El Nino-Southern Oscillation Signal Research Articles

The curious case of a strong relationship between ENSO and Indian summer monsoon in CFSv2 model

An ensemble of forecasts is necessary to identify the uncertainty in predicting a non-linear system like climate. While ensemble averages are often used to represent the mean state and diagnose physical mechanisms, they can lead to information loss and inaccurate assessment of the model’s characteristics. Here, we highlight an intriguing case in the seasonal hindcasts of the Climate Forecast System version 2 (CFSv2). While all ensemble members often agree on the sign of predicted El Niño Southern Oscillation (ENSO) for a particular season, non-ENSO climate forcings, although present in some of the individual members, are disparate. As a result, an ensemble mean retains ENSO anomalies while diminishing non-ENSO signals. This difference between ENSO and non-ENSO signals significantly influences moisture convergence and Indian summer monsoon rainfall (ISMR). This stronger influence of ENSO on seasonal predictions increases ENSO–ISMR correlation in ensemble mean seasonal hindcasts. Thus, this discrepancy in the ENSO–ISMR relationship is not present in the individual ensemble members, considered individually or together (without averaging) as independent realizations. Therefore, adequate care should be taken while evaluating physical mechanisms of teleconnection in ensemble mean predictions that can often be skewed due to constructive or destructive superposition of different impacts.

Unraveling the role of the western North Pacific circulation anomaly in modulating Indian summer monsoon rainfall variability beyond ENSO

The Indian summer monsoon (ISM) rainfall interannual variability is known to be strongly linked to the El-Niño–Southern Oscillation (ENSO). This linear relationship is the primary factor in controlling the interannual variation in ISM precipitation. However, there are many outlier cases, and such deviations pose significant challenges in seasonal prediction over this region. Here we show that such challenges can be attributed to anomalous atmospheric pressure patterns in the Western North Pacific (WNP) region. The anticyclonic circulation anomaly over WNP region causes the easterly wind toward the Indian subcontinent, leading to positive precipitation anomalies with stronger low-level moist convergence, while the cyclonic circulation decreases ISM precipitation. The linear baroclinic model simulation results further support that the WNP circulation pattern can serve as an independent factor for forecasting precipitation over India. The WNP circulation anomaly play the crucial role generating ISM precipitation particularly for July and September. Our study suggests that the role of the WNP circulation anomaly should be carefully considered as the secondary prevailing mechanism on the subseasonal timescale during the boreal summer in addition to the ENSO signal.

Indian Ocean Dipole (IOD) forecasts based on convolutional neural network with sea level pressure precursor

Abstract Forecasting the Indian Ocean Dipole (IOD) is crucial because of its significant impact on regional and global climates. While traditional dynamic and empirical models suffer from systematic errors due to nonlinear processes, convolutional neural networks (CNN) are nonlinear in nature and have demonstrated remarkable El Niño Southern Oscillation (ENSO) and IOD forecasting skills based on oceanic predictors, particularly sea surface temperature and heat content. However, it is difficult to measure heat content and easily introduces uncertainties, prompting the need to explore atmospheric predictors for IOD forecasts. Based on sensitivity prediction experiments, we identified the sea level pressure (SLP) signal as a crucial predictor, which forecasts IOD at a 7 month lead. In addition, the CNN model improves monthly forecasting accuracy while reducing errors by 13.43%. Utilizing the heatmap analysis, we elucidated that the multi-seasonal predictability of the IOD primarily originates from mid-latitude climate variability. Besides ENSO signals in the Pacific Ocean, our study highlights the significant impact of remote climate forcing in the South Indian Ocean, tropical North Indian Ocean, and Northwest Pacific Ocean on IOD forecasts. By introducing the SLP precursor and extratropical zones into IOD forecasts, our study offers fresh insights into the underlying dynamics of IOD evolution.

