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The climatology and variability of the January to February (JF) season in eastern Africa's (EA) precipitation are examined during the 1960–2020 period, as off‐season climate could have dire consequences, considering agricultural practices tie to the seasonal cycle of precipitation. The analysis in this study is divided into four parts. The first is the climatological background of variability during the JF season. Second, the spatiotemporal variability of the leading mode of the JF precipitation is described using an empirical orthogonal function (EOF) method. Third, anomalous atmospheric circulations linked to the variability of the JF precipitation were examined through composite analysis. Fourth, the link between JF precipitation and sea surface temperature (SST) is explored using composite and correlation analyses. The leading mode (EOF1) shows a monopole variation, with a positive anomaly in the entire region accounting for 55.1% of the total variance. EOF1 is linked to the SST anomaly (SSTA) over the tropical Indian Ocean (TIO). A warm (cool) SSTA in the TIO induces diabatic warming/adiabatic cooling (diabatic cooling/adiabatic warming). This leads to the rising (sinking) of warm and moist air (cold and dry air) from the lower to higher (higher to lower) troposphere via the ascending (descending) branch of the Walker circulation and contributes to the upper warm (cold) temperature anomaly centred at ~300 hPa. The warm (cold) anomaly is closely associated with the upper‐level westerly (easterly) and divergence (convergence) anomalies at the upper side of the warm (cold) core, coupled with ascending (descending) and deep wet (dry) anomalies below the warm (cold) core. This induces moisture convergence (divergence) and unstable (stable) conditions that favour (suppresses) precipitation over EA. Consequently, this study may facilitate the prediction of the JF precipitation and decrease in socio‐economic losses in EA. [ABSTRACT FROM AUTHOR]

The interannual relationship between the quasi‐biennial oscillation (QBO) and the South China Sea (SCS) summer monsoon intensity (SCSSMI) is investigated using the ERA5 data set in the period of 1953–2020. The QBO–SCSSMI connection is weak in El Niño‐Southern Oscillation (ENSO)‐related summer, but significant in neutral ENSO summer. In neutral ENSO summer, there is significant positive correlation between QBO and SCSSMI, that is, the SCSSMI is enhanced in the QBO westerly phase, while it is weakened in the QBO easterly phase with larger amplitude of the SCSSMI anomaly. QBO may modulate the SCSSMI in three possible ways. The QBO‐related temperature anomalies downward to the lower stratosphere directly affect the SCS monsoon convection by modulating the upper‐tropospheric stability and tropopause height over the SCS region. Moreover, the QBO‐related anomalous zonal wind downward to the lower stratosphere may regulate cross‐tropopause vertical wind shear and then modulate the convection development. Besides, the QBO‐related zonal wind and temperature signals propagating downward into the upper troposphere can affect the horizontal divergence by influencing the vertical vorticity and the geopotential via thermal forcing. Through those three processes, QBO directly affects the deep convection activities and then modulates the SCSSMI via internal dynamic adjustment. In the southern SCS, cross‐tropopause vertical wind shear makes a main contribution to deep convections, while upper‐tropospheric static stability makes a major contribution to deep convection in the northern SCS. The conclusions are also generally verified by the CMIP6 model simulation. [ABSTRACT FROM AUTHOR]

The present study reveals the impact of the interannual variability of the South China Sea monsoon trough (SCSMT) on tropical cyclone (TC) activity over the South China Sea (SCS) and the western North Pacific (WNP) from June to August during the period of 1958–2019. The SCSMT can influence TC genesis and occurrence through affecting the large‐scale environmental conditions and barotropic energy conversion. The intensification of the SCSMT can lead to enhanced low‐level vorticity and upper‐level divergence, decreased vertical wind shear, and increased midlevel humidity over the northern SCS and subtropical WNP. The strengthening of the SCSMT is accompanied by an intensification and eastward extension of the WNP monsoon trough and eastward retreat of the WNP subtropical high. All these changes associated with the SCSMT are favourable for TC genesis and occurrence over the north SCS and WNP. More TCs form over the WNP around 20° N, while higher TC occurrence frequency appears over the north SCS and WNP north of 20° N. Diagnostic results of eddy kinetic energy distribution and barotropic energy conversion provide a possible explanation for the impact of the SCSMT on TC activity. The synoptic‐scale disturbances are more active in the WNP region north of 20° N which might lead to an increase in TC occurrence frequency. While the barotropic energy conversion is intensified in the northern SCS and east of Philippines, more eddy energy converted from the mean flow is favourable for TC genesis. The meridional shear of mean zonal wind and the meridional convergence of mean meridional wind, which are related with the interannual variability of the SCSMT, primarily contribute to the barotropic energy conversion. [ABSTRACT FROM AUTHOR]

This study addresses the interannual variability and trends in weather regimes in subtropical South America and their impact over Uruguay. Weather regimes are found by applying a clustering method to the sea level pressure field in every season. We evaluate for all seasons their frequency of occurrence and transition pathways among them, their statistical relationship with global climate variability modes and their impact on precipitation and temperature in Uruguay. Overall, we find that there are five weather regimes that represent well the atmospheric dynamics over all seasons with two additional regimes in austral winter. The interannual variability in their frequency of occurrence and preferred transitions are sensitive to the phases of El Niño–Southern Oscillation and to other modes of variability. In addition, some weather regimes show positive and others negative trends in their frequency of occurrence over the last 70 years. In particular, we find that the weather regime that represents the development of a low‐pressure system over Uruguay has a negative trend (10–15% over 70 years) in all seasons except spring. [ABSTRACT FROM AUTHOR]

The occurrence of severe weather is an annual problem for much of the United States and North America and maximizes from March through June. With the increased interest in subseasonal weather forecasting, there have been attempts to anticipate the occurrence of anomalous weather on the timescale of one to 4 weeks including the occurrence of severe weather. Previous research has shown that teleconnection indices, associated with long period Rossby wave activity, or persistent large‐scale flow regimes have been useful tools in this endeavour. Here, abrupt changes over a 24–72‐hr period (10 or more units per day or 20 or more units over 3 days) in the Southern Oscillation Index (SOI) time series will be used to demonstrate that these changes can be associated with the possible occurrence of major severe weather event‐days defined as; 20 or more tornado, 155 or more wind speed events >25.9 m·s−1, or 135 or more hail diameter larger than 25.4 mm, reports over the United States one to 3 weeks in advance, especially during the March through June period. The severe weather events obtained from the archive at the Storm Prediction Center (SPC) from 1991 through 2020 were used. The results here demonstrate that more than 7 in 10 major severe weather occurrences were associated with abrupt positive and negative changes in the daily SOI when using signal detection methods. [ABSTRACT FROM AUTHOR]

The atmospheric energy is an essential property of atmosphere, mainly composed of internal, potential and latent energies, and could be changed by energy balance processes. The recent Arctic warming and wetting could have direct impacts on the internal and latent energies, and indirect impacts on energy transport and transformation. It is necessary to understand if and how the Arctic atmosphere, described by energy, has been changed under Arctic warming. This study attempts to analyse and discuss the changes of tropospheric energy over the Arctic (north of 66°N) from 1979 to 2018, including the general status, seasonal variation, long‐term trend, interannual variability and extremes. Three latest reanalysis products, ERA5, CFSR and JRA55, are used in this study, with acceptable reliability in validation with radiosonde observations. The reanalysis products exhibit high consistence in energy changes over the Arctic. The main results are obtained as (a) the Arctic troposphere has a total energy of 3.42 × 1022 J, dominated by internal energy; (b) an increase of total energy in the Arctic troposphere is found on annual basis, with a trend of 7.20 ± 5.47 × 1019 J·decade−1, and possibly enhanced in last 20 years, mainly contributed by internal energy; (c) the tropospheric energy over the Arctic had a interannual variability of 1.95 × 1020 J, mainly supported by internal and potential energies, and the extremes of high energies were gradually enhanced. Therefore, the Arctic atmosphere, described by energy, has been changed under Arctic warming, evidenced by significant increases of total energy and its components in the Arctic troposphere on annual basis. [ABSTRACT FROM AUTHOR]

Rainfall over Uruguay is characterized by large interannual variability partially due to the influence of El Niño–Southern Oscillation (ENSO), such that El Niño (La Niña) events on average lead to increased (decreased) rainfall during austral spring (October–December) particularly in the northern portion of the country. Here we document a clear shift in the late 1980s of the influence of ENSO on rainfall over Uruguay during this season. Analyses based on observed station data and a reanalysis show that before the late 1980s ENSO influence was relatively weak and only in the northern portion of the country. After the late 1980s the ENSO signal became stronger and affected both the southern and northern portions of Uruguay. Moreover, we found that this shift was mainly related to the impact of El Niño events, not of La Niña. Before the 1980s there is a negative relationship between the magnitude of El Niño events and rainfall anomalies over Uruguay, while after this date the relationship became positive so that rainfall anomalies are largest for strong El Niño events. The change in the impact of El Niño events is traced back to distinct extratropical teleconnections. To explore this, we defined a South American Teleconnection Index (ITSA), which characterizes the geopotential height anomalies over South America that determine the positive rainfall anomalies during El Niño over Uruguay. It is shown that after the late 1980s the ITSA is well connected to ENSO, while before late 1980s ITSA is more dependent on other variability modes such as the Southern Annular Mode. [ABSTRACT FROM AUTHOR]

