Search

ABSTRACT Flash drought events can be characterised by the quick depletion of crop root zone soil moisture (rapid intensification) and hence can be termed as agricultural flash droughts. These events can have devastating impacts, such as increasing the risk of agricultural yield loss, heatwaves and increased wildfire risk, which further have cascading impacts on the socio‐economic conditions. The regional hotspots of flash droughts are analysed for winter, pre‐monsoon, monsoon and post‐monsoon seasons over India from 1981 to 2020. We assess the impact of the El Niño‐Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) on the flash drought frequency (FDF: number of flash drought events). The causal connection of the FDF with the ENSO and IOD is analysed using the PCMCI (Peter and Clark's algorithm combined with the Momentary Conditional Independence) algorithm. The monsoon season (June–September) is found to be more prone to flash droughts with higher spatial/regional average values of total per pixel FDF during the 40‐year period over the Central Northeast (~54) and West Central (~41) regions. It is observed that the fraction of the total number of flash droughts during the El Niño years (38.8%) is higher as compared with that in La Niña years (25.7%). It is also found that the co‐occurrence of positive/negative IOD with the El Niño phase can alter the seasonal fraction of FDF over India, highlighting the high complexity in the ENSO–IOD interactions. The causal analysis shows that only the Southern Peninsula and West Central regions have significant direct and lagged causal links of average per pixel FDF with IOD. Whereas, similar (direct and lagged) causal connections are observed between the ENSO and IOD. This study reveals that flash droughts and their teleconnections vary greatly among the seasons and regions in India, limiting its accurate predictions and increasing the risk to agricultural communities. [ABSTRACT FROM AUTHOR]

Studies related to monitoring changes in frequency, intensity and duration of precipitation extremes are key to creating efficient climate change measures and forest conservation policies. This study describes new insights into rainfall precipitation extremes over the Amazon basin (AB) during the last four decades (1981–2021) from the Climate Hazards Group InfraRed Precipitation with Station data (CHIRPSv2). Here we analysed the trends of daily extreme precipitation indices proposed by the Expert Team on Climate Change Detection and Indices (ETCCDI) at the seasonal scale, using the trend‐empirical orthogonal function (TEOF). Our results indicate that the frequency of precipitation extremes increased over Peruvian Amazonia and northeastern Brazilian Amazonia, and reduced in the centre of AB, mainly during the first seasons of the year: December–January–February (DJF) and March–April–May (MAM). The cooling trend over the eastern and central tropical Pacific and the warming trend over the tropical and western subtropical Pacific could associate with the increase in frequency of precipitation extremes in DJF. Furthermore, during June–July–August (JJA) and September–October–November (SON), rainfall intensity indices showed a decrease in Colombia and the Bolivian Amazon; in contrast, northern and southern Peru delivered an increased pattern. The trend pattern in the JJA and SON seasons could be associated with a warming trend over most of the North Atlantic and a cooling in the 40°–60° S band. Our results demonstrate that the precipitation extremes over the AB have spatially varying trends. These heterogeneous trends over the space might be take into account for robust adaptation policies over the countries that are parts of the AB, such as Bolivia, Brazil, Colombia, Ecuador, Guyana, Perú, Surinam and Venezuela. [ABSTRACT FROM AUTHOR]

This study demonstrates asymmetric relationships between El Niño–Southern Oscillation (ENSO) and tropical cyclones (TCs) affecting the Philippines during October–December. In El Niño or La Niña years, the number of TCs impacting the Philippines may increase or decrease. These variations result in four ENSO–TC variability types all of which exhibit strong sea surface temperature (SST) anomalies across the equatorial eastern Pacific. The major difference between the active and inactive types in terms of El Niño or La Niña years is related to the magnitude of SST anomalies in the tropical western Pacific (TWP) over the 120°–150°E region. During El Niño years, moderate cold SST anomalies in this TWP region cause an anomalous divergent centre around the 120°–130°E zone to evoke an anomalous cyclone east of the Philippines. In the western North Pacific (WNP), this anomalous cyclone causes more TCs to form and move toward the Philippines, resulting in active TC activity. For the inactive TC type during El Niño years, very weak cold SST anomalies in the aforementioned TWP region correspond with a northeastward‐extended anomalous divergent centre over the 120°–140°E, 10°S–20°N zone and an anomalous anticyclone across the Philippines and its eastern side. Decreases in the formation of the WNP TC and movement toward the Philippines lead to inactive TC activity. The large‐scale anomalies and regulating processes are mainly opposite between the active TC type during El Niño years and the inactive TC type during La Niña years. These two types are influenced by interdecadal variability of the Pacific decadal oscillation. Opposite anomalies and regulating processes also occur between the inactive TC type during El Niño years and the active TC type during La Niña years. The former type is jointly modulated by the positive Indian Ocean Dipole mode and central‐Pacific El Niño. [ABSTRACT FROM AUTHOR]

This study investigated relationships between year‐to‐year variability in precipitation in the rainy season in Mozambique and major modes of climate variability in the Tropics and subtropics. The Niño3.4 index was strongly negatively correlated with precipitation in Mozambique's southern and central regions. We suggest that Rossby wave propagation reaching Southern Africa from the tropical Pacific is key to the relationship between precipitation in Mozambique and El Niño–Southern Oscillation. Subtropical Indian Ocean Dipole did not lead rainy‐season precipitation, but showed a simultaneous correlation with precipitation in southern, central and northeastern regions. Benguela Niño was found to have a significant positive lead correlation by 6 months with precipitation in the southern, central and northwestern regions. In contrast, Indian Ocean Dipole led precipitation in the southern, central and northeastern regions by 3 months. Overall, the modes of climate variability exerted stronger control over precipitation variability in southern and central Mozambique, and weaker control in northern Mozambique, particularly in the northwestern region. [ABSTRACT FROM AUTHOR]

El Niño–Southern Oscillation (ENSO) events are a major predictor of Asian spring precipitation. However, it is unclear whether ENSO events with different intensities have similar or different impacts on Asian spring precipitation. This question is explored in this study. According to their intensity, ENSO events are divided into strong and moderate (S‐ENSO and M‐ENSO) events. The analysis indicates that S‐ENSO events have stronger sea surface temperature (SST) anomalies over the tropical Pacific and Indian oceans, and warm/cold events can lead to significantly more/less precipitation over Central Asia and East China as well as less/more precipitation over the Indo‐China Peninsula, Tibetan Plateau, and Maritime Continent. The S‐ENSO events significantly affect the Walker circulation and West Pacific anticyclone and excite an eastward‐propagating wave train over the midlatitude Northern Hemisphere. The changes in these atmospheric circulations further alter the dynamic and moisture conditions over the aforementioned Asian regions, consequently leading to precipitation anomalies. In contrast, the M‐ENSO events have relatively weak SST anomalies over the tropics, and the warm/cold events can result in more/less precipitation over Central Asia, Mongolia and northern China and less/more precipitation over northeastern Siberia, South China and the Maritime Continent. The impact of M‐ENSO events on spring precipitation over the Asian regions mainly occurs due to modulating effects on the western Pacific pattern, Aleutian Low, and anomalous anticyclones over northwestern Asia. The results in this study indicate that ENSO events of different intensities may have different impacts on Asian spring precipitation, which should be considered when predicting it. [ABSTRACT FROM AUTHOR]

Recently, we developed seasonal prediction schemes with improved skill to predict tropical cyclone (TC) activity up to 3 months in advance for the Solomon Islands (SI) region (5°–15°S, 155°–170°E) using sophisticated Bayesian regression techniques. However, TC prediction at subseasonal timescale (i.e., 1–4 weeks in advance) is not being researched for that region despite growing demands from decision makers at sectoral level. In this paper, we first assess the feasibility of developing subseasonal prediction frameworks for the SI region using a pool of predictors that are known to affect TC activity in the region. We then evaluate multiple predictor combinations to develop the most appropriate models using a statistical approach to forecast weekly TC activity up to 4 weeks in advance. Predictors used include indices of various natural climate variability modes, namely the Madden–Julian Oscillation (MJO), the El Niño–Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD) and the Interdecadal Pacific Oscillation (IPO). These modes often have robust physical and statistical relationships with TC occurrences in the SI region and the broader southwest Pacific territory as shown by preceding studies. Additionally, we incorporate TC seasonality as a potential predictor given the persistence of TCs occurring more in certain months than others. Note that a model with seasonality predictor alone (hereafter called the "climatology" model) forms a baseline for comparisons. The hindcast verifications of the forecasts using leave‐one‐out cross‐validation procedure over the study period 1975–2019 indicate considerable improvements in prediction skill of our logistic regression models over climatology, even up to 4 weeks in advance. This study sets the foundation for introducing subseasonal prediction services, which is a national priority for improved decision making in sectors like agriculture and food security, water, health and disaster risk mitigation in the Solomon Islands. [ABSTRACT FROM AUTHOR]