Links between the Botswana High and drought modes over southern Africa

AbstractDrought is one of the most devastating threats to the livelihoods of the southern African population, who mainly rely on rain‐fed agriculture for income. Previous studies have highlighted that the Botswana High influences drought over the region; however, its influence on the spatial modes of drought remains unknown. This study examines the spatiotemporal structures of drought modes (DMs) over southern Africa and their link with the Botswana High in observation, reanalysis and Model for Prediction Across Scales (MPAS). To characterize droughts, the study uses the 3‐month scale standardized precipitation index (SPI) and the standardized precipitation evapotranspiration index (SPEI). Spatiotemporal characteristics of the DMs are identified using empirical orthogonal function (EOF) analysis on SPI and SPEI. EOF analysis is also used to identify the spatiotemporal characteristics of the Botswana High. The relationship between each DM and the Botswana High is quantified using correlation and R2 analysis. In all the datasets (Climate Research Unit (CRU), European Centre for Medium‐Range Weather Forecasts version 5 (ERA5), 20th Century reanalysis II (20C) and MPAS), the most dominant five DMs (hereafter DM1–DM5) over southern Africa jointly explain more than 60% of the interannual variability in the 3‐month scale summer droughts for SPEI and SPI. CRU, ERA5 and MPAS agree that the Botswana High correlates with the interannual variability of DM1, with a stronger correlation in ERA5 (r = −0.85) compared to MPAS (r = −0.42) and CRU (r = −0.35). Additionally, wet years (+ve SPEI and SPI) are characterized by a weak Botswana High and drought years (−ve SPEI and SPI) by a strong Botswana High. The wet and dry years correspond to the −ve and +ve phases of El Niño–Southern Oscillation (ENSO), respectively. Given this, the results of this study suggest that the Botswana High might be a teleconnection pattern through which ENSO signals influence DM1 over the region.

ENSO Disrupts Boreal Winter CRE Feedback

Abstract Twenty years of satellite-based cloud and radiation observations allow us to examine the observed cloud radiative effect (CRE) feedback (i.e., CRE change per unit change in global mean surface temperature). By employing a decomposition method to separate the contribution of “internal changes” and “relative-frequency-of-occurrence (RFO) changes” of distinct cloud regime (CR) groups, notable seasonal contrasts of CRE feedback characteristics emerge. Boreal winter CRE feedback is dominated by the positive shortwave CRE (SWCRE) feedback of oceanic low-thick clouds, due to their decreasing RFO as temperature rises. This signal is most likely due to El Niño–Southern Oscillation (ENSO) activity. When ENSO signals are excluded, boreal winter CRE feedback becomes qualitatively similar to the boreal summer feedback, where several CR groups contribute to the total CRE feedback more evenly. Most CR groups’ CRE feedbacks largely come from changing RFO (e.g., the predominant transition from oceanic cumulus to broken clouds and more occurrences of higher convective clouds with warming temperature). At the same time, low-thick and broken clouds experience optical thinning and decreasing cloud fraction, and these features are more prominent in boreal summer than winter. Overall, the seasonally asymmetric patterns of CRE feedback, primarily due to ENSO, introduce complexity in assessments of CRE feedback.

The impact of global warming on ENSO from the perspective of objective signals

Most studies on the relationship between global warming and El Niño-Southern Oscillation (ENSO) rely on simulation experiments, but they have produced contradictory results, which cannot guarantee the reliability of the experimental findings. To address this challenge, we approached the issue from the perspective of objective ENSO and global warming signals. The effects of global warming on ENSO are not limited to the Earth's surface, but also present in the atmosphere. While objective monitoring indices exist for surface global warming and ENSO signals, objective ENSO and global warming signals in the atmosphere are difficult to obtain. To obtain high-precision ENSO and global warming signals in the atmosphere, we propose using the strategy of ICA (Independent Component Analysis) combined with a non-parametric method. The results of the objective ENSO and global warming signals suggest that the global warming signal leads the ENSO signal by approximately three months and exhibits a clear positive correlation with the ENSO signal, especially the occurrence of strong ENSO events is closely related to global warming, indicating that global warming promotes the occurrence of ENSO events. Under the influence of global warming, the frequency and intensity of ENSO increase, the characteristics and complexity of ENSO atmospheric teleconnections may be altered, and there is a greater tendency for more destructive El Niño events to occur. Since the beginning of the 21st century, the probability of ENSO event has increased significantly. Furthermore, moderate or strong ENSO events tend to develop earlier, and extreme ENSO events not only develop earlier but also prolong their decay phase, leading to an extended lifespan of ENSO events. This paper investigates the effect of global warming on ENSO from the perspectives of objective ENSO and global warming signals, which well supplements the objective factual basis for the previous simulation experimental studies.

