South Asian High and Asian-Pacific-American Climate Teleconnection

Growing evidence indicates that the Asian monsoon plays an important role in affecting the weather and climate outside of Asia. However, this active role of the monsoon has not been demonstrated as thoroughly as has the variability of the monsoon caused by various impacting factors such as sea surface temperature and land surface. This study investigates the relationship between the Asian monsoon and the climate anomalies in the Asian-Pacific-American （APA） sector. A hypothesis is tested that the variability of the upper-tropospheric South Asian high （SAH）, which is closely associated with the overall heating of the large-scale Asian monsoon, is linked to changes in the subtropical western Pacific high （SWPH）, the midPacific trough, and the Mexican high. The changes in these circulation systems cause variability in surface temperature and precipitation in the APA region. A stronger SAH is accompanied by a stronger and more extensive SWPH. The enlargement of the SWPH weakens the mid-Pacific trough. As a result, the southern portion of the Mexican high becomes stronger. These changes are associated with changes in atmospheric teleconnections, precipitation, and surface temperature throughout the APA region. When the SAH is stronger, precipitation increases in southern Asia, decreases over the Pacific Ocean, and increases over the Central America. Precipitation also increases over Australia and central Africa and decreases in the Mediterranean region. While the signals in surface temperature are weak over the tropical land portion, they are apparent in the mid latitudes and over the eastern Pacific Ocean.

Subseasonal features of the Asian summer monsoon in the NCEP climate forecast system

The operational climate forecast system （CFS） of the US National Centers for Environmental Prediction provides climate predictions over the world, and CFS products are becoming an important source of information for regional climate predictions in many Asian countries where monsoon climate dominates. Recent studies have shown that, on monthly-to-seasonal time-scales, the CFS is highly skillful in simulating and predicting the variability of the Asian monsoon. The higher-frequency variability of the Asian summer monsoon in the CFS is analyzed, using output from a version with a spectral triangular truncation of 126 waves in horizontal and 64 sigma layers in vertical, focusing on synoptic, quasi-biweekly, and intraseasonal time-scales. The onset processes of different regional monsoon components were investigated within Asia. Although the CFS generally overestimates variability of monsoon on these time-scales, it successfully captures many major features of the variance patterns, especially for the synoptic timescale. The CFS also captures the timing of summer monsoon onsets over India and the Indo-China Peninsula. However, it encounters difficulties in simulating the onset of the South China Sea monsoon. The success and failure of the CFS in simulating the onset of monsoon precipitation can also be seen from the associated features of simulated atmospheric circulation processes. Overall, the CFS is capable of simulating the synoptic-to-intraseasonal variability of the Asian summer monsoon with skills. As for seasonal-tointerannual time-scales shown previously, the model is expected to possess a potential for skillful predictions of the high-frequency variability of the Asian monsoon.

Impacts of Land Process on the Onset and Evolution of Asian Summer Monsoon in the NCEP Climate Forecast System

Impacts of land models and initial land conditions （ICs） on the Asian summer monsoon, especially its onset, were investigated using the NCEP Climate Forecast System （CFS）. Two land models, the Oregon State University （OSU） land model and the NCEP, OSU, Air Force, and Hydrologic Research Laboratory （Noah） land model, were used to get parallel experiments NCEP/Department of Energy （DOE） Global Reanalysis 2 System （GLDAS）. The experiments also used land ICs from the （GR2） and the Global Land Data Assimilation Previous studies have demonstrated that, a systematic weak bias appears in the modeled monsoon, and this bias may be related to a cold bias over the Asian land mass. Results of the current study show that replacement of the OSU land model by the Noah land model improved the model＇s cold bias and produced improved monsoon precipitation and circulation patterns. The CFS predicted monsoon with greater proficiency in E1 Nifio years, compared to La Nifia years model in monsoon predictions for individual years. and the Noah model performed better than the OSU These improvements occurred not only in relation to monsoon onset in late spring but also to monsoon intensity in summer. Our analysis of the monsoon features over the India peninsula, the Indo-China peninsula, and the South Chinese Sea indicates different degrees of improvement. Furthermore, a change in the land models led to more remarkable improvement in monsoon prediction than did a change from the GR2 land ICs to the GLDAS land ICs.

