【作者】 楚翠姣；

【导师】 杨修群；

【作者基本信息】 南京大学 ， 气象学， 2014， 博士

【摘要】 热带海表温度(SST)异常是全球许多区域气候异常的重要热力强迫。观测事实表明,1976／77年以来热带印度洋-热带太平洋表现为显著的年代际增暖,认识这种增暖现象对北半球冬季大气环流和气候年代际变化的影响及其机理,对于理解和预测气候年代际变化具有重要的科学意义。热带印-太海盆增暖对全球气候年代际变化如何影响仍然是一个没有很好解决的重要科学问题。本文通过利用ERA-40和NCEP再分析资料的诊断分析以及设计一系列大气环流模式(CCM3/NCAR)数值试验研究了热带印度洋-热带太平洋增暖对北半球冬季气候年代际变化的影响及其机制,其中重点分析了热带印太海盆增暖对北半球冬季不同区域大气环流年代际变化的影响、对青藏高原冬季积雪及东亚冬季降水的影响以及对北太平洋和北大西洋风暴轴年代际变化的影响等。得到主要结论如下：一、1980年代以来,热带印度洋-太平洋增暖对北大西洋、北太平洋、欧洲、东亚、北美地区冬季气候年代际变化影响的重要性明显不同。其中,热带印度洋增暖可能是导致北大西洋涛动(NAO)正位相的年代际异常的重要原因；而热带中东太平洋增暖可能是影响北太平洋地区PNA异常型的重要原因；热带太平洋增暖和中纬度北太平洋SST负异常密切相关,两者共同作用决定了北太平洋位势高度场PNA型异常。1976／77年北太平洋出现了一次显著的年代际突变现象,在海洋方面,热带中东太平洋海表面温度年代际异常增高,北太平洋中部异常变冷,北美沿岸和阿拉斯加湾SST增高；在大气方面,北太平洋500hPa位势高度场明显降低,阿留申低压异常加深、东移并偏南。伴随着太平洋海表温度上述年代际振荡,热带印度洋也表现为显著地年代际增暖。本文利用CCM3模式模拟了热带印度洋、太平洋海温历史变化对北半球大气环流影响,数值试验结果表明,热带印度洋、太平洋SST变化对北大西洋、北太平洋、欧洲、东亚、北美地区的冬季气候影响的重要性明显不同。热带印度洋增暖可能是影响北大西洋涛动(NAO)正位相异常的重要因子,而热带中东太平洋增暖不能导致NAO正位相的产生。由热带印度洋增暖引起一个纬向环绕中高纬度地区的位势高度场异常,在北大西洋地区为NAO正位相异常,同时该NAO正位相一直延伸至欧洲地区。热带中东太平洋增暖可能是影响北太平洋地区PNA异常型的重要原因,而独立的北太平洋SST年代际异常对北太平洋地区PNA影响并不显著。热带印度洋增温引起了北太平洋PNA负位相异常,同时伴随着东亚急流的减弱。对于东亚地区,热带印度洋、太平洋海温变化对该地区气候的年代际变化的影响同样重要。对于加拿大和美国,热带印度洋增暖引起的NAO正位相以及由热带太平洋增暖引起的PNA正位相共同影响着该地区的环流形式。同时,北半球冬季大气环流场的异常与低层水平风场和高空急流以及降水引起的潜热释放异常关系密切。二、1980年代以来热带印度洋SST增加有利于青藏高原东南侧的水汽输送及该地区印缅槽的加深,是该时段高原雪深增加的重要影响因子；同时,热带印度洋SST增加有利于同时期中国南方地区冬季降水的增加。1970s末到1990s青藏高原冬季积雪增加,同时期中国南方地区冬季降水增加。基于热带印太海盆SST年代际变化的观测事实,利用CCM3/NCAR模式模拟研究了1950-1999年间热带印度洋-热带太平洋海域SST历史变化对青藏高原冬季雪深以及中国南方地区冬季降水的影响。结果表明,当热带印度洋海温增加时,孟加拉湾的南风异常加强,印缅槽加深。加深的印缅槽有利于北方冷空气南下,来自青藏高原南部的南风异常有利于带来更多的水汽供应。此外,东亚地区加强的上升运动也有利于青藏高原东部强降雪的产生,以及中国南方地区冬季降水的增加。因此,热带印度洋增暖可能是1980s以后青藏高原冬季雪深增加的一个重要原因。与热带印度洋SST增暖相比,热带西太平洋SST不利于青藏高原冬季积雪及中国南方冬季降水增加。三、热带印度洋-西太平洋(IWP)增暖可能是1980s以来北大西洋风暴轴年代际异常的一个重要原因。利用ERA40再分析资料分析表明：北大西洋风暴轴年代际变化表现为风暴轴气候态轴线以北强度加强,以南强度减弱。给定一个理想化的热带印度洋-西太平洋(IWP)年代际异常,利用HadAM3/UKMO和GAMIL/IAP两个大气环流模式的模拟试验表明,IWP增暖引起北大西洋地区位势高度场出现NAO正位相异常,这样的位势高度场异常加强了急流轴北部的西风异常,以及低层大气斜压性的加强,因此该地区的天气尺度活动更加活跃。然而对于北太平洋风暴轴的年代际异常,敏感试验的结果与再分析资料结果相反。IWP增暖引起一个纬向环绕中高纬度地区的位势高度场异常,表现在北太平洋地区为正的位势高度场异常,这与再分析资料中北太平洋地区负的位势高度场异常相反。IWP增暖后急流北部加强南部减弱,伴随的低层大气斜压增长率也是北部加强南部减弱,因此天气尺度活动北部活跃加强。而再分析资料中北太平洋地区风暴轴的年代际变化表现为风暴轴气候态轴线以南强度加强,北部强度减弱。因此IWP增暖对北太平洋风暴轴年代际异常起着相反的作用。更多还原

【Abstract】 The sea surface temperature (SST) anomaly in the tropical oceans is an important thermal force that can affect climate anomalies over many regions of globe. The observational analysis suggests that the SST of the tropical Indian-Pacific oceans showed a pronounced decadal warming since the late1970s. Understanding the impact and mechanism of the warming on decadal change of Northern Hemisphere (NH) winter climate is of great scientific significance to understand and forecast decadal climate change. The impact of tropical Indian-Pacific oceans warming on decadal change of global climte change is still an open question. In this study, with the ERA-40and NCEP reanalysis data and a series of atmospheric general circulation model (CCM3/NCAR) numerical simulations, some key issues associated with the impact and mechanism of the Indian-Pacific ocean warming on decadal change of NH winter climate are investigated, in which the impact on decadal change of winter atmospheric general