Based on 40 years of meteorological data, we characterized and analysed long‐term climatic trends in the Chamela‐Cuixmala Biosphere Reserve on the Pacific coast of central Mexico. The region is still covered by a large proportion of well‐preserved tropical dry forest, characterized by a short period of rain and a dry season of around 8 months. We found a sustained temperature increase which is likely driven by global climate change. An increasing trend in rainfall was also found, but the trend was detected only during the wet season. Correspondingly, an increase in runoff during the rainfall season was also detected; this seems to be linked with an increasing number of intense rainfall events, rather than an increase in the total number of rainy days. El Niño years are likely to present below average precipitation during the wet season and above average precipitation during the dry season. The opposite is expected during La Niña years, when tropical cyclones are likely to come closer to the coast and cause intense rainfall events during the hurricane season. As many of the studied variables are likely to change under climate change scenarios, our results highlight the need to understand the expected impacts of global climate change on tropical dry forests. In particular, it is necessary to monitor changes in water availability to anticipate its consequences for the forest and the human communities that depend on it. [ABSTRACT FROM AUTHOR]

Extreme events contribute significantly to rainfall variability in semi‐arid regions like South Africa. Here, following the definition of a novel typology of rainfall extremes, disentangling large‐ and small‐scale events in Part I, we use quality‐controlled observational databases in South Africa, the ERA5 reanalysis and satellite estimates TRMM‐3B42 to examine the relationship between these two types of rainfall extremes and different modes of climate variability at various timescales. At low frequencies, rainfall extremes are assessed at interannual (IV: 2–8 years) and quasi‐decadal (QDV: 8–13 years) timescales, which are primarily associated with the El Niño–Southern Oscillation (ENSO) and the Interdecadal Pacific Oscillation (IPO), respectively. At subseasonal timescales, the typology of rainfall extremes is analysed depending on the synoptic configurations, as inferred by seven convective regimes including tropical temperate troughs (TTTs: 3–7 days), and the intraseasonal variability associated with the Madden–Julien Oscillation (MJO: 30–60 days). At the IV timescale, the occurrence of large‐scale extremes is substantially higher during its wet phases thereby suggesting a 400% rise in the occurrence of large‐scale extremes as compared to its dry phases. At the QDV timescale, variability mostly relates to the modulation of small‐scale extremes during its wet phases. Teleconnections with global sea surface temperature (SST) confirm that La Niña conditions favour overall wet conditions and extremes in South Africa. The numbers of large‐scale extremes are consistently related to warmer SSTs in the North Atlantic, while their link with warmer Indian and tropical South Atlantic oceans is found to be statistically independent of the state of ENSO. At the subseasonal timescales, large‐scale extremes largely occur during three out of the seven convective regimes identified in the southern African region whereas small‐scale extremes are nearly equiprobable during all convective regimes. The occurrence of large‐scale extremes during continent‐rooted TTT is further enhanced during the locally wet phases of the MJO and is symmetrically weakened during its dry phases. [ABSTRACT FROM AUTHOR]

Characteristics and physical features related to low precipitation across many years in Southern Africa that lead to societal disruptions are diagnosed using observed analyses and an ensemble of historical coupled climate model simulations during 1921 to 2014. Four regions are evaluated, as identified through a hierarchical clustering algorithm applied to the Standardized Precipitation Index (SPI) during the October–April precipitation season. Although dryness spanning many October–April occurs periodically in each region, they seldom occur simultaneously, consistent with largely insignificant SPI cross‐correlations between them. However, characteristics relevant to low precipitation across many years are generalizable between the four regions, including the serial persistence of October–April precipitation, the likelihood of consecutive dry October–April, and the likelihood of dry October–April in temporal extents of up to 10 consecutive such 7‐month seasons. Systematic precipitation persistence is not a feature in any of the four Southern Africa regions, as serial correlations of October–April SPI are not statistically significant at any time lags. It follows that there is an exponential‐folding decay in the likelihood of consecutive October–April for various SPI thresholds and that there is a large spread in the likelihood of low October–April SPI across many years. In terms of physical features, low October–April SPI in each Southern Africa region is closely related to local atmospheric circulations; however, they are not as closely related to sea surface temperatures (SSTs). These results suggest that dryness spanning many years is determined primarily by persistent local circulations related to atmospheric variability and to a lesser extent variability related to SST anomalies, including the El Niño–Southern Oscillation. [ABSTRACT FROM AUTHOR]

This study analyses the spring climate anomalies under different configurations of western North Pacific anticyclone (WNPA) and Siberian high (SH). Compared to the WNPA alone with increased precipitation over southern China, the out‐of‐phase configuration (strong‐WNPA–weak‐SH) can enhance the WNPA‐related southwesterlies over eastern China, favouring increased precipitation with larger amplitude over southern China and southern Northeast China. Differently, the in‐phase configuration (strong‐WNPA–strong‐SH) decreases and confines WNPA‐related southwesterlies over southern China, causing decreased precipitation over eastern Southwest China; additionally, the WNPA‐related southwesterlies converge with SH‐related northwesterlies over the lower reaches of the Yangtze River, causing increased precipitation there. For surface air temperature, the WNPA alone induces warming over western Southwest China through changing heat flux, while the SH alone induces cooling over northern China through changing temperature advection, and over Northwest and southern China through changing heat flux. In the out‐of‐phase configuration, warming expands into Northern China due to the cooperation of weakening SH. In the in‐phase configuration, SH‐related cooling mainly occurs to the north of the Yangtze River because of the obstruction of WNPA. Further analysis indicates the eastern Pacific (EP) and central Pacific (CP) El Niño‐Southern Oscillation (ENSO), European low and Ural high contribute to the different configurations of WNPA and SH. The EP ENSO‐related zonal overturning circulation intensifies the WNPA and European low‐induced warm advection weakens the SH, resulting in the out‐of‐phase configuration. The CP ENSO‐related zonal overturning circulation intensifies the WNPA with a northeast–southwest tilt and Ural high‐induced cold advection intensifies SH, resulting in the in‐phase configuration. [ABSTRACT FROM AUTHOR]

This study examines the climatology, variability, and trends of tropical cyclones (TCs) affecting the Solomon Islands (SI) territory, in the wider southwest Pacific (SWP), using the South Pacific Enhanced Archive for Tropical Cyclones (SPEArTC) database. During the period 1969/1970–2018/2019, 168 TCs were recorded in the SI territory. A cluster analysis is used to objectively partition these tracks into three clusters of similar TC trajectories to obtain better insights into the effects of natural climate variability, particularly due to the El Niño–Southern Oscillation (ENSO) phenomenon, which otherwise is not very apparent for TCs when considered collectively in the SI region. We find that TCs in clusters 1 and 3 show enhanced activity during El Niño phase, whereas TCs in cluster 2 are enhanced during La Niña and neutral phases. In addition to being modulated by ENSO, TCs in clusters 2 and 3 show statistically significant modulation at an intraseasonal timescale due to the Madden–Julian Oscillation (MJO) phenomenon. There are also some indications through sophisticated Bayesian modelling that TCs in clusters 2 and 3 are slightly influenced by the Interdecadal Pacific Oscillation (IPO). These results can have substantial implications for cluster‐specific development of TC prediction schemes for the SI region. [ABSTRACT FROM AUTHOR]

Positive/negative Indian Ocean Dipole (IOD) tends to co‐occur with El Niño/La Niña. Five positive IODs, however, are observed under La Niña condition in 1961, 1966, 1967, 2007, and 2011. This study uses the monthly ocean temperature and atmospheric reanalysis datasets from ECMWF to investigate how these positive IOD events occurred. It is found that the five positive IODs are consistently accompanied by a moderately strong positive subsurface IOD (Sub‐IOD) event that initiated earlier. The Sub‐IOD‐related oceanic dynamical processes including oceanic Kelvin and Rossby waves help to promote the development of the positive IODs. More specifically, the equatorial easterly anomalies in the Tropical Indian Ocean (TIO) that are crucial for the development of the positive Sub‐IODs and IODs in 1961/2011 are triggered by an anticyclonic curl over South Indian Ocean that is induced by a Southern Hemispheric Indian Ocean Subtropical Dipole mode. The crucial equatorial easterly anomalies in 1967 are triggered by a thermocline warming in the southwestern TIO induced by westward downwelling oceanic Rossby waves, while in 1966/2007, the equatorial easterly anomalies are connected with the anomalous Walker circulation resulting from the certain sea surface temperature anomaly pattern over the equatorial Pacific. On the other hand, the remote effects of La Niña on TIO are relatively weaker, being overwhelmed by the above triggering processes. [ABSTRACT FROM AUTHOR]