Orographic amplification of El Niño teleconnections on winter precipitation across the Intermountain West of North America

A large proportion of western North America experiences regular water stress, compounded by high seasonal and interannual variability. In the Intermountain West region, the El Niño/Southern Oscillation (ENSO) is a critical control on winter precipitation, but the nature of this signal is entangled with a combination of orographic effects and long-term climate trends. This study employs a spatially distributed, nonlinear spline model to isolate ENSO impacts from these other factors using gauge-based observations starting in 1871. In contrast to previous modelling approaches, our approach uses original gauge data, without shortening the record to accommodate a common period. This enables more detailed separation of ENSO effects from the confounding influence of topography and long-term trends, whereas the longer time frame permits more robust correlation with the ENSO signal. Here we show that the complex topography of the Intermountain West exaggerates the underlying ENSO signal, producing a 2.3–5.8 times increase in the range of ENSO-induced precipitation changes along high-elevation western slopes relative to lower elevations. ENSO effects on winter precipitation can be as large as ± 100 mm at high elevations. Further, our approach reveals that the previously recognized dipolar pattern of positive (negative) association of ENSO with precipitation in the south (north) manifests as an incremental relationship in the south but as a near-binary switch in effects between El Niño and La Niña in the north. The location and extent of the strongest precipitation differences vary during the positive and negative ENSO phases within each region. The intricacies of these spatial- and elevation-based modulations of ENSO impacts are especially informative for the northern centre of this dipole, where ENSO-precipitation relationships have previously been difficult to resolve.

Wavelet Analysis of Ozone Driving Factors Based on ~20 Years of Ozonesonde Measurements in Beijing

A long-term vertical ozone observational dataset has been provided during 2001–2019 by ozonesonde measurements in Beijing on the North China Plain. Previous studies using this dataset primarily focused on the vertical characteristics of climatological ozone and its variation; however, the driving factors of ozone variation have not been well discussed. In this study, by applying the wavelet analysis method (including continuous wavelet transform and cross wavelet) and sliding correlation coefficients to ~20 years of ozonesonde measurements collected in Beijing, we analyzed the dominant modes of ozone column variability within three height ranges over Beijing (total column ozone: TOT; stratospheric column ozone: SCO; and tropospheric column ozone: TCO). Moreover, we also preliminarily discussed the relationship between these three ozone columns and the El Niño Southern Oscillation (ENSO), Quasi-biennial Oscillation (QBO), and 11-year solar activity cycle. The results revealed that the ozone columns within the three height ranges predominantly adhered to interannual variability patterns, and the short-term variabilities in TOT and SCO may have been related to eruptive volcanic activity. In comparison to the TOT and SCO, the TCO was more susceptible to the forcing influences of high-frequency factors such as pollutant transport. Similar to the results in other mid-latitude regions, strong ENSO and QBO signals were revealed in the interannual ozone column variability over Beijing. The TOT and SCO showed positive anomalous responses to ENSO warm-phase events, and the peak of the ENSO warm phase led the winter peaks of the TOT and SCO by approximately 3–6 months. During the strong cold–warm transition phase in 2009–2012, the TOT and SCO showed a significant positive correlation with the ENSO index. The strong seasonality of the meridional circulation process driven by the QBO led to a significant positive correlation between the QBO index and the TOT and SCO in the interannual cycle, except for two periods of abnormal QBO fluctuations in 2010–2012 and 2015–2017, whereas the TCO showed a time-lagged correlation of approximately 3 months in the annual cycle relative to the QBO due to the influence of the thermodynamic tropopause. In addition, analysis of the F10.7 index and the ozone columns revealed that the ozone columns over Beijing exhibited lagged responses to the peaks of sunspot activity, and there was no obvious correlation between ozone columns and 11-year solar activity cycle. Given the complex driving mechanism of the climatic factors on local ozone variability, the preliminary results obtained in this study still require further validation using longer time series of observational data and the combination of chemical models and more auxiliary data.

