Prediction skill of monthly SST in the North Atlantic Ocean in NCEP Climate Forecast System version 2

This work evaluates the skill of retrospective predictions of the second version of the NCEP Climate Forecast System (CFSv2) for the North Atlantic sea surface temperature (SST) and investigates the influence of El Niño-Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO) on the prediction skill over this region. It is shown that the CFSv2 prediction skill with 0–8 month lead displays a “tripole”-like pattern with areas of higher skills in the high latitude and tropical North Atlantic, surrounding the area of lower skills in the mid-latitude western North Atlantic. This “tripole”-like prediction skill pattern is mainly due to the persistency of SST anomalies (SSTAs), which is related to the influence of ENSO and NAO over the North Atlantic. The influences of ENSO and NAO, and their seasonality, result in the prediction skill in the tropical North Atlantic the highest in spring and the lowest in summer. In CFSv2, the ENSO influence over the North Atlantic is overestimated but the impact of NAO over the North Atlantic is not well simulated. However, compared with CFSv1, the overall skills of CFSv2 are slightly higher over the whole North Atlantic, particularly in the high latitudes and the northwest North Atlantic. The model prediction skill beyond the persistency initially presents in the mid-latitudes of the North Atlantic and extends to the low latitudes with time. That might suggest that the model captures the associated air-sea interaction in the North Atlantic. The CFSv2 prediction is less skillful than that of SSTA persistency in the high latitudes, implying that over this region the persistency is even better than CFSv2 predictions. Also, both persistent and CFSv2 predictions have relatively low skills along the Gulf Stream.     

印度洋海温异常对西北太平洋台风的影响及机制研究

基于近40 a NCEP/NCAR再分析月平均高度场、风场、涡度场、垂直速度场以及NOAA重构的海面温度（sea surface temperature,SST）资料和美国联合台风预警中心（Joint Typhoon Warning Center, JTWC）热带气旋最佳路径资料,利用合成分析方法,研究了前期春季及同期夏季印度洋海面温度同夏季西北太平洋台风活动的关系。结果表明：1）前期春季印度洋海温异常（sea surface temperature anoma1y,SSTA）尤其是关键区位于赤道偏北印度洋和西南印度洋地区对西北太平洋台风活动具有显著的影响,春季印度洋海温异常偏暖年,后期夏季,110°～180°E的经向垂直环流表现为异常下沉气流,对应风场的低层低频风辐散、高层辐合的形势,这种环流形势使得低层水汽无法向上输送,对流层中层水汽异常偏少,纬向风垂直切变偏大,从而夏季西北太平洋台风频数偏少、强度偏弱,而异常偏冷年份则正好相反。2）春季印度洋异常暖年,西北太平洋副热带高压加强、西伸;而春季印度洋异常冷年,后期夏季西北太平洋副热带高压减弱、东退,这可能是引起夏季西北太平洋台风变化的另一原因。     

Impact of Northwest Pacific anticyclone on the Indian summer monsoon region

Influence of northwest (NW) Pacific anticyclone on the Indian summer monsoon (ISM), particularly over the head Bay of Bengal and monsoon trough region, is investigated. Strong NW Pacific anticyclone during summer induces negative precipitation anomalies over the head Bay of Bengal and Gangetic Plain region. Westward extension of moisture divergence and dry moisture transport from NW Pacific associated with anticyclone (ridge) and local Hadley cell-induced subsidence are responsible for these negative precipitation anomalies. The impact is maximum when the anticyclone and Indian Ocean basin warming co-occur. This contributes significantly to year-to-year variability of ISM.     