Mitigating the impacts of droughts and floods over East Africa (EA) may largely be possible through accurate predictions of the long rains (March–May). Though the close interannual variability between the long rains, Indian and Pacific Oceans has been widely studied, Tanzania's long rain variability remains unclear. This paper examines the Tanzanian long rains dominant modes through an empirical orthogonal function (EOF) analysis, and their relationship with winter sea surface temperature (SST) for the period 1951–2018. Correlation and composite analyses were applied to explore the associated mechanisms. The first EOF (EOF1) mode is characterized by negative anomalies over Tanzania. The second EOF (EOF2) mode depicts positive (negative) anomalies over northern (southern) Tanzania. The third EOF (EOF3) mode reveals positive (negative) anomalies over eastern (western) Tanzania. EOF1 is closely related to the SST anomalies (SSTAs) in the southeastern Indian Ocean (SEIO). When the SSTAs are positive, they subsequently linked to upper (lower) level convergence (southeasterly winds) over Tanzania. Associated with this convergence entails a sinking motion, leading to a negative rainfall anomaly. The southeasterly winds indicate an early onset of the southeast monsoon, leading to normal rainfall been recorded. EOF2 is linked to the SSTAs in the central Pacific (CP). Positive SSTAs are related to reversed Walker circulation (WC) over northern Tanzania, while the upper‐level convergence was observed around southern Tanzania. This meridional dipole vertical motion pattern enhances (suppresses) rainfall anomalies. EOF3 is associated with mega El Niño–Southern Oscillation (ENSO)‐like SSTAs. The SSTAs are related to eastward shift of WC over the Indian Ocean, and zonal cell over the western Indian Ocean with rising limb is located east of Tanzania. The ascending motion enhances rainfall over eastern Tanzania. Predictions based on the SEIO, CP, and mega ENSO index may significantly support long rains prediction efforts and provide early warning. [ABSTRACT FROM AUTHOR]

There is a lack of studies elucidating the characteristics of interannual variability in different models, especially for northern Eurasia. Therefore, spatial patterns of interannual variability in summer precipitation over northern Eurasia across 15 models of Coupled Model Intercomparison Project Phase 5 (CMIP5) were investigated using empirical orthogonal function (EOF) analysis and cluster analysis. The spatial patterns of EOF1‐3 obtained from the models exhibited similar features across the models. The spatial pattern characteristics of the interannual variability of models were similar to those of the actual observations. However, their contribution rates and the order of magnitude of the EOFs in each model differ among models and from observations. Despite the similarity between the patterns of the EOF in the models and the observations, the atmospheric circulations causing the pattern differed in the models and actual observations, resulting in uncertainty in the models. This indicates that in the analysis of extreme events and their impact, the reproducibility and uncertainty of climate models with regard to interannual variability need to be adequately evaluated. In addition, similar to many climate models, the spatial pattern of interannual variability, which is different from any of EOF1‐3 for the observations, has been more strongly represented than the actual observations. Clarification of the factors governing the common but unsuitable variability across models may be expected to decrease the uncertainty of the models in terms of interannual variability. [ABSTRACT FROM AUTHOR]

The respective influences of the Indian Ocean dipole (IOD) and El Niño‐Southern Oscillation (ENSO) on the thermocline depth (represented by the 20°C isotherm depth, D20) variability in the south‐central Indian Ocean (SCIO) are investigated for the period of 1960–2020. The results show that pronounced interannual variations in D20 mainly occur in the SCIO and display a close relationship with the two tropical climate modes, with positive (negative) phase being associated with SCIO D20 deepening (shoaling). The D20 anomalies (D20A) induced by IOD and ENSO events have the same intensities but present different spatial–temporal features, as demonstrated by partial regression analysis. The IOD‐related D20A occurs during the mature phase of the event and behaves as a local response to anomalous wind stress curl induced by IOD, while the ENSO signature, dominated by the westward‐propagating oceanic Rossby waves, lasts much longer and extends further west. [ABSTRACT FROM AUTHOR]

This paper uses monthly surface air temperature (SAT) data from the Climatic Research Unit dataset and the Coupled Model Intercomparison Project phase 6 multimodel ensemble simulations to investigate the variations in the annual‐mean and four seasons SAT across the Eurasian and North Africa landmasses and the possible causes. The SAT over multiple timescales has significant seasonal and regional differences. The trend, variance of decadal variation (DV), and variance of interannual variability (IV) show that the differences between Siberia and mid‐low latitudes are much larger in winter and spring than in summer and autumn. The SAT exhibits a robust warming trend over Siberia in winter (0.21–0.42°C·decade−1) than over mid–low latitudes in summer (<0.07°C·decade−1). The cause is the combined diversity effects of greenhouse gases (GHG) and anthropogenic aerosols (AA) in four seasons over different regions. The warming trend of Siberia in winter is affected by the combination of strong warming effect by GHG and the weak cooling effect by AA, and the trend of mid–low latitudes in summer is opposite. The leading modes of the DV show a consistent spatial pattern over the whole Eurasia in four seasons while the IV show a meridional dipole pattern across 40°N except in summer. The internal climate variability is the main driver of the SAT IV and DV, but the effect of anthropogenic activity and natural forcing may reinforce the IV and DV in some regions. The combined role of GHG and AA forcings can significantly enlarge the DV in many parts of Eurasia, and the AA and natural forcing have significant effect on the IV in the high latitudes. [ABSTRACT FROM AUTHOR]

Previous studies have suggested that interocean interactions can improve our understanding of climate's impacts on extreme weather events. The present study investigates how the interaction between the tropical eastern Pacific and Atlantic Oceans impacts extreme weather events over Tanzania. Results illustrate that the proposed Trans‐Atlantic‐Pacific Ocean Dipole index (TAPODI) is significantly correlated with the short rains over Tanzania. The wavelet coherence analysis showed that the variability of the short rains was influenced principally by the TAPODI on the interannual (1–5 years) scale. The association between TAPODI and short rains is in‐phase, with the phase of TAPODI lags that of the short rains by 90°. The positive phase of the TAPODI is associated with the advection of moist air from the Congo basin that favours ascent motion and leads to above‐normal rainfall over Tanzania. The negative phase of the TAPODI is related to sinking motions, accompanied by divergent winds at low level that block winds from the Congo basin and the western Indian Ocean, hence leads to below‐normal rainfall over Tanzania. Therefore, the study revealed the interactions between the tropical Pacific and Atlantic Ocean, which will improve the climate predictability and prediction of the short rains over Tanzania. [ABSTRACT FROM AUTHOR]

The Accumulated Winter Season Severity Index (AWSSI) describes the “harshness” of the cold season. This work utilizes AWSSI to explore the interannual teleconnective influences that shaped winter severity between 1951 and 2020 across the Midwest U.S. Annual AWSSI total variability clusters into four distinct geographic areas, each shaped by different teleconnective influences. The regions are closely delineated by the main river basins and include the Ohio (Region 1), upper Mississippi and Missouri (Region 2), central Mississippi and southern Missouri (Region 3), and the Great Lakes (Region 4). Of outmost influence in shaping severe winters across the Midwest have been synoptic flow modifications associated with significant meandering and southern displacement of the Polar jet over the conterminous United States and/or the Midwest that depress near-surface air temperatures and increase the likelihood of snow fall and/or snow depth. These flows are most closely related to the combined influence of the negative (positive) phases of the NAO (PNA and PDO) in Region 1, PNA− in Region 2, NAO− in Region 3, and the negative (positive) phase of the PNA (TNH) across Region 4. Mild winters have been shaped by more zonal flows with the Polar jet positioned north of the Midwest and bringing milder temperatures, less snowfall, and reduced winter severity to the impacted areas. Such modifications have been related to TNH−, PNA+, and NAO+ across Regions 1, 2, and 3, respectively. Mild winters have been shaped by TNH− and to a limited extent NAO+, across Region 4. [ABSTRACT FROM AUTHOR]