Seasonal Dependence of the Pacific–North American Teleconnection Associated with ENSO and Its Interaction with the Annual Cycle

Abstract The Pacific–North American (PNA) teleconnection pattern is one of the prominent atmospheric circulation modes in the extratropical Northern Hemisphere, and its seasonal to interannual predictability is suggested to originate from El Niño–Southern Oscillation (ENSO). Intriguingly, the PNA teleconnection pattern exhibits variance at near-annual frequencies, which is related to a rapid phase reversal of the PNA pattern during ENSO years, whereas the ENSO sea surface temperature (SST) anomalies in the tropical Pacific are evolving much slower in time. This distinct seasonal feature of the PNA pattern can be explained by an amplitude modulation of the interannual ENSO signal by the annual cycle (i.e., the ENSO combination mode). The ENSO-related seasonal phase transition of the PNA pattern is reproduced well in an atmospheric general circulation model when both the background SST annual cycle and ENSO SST anomalies are prescribed. In contrast, this characteristic seasonal evolution of the PNA pattern is absent when the tropical Pacific background SST annual cycle is not considered in the modeling experiments. The background SST annual cycle in the tropical Pacific modulates the ENSO-associated tropical Pacific convection response, leading to a rapid enhancement of convection anomalies in winter. The enhanced convection results in a fast establishment of the large-scale PNA teleconnection during ENSO years. The dynamics of this ENSO–annual cycle interaction fills an important gap in our understanding of the seasonally modulated PNA teleconnection pattern during ENSO years.

Solar Forcing of ENSO on Century Timescales

AbstractUnderstanding how El Niño‐Southern Oscillation (ENSO) responds to natural variability is of key importance for future climate projections under a warming climate. However, there is no clear consensus on what drives ENSO's variability on centennial timescales. Here, we find that the epikarst in southeastern Alaska is effective at filtering ENSO and solar irradiance signals from the Aleutian Low regional climate, which are subsequently recorded in speleothem proxy data. By applying a correlation test, we find that ENSO was significantly influenced by solar irradiance over the past ∼3,500 years. This relationship dissolved after ∼1970 CE, with ENSO now being dominated by anthropogenic forcing. This implies a new ENSO mean state that will need to be incorporated into future climate projections.

Quantifying the relative contributions of climate change and ENSO to flood occurrence in Bangladesh

Bangladesh is highly vulnerable to flood hazards, and its flood risk is projected to increase with global warming. In addition to climate change, internal climate variation, such as the El Niño–Southern Oscillation (ENSO), influences flooding in many rivers worldwide. However, the impact of internal climate variability on flooding in Bangladesh remains unclear due to the limited observations. Here, we assess the impacts of ENSO and climate change on flood occurrence in Bangladesh using a large-ensemble climate simulation dataset and a global river model (CaMa-Flood). After separating 6000 years of simulation (100-member ensemble river simulations for 1950–2010) into El Niño, La Niña, and neutral years, we calculated the extent to which each ENSO stage increased flood occurrence probability relative to the neutral state using the fraction of attributable risk method. In addition, we estimated the impact of historical climate change on past flood occurrence through a comparison of simulations with and without historical global warming. Under the no-global-warming climate, La Niña increased the occurrence probability of a 10 year return period flood at Hardinge Bridge on the Ganges River by 38% compared to neutral years. The influence of La Niña or El Niño state on flood occurrence probability in the Brahmaputra River at Bahadurabad station is negligible. Historical global warming increased the occurrence of flooding in the Ganges River, the Brahmaputra River, and their confluence by 59%, 44%, and 55%, respectively. The impact of ENSO on flood occurrence probability decreased in the historical simulation, likely due to the conflation of ENSO and climate change signals, and no significant correlation between ENSO and flood occurrence was detected when only small-ensemble simulations were used. These findings suggest that the use of large-ensemble climate simulation datasets is essential for precise attribution of the impacts of internal climate variability on flooding in Bangladesh.

The Impact of Eastern Pacific Warming on Future North Atlantic Tropical Cyclogenesis

AbstractTropical cyclogenesis in the Atlantic is influenced by environmental parameters including vertical wind shear, which is sensitive to forcing from the tropical Pacific. Reliable projections of the response of such parameters to radiative forcing are key to understanding the future of hurricanes and coastal risk. One of the least certain aspects of future climate is the warming of the eastern tropical Pacific Ocean. Using climate model experiments isolating the warming of the eastern Pacific and controlling for other factors including El Niño‐Southern Oscillation (ENSO), changes in Atlantic tropical cyclogenesis potential by the end of this century are ∼20% lower with enhanced eastern Pacific warming. The ENSO signal in Atlantic tropical cyclogenesis potential amplifies with global warming, and that amplification is larger with enhanced eastern Pacific warming. The largest changes and dependencies on eastern Pacific warming are found in the south‐central main development region, attributable to changes in zonal overturning.

