On the Utility of ENSO and PNA Indices for Long-Lead Forecasting of Summer Weather over the Crop-Growing Region of the Canadian Prairies

Summary The well known ENSO (El Ni?o-Southern Oscillation) phenomenon is quantitatively identified in terms of SST (sea surface temperature) anomalies over the equatorial central and eastern Pacific and sea level pressure difference over eastern and western regions of the equatorial Pacific. The PNA (Pacific North American) atmospheric flow pattern, originally identified by Wallace and Gutzler (1981) is representative of a certain preferred configuration of the mid-tropospheric geopotential height field in the longitudinal sector extending from the mid-Pacific to the southeastern United States. The PNA index is defined as a linear combination of normalized geopotential height anomalies at the 700 mb level at four selected locations. Using multiple regression analysis, suitable linear combinations of predictors based on monthly values of ENSO and PNA indices are obtained which can foreshadow the summer season’s weather over the crop-growing region of the Canadian prairie provinces with a lead time of 2 to 7 months. The utility of the ENSO and PNA indices for advance indication of summer weather with implications for grain yields over the Canadian prairies is further discussed. Received September 10, 1996 Revised October 13, 1997     

ENSO对中国冬半年降水影响的不对称性及机制分析

利用1979—2010年观测和再分析资料,诊断分析ENSO对中国华南冬半年降水的影响及其机制。结果表明,在El Ni?o冬半年期间,东亚沿岸上空对流层低层南风的增强导致了水汽输送明显偏多,水汽在华南辐合,使得大气可降水量和比湿增加,降水显著偏多。而在La Ni?a冬半年期间,这些大气要素并没有呈现显著的相反变化,负异常的量值很弱并在统计上不显著。通过进一步分析El Ni?o和La Ni?a冬半年期间季节内振荡的特点,给出一种华南冬半年降水对ENSO信号不对称响应的物理解释。El Ni?o期间,热带西太平洋到南海地区的季节内振荡不活跃,与El Ni?o相联系的西北太平洋反气旋性环流异常造成的水汽输送以及水汽辐合在华南能够稳定维持,致使华南降水明显偏多。但在La Ni?a冬半年期间,季节内振荡很活跃,与La Ni?a相联系的西北太平洋气旋性环流异常受到季节内时间尺度的扰动影响,ENSO的年际变化信号被季节内振荡破坏,使得西北太平洋和华南的年际异常信号不能得到稳定维持,导致与ENSO信号相联系的年际变化在统计上不显著。因此,热带西太平洋到南海地区的季节内振荡强度在El Ni?o和La Ni?a冬半年期间的差异,是华南冬半年降水对ENSO信号不对称响应的一个主要原因。      

ENSO与中国夏季降水的联系:冬季QBO的调制作用

使用NCEP/NCAR再分析资料、中国气象局台站降水资料和GPCC降水资料,系统研究了在冬季平流层准两年振荡(Quasi-Biennial Oscillation, QBO)调制下,厄尔尼诺-南方涛动(El Ni?o-Southern Oscillation, ENSO)不同阶段与中国夏季降水的可能联系。根据两者的位相和强度,可将它们的配置分为QBO西风/El Ni?o、QBO西风/La Ni?a、QBO东风/El Ni?o、QBO东风/La Ni?a。研究结果表明,在年际时间尺度上,ENSO和QBO无显著相关关系。冬季QBO西风位相时,El Ni?o发展年夏季,我国整体偏旱,而华南偏涝;衰减年夏季,华南、华东北部偏旱,东北、长江流域偏涝。La Ni?a发展年夏季,我国东部降水异常呈负-正-负的三极分布;衰减年夏季,东南沿海偏涝。冬季QBO东风位相时,El Ni?o发展年夏季,长江以北偏旱;衰减年夏季,我国东部降水异常呈负-正-负的三极分布。La Ni?a发展年夏季,江淮和华南南部偏旱;衰减年夏季,我国东部沿海偏涝。ENSO是影响我国夏季降水异常的重要因子,而QBO的调制作用在ENSO衰减年夏季更为显著。相比冬季QBO东(西)风位相,QBO西(东)风位相时El Ni?o (La Ni?a)期间赤道西太平洋负(正)海温异常更强,衰减年夏季位于西太平洋的异常下沉(上升)运动和印度洋的异常上升(下沉)运动更强更深厚,西太平洋副热带高压范围更大(小),南亚高压更偏东(西)。      