The Arctic Oscillation (AO) is a well‐known mode that affects climate variability in the Northern Hemisphere. The equal‐weighed multi‐model ensemble (MME) of six state‐of‐the‐art models from the Copernicus Climate Change Service (C3S) and Pusan National University (PNU) was analysed to understand the wintertime AO performance for the hindcast period December–February 1993/1994–2016/2017. The hindcasts were chosen with lead times of 1 and 4 months with respect to the initialization date (August and November, respectively). The spread of the AO prediction skills of the individual models was significant. In general, the MME demonstrates superior skill compared to the average of single‐model skills in representing the AO pattern at lead times of 1 and 4 months. The AO‐related vertical structure predicted by MME is similar to the observation, but the upper‐level structure is relatively poor compared to the structure of the hindcasted lower‐ or mid‐level atmosphere. Both observation and MME indicate that since the mid‐1990s, the relationship between the AO and East Asian winter monsoon (EAWM) has been weak compared to the connection between the AO and El Niño–Southern Oscillation (ENSO). Simultaneously, the North Pacific centre of the AO moved eastward during the observational period. The MME showed an AO pattern similar to that observed. The eastward shift of the North Pacific centre of the AO may contribute to deepening the Aleutian low and its effect on the tight AO–ENSO relation demonstrated in observations and MME. Strong AO–ENSO relations and weak AO–EAWM connections are found in both observations and MME model. The observation and MME represent the wave activity flux from 60°N to the equator in the troposphere; consequently, the wave activity flux may contribute to the AO and ENSO connection in both observation and MME. [ABSTRACT FROM AUTHOR]

The rise of global surface temperature due to warming climate is expected to increase the intensity and occurrence of extreme precipitation events. Previous studies in Southeast Asia revealed complex variations in changes of precipitation extremes. This study presents a spatial–temporal analysis on changes of precipitation extremes in Peninsular Malaysia by utilizing long‐term daily rainfall records at 64 observed stations during 1989–2018. The modified Mann–Kendall and Sen's slope tests were performed to detect the significance and magnitude of trends in eight extreme precipitation indices recommended by the Expert Team on Climate Change Detection and Indices. Statistically significant increasing trends are observed for four of these extreme indices in the annual assessment. Spatial analysis demonstrates an obvious contrast between wet and dry regions in patterns of precipitation extremes. Seasonal analysis reveals the intensity and frequency of wet extremes are enhanced significantly during the northeast monsoon season. Significant correlations are found between precipitation extremes and El Niño–Southern Oscillation, particularly in the northern, eastern and southwest regions. Collectively, the evidence presented suggests that the occurrence of precipitation extremes in Peninsular Malaysia tends to be more frequent and intense over the year and is closely associated with the combined effects of tropical monsoon cycles and El Niño–Southern Oscillation. [ABSTRACT FROM AUTHOR]

There is growing interest in the international scientific community in characterizing climate variability in Antarctica because of the continent's fundamental role in regulating the world's climate. Researchers have intensively studied the Antarctic Peninsula since the warming that began in the mid‐1950s. This was followed by a subsequent cooling period over the last decades. For this paper, using the available data, we analysed the variability in surface air temperatures at five meteorological stations located on King George Island (KGI) (a subantarctic island that is part of the South Shetland Islands); we also investigated the relationships between the air temperatures and large‐scale atmospheric patterns from 1968 to 2019. In this study we found that summer temperatures are above 0°C from December to March and close to melting temperatures (extreme values) in spring and autumn; consequently, a small increase in temperature can have a significant impact on the cryosphere. The statistical analysis of the mean temperatures confirmed a trend toward cooling during the summer and in the mean monthly maximum temperatures over the 1990s at most of the weather stations whose data we analysed. Analysing the teleconnection patterns showed that the Southern Annular Mode (SAM) had strong, direct, and positive correlations during the autumn and less strong connections in spring, winter, and on an annual scale. Furthermore, we observed a lesser influence of El Niño‐Southern Oscillation (ENSO). [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]

In this paper, we report on the frequency of false onsets of wet season rainfall in the case of the Northern Australian wet season and investigate the role of large‐scale tropical climate processes such as the El Niño–Southern Oscillation, Indian Ocean Dipole (IOD) and Madden–Julian Oscillation. A false onset occurs when a wet season rainfall onset criterion is met, but follow‐up rainfall is not received for weeks or months later. Our analysis of wet season rainfall data from 1950 through 2020 shows a false onset occurs, on average, between 20 and 30% of wet seasons across all of northern Australia. This increases at a regional and local level such as at Darwin, the Northern Territory (NT), and parts of Queensland's north coast to over 50%. Seasonal climate influences, such as a La Niña pattern and a negative IOD that typically expedite the wet season rainfall onset, also increase the likelihood of a false onset over northern Australia. Our analysis also finds that periods of false onsets can sometimes, but not always, coincide with periods of rapid soil moisture depletion. The false rainfall onsets that develop into flash drought can be potentially disruptive and costly and are of potential significance for agriculture and fire management in northern Australia, and in other monsoonal climates that also typically experience a slow build‐up to the seasonal monsoon. In conclusion, effective rainfall indicates that many seasons experience 'false onsets' with dry conditions after early rainfall. We propose that false onsets are a physical characteristic of the climate of northern Australia which occurs with relatively high frequency. In addition, these false onsets may sometimes co‐occur with a flash drought. [ABSTRACT FROM AUTHOR]

Previous studies universally found that tree‐ring oxygen isotope (δ18OTR) in southeastern China (SEC) significantly recorded the El Niño–Southern Oscillation (ENSO) signal. However, the correlation between the climate in SEC and ENSO is insignificant, so the local climatic response of SEC δ18OTR fails to explain the significant positive correlations between these δ18OTR records and ENSO. In this study, based on four δ18OTR series from SEC, meteorological data, simulated precipitation oxygen isotope (δ18OP) data, and a moisture uptake atlas deduced from Lagrangian backward trajectory experiments, a conceptual model was established to explain how the ENSO signal is recorded by SEC δ18OTR. During the El Niño decaying years, the Northwest Pacific (NWP) convective activity weakens; the convective weakening areas coincide with the main moisture sources for SEC, which enriches 18O in moisture that is transported to SEC, resulting in positive anomalies of δ18OP in SEC; finally, the ENSO‐modulated δ18OP signal is inherited by SEC δ18OTR. The opposite situations occur in the La Niña events. It was also indicated that the change in the moisture contribution percentage from different sources may not be a primary factor that connects SEC δ18OTR with ENSO. These findings contribute to understanding climatic signals represented by δ18OTR and δ18OP in the East Asian monsoon region. [ABSTRACT FROM AUTHOR]

The Tumaco multivariate index (TMI) is a multidecadal monthly index constructed with unique time series of sea surface temperature, surface air temperature and rain measured at Tumaco bay, in the southern Pacific coast of Colombia, and available since 1961. In this work, this index is re‐evaluated after the addition of in situ sea level data, and its properties for different standardization periods are compared against oceanic El Niño–Southern Oscillation (ENSO) and other derived indices. In particular, we propose a modified TMI, hereinafter referred as TMI4, whose potential to identify the expected sign and the amount of future variations of rain induced by ENSO events in Colombia is analysed for selected extreme episodes. Results indicate that after the inclusion of sea level data, TMI4 can anticipate the development of El Niño events before the ENSO 3 and some other sea surface temperature‐based regional indices, although its predictability depends on the ENSO type (canonical or Modoki). The explanation is that sea level includes new information into TMI4 on the onset of El Niño events. In particular, the signal of intraseasonal sea level anomalies carried by downwelling Kelvin waves is detected at Tumaco tide‐gauge. Moreover, the analysis of the differences, both in magnitude and spatial distribution, of rainfall anomalies induced by positive (El Niño) and negative (La Niña) ENSO events characterized by TMI4 are regionally presented. As a result, we find that TMI4 is especially suited for extensive northern and western areas of mainland Colombia. For completeness, in the appendix we briefly introduce the semi‐automated implementation of TMI4, including a visual interface, which is currently being tested by personnel within the operational oceanography area at Centro de Investigaciones Oceanográficas e Hidrográficas del Pacífico (Dimar‐CCCP). [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]

The rainfall is essential to Brazil's hydrological cycle, agricultural development, and power generation, mainly Cerrado biome. Thus, the study assessed the influence of the El Niño–Southern Oscillation (ENSO) and synoptic systems on rainfall variability over the Brazilian Cerrado. To evaluate this variability, it used monthly rainfall data from the second version of the Climate Hazard Group InfraRed Precipitation with Station (CHIRPS) database (1981–2020), based on the annual densidrograms and thematic maps. In Annual terms, the average climatological was 1,417 mm·year−1. The highest precipitation values (>1,500 mm·year−1) occurred at the boundaries between Cerrado and Amazon, mainly in the states of the Maranh~ao, Mato Grosso, and Tocantins due to physiography (<500 m). The seasonal rainfall was more frequent in summer (on average, 48% of the annual total) distributed over the biome due to the South American monsoon system and South Atlantic convergence zone. The climatological dry period occurred between November and March (on average, 8% of the annual total) due to the Atmospheric Blocking called as Bolivia High that inhibit the passage of synoptic system transients in Central Brazil. In monthly terms, January (July) is the wetter month with 18% (drier month with 1%) of the annual total. The annual densidrograms appointed the following wetter (drier) years: 1985, 1989, and 2009 (1990, 1993, and 2012), influenced by ENSO events. The year 1985 appointed an increase of 19% in the mean annual rainfall, but in 1990 showed a reduction of 18% in climatology (1981–2020). In terms of seasonal anomalies, the summer of 1985 (1990) appointed an increase of 26% above (2.5% below) mean seasonal climatological, verified in the states of Maranh~ao, Piauí, and Tocantins (states of Mato Grosso). The La Niña (El Niño) events influenced positively (negatively) the annual rainfall anomalous pattern in Cerrado. [ABSTRACT FROM AUTHOR]

