosphere–ocean–sea ice model ECHAM5/MPI-OM. The ensembles consist of six members and are initialized in January and July from different years of the model’s 300-year control integration. The potential prognostic predictability is analyzed for a set of oceanic and atmospheric climate parameters. The predictability of the atmospheric circulation is small except for southeastern Europe, parts of North America and the North Pacific, where significant predictability occurs with a lead time of up to half a year. The predictability of 2 m air temperature shows a large land–sea contrast with highest predictabilities over the sub polar North Atlantic and North Pacific. A combination of relatively high persistence and advection of sea surface temperature anomalies into these areas leads to large predictability. Air temperature over Europe, parts of North America and Asia shows significant predictability of up to half a year in advance. Over the ice-covered Arctic, air temperature is not predictable at time scales exceeding 2 months. Sea ice thickness is highly predictable in the central Arctic mainly due to persistence and in the Labrador Sea due to dynamics. Surface salinity is highly predictable in the Arctic Ocean, northern North Atlantic and North Pacific for several years in advance. We compare the results to the predictability due to persistence and show the importance of dynamical processes for the predictability.