The Optimal Precursors for ENSO Events Depicted Using the Gradientdefinition-based Method in an Intermediate Coupled Model

The predictability of El Ni?o?Southern Oscillation (ENSO) has been an important area of study for years. Searching for the optimal precursor (OPR) of ENSO occurrence is an effective way to understand its predictability. The CNOP (conditional nonlinear optimal perturbation), one of the most effective ways to depict the predictability of ENSO, is adopted to study the optimal sea surface temperature (SST) precursors (SST-OPRs) of ENSO in the IOCAS ICM (intermediate coupled model developed at the Institute of Oceanology, Chinese Academy of Sciences). To seek the SST-OPRs of ENSO in the ICM, non-ENSO events simulated by the ICM are chosen as the basic state. Then, the gradient-definition-based method (GD method) is employed to solve the CNOP for different initial months of the basic years to obtain the SST-OPRs. The experimental results show that the obtained SST-OPRs present a positive anomaly signal in the western-central equatorial Pacific, and obvious differences exist in the patterns between the different seasonal SST-OPRs along the equatorial western-central Pacific, showing seasonal dependence to some extent. Furthermore, the non-El Ni?o events can eventually evolve into El Ni?o events when the SST-OPRs are superimposed on the corresponding seasons; the peaks of the Ni?o3.4 index occur at the ends of the years, which is consistent with the evolution of the real El Ni?o. These results show that the GD method is an effective way to obtain SST-OPRs for ENSO events in the ICM. Moreover, the OPRs for ENSO depicted using the GD method provide useful information for finding the early signal of ENSO in the ICM.     

近65年ENSO事件强度变化及时频特征研究

利用海洋尼诺指数(ONI)、南方涛动指数(SOI)和多变量ENSO指数(MEI)等ENSO特征值分析了1951年1月—2016年5月近65年ENSO事件的强度与时频特征,并将其强度划分为5个等级。结果表明:近65年共发生22次暖事件（El Ni?o）和13次冷事件（La Ni?a）;对ENSO特征值进行频次分析发现,强El Ni?o月份所占比例比强La Ni?a多;使用连续小波、交叉小波和小波相干分析得出,ENSO循环主要具有2~7 a的周期,还具有10~16 a的年代际变化。      

Seasonal variability of Indonesian rainfall in ECHAM4 simulations and in the reanalyses: The role of ENSO

Summary A study of the skill of the ECHAM version 4 atmospheric general circulation model and two reanalyses in simulating Indonesian rainfall is presented with comparisons to 30 years of rain gauge data. The reanalyses are those performed by the European Centre for Medium-Range Weather Forecasts and of the National Centers for Environmental Prediction jointly with National Center for Atmospheric Research. This study investigates the skill of the reanalyses and ECHAM4 with regard to three climate regions of Indonesia, the annual and interannual variability of rainfall and its responses to El Ni?o-Southern Oscillation (ENSO) events. The study is conducted at two spectral resolutions, T42 and T106. The skill of rainfall simulations in Indonesia depends on the region, month and season, and the distribution of land and sea. Higher simulation skills are confined to years with ENSO events. With the exception of the northwest region of Indonesia, the rainfall from June (Molucca) and July (south Indonesia) to November is influenced by ENSO, and is more sensitive to El Ni?o than La Ni?a events. Observations show that the Moluccan region is more sensitive to ENSO, receives a longer ENSO impact and receives the earliest ENSO impact in June, which continues through to December. It is found that the reanalyses and the climate model simulate seasonal variability better than monthly variability. The seasonal skill is highest in June/July/August, followed by September/October/November, December/January/February and March/April/May. The correlations usually break down in April (for monthly analysis) or in the boreal spring (for seasonal analysis). This period seems to act as a persistent barrier to Indonesian rainfall predictability and skill. In general, the performance of ECHAM4 is poor, but in ENSO sensitive regions and during ENSO events, it is comparable to the reanalyses.     

