Warm El Nino-Southern Oscillation Events Research Articles

Diurnal to interannual variability of low‐level cloud cover over western equatorial Africa in May–October

AbstractThis study examines the diurnal to interannual variations of the stratiform cloud cover in May–October (1971–2019) from a 3‐hourly station database and from ERA5 reanalyses over western equatorial Africa (WEA). The main diurnal variations of the local‐scale fraction and genus of stratiform clouds are synthesized into three canonical diurnal types (i.e., “clear,” “clear afternoon,” “cloudy” days). The interannual variations of frequencies of the three diurnal types during the cloudiest months (JJAS) are mostly associated with two main mechanisms: a meridional shallow overturning cell associating more “cloudy” and less “clear” and “clear afternoon” days to anomalous southerlies below 900 hPa over and around WEA, anomalous ascent around 5°–7°N, anomalous northerlies between 875 and 700 hPa, and anomalous subsidence over the equatorial Atlantic. This circulation is strongly related to interannual variations of the equatorial Atlantic upwelling (i.e., more clouds when the upwelling is strong) associated with a meridional shift of the Intertropical Convergence Zone over the Tropical Atlantic and adjacent continents. The second mechanism operates mostly in the zonal direction and involves again the coupled ocean–atmosphere system over the equatorial Atlantic, but also the remote El Niño–Southern Oscillation (ENSO). An anomalously cold equatorial Atlantic drives increased low‐level westerlies toward the Congo Basin. Warm ENSO events promote broad warm and easterly anomalies in the middle and upper troposphere, which increase the local static stability, and thus the local stratiform cloud cover over WEA. The present study suggests new mechanisms responsible for interannual variations of stratiform clouds in WEA, thus providing avenues of future research regarding the stability of the stratiform cloud deck under the ongoing differential warming of tropical ocean and land masses.

Combined influence of ENSO and North Atlantic Oscillation (NAO) on Eurasian Steppe during 1982–2018

As the most influential atmospheric oscillation on Earth, the El Niño/Southern Oscillation (ENSO) can significantly change the surface climate of the tropics and subtropics and affect the high latitudes of northern hemisphere areas through atmospheric teleconnection. The North Atlantic Oscillation (NAO) is the dominant pattern of low-frequency variability in the Northern Hemisphere. As the dominant oscillations in the Northern Hemisphere, the ENSO and NAO have been affecting the giant grassland belt in the world, the Eurasian Steppe (EAS), in recent decades. In this study, the spatio-temporal anomaly patterns of grassland growth in the EAS and their correlations with the ENSO and NAO were investigated using four long-term leaf area index (LAI) and one normalized difference vegetation index (NDVI) remote sensing products from 1982 to 2018. The driving forces of meteorological factors under the ENSO and NAO were analyzed. The results showed that grassland in the EAS has been turning green over the past 36 years. Warm ENSO events or positive NAO events accompanied by increased temperature and slightly more precipitation promoted grassland growth, and cold ENSO events or negative NAO events with cooling effects over the whole EAS and uneven precipitation decreased deteriorated the EAS grassland. During the combination of warm ENSO and positive NAO events, a more severe warming effect caused more significant grassland greening. Moreover, the co-occurrence of positive NAO with cold ENSO or warm ENSO with negative NAO kept the characteristic of the decreased temperature and rainfall in cold ENSO or negative NAO events, and deteriorate the grassland more severely.