Interannual variability of the South Pacific Ocean in observations and simulated by the NCEP Climate Forecast System,version 2

The mechanism of the South Pacific Ocean Dipole (SPOD) mode is examined, using a 50-year simulation of the Climate Forecast System, version 2 (CFSv2) and 50-year observation-based ocean–atmosphere analyses (1961–2010). It is shown that the SPOD, a sea surface temperatures (SST) seesaw between the subtropics and extratropics, is the dominant mode of the interannual variability in the South Pacific in both observations and CFSv2 simulation. CFSv2 also reproduces the seasonal phase-locking of the observed SPOD, with the anomaly pattern developing in austral spring, peaking in summer, and decaying in autumn. Composite analyses based on both observational and model data suggest that in the warm phase of SPOD, positive SST anomaly (SSTA) is initiated by weakened westerly winds over the central South Pacific in austral spring, which suppress the surface evaporative heat loss and reduce the oceanic mixed layer depth, both contributing to the SST warming. The wind-SST-mixed layer anomalies then evolve coherently over the next two seasons while the cold SSTA develops to the north. The wind perturbations are in turn a response to El Niño-Southern Oscillation (ENSO), which forces an atmospheric planetary wave train, the Pacific-South American pattern, emanating from an anomalous heat source in the tropical western Pacific. Moreover, SPOD is significantly correlated with the southern annular mode (SAM) while the latter is also significantly correlated with the ENSO index. This suggests that ENSO’s influence on the SPOD may be partially conveyed through SAM.     

印度洋海面温度对黄淮地区夏季降水影响的年代际变化研究

利用1961—2016年我国400个气象台站夏季降水资料、NCEP/NCAR再分析资料和NOAA的海面温度资料,分析了黄淮地区夏季降水与同期大气环流及前期海面温度关系的年代际变化特征。结果显示,20世纪80年代之前（后）,前期冬季太平洋海面温度发生厄尔尼诺时,黄淮地区夏季降水偏少（多）,20世纪80年代中期之前,前期冬季印度洋海面温度全区一致型模态与黄淮地区夏季降水关系较弱,但是从20世纪80年代中期开始,正相关关系明显增强。进一步研究发现,前期印度洋海面温度一致偏高时,夏季500 hPa高度场上,副热带高压明显增强,这种相关关系20世纪80年代中期之后明显强于20世纪80年代之前,海平面气压场上,高相关区在20世纪80年代之后呈现明显的南方涛动模态,印度洋海面温度通过影响决定黄淮地区夏季降水异常的大气环流关键区域,使得其与黄淮地区夏季降水的关系随着年代际变化增强。     

厄尔尼诺年西北太平洋异常反气旋的年际变化特征及其影响

基于1901-2000年多种海-气资料,分析了厄尔尼诺成熟年冬季-初夏西北太平洋异常反气旋（WNPAC）的年际变化特征及其对东亚气候的影响。结果表明,无论是厄尔尼诺事件成熟期的冬季还是次年的春季和初夏,WNPAC的年际变化主要存在两个空间变化型,即反映其强度变化的经验正交函数分解第1模态和反映其位置变化的第2模态。厄尔尼诺成熟年冬季WNPAC强度不仅与赤道中东太平洋海温异常有关,而且与太平洋西部（WP）型遥相关的强度有关,而其位置的变化则主要与西北太平洋局地海温异常以及北极涛动（AO）有关;次年春季,WNPAC的强度除了与赤道中东太平洋海温异常和太平洋西部型遥相关存在显著相关外,还与赤道大西洋海温异常有关,而其位置的变化则主要与西北太平洋局地海温异常和太平洋西部型遥相关有关;次年初夏,WNPAC强度主要与西北印度洋和西南印度洋的海温异常以及东亚-太平洋（EAP）型遥相关的强度有关。进一步分析表明,成熟年冬季-初夏WNPAC的强度和位置的变化均可对东亚地区降水异常分布产生影响,这对预测厄尔尼诺事件发生后冬季及后期春、夏季节东亚地区降水异常分布具有一定的指示意义。此外,次年初夏,WNPAC强度变化与西北太平洋台风发生频数存在显著负相关,即WNPAC越强,台风发生的频数越少,反之亦然。     