ENSO多样性研究进展

El Ni?o是热带中东太平洋异常偏暖的现象,发展过程具有显著的季节锁相特征。近年来,新形态事件更频繁发生引起了科学界广泛关注。学者们根据空间分布形态或爆发时间将ENSO事件分为两类,虽然选取标准不同,分类结果却有诸多相似点:中太平洋(Dateline、Modoki、CP、WP及SU型)El Ni?o事件发展至成熟时,正SSTA中心位于赤道太平洋中部;东太平洋(传统、EP、CT及SP型)El Ni?o发展至成熟时,正SSTA中心位于赤道东太平洋,低层西风异常更强,向东传输的距离也更远。研究结果显示,东太平洋El Ni?o比中太平洋El Ni?o持续时间更长,强度也更强;两类事件对全球气候的影响模态有很大的差异。近几十年,中太平洋El Ni?o出现频率有所增加,但其原因尚未清楚。关于两类事件生成发展和位相转换的动力原因,目前科学界普遍认为东太平洋El Ni?o是一个海盆尺度的海气耦合过程,其生消过程与温跃层的变化有紧密联系,但对中太平洋El Ni?o的动力机制尚未有统一的认识。      

Decadal Changes in Interannual Dependence of the Bay of Bengal Summer Monsoon Onset on ENSO Modulated by the Pacific Decadal Oscillation

Interannual variations of the Bay of Bengal summer monsoon (BOBSM) onset in association with El Ni?o?Southern Oscillation (ENSO) are reexamined using NCEP1, JRA-55 and ERA20C atmospheric and Hadley sea surface temperature (SST) reanalysis datasets over the period 1900?2017. Decadal changes exist in the dependence of the BOBSM onset on ENSO, varying with the Pacific Decadal Oscillation (PDO). A higher correlation between the BOBSM onset and ENSO arises during the warm PDO epochs, with distinct late (early) onsets following El Ni?o (La Ni?a) events. In contrast, less significant correlations occur during the cold PDO epochs. The mechanism for the PDO modulating the ENSO?BOBSM onset relationship is through the variations in SST anomaly (SSTA) patterns. During the warm PDO epochs, the superimpositions of the PDO-related and ENSO-related SSTAs lead to the SSTA distribution of an El Ni?o (La Ni?a) event exhibiting significant positive (negative) SSTAs over the tropical central?eastern Pacific and Indian Ocean along with negative (positive) SSTAs, especially over the tropical western Pacific (TWP), forming a strong zonal interoceanic SSTA gradient between the TWP and tropical Indian Ocean. Significant anomalous lower tropospheric easterlies (westerlies) together with upper-tropospheric westerlies (easterlies) are thus induced over the BOB, favoring an abnormally late (early) BOBSM onset. During the cold PDO epochs, however, the superimpositions of PDO-related SSTAs with El Ni?o-related (La Ni?a-related) SSTAs lead to insignificant SSTAs over the TWP and a weak zonal SSTA gradient, without distinct circulation anomalies over the BOB favoring early or late BOBSM onsets.     

Contrasting Regional Responses of Indian Summer Monsoon Rainfall to Exhausted Spring and Concurrently Emerging Summer El Ni?o Events

The inverse relationship between the warm phase of the El Ni?o Southern Oscillation(ENSO) and the Indian Summer Monsoon Rainfall(ISMR) is well established. Yet, some El Ni?o events that occur in the early months of the year(boreal spring) transform into a neutral phase before the start of summer, whereas others begin in the boreal summer and persist in a positive phase throughout the summer monsoon season. This study investigates the distinct influences of an exhausted spring El Ni?o(springtime)...     