High variabilities in rainfall and the subsequent consequences have caused severe water‐related issues for Thailand over recent decades. In this study, the factors that affect rainfall variability over Thailand were determined. The spatio‐temporal structures of the monthly rainfall in Thailand between 1979 and 2018 were extracted via an empirical orthogonal function (EOF) analysis. With a unimodal temporal structure, the first EOF mode suggests the overall phenomenon governing rain characteristics in Thailand. The higher modes, consisting of bimodal temporal structures, indicate different rain phenomena in various regions. A significantly high correlation between the 3‐month average of the monsoon trough index and rainfall was observed, while wet adiabatic front and convective available potential energy (CAPE) data had a moderately high correlation. Only the local front index, within the Gulf of Thailand, correlated to the rain characteristics along the east coast of southern Thailand. The cross‐wavelet analysis indicated that most of the rainfall variability correlates with monsoon trough. However, the front can involve extreme events. The interannual variability of monsoon trough and rainfall with a dominant time scale between 3.0 and 6.0 years has significantly increased since 1995. Meanwhile, the intra‐annual variability is more intermittent, indicating rare intensified strong tropical cyclones along the monsoon troughs in Thailand. The interannual variability of the monsoon trough and front indices also intermittently connects with the Southern Oscillation Index (SOI). However, the relationship in annual and intra‐annual cycles is mostly weak. Therefore, the dependence of interannual variability of rainfall in Thailand on SOI or monsoon trough is comparable. Notably, the intra‐annual rainfall variability is heavily influenced by the monsoon trough and front. This study is the first to identify the crucial factors behind the seasonal rain phenomena in Thailand. This information will benefit seasonal rainfall predictors and increase preparedness for hydro‐climatic related disasters in the Indo‐China region. [ABSTRACT FROM AUTHOR]

Australia experiences some of the world's most variable rainfall. Previous studies have mostly focused on understanding rainfall variability in terms of frequency and intensity. However, understanding the timing of when extreme rainfall occurs is crucial for seasonal prediction, although it largely remains unexplored. Here we investigate the timing of extreme rainfall in Australia and the spatial variability of this timing. This study examines how some of the large‐scale drivers, such as the El Niño–Southern Oscillation (ENSO) and the Interdecadal Pacific Oscillation (IPO), determine the timing and interannual variability of the timing of extreme rainfall in Australia. Our results show that there is a clear spatial north–south delineation in the season when extreme rainfall occurs in Australia, shown by a contour diagonally extending roughly from 21°S in the west of Australia to 33°S in the east. North of this contour, extreme rainfall usually occurs in austral summer, with the smallest interannual variability in the timing of extreme rainfall in this region. In the south, extreme rainfall usually occurs in autumn/winter months; however, the timing is highly variable. In southeast Australia (SEA), extreme rainfall can fall at any time of the year, which makes seasonal prediction extremely challenging for this region. Both observation and reanalysis data show that the area where extreme rainfall occurs in summer extends further south during negative IPO years. We also find that IPO and ENSO phases, and the interaction between them, play significant roles in both determining the timing of extreme rainfall and constraining the interannual variability, especially in SEA. We focus on SEA for further analysis as this region shows the greatest shift in seasonality of extremes in response to large‐scale variability. We conclude that studying the relationship between rainfall and large‐scale drivers is important for verification and improvement of the seasonal prediction of extreme rainfall. [ABSTRACT FROM AUTHOR]

The increased trends of extreme precipitation events over China have been partially attributed to global warming, while it is unclear how the natural climate variability affect these variations in extreme rainfall trends. Here we examine the contributions of large‐scale modes climate variability from the Pacific Ocean, which are represented by El Niño‐Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO), on the trends of seasonal extreme events across China. Results show that the effects of climate variability modes on precipitation extremes exhibit evidently spatiotemporal characteristics at the seasonal scale. A significant positive trend is found over southeastern and northwestern China in four seasons, and substantial increases in two types of extreme events are also seen over northwestern China in spring and winter. Most of the seasonal trends of extreme precipitation events can be explained by the trends of the principal components. El Niño (La Niña) and PDO–like sea surface temperature (SST) anomalies over the Pacific have a close linkage with the variations in precipitation extremes. By modulating the large‐scale atmospheric circulations, these Pacific SST anomalies associated with climatic oscillations, such as ENSO and PDO, provide the conducive conditions responsible for the occurrences of seasonal precipitation extremes across China. The seasonal trends of extreme events can be largely explained by the variability of the ENSO and PDO episodes, with a relatively larger modulation of PDO as compared to ENSO. These researches have important implications for improving seasonal flood disaster predictability and climate model performances. [ABSTRACT FROM AUTHOR]

The present study investigates how the 20th‐century atmospheric reanalyses (ERA‐20C and NOAA‐20CR) reproduce the known impacts of El Niño–Southern Oscillation (ENSO) on the tropical North Atlantic (TNA) atmospheric circulation from the boreal winter to spring and the consequences on northeastern Brazil (NEB) precipitation during its rainy season (boreal spring). For the period 1979–2010, comparisons of atmospheric patterns associated with ENSO events are made through the use of anomaly composites of variables provided by ERA‐20C, NOAA‐20CR and satellite‐era reanalyses (ERA‐Interim and NCEP‐Reanalysis 2). We also evaluate the ability of the reanalyses to reproduce the interannual variability of precipitation during the boreal spring over Ceará state (located in NEB) by statistically comparing them with Funceme rain‐gauge and Global Precipitation Climatology Center (GPCC) datasets. As the main result, both 20th‐century reanalyses capture the major features in the tropical Pacific representing the climatic system during ENSO years as well as its impacts on the tropical Atlantic, in agreement with previous studies. However, some differences are noted, such as (a) NOAA‐20CR differs from the other reanalyses since it indicates stronger (weaker) near‐surface (upper levels) wind anomalies over the central‐western equatorial Pacific in both El Niño and La Niña composites; (b) over the tropical Atlantic, all reanalyses indicate the existence of an omega anomalies dipole between 10°N and 10°S for both ENSO phases although a weaker signal is seen in NOAA‐20CR; (c) NOAA‐20CR underestimates the intensity of the precipitation anomalies over the NEB during El Niño and La Niña years, and this is associated with the weaker vertical velocity anomalies. These results indicate that ERA‐20C is the most appropriate 20th‐century reanalysis to study the long‐term interannual variability in the tropical Atlantic associated with ENSO and its impacts on precipitation in northeastern Brazil. [ABSTRACT FROM AUTHOR]

The synergetic effect of Indian Ocean Dipole (IOD) and El Niño‐Southern Oscillation on the particulate matter (PM2.5) concentration in Guangdong for October–November–December 1980–2016 has been investigated. Results indicate that, during a typical polluted episode, Guangdong is usually under the influence of a high‐pressure system behind the cold front, which is characterized by weakened southwesterly, subsidence/divergence, and stable atmospheric conditions. Persistent low values of PM2.5 are found to prevail during the years when concurrent IOD positive phase and El Niño occur. However, separate occurrence of El Niño and the positive phase of IOD had only marginal effect on PM2.5. The remarkable synergetic effect of El Niño and the positive phase of IOD is caused by a significantly strengthened ascending motion associated with anomalous southwesterly, which facilitates the transport of abundant moisture, increasing rainfall and thus suppressing PM2.5 pollution in Guangdong. [ABSTRACT FROM AUTHOR]

The decreasing trend in diurnal temperature range (DTR) since the 1950s has been widely observed in many regions of the world, including the Tibetan Plateau (TP). However, scarce instrumental records on the TP before the 1950s limit the understanding of DTR variation on a longer timescale, and its internal forcing mechanisms remain unclear. Here, we present two regional records of May–June DTR since 1753 reconstructed from tree rings on the northeastern TP (NETP) and southeastern TP (SETP), respectively. The decreasing trend in DTR in the second half of the twentieth century also occurred in the time earlier than the 1950s, indicating that the reduction in DTR is not unique to global warming. Spatially, both the instrumental and reconstructed DTR show different modes on the NETP and SETP. Composite analysis and superposed epoch analysis reveal the linkage between El Niño‐Southern Oscillation (ENSO) and the DTR on the SETP. La Niña (El Niño) winters are generally followed by the stronger (weaker) early Indian summer monsoon so that may lead to the decrease (increase) in May–June DTR. The positive correlation between Niño 3.4 index and the DTR on the SETP over the past two and a half centuries is accompanied by a stable response of the decreased DTR to La Niña events. These results suggest that ENSO is a crucial driver for the DTR variation on the SETP through oceanic‐atmospheric modulation. [ABSTRACT FROM AUTHOR]