ENSO statistics, teleconnections, and atmosphere–ocean coupling in the Taiwan Earth System Model version 1

Abstract. This study provides an overview of the fundamental statistics and features of the El Niño–Southern Oscillation (ENSO) in the historical simulations of the Taiwan Earth System Model version 1 (TaiESM1). Compared with observations, TaiESM1 can reproduce the fundamental features of observed ENSO signals, including seasonal phasing, thermocline coupling with winds, and atmospheric teleconnection during El Niño events. However, its ENSO response is approximately 2 times stronger than observed in the spectrum, resulting in powerful teleconnection signals. The composite of El Niño events shows a strong westerly anomaly extending fast to the eastern Pacific in the initial stage in March, April, and May, initiating a warm sea surface temperature anomaly (SSTA) there. This warm SSTA is maintained through September, October, and November (SON) and gradually diminishes after peaking in December. Analysis of wind stress–SST and heat flux–SST coupling indicates that biased positive SST–shortwave feedback significantly contributes to the strong warm anomaly over the eastern Pacific, especially in SON. Our analysis demonstrates TaiESM1's capability to simulate ENSO – a significant tropical climate variation on interannual scales with strong global impacts – and provides insights into mechanisms in TaiESM1 related to ENSO biases, laying the foundation for future model development to reduce uncertainties in TaiESM1 and climate models in general.

Oxygen isotope variability in precipitation, dripwater, and modern calcite responding to ENSO based on 11 years’ monitoring in Yuhua Cave, Southeast China

The scientific explanation of cave δ18O in the East Asian Summer Monsoon (EASM) region is a crucial issue restricting stalagmite-inferred paleoclimate research. The connection between El Niño Southern Oscillation (ENSO) and monsoonal precipitation/cave δ18O and the mechanism involved also remain contentious. Long-term cave monitoring is an effective solution to deal with these issues. Here, we present 11-year-long precipitation, dripwater, and modern calcite δ18O data from Yuhua Cave in Southeast China, located in the frontal zone affected by the EASM. The upstream convection and rainout processes mainly regulate seasonal and interannual variations in precipitation δ18O (δ18Op). The δ18Op series shows a strong response to EASM activities associated with the Intertropical Convergence Zone migrations and the activities of the West Pacific Subtropical High affected by ENSO activities. Due to the influence of the epikarst reservoir mixing effect, the seasonal variation of dripwater δ18O (δ18Od) is irregular, and its amplitude is much smaller than that of δ18Op. The δ18Od can reflect integrated δ18Op changes within the residence time of rainwater in the karst reservoir and can inherit the ENSO signal in δ18Op. The inferred recharge models for the Yuhua drip sites reveal differences in the response to ENSO in terms of both timing and amplitude, which can be attributed to variations in the residence time of rainwater and the mixing ratio of new and old water within the karst aquifer. Higher rainfall would lead to a more sensitive response of δ18Od to ENSO activities, with a shorter time lag and greater amplitude. The modern calcite δ18O (δ18Oc) series show significant seasonal variations controlled by the fractionation coefficient varied with cave temperature. On the inter-annul time scale, the change of δ18Oc is mainly controlled by δ18Od and can record the ENSO-related variations. Our findings imply that high-resolution stalagmite δ18O reconstructions from Yuhua Cave may be able to identify historical ENSO-related variability in the EASM on annual to decade time scales.