前冬印度洋海盆一致模对ENSO衰减期华南春季降水的影响

利用逐月台站观测降水、HadISST1.1海温和ERA5大气再分析资料,研究了前冬印度洋海盆一致模（Indian Ocean Basin,IOB）对华南春季降水（SCSR）与ENSO关系的影响,并分析了IOB通过调控ENSO环流异常进而影响SCSR的可能机制。结果表明:当前冬El Ni?o（La Ni?a）与IOB暖（冷）位相同时发生时,SCSR显著增多（减少）;而当El Ni?o或La Ni?a单独发生而IOB处于中性时,SCSR并无明显多寡倾向。其原因在于,当El Ni?o与IOB暖相位并存时,前冬热带印度洋和赤道中东太平洋均为正海温异常（Sea-Surface Temperature Anomaly,SSTA）,且印度洋SSTA强度可一直维持至春季。在对流层低层,春季赤道中东太平洋的正SSTA激发出异常西北太平洋反气旋（Western North Pacific Anticyclone,WNPAC）。而热带印度洋的正SSTA在副热带印度洋激发出赤道南北反对称环流,赤道以北的东风异常有利于异常WNPAC西伸;赤道以南的西风异常与来自赤道西太平洋的东风异常在东印度洋辐合上升,气流至西北太平洋下沉,形成经向垂直环流,有利于春季WNPAC维持。在对流层高层,印度洋的正SSTA在热带印度洋上空激发出位势高度正异常,随之形成的气压经向梯度加强了东亚高空副热带西风急流,进而在华南上空形成异常辐散环流。WNPAC的西伸和加强可为华南提供充足的水汽,同时高空辐散在华南引发水汽上升运动,共同导致SCSR正异常。而若El Ni?o发生时IOB处于中性状态,El Ni?o相关的SSTA衰减较快,春季WNPAC不显著,SCSR无明显多寡趋势。      

The superior influence of Darwin Sea level pressure anomalies over ENSO as a simple drought predictor for Southern Africa

Summary The dominant climatic mode responsible for seasonal rainfall variability across central southern Africa has been well-established as ENSO. Hence, the El Ni?o signal of the equatorial Pacific has been used extensively to predict droughts in this sub-region. Although this paper acknowledges that El Ni?o influences rainfall deficits over eastern southern Africa, an earlier signal of extreme positive sea level pressure (SLP) anomalies at Darwin for the averaged March to June period (MAMJ Darwin) has proved to have a superior remote connection to droughts in the sub-region. Simple linear statistical tools including composite techniques and correlation methods have been employed on century long data sets (1901–2000) to identify the emerging paramount connection between MAMJ Darwin SLP anomalies and southern African rainfall. Both MAMJ Darwin SLP anomalies and the Zimbabwe seasonal rainfall time series are significantly correlated (above the 95% significant level) with sea surface temperature anomalies. These represent the Indian Ocean Dipole mode in the tropical Indian Ocean and ENSO in the tropical Pacific for the averaged September to December period. ‘Pure’ MAMJ Darwin (that occur in the absence of El Ni?o in the Pacific) coincide with droughts more significantly (83% hit rate) than ‘pure’ El Ni?o events (not preceded by a high MAMJ Darwin) (38% hit rate). Co-occurrences (MAMJ Darwin preceded by El Ni?o) do not only have the highest hit rate of 93% but subsequent droughts are noticeably more severe. The ‘pure’ El Ni?os however, are not only poorly related to Zimbabwe seasonal rainfall deficits, but are apparently not connected to extreme droughts of the 20^th century. Thus, MAMJ Darwin is a good simple predictor of droughts associated with or without ENSO in the Pacific. The high prediction skill of these results, especially the inherent longer lead-time than ENSO, makes MAMJ Darwin SLP anomalies an ideal additional input candidate for sub-regional drought monitoring and forecasting schemes. In this way, drought early warning and disaster preparedness activities can be enhanced over the sub-region. Authors’ addresses: D. Manatsa, W. Chingombe, H. Matsikwa, Faculty of Science, Bindura University of Science Education, P. Bag 1020, Bindura, Zimbabwe; C. H. Matarira, Department of Geography and Environmental Science, National University of Lesotho, Roma 180, Lesotho.     

Hydro-climatic variability over the Andes of Colombia associated with ENSO: a review of climatic processes and their impact on one of the Earth��s most important biodiversity hotspots