The Dynamics of ENSO Anomaly as Revealed in Ensemble Climate Simulations-Impact of Mean Stationary Wave

A series of climate ensemble experiments using the climate model from National Centers for Environmental Prediction (NCEP) were performed to exam impact of sea surface temperature (SST) on dynamics of El-Nino/South-crn Oscillation (ENSO).A specific question addressed in this paper is how important the mean stationary wave influences anomalous Rossby wave trains or teleconnection patterns as often observed during ENSO events.Evidences from those ensemble simulations argue that ENSO anomalies,especially over Pacific-North America (PNA) region,appear to be a result of modification for climatological mean stationary wave forced by persistent tropical SST anomalies Therefore,the role of SST forcing in maintaining climate basic state is emphasized.In this argument,the interaction between atmospheric internal dynamics and external forcing,such as SST is a key element to understand and ultimately predict ENSO.

Ocean and climate sciences taking grand challenges and extraordinary opportunities:introduction to COAA 2007 special sections

In this universe, the earth is a unique breeding ground for living beings. It bares the evolutions, sustains the survival and enriches the development of human species. The studies of history have indicated that minute changes of the earth＇ s environment may have caused great impacts on the vulnerable balanced ecological systems, endanger the living conditions for human beings and associated cultures. Thus, understanding the nature＇s governing rules of the earth, and clarifying its fundamental laws of sciences are everlasting research topics for the human pursuits.

The 2022 Extreme Heatwave in Shanghai,Lower Reaches of the Yangtze River Valley:Combined Influences of Multiscale Variabilities

In the summer of 2022,China(especially the Yangtze River Valley,YRV)suffered its strongest heatwave(HW)event since 1961.In this study,we examined the influences of multiscale variabilities on the 2022 extreme HW in the lower reaches of the YRV,focusing on the city of Shanghai.We found that about 1/3 of the 2022 HW days in Shanghai can be attributed to the long-term warming trend of global warming.During mid-summer of 2022,an enhanced western Pacific subtropical high(WPSH)and anomalous double blockings over the Ural Mountains and Sea of Okhotsk,respectively,were associated with the persistently anomalous high pressure over the YRV,leading to the extreme HW.The Pacific Decadal Oscillation played a major role in the anomalous blocking pattern associated with the HW at the decadal time scale.Also,the positive phase of the Atlantic Multidecadal Oscillation may have contributed to regulating the formation of the double-blocking pattern.Anomalous warming of both the warm pool of the western Pacific and tropical North Atlantic at the interannual time scale may also have favored the persistency of the double blocking and the anomalously strong WPSH.At the subseasonal time scale,the anomalously frequent phases 2-5 of the canonical northward propagating variability of boreal summer intraseasonal oscillation associated with the anomalous propagation of a weak Madden-Julian Oscillation suppressed the convection over the YRV and also contributed to the HW.Therefore,the 2022 extreme HW originated from multiscale forcing including both the climate warming trend and air-sea interaction at multiple time scales.

热带大气季节内振荡(MJO)实时监测预测业务

参考目前国际上普遍认可的Wheeler和Hendon设计的MJO监测指标,设计了适合开展实时业务监测的MJO计算方法,初步在国家气候中心建立了逐日的MJO实时监测业务,通过与国外同类监测结果的比较分析表明,监测指标可以很好地描述MJO的强度和传播特征,与国外同类监测产品有很好的一致性。另外,引入了两种统计方法进行了针对MJO指数的实时预测,对预测结果的检验表明,对MJO在两周内有较好的预测技巧,其中利用滞后线性回归方法(PCL)的预测技巧要高于自回归模型(ARM)。

1991年5-7月江淮洪涝：水汽和冷空气的作用比较

对1991年5—7月江淮流域持续性暴雨的环流异常进行分析,比较造成此次洪涝的水汽输送和冷空气活动重要性。结果表明:(1)东亚大槽维持在中国东北上空发展明显,同时鄂霍次克海上空没有建立强大阻高,这种形势有利于冷空气持续向南侵袭,盛行经向环流。此外,低纬度地区,西太平洋副热带高压主体位置于常年同期相比,明显偏西、偏强,有利于暖湿气流沿副热带高压北上到达江淮流域,与北方冷空气辐合形成强降水。(2)通过定义IT和Iq分别表征温度与水汽对降水的贡献,发现此次江淮流域地区强降水是由对流层低层水汽异常增多和气温异常偏低共同造成的,作用基本相当。