Diurnal Cycle Dependence of ENSO Influence on the Winter Surface Air Temperature in Southeastern China

Abstract It is well known that El Niño–Southern Oscillation (ENSO) can influence the East Asian winter climate by modifying the atmospheric circulation over the western North Pacific (WNP). While the impact on precipitation in southeastern China has been extensively studied, the ENSO signal in surface air temperature (SAT) remains overlooked. In this paper, we identify robust ENSO footprints in the winter daily minimum SAT in southeastern China, with El Niño winters generally accompanied by warmer-than-normal minimum SAT anomalies. In contrast, the responses of maximum SAT are weak and negligible, suggesting a diurnal cycle–dependent ENSO influence. Further analysis indicates that this diurnal cycle dependence stems primarily from the disparate surface radiative heating between day and night induced by ENSO-related local total cloud cover (TCC) change. The warmer minimum SAT occurring in the early morning of El Niño winters is mainly caused by the enhanced surface downward longwave radiative heating as a result of the TCC increase. However, in the afternoon of El Niño winters, although the anomalous horizontal advection of warm air plays a role, there is surface radiative cooling as the weakening of solar radiation due to TCC reflection overwhelms the increase in downward longwave radiation, which leads to a weakened sensible heat flux and thus has a cooling effect on the SAT. Ultimately, these two processes effectively cancel each other out and together produce insignificant maximum SAT responses. Our conclusions carry important implications for the seasonal to interannual winter climate prediction in southeastern China. Significance Statement The purpose of this study is to better understand how ENSO influences winter surface air temperature in southeastern China. Despite insignificant responses of winter daily maximum temperature to ENSO, we find here a robust ENSO influence on the daily minimum temperature, with warm anomalies tending to occur during El Niño events. Further evidence suggests that this diurnal cycle–dependent ENSO influence is mainly caused by ENSO-induced change in local cloudiness, which has a different radiative heating effect between day and night. Our findings are of great importance for improving regional climate prediction, which will benefit social and economic activities.

Space-Time Causality Analysis of Regional Impacts of ENSO on Terrestrial and Oceanic Precipitation

Future changes are expected in precipitation under climate change, therefore, changes are projected in the oceanic and terrestrial components. However, it remains poorly elucidated how the El Niño–Southern Oscillation (ENSO) can influence these changes. Therefore, we aimed to perform a space-time causality analysis of regional ENSO impacts on terrestrial and oceanic precipitation by using the Granger causality method as a function of eight temporal lags (lags 1–8). The monthly values of total precipitation obtained using the Lagrangian approach and their respective terrestrial (PLT) and oceanic (PLO) components were used. The analysis was performed for the two regions of western North America (WNA) and eastern South America (ESA) with strong ENSO signals. For the WNA region in winter, the maximum Granger causality was observed in the component of oceanic origin for temporal lags 1 and 2 (3 and 6 months), with a predominance of both positive and negative ENSO conditions. For the ESA region, it was verified that the causality of the ENSO index was maximum for PLT. Temporal lags 2–5 (6–15 months) stood out in winter when there was a marked region of the Granger causality over the La Plata Basin. In autumn, for lags 1–4 (3–12 months), the Granger causality values were predominant in the southern and western areas of ESA and showed a tendency to move northward with an increased temporal lag. Finally, it was shown that high correlation values did not imply the causality of the relationship between the ENSO index and precipitation in the two regions.

Revisiting the Influence of ENSO on the Arctic Stratosphere in CMIP5 and CMIP6 Models

The Arctic stratospheric response to El Niño–Southern Oscillation (ENSO) is assessed using the historical simulations provided by the Coupled Model Intercomparison Project Phases 5 and 6 (CMIP5 and CMIP6, respectively). CMIP6 models can well reproduce the ENSO signals in the Arctic stratosphere and have an ameliorated performance compared to CMIP5 models. Specifically, El Niño is associated with an intensified Pacific–North American pattern that leads to a considerable enhancement of planetary wavenumber 1 but a small reduction of planetary wavenumber 2, and thus, a warm and weakened stratospheric polar vortex. The case for La Niña is nearly the opposite, with a cool and strengthened stratospheric polar vortex. In CMIP6, the ENSO-related stratospheric signal matures in the February–March–April season and increases with ENSO magnitude, regardless of the ENSO phase. However, the stratospheric response to strong El Niño (La Niña) is weaker (stronger) than that which should be achieved if the response changes linearly with the amplitude of El Niño (La Niña). An asymmetric time evolution of stratospheric signals exists between strong El Niño and La Niña events. The stratospheric response caused by strong El Niño is weaker from late winter to early spring but stronger in middle and late spring compared to that caused by strong La Niña. By contrast, the Arctic stratospheric signal in moderate El Niño events is larger than that in moderate La Niña. Compared to ENSO-neutral winters, stratospheric sudden warming occurs more (less) frequently in El Niño (La Niña), as simulated by CMIP6 high-top models.