The hydro-climatic variability of the Colombian Andes associated with El Ni?o?CSouthern Oscillation (ENSO) is reviewed using records of rainfall, river discharges, soil moisture, and a vegetation index (NDVI) as a surrogate for evapotranspiration. Anomalies in the components of the surface water balance during both phases of ENSO are quantified in terms of their sign, timing, and magnitude. During El Ni?o (La Ni?a), the region experiences negative (positive) anomalies in rainfall, river discharges (average and extremes), soil moisture, and NDVI. ENSO??s effects are phase-locked to the seasonal cycle, being stronger during December?CFebruary, and weaker during March?CMay. Besides, rainfall and river discharges anomalies show that the ENSO signal exhibits a westerly wave-like propagation, being stronger (weaker) and earlier (later) over the western (eastern) Andes. Soil moisture anomalies are land-cover type dependant, but overall they are enhanced by ENSO, showing very low values during El Ni?o (mainly during dry seasons), but saturation values during La Ni?a. A suite of large-scale and regional mechanisms cooperating at the ocean?Catmosphere?Cland system are reviewed to explaining the identified hydro-climatic anomalies. This review contributes to an understanding of the hydro-climatic framework of a region identified as the most critical hotspot for biodiversity on Earth, and constitutes a wake-up call for scientists and policy-makers alike, to take actions and mobilize resources and minds to prevent the further destruction of the region??s valuable hydrologic and biodiversity resources and ecosystems. It also sheds lights towards the implementation of strategies and adaptation plans to coping with threats from global environmental change.     

Dependency of typhoon intensity and genesis locations on El Ni?o phase and SST shift over the western North Pacific

The effects of the El Ni?o-Southern Oscillation (ENSO) phase and the shifting of the ENSO sea surface temperature (SST) on the intensity of tropical cyclones (TC) have been extensively investigated in terms of TC genesis locations in the western North Pacific (WNP). To advance the hypothesis for a relation of genesis location–intensity that the TC formation location hints its intensity, two cases have been compared, which include the phase of the decaying El Ni?o turning over to La Ni?a (type I) and the phase that recovers to a neutral condition (type II). In addition, the shift of ENSO SST to the central Pacific warming (CPW) from the East Pacific warming (EPW) has been examined. The genesis potential index (GPI) and the accumulated cyclone energy have been applied to compare the differences between the ENSO phase and the TC formation location. It was apparent that ENSO influences the WNP typhoon formation location depending on the cycle of the ENSO phase. In addition, the typhoon activity was affected by the zonal shift of the El Ni?o SST. The CPW, which has maximum SST over the central Pacific, tends to have a persistently high GPI over the WNP in September–November and June–August, demonstrating that the formation locations of strong TCs significantly shift southeastward compared with the EPW having SST maximum over the eastern Pacific. CPW years revealed a distinguishable relationship between the TC formation location and the TC between the tropical depression (TD) + tropical storm (TS) and the intense typhoon of category 4?+?5.     

Resonance Effect in Interaction Between South Asian Summer Monsoon and ENSO During 1958–2018

The study has shown that the shear component of the vertical integrated kinetic energy (Ks) over the box (40oE–100oE, 0o–20oN) can be used as a measure of the intensity of the South Asian summer monsoon (SASM). Based on its value averaged between June and August, the SASM can be divided into strong and weak monsoon episodes. Between 1958 and 2018, there existed 16 (16) strong (weak) monsoon episodes. Based on the calendar year, the relationship between the SASM and the ENSO episodes can be grouped into six patterns: weak monsoon - El Ni?o (WM-EN), normal monsoon - El Ni?o (NM-EN), weak monsoon - non ENSO (WM-NE), strong monsoon - La Ni?a (SM-LN), normal monsoon - La Ni?a (NM-LN) and strong monsoon - non ENSO (SM-NE). Previous studies have suggested that the WM-EN and SM-LN patterns reflect the correlated relationship between the SASM and El Ni?o/Southern Oscillation (ENSO) events. Therefore, we name these two strongly coupled categories WM-EN and SM-LN as the resonance effect. Two important circulations, Walker circulation (WC) and zonal Asian monsoon circulation (MC), in the vertical plane are found to be not always correlated. MC is controlled by thermal gradients between the Asian landmass and the tropical Indian Ocean, while the WC associated with the ENSO event is primarily the east-west thermal gradient between the tropical South Pacific and the tropical Indian Ocean. Furthermore, the gradient directions caused by different surface thermal conditions are different. The main factor for the resonance effect is the phenomenon that the symbols of SSTA in the tropical Indian Ocean and the equatorial eastern Pacific are the same, but are opposite to that of the SSTA near the maritime continent.     