The number of tropical cyclones (TCs) over the western North Pacific (WNP) during spring (March–May) has a significant inverse correlation with concurrent El Niño‐Southern Oscillation (ENSO) conditions during the period from 1979 to 2018. This relationship is different from the previously‐documented weak relationship between TC frequency and ENSO during the climatologically most active portion of the TC season. In general, TCs seldom occur in El Niño years during March–May, whereas they frequently form over the western part of the WNP, particularly to the southeast of the Philippines, in La Niña years. This difference can be largely explained by ENSO‐driven differences in the genesis potential index as derived from environmental variables. In La Niña years, the abnormally moist mid‐troposphere, which relates to the strengthened vertical transport of water vapour induced by the enhanced Walker Circulation, primarily favours TC development, while increased sea surface temperatures and positive low‐level relative vorticity anomalies appear to play a lesser role in impacting TC formation. [ABSTRACT FROM AUTHOR]

The eastern Pacific (EP) El Niño‐Southern Oscillation (ENSO) is an important impact factor for the East Asian trough (EAT) variability during boreal winter. However, not all ENSO events can result in anomalous EAT events, which indicates that there could be some other factors modulating the relationship between ENSO and the EAT. The analysis of atmospheric circulations indicates that the intensity of the Asian Polar vortex (PV) plays an important role in the El Niño‐EAT connection. The strengthened Asian PV favours the wave‐train triggered by El Niño events propagates eastward from the North Pacific to East Asia and finally weakens the EAT. In contrast, during El Niño years with the weakened Asian PV, El Niño's influences are confined over western Siberia and the wave activity flux over Eurasia is weak, which consequently leads to a weak response of the EAT. For the La Niña‐EAT connection, the North Pacific Oscillation (NPO) is a potential modulating factor. The La Niña and negative‐phase NPO have a strengthened effect on the EAT, resulting in a strong response of the EAT to La Niña events. In contrast, the effects of La Niña and positive‐phase NPO on the EAT are offset, leading to a weak response of the EAT to La Niña events. The above results are further confirmed by a numerical simulation of the pre‐industrial Control experiment. Therefore, the simultaneous occurrence of the Asian PV and NPO should be a great concern when understanding and predicting the EAT based on ENSO information. [ABSTRACT FROM AUTHOR]

The time‐lagged correlations of monthly precipitation in Myanmar and the Indochina Peninsula (ICP) with several climate indices were investigated by using Global Precipitation Climatology Centre dataset after checking its quality with rain‐gauge data. The results showed significant time‐lagged correlations of several climate indices related to El Niño‐Southern Oscillation and El Niño Modoki with precipitation anomalies in Southern Myanmar during pre‐monsoon (April) and post‐monsoon (October) months. Composite analysis of extremes and Empirical Orthogonal Function (EOF) analysis on precipitation variations over the whole ICP for these 2 months were carried out. The analyses showed a good association between precipitation in Southern Myanmar and a large‐scale precipitation structure that affects the Southern Myanmar and ICP simultaneously during these 2 months. An analysis of moisture flux over a wide area of ICP, including the surrounding seas, showed that the regressed variability of its convergence with the first principal component (PC1) of EOF analysis on precipitation over the whole ICP is statistically significant over the wide area from the Bay of Bengal to Southern ICP and South China Sea for the 2 months. The significant time‐lagged correlations were confirmed by regressing sea surface temperature (SST) in the Pacific and Indian Oceans and 850‐hPa zonal wind (U850) upon the PC1. We propose an area‐averaged SST, U850, or the combination of these variables at each corresponding best location in the equatorial region to the highest correlation searched in this study as a good predictor for long‐range statistical prediction of monthly precipitation in the southern part of ICP, particularly for the pre‐monsoon month. Using a linear model and the proposed predictors, we demonstrate a successful hindcast in predicting the pre‐monsoon and post‐monsoon precipitation in the southern part of ICP (south of 20 N) up to 6 and 3 months ahead, respectively. [ABSTRACT FROM AUTHOR]

At subannual timescales, terrestrial water storage (TWS) plays a critical role in the partitioning of precipitation into runoff and evapotranspiration, and Amazonia stands out due to the great amplitude of the annual cycles. In this study, we analyse the monthly variation of the water balance and extreme hydrological events using GRACE data for a watershed situated in the "arc‐of‐deforestation" in the Eastern Brazilian Legal Amazon. The existence of a correlation and lag time response between the monthly sea surface temperatures in the Pacific and North Atlantic and the variables of the water balance were also investigated. The results showed a weak seasonal cycle of evapotranspiration, with higher water intercepted by the vegetation canopy in the rainy season and higher potential evapotranspiration supplied by TWS in the dry season. The lowest value of water storage was reached in December 2015 during the strongest El Niño event observed in the analysed period (2002–2016). This value impacted the streamflow for at least the entire rainy season. The correlation analysis showed that positive anomalies of the Niño 3.4 index from October to March caused a significant decrease in precipitation in the following March to May and in November, a significant increase in the reference evapotranspiration in April and May, and longer lasting effects on the streamflow and TWS. Although the underlying mechanisms contributing to the time delays remain incompletely understood, the results indicate that consideration of the ENSO influence on the water balance and of the TWS influence on streamflow would improve flood and drought forecasts with benefits for water management. [ABSTRACT FROM AUTHOR]

The study of the interannual and intraseasonal modulation of tropical cyclogenesis (TCG) in the northwest Pacific (NWP) basin has received significant attention in the past. Most previous studies have focused on the individual impact of El Niño‐southern oscillation (ENSO) or Madden–Julian Oscillation on TCG, whereas the combined effect of ENSO and another leading intraseasonal oscillation mode, quasi‐biweekly oscillation (QBWO), on NWP TCG has rarely been studied. Therefore, this study systemically investigates the modulation of NWP TCG by the QBWO under different ENSO phases. A significant modulation of TCG by QBWO in the ENSO neutral years is observed, with significantly increased (decreased) TCG over the NWP in QBWO active (inactive) phases. Moreover, during El Niño years and La Niña years, the modulation of TCG by QBWO is not significant and no distinctive difference in the TCG frequency can be detected. Further analyses show that the modulation of TCG by the QBWO is mainly related to the changes in large‐scale environment and energy that affect TCG associated with active or inactive phases of the QBWO under different ENSO phases. Large‐scale environmental factors and energy are more favourable for TCG during ENSO neutral years compared with El Niño and La Niña years. These results are associated with QBWO cycle under the impact of background (e.g., boundary layer moisture and zonal wind shear) which is determined by ENSO conditions. [ABSTRACT FROM AUTHOR]

This article evaluates the linear isolated impacts of the East Asian winter monsoon (EAWM) and El Niño–Southern Oscillation (ENSO) on the evolution of the western North Pacific anticyclone (WNPAC) using 18 Coupled Model Inter‐comparison Project phase 5 (CMIP5) models. The independent effects of ENSO and EAWM (ENSOres and EAWMres) are obtained by removing the ENSO–EAWM interaction via linear regression. For the ENSO high‐skill (HS) models, which can well simulate an ENSO event, the anomalous WNPAC is confined to low latitudes and sustains from winter to late summer, which bears great resemblance to observational results. In contrast, the ENSO low‐skill (LS) models induce an insignificant and weak WNPAC from winter to the following summer. This is related to the simulation biases of ENSOres sea surface temperature anomalies (SSTAs). The ENSO LS models simulate a weak and slowly decaying ENSOres. This bias can induce unreasonable evolution of the WNPAC via producing an unrealistic intensity and location of the anomalous convection activity over the western Pacific, which is the key factor for yielding an ENSOres‐related WNPAC. The observed EAWMres‐related WNPAC appears only in winter and covers a large region. The EAWM HS models, which can well simulate EAWM features, reasonably reproduce the WNPAC in winter. However, the WNPAC is significant and persists to the following summer, which is inconsistent with observations. This bias is related to the EAWMres‐induced negative SSTA bias in the tropical central Pacific from spring to summer in the EAWM HS models. In contrast, the EAWM LS models, which cannot reasonably simulate EAWM features, are unable to reproduce the WNPAC in winter and the following seasons. Among the 18 state‐of‐the‐art CMIP5 models, the CNRM‐CM5 model yields a better performance in simulating the evolution of WNPAC compared with the ENSO HS models and EAWM HS models. [ABSTRACT FROM AUTHOR]

