印度洋不同海温模态对两类厄尔尼诺事件与我国南方秋季降水关系的影响

利用1951—2015年NOAA气候预测中心的SST扩展重建资料（ERSST V3b）、国家气候中心提供的我国160站月降水量资料、美国国家环境预报中心/大气研究中心（NCEP/NCAR）提供的各气压层的水平风速、垂直速度和比湿资料,研究了印度洋不同海温模态对两类厄尔尼诺事件与我国南方秋季降水关系的影响。结果表明,虽然东部型（中部型）厄尔尼诺年秋季我国长江以南地区降水偏多（少）,但当东部型厄尔尼诺与印度洋正偶极子同时发生年秋季,我国长江以南地区降水偏多的程度显著提高;当中部型厄尔尼诺与印度洋正偶极子同时发生年秋季,我国西南地区降水转为偏多,其他南方地区降水仍然偏少;当中部型厄尔尼诺与印度洋一致增暖型海温同时发生年秋季,我国整个长江以南地区降水偏少,且偏少的幅度要显著高于不考虑印度洋海温异常的情况。此外还对印度洋不同海温模态对两类厄尔尼诺事件与我国南方秋季降水关系的影响的环流成因进行了分析。     

印度洋偶极子中的西极子对西藏高原盛夏降水的影响

利用西藏高原地区1987-2016年的逐月夏季降水资料和印度洋偶极子指数资料分析了两者的关系,结果表明：高原地区盛夏降水与表征西印度洋异常海温的西极子指数表现出良好的相关关系,在西极子指数正异常年时高原降水偏多10%~30%,其中高原中部偏多最为显著,而在负异常年时与之相反。分析其机理研究发现,在正西极子异常年,南海和西太暖池区域的深对流加强、西太副高偏西偏南和印度热低压的减弱使得来自热带的水汽更容易深入高原腹地,其次,南亚高压东体异常增强,配合低空异常辐合,都使得高原降水偏多。同时,高原上空局地纬圈环流在高原中部（90 °E附近）上空（400 hPa以上）有异常辐合上升区,使得高原中部更容易发展暖湿切变线、高原低涡等中尺度涡旋低值系统,造成更多的降水。本研究从高原气候变化响应海洋年际变化的角度分析了区域降水的季节差异,可以为高原气候预测提供新的思路。     

Influence of the North American Dipole on ENSO onset as simulated by a coupled ocean-Atmosphere model

北美偶极子(NAD)是热带北大西洋西部和北美东北部的南北向海平面气压异常偶极型模态.以往的观测研究表明,NAD可以有效地影响ENSO事件的爆发.本文利用全球耦合模式FGOALS-g2,评估了NAD与ENSO的关系.结果表明,该模式能较好地重现NAD模态.进一步的分析验证了冬季NAD可以通过强迫冬末春初副热带东北太平洋上空的反气旋和暖海温的出现,在随后的冬季触发El Ni?o事件.此外,在同化NAD实验中,发生El Ni?o事件的概率增加了将近一倍.相比之下,NAO未能在副热带东北太平洋上空引起表面风和海温的异常,因而不能有效地激发次年冬季ENSO事件.     

非球形冰晶的毫米波k-Ze关系研究

针对毫米波雷达处理数据的实际需要,应用离散偶极子近似法DDA,获得了非球形冰晶的后向散射及衰减截面并进行了参数化,并主要基于细化的冰云模型,假设冰晶粒子谱为Γ分布,通过模拟取样各1330次（代表1330种粒子分布）,分别计算得到了W波段（94 GHz）与Ka波段（35 GHz）毫米波雷达探测的冰云衰减系数k及雷达反射率因子Z_e,而且利用数值模拟的方法,建立了k-Z_e关系的具体表达式。计算表明,非球形和非瑞利散射对W波段毫米波雷达衰减的影响较大,而且在同样滴谱分布条件下,W波段毫米波雷达的衰减比Ka波段毫米波雷达的大几倍,此外细化的冰云模型对k-Z_e关系具有影响。本研究对中纬度非降水性冰云的毫米波雷达的衰减订正具有参考价值,并对中国的毫米波雷达应用具有借鉴作用。     

