过去60年中3—5年时间尺度的强ENSO过程与平流层环流异常的滞后耦合及其机理

基于1950—2009年60a月平均Nino3指数和NCEP/NCAR第一套等压面再分析资料,关注3—5a时间尺度的强ENSO过程与平流层环流年际异常的时空联系及其机理,通过对此期间出现在3次持续强ENSO阶段中的11次3—5a时间尺度的强ENSO过程的诊断表明,平流层环流的年际尺度异常与3—5a时间尺度的强ENSO循环过程密切耦合。极夜急流强度趋于在ENSO暖/冷峰值之后减弱/加强,最大异常值多滞后ENSO峰值约1/4周期(接近1a),出现在ENSO峰值之后的下一年冬季;且3—5a时间尺度的ENSO峰值愈强,滞后约1/4周期出现的热带外平流层纬向风的年际异常也愈强;平均而言,这种年际时间尺度的耦合关系,也对实际的季节尺度平流层极涡振荡的强度和性质有显著的调制作用。进一步研究这种滞后耦合关系与年际时间尺度的行星波活动异常的联系发现,在暖ENSO峰值所在的当年冬季,对流层高层被强迫出年际时间尺度的太平洋-北美(PNA)型异常环流,而与冷ENSO峰值相对应的是相反的太平洋-北美异常型;这种太平洋-北美型与平流层热带外地区的行星波1波的发展相联系;在ENSO峰值之后的下一年冬季,太平洋-北美型环流减弱但对流层高层的主要异常分布在中高纬度,多对应着平流层行星波2波的显著增强,与平流层极区最强的高度异常相联系。行星波活动所引起的经向动量通量和经向热量通量的辐合、辐散异常对平流层滞后异常响应的贡献,存在显著的阶段性差异,在不同的阶段两者可以共同起作用,也可以分别起作用。     

Pacific decadal oscillation and the decadal change in the intensity of the interannual variability of the South China Sea summer monsoon

本研究表明,二十世纪南海夏季风的年际变率强度在一定程度上受到太平洋年代际振荡(PDO)的调控,PDO处于暖(冷)位相时南海夏季风的年际变率强度偏强(弱)。热带太平洋海温的年际变率强度及南海夏季风与ENSO的关系在上述调控中起到重要作用。PDO处于暖位相时,热带太平洋海温变率偏大,ENSO事件偏强,因而沃克环流及西北太平洋反气旋异常的位置和强度均发生改变,最终导致南海夏季风与热带太平洋海温的相互作用更强,南海夏季风年际变率强度增大,反之亦然。     

太平洋年代际振荡冷、暖背景下ENSO循环的特征

利用英国气象局哈德莱中心的月平均海温距平资料、美国Scripps海洋研究所联合环境分析中心(JEDAC)的海表和次表层海温观测资料以及NCEP/NCAR再分析资料,研究了太平洋年代际振荡(PDO)不同背景下ENSO循环的特征.结果表明,PDO为ENSO循环提供了一个年代际气候背景,在PDO的暖位相时期,El Nino事件发生的频率较高,强度较强;反之,在PDO的冷位相时期,La Nina事件发生的频率较高,强度较强.而且在不同的太平洋年代际振荡背景下,ENSO循环表现出不同的特征.在PDO冷位相时期,发生El Nino(La Nina)事件时,正(负)的SOTA从西太平洋沿温跃层向东传播,正(负)的SSTA从赤道东太平洋向西扩展到中太平洋,ENSO事件先在赤道东太平洋爆发.在PDO的暖位相时期,发生El Nino(La Nina)事件时,正(负)的SOTA首先出现在赤道中太平洋,然后沿温跃层向东传播,正(负)的SSTA从赤道中太平洋向东扩展到东太平洋,ENSO事件首先在中太平洋爆发.这为ENSO预测提供了新的线索.     