circulation in different regions of NH, the winter snow depth over Tibetan Plateau (TP) and the East Asian winter precipitation, and NH storm tracks is particularly emphasized. The main conclusions are as follows:(1) Since the1980s, the impacts of the tropical Indian-Pacific ocean warming on the decadal change of winter climate over northern Atlantic, northern Pacific, Europe, East Asia, and northern America are quite different. The tropical Indian Ocean (TIO) warming is the principal contributor to a decadal positive-phase North Atlantic oscillation (NAO) anomaly, while the tropical central-eastern Pacific (TCEP) warming is concurrent with the anomalies associated with the Pacific-North American (PNA) pattern. The tropical Pacific warming is closely associated with the northern Pacific decadal cooling, and their interaction determines the decadal anomaly of the PNA pattern.The Pacific ocean-atmosphere system coherently experienced a significant abrupt change around1976/77, which caused a warming in the tropical central-to-eastern Pacific, and a cooling in the midlatitude North Pacific, together with a strengthened Aleutian low with its position shifting southeastward, and a PNA pattern in its positive phase. Accompanied with the Pacific Decadal Oscillation (PDO), the TIO also showed a significant decadal warming. A series of atmospheric general circulation model (CCM3) experiments with prescribed SST are conducted to examine the impact of the Indian-Pacific SST change on the northern Hemisphere wintertime atmospheric circulation. The results show that the TIO SST and the tropical Pacific SST act on the decadal change of northern Atlantic, northern Pacific, Europe, East Asia, and northern America winter climate quite differently. The TIO warming is the principal contributor to a decadal positive-phase NAO anomaly over the North Atlantic, but the TCEP warming can not induce the NAO positive phase. The TIO warming gives rise to an anticyclonic anomaly over the midlatitude North Pacific and a positive-phase NAO anomaly over the North Atlantic that extends to the Europe. The TCEP warming is a key factor to cause the decadal PNA pattern, while the impact of independent northern Pacific cooling on PNA is not remarkable. The TIO warming causes a negative PNA pattern, along with a weakened East Asian jet. To East Asia, both the impact of TIO and TCEP SST change are important. The NAO positive phase caused by TIO warming and PNA positive pattern caused by Pacific SST change have great effect on decadal change of Canadian and American winter climate. Such a consistent effect on the Northern Hemisphere atmospheric circulations is closely associated with the changes in the high-level jet stream and the diabatic heating caused by the precipitation. (2) The tropical Indian Ocean warming can cause more moisture supply over southeast Tibetan Plateau (TP) and a deeper India-Burma trough, therefore leading to a significant decadal increase of snow depth over eastern Tibetan Plateau during the1980s-1990s. The decadal TIO warming is in favor of more winter precipitation in South China.The snow depth over eastern TP from the late1970s