Rainfall erosivity variability over the United States associated with large-scale climate variations by El Niño/southern oscillation

A comprehensive investigation of the contiguous Unites States rainfall erosivity patterns in relation to the warm and cold phases of El Niño/southern Oscillation (ENSO) was described using a set of empirical and statistical analyses, such as harmonic analysis, annual cycle composites, and cross-correlation analysis. Monthly rainfall erosivity index (REI) composites for the first harmonic, covering 24-month ENSO events, are formed for all climate divisions over the United States spanning up to 29 ENSO episodes. From the harmonic vectorial maps plotted on the study area, each vector reveals both intensity and temporal phase of the ENSO-related REI teleconnection, and the corresponding candidate and core regions are determined using a machine learning technique of Gaussian Mixture Model (GMM) based on magnitude and temporal phase of climate signal, and Köppen climate classification. As a result of vectorial mapping, five core regions were designated as the northwest (NW), the north central (NC), the northeast coastal (NEC), the southeast (SE), and the southwest/middle-inland (SWM) regions. During fall (0) to spring (+) seasons, the results of this analysis show negative (positive) rainfall erosivity response to the El Niño events at the NW and NC regions (NEC, SE, and SWM regions), while the opposite patterns are detected for the cold phase of ENSO. The temporal consistency values were 0.62 to 0.86 (0.73 to 0.82), and spatial coherence values ranged from 0.93 to 0.98 (0.94 to 0.97) for the El Niño (La Niña) events. Comparative analyses of rainfall erosivity responses to both warm and cold ENSO events reveal the high significance level of the ENSO-REI correlation with an opposite tendency in monthly rainfall erosivity anomalies. Above normal rainfall erosivity anomalies during the El Niño thermal forcing are more significant than below normal rainfall erosivity departures during the La Niña events. Consequently, middle latitude rainfall erosivity responses to the El Niño and La Niña phenomena are detectable over the contiguous United States.

The Recent Droughts of 2019/20 in Southern Africa and Its Teleconnection with ENSO Events

This study is motivated to highlight the variability of recent drought hotspots in the region of southern Africa in terms of the seasonal and annual rainfall regimes and their possible spatial linkage with the 1950-2020 seasonal El Niño/Southern Oscillation (ENSO). Some evidence is found on possible links between the occurrence of drought hotspots in the region in terms of seasonal and mean annual runoff and warm ENSO events. This was revealed by the existence of a strong and nearly-strong positive linear correlation between Seasonal and annual rainfall depths and the warm seasonal ENSO indices explained by the southern oscillation index represented by the sea level pressure (SLP) anomalies data obtained from the National Oceanographic and Aeronautics Administration (NOAA). Considering the entire southern African region, 41% of the surface areas exhibit moderate (r > 0.25) and strong (r > 0.5) correlation coefficients in terms of the December to February quarter rainfall and ENSO indices. Above 50% confidence interval in the correlation between seasonal rainfall and ENSO during DJF quarters is found in 74% of the surface area of the region of southern Africa. The high confidence interval of the positive correlation coefficients is an indication that substantial variance of precipitation during ENSO years is accounted for by the warm ENSO events. The areas with pronounced lower rainfalls and droughts associated with ENSO activity in the region include larger and some pockets of various countries in southern Africa, including but not limited to Angola, Botswana, Lesotho, Namibia, Zambia, Zimbabwe, South Africa and Mozambique. The recent drought events of 2019/2020, and previously in 2015/16 in this region with wider regional impacts can be explained by the ENSO phenomena.

Teleconnection between climate oscillations and riverine nutrient dynamics in Southeast China based on wavelet analysis.

Nonpoint source (NPS) pollution is mainly driven by hydrological processes; climate oscillation can affect regional water cycle processes. However, the relationship between climate oscillation and NPS pollution is still unclear, which increases the difficulty of water quality prediction and management. In this study, Mann-Kendall test and wavelet transform were adopted to investigate the teleconnection between ENSO (El Niño-Southern Oscillation) phenomenon and riverine NPS load dynamics in an agricultural watershed of Southeast China from 2003 to 2016. Results showed that annual precipitation, streamflow, and riverine nutrient load increased significantly during the study period. The change point for long-term riverine TN load and TP load appeared in 2009 and 2007, respectively. Rainfall, streamflow, nutrient loads, and Niño 3.4 sea temperature (SST) shared a common periodicity of 10-16 months. The southern oscillation index (SOI) and Niño 3.4 SST shared a common periodicity of 28-36 months. Moreover, Niño 3.4 SST showed a positive correlation with riverine nutrient loads at a periodicity of 10-16 months, while SOI showed a weakly negative correlation with riverine nutrient loads at a periodicity of 28-36 months. These findings indicate that the increasing frequency of warm ENSO events would enhance the risk of nutrient export to rivers in Southeast China and more attention should be paid to large-scale climate oscillation in the prediction of agricultural nutrient pollution and management of water quality in agricultural watersheds.