热带加热异常影响冬季平流层极涡强度的数值模拟

本文利用大气环流模式SAMIL/LASG,通过选择两种对流参数化方案,研究了热带加热异常对热带外平流层模拟的影响。结果表明,因不同对流参数化方案引起的热带对流加热状况的差异,可显著影响模式对北半球冬季平流层极涡强度的模拟偏差。与采用Manabe对流参数化方案相比,采用Tiedtke参数化方案可以显著改善对平流层极涡强度的模拟,使平流层极涡“过强”及极区“过冷”的模拟偏差得到明显改善。研究其中的影响过程发现,由于Manabe方案最大凝结潜热加热高度过低,在对流层中低层;而Tiedtke方案的最大凝结潜热加热位置在对流层中上层,因而Tiedtke(Manabe)方案时热带大气温度在对流层中上层较为偏暖(偏冷),在平流层低层较为偏冷(偏暖)。自秋季开始,与热带对流层高层温度的暖偏差相联系,热带外对流层高层以及热带平流层低层出现伴随的温度冷偏差;与之对应,平流层中纬度从秋季开始也出现持续的温度暖偏差。另外,随着秋冬季节平流层行星波活动的出现,Tiedtke方案时热带外地区行星波1波的强度也明显强于Manabe方案,使得秋冬季节涡动引起的向极热通量在Tiedtke方案时明显偏强,从而造成了冬季平流层极区温度偏暖、极涡强度偏弱。     

印度洋偶极子向海盆一致模转变的年代际变化及其成因

利用3种百年时间尺度的海温资料,探讨了印度洋偶极子(India Ocean Dipole,IOD)向海盆一致模(Indian Ocean Basin,IOB)年际转变的年代际变化,结果发现1940—1970年几乎不存在这一转变现象,1970年以后该现象则十分显著。研究表明,IOD与ENSO之间海气耦合作用的年代际变化是这一转变现象的主要原因,1940—1970年IOD与ENSO之间发生发展相互独立,而1970年以后联系密切。通过进一步对物理量场的诊断分析,揭示了其中主要的动力机理:1970年以前,热带印度洋上空形成的季风环流异常无法与热带太平洋的沃克环流异常进行耦合,IOD事件发生时无法与热带太平洋产生联系。反之,1970年以后,热带两大洋上空两个纬圈环流异常之间耦合作用强烈,正(负) IOD事件发生时,通过海气相互作用,促进El Niño (La Niña)发展,印度洋又会受到来自ENSO的正反馈作用。因此这种“齿轮式”耦合模型能一直持续到冬季和次年春季,热带印度洋上空持续受到东(西)风异常的影响和低层环流的引导,西印度洋有次表层暖水的流入(出),加上印度洋本身海盆尺度较小,西边的暖(冷)水区显著增大,东西海温异常差异迅速减小并向海盆一致变暖(冷)转变,导致了后期冬季、春季正(负) IOB事件的出现。     

赤道西太平洋海面增暖对早春北极平流层极涡的调控

平流层极涡异常对冬春季极端天气和极地臭氧亏损有重要指示意义。在1979～2020年ERA5再分析资料中,早春北极平流层极涡年际变率与热带太平洋海温第二模态（主要空间型为赤道西太平洋海温异常）有关。通过CAM5数值模拟,进一步揭示了赤道西太平洋海温异常影响平流层极涡的途径：冬春季赤道西太平洋增暖时,在暖海温区深对流降水加强,异常的潜热加热通过Matsuno-Gill响应在其西北侧激发了Rossby波（对流层上层的高压异常）。该Rossby波沿大圆路径在北太平洋调整了北半球最强的槽脊系统的强度和位置,从而使得经向风1波分量的振幅减小,经向风3波和温度3波分量的位相差增大。两者分别减弱了下平流层的1波和3波的波作用通量上传,更少的涡动热量通量向极输送促进了春季北极平流层极涡增强。     

北半球平流层极涡崩溃早晚的环流特征及其对5月南亚降水的影响

本文采用1979—2014年NCEP/NCAR月平均再分析资料、CMAP和GPCP月平均降水资料,分析了北半球平流层极涡崩溃早晚的环流特征及其与南亚降水的关系。结果表明,北半球平流层极涡崩溃时间存在明显的年际变化特征。极涡崩溃偏早(偏晚)年,自3月开始异常信号从平流层向下传播,之后的4月,从平流层到对流层高层极区温度异常偏高(偏低),极涡异常偏弱(偏强),极夜急流异常偏弱(偏强)。结果还表明,5月南亚降水异常与平流层极涡崩溃时间的早晚存在显著相关,5月南亚降水异常与平流层极涡崩溃早晚年平流层异常信号的下传有关。当平流层极涡崩溃偏晚年,4月平流层极区表现为位势高度异常偏低,而中纬度则位势高度场异常偏高,并伴随位势高度异常场的向下传播,5月该位势高度异常场下传至阿拉伯海北部大陆上空对流层顶,形成有利于降水的环流场,导致南亚降水偏多。反之,则相反。     