The present paper investigates the impact of El Niño‐Southern Oscillation (ENSO) on the Bay of Bengal tropical cyclone (TC) activity and associated alterations in environmental conditions during post‐monsoon (October–December) season for a period of 44 years (1972–2015). The analysis reveals that the post‐monsoon season TCs frequency, accumulated cyclone energy (ACE) and power dissipation index (PDI) values are negatively correlated with the Niño 3.4 sea surface temperature (SST) anomalies (significant at the 95% confidence level). La Niña years are characterized by more frequent and intense cyclonic events compared with El Niño years. The mean ACE and PDI values are approximately two times higher in La Niña than El Niño years. The mean number of TC days per year is also higher in La Niña (7.64 days) than El Niño (3.68 days) years (significant at 95% confidence level). In addition, a significant shift in genesis locations, tracks and landfalling locations of TCs has been observed under different ENSO phases. The mean genesis location of TCs have shifted eastward with tendency of more recurving tracks in La Niña than El Niño years. The presence of strong convective activity, reduced vertical wind shear, high SST (≥28°C), enhanced mid‐tropospheric relative humidity and low‐level cyclonic circulation aids the TCs formation and strengthening during La Niña and vice‐versa in El Niño conditions. [ABSTRACT FROM AUTHOR]

Based on a revised and extended multi‐station Hawaiʻi Temperature Index (HTI), the mean air temperature in the Hawaiian Islands has warmed significantly at 0.052°C/decade (p < 0.01) over the past 100 years (1917–2016). The year 2016 was the warmest year on record at 0.924°C above the 100‐year mean (0.202°C). During each of the last four decades, mean state‐wide positive air temperature anomalies were greater than those of any of the previous decades. Significant warming trends for the last 100 years are evident at low‐ (0.056°C/decade, p < 0.001) and high‐elevations (0.047°C/decade, p < 0.01). Warming in Hawai'i is largely attributed to significant increases in minimum temperature (0.072°C/decade, p < 0.001) resulting in a corresponding downward trend in diurnal temperature range (−0.055°C/decade, p < 0.001) over the 100‐year period. Significant positive correlations were found between HTI, the Pacific Decadal Oscillation, and the Multivariate ENSO Index, indicating that natural climate variability has a significant impact on temperature in Hawaiʻi. Analysis of surface air temperatures from NCEP/NCAR reanalysis data for the region of Hawaiʻi over the last 69 years (1948–2016) and a mean atmospheric layer temperature time series calculated from radiosonde‐measured thickness (distance between constant pressure surfaces) data over the last 40 years (1977–2016) give results consistent with the HTI. Finally, we compare temperature trends for Hawaii's highest elevation station, Mauna Loa Observatory (3,397 m), to those on another mountainous subtropical island station in the Atlantic, Mt. Izaña Observatory (2,373 m), Tenerife, Canary Islands. Both stations sit above the local temperature inversion layer and have virtually identical significant warming trends of 0.19°C/decade (p < 0.001) between 1955 and 2016. [ABSTRACT FROM AUTHOR]

Providing reliable seasonal forecasts of the climate system is essential to make decisions for resource management and planning, such that advances in seasonal forecasting by dynamical downscaling can bring significant socio‐economic benefits. A finer resolution Regional Climate Model version 4 (RegCM4) is nested within the large‐scale fields of the coarser CSIRO‐Mk3.6 general circulation model (GCM) to dynamically downscale outputs of CSIRO‐Mk3.6 over Iran in Southwest Asia. To this end, 120 6‐month‐long simulations were conducted. Starting from January 2007, each simulation run forwards 1 month compared to the previous run, altogether covering the period 2007–2016. Comparing model results with ERA‐Interim and CRU datasets indicated that RegCM4 is able to add some value to the driving CSIRO‐Mk3.6, particularly in low lands of central to eastern Iran where CSIRO‐Mk3.6 depicts significant cold biases. Nevertheless, relatively large biases in simulations of RegCM4 are found over some regions in Iran caused by propagation of errors from the driving GCM. It is also found that errors in prediction of different phases of the El Niño‐Southern Oscillation (ENSO) events in the driving CSIRO‐Mk3.6 significantly degrade the nested model simulations, such that associated with La Niña and El Niño, large warm and cold biases are found, respectively, over Iran. Improvements should be made in physics of the CSIRO‐Mk3.6 GCM to better simulate different phases of ENSO. Results also indicated that performance of RegCM4 in simulating 2‐m temperature is strongly related to the forecast lead time, such that model errors generally become larger when the forecast time increases. [ABSTRACT FROM AUTHOR]

Based on reanalysis data, we investigate the respective roles of the East Asian winter monsoon (EAWM) and El Niño–Southern Oscillation (ENSO) in building an anomalous western North Pacific anticyclone (WNPAC) in winter to the succeeding summer. The isolated ENSO and EAWM parts (termed ENSOres and EAWMres, respectively) are acquired by removing the ENSO–EAWM interdependent part through linear regression. The results suggest that both ENSOres and EAWMres can produce a WNPAC in winter. The ENSOres‐related WNPAC dominantly covers the southern part of the western North Pacific, whereas the EAWMres‐related WNPAC over the northern part of the western North Pacific. In the subsequent spring and summer, only ENSOres yields a WNPAC, while EAWMres has no effect on the atmospheric circulation over the western North Pacific. Additionally, the Pacific Decadal Oscillation (PDO) can modulate the influence of ENSOres and EAWMres on the WNPAC. There tends to be a much stronger WNPAC associated with ENSOres (EAWMres) during positive (negative) PDO phases than during negative (positive) ones. [ABSTRACT FROM AUTHOR]

Tropical cyclones (TCs) initially forming in the western North Pacific (WNP) and later moving into the South China Sea (SCS) are defined as entrance TCs. El Niño–Southern Oscillation (ENSO) and inter‐annual variability of entrance TCs during fall exhibit four types of asymmetric relationships: La Niña‐more, La Niña‐less, El Niño‐more, and El Niño‐less. Large‐scale modulation of the La Niña‐more (La Niña‐less) type features negative sea surface temperature (SST) anomalies in the tropical central‐eastern Pacific with a maximum centre in the eastern (central) Pacific. The responding circulation pair extends northwestwards (northwards) with a cyclonic anomaly in the SCS and an anticyclonic anomaly in the North Pacific (NP). Above‐normal TCs forming to the east/northeast of the Philippines are steered by anomalous southeasterly (southerly) flows to enter the SCS (NP), leading to more (less) entrance TCs. For the El Niño‐less (El Niño‐more) type, positive SST anomalies over the tropical Pacific exhibit a centre in the eastern (central) Pacific. A circulation pair characterizes an anticyclonic anomaly over the entire (southwestern) SCS and a cyclonic anomaly within the NP (displacing westwards into the northern SCS) inducing anomalous flows from the northern (central and southern) SCS into the tropical WNP. TC entrance in both El Niño‐less and El Niño‐more types is hindered by inter‐annual flows. More entrance TCs in the El Niño‐more type are facilitated by the intra‐seasonal oscillation (ISO). Entrance TCs move into the SCS along a favourable environment provided by a northwest–southeasterly elongated 30–60‐day cyclonic anomaly. Regarding the relative role of the inter‐annual and intra‐seasonal modes, ISO (ENSO) exerts major modulatory effects on entrance TCs in the El Niño‐more (El Niño‐less) type. Both ISO and ENSO jointly facilitate (suppress) entrance TCs in the La Niña‐more (La Niña‐less) type. The 1960–2010 time series of the number of entrance TCs from the WNP into the SCS during fall. The long‐term mean is 2.39, indicated by a horizontal line. [ABSTRACT FROM AUTHOR]

The significant wave height (Hs) variability caused by wind anomalies associated with the co‐occurrence of the Madden‐Julian Oscillation (MJO) and El Niño–Southern Oscillation (ENSO) was investigated in the New Zealand region. For this purpose, Hs and wind anomalies composites were created using 23 years (1979–2002) of modelled data during November–March periods, when simultaneous ENSO‐MJO phase pairs are potentially most active. The results show striking features: El Niño‐related wave conditions (which consist of increased Hs along the west and south coasts of New Zealand) are reinforced during MJO phase 8, whereas the wave conditions associated with La Niña (which consist of larger Hs along the north coast) are enhanced during MJO phase 6; Similar wave anomalies are generated during opposing ENSO phases (La Niña and El Niño) when these are combined with MJO phases 3 and 5, respectively; The majority of statistically significant Hs anomalies disappear from the study area during El Niño‐MJO phase 6, El Niño‐MJO phase 2, and La Niña‐MJO phase 4, showing the neutralizing nature of some phase combinations; Finally, negative Hs anomalies are experienced off the country's west coast during El Niño‐MJO phase 4, in contrast to the positive anomalies expected during El Niño events. These results clearly show the importance of remote forcing to wave anomalies in the New Zealand region, and highlight the need to assess atmospheric and oceanic conditions considering multiple climate oscillations. Significant wave height (Hs) anomalies are substantially modified during simultaneous ENSO‐MJO phase pairs (as shown by the statistically significant Hs daily anomaly composites in the image) relative to those anomalies expected during periods in which only the ENSO is active (e.g., increased Hs occurs along the west and south coasts of New Zealand during El Niño). Changes in Hs anomalies comprise reinforcement, nullification, and even sign reversal. Some phase combinations are associated with hazardous conditions along the New Zealand coastline. [ABSTRACT FROM AUTHOR]