Análisis de eventos fríos y cálidos por medio de datos de buceo: un enfoque de ciencia ciudadana en el Golfo de Papagayo, Costa Rica

Introduction: In January 2011 and as a Citizen Science initiative, the owners of the Deep Blue Diving Shop in Playa del Coco, Guanacaste, Costa Rica, began to collect daily temperature data in the seawater column during their diving activities, to inform tourists before traveling to Costa Rica and help them properly select their diving equipment and thus maximize the enjoyment of the experience in the Gulf of Papagayo. This data collection remained constant until January 2020 due to the COVID-19 pandemic. Objective: Identify cold and warm events of the sub-surface temperature of the sea in the Gulf of Papagayo, Guanacaste, Costa Rica and their relationship with known sources of climate variability as El Nino-Southern Oscillation (ENSO) and synoptic systems such as incursions of cold fronts in the Caribbean Sea. Methods: Sea Subsurface Temperature data corresponding to the lowest temperature of the seawater column were used, at an approximate depth of 25-35m. Observations were made daily, from 01/01/2011 to 01/31/2020. Results: The mean temperature was 25.7 degrees C. Cooler temperatures were observed in February-March, below 22.5 degrees C with a secondary minimum in July. There were two peaks in May and August with temperatures above 27.4 degrees C. The drops during cold events reached 16-17 degrees C. All cold events were associated with the passage of cold fronts through the Caribbean Sea, due to reinforcement in the intensity of the trade winds, with a zonal component from the East, which causes seasonal upwelling. The warm events presented temperatures at their maximum of 30-31 degrees C. The latter events were associated with the development and maturity of warm El Nino-type ENSO events. ENSO is an important modulator of sea temperature variability in the Gulf of Papagayo, since El Nino events are related to positive anomalies in sea temperature in the Eastern Tropical Pacific. Conclusions: The Citizen Science initiative presented in this study proved to be very useful for monitoring sea temperature in the Gulf of Papagayo. The results of this study indicate that Dive Masters can provide data on sea temperature of sufficient quality and with high temporal resolution. Divers can profitably support monitoring and Citizen Science can contribute positively to social well-being by influencing the questions that are being addressed and by giving people a voice in local environmental decision-making. The information generated in this study returns to the tour operators and enhance the understanding of the variability observed in the data collected by them, a knowledge that is later transmitted to their clients to improve their experience.

Interannual variability of surface satellite-derived chlorophyll concentration in the bay of La Paz, Mexico, during 2003–2018 period: The ENSO signature