Influence of the Preceding Austral Summer Southern Hemisphere Annular Mode on the Amplitude of ENSO Decay

There is increasing evidence of the possible role of extratropical forcing in the evolution of ENSO.The Southern Hemisphere Annular Mode(SAM) is the dominant mode of atmospheric circulation in the Southern Hemisphere extratropics.This study shows that the austral summer(December–January–February; DJF) SAM may also influence the amplitude of ENSO decay during austral autumn(March–April–May;MAM).The mechanisms associated with this SAM–ENSO relationship can be briefly summarized as follows:The SAM is positively(negatively) correlated with SST in the Southern Hemisphere middle(high) latitudes.This dipole-like SST anomaly pattern is referred to as the Southern Ocean Dipole(SOD).The DJF SOD,caused by the DJF SAM,could persist until MAM and then influence atmospheric circulation,including trade winds,over the Nio3.4 area.Anomalous trade winds and SST anomalies over the Nio3.4 area related to the DJF SAM are further developed through the Bjerkness feedback,which eventually results in a cooling(warming) over the Nio3.4 area followed by the positive(negative) DJF SAM.     

Investigating the Initial Errors that Cause Predictability Barriers for Indian Ocean Dipole Events Using CMIP5 Model Outputs

By analyzing the outputs of the pre-industrial control runs of four models within phase 5 of the Coupled Model Intercomparison Project, the effects of initial sea temperature errors on the predictability of Indian Ocean Dipole events were identified. The initial errors cause a significant winter predictability barrier(WPB) or summer predictability barrier(SPB).The WPB is closely related with the initial errors in the tropical Indian Ocean, where two types of WPB-related initial errors display opposite patterns and a west–east dipole. In contrast, the occurrence of the SPB is mainly caused by initial errors in the tropical Pacific Ocean, where two types of SPB-related initial errors exhibit opposite patterns, with one pole in the subsurface western Pacific Ocean and the other in the upper eastern Pacific Ocean. Both of the WPB-related initial errors grow the fastest in winter, because the coupled system is at its weakest, and finally cause a significant WPB. The SPB-related initial errors develop into a La Ni ?na–like mode in the Pacific Ocean. The negative SST errors in the Pacific Ocean induce westerly wind anomalies in the Indian Ocean by modulating the Walker circulation in the tropical oceans. The westerly wind anomalies first cool the sea surface water in the eastern Indian Ocean. When the climatological wind direction reverses in summer, the wind anomalies in turn warm the sea surface water, finally causing a significant SPB. Therefore, in addition to the spatial patterns of the initial errors, the climatological conditions also play an important role in causing a significant predictability barrier.     

南太平洋副热带偶极子对南太平洋辐合带的影响

为研究南太平洋副热带偶极子的局地气候效应,利用Hadley中心的海温数据集Had ISST以及NCEP-NCAR的大气再分析数据,分析了南太平洋副热带偶极子(South Pacific Subtropical Dipole,SPSD)对南太平洋辐合带(South Pacific Convergence Zone,SPCZ)的影响,并探讨了相应的物理过程。研究结果显示,南太平洋副热带偶极子事件线性独立于ENSO(El Ni?o-Southern Oscillation)事件,有明显的季节锁相,于12~2月达到峰值并显著影响SPCZ降水带的位置。其中,正偶极子事件(简称"正事件")期间偶极子东北极区域(暖海温)水汽辐合上升,降水增多;而在偶极子西南极区域(冷海温)水汽辐散下沉,降水减少,因此SPCZ降水带偏北。负偶极子事件(简称"负事件")则相反,东北极降水减少的同时西南极降水增多,从而SPCZ降水带偏南。本研究关于SPSD与SPCZ关系的分析,将有助于更好地理解南太平洋的年际气候变异和海气相互作用。     