热带太平洋盐度年际变化对海表温度异常作用比较：1997/1998、2014/2015和2015/2016年El Niño事件

海洋盐度变化为研究气候变化的机制提供了一个新的视角。本文通过对比1997/1998年、2015/2016年两次强厄尔尼诺（El Ni?o）事件和2014/2015年特殊El Ni?o事件,对盐度变化及其影响海表面温度异常（SSTA）的物理过程进行了比较分析。研究表明,El Ni?o和南方涛动（El Ni?o–Southern Oscillation, ENSO）发展的强弱与热带西太平洋大范围海表层盐度异常（SSSA）及其向东扩散的差异有明显关联。1997/1998、2015/2016年赤道东太平洋SSTA的增暖,对应两次强El Ni?o事件,在发生年4月,中西太平洋海域出现了明显的负SSSA,之后东移至日期变更线以西,SSSA引发的混合层深度（MLD）变浅、障碍层厚度（BLT）变厚,导致热带中—西太平洋表层升温增强,促使了赤道中太平洋的早期变暖;2014/2015年弱El Ni?o事件虽然在发生年4月,位于赤道中西太平洋出现了负SSSA,但没有发展东移,导致BLT的增厚过程减弱,对表层温度的调制作用减弱甚至消失。三次事件对应的盐度变化过程中,水平平流和淡水通量（FWF）引起的表层强迫是影响盐度收支的主要因子,水平平流影响盐度异常的前期变化,触发事件的发生;热带太平洋西部降水引起的FWF负异常的影响最为显著,对ENSO异常信号出现后SSSA的维持起决定性作用。相比较两次强El Ni?o事件,2014/2015年El Ni?o对应的早期FWF负异常没有发展和东移,并且之后迅速减弱,导致中西太平洋盐度负趋势减缓,MLD加深,BLT变薄,促使上表层海水冷却,抑制了赤道东太平洋的早期变暖和ENSO发展。研究结果表明,盐度变化与ENSO密切相关,热带中西太平洋海域早期表层盐度变化可能可以作为SSTA的指数。特别地,SSSA在调节SSTA时,不仅影响它的强度,而且可以作为判断ENSO是否发展及其强弱的前兆因子。     

The Asymmetric Connection of SST in the Tasman Sea with Respect to the Opposite Phases of ENSO in Austral Summer

This study uses linear regression and composite analyses to identify a pronounced asymmetric connection of sea surface temperature (SST) in the Tasman Sea with the two opposite phases of El Ni?o-Southern Oscillation (ENSO) during austral summer. In El Ni?o years, the SST anomalies (SSTAs) in the Tasman Sea exhibit a dipolar pattern with weak warmth in the northwest and modest cooling in the southeast, while during La Ni?a years the SSTAs exhibit a basin-scale warmth with greater amplitude. Investigations into the underlying mechanism suggest that this asymmetry arises from a mechanism related to oceanic heat transport, specifically the anomalous Ekman meridional heat transport induced by the zonal wind stress anomalies, rather than the surface heat fluxes on the air-sea interface. Further analysis reveals that the asymmetry of oceanic heat transport between El Ni?o and La Ni?a years is driven by the asymmetric atmospheric circulation over the Tasman Sea stimulated by the asymmetric diabatic heating in the tropical Pacific between the two opposite ENSO phases.     

Influence of Intraseasonal Oscillation on the Asymmetric Decays of El Ni?o and La Ni?a

Warm and cold phases of El Nino–Southern Oscillation (ENSO) exhibit a significant asymmetry in their decay speed. To explore the physical mechanism responsible for this asymmetric decay speed, the asymmetric features of anomalous sea surface temperature (SST) and atmospheric circulation over the tropical Western Pacific (WP) in El Nino and La Nina mature-to-decay phases are analyzed. It is found that the interannual standard deviations of outgoing longwave radiation and 850 hPa zonal wind anomalies over the equatorial WP during El Nino (La Nina) mature-to-decay phases are much stronger (weaker) than the intraseasonal standard deviations. It seems that the weakened (enhanced) intraseasonal oscillation during El Nino (La Nina) tends to favor a stronger (weaker) interannual variation of the atmospheric wind, resulting in asymmetric equatorial WP zonal wind anomalies in El Nino and La Nina decay phases. Numerical experiments demonstrate that such asymmetric zonal wind stress anomalies during El Nino and La Nina decay phases can lead to an asymmetric decay speed of SST anomalies in the central-eastern equatorial Pacific through stimulating di erent equatorial Kelvin waves. The largest negative anomaly over the Nino3 region caused by the zonal wind stress anomalies during El Nino can be threefold greater than the positive Nino3 SSTA anomalies during La Nina, indicating that the stronger zonal wind stress anomalies over the equatorial WP play an important role in the faster decay speed during El Nino.     