An enhanced influence of sea surface temperature in the tropical northern Atlantic on the following winter ENSO since the early 1980s

已有研究指出春季热带北大西洋海温对随后冬季ENSO事件的发生存在显著的影响。该研究发现它们的联系在20世纪80年代初以后显著增强。20世纪80年代初以后,春季热带北大西洋海温为正异常时,副热带东北太平洋存在显著的异常气旋环流,同时异常沃克环流在热带中东太平洋引起显著的异常下沉运动。异常气旋环流西侧的东北风异常增强气候态的风速,导致负海温异常。负海温异常通过Gill型大气响应使得热带西北太平洋产生异常的反气旋环流。同时,热带中东太平洋的沃克环流异常下沉运动也对热带西北太平洋异常反气旋环流的形成起到一定作用。热带西北太平洋反气旋环流南侧的东风异常通过海洋动力过程对随后冬季ENSO产生影响。因此,春季热带北大西洋海温对冬季ENSO存在显著的影响。然而,20世纪80年代以前,春季热带北大西洋海温相关的热带东北太平洋异常气旋环流和热带地区的异常沃克环流不显著,从而不能在热带西北太平洋产生异常反气旋。所以,20世纪80年代以前,春季热带北大西洋海温对冬季ENSO的影响不显著。进一步的分析指出,春季热带北大西洋海温对ENSO影响的年代际变化可能与热带北大西洋降水气候态的年代际变化有关。     

热带印度洋海温异常单、偶极模态及其相互作用

利用最近50多年的GISST和NCEP的OISST海表温度资料研究了印度洋海温变化的空间分布型和多重时间尺度及其相互作用.结果表明,热带印度洋海温主要存在两种空间分布型,即全海盆符号一致的单极和东、西部符号相反的偶极.单极既存在长期增暖趋势,也存在年际振荡;偶极则以年际变化为主.在去掉由EOF重建的单极后,热带印度洋东、西部海温表现为显著的反相关关系;对17次典型偶极子个例的分析表明,对偶极子本身而言,偶极子的演变更像是一种翘翘板似的局地振荡.单极在长期趋势和年际时间尺度上对偶极的影响是不同的.长期趋势缩短了偶极子的生命期,在冷期,印度洋海温经历了由负单极到正偶极再到负单极的演变,偶极子的异常信号最早出现在热带西印度洋;在暖期,印度洋海温经历了由正单极到正偶极再到正单极的演变,偶极子的异常信号最早出现在热带东印度洋.对年际时间尺度的变化而言,印度洋海温异常由负单极向正偶极再到正单极转换,偶极子位于一种单极向另一种单极的转换过程之中,在此过程中,印度洋海温表现为明显自西向东的传播特征.     

关于ENSO本质的进一步研究

基于ENSO是热带太平洋海气相互作用产物的科学观点,一系列的分析研究表明:赤道太平洋次表层海温异常(SOTA)有明显的年际变化(循环),并且与ENSO发生密切相关;ENSO的真正源区在赤道西太平洋暖池,赤道西太平洋暖池正(负)SOTA沿赤道温跃层东传到东太平洋,导致El Nino(La Nina)的爆发;在暖池正(负)SOTA沿赤道温跃层东传的同时,将有负(正)SOTA沿10°N和10°S两个纬度带向西传播,从而构成SOTA的循环;热带太平洋SOTA年际循环的驱动者主要是由异常东亚季风所引起的赤道西太平洋纬向风的异常.进而,可以提出关于ENSO本质的一种新理论,即ENSO实质上主要是由异常东亚季风引起的赤道西太平洋异常纬向风所驱动的热带太平洋次表层海温距平的年际循环.       