The Bay of La Paz (BLPZ), located in the southwestern Gulf of California, is a region of significant biological productivity and strong environmental variability that remains to be understood. In particular, the spatial signature of different El Niño-Southern Oscillation (ENSO) conditions and their impact on the ecology of the bay remains puzzling because of contrasting field observations. In this context, the available satellite-derived data on surface chlorophyll (ChlSAT), sea surface temperature (SST) derived from the MODIS-Aqua sensor, together with surface wind intensity (WI) derived from reanalysis datasets from the period 2003–2018 were analyzed to identify the influence of local and remote forcing over the interannual variability of chlorophyll pigment concentration in BLPZ using WI, SST, and the Oceanic El Niño Index (ONI) as environmental factors. Significant relationships between environmental factors and ChlSAT were found using exponential models, with the higher slopes and correlations during La Niña events. Empirical orthogonal function (EOF) analysis was used to extract the principal mode of the ChlSAT and SST interannual variability. The first mode (EOF1) accounted for 64% and 89% of the ChlSAT and the SST interannual variance, respectively, showing a strong gradient along the coast of the bay. The corresponding amplitude of the ChlSAT mode was significantly correlated with the independent variables PC1SST, ONI, and WI anomalies (RSST = − 0.60, RONI = − 0.57, and RWI = 0.44, P < 0.001). Monthly time series of ChlSAT anomalies along the coast where examined as a function of each independent variable using a multiple linear regression analysis (LRA), which revealed that during the 2005 to 2013 period, high peaks of positive ChlSAT anomalies in the central and southern coast were related to cold ENSO events and favorable wind forcing, which smoothed the effect of weak and moderate El Niño episodes. This period of high biological productivity was only interrupted by the strong and long 2015–2016 El Niño event. Since late 2013, the ChlSAT and the WI anomalies showed a sharp drop to negative anomaly values, which may be the result of complex interactions of physical and biological processes observed during strong warm ENSO events, showing one peak of persistent warm SST anomalies via a weakening of the wind intensity (i.e., from atmospheric teleconnection) and a second one due to the oceanic connection. It is confirmed that BLPZ maintains high pigment concentration levels, comparable to those reported on the mainland coast of GC, where only strong warm ENSO episodes cause changes in chlorophyll concentration.

Influence of Recent Climate Shifts on the Relationship Between ENSO and Asian Monsoon Precipitation Oxygen Isotope Ratios

AbstractRecent studies have revealed robust in‐phase relationships between El Niño–Southern Oscillation (ENSO) and Asian Monsoon precipitation δ18O values (i.e., warm ENSO events with high δ18O values), and this relationship has been used in an attempt to reconstruct past ENSO activity. However, whether this relationship holds in the past is unknown. Here we use precipitation δ18O data from Hong Kong (East Asia) and Bangkok (Southeast Asia) and an ice core δ18O record from Dasuopu glacier (South Asia) to examine the δ18O‐ENSO relationship across two recent climate shifts that occurred during the winters of 1976/1977 and 1988/1989. On an annual scale, the δ18O‐ENSO relationship is weak prior to 1977 and strongest after 1988. We show that the changing δ18O‐ENSO relationship mainly originates from changes in the dry season isotope/climate relationship (which is significant only after 1988), whereas the rainy season relationship is relatively stable. We confirm that, consistent with earlier work on the rainy season, the significant δ18O‐ENSO relationship in the dry season post‐1988 is associated with ENSO's influence on regional convection (Bay of Bengal to South China Sea region). We suggest the insignificant dry season relationship prior to 1989 is due to limited ENSO impacts on convection in the Bay of Bengal to South China Sea region, which is supported by the insignificant relationship between ENSO and vertical velocity at 500 hPa. These findings suggest that without additional constraints, systematic variability in isotope/climate relationships will lead to large uncertainties in ENSO reconstructions based on Asian Monsoon region δ18O data.

El Niño and La Niña influence on mean river flows of southern South America in the 20th century

ABSTRACTThe influence of the El Niño Southern Oscillation (ENSO) phenomenon on monthly mean river flows of 12 rivers in the extreme south of South America in the 20th century is analysed. The original dataset of each river is divided into two subsets, i.e. warm ENSO events or El Niño, and cold ENSO events or La Niña. The elements of the subsets are composites of 24 consecutive months, from January of the year when the ENSO event begins to December of the following year. The ENSO signal is analysed by comparing the monthly mean value of each subset to the long-term monthly mean. The results reveal that, in general, monthly mean El Niño (La Niña) river flows are predominantly larger (smaller) than the long-term monthly mean in the rivers studied. The anomalies are more evident during the second half of the year in which the event starts and the first months of the following year.