Previous studies in this region focused on interannual and interdecadal precipitation variability during Dec–Feb (summer), relating the El Niño (EN) phase to droughts. Many studies examine this variability over different parts of the Altiplano, and mountainous terrain is well‐known for producing complex climate signals as well as variability over relatively small regions. Studies that have examined climate variability over the Altiplano suggest that the El Nino Southern Oscillation (ENSO) influence is complex. The monthly precipitation, maximum and minimum temperature from 1979 to 2017 are used to examine how ENSO and PDO (Pacific decadal oscillation) influence climate conditions. Power spectra analysis determined the periodicities, and analysis of variance was applied to standardized anomalies of the three variables to evaluate PDO and ENSO as variance factors. Frequency distributions were calculated by PDO and ENSO phases, and the differences were tested using the Chi‐square test. Our results suggest a PDO and ENSO influence, displaying more dry anomalies during DJF for EN/PDO(+), in agreement with a northward position of the Bolivian high. More wet anomalies during SON (Sep–Oct–Nov) in Neutral (NEU)/PDO(+) due to an upper‐level weak westerly flow during PDO(+). For maximum temperature, a reduction in cold anomalies during EN/PDO(−) was observed along with greater warm anomalies in JJA (Jun–Jul–Aug), SON and MAM (Mar–Apr–May). Similar results were observed for NEU phase, due to a greater occurrence of high pressures centres over the region during the PDO(−). Minimum temperature results displayed more variation within stations and a less clear response to ENSO compared to PDO. The research found a clear influence of PDO in the regional climatology associated with the influence of the upper‐level jet stream using the NCEP‐NCAR composites. This provided a better understanding of these two phenomena, which would allow the production of seasonal forecasts based on ENSO and PDO using both surface and upper‐level information. [ABSTRACT FROM AUTHOR]

Interannual variability of summer (June–August) tropical cyclones (TCs) in the northwestern North Pacific (NP) region (120°–135°E, 20°–35°N) and El Niño‐Southern Oscillation (ENSO) exhibit four asymmetric ENSO‐TC relationships. The major relationship types are El Niño‐enhanced and La Niña‐suppressed, while the minor relationship types are El Niño‐suppressed and La Niña‐enhanced. The major modulatory processes for the El Niño‐enhanced type feature an elongated anomalous cyclone extending from the tropical western NP (WNP) northwestward toward the northwestern NP. This feature intensifies TC formation in the tropical WNP and guides them northward/northwestward increasing TC frequency in the northwestern NP. The El Niño‐suppressed type has its anomalous cyclone displacing eastward in the tropical WNP with an accompanying anomalous anticyclone across the northwestern NP. TC formation reduces in the tropical WNP and TC movement is blocked toward the northwestern NP, resulting in decreased TC frequency. The La Niña‐suppressed type features a dominant elongated anomalous anticyclone in the WNP to suppress TC formation and movement toward the northwestern NP, yielding reduced TC frequency. The La Niña‐enhanced type contains an anomalous anticyclone south of 20°N in the tropical WNP in company with an anomalous cyclone over the northwestern NP. Under this anomalous cyclone, enhanced TC formation and movement in the northwestern NP result in increased TC frequency. For all relationship types, 30–60‐day intraseasonal oscillation (ISO) anomalies feature a dominant anomalous cyclone over the WNP providing favourable conditions that guide TC movement toward its central region over the northwestern NP. Both ENSO and ISO make positive contributions that increase TC frequency in the northwestern NP for the El Niño‐enhanced and La Niña‐enhanced types. ENSO is the major factor reducing TC frequency in the El Niño‐suppressed and La Niña‐suppressed types. [ABSTRACT FROM AUTHOR]

Heat waves generally hit the Sichuan Basin (SCB) in summer, especially in July and August. This study investigates the interannual variability of mid‐summer heat wave frequency (HWF) over the SCB. The leading mode of the SCB mid‐summer HWF, which explains 48% of the total variance, exhibits an uniform spatial pattern and an increasing trend. This leading pattern mainly dominated by two factors: the mid‐to‐upper‐level height anomalies over western Europe and tropical convection anomalies. On one hand, the mid‐level height anomalies over western Europe could affect the HWF over most parts of the SCB by inducing an eastward‐propagating atmospheric teleconnection wave train to northeastern Asia, which causes a northwestward shifted western North Pacific subtropical high, leading to significant subsidence anomalies over the SCB, hence induces less cloudiness and below‐normal precipitation in‐situ. Consequently, there are increased surface solar radiation and decreased soil moisture over the region, favouring the occurrence of HWs there. On the other hand, the enhanced Philippine Sea convection and suppressed tropical eastern Indian Ocean convection excite an anomalous lower‐tropospheric cyclone dominant over the western North Pacific and southern China, which could weaken the transport of water vapour from the tropic oceans to the northeastern SCB and then contributes to decreased precipitation, favouring a higher HWF. [ABSTRACT FROM AUTHOR]

Most of the rainfall over South Asia occurs from June to September, and its variation is strongly related to the fluctuation of sea surface temperature (SST) on a global scale. In this study, we investigate how and to what degree the interannual variance (IAV) of South Asian summer monsoon (SASM) rainfall is regulated by SST anomalies through comparing the multi‐model simulations forced by observed interannual SST variation and climatological SST just with annual cycle. Results indicate that the interannual change in global SST can explain nearly 50% of the IAV of SASM rainfall, in which dynamic process (via atmospheric circulation), instead of thermodynamic process (via water‐vapour effect), plays a dominated role. SST influences the IAV of SASM rainfall mainly through changing the lower‐ and upper‐level atmospheric circulations, namely, the large‐scale vertical wind shears. Additional sensitivity experiments with the NCAR Community Atmosphere Model show that the effect of tropical Pacific SST (~24.0%) is more important than that of tropical Indian Ocean SST (~15.3%) on regulating the IAV of SASM rainfall. [ABSTRACT FROM AUTHOR]

For the first time, the spatial and temporal variability of rainfall in the Republic of Djibouti is investigated using data from 14 weather stations over the period 1946–2017. Due to limited data availability, high‐resolution long‐term satellite rainfall products (CHIRPS, PERSIANN‐CDR, TAMSATv3, ARC2) and ERA5 reanalysis also contribute to document time–space rainfall variability at monthly, seasonal and annual scales. Principal component analysis identifies two spatially coherent regions of rainfall variability in the east (coastal zone) and the west (inland zone) of the country. Annual rainfall amounts are everywhere very low (60–300 mm), but with contrasted regimes. At seasonal scale, the highest rainfall amounts in the eastern part of the country are found between October–December (OND) and March–May (MAM), while July–September is the wettest season in the western part. The monthly rainfall regimes are relatively well reproduced by most products. ERA5 displays the highest monthly correlations with observations, followed by PERSIANN‐CDR and CHIRPS. Trend analysis since 1983 shows a significant decrease of rainfall during MAM which is in agreement with other parts of East Africa. On the other hand, nonsignificant decreasing trends are observed in January–February (JF) and OND. Only June–September (JJAS) revealed a nonsignificant increasing trend, but it follows a prior drying trend since the 1950s. The impact of large‐scale background climate on rainfall variability is assessed with focus given on El Niño Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). Rainfall variability during OND shows a significant correlation with IOD, while in JJAS it is significantly negatively correlated with ENSO. In general, ERA5, CHIRPS and PERSIANN datasets are best able to reproduce rainfall patterns in Djibouti and suitable for further analysis. The fact that the interannual and decadal‐scale rainfall variations in Djibouti show large‐scale teleconnections with global sea‐surface temperature fields, as demonstrated in this study, provides good prospects for the prediction of rainfall variations at a range of different temporal scales. [ABSTRACT FROM AUTHOR]

The present study examines the ability of Coupled Model Inter‐comparison Project phase 6 (CMIP6) models in representing the dominant modes of tropical Indian Ocean (TIO) sea surface temperature (SST) variability on the interannual and decadal time scale. Historical simulations from 27 CMIP6 models are assessed against Extended Reconstructed SST over the period of 1854 to 2014. Spectrum analysis reveals that many models reproduce interannual and decadal variability of TIO SST but underestimate the amplitude of variability with some disparity in the periodicity. All models can reproduce the dominant basin‐wide mode of interannual and decadal variability of TIO SST reasonably well. Skill score analysis of TIO SST variability reveals that KACE‐1‐0‐G has highest skill, followed by FGOALS‐f3‐L, EC‐Earth3‐Veg‐LR, ACCESS‐ESM1‐5, CanESM5‐CanOE on the interannual timescale and FGOALS‐f3‐L, CanESM5‐CanOE, KACE‐1‐0‐G and CanESM5, respectively, showed highest skills for decadal variability. It is found that variations in radiation and latent heat flux are primarily responsible for interannual variability in TIO SST, the basin‐wide warming, in the observations. Taylor diagram analysis reveals that all the models exhibit better skill for the radiative flux; however, skill for the latent heat and momentum flux varies from model to model. It is important to note that the models in which the latent heat flux and zonal wind are better represented have produced better TIO SST variability compared to other models. A higher discrepancy in latent heat and zonal momentum flux leads to improper wind‐evaporation‐SST and wind‐circulation‐SST feedback, which in turn restricts the model skill. Besides, model that has realistic central and eastern Pacific SST variability show better skill for TIO SST variability in both interannual and decadal time scales. The present study advocates that better representation of latent heat flux and zonal wind in coupled models is important for the accurate simulation of interannual and decadal variability in TIO SST. [ABSTRACT FROM AUTHOR]