Seasonality of the Lagged Relationship between ENSO and the Northern Hemispheric Polar Vortex Variability

This paper reports the seasonal feature of the relationship between ENSO and the stratospheric Polar Vortex Oscillation (PVO) variability in the Northern Hemisphere.It is shown that the lagged ENSO-PVO coupling relationship exhibits distinct seasonal feature,due to the strong seasonality of PVO and ENSO.Specifically,the PVO variability not only during winter,but also in autumn and spring months,is significantly correlated with ENSO anomalies leading by seasons;however,no significant effect of ENSO is found on the PVO variability in winter months of November and February.Although a significant ENSO effect is primarily observed when ENSO leads PVO by about one year,a significant correlation is also found between PVO in the following spring months (M +1 A +1) and ENSO anomalies in the previous autumn (A-1 S-1 O- 1 N -1) when ENSO anomalies lead by about 18 months.The significant correlation between PVO in various seasons and the corresponding ENSO anomalies leading by seasons could be explicitly verified in most of the individual years,confirming that the lagged ENSO effect can largely modulate the seasonal timescale variability of PVO.Moreover,the composite spatial patterns of the zonal-mean temperature anomalies further show that the ENSO effect on the PVO in various seasons is related to the interannual variability of the seasonal timescale PVO events.     

A timescale decomposed threshold regression downscaling approach to forecasting South China early summer rainfall

A timescale decomposed threshold regression(TSDTR) downscaling approach to forecasting South China early summer rainfall(SCESR) is described by using long-term observed station rainfall data and NOAA ERSST data. It makes use of two distinct regression downscaling models corresponding to the interannual and interdecadal rainfall variability of SCESR.The two models are developed based on the partial least squares(PLS) regression technique, linking SCESR to SST modes in preceding months on both interannual and interdecadal timescales. Specifically, using the datasets in the calibration period 1915–84, the variability of SCESR and SST are decomposed into interannual and interdecadal components. On the interannual timescale, a threshold PLS regression model is fitted to interannual components of SCESR and March SST patterns by taking account of the modulation of negative and positive phases of the Pacific Decadal Oscillation(PDO). On the interdecadal timescale, a standard PLS regression model is fitted to the relationship between SCESR and preceding November SST patterns. The total rainfall prediction is obtained by the sum of the outputs from both the interannual and interdecadal models. Results show that the TSDTR downscaling approach achieves reasonable skill in predicting the observed rainfall in the validation period 1985–2006, compared to other simpler approaches. This study suggests that the TSDTR approach,considering different interannual SCESR-SST relationships under the modulation of PDO phases, as well as the interdecadal variability of SCESR associated with SST patterns, may provide a new perspective to improve climate predictions.     

Interannual to decadal variability of the winter Aleutian Low intensity during 1900–2004

A new winter Aleutian Low (AL) intensity index was defined in this paper. A centurial-long time series of this index was constructed using the sea level pressure (SLP) data of nearly 100 years. The features of interannual and decadal variability of the winter AL intensity since 1900 were analyzed by applying the wavelet analysis. The relationship between the winter AL intensity and atmospheric circulation was examined. The cross-wavelet analysis technique was used to further reveal the relationship between the AL intensity and sea surface temperature (SST) in the equatorial eastern Pacific (EEP) and tropical Indian Ocean (TIO) in winter. The results indicate that: 1) On the interannual timescale, the winter AL intensity displays 3–7-yr oscillations, while on the decadal timescale, 8–10-yr and 16–22-yr oscillations are more obvious. 2) Of the linkage to atmospheric circulation, both AO (Arctic Oscillation) and PNA (Pacific North America pattern) are closely associated with winter AL intensity on the interannual timescale, but only PNA contributes to the variation of winter AL intensity on the decadal timescale. 3) As to the ocean impact, winter EEP SST is a major factor affecting the winter AL intensity on the interannual timescale, especially on the 3–7-yr periods. However, on the decadal timescale, though both the TIO and EEP SSTs are associated with the AL intensity in winter, the TIO SST impact is more significant.     