The Record-breaking Mei-yu in 2020 and Associated Atmospheric Circulation and Tropical SST Anomalies

The record-breaking mei-yu in the Yangtze-Huaihe River valley(YHRV)in 2020 was characterized by an early onset,a delayed retreat,a long duration,a wide meridional rainbelt,abundant precipitation,and frequent heavy rainstorm processes.It is noted that the East Asian monsoon circulation system presented a significant quasi-biweekly oscillation(QBWO)during the mei-yu season of 2020 that was associated with the onset and retreat of mei-yu,a northward shift and stagnation of the rainbelt,and the occurrence and persistence of heavy rainstorm processes.Correspondingly,during the mei-yu season,the monsoon circulation subsystems,including the western Pacific subtropical high(WPSH),the upper-level East Asian westerly jet,and the low-level southwesterly jet,experienced periodic oscillations linked with the QBWO.Most notably,the repeated establishment of a large southerly center,with relatively stable latitude,led to moisture convergence and ascent which was observed to develop repeatedly.This was accompanied by a long-term duration of the mei-yu rainfall in the YHRV and frequent occurrences of rainstorm processes.Moreover,two blocking highs were present in the middle to high latitudes over Eurasia,and a trough along the East Asian coast was also active,which allowed cold air intrusions to move southward through the northwestern and/or northeastern paths.The cold air frequently merged with the warm and moist air from the low latitudes resulting in low-level convergence over the YHRV.The persistent warming in the tropical Indian Ocean is found to be an important external contributor to an EAP/PJ-like teleconnection pattern over East Asia along with an intensified and southerly displaced WPSH,which was observed to be favorable for excessive rainfall over YHRV.

Time-Frequency Characteristics of the Relationships Between Tropical Indo-Pacific SSTs

In this study, several advanced analysis methods are applied to understand the relationships between the Nino-3.4 sea surface temperatures （SST） and the SSTs related to the tropical Indian Ocean Dipole （IOD）. By analyzing a long data record, the authors focus on the time-frequency characteristics of these relationships, and of the structure of IOD. They also focus on the seasonal dependence of those characteristics in both time and frequency domains. Among the Nino-3.4 SST, IOD, and SSTs over the tropical western Indian Ocean （WIO） and eastern Indian Ocean （EIO）, the WIO SST has the strongest annual and semiannual oscillations. While the Nino-3.4 SST has large inter-annual variability that is only second to its annual variability, the IOD is characterized by the largest semiannual oscillation, which is even stronger than its annual oscillation. The IOD is strongly and stably related to the EIO SST in a wide range of frequency bands and in all seasons. However, it is less significantly related to the WIO SST in the boreal winter and spring. There exists a generally weak and unstable relationship between the WIO and EIO SSTs, especially in the biennial and higher frequency bands. The relationship is especially weak in summer and fall, when IOD is apparent, but appears highly positive in winter and spring, when the IOD is unimportantly weak and even disappears. This feature reflects a caution in the definition and application of IOD. The Nino-3.4 SST has a strong positive relationship with the WIO SST in all seasons, mainly in the biennial and longer frequency bands. However, it shows no significant relationship with the EIO SST in summer and fall, and with IOD in winter and spring.

The Extreme Mei-yu Season in 2020:Role of the Madden-Julian Oscillation and the Cooperative Influence of the Pacific and Indian Oceans