Utilizing the self‐organizing map (SOM), a type of artificial neural network, a new classification of the climate of Japan is proposed. The SOM is applied on the monthly mean surface air temperature (SAT) anomalies, extracted from 762 stations. Considering the strong seasonal cycle in mid‐latitudes, the classification is performed for two distinct seasons, boreal winter and boreal summer. Applied on monthly average temperature, to capture the seasonal signal, the SOM is an easily implementable interesting tool to (a) objectively capture the patterns present in the input data, and (b) identify the source of interannual variability, which is crucial for power demand forecasting. While modulated by local conditions, SAT in Japan is mainly influenced by large‐scale circulation. It is found in this study that stronger relationships exist for tropics with southern regions and for extra‐tropics with northern regions, in the seasonally oriented teleconnection patterns. In winter, the regions are organized along a north–south orientation, with a secondary west–east orientation in the central part of the country. This organization is a function of the strength of the link with the tropical Pacific and Indian Oceans sea‐surface temperatures. Changes in the speed of the westerly jet, together with the modulation of the Western Pacific (WP)‐like and Eurasian (EU)‐like patterns, are also important for SAT in Japan in winter. In summer, the main organization follows a west–east orientation. The Pacific Japan (PJ)‐like pattern plays an important role in the geographical distribution of the SAT, together with the existence of mid‐latitude and subtropical wave trains, like the Silk Road (SR)‐like pattern. [ABSTRACT FROM AUTHOR]

The potential role of low‐level circulation on the transport of atmospheric moisture towards Central America and northern South America is studied with the Dynamical Recycling Model (DRM) for the period 1981–2015. The net 2D field for advection of moisture in the DRM is estimated based on two different approaches: from the Vertically Integrated Moisture Flux (VIMF) and from a linear combination of low‐level winds (Ueff). The relative importance of low‐level wind structures is inferred from the comparison of both DRM approaches. The Ueff approach yields larger long‐term estimates of transport from the Pacific compared to the VIMF approach, largely associated with the effect of the low‐level circulation over the eastern Pacific, known as the Choco Jet (CJ). Large differences in the transport estimated by both approaches also appear to be associated with different phases of El Niño‐Southern Oscillation (ENSO), when differences in the low‐level circulation take place over the eastern Pacific and the tropical Atlantic. Similar differences are found for anomalous activity of the CJ and the Caribbean Low‐level Jet (CLLJ). The differences in the magnitude of long‐term estimates and anomalies from both 2D approaches show that details in vertical wind shear are important for the estimation of water vapour transport. However, the sign of the anomalies and correlation patterns under different circulation regimes were the same for both methods. Furthermore, with both approaches, it is found that variability in the low‐level circulation over the eastern Pacific and the tropical Atlantic is substantially related to changes in the vapour transport towards Central America and northern South America. In addition, the robustness of this signal suggests that a simple 2D tool like the DRM may be effectively used to study transport changes under different regimes, like different ENSO phases or climate change scenarios. [ABSTRACT FROM AUTHOR]

The South China Sea monsoon trough (SCSMT) presents remarkable year‐to‐year variability, which exerts great influences on the summer climate over the South China Sea and surrounding regions. The variation of SCSMT may be forced by the tropical Indo‐Pacific sea surface temperature anomalies (SSTAs). The relationship between interannual variability of the SCSMT and the SSTA in tropical Indo‐Pacific oceans experienced several interdecadal changes during 1958–2019, namely around the late 1970s, early 1990s, and mid‐2000s, respectively. In epoch 1 (1958–1977), the significant SSTA pattern associated with the weakening of the SCSMT features the warming in the eastern tropical Indian Ocean (TIO) and the Maritime Continent (MC) which contributes to interannual variation of the SCSMT via inducing anomalous local meridional circulation. In epoch 2 (1978–1992), there is strong warming in the whole Indian Ocean basin which induces southwest–northeast vertical circulation anomalies and excites warm Kelvin wave in tropospheric atmosphere, leading to the weakening of the SCSMT. The SSTA in epoch 3 (1993–2003) features significant warming in the eastern TIO and MC which influences the SCSMT via abnormal meridional circulation and Kelvin wave, and weak cooling in the central Pacific which partly contributes to the weakening of the SCSMT via Rossby wave response. The SSTA pattern in epoch 4 (2004–2014) is similar to that in epoch 3, while the warming in the MC is weaker with insignificant meridional circulation and Kelvin‐wave response, and the most significant feature is the cooling in the central to eastern Pacific (CEP) with strong Rossby wave response. These results suggest that, during different epochs, interannual variability of the SCSMT is modulated by SSTA in different key areas through different mechanisms, and the key area shifts eastward from the TIO to the CEP with time. [ABSTRACT FROM AUTHOR]

The spatial inhomogeneity of air pressure drives the atmospheric circulation. The interannual variation of the pressure can be affected by both air temperature and density. The goal of this study is to identify, for the near‐surface air, whether the variation of the density or temperature is more important in the interannual variation of the pressure. The physical relation of the pressure (P) with density (D) and temperature (T), denoted as P = D ⋅ T for simplicity, is nonlinear. To be convenient for estimating the relative importance, a normalized linear regression is used to fit the relation, which is finally written as P = AD + BT. Tests show that the linear fitting is robust; it is perfect everywhere in the globe. Because of the normalizations, the D and T are in magnitude of 1, but can be positive or negative. Derivations and calculations indicate that the coefficients A and B are both positive. What they reflect are the partial correlations of the pressure with the D and T, which are both equal to 1 across the entire field. For the normalized fitting, the A and B can be used to estimate the contributions of the D and T to the variation of the pressure. The linear fitting method can provide quantitative results for the dominance. The original nonlinear relation, due to the special form for the issue, can be used to qualitatively deduce the dominance. Comparisons suggest that the results obtained from the linear fitting are consistent with those qualitatively reached from the nonlinear physical relation. The consistency is true, no matter what the relation is between the two influencing factors. Corresponding to the negative relation between the density and temperature, there are two patterns. One is that the pressure increases with density but decreases with temperature. In this case, density dominates the variation of pressure. The second is that the pressure increases with temperature but decreases with density, and thus temperature dominates the variation of pressure. The third pattern is for the positive relation between density and temperature. In this case, density and temperature both have positive contributions to the variation of pressure, and which of them is more important can further be assessed with the linear fitting method. [ABSTRACT FROM AUTHOR]

This paper compares the characteristics of the Tropical Easterly Jet (TEJ) and upper‐level winds in six reanalysis products, compares them with soundings at seven West African locations, examines the relationship between Sahel rainfall and the TEJ, and examines factors influencing the TEJ. The jet characteristics assessed by MERRA2, NCEP 1, JRA 55, and ERA 5 are similar. CFSR and 20th Century Reanalysis are outliers in nearly every analysis, overestimating wind speeds by as much as 25 to 40% compared to other reanalyses. Over the period 1948 to 2014, the correlation between rainfall and TEJ magnitude is.72. Arguments based on observations and modelling studies provide evidence that on interannual scales changes in the TEJ are not forced by rainfall, that large‐scale factors drive the TEJ. Potential mechanisms are discussed for a causal relationship such that a strong jet leads to high rainfall. However, further modelling efforts are needed to conclusively determine whether the TEJ/Sahel rainfall link is a result of common forcing factors. The factors that appear to control jet strength include sea‐surface temperature (SST) contrast between the central equatorial Pacific and central equatorial Indian Ocean (correlation of −.64), SST contrast between the central equatorial and the southern subtropical Indian Ocean (correlation of −.39), the latitude of the shift between upper‐tropospheric easterlies and westerlies in the Southern Hemisphere (correlation of −.84 at 150 hPa), and the intensity of the Southern Hemisphere westerlies (correlation of +.52 at 200 hPa). This suggests considerable control on the TEJ by extra‐tropical circulation in the Southern Hemisphere. [ABSTRACT FROM AUTHOR]