An empirical adjusted ENSO ocean energetics framework based on observational wind power in the tropical Pacific

According to El Nino–Southern Oscillation (ENSO) energetics theory, the work done by the wind on the ocean, known as wind power, has a strong forcing relationship with ENSO sea surface temperature (SST) changes, wherein negative (positive) tropical Pacific wind power anomalies contribute to warm (cold) ENSO events. The ENSO energetics framework assumes a mean state characterized by easterly winds, westward currents and a thermocline shoaling from west to east such that positive zonal wind power anomalies will induce a La Nina state and negative anomalies will induce an El Nino state. In this study, tropical Pacific wind power is computed using satellite data and its correlations with Nino 3.4 SST anomalies evaluated and compared to the conventional dynamical predictors, namely warm water volume and wind stress. Analysis of the spatial and temporal structure of climatological wind power and its variability during individual ENSO events reveals sign inconsistencies in which certain wind power anomalies (e.g. those associated with westerly wind events in the far west) are positive despite forcing the system towards an El Nino state. These results show that the conventional ENSO energetics framework makes assumptions about the climatological state that are not always consistent with observations. We apply sign adjustments to the computation of a tropical Pacific wind power index that include the directional aspect of the wind power perturbations and find that these adjustments greatly enhance the lead correlations between wind power and Nino3.4 SST anomalies, which are now comparable to conventional dynamical predictors.

Asymmetry of the Predictability Limit of the Warm ENSO Phase

AbstractA nonlinear local Lyapunov exponent method based on monthly sea surface temperature data is employed to explore the predictability limit of warm El Niño–Southern Oscillation (ENSO) events. Results using observational data show an asymmetry of the predictability limit between the developing and decaying stages of the warm ENSO phase. To wit, predictability of the developing stage of warm ENSO events is found to approach a limit of 10 months, less than that of the mature and decaying stages. This asymmetrical predictability limit is also found in a long climate model simulation and may explain the asymmetry in operational forecast skill of warm ENSO events. Through exploring the error growth rate as represented by nonlinear local Lyapunov exponent and the instantaneous error growth rate, it is shown that error growth, especially during the first 8‐month lead forecasts, is the primary contributor to the asymmetry of the predictability limit of warm ENSO events.

Response of O&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and pH to ENSO in the California Current System in a high-resolution global climate model

Abstract. Coastal upwelling systems, such as the California Current System (CalCS), naturally experience a wide range of O2 concentrations and pH values due to the seasonality of upwelling. Nonetheless, changes in the El Niño–Southern Oscillation (ENSO) have been shown to measurably affect the biogeochemical and physical properties of coastal upwelling regions. In this study, we use a novel, high-resolution global climate model (GFDL-ESM2.6) to investigate the influence of warm and cold ENSO events on variations in the O2 concentration and the pH of the CalCS coastal waters. An assessment of the CalCS response to six El Niño and seven La Niña events in ESM2.6 reveals significant variations in the response between events. However, these variations overlay a consistent physical and biogeochemical (O2 and pH) response in the composite mean. Focusing on the mean response, our results demonstrate that O2 and pH are affected rather differently in the euphotic zone above ∼ 100 m. The strongest O2 response reaches up to several hundreds of kilometers offshore, whereas the pH signal occurs only within a ∼ 100 km wide band along the coast. By splitting the changes in O2 and pH into individual physical and biogeochemical components that are affected by ENSO variability, we found that O2 variability in the surface ocean is primarily driven by changes in surface temperature that affect the O2 solubility. In contrast, surface pH changes are predominantly driven by changes in dissolved inorganic carbon (DIC), which in turn is affected by upwelling, explaining the confined nature of the pH signal close to the coast. Below ∼ 100 m, we find conditions with anomalously low O2 and pH, and by extension also anomalously low aragonite saturation, during La Niña. This result is consistent with findings from previous studies and highlights the stress that the CalCS ecosystem could periodically undergo in addition to impacts due to climate change.