to the end of the1990s shows a pronounced increase, while the amount of winter precipitation over East Asia increased in the corresponding period. Uncoupled atmospheric general circulation model (CCM3) experiments with prescribed SST are conducted to examine the impact of different tropical oceanic basins (the Indian Ocean versus the tropical Pacific Ocean) on the wintertime snow depth and precipitation variability over TP as well as in South China. The results show that the TIO warming played a significant role in the increase of the snow depth over eastern TP from the1980s to the end of the1990s, and also has contributed to the increase of wintertime rainfall in the South China during the same period. Such an impact of TIO warming is in contrast to the impact of the tropical western Pacific warming, and is also different from the tropical central and eastern Pacific warming. It is found that the increased winter snow depth over the eastern TP after the mid-1970s is concurrent with a deeper India-Burma trough, that is beneficial to more cold air flowing southward to TP due to the warming of TIO. Additional factors for the excessive snowfall depth include more moisture supply associated with the intensification of the southerly flow over the Bay of Bengal and an increase of humidity over the Indian Ocean.(3) The Indian-western Pacific Ocean (IWP) warming is a key factor to cause decadal northward shift of the North Atlantic storm track since the1980s.The ERA-40reanalysis data shows that an increased transient eddy (TE) activity north of the climatological north Atlantic storm track axis but a reduced TE activity south of it. With40-yr integration output of two atmospheric general circulation models (GAMIL/IAP and HadAM3/UKMO) forced with identical prescribed seasonally-varying sea surface temperature, this study examines the effect of the observed IWP warming on the Northern Hemisphere storm tracks (NHSTs). Both models indicate that the observed IWP warming tends to cause both the North Pacific storm track (NPST) and the North Atlantic storm track (NAST) to move northward. The IWP warming can excite a wavelike circum-global teleconnection in the geopotential height that gives rise to an anticyclonic anomaly over the midlatitude North Pacific and a positive-phase NAO anomaly over the North Atlantic. These geopotential height anomalies tend to enhance upper-level zonal westerly winds north of the climatological jet axes and increase low-level baroclinicity and eddy growth rates, thus favoring transient eddy more active north of the climatological storm track axes, responsible for the northward shift of the both storm tracks. The IWP warming-induced northward shift of the NAST is quite similar to the observed, suggesting that the IWP warming can be a key factor to cause decadal northward shift of the NAST since the1980s. However, the IWP warming-induced northward shift of the NPST is completely opposite to the observed, implying that the observed southward shift of the NPST since the1980s would be primarily attributed to other reasons, although the IWP warming can have a cancelling effect against those reasons.更多还原