Influence of sea surface temperature on the intraseasonal variability of the South China Sea summer monsoon

The objective of this study is to examine, based on recently available high resolution satellite and observational data, the evolution and role of sea surface temperature (SST) in influencing the intraseasonal variability of the South China Sea (SCS) summer monsoon (SM). The study focuses on the 30–60?day timescale when the northward propagating anomalies are dominant over the SCS. Composite analysis of the SST maximum events during SCS SM shows that increased SST anomalies over the SCS are significantly influenced by the downward shortwave radiation flux anomalies, with the suppressed surface latent heat flux anomalies supplementing to it. A thermal damping of the positive SST anomalies induces positive upward heat fluxes, which then destabilize the lower atmosphere between 1,000 and 700?hPa. The positive SST anomalies lead the positive precipitation anomalies over the SCS by 10?days, with a significant correlation (r?=?0.44) between the SST-precipitation anomalies. The new findings here indicate an ocean-to-atmosphere effect over the SCS, where underlying SST anomalies tend to form a favorable condition for convective activity and sustain enhanced precipitation during the SCS SM. It is also argued, based on our observations, that the negative sea level pressure anomalies induced by the positive SST anomalies play a role in enhancing the northward propagation of the intraseasonal anomalies over the SCS.     

Mechanisms of Atlantic Meridional Overturning Circulation (AMOC) Variability in a Coupled Ocean–Atmosphere GCM简

The mechanisms involved in the variability of Atlantic Meridional Overturning Circulation （AMOC） are studied using a 2000-yr control simulation of the coupled Fast Ocean-Atmosphere Model （FOAM）.This study identifies a coupled mode between SST and surface heat flux in the North Atlantic at the decadal timescale,as well as a forcing mode of surface heat flux at the interannual timescale.The coupled mode is regulated by AMOC through meridional heat transport.The increase in surface heating in the North Atlantic weakens the AMOC approximately 10 yr later,and the weakened AMOC in turn decreases SST and sea surface salinity.The decreased SST results in an increase in surface heating in the North Atlantic,thus forming a positive feedback loop.Meanwhile,the weakened AMOC weakens northward heat transport and therefore lowers subsurface temperature approximately 19 yr later,which prevents the AMOC from weakening.In the forcing mode,the surface heat flux leads AMOC by approximately 4 yr.     

夏季赤道西印度洋对大气的强迫作用及其机制研究

基于1982—2015年高分辨率海气资料,从海表面温度(Sea Surface Temperature, SST)和海表面风速相关关系的角度研究了年际尺度上赤道印度洋的海气关系。结果表明,印度洋的海气关系具有明显区域性和季节性特征,即整个印度洋除赤道东南印度洋和赤道西印度洋SST与海表风速在夏季(7—9月)为显著正相关关系,主要表现为海洋影响大气;其他地区和月份均为负相关关系,主要表现为大气对海洋的强迫作用。回归分析发现,夏季赤道西印度洋SST异常可能通过海平面气压调整机制影响海表面风场,即海温增温使边界层空气增暖,海表面风场辐合增强;反之则相反。此外,还利用AM2.1模式进行模拟试验,试验结果成功地再现了夏季赤道西印度洋海表面温度与海表风速之间的正相关关系。     

Impact of the North Atlantic sea surface temperature tripole on the East Asian summer monsoon

A strong (weak) East Asian summer monsoon (EASM) is usually concurrent with the tripole pattern of North Atlantic SST anomalies on the interannual timescale during summer, which has positive (negative) SST anomalies in the northwestern North Atlantic and negative (positive) SST anomalies in the subpolar and tropical ocean. The mechanisms responsible for this linkage are diagnosed in the present study. It is shown that a barotropic wave-train pattern occurring over the Atlantic-Eurasia region likely acts as a link between the EASM and the SST tripole during summer. This wave-train pattern is concurrent with geopotential height anomalies over the Ural Mountains, which has a substantial effect on the EASM. Diagnosis based on observations and linear dynamical model results reveals that the mechanism for maintaining the wave-train pattern involves both the anomalous diabatic heating and synoptic eddy-vorticity forcing. Since the North Atlantic SST tripole is closely coupled with the North Atlantic Oscillation (NAO), the relationships between these two factors and the EASM are also examined. It is found that the connection of the EASM with the summer SST tripole is sensitive to the meridional location of the tripole, which is characterized by large seasonal variations due to the north-south movement of the activity centers of the NAO. The SST tripole that has a strong relationship with the EASM appears to be closely coupled with the NAO in the previous spring rather than in the simultaneous summer.     