ENSO and the recent warming of the Indian Ocean

ABSTRACTThe recent Indian Ocean (IO) warming and its relation with the El Niño Southern Oscillation (ENSO) is investigated using available ocean and atmospheric reanalyses. By comparing the events before and after 1976 (identified as a threshold separating earlier and recent decades with respect to global warming trends), our results indicate that the IO had experienced a distinct change in the warming pattern. After 1976, during the boreal summer season the cold anomalies in the IO were replaced by warm anomalies in both warm (El Niño) and cold (La Niña) ENSO events. Strong sinking by upper level winds and the associated anomalous equatorial easterly winds created favourable conditions for the IO warming from 90°E towards the western IO. Our study highlights that after 1976, atmospheric and oceanic fields changed mostly during La Niña, with both ENSO phases contributing to the warming of the IO. Warm anomalies of 0.2 °C are seen over large areas of the IO in the post‐1976 La Niña composites. Our analysis suggests that the IO warming during La Niña events after 1976 may have a relation to the warm anomalies persisting from the preceding strong El Niño events.

Characterization of Heat Waves in the Sahel and Associated Physical Mechanisms

Great effort is made to address heat waves (HWs) in developed countries because of their devastating impacts on society, economy, and environment. However, HWs are still understudied over developing countries. This is particularly true in West Africa, and especially in the Sahel, where temperatures recurrently reach critical values, such as during the 2010 HW event in the western Sahel. This work aims at characterizing the Sahelian HWs during boreal spring seasons (April–May–June) and understanding the mechanisms associated with such extreme events. Over the last three decades, Sahelian HWs have been becoming more frequent, lasting longer, covering larger areas, and reaching higher intensities. The physical mechanisms associated with HWs are examined to assess the respective roles of atmospheric dynamics and radiative and turbulent fluxes by analyzing the surface energy budget. Results suggest that the greenhouse effect of water vapor is the main driver of HWs in the western Sahel, increasing minimum temperatures by enhanced downward longwave radiation. Atmospheric circulation plays an important role in sustaining these warm anomalies by advecting moisture from the Atlantic Ocean and the Guinean coasts into the Sahel. Maximum temperature anomalies are mostly explained by increased downward shortwave radiation due to a reduction in cloud cover. Interannual variability of HWs is affected by the delayed impact of El Niño–Southern Oscillation (ENSO), with anomalous temperature warming following warm ENSO events, resulting from an amplified water vapor feedback.

Precipitation, temperature, and moisture transport variations associated with two distinct ENSO flavors during 1979–2014

Interannual precipitation and temperature variations during 1979–2014 are investigated by examining the effects of two distinct flavors of the El Nino-Southern Oscillation (ENSO), i.e., the tropical eastern Pacific (EP) and central Pacific (CP) ENSO events. Satellite- and ground-based observations with global coverage are applied including the monthly precipitation data from the Global Precipitation Climatology Project (GPCP) and surface temperature anomalies from the NASA-GISS surface temperature anomaly analysis. Related variations in other water-cycle components including atmospheric moisture transport are also examined by using the outputs from the NASA-Modern Era Retrospective-analysis for Research and Applications (MERRA). While the second leading mode from an EOF analysis of sea surface temperature (SST) anomalies between 30°N and 30°S is dominated by interdecadal-scale variability that is not a focus of this study, the first and third leading modes represent well the EP and CP events, respectively. The corresponding principal components (PC1 and PC3) are then applied as indices to estimate the influences of the two ENSO flavors on various physical components through linear regression. Because of their distinct SST configurations in the tropical Pacific, the two ENSO flavors manifest different spatial features of precipitation anomalies as shown in past studies. Differences can also be readily seen in satellite-retrieved tropospheric layered temperatures and oceanic columnar water vapor content. General agreements between observations and MERRA outputs can be obtained as judged by consistent respective anomalies corresponding to the two ENSO flavors, suggesting that MERRA could provide an accurate account of variations on the interannual time scale. Interannual variations in MERRA vertically integrated moisture transport (VIMT) are further examined to explore possible relations between precipitation and tropospheric moisture transport corresponding to the two flavors during two contrasting seasons: December–March (DJFM) and June–September (JJAS). Anomalies of zonal moisture transport in the deep tropics following the variations in the Pacific Walker Circulation are distinctly different for two ENSO flavors and also manifest evident seasonal variations for each flavor. Differences in the zonal mean VIMT (both zonal and meridional components) are also evident between the two flavors, consistent with the differences in zonal mean precipitation anomalies from both GPCP and MERRA. Furthermore, the ENSO flavors are associated with distinct precipitation anomaly patterns over various land areas, which can be further traced to the differences in their associated VIMT anomalies, particularly during DJFM when the warm ENSO events usually reach their mature phase.