夏季热带北大西洋海温异常对西北太平洋热带气旋生成的影响

利用1979—2012年西北太平洋热带气旋最佳路径资料,Hadley中心的海温资料和NCEP/NCAR再分析资料等,研究了夏季(6—10月)热带北大西洋海温异常与西北太平洋热带气旋(Tropical Cyclone,TC)生成的关系及其可能机制。结果表明,夏季热带北大西洋海温异常与同期西北太平洋TC生成频次之间存在显著的负相关关系。热带北大西洋海温的异常增暖可产生一对东—西向分布的偶极型低层异常环流,其中气旋性异常环流位于北大西洋/东太平洋地区,反气旋异常环流位于西北太平洋地区。该反气旋环流异常使得TC主要生成区的对流活动受到抑制、低层涡度正异常、中低层相对湿度负异常、中层下沉气流异常,这些动力/热力条件均不利于TC生成。此外,西北太平洋地区低层涡旋动能负异常,同时来自大尺度环流的涡旋动能的正压转换也受到抑制,不能为TC的生成和发展提供额外能量源。反之亦然。     

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     

Local intraseasonal air-sea relationship over the North Indian Ocean and western North Pacific during the spring-to-summer transition

该工作研究了1998-2013年春夏转换期间的两种主要模态的局地季节内海气相互作用。大气要素场和海表热通量在高频尺度上(10-20天)显示出更大的变率。在北印度洋,南海和菲律宾海,30-60天的海温和海表热通量的显著相关区域更大,并且相关系数更高。结果表明在两个时间尺度上在北印度洋,南海和西北太平洋存在强烈的局地海气相互作用,然而海气相互作用的强度取决于区域和要素的选取方式。     

Subseasonal forecast skills and biases of global summer monsoons in the NCEP Climate Forecast System version 2

Subseasonal forecast skills and biases of global summer monsoons are diagnosed using daily data from the hindcasts of 45-day integrations by the NCEP Climate Forecast System version 2. Predictions for subseasonal variability of zonal wind and precipitation are generally more skillful over the Asian and Australian monsoon regions than other monsoon regions. Climatologically, forecasts for the variations of dynamical monsoon indices have high skills at leads of about 2 weeks. However, apparent interannual differences exist, with high skills up to 5 weeks in exceptional cases. Comparisons for the relationships of monsoon indices with atmospheric circulation and precipitation patterns between skillful and unskillful forecasts indicate that skills for subseasonal variability of a monsoon index depend partially on the degree to which the observed variability of the index attributes to the variation of large-scale circulation. Thus, predictions are often more skillful when the index is closely linked to atmospheric circulation over a broad region than over a regional and narrow range. It is also revealed that, the subseasonal variations of biases of winds, precipitation, and surface temperature over various monsoon regions are captured by a first mode with seasonally independent biases and a second mode with apparent phase transition of biases during summer. The first mode indicates the dominance of overall weaker-than-observed summer monsoons over major monsoon regions. However, at certain stages of monsoon evolution, these underestimations are regionally offset or intensified by the time evolving biases portrayed by the second mode. This feature may be partially related to factors such as the shifts of subtropical highs and intertropical convergence zones, the reversal of biases of surface temperature over some monsoon regions, and the transition of regional circulation system. The significant geographical differences in bias growth with increasing lead time reflect the distinctions of initial memory capability of the climate system over different monsoon regions.     

Upscale feedback of high-frequency winds on seasonal SST change over the tropical western North Pacific during boreal summer

Climate Dynamics - Present study compares the contribution of low-frequency (> 90-day) and high-frequency (< 90-day) variation-related latent heat flux (LHF) to seasonal...     