The middle and lower reaches of the Yangtze River in eastern China during summer 2020 suffered the strongest mei-yu since 1961.In this work,we comprehensively analyzed the mechanism of the extreme mei-yu season in 2020,with focuses on the combined effects of the Madden-Julian Oscillation(MJO)and the cooperative influence of the Pacific and Indian Oceans in 2020 and from a historical perspective.The prediction and predictability of the extreme mei-yu are further investigated by assessing the performances of the climate model operational predictions and simulations.It is noted that persistent MJO phases 1−2 during June−July 2020 played a crucial role for the extreme mei-yu by strengthening the western Pacific subtropical high.Both the development of La Niña conditions and sea surface temperature(SST)warming in the tropical Indian Ocean exerted important influences on the long-lived MJO phases 1−2 by slowing down the eastward propagation of the MJO and activating convection related to the MJO over the tropical Indian Ocean.The spatial distribution of the 2020 mei-yu can be qualitatively captured in model real-time forecasts with a one-month lead.This can be attributed to the contributions of both the tropical Indian Ocean warming and La Niña development.Nevertheless,the mei-yu rainfall amounts are seriously underestimated.Model simulations forced with observed SST suggest that internal processes of the atmosphere play a more important role than boundary forcing(e.g.,SST)in the variability of mei-yu anomaly,implying a challenge in quantitatively predicting an extreme mei-yu season,like the one in 2020.

Causes and Predictability of the 2021 Spring Southwestern China Severe Drought

In the spring of 2021,southwestern China(SWC)experienced extreme drought,accompanied by the highest seasonal-mean temperature record since 1961.This drought event occurred in the decaying phase of a La Niña event with negative geopotential height anomalies over the Philippine Sea,which is distinct from the historical perspective.Historically,spring drought over SWC is often linked to El Niño and strong western North Pacific subtropical high.Here,we show that the extreme drought in the spring of 2021 may be mainly driven by the atmospheric internal variability and amplified by the warming trend.Specifically,the evaporation increase due to the high temperature accounts for about 30%of drought severity,with the contributions of its linear trend portion being nearly 20%and the interannual variability portion being about 10%.Since the sea surface temperature forcing from the tropical central and eastern Pacific played a minor role in the occurrence of drought,it is a challenge for a climate model to capture the 2021 SWC drought beyond one-month lead times.

Simulations of Eurasian Winter Temperature Trends in Coupled and Uncoupled CFSv2

Conflicting results have been presented regarding the link between Arctic sea-ice loss and midlatitude cooling, particularly over Eurasia. This study analyzes uncoupled(atmosphere-only) and coupled(ocean–atmosphere) simulations by the Climate Forecast System, version 2(CFSv2), to examine this linkage during the Northern Hemisphere winter, focusing on the simulation of the observed surface cooling trend over Eurasia during the last three decades. The uncoupled simulations are Atmospheric Model Intercomparison Project(AMIP) runs forced with mean seasonal cycles of sea surface temperature(SST)and sea ice, using combinations of SST and sea ice from different time periods to assess the role that each plays individually,and to assess the role of atmospheric internal variability. Coupled runs are used to further investigate the role of internal variability via the analysis of initialized predictions and the evolution of the forecast with lead time.The AMIP simulations show a mean warming response over Eurasia due to SST changes, but little response to changes in sea ice. Individual runs simulate cooler periods over Eurasia, and this is shown to be concurrent with a stronger Siberian high and warming over Greenland. No substantial differences in the variability of Eurasian surface temperatures are found between the different model configurations. In the coupled runs, the region of significant warming over Eurasia is small at short leads, but increases at longer leads. It is concluded that, although the models have some capability in highlighting the temperature variability over Eurasia, the observed cooling may still be a consequence of internal variability.

Roles of Multi-Scale Disturbances over the Tropical North Pacific in the Turnabout of 1997-98 El Nino

The space-time features of major vorticity disturbances over the western North Pacific during the 1997-98 El Nino ranked as one of the strongest events on record was investigated in this study. We distinguished the different roles that these disturbances had on different timescales in causing the reversal or turnabout of the El Nino event. Remarkable differences in the various disturbances of synoptic, intraseasonal, and interannual timescales were found in the time evolution, propagation, and in their contributions to the changes in nearequatorial zonal flow, which was crucial to the demise of the warm sea surface temperature anomalies in the central-eastern Pacific. It is hypothesized that the westward-traveling synoptic and intraseasonal oscillations in the western North Pacific might be considered as a self-provided negative feedback from the El Nino and played an additional role in its reversal in comparison with other interannual internal and external forcings. In this case, the off-equatorial synoptic and intraseaonal fluctuations served as a stochastic forcing for the tropical ocean and gave rise to the aperiodicity or irregularity of the El Nino-Southern Oscillation.