Modeling the Mechanisms of Linear and Nonlinear ENSO Responses to the Pacific Meridional Mode

Abstract Interactions between the Pacific meridional mode (PMM) and El Niño–Southern Oscillation (ENSO) are investigated using the National Center for Atmospheric Research (NCAR) Community Earth System Model (CESM) and an intermediate coupled model (ICM). The two models are configured so that the CESM simulates the PMM but not ENSO, and the ICM simulates ENSO but not the PMM, allowing for a clean separation between the PMM evolution and the subsequent ENSO response. An ensemble of CESM simulations is run with an imposed surface heat flux associated with the North Pacific Oscillation (NPO) generating a sea surface temperature (SST) and wind response representative of the PMM. The PMM wind is then applied as a forcing to the ICM to simulate the ENSO response. The positive (negative) ensemble-mean PMM wind forcing results in a warm (cold) ENSO event although the responses are not symmetric (warm ENSO events are larger in amplitude than cold ENSO events), and large variability between ensemble members suggests that any individual ENSO event is strongly influenced by natural variability contained within the CESM simulations. Sensitivity experiments show that 1) direct forcing of Kelvin waves by PMM winds dominates the ENSO response, 2) seasonality of PMM forcing and ENSO growth rates influences the resulting ENSO amplitude, 3) ocean dynamics within the ICM dominate the ENSO asymmetry, and 4) the nonlinear relationship between PMM wind anomalies and surface wind stress may enhance the La Niña response to negative PMM variations. Implications for ENSO variability are discussed.

Do strong warm ENSO events control the phase of the stratospheric QBO?

AbstractAlthough there in general are no significant long‐term correlations between the quasi‐biennial oscillation (QBO) and the El Niño–Southern Oscillation (ENSO) in observations, we find that the QBO and the ENSO were aligned in the 3 to 4 years after the three warm ENSO events in 1982, 1997, and 2015. We investigate this indicated relationship with a version of the EC‐Earth climate model which includes nonorographic gravity waves. We analyze the modeled QBO in ensembles forced with climatological sea surface temperatures (SSTs) and observed SSTs. In the ensemble with observed SSTs we find a strong and significant alignment of the ensemble members in the equatorial stratospheric winds in the 2 to 4 years after the strong ENSO event in 1997. This alignment also includes the observed QBO. No such alignment is found in the ensemble with climatological SSTs. These results indicate that strong warm ENSO events can lock the phase of the QBO.

Impacts of ENSO events on cloud radiative effects in preindustrial conditions: Changes in cloud fraction and their dependence on interactive aerosol emissions and concentrations

AbstractWe use three 150 year preindustrial simulations of the Community Earth System Model to quantify the impacts of El Niño–Southern Oscillation (ENSO) events on shortwave and longwave cloud radiative effects (CRESW and CRELW). Compared to recent observations from the Clouds and the Earth's Radiant Energy System data set, the model simulation successfully reproduces larger variations of CRESW and CRELW over the tropics. The ENSO cycle is found to dominate interannual variations of cloud radiative effects. Simulated cooling (warming) effects from CRESW (CRELW) are strongest over the tropical western and central Pacific Ocean during warm ENSO events, with the largest difference between 20 and 60 W m−2, with weaker effects of 10–40 W m−2 over Indonesian regions and the subtropical Pacific Ocean. Sensitivity tests show that variations of cloud radiative effects are mainly driven by ENSO‐related changes in cloud fraction. The variations in midlevel and high cloud fractions each account for approximately 20–50% of the interannual variations of CRESW over the tropics and almost all of the variations of CRELW between 60°S and 60°N. The variation of low cloud fraction contributes to most of the variations of CRESW over the midlatitude oceans. Variations in natural aerosol concentrations explained 10–30% of the variations of both CRESW and CRELW over the tropical Pacific, Indonesian regions, and the tropical Indian Ocean. Changes in natural aerosol emissions and concentrations enhance 3–5% and 1–3% of the variations of cloud radiative effects averaged over the tropics.