Using observed and reanalysed data sets, we investigate the potential role of Atlantic Multi‐decadal Oscillations (AMO) in weakening the in‐phase association between Indian summer monsoon rainfall (ISMR) and following Australian summer monsoon rainfall (ASMR) during the 1932–1966 period. This weakened inter‐monsoon link was earlier attributed to a complete breakdown of the El Niño–Southern Oscillation (ENSO) links with the ASMR. We now find that this weakening was associated with strengthening of low‐level westerlies and an increase in local low‐level convergence in the northern Australia associated with the warm phase of the AMO during this period. This increased background convergence resulted in relatively enhanced ASMR during the El Niño summers during this period. Using observed and reanalysed data sets, we investigate the potential role of Atlantic Multi‐decadal Oscillations (AMO) in weakening the in‐phase association between Indian summer monsoon rainfall (ISMR) and following Australian summer monsoon rainfall (ASMR) during the 1932–1966 period. In the figure, a schematic representation of the proposed mechanism by which the AMO influence the ASMR–ENSO relation. [ABSTRACT FROM AUTHOR]

El Niño‐Southern Oscillation (ENSO) has a large impact on the succeeding East Asian summer monsoon (EASM) variability via modulating the western North Pacific (WNP) anticyclonic anomaly. This study reveals that spring northern tropical Atlantic (NTA) sea surface temperature (SST) has a prominent modulation of the ENSO–EASM relationship. When the spring NTA SST anomalies are of the same sign as the preceding winter equatorial central‐eastern Pacific SST anomalies, the ENSO–EASM connection is strong. The connection between ENSO and EASM becomes very weak when the spring NTA and preceding winter equatorial central‐eastern Pacific SST anomalies are opposite. This result implies that it is crucial to consider the spring NTA SST status when forecasting the EASM based on the winter equatorial central‐eastern Pacific SST. The modulation of spring NTA SST anomalies on the ENSO–EASM relation is through its destructive or constructive contribution to the anomalous anticyclone over the WNP. When the spring NTA SST anomalies and the equatorial central‐eastern Pacific SST anomalies have the same (opposite) signs, the summer WNP anticyclonic anomaly generated by the ENSO was largely enhanced (weakened) by the spring NTA SST‐generated atmospheric anomalies, which result in strong (weak) influence of ENSO on the EASM variability. From the above figure, it shows that spring northern tropical Atlantic (NTA) SST has a pronounced modulation of the relationship between preceding ENSO and following EASM. When winter Niño3.4 SST and spring NTA SST anomalies have the same (opposite) sign, the ENSO–EASM connection is strong (weak). The modulation of spring NTA SST anomalies on the ENSO–EASM relation is through its destructive or constructive contribution to the anomalous anticyclone over the WNP. [ABSTRACT FROM AUTHOR]

Previous studies have indicated that the relationship between El Niño‐Southern Oscillation (ENSO) and the East Asian winter monsoon (EAWM) weakened significantly after the late 1970s. In this study, the relationship is further analysed based on the impact of different ENSO types. It is found that the relationship between ENSO and the EAWM is related to the ENSO's types. The relationship with the EAWM is strong for eastern Pacific (EP) ENSO but weak for central Pacific (CP) ENSO. The blend of EP and CP ENSO events in the Niño3 index results in a weakened statistical relationship between ENSO and the EAWM. If the pure CP ENSO events are removed from the Niño3 index, the results show that EP ENSO still exerts influence on the EAWM after the late 1970s via its strong connections with the Philippine Sea anticyclone (PSAC) and East Asian trough (EAT), similar to the period before the late 1970s. In contrast, the connections between CP ENSO and the PSAC and EAT are relatively weak, consequently resulting in a weakened relationship between CP ENSO and the EAWM. The influence of mixed (MIX) ENSO on the EAWM is similar to that of EP ENSO. Thus, the EP and MIX ENSO events have had a stable influence on the East Asian winter climate over the past half century, which is significant for the prediction of the East Asian winter climate. Scatterplot of the normalized EAWMI‐1 and Niño3 indices during the period after the late 1970s. The light‐coloured circles represent the normal years. The dark‐coloured rectangles and diamonds represent the EP and CP ENSO events, respectively. A symbol overlaid with a pentagram represents a MIX ENSO event. The results indicate that the EP and MIX ENSO (CP ENSO) events have a strong (weak) influence on the East Asian winter climate. [ABSTRACT FROM AUTHOR]

The features and possible causes of prolonged seasonal drought (PSD) events over northern China (NC) during 1960–2012 are explored in this study using the multi‐scale standardized precipitation evapotranspiration index (SPEI). The results show that PSD events (i.e., drought events with a timescale of 9 months or less) have had a relatively high probability of occurrence from spring to autumn during the past five decades. The results of the empirical orthogonal function (EOF) analysis show that the leading mode of the SPEI09 in autumn reflects a broad pattern of drought in NC, which confirms the existence of the PSD events. Furthermore, we conclude that two factors, teleconnection patterns in the mid‐latitudes of Eurasia and transitions from La Niña to El Niño, are mainly responsible for the PSD events. On the one hand, the associated teleconnection patterns in the upper‐level troposphere show an Eurasian (EU)‐like pattern in spring, the Silk Road (SR) pattern or the circumglobal teleconnection (CGT) pattern in summer, and a Scandinavia (SCA)‐like pattern in autumn, respectively. Associated with these teleconnection patterns, descending anomalies are detected in the whole atmosphere over NC in all seasons. On the other hand, the transition from La Niña (winter) to El Niño (the following autumn) also exerts effects on the PSD through modulating the Walker circulation and the meridional vertical circulation along the coast of East Asia. However, this process is more effective in spring relative to summer and autumn. In addition, physical connections likely exist among these factors, and their combined effects, together with the insufficient moisture supply, regulate the PSD events over NC. As shown in the figure, the drought events over northern China exhibit a high probability of persisting over multiple seasons (with a timescale of 9 months or less), and such events may exert more catastrophic impacts on agriculture, ecosystems, and socioeconomic development than the single‐season drought events. The present study found that the prolonged seasonal drought (PSD) events can be attributed to two possible causes, that is, the teleconnection patterns in the mid‐latitudes of Eurasia and transitions from La Niña to El Niño. We provide clues for improving the understanding and prediction of the PSD events. [ABSTRACT FROM AUTHOR]

The characteristics of the northern and southern modes of the East Asian winter monsoon (EAWM) and their relationships with El Niño‐Southern Oscillation (ENSO) are further investigated in this paper using reanalysis data. The results demonstrate that the northern mode shows strong inter‐decadal variations, whereas the southern mode is marked by noticeable inter‐annual variations. The west–east pressure gradient plays a leading role in influencing northern mode, and both the west–east and north–south pressure gradients have significant impacts on the southern mode. The two modes are closely related, but their relationship shows inter‐decadal variations. The relationship between the southern mode and ENSO is significant and stable, but the relationship between the northern mode and ENSO is unstable and shows obvious inter‐decadal variations. The relationship between the northern mode and ENSO is significant before the late 1960s and after late 1990s, but it is not significant during the period between the late 1960s and late 1990s. ENSO influences the activity of northern mode mainly by changing the west–east pressure gradient. During the periods of a weak relationship between northern mode and ENSO, the variations in west–east pressure gradient may be dominated by the climate systems at mid–high latitudes, and the effects of ENSO are confined or offset. The inter‐decadal variations in the relationship between northern mode and ENSO may be modulated by the Pacific Decadal Oscillation (PDO) and Atlantic Multi‐decadal Oscillation (AMO), but their roles and importance have varied over time. The relationship between EAWM and ENSO and its inter‐decadal variations vary with the EAWM index, which is induced by the differences between the northern and southern modes. When the EAWM index is more closely correlated with the southern (northern) mode, its relationship with ENSO is (not) significant and stable (unstable). [ABSTRACT FROM AUTHOR]