Robust Changes in North America's Hydroclimate Variability and Predictability

AbstractClimate change adaptation planning requires a robust understanding of the projected change in hydroclimate variability and predictability. We use two large ensemble data sets to quantify the projected change in land hydroclimate variability and its potential predictability. Additionally, we use a “reddened El Niño‐Southern Oscillation (ENSO)” framework that partitions annually averaged root‐zone soil moisture variability into ENSO, land surface memory, and internal variability to understand drivers of the changes in hydroclimate variability and predictability. Even when global warming is projected to increase ENSO and its teleconnected precipitation variability over North America, we find that the corresponding change in soil moisture variability is relatively small and even decreases. This counter‐intuitive result occurs since there is also a concurrent projected reduction in land surface memory due to global warming, which leads to reduced year‐to‐year persistence of soil moisture variability. Further, we find that regional mean state land surface (soil moisture) changes primarily drive future drought and pluvial risks, suggesting that infrastructure planning can incorporate robust mean state changes despite uncertainty in the variability projections. For the regions and in the models where the ENSO signal increases, we also find a concomitant shift in the frequency of drought and pluvial events, with higher power on inter‐annual time scales but less power on decadal time scales, enhancing inter‐annual hydroclimate predictability.

Heatwave Variability and Structure in South Africa during Summer Drought

Pronounced subsidence leading to summer drought over southern Africa causes warmer than average surface air temperatures or even heatwave (HW) conditions. We investigated the occurrence of HWs during the summer drought over South Africa based on station data and the ECMWF ERA5 reanalyses. Temperature observations from the South African Weather Service were analyzed for seasonality and long-term trends (1981–2020) as background to the occurrence and variability of HWs. We focused on three severe El Niño Southern Oscillation (ENSO)-induced drought seasons, i.e., 1982/83, 1991/92, and 2015/16, to investigate HW characteristics. While 1997/98 was among the strongest El Niño seasons, the impacts were not as severe because it coincided with an intense Angola low, which allowed for rain-bearing cloud bands to form. Results showed that the hottest months were spread across the austral summer season from December to February. Regions experiencing high mean maximum temperatures and high HW frequencies exhibited a strong ENSO signal, with record HWs occurring during 2015/16. The establishment and persistence of a middle-level high-pressure system over Botswana/Namibia (Botswana High) appears to trigger the longest-lasting HWs during drought seasons. The Botswana high is usually coupled with a near-surface continental heat low and/or tropical warm air advection towards the affected region. It was also found that intense ENSO-induced drought events coincided with high HW frequency over South Africa, such as during 1982/83, 1991/92, and the recent 2015/16 events. The results of this study contribute to understanding drought and heat wave dynamics in a region experiencing rapid warming as a result of climate change.

Recent progress in understanding the interaction between ENSO and the East Asian winter monsoon: A review

This paper reviews recent advances in understanding the interaction between the East Asian winter monsoon (EAWM) and El Niño-Southern Oscillation (ENSO). The achievements are summarized into two aspects: 1) the impacts of ENSO on the EAWM, and 2) effects of the EAWM on ENSO. For the first aspect, the results show that: the current climate model simulations of ENSO impacts on the EAWM have a common weaker bias than in the observations; The influence of central Pacific type ENSO on the EAWM is generally weaker than that of the eastern Pacific type ENSO; The precipitation anomalies in the tropical Indian Ocean tend to contribute to the intra-seasonal transition of ENSO teleconnection over East Asia; The ENSO-EAWM relationship is unstable and subject to non-linear modulation by the state of oceans and extratropical atmospheric phenomena, which include the Pacific Decadal Oscillation and the Arctic Oscillation. Regarding the second aspect, studies have shown that the “pure” EAWM (denoted as EAWMres), which is independent of the ENSO signal, can lead to significant variations in the tropical convection over the western Pacific, the local Hadley circulation over East Asia, and the Walker circulation over the equatorial latitudes; The CMIP6 models can preproduce the above EAWMres effects well, although with some weaker bias. The changes in tropical convection and extratropical zonal flow associated with the EAWMres tend to have a significant modulating effect on the ENSO atmospheric teleconnection over North America. A strong EAWM and a strong Australian summer monsoon coherently provide favorable conditions for the onset of El Niño.