THE WARMING MECHANISM IN THE SOUTHERN ARABIAN SEA DURING THE DEVELOPMENT OF INDIAN OCEAN DIPOLE EVENTS

This study aims to explore the relative role of oceanic dynamics and surface heat fluxes in the warming of southern Arabian Sea and southwest Indian Ocean during the development of Indian Ocean Dipole (IOD) events by using National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) daily reanalysis data and Global Ocean Data Assimilation System (GODAS) monthly mean ocean reanalysis data from 1982 to 2013, based on regression analysis, Empirical Orthogonal Function (EOF) analysis and combined with a 2? layer dynamic upper-ocean model. The results show that during the initial stage of IOD events, warm downwelling Rossby waves excited by an anomalous anticyclone over the west Indian Peninsula, southwest Indian Ocean and southeast Indian Ocean lead to the warming of the mixed layer by reducing entrainment cooling. An anomalous anticyclone over the west Indian Peninsula weakens the wind over the Arabian Sea and Somali coast, which helps decrease the sea surface heat loss and shallow the surface mixed layer, and also contributes to the sea surface temperature (SST) warming in the southern Arabian Sea by inhibiting entrainment. The weakened winds increase the SST along the Somali coast by inhibiting upwelling and zonal advection. The wind and net sea surface heat flux anomalies are not significant over the southwest Indian Ocean. During the antecedent stage of IOD events, the warming of the southern Arabian Sea is closely connected with the reduction of entrainment cooling caused by the Rossby waves and the weakened wind. With the appearance of an equatorial easterly wind anomaly, the warming of the southwest Indian Ocean is not only driven by weaker entrainment cooling caused by the Rossby waves, but also by the meridional heat transport carried by Ekman flow. The anomalous sea surface heat flux plays a key role to damp the warming of the west pole of the IOD.     

我国西北和华北夏季降水的偶极子关系及其环流特征

用全国35年以上资料的672个站,讨论了华北和西北夏季降水关系及其环流特征,结果表明:(1)华北和西北夏季降水存在跷跷板式的偶极子关系,当华北夏季降水偏多时,西北降水偏少,反之亦然。华北夏季降水指数与西北夏季降水指数之间的相关系数为-0.28,超过了0.05显著性水平检验。这两条曲线的10年滑动平均的相关系数达到-0.61;而滤去年代际振荡后,它们的相关系数为-0.23,显著性水平接近0.05。这表明华北与西北夏季降水的偶极子关系主要是由年代际振荡引起的,但年际变化也是一个不可忽略的因素。(2)环流分析表明:华北与西北地区夏季降水的偶极子关系与贝加尔湖南部地区(45°~55°N,80°~110°E之间的8个格点)和西部巴尔喀什湖地区(35°~40°N,70°~90°E之间的5个格点)500 hPa高度距平符号相反有关,由它们之差(G-D)组成的华北和西北夏季降水的偶极子关系的环流指数,能很好地解释华北和西北夏季降水的偶极子关系。     

A model of tidal flushing of an estuary by dipole formation

We discuss a laboratory model of the flushing and mixing between a bay and a sea connected by a narrow channel. Tidal changes in water elevation lead to periodic flow in and out of the bay through the narrow channel. Due to flow separation there is an asymmetry between the flow entering and leaving the channel on different phases of the tide. A consequence of this asymmetry is that every tidal cycle a fraction of fluid is not returned through the channel, so that a passive tracer in the bay is successively flushed away. We consider what happens when the creation of vorticity in the channel leads to the formation of a dipole that can propagate away from the returning flow of the later phase of the tide. When the dimensionless ratio of maximum channel velocity U, width of channel W and tidal period T is such that W/UT<0.13, we find that dipoles can propagate away from the source region without being drawn back into the sink. This process leads to a larger exchange of water between the estuary and sea, than the classical model of tidal flushing by the jet-sink asymmetry of [Stommel, H., Farmer, H. G., 1952. On the nature of estuarine circulation WHOI Tech. Rep. 52–88, 131].