This work investigated the simulation and future changes in East Asian summer mean rainfall, interannual variability, and associated uncertainty based on the historical and representative concentration pathway (RCP) 4.5 runs of 26 CMIP5 models. It is noted that the present‐day mean rainfall was well simulated by most of the models, though underestimation is over the Meiyu/Baiu/Changma (MBC) rainfall belt, and overestimation is in the western North Pacific (WNP). Nevertheless, it is still a challenge for the CMIP5 models to well simulate the interannual variability. The approach of the multi‐model ensemble greatly improves the credibility of model simulations, especially for the rainfall variability. The East Asian summer rainfall is projected to a relatively consistent increase in the near future, while the spatial distributions of the summer rainfall changes by the end of the 21st century are similar to those in the near future, with twice magnitude of the mean rainfall amount increase. The rainfall changes are associated with the strengthened low‐level southerly wind over East China and intensified western North Pacific high. Also, the variation of rainfall interannual variability by the end of the 21st century over East Asia is larger than that in the near future, with similar spatial patterns. It may imply that the stronger rainfall interannual variability is associated with the increased mean rainfall amount in the future. The inter‐model spreads of both the mean and interannual variability changes are larger in the low latitudes than in the high latitudes over East Asia. The signal‐noise‐ratio (SNR) is largely determined by the rainfall change (signal) in the future. Lower SNR of interannual variability than that of mean rainfall over East Asia implies that the CMIP5 models are more robust to a certain extent in simulating the summer mean rainfall change in East Asia than the change of interannual variability. [ABSTRACT FROM AUTHOR]

This study investigated the interannual variability of the frequency of extreme high‐temperature events (FEHE) during summer over eastern China and associated mechanisms. Two leading modes of interannual variability of FEHE over eastern China during summer are obtained by Empirical Orthogonal Function (EOF) analysis. The first EOF (EOF1) mode of FEHE shows a diploe pattern characterized by positive (negative) anomalies in northeastern (southern) China. The second EOF (EOF2) mode of FEHE shows a monopole pattern characterized by positive anomalies in the Yangtze River valley (YRV) region. The potential mechanisms of the two leading modes are different. The EOF1 of FEHE is modulated by the EOF1 of 500‐hPa geopotential height (Z500) over Eurasia, which exhibits a Eurasian wave train associated with the air–sea interaction over the North Atlantic. Besides, anomalous SSTs over the western tropical Pacific and associated anomalous convective activity over the Maritime continent may also exert an impact on the EOF1 of FEHE via stimulating a meridional wave train, the so‐called "Pacific‐Japan/East Asia‐Pacific" teleconnection. The EOF2 of FEHE is mainly associated with the NAO mode. [ABSTRACT FROM AUTHOR]

This study investigates the interannual variability of rainfall over the middle and lower reaches of Yangtze River Valley (MLYRV) and that over the South China Sea and Philippine Sea (SCS‐PS) during boreal summer (June–August) from 1979 to 2012. Results exhibit out‐of‐phase rainfall variations between the MLYRV and the SCS‐PS, which is closely related to the tropical zonal sea surface temperature gradient (ZSG) between the northern Indian Ocean (NIO; 5°–25°N, 60°–100°E) and the equatorial central and eastern Pacific (CEP; 5°S–5°N, 180°–130°W). The ZSG can explain as much as 40% of the total variance of the summer rainfall over the MLYRV and the SCS‐PS in the past 40 years. This is much higher than that explained by the NIO SST (24%), CEP SST (14%) and Niño‐3.4 index (16%) alone. A positive ZSG between the warm NIO and the cold CEP tends to increase rainfall over the MLYRV and decrease rainfall over the SCS‐PS, whereas a negative ZSG between the cold NIO and the warm CEP is generally favourable for rainfall over the SCS‐PS and unfavourable for rainfall over MLYRV. The close connection between the ZSG and the out‐of‐phase interannual rainfall variation over the two regions can be explained by influences of the ZSG on atmospheric circulation over East Asia. A positive ZSG induces anomalous easterlies and Walker‐like circulation in the tropics, which results in an anomalous subsidence and boundary layer divergence over the SCS‐PS. As a result, summer rainfall decreases in this region. Meanwhile, moisture transport increases due to the anomalously strong southwesterlies along the northwestern flank of the intensified western Pacific subtropical high, providing more precipitable water to the MLYRV region. In contrast, a negative ZSG induces surface westerlies and favourable environmental condition for rainfall over the SCS‐PS. Dry descending flow induced by local anomalous Hadley circulation develops over the MLYRV around 30°N, which is unfavourable for rainfall over the MLYRV. The mechanism further examined using numerical experiments. These thermal‐dynamical processes induced by the ZSG work together to cause the out‐of‐phase interannual changes of rainfall between the MLYRV and the SCS‐PS, suggesting that the ZSG is highly indicative of the interannual change of summer rainfall in the two regions. [ABSTRACT FROM AUTHOR]

Unlike in other parts of the Asian Monsoon region, the summer monsoon in the Northwestern Maritime Continent (NwMC) is relatively dry. Hence, an objective definition is needed in order to determine the arrival of the dry season in this particular sub‐region. Criteria for the objective definition of summer monsoon onset in the NwMC were defined based on reanalysis data from the National Centers for Environment Prediction (NCEP) and Global Precipitation Climatology Project (GPCP) databases. Considering the Matsuno‐Gill theoretical relationship between tropical precipitation and low‐level winds, the summer monsoon onset date can be determined objectively based on the commencement of the sustained 850‐hPa zonal winds averaged over the area from 5° to 10°N and from 100° to 115°E. The establishment of steady westerly/southwesterly winds over the NwMC region is consistent with the abrupt shift of rainfall from the equatorial region (5°S–5°N) to areas north of the equator. The onset is also associated with the northeastward propagation of anomalous convection from the western equatorial Indian Ocean on an intraseasonal time scale. In view of its representation of large‐scale circulation and objectiveness, this new monsoon onset index will provide a useful metric with which to predict NwMC summer monsoon onset and for studying the interannual variation in monsoon onset. A simple lead–lag correlation shows the monsoon onset in NwMC has a close relationship with ENSO. Generally an early (late) monsoon onset can be expected when cold (warm) ENSO event occurred in the preceding summer. [ABSTRACT FROM AUTHOR]

Atmospheric rivers (ARs) are narrow pathways of atmospheric humidity which have a critical role in the global water cycle and can affect natural phenomena such as severe storms and floods. In this article, their temporal and spatial changes in the MENA (Middle East and North Africa) region have been investigated during a period of 30 years (1984–2013). After extracting the days with ARs, their frequency and sources were identified. In order to study the formation and moisture feeding mechanism of the ARs, the jet stream and atmospheric stream maps were produced, and their sources and movement paths were identified. The results showed that each year, about 13 ARs were observed and most of them occurred in the fall and winter seasons. The major reason for the formation of ARs is jet streams which, in all selected cases, were located about 100–150 hPa above the ARs. Most ARs originated over the North Atlantic Ocean and entered Africa from Mauritania and Senegal and after passing Egypt, Saudi Arabia and Iran entered Afghanistan and Pakistan. The Red Sea is a major moisture source which feeds ARs on their path. The results showed that the ARs in the MENA region experience convergence over this region which causes these events to narrow. A large number of the countries that studied is desert, the findings can be utilized in understanding the ways ARs can be useful and shortage of water resources in the region can be compensated. [ABSTRACT FROM AUTHOR]

Temporal variability is an important feature of climate, comprising systematic variations such as the annual cycle, as well as residual temporal variations such as short‐term variations, spells and variability from interannual to long‐term trends. The EU‐COST Action VALUE developed a comprehensive framework to evaluate downscaling methods. Here we present the evaluation of the perfect predictor experiment for temporal variability. Overall, the behaviour of the different approaches turned out to be as expected from their structure and implementation. The chosen regional climate model adds value to reanalysis data for most considered aspects, for all seasons and for both temperature and precipitation. Bias correction methods do not directly modify temporal variability apart from the annual cycle. However, wet day corrections substantially improve transition probabilities and spell length distributions, whereas interannual variability is in some cases deteriorated by quantile mapping. The performance of perfect prognosis (PP) statistical downscaling methods varies strongly from aspect to aspect and method to method, and depends strongly on the predictor choice. Unconditional weather generators tend to perform well for the aspects they have been calibrated for, but underrepresent long spells and interannual variability. Long‐term temperature trends of the driving model are essentially unchanged by bias correction methods. If precipitation trends are not well simulated by the driving model, bias correction further deteriorates these trends. The performance of PP methods to simulate trends depends strongly on the chosen predictors. [ABSTRACT FROM AUTHOR]