A New Index About the Walker Circulation

The Walker circulation (WC) has always been an important issue in atmospheric science research due to the association between the WC and tropical Pacific sea surface temperature (SST), and between the WC and ENSO events. In this paper, a new index-Omega index (OMGI) - is constructed for WC characterization based on the NCEP / NCAR reanalysis data of monthly mean vertical velocity in recent 70 years (1948-2017). Results show that the OMGI can accurately depict the variation characteristics of WC on seasonal, annual and decadal time-scales. There is a significant inverse correlation between the OMGI and equatorial eastern and central Pacific SST. Meanwhile, the peak of the OMGI appears ahead of the ENSO peak, and therefore is able to reflect the SST in the equatorial Pacific. Especially, in 35 ENSO events, the peak of the OMGI appears earlier than Ni?o 3.4 index for 19 times with 2.6 months ahead on average. In 16 El Ni?o events, the peak of the OMGI occurs ahead of the El Ni?o for 9 times with 4 months ahead on average. In 19 La Ni?a events, the OMGI peak arises 10 times earlier than the La Ni?a peak, with an average of 1.4 months ahead. OMGI shows obvious leading effect and stability over ENSO events with different strengths and types of single peak and multi peaks: the peak of the OMGI keeps about 2-3 months ahead of the ENSO. Compared with other WC indexes such as UWI and SPLI, OMGI has some advantages in the ability to describe WC changes and present the probability and thetime of prediction of ENSO event peaks.     

Why Does Extreme Rainfall Occur in Central China during the Summer of 2020 after a Weak El Ni?o?

In summer 2020, extreme rainfall occurred throughout the Yangtze River basin, Huaihe River basin, and southern Yellow River basin, which are defined here as the central China (CC) region. However, only a weak central Pacific (CP) El Ni?o happened during winter 2019/20, so the correlations between the El Ni?o–Southern Oscillation (ENSO) indices and ENSO-induced circulation anomalies were insufficient to explain this extreme precipitation event. In this study, reanalysis data and numerical experiments are employed to identify and verify the primary ENSO-related factors that cause this extreme rainfall event. During summer 2020, unusually strong anomalous southwesterlies on the northwest side of an extremely strong Northwest Pacific anticyclone anomaly (NWPAC) contributed excess moisture and convective instability to the CC region, and thus, triggered extreme precipitation in this area. The tropical Indian Ocean (TIO) has warmed in recent decades, and consequently, intensified TIO basinwide warming appears after a weak El Ni?o, which excites an extremely strong NWPAC via the pathway of the Indo-western Pacific Ocean capacitor (IPOC) effect. Additionally, the ENSO event of 2019/20 should be treated as a fast-decaying CP El Ni?o rather than a general CP El Ni?o, so that the circulation and precipitation anomalies in summer 2020 can be better understood. Last, the increasing trend of tropospheric temperature and moisture content in the CC region after 2000 is also conducive to producing heavy precipitation.     

海温异常对东亚夏季风影响机理的研究进展

从短期气候预测关注的外强迫信号角度出发,回顾了国内外在海温异常对东亚夏季风和我国汛期降水影响机理方面的主要研究进展,重点评述了热带太平洋ENSO循环、热带印度洋全区一致型海温模态、热带印度洋海温异常偶极子、南印度洋偶极子和北大西洋海温三极子模态的年际变化及其对东亚夏季风年际变率的影响。从研究成果在短期气候预测业务中应用的角度,重点关注海温异常和东亚夏季风年际变率以及我国汛期降水多雨带位置的关系,总结了海温异常作为外强迫信号对我国汛期降水预测的指示意义以及汛期降水预测的难度。最后指出气候预测业务对东亚夏季风影响的机理研究和动力气候模式发展方面的需求。