Will the Historic Southeasterly Wind over the Equatorial Pacific in March 2022 Trigger a Third-year La Niña Event?

Based on the updates of the Climate Prediction Center and International Research Institute for Climate and Society(CPC/IRI)and the China Multi-Model Ensemble(CMME)El Niño-Southern Oscillation(ENSO)Outlook issued in April 2022,La Niña is favored to continue through the boreal summer and fall,indicating a high possibility of a three-year La Niña(2020-23).It would be the first three-year La Niña since the 1998-2001 event,which is the only observed three-year La Niña event since 1980.By examining the status of air-sea fields over the tropical Pacific in March 2022,it can be seen that while the thermocline depths were near average,the southeasterly wind stress was at its strongest since 1980.Here,based on a quaternary linear regression model that includes various relevant air-sea variables over the equatorial Pacific in March,we argue that the historic southeasterly winds over the equatorial Pacific are favorable for the emergence of the third-year La Niña,and both the anomalous easterly and southerly wind stress components are important and contribute~50%of the third-year La Niña growth,respectively.Additionally,the possible global climate impacts of this event are discussed.

Seasonal Phase-Locking of Peak Events in the Eastern Indian Ocean

The sea surface temperature （SST） anomaly of the eastern Indian Ocean （EIO） exhibits cold anomalies in the boreal summer or fall during E1 Nino development years and warm anomalies in winter or spring following the E1 Nino events. There also tend to be warm anomalies in the boreal summer or fall during La Nina development years and cold anomalies in winter or spring following the La Nina events. The seasonal phase-locking of SST change in the EIO associated with E1 Nino/Southern Oscillation is linked to the variability of convection over the maritime continent, which induces an atmospheric Rossby wave over the EIO. Local air-sea interaction exerts different effects on SST anomalies, depending on the relationship between the Rossby wave and the mean flow related to the seasonal migration of the buffer zone, which shifts across the equator between summer and winter. The summer cold events start with cooling in the Timor Sea, together with increasing easterly flow along the equator. Negative SST anomalies develop near Sumatra, through the interaction between the atmospheric Rossby wave and the underneath sea surface. These SST anomalies are also contributed to by the increased upwelling of the mixed layer and the equatorward temperature advection in the boreal fall. As the buffer zone shifts across the equator towards boreal winter, the anomalous easterly flow tends to weaken the mean flow near the equator, and the EIO SST increases due to the reduction of latent heat flux from the sea surface. As a result, wintertime SST anomalies appear with a uniform and nearly basin-wide pattern beneath the easterly anomalies. These SST anomalies are also caused by the increase in solar radiation associated with the anticyclonic atmospheric Rossby wave over the EIO. Similarly, the physical processes of the summer warm events, which are followed by wintertime cold SST anomalies, can be explained by the changes in atmospheric and oceanic fields with opposite signs to those anomalies described above.

Monthly Forecast of Indian Southwest Monsoon Rainfall Based on NCEP’s Coupled Forecast System

The monthly forecast of Indian monsoon rainfall during June to September is investigated by using the hindcast data sets of the National Centre for Environmental Prediction (NCEP)’s operational coupled model (known as the Climate Forecast System) for 25 years from 1981 to 2005 with 15 ensemble members each. The ensemble mean monthly rainfall over land region of India from CFS with one month lead forecast is underestimated during June to September. With respect to the inter-annual variability of monthly rainfall it is seen that the only significant correlation coefficients (CCs) are found to be for June forecast with May initial condition and September rainfall with August initial conditions. The CFS has got lowest skill for the month of August followed by that of July. Considering the lower skill of monthly forecast based on the ensemble mean, all 15 ensemble members are used separately for the preparation of probability forecast and different probability scores like Brier Score (BS), Brier Skill Score (BSS), Accuracy, Probability of Detection (POD), False Alarm Ratio (FAR), Threat Score (TS) and Heidke Skill Score (HSS) for all the three categories of forecasts (above normal, below normal and normal) have been calculated. In terms of the BS and BSS the skill of the monthly probability forecast in all the three categories are better than the climatology forecasts with positive BSS values except in case of normal forecast of June and July. The “TS”, “HSS” and other scores also provide useful probability forecast in case of CFS except the normal category of July forecast. Thus, it is seen that the monthly probability forecast based on NCEP CFS coupled model during the southwest monsoon season is very encouraging and is found to be very useful.