A realistic simulation of rainfall over the Tibetan Plateau (TP) is a big challenge for both regional and global climate models. In this study, we investigate the simulations of summer rainfall over the TP from 1979 to 2010 by the Weather Research and Forecasting (WRF) model with various horizontal resolutions and cumulus schemes, with a focus on the difference in the model's skill in simulating interannual variability of rainfall between early and high summer. The WRF captures spatial pattern of climatological summer mean rainfall over the TP. However, it produces apparent wet bias, especially in southern and eastern edges of the TP. Despite this climatological bias, WRF skilfully reproduces the interannual variability of summer rainfall over the southeastern TP, where maximum rainfall is located. An increase in horizontal resolution or an appropriate cumulus scheme mostly improves the simulation of climatological mean rainfall. However, the phase of interannual variability of simulated rainfall is not sensitive to horizontal resolution and cumulus scheme. Instead, it is sensitive to mechanisms responsible for interannual variability of rainfall. WRF has a high skill in simulating the interannual variability of rainfall over the southeastern TP in July and August, but a low skill in June. The WRF's high skill is attributed to that the interannual variability of July–August rainfall is largely driven by large‐scale circulation, while its low skill for June rainfall may be ascribed to the overestimation of snow and its relationship with rainfall. This figure shows normalized rainfall averaged over the southeastern TP from rain gauge observation and four model simulations. The WRF has high skill in simulating interannual variability of rainfall in July and August when it is significantly affected by large‐scale circulation anomalies. However, it exhibits low skill for interannual variability of rainfall in June, which may be attributed to its deficiencies in snow simulation. [ABSTRACT FROM AUTHOR]

It is generally accepted that El Niño–Southern Oscillation (ENSO) is the main modulator of rainfall variability over Northern South America in the interannual scale. Assuming that, an index is proposed to quantify this expected interannual variability in time series of rainfall. The result is the exposure assessment to the effects of droughts, measured with the Standardized Precipitation Index (SPI) for the monthly scale. The SPI is calculated from rainfall series, and wavelet analysis is used to estimate the variance for different frequencies present in the signal. The Wavelet Interannual Variability Index (WIVI) is calculated as the sum of the wavelet coefficients over a predetermined range of modes of variability (with periods longer than 2 year and shorter than 8 years). The index was tested using a dataset of rainfall records from Tocantins state, Central Brazil. Most of the series ranged from 1974 to 2012. On average, the series had 3.2% of gaps which were not filled to avoid the effect of artificial trends on the data. The state lies mostly over the Cerrado biome and is a new frontier of agricultural development in Brazil. According to the results, the Northern region is under higher exposure of interannual variability, with higher values of WIVI. The assessment is in agreement with large‐scale features of South American climate, specifically considering the influences of the Pacific and Atlantic Oceans and their patterns of sea surface temperature (SST). Interannual variability of rainfall was quantified by applying wavelet decomposition to time series of Standardized Precipitation Index (SPI). The wavelet variance for time scales of 2–8 years was quantified for 36 stations over Tocantins, Central Brazil. The variability within those scales was normalized as the Wavelet Index of Interannual Variability (WIVI). According to results, the Northern region is under higher exposure of interannual variability, with higher values of WIVI. The assessment is in agreement with large‐scale features of South American climate, specifically considering the influences of the Pacific and Atlantic Oceans and their patterns of sea surface temperature (SST). [ABSTRACT FROM AUTHOR]

We introduce a new hybrid scheme for seasonal precipitation prediction over the Mexican territory. The scheme takes as input sea surface temperature (SST) predicted from the ECMWF System 4 coupled model (Sys4) identifying three predictors, namely SST over the western Pacific (WP), eastern Pacific (EP) and tropical Atlantic (TA); these predictors are used to forecast precipitation on a high‐resolution grid via a multiple regression model. Comparison of the performance of the hybrid scheme with the Sys4 model for ensemble re‐forecasts for the period 1982–2010 shows that the hybrid model adds skill to the Sys4 forecast, with higher correlations and Heidke skill scores throughout most of the Mexican territory, both in winter and summer. In winter the contribution of the EP is dominant in modulating precipitation via the effects of El Niño‐Southern Oscillation (ENSO), while in summer the contributions of the WP and TA are dominant in different Mexican regions. The hybrid scheme, applied to high‐resolution precipitation observations, also produces significant fine‐scale variability of forecast skill and mechanism contributions. Overall, we show that our hybrid approach can provide a useful tool to improve seasonal precipitation forecasts over Mexico. [ABSTRACT FROM AUTHOR]

The Colorado River basin (CRB) is the primary source of water in the southwestern United States. A key step to reduce the uncertainty of future streamflow projections in the CRB is to evaluate the performance of historical simulations of general circulation models (GCMs). In this study, this challenge is addressed by evaluating the ability of 19 GCMs from the Coupled Model Intercomparison Project Phase 5 (CMIP5) and four nested regional climate models (RCMs) in reproducing the statistical properties of the hydrologic cycle and temperature in the CRB. To capture the transition from snow‐dominated to semi‐arid regions, analyses are conducted by spatially averaging the climate variables in four nested sub‐basins. Most models overestimate the mean annual precipitation (P) and underestimate the mean annual temperature (T) at all locations (up to +140% and −4.9 °C, respectively). A group of models capture the mean annual run‐off at all sub‐basins with different strengths of the hydrological cycle, depending on the level of P overestimation. Another set of models overestimate the mean annual run‐off, due to a weak cycle in the evaporation channel. An abrupt increase in the mean annual T of ~0.8 °C is detected at all locations around 1980 from the observed and most of the simulated time series. However, no statistically significant monotonic trends emerge for both P and T. All models simulate the seasonality of T quite well. The phasing of the seasonal cycle of P is reproduced fairly well in one of the upper, snow‐dominated sub‐basins. Model performances degrade in the larger sub‐basins that include semi‐arid areas, because several GCMs are not able to capture the effect of the North American monsoon. Finally, the relative performances of the climate models in reproducing the climatologies of P and T are quantified to support future impact studies in the basin. [ABSTRACT FROM AUTHOR]

ABSTRACT: The purpose of this study was to investigate the seasonal to interannual variability of the temporal and spatial distributions of land–atmosphere coupling (LAC) at the mesoscale within the Southern Great Plains (SGP) of the United States. The North American Regional Reanalysis data set from 1979 to 2014 was used to complete this study. To further expand the relationship between soil moisture and precipitation, LAC was examined for the effects of soil moisture variability on latent heat flux (SM‐E) and the impact of latent heat flux variability on precipitation (E‐P). Results revealed that within the SGP there is a temporal and spatial seasonal evolution of the SM‐E relationship and dry boreal summer month (June, July and August, JJA) periods exhibit a stronger E‐P relationship relative to pluvial boreal summer month periods. Further, the variability of coupling was large both within‐season (i.e. JJA) as well as at the interannual scale while the interannual spatial and temporal coherence was such that no specific locations showed consistent coupling within the domain. Thus, the results indicate that while the SGP domain is sensitive to coupling, the location of preferred coupling is likely due to non‐local factors at the mesoscale embedded within synoptic conditions as well as the regional climate. [ABSTRACT FROM AUTHOR]

ABSTRACT The characteristics of Madden-Julian Oscillation ( MJO), especially over the equatorial western Pacific, during the boreal winter (November to March) have got a further investigation based on the reanalysis data. The MJO activity prevails over the equatorial Indian Ocean and western Pacific and its main active centres are in the Southern Hemisphere. The MJO also has the highest percentage in the atmospheric variability and strong interannual variability here. The MJO over the equatorial western Pacific shows a prominent interannual variability with a period of 2-8 years and an interdecadal variability with a period of 12-16 years. These variabilities of MJO are closely related to the intensity and duration of strong discrete MJO events over there. During the equatorial western Pacific MJO activity strong winter, the MJO is intensified throughout the tropics and its eastward propagation is more continuous, more coherent, and further east. Anomalous ascending motion and moisture convergence over the equatorial western Pacific could enhance the Walker circulation over the equatorial Pacific and favour the activity of MJO over the equatorial western Pacific. The further eastward propagation of MJO in strong MJO activity winter is mainly due to the strong moistening at the lower troposphere to the east of the MJO convection centre. This moistening is dominated by the vertical advection of moisture. In weak MJO activity winter, the above characteristics are reversed. Furthermore, the MJO activity over the Indian Ocean and Pacific has undergone several interdecadal changes in both of its intensity and propagation during the past 150 years. [ABSTRACT FROM AUTHOR]

This paper presents a simple methodology to identify sea breeze frontal passage in the city of São Paulo, Brazil, based in hourly wind direction data. Fifty-year time series of annual frequency of sea breeze frontal passage and of its time of arrival in São Paulo are generated and related to global climate indicators and local variables. The frequency of occurrence and time of arrival of sea breeze shows long-term oscillations in the interannual and decadal time scales. Multiple regression is used to identify the variables that explain the observed variance of both frequency of occurrence and timing of the sea breeze in São Paulo. Local variables such as air temperature in São Paulo and sea surface temperature (SST) in the vicinity of the coast explain more than 95% of the total R2 (R2 = 51%) in a multiple regression model of the frequency of sea breeze frontal passage. Global climate indices such as the Atlantic Multidecadal Oscillation, the Southern Oscillation Index (SOI) and the Pacific Decadal Oscillation explain about 85% of the total R2 (R2 = 52%) in a multiple regression model of the time of arrival of the sea breeze in São Paulo. The sea breeze circulation is shown to impact the rainfall in São Paulo and vicinity and to be associated with extreme rainfall events. [ABSTRACT FROM AUTHOR]