MJO prediction in the NCEP Climate Forecast System version 2

The Madden–Julian Oscillation (MJO) is the primary mode of tropical intraseasonal climate variability and has significant modulation of global climate variations and attendant societal impacts. Advancing prediction of the MJO using state of the art observational data and modeling systems is thus a necessary goal for improving global intraseasonal climate prediction. MJO prediction is assessed in the NOAA Climate Forecast System version 2 (CFSv2) based on its hindcasts initialized daily for 1999–2010. The analysis focuses on MJO indices taken as the principal components of the two leading EOFs of combined 15°S–15°N average of 200-hPa zonal wind, 850-hPa zonal wind and outgoing longwave radiation at the top of the atmosphere. The CFSv2 has useful MJO prediction skill out to 20 days at which the bivariate anomaly correlation coefficient (ACC) drops to 0.5 and root-mean-square error (RMSE) increases to the level of the prediction with climatology. The prediction skill also shows a seasonal variation with the lowest ACC during the boreal summer and highest ACC during boreal winter. The prediction skills are evaluated according to the target as well as initial phases. Within the lead time of 10 days the ACC is generally greater than 0.8 and RMSE is less than 1 for all initial and target phases. At longer lead time, the model shows lower skills for predicting enhanced convection over the Maritime Continent and from the eastern Pacific to western Indian Ocean. The prediction skills are relatively higher for target phases when enhanced convection is in the central Indian Ocean and the central Pacific. While the MJO prediction skills are improved in CFSv2 compared to its previous version, systematic errors still exist in the CFSv2 in the maintenance and propagation of the MJO including (1) the MJO amplitude in the CFSv2 drops dramatically at the beginning of the prediction and remains weaker than the observed during the target period and (2) the propagation in the CFSv2 is too slow. Reducing these errors will be necessary for further improvement of the MJO prediction.     

Prediction and mechanistic analysis of May precipitation in North China based on April Indian Ocean SST and the Northwest Pacific Dipole

North China May precipitation (NCMP) accounts for a relatively small percentage of annual total precipitation in North China, but its climate variability is large and it has an important impact on the regional climate and agricultural production in North China. Based on observed and reanalysis data from 1979 to 2021, a significant relationship between NCMP and both the April Indian Ocean sea surface temperature (IOSST) and Northwest Pacific Dipole (NWPD) was found, indicating that there may be a link between them. This link, and the possible physical mechanisms by which the IOSST and NWPD in April affect NCMP anomalies, are discussed. Results show that positive (negative) IOSST and NWPD anomalies in April can enhance (weaken) the water vapor transport from the Indian Ocean and Northwest Pacific to North China by influencing the related atmospheric circulation, and thus enhance (weaken) the May precipitation in North China. Accordingly, an NCMP prediction model based on April IOSST and NWPD is established. The model can predict the annual NCMP anomalies effectively, indicating it has the potential to be applied in operational climate prediction.摘要尽管华北区域五月降水 (NCMP) 占华北区域年总降水量的比率较少, 但是其气候变率较大, 对华北区域气候和农业生产等具有重要影响. 基于观测和再分析资料, 发现NCMP与前期四月的印度洋海温 (IOSST) 和西北太平洋偶极子 (NWPD) 具有显著关系, NCMP可能受到IOSST和NWPD的协同影响. 进一步分析表明, 前期四月暖 (冷) 的IOSST和正 (负) 位相的NWPD能够分别通过调节印度洋和西北太平洋区域的局地环流增强 (减弱) 从印度洋和西北太平洋向华北区域输送的水汽, 进而增强 (减弱) NCMP. 最后基于四月IOSST和NWPD构建了NCMP异常的预测模型, 后报检验显示该模型对NCMP异常具有较好的预测能力.     

The Relationship Between Indian Ocean SST and Tropical Cyclone Genesis Frequency over North Indian Ocean in May

Tropical cyclone (TC) activities in the North Indian Ocean (NIO) peak in May during the pre-monsoon period, but the TC frequency shows obvious inter-annual variations. By conducting statistical analysis and dynamic diagnosis of long-term data from 1948 to 2016, the relationship between the inter-annual variations of Indian Ocean SST and NIO TC genesis frequency in May is analyzed in this paper. Furthermore, the potential mechanism concerning the effect of SST anomaly on TC frequency is also investigated. The findings are as follows: 1) there is a broadly consistent negative correlation between NIO TC frequency in May and SST in the Indian Ocean from March to May, with the key influencing area located in the southwestern Indian Ocean (SWIO); 2) the anomalies of SST in SWIO (SWIO-SST) are closely related to a teleconnection pattern surrounding the Indian Ocean, which can significantly modulate the high-level divergence, mid-level vertical motion and other related environmental factors and ultimately influence the formation of TCs over the NIO; 3) the increasing trend of SWIO-SST may play an essential role in the downward trend of NIO TC frequency over the past 69 years.     