STUDY ON VARIATION CHARACTERISTICS OF SST OBSERVED IN THE PAST 40 YEARS IN THE COASTAL REGION OF SOUTH CHINA

To study the variation characteristics of SST in coastal south China, observed SST was analyzed for the past 44 years. The results show that the monthly and yearly averaged SST have rising trends, the yearly averaged SST's linearity rising rate is 0.12-0.19 ℃/decade in the years, the rising extents in winter are higher than those in summer. The variability in winter is larger than that in summer, with the largest in February and March. A majority of significant cold or warm events occurred in winter. A wavelet multiresolution method was used to analyze periodical characteristics of SST in coastal south China, all timescale decomposition series are very similar, and high-frequency decompositions within the year are dominant. Low-frequency decompositions show rising trends since the mid-1970.     

The 30–60-day Intraseasonal Variability of Sea Surface Temperature in the South China Sea during May–September

This study investigates the structure and propagation of intraseasonal sea surface temperature(SST) variability in the South China Sea(SCS) on the 30–60-day timescale during boreal summer(May–September). TRMM-based SST, GODAS oceanic reanalysis and ERA-Interim atmospheric reanalysis datasets from 1998 to 2013 are used to examine quantitatively the atmospheric thermodynamic and oceanic dynamic mechanisms responsible for its formation. Power spectra show that the 30–60-day SST variability is predominant, accounting for 60% of the variance of the 10–90-day variability over most of the SCS. Composite analyses demonstrate that the 30–60-day SST variability is characterized by the alternate occurrence of basin-wide positive and negative SST anomalies in the SCS, with positive(negative) SST anomalies accompanied by anomalous northeasterlies(southwesterlies). The transition and expansion of SST anomalies are driven by the monsoonal trough–ridge seesaw pattern that migrates northward from the equator to the northern SCS. Quantitative diagnosis of the composite mixed-layer heat budgets shows that, within a strong 30–60-day cycle, the atmospheric thermal forcing is indeed a dominant factor, with the mixed-layer net heat flux(MNHF) contributing around 60% of the total SST tendency, while vertical entrainment contributes more than 30%. However, the entrainment-induced SST tendency is sometimes as large as the MNHF-induced component, implying that ocean processes are sometimes as important as surface fluxes in generating the30–60-day SST variability in the SCS.     

Causes of the Intraseasonal SST Variability in the Tropical Indian Ocean

Satellite observations reveal a much stronger intraseasonal sea surface temperature (SST) variability in the southern Indian Ocean along 5-10oS in boreal winter than in boreal summer. The cause of this seasonal dependence is studied using a 2?-layer ocean model forced by ERA-40 reanalysis products during 1987-2001. The simulated winter-summer asymmetry of the SST variability is consistent with the observed. A mixed-layer heat budget is analyzed. Mean surface westerlies along the ITCZ (5-10oS) in December-January-February (DJF) leads to an increased (decreased) evaporation in the westerly (easterly) phase of the intraseasonal oscillation (ISO), during which convection is also enhanced (suppressed). Thus the anomalous shortwave radiation, latent heat flux and entrainment effects are all in phase and produce strong SST signals. During June-July-August (JJA), mean easterlies prevail south of the equator. Anomalies of the shortwave radiation tend to be out of phase to those of the latent heat flux and ocean entrainment. This mutual cancellation leads to a weak SST response in boreal summer. The resultant SST tendency is further diminished by a deeper mixed layer in JJA compared to that in DJF. The strong intraseasonal SST response in boreal winter may exert a delayed feedback to the subsequent opposite phase of ISO, implying a two-way air-sea interaction scenario on the intraseasonal timescale. Citation: Li, T., F. Tam, X. Fu, et al., 2008: Causes of the intraseasonal SST variability in the tropical Indian ocean, Atmos. Oceanic Sci. Lett., 1, 18-23