The purpose of this study is to assess the ability of Coupled Model Intercomparison Project 5 (CMIP5) models in reproducing the variability and change of the austral summer precipitation observed in Southeastern South America (SESA) along the 20th century and beginning of the 21st. Models show a reduction in mean precipitation biases and inter-model dispersion, and a significant improvement in the representation of the leading pattern of precipitation interannual variability (EOF1), in comparison with Coupled Model Intercomparison Project 3 (CMIP3) models. Changes of the EOF1 activity in the present climate, as represented by both, climate model simulations and rainfall gridded datasets, evidence an increase of the frequency of EOF1 positive events (associated with positive precipitation anomalies in SESA and negative ones in the South Atlantic Convergence Zone) and a decrease of the frequency of EOF1 negative events. Nevertheless there are still large uncertainties due to model differences and the internal variability of the climate system. In order to reduce the impact of model uncertainties, an ensemble of the climate simulations that represent better the features associated with EOF1 activity was built, regardless to which model they correspond. The results obtained with this ensemble confirm that largest precipitation trends in SESA are those represented by climate simulations associated with an increase (decrease) of EOF1 positives (negative) events. It was also found that positive precipitation trends in SESA resulted from climate simulations forced by anthropogenic sources are the largest and significantly different from those from simulations forced by natural sources only, which are not significantly different from zero. [ABSTRACT FROM AUTHOR]

ABSTRACT Valleys can trap cold air to form Valley Cold Pools ( VCPs). Characterized by stable stratification and weak winds, VCPs, especially those that last over multiple days, can produce adverse effects such as poor visibility and severe air pollution. Using the gridded data set of the North American Regional Reanalysis ( NARR) for the period of 1979 through 2012, this study examines the climatology of VCPs in the western United States with a focus on spatial and temporal variability. The results reveal a widespread occurrence of short-lived (1-2 days) VCPs over both the mountainous areas of the West and the Northern Plains. Longer VCP episodes, however, tend to be limited to large basins/valleys in the Northwest and the Intermountain West. The leading mode of variability in the annual number of cold-season multi-day VCP event anomalies appears to be linked to a sea-surface temperature anomaly pattern typically found during the warm phase of Pacific Decadal Oscillation. Corresponding to the positive phase of the leading mode of variability is an anomalous 500- hPa ridge over the western United States induced by a Rossby wave train, which, by blocking cold-air intrusions and producing mid-level subsidence warming, leads to more persistent VCP episodes in the western United States. In addition to large-scale wintertime circulation anomalies, local surface temperature anomalies also contribute to the variability. [ABSTRACT FROM AUTHOR]

ABSTRACT Extratropical and subtropical cyclones, and their associated fronts, produce the majority of rainfall and extreme weather in the mid-latitudes around the world. In the Southern Hemisphere in particular, where observations are sparse, we have limited knowledge of their long-term variability and trends. While the long time period of the 20th Century Reanalysis ( 20CR) offers potential to assess longer-term changes, the lack of observational data sets makes it difficult to assess its reliability. We assess the skill of the 20CR at representing mid-latitude cyclones through a case study on the east coast of Australia. In this region, a comprehensive database has recently been developed of all severe floods between 1871 and 2012, of which >70% are associated with a coastal low-pressure system. Through use of the full 20CR 56-member ensemble, we are able to identify the majority of severe Australian east coast lows as early as the late 19th century, as well as recreate the cyclone tracks for some of the most severe events. The analysis shows that the full ensemble can be used for a long-term assessment of interannual variability in cyclone frequency from at least 1911, despite large inhomogeneities in the ensemble mean pressure fields. [ABSTRACT FROM AUTHOR]

ABSTRACT This study investigates the interannual variation of heat wave frequency ( HWF) in South Korea during the past 42 years (1973-2014) and examines its connection with large-scale atmospheric circulation changes. Korean heat waves tend to develop most frequently in late summer during July and August. The leading Empirical Orthogonal Function accounting for 50% of the total variance shows a mono-signed pattern over South Korea, suggesting that the dominant mechanisms responsible for the heat wave are linked in a spatial scale much larger than the nation. It also exhibits a regional variation with more occurrences in the southeastern inland area. The regression of the leading principal component ( PC) time series of HWF with large-scale atmospheric circulation identifies a north-south dipole pattern between the South China Sea and Northeast Asia. When this large-scale circulation mode facilitates deep convection in South China Sea, it tends to weaken moisture transport from the South China Sea to Northeast Asia. Enhanced deep convection in the South China Sea triggers a source of Rossby wave train along southerly wind that generates positive geopotential height anomalies around Korea. The anomalous high pressure pattern is accompanied by large-scale subsidence in Korea, thereby providing a favourable condition for extreme hot and dry days in Korea. This study highlights that there is a decadal change of the relationship between Korean heat waves and large-scale atmospheric circulation patterns. The tropical forcing tends to be weakened in the recent decade, with more influences from the Arctic variability from the mid-1990s. [ABSTRACT FROM AUTHOR]

ABSTRACT In this paper, interannual variations of convective and stratiform precipitations in summer East Asian monsoon ( SEAM) dominion are analysed using 16-year (1998-2013) Tropical Rainfall Measuring Mission ( TRMM) precipitation radar ( PR) rain products ( 2A25 and 3A25). It is found that the climatological mean patterns of convective and stratiform rain rates ( RRs) are generally similar to each other, but the area fraction ( AF) of stratiform precipitation can always reach about 80% and dominates in summer East Asia. Nonetheless, convective precipitation produces similar rainfall quantity with stratiform rainfall. For the RR, AF and contribution fraction ( CF), their interannual standard deviation and coefficient of variation ( CV) spatial patterns are similar between summer convective and stratiform precipitations in East Asia, because of the fact that they are all maybe influenced by SEAM. Differently, only the intensity of interannual variability (i.e. CV) for convective precipitation AF is much greater than that for stratiform precipitation, while CV for both convective and stratiform precipitations RR/ CF are similar in most areas. Moreover, the interannual variability of SEAM is more highly correlated with that of RR for convective precipitation than that for stratiform precipitation in most land regions. While the opposite conclusion is found in ocean regions. Generally, the SEAM intensity in the lower troposphere mainly controls interannual variability of the atmospheric stability and both stratiform and convective precipitations RR. Then, atmospheric stability variation further modulates the AF and CF variations of both stratiform and convective precipitations on the interannual scale. This work mainly provides possible evidence of linkage between large-scale circulation and convective and stratiform precipitations on the interannual scale. It suggests that the possible dominant mechanism should be explored using more observations and together with model simulations in the future work. [ABSTRACT FROM AUTHOR]

ABSTRACT Apart from the external forcings, the interannual variability of the South Asian summer monsoon can arise from atmospheric internal dynamics, although its mechanism has not been fully understood. Based on a three-dimensional mean kinetic energy ( MKE) budget equation, the contributions of internally generated processes associated with mean flow-transient eddy interaction and circulation-heating feedback to the interannual variability of the South Asian summer monsoon were diagnosed using two reanalysis data sets. In years with enhanced convections over South Asia, a strengthened low (high)-level MKE related to southwesterly (easterly) jet is observed over the Arabian Sea. The enhancement of MKE at the low level is maintained by both the anomalous barotropic energy conversion between eddy kinetic energy ( EKE) and MKE and the baroclinic energy conversion from mean available potential energy ( MAPE) to MKE. Among the three-dimensional barotropic energy conversion processes, the vertical eddy momentum flux interacting with the monsoon vertical wind shear anomaly plays a leading role in supporting the MKE anomaly. Moreover, the contribution of eddy activities related to the intraseasonal oscillation ( ISO) of 20-90 days is larger than that of high-frequency (<20 days) disturbances. This mean flow- ISO interaction dominates the atmospheric energy conversion processes and external boundary factors that may exert the interannual variations of monsoonal MKE in the lower troposphere. Different from the low-level dynamics, scale interaction between eddies and the mean flow shows an insignificant contribution to the strengthening of the easterly jet in the higher troposphere. The Niño3.4 sea surface temperature anomaly appears to be a key factor modulating the interannual variations of the high-level monsoon easterly. [ABSTRACT FROM AUTHOR]