Advances in tropical cyclone prediction on subseasonal time scales

This review describes advances in understanding and forecasting tropical cyclone(TC)subseasonal variability during the past four years.A large effort by the scientific community has been in understanding the sources of predictability at subseasonal timescales beyond the well-known modulation of TC activity by the Madden-Julian Oscillation(MJO).In particular,the strong modulation of TC activity over the western North Pacific by the Boreal Summer Intra-Seasonal Oscillation(BSISO)has been documented.Progress has also been realized in understanding the role of tropical-extratropical interactions in improving subseasonal forecasts.In addition,several recent publications have shown that extratropical wave breaking may have a role in the genesis and development of TCs.Analyses of multi-model ensemble data sets such as the Subseasonal to Seasonal(S2S)and Subseasonal Experiment(SubX)have shown that the skill of S2S models in predicting the genesis of TCs varies strongly among models and regions but is often tied to their ability to simulate the MJO and its impacts.The skill in select models has led to an increase over the past four years in the number of forecasting centers issuing subseasonal TC forecasts using various techniques(statistical,statistical-dynamical and dynamical).More extensive verification studies have been published over the last four years,but often only for the North Atlantic and eastern North Pacific.

Interdecadal Variations of ENSO around 1999/2000

This paper discusses the interdecadal changes of the climate in the tropical Pacific with a focus on the correspond- ing changes in the characteristics of the E1 Nifio-Southern Oscillation （ENSO）. Compared with 1979-1999, the whole tropical Pacific climate system, including both the ocean and atmosphere, shifted to a lower variability regime after 1999/2000. Meanwhile, the frequency of ENSO became less regular and was closer to a white noise process. The lead time of the equatorial Pacific＇s subsurface ocean heat content in preceding ENSO decreased remarkably, in addition to a reduction in the maximum correlation between them. The weakening of the correlation and the shorten- ing of the lead time pose more challenges for ENSO prediction, and is the likely reason behind the decrease in skill with respect to ENSO prediction after 2000. Coincident with the changes in tropical Pacific climate variability, the mean states of the atmospheric and oceanic components also experienced physically coherent changes. The warm an- omaly of SST in the western Pacific and cold anomaly in the eastern Pacific resulted in an increased zonal SST gradi- ent, linked to an enhancement in surface wind stress and strengthening of the Walker circulation, as well as an in- crease in the slope of the thermocline. These changes were consistent with an increase （a decrease） in precipitation and an enhancement （a suppression） of the deep convection in the western （eastern） equatorial Pacific. Possible con- nections between the mean state and ENSO variability and frequency changes in the tropical Pacific are also dis- cussed.

Evolution of the 2015/16 El Nio and historical perspective since 1979

The 2015/16 El Nio developed from weak warm conditions in late 2014 and NINO3.4 reached 3℃ in November 2015. We describe the characteristics of the evolution of the 2015/16 El Nio using various data sets including SST, surface winds,outgoing longwave radiation and subsurface temperature from an ensemble operational ocean reanalyses, and place this event in the context of historical ENSO events since 1979. One salient feature about the 2015/16 El Nio was a large number of westerly wind bursts and downwelling oceanic Kelvin waves(DWKVs). Four DWKVs were observed in April-November 2015 that initiated and enhanced the eastern-central Pacific warming. Eastward zonal current anomalies associated with DWKVs advected the warm pool water eastward in spring/summer. An upwelling Kelvin wave(UWKV) emerged in early November 2015 leading to a rapid decline of the event. Another outstanding feature was that NINO4 reached a historical high(1.7℃), which was 1℃(0.8℃) higher than that of the 1982/83(1997/98) El Nio . Although NINO3 was comparable to that of the 1982/83 and 1997/98 El Nio , NINO1+2 was much weaker. Consistently, enhanced convection was displaced 20 degree westward, and the maximum D20 anomaly was about 1/3.1/2 of that in 1997 and 1982 near the west coast of South America.