ABSTRACT: The North Pacific subtropical front (NPSTF) of sea surface temperature (SST) is an important subtropical feature in the North Pacific. In this study, we reveal that the inter‐annual variability of the spring NPSTF has a robust correlation with the El Niño‐Southern Oscillation (ENSO) in the following winter over the period of 1961–2016. When there is a strong spring NPSTF, anomalous cyclonic and anticyclonic circulations in the low troposphere occur to its north and south, respectively. The northeasterly wind anomaly associated with the anticyclonic circulation increases the climatological northeasterly wind over the subtropical Northeast Pacific, resulting in SST decrease via the wind–evaporation–SST feedback. The SST and wind perturbations propagate southwards from the subtropical to the eastern central equatorial Pacific in summer and grow via the Bjerknes feedback from summer to winter, eventually leading to a La Niña event in winter. A strong spring NPSTF induces an anomalous anticyclonic circulation to its south through both transient eddy activity and atmospheric heat source anomaly. The enhanced air temperature gradient due to a strong NPSTF increases the atmospheric baroclinicity, favouring an enhanced transient eddy activity over the NPSTF and to the north. The enhanced transient eddy activity triggers a significant negative height anomaly to the north via high‐frequency transient eddy feedback forcing, which is conductive to an anticyclonic circulation over the subtropical North Pacific through the downstream effect of Rossby waves. On the other hand, the convergence and increased atmospheric baroclinicity related to the strong NPSTF act to enhance precipitation over the NPSTF and to the north, corresponding to the increased atmospheric heat source, which can also excite an anomalous anticyclonic circulation over the subtropical North Pacific. [ABSTRACT FROM AUTHOR]

The relationships between the synoptic climatology, large-scale teleconnections and the regional avalanche activity index (RAAI) inferred from tree-rings were evaluated for the Presidential Range in the White Mountains (New Hampshire, USA). During the period 1936-2012, 18 years of regional avalanche activity were compared with the winter-scale prevailing joint temperature/climatic modes (cold/wet (CW), cold/dry (CD), warm/wet (WW) and warm/dry (WW)), the North Atlantic Oscillation (NAO), the El Nino-Southern Oscillation (ENSO), the 500-mbar composite anomaly maps and the ratio of snow from different storm tracks. The total winter snowfall and the NAO negatively correlate with the RAAI. There is also a significant difference in avalanche activity between winters with a NAO under or above -3. Winters of regional avalanche activity were present in the four climatic modes, albeit the ratio of avalanche/non-avalanche years is superior for CW winters compared to the three other modes, as well as for wet winters compared to dry winters. CW, CD and WW winters exhibit a negative NAO anomaly, which is eastbound for the wet years. Cold winters (CW, CD) receive more snow from the Great Lakes, whereas coastal depressions are more important during wet winters (CW, WW). The NAO is an adequate predictor of snowfall, but does not provide information about the storm tracks. On the contrary, the ENSO is poorly correlated with snowfall, but its relationship is significant with the ratio of snow produced by coastal depressions (positive relationship) and Great Lakes storms (negative relationship). These are the first results quantifying the atmospheric circulation – synoptic meteorology – snow avalanche relationships in Northeastern North America. [ABSTRACT FROM AUTHOR]

The dominant seasonal surface air temperature (SAT) anomalous modes over entire South America (SA) for the 1951–2010 period were investigated. The El Niño-Southern Oscillation (ENSO) and non-ENSO (residual - RES) characteristics of the SAT anomalous modes and the associated precipitation and temperature advection anomalies were discussed. The SAT (referred to as TOT) and RES modes were obtained from the empirical orthogonal function analyses, and the ENSO modes from the correlation analyses between the SAT anomalies and the Oceanic Niño index. The SAT TOT modes are in general, weakly linked to the ENSO, except for the first summer and the fourth spring TOT modes. For these two modes, the precipitation variations are more crucial than the temperature advection variations in defining the SAT anomaly patterns. On the other hand, the first winter and autumn TOT modes contain mostly the non-ENSO components and feature an extensive warmed area in tropical and subtropical SA. For these modes, the subtropical warming is due to the temperature advection. The TOT modes with a weak relation to the ENSO and the corresponding RES modes show a dipolar pattern with a warm node in tropical SA and the cold one in extratropical SA, and is determined mostly by the precipitation. Nevertheless, the cold advection reinforces the cold node of the winter and summer modes. In some cases, the regional systems contribute to the SAT anomalies. The South American low-level jet contributes to the positive SAT anomalies in central and eastern Brazil for the second winter and first spring RES modes and the South Atlantic Convergence Zone influences the first summer RES mode. The SAT modes describe mostly the interannual variations of the SAT patterns over SA, but some of them also contain decadal and multidecadal time scale variations. [ABSTRACT FROM AUTHOR]

ABSTRACT The impacts of El Niño ( EN) on the rainfall over South America are investigated considering three EN types differing in the locations of the maximum sea surface temperature ( SST) anomalies in the equatorial Pacific: the Central-Pacific ( CP), Eastern-Pacific ( EP) and the Mixed ( MIX) types. The largest positive ( SST) anomalies for the EP and CP types occur, respectively, in the eastern and central sectors of the tropical Pacific during all seasons. The SST anomaly pattern for the MIX-EN resembles that of the EP-EN during its onset stage, and of the CP-EN during its mature and demise stages. The different SST anomaly patterns affect the large-scale (Walker circulation and the tropospheric Rossby-wave trains) and local (South American low-level jet - SALLJ) atmospheric circulation patterns in different ways and lead to distinct precipitation anomaly patterns over South America. Variations in the position and longitudinal extension of the downward motions of the EN-related eastward-displaced Walker circulation explain the differences in the dryness over equatorial South America. For the CP-EN, a double Walker cell defines a more zonal configuration of the equatorial dryness over South America during the first three analysed seasons. This feature is not noted for the other two EN types. The Rossby-wave train pattern path depends on the EN types and seasons. In consequence, the associated local atmospheric circulation patterns depend also on the season and EN types. In all seasons, an intense SALLJ for the EP EN contrasts with weak or inexistence SALLJ for the other two EN types. Thus, a wetter condition over southeastern South America, southern and eastern Brazil occurs for the EP EN in relation to the other EN types. The results shown here, in particular considering the MIX EN type, have not been discussed before and might be useful mainly for climate monitoring purposes. [ABSTRACT FROM AUTHOR]

ABSTRACT Heavy rainfall months of more than 450 mm occur in all 56 meteorological stations in eight climatic zones of Vietnam during the rainy season from April to September in the north (>20°N), from August to December in the centre and from May to November in the south (<12°N). The severity of an El Niño Southern Oscillation ( ENSO) episode, expressed as the integral of sea surface temperature anomaly ( SSTA) in the central tropical Pacific over the duration, shows a 4.6-fold (2.3-fold) increase in number of heavy rainfall months during La Niña (El Niño) per unit change in severity during the 1960-2009 period, suggesting a twin peak occurrence with both ENSO extremes. A heavy rainfall index ( HRI) links heavy rainfall months to the rainy season duration, and allows evaluation of the rainfall severity per station, climatic zone and ENSO cycle. For the deltas and central climatic zones, seasonal rainfall and number of heavy rainfall months are significantly higher at the p < 0.05 level during La Niña than during El Niño episodes. Interpolated seasonal rainfall shows distinct differences between regions, with location having a larger effect than ENSO cycles on monthly rainfall amounts. Twenty-year return monthly rainfall derived from generalized Pareto distributions for peak over thresholds range from 475 mm in the central highlands to 2185 mm in the central coast. The spatial and temporal patterns of heavy monthly rainfall help explain flooding and paddy inundation which occur at least twice as frequent during La Niña as compared to El Niño conditions, particularly in Central Vietnam. The relation of HRI with both 20-year return levels and ENSO cycles offers opportunities for fast screening of impacts in a wider region of Southeast Asia. Because ENSO cycles have an impact on flooding and paddy inundation, it provides prospects for early warning, differentiated for different zones and rainfall regimes. [ABSTRACT FROM AUTHOR]

ABSTRACT The simulated response of the East Asian winter monsoon ( EAWM) to ENSO-like sea surface temperature anomalies ( SSTAs) forcing in three U.S. Climate Variability and Predictability (U.S. CLIVAR) Project models ( GFDL, CCM3, and CCM3.5) are evaluated. The time series of the ENSO-like pattern significantly correlates with the EAWM index for the period 1949-2004 based on the HadISST SST data set and the NCEP atmospheric reanalysis. Their correlation coefficient is −0.32 at the 95% confidence level. The warm (cold) phase of ENSO-like SSTAs is associated with a weakened (strengthened) Siberian High and an eastern-positioned (western-positioned) Aleutian Low, anomalous southerly (northerly) wind at 850 hPa and positive (negative) surface air temperature anomalies over East Asia and western North Pacific, while 300 hPa zonal wind anomalies emerge with a reduced (accelerated) East Asian jet stream. Further model examination reveals that the simulated response over East Asia forced by warm (cold) ENSO-like SSTAs in CCM3 and the three-model ensemble corresponds to the weakened (strengthened) EAWM. The spatial correlation coefficients between the three-model ensemble and observation in the atmospheric circulation and surface air temperature range from 0.15 to 0.80 at the 99% confidence level. This success in simulating relationship between ENSO-like SSTAs and EWAM using three-model ensemble proposes a new idea for the EAWM prediction. [ABSTRACT FROM AUTHOR]