基于ConvLSTM的西北太平洋海表温度中短期预报

尽管海表温度(Sea Surface Temperature,SST)短期变化较小,但这种变化对海洋涡旋、海洋锋以及热带气旋的发生发展仍有着重要的影响,因此短期SST预报意义重大,且对预报精度的要求较高。本文基于ConvLSTM的深度学习模型,利用SST和温度平流双预报因子对西北太平洋划定区域内SST进行7 d的连续预报,将其结果与仅使用SST预报因子ConvLSTM以及混合坐标海洋模型(HYbrid Coordinate Ocean Model,HYCOM)的预报结果分别进行了对比。结果表明,在7 d的预报时效内,温度平流预报因子的加入可使得ConvLSTM模型预报技巧大幅提升,明显优于HYCOM模式。此外,本文将预报时效进一步延长至30 d,对模型在不同季节的预报能力进行了分析,发现ConvLSTM模型在春、秋季(夏、冬季)的预报效果相对较好(差)。     

The impact of ENSO periodicity on North Pacific SST variability

The periodicity of ENSO in nature varies. Here we examine how changes in the frequency of ENSO impacts remote teleconnections in the North Pacific. The numerical experiments presented here are designed to simulate perfectly periodic ENSO in the tropical Pacific, and to enable the air–sea interaction in other regions (i.e., the North Pacific) via a simple mixed layer ocean model. The temporal evolution and spatial structure of the North Pacific SST teleconnection patterns are relatively insensitive to the frequency of ENSO, but the amplitude of the variability is sensitive. Specifically, the 2-year period ENSO experiment (P2) shows weak event-by-event consistency in the ENSO response mature pattern. This is because there is not enough time to damp the previously forced ENSO teleconnections (i.e., 1 year earlier). The 4-year period ENSO experiment (P4) has 1 year damping time before a successive ENSO event matures, so the structure of the response pattern is stably repeated. However, the event-by-event variance of anomaly magnitude, specifically responding to El Niño, is still larger than that in the 6-year ENSO experiment (P6), which has 2-year damping time between consecutive ENSO events. In addition, we tested whether the variability due to tropical remote forcing is linearly independent of the extratropical local variability. Statistical tests indicate that tropical remote forcing can constructively or destructively interfere with local variability in the North Pacific. Lastly, there is a non-linear rectification of the ENSO events that can be detected in the climatology.     

Spring Indian Ocean-western Pacific SST contrast and the East Asian summer rainfall anomaly

studying the relationship between SST in the tropical Indian Ocean （TIO）, tropical western Pacific （TWP）, and tropical eastern Pacific （TEP） and East Asian summer rainfall （EASR）, using data provided by NOAA/OAR/ESRL PSD and the National Climate Center of China for the period 1979-2008, an index, SSTDI, was defined to describe the SST difference between the TIO and TWP. In comparison with the winter ENSO, the spring SST contrast between the TIO and TWP was found to be more significantly associated with summer rainfall in East Asia, especially along the EASR band and in Northeast China. This spring SST contrast can persist into summer, resulting in a more significant meridional teleconnection pattern of lower-tropospheric circulation anomalies over the western North Pacific and East Asia. These circulation anomalies are dynamically consistent with the summer rainfall anomaly along the EASR band. When the SSTDI is higher （lower） than normal, the EASR over the Yangtze River valley, Korea, and central and southern Japan is heavier （less） than normal. The present results suggest that this spring SST contrast can be used as a new and better predictor of EASR anomalies.