ABSTRACT Earlier studies have identified a teleconnection between the Atlantic zonal mode ( AZM) and Indian summer monsoon rainfall ( ISMR), both of which are active during the boreal summer ( AZM: June-August; ISMR: June-September). It is known that El Niño-Southern Oscillation ( ENSO)-like coupled dynamics are operational in the tropical Atlantic during the AZM events. Our goal here is to extend this process understanding to seek a predictive relation between the tropical Atlantic and the ISMR based on these known teleconnections. Monthly composite analysis of the zonal surface winds, heat content, and sea surface temperature ( SST) in the equatorial Atlantic tells us that signatures of a warm or cold AZM event begin to emerge as early as January of that year. We found significant correlations between the ISMR and the low level zonal winds in the western equatorial Atlantic and heat content in the eastern equatorial Atlantic in the boreal spring season. Tracking coherent changes in these winds and the evolution of the heat content in the deep tropical Atlantic in the boreal spring may offer the potential for skillful predictions of the ensuing summer monsoon anomalies, especially during non- ENSO years when the predictability of ISMR tends to be low. [ABSTRACT FROM AUTHOR]

ABSTRACT This paper investigates the interannual variability of precipitation over the South Asian monsoonal region in May, during which the South Asian summer monsoon is generally established. The South Asian monsoonal region in this study is referred to as a broad area that includes the Arabian Sea, the Indian peninsula, the Bay of Bengal, and the Indochina peninsula. The precipitation exhibits a significant enhancement in May, compared to that in the previous month. The present results indicate that the interannual variability of May precipitation in this region is significantly regulated by two independent factors: The Europe-West Asia ( EWA) teleconnection and El Niño Southern Oscillation. The EWA teleconnection pattern is characterized by the see-saw pattern of upper-tropospheric geopotential height anomalies between the Europe and the Iranian plateau. Both the factors modulate the meridional gradient of tropospheric temperature and lower-tropospheric zonal winds, and thus affect the rainfall in the monsoonal region. Therefore, both the extratropical circulation teleconnection and tropical forcing are crucial for inducing rainfall variability in May over the monsoonal region. [ABSTRACT FROM AUTHOR]

ABSTRACT The Pacific-Japan ( PJ) pattern affects interannual variability in the East Asian and western North Pacific ( WNP) summer monsoons. This teleconnection pattern is characterized by a meridional dipole of anomalous circulation and precipitation between the tropical WNP and the midlatitudes. This study develops a long index of the PJ pattern using station-based atmospheric pressure data to track the PJ variability from 1897 to 2013. This index is correlated with a wide array of climate variables including air temperature, precipitation, Yangtze River flow, Japanese rice yield and the occurrence of tropical cyclones over the WNP (especially those that make landfall on the Chinese and Korean coast). For the recent three decades, the PJ index reproduces well-known correlations with El Niño-Southern Oscillation ( ENSO) in the preceding boreal winter and Indian Ocean temperature in the concurrent summer. For the 117-year period, this ENSO-PJ relationship varies on interdecadal time scales, with low correlations in the 1920s and from the 1940s to 1970s, and recurrences of significant correlations at the beginning of the 20th century and the 1930s. In accordance with the modulation, the magnitude and regional climate effect of the PJ variability have changed. These results highlight the importance of interdecadal modulations of climate anomalies in the summer WNP and the need of long-term observations to study such modulations. [ABSTRACT FROM AUTHOR]

ABSTRACT The output of four regional climate models ( RCMs) from the Coordinated Regional Climate Downscaling Experiment ( CORDEX)-North America (NA) region was analysed for the 1990-2008 period, with particular interest on the mechanisms associated with wet and dry years over the North American Monsoon ( NAM) core region. All RCMs (RCA3.5, HadGEM3-RA, REMO, and RegCM4) were forced by the ERA-Interim reanalysis. Model precipitation was compared against several observational gridded data sets at different time scales. Most RCMs capture well the annual cycle of precipitation and outperform ERA-Interim, which is drier than the observations. RCMs underestimate (overestimate) the precipitation over the coastal plains (mountains) and have some problems to reproduce the interannual variability of the monsoon. To further investigate this, two extreme summers that showed the largest consistency among observations and RCMs were chosen: one wet (1990) and one dry (2005). The impact of the passage of tropical cyclones, the size of the Western Hemisphere Warm Pool ( WHWP), the Intertropical Convergence Zone ( ITCZ) position, and the initial intensity of the land-sea thermal contrast ( LSTC) were analysed. During the wet year, the LSTC was stronger than the 2005 dry monsoon season and there were a larger number of hurricanes near the Gulf of California, the WHWP was more extended, and the ITCZ was located in a more northerly position than in 2005. All these processes contributed to a wetter NAM season. During the dry year, the LSTC was weaker, with a later onset, probably due to a previous very wet winter. The inverse precipitation relationship between winter and summer in the monsoon region was well captured by most of the RCMs. RegCM4 showed the largest biases and HadGEM3-RA the smallest ones. [ABSTRACT FROM AUTHOR]

ABSTRACT Annual and seasonal variations in surface air temperature ( SAT) during the period 1961-2011 were analysed using daily mean temperature data sets from regular grid points (10 × 10 km2) throughout Finland. The Mann-Kendall nonparametric test was used to detect significant historical trends in SAT and Spearman's correlation coefficient ( ρ) to test the relationships between SAT patterns and various atmospheric circulation patterns over the northern hemisphere. The results showed that mean annual SAT in Finland increased ( p < 0.05) by 0.4 ± 0.2 °C per decade during the study period and that the SAT was significantly ( ρ = 0.58, p < 0.05) positively correlated with the Arctic Oscillation ( AO) index. However, there were spatial differences within Finland for both the trends and relationships with the atmospheric circulation. Analysis of seasonal mean SAT identified significant ( p < 0.05) warming trends for both spring (by 0.4 ± 0.2 °C per decade) and summer (by 0.3 ± 0.2 °C per decade). Winter and spring mean SATs were most strongly ( p < 0.05) correlated with the AO index ( ρ = 0.72 and 0.42, respectively), while the most significant teleconnection pattern for mean SAT in summer was the East Atlantic ( EA) pattern ( ρ = 0.43, p < 0.05); and in autumn the EA/West Russia ( WR) pattern ( ρ = −0.59, p < 0.05). These results provide a detailed spatial picture of climate warming in Finland in recent decades and reveal that interannual variation of the SAT in Finland is closely linked with a number of atmospheric circulation patterns, not just the AO and North Atlantic Oscillation ( NAO). Annual and cold-season SAT are mainly influenced by the AO and NAO, whereas the EA, EA/ WR, Scandinavia ( SCA) and West Pacific ( WP) patterns play an important role for warm-season SAT. [ABSTRACT FROM AUTHOR]

ABSTRACT Heat waves (HWs) and cold spells (CSs) have pronounced impacts on the natural environment and society. The main aim of this study was to identify major Central European HWs and CSs since 1950 and assess their severity not only from the viewpoint of temperature and duration but also as to the affected area. The HWs and CSs were delimited from the E-OBS gridded data set. An extremity index was proposed that captures joint effects of spatial extent, temperature and duration of HWs and CSs. During the 1950-2012 period, we identified 18 major HWs and 24 major CSs over Central Europe. The most severe HW occurred in summer 1994, followed by the 2006 HW; both these events were far more extreme over Central Europe than HWs in the well-known 2003 and 2010 summers. The most severe CSs occurred in the winters of 1955/1956 and 1962/1963. The recent winter of 2011/2012 was found to be the sixth coldest since 1950/1951 according to the seasonal sum of the extremity index. The HWs and CSs were classified through a hierarchical cluster analysis of their characteristics (temperature amplitude, spatial extent of the core and duration) into four basic types. The established list of major Central European HWs and CSs might be utilized in further analyses. The extremity index may be applied over different areas to perform comparative studies and used also for evaluation of regional climate model simulations. [ABSTRACT FROM AUTHOR]

ABSTRACT The potential role of surface soil moisture ( SSM) in improving the simulation of interannual East Asian summer precipitation is investigated using a coupled land-atmosphere climate model, the Community Atmosphere Model version 3 ( CAM3). Forced by specified observational sea surface temperature ( SST), two ensemble simulations for 22 boreal-summer seasons (1979-2000) are conducted, one with interannually varying SSM ( SSMinter) forcing and another with climatological SSM ( SSMclim) forcing. Results show that, relative to the SSMclim run, interannual variability of East Asian summer precipitation is better simulated in the SSMinter run over the mid- and high-latitudes of East Asia, especially in northwest China, where the correlation coefficient between precipitation simulated and observed from the Climate Prediction Center Merged Analysis of Precipitation (CMAP) is increased from 0.12 to 0.56 during 1979-2000. Meanwhile, positive relationships between anomalies of local soil evaporation and summer precipitation in northwest China can be better reproduced in the SSMinter run than in the SSMclim run. Possible mechanisms for the improved simulations of the interannual summer precipitation variability in northwest China are analysed; the improvement is resulted from the reasonable reproduction of anomalous atmospheric circulation in Siberia and Iranian plateau. [ABSTRACT FROM AUTHOR]