Trends of mean temperatures and warm extremes in northern tropical Africa (1961–2014) from observed and PPCA‐reconstructed time series

AbstractTrends in daily maximum (TX) and minimum (TN) temperatures and indices of warm extremes are studied in tropical North Africa, west of the eastern African highlands, from 1961 to 2014. The analysis is based on the concatenation and cross‐checking of two observed databases. Due to the large number of missing entries (~25%), a statistical infilling using probabilistic principal component analysis was applied. Averaged over 90 stations, the linear trends of annual mean TX and TN equal respectively +0.021 °C/yr and +0.028 °C/yr. The frequency of very hot days (TX &gt; 35°C) and tropical nights (TN &gt; 20°C), as well as the frequency of daily TX and TN above the 90th percentile (p90) (“warm days” and “warm nights”), roughly follows the variations of mean TX and TN, respectively. Heat spells of TX or TN &gt; p90 are often short (usually &lt;2–3 days), and the interannual variation of their mean duration is noisier than for the other indices. Nevertheless, heat spells tend to last longer, with almost constantly positive anomalies since the mid‐1990s. The trends in March–June, the warmest season across the Sahelian and Sudanian belts, show similar variations as annual means. Overall, the local‐scale warming in annual temperatures, and in March–June, may be viewed merely as a simple shift of the probability distribution function of daily TX and TN. The correlations between the thermal indices and the 2 m temperatures suggest that the low‐frequency (&gt;8 years) variations may be viewed as a regional‐scale fingerprint of the global warming, with largest correlations in the tropical Atlantic and Indian basins, while the high‐frequency (&lt;8 years) variations should be mostly viewed as a delayed remote impact of El Niño–Southern Oscillation (ENSO) events over the region, with warm (cold) anomalies tending to follow warm (cold) ENSO events.

The interdecadal change of the leading mode of the winter precipitation over China

The interdecadal change of the leading mode of the mean winter precipitation over China has been investigated using observational data for the period from 1960 to 2012. The leading empirical orthogonal function (EOF) mode (EOF1) of the winter precipitation over China displays a mono-sign pattern over southeastern China, accounting for 49.7 % of the total variance in the precipitation. Both the El Nino-Southern Oscillation (ENSO) and the East Asian winter monsoon (EAWM) can impact EOF1. A positive (negative) EOF1 is accompanied by warm (cold) ENSO events and weak (strong) EAWM, and the latters can cause anomalous southerlies (northerlies) along the coast of southeastern China, accompanied by the transportation of water vapor from the Bay of Bengal and the South China Sea favoring a wet (dry) winter over southeastern China. An abrupt transition of the EOF1 is observed around the mid-1980s. Therefore, the data are divided into two subperiods, i.e., 1960–1987 (P1) and 1988–2009 (P2). Significant differences in the large scale atmospheric circulation and sea surface temperature anomalies associated with EOF1 during these two subperiods are observed. EOF1 is closely related to the mid- to high-latitude atmospheric circulation in P1, while its relationship to the tropics obviously increases during P2. The partial regression analysis results show that the interdecadal change of EOF1 is caused by both the interdecadal changes of the EAWM and ENSO around the mid-1980s. In P1, the lower-level anomalous southerlies along the coastal southeastern China accompanied by water vapor transportation that causes above-average precipitation are related to an anti-cyclonic system centered over the mid-latitude western North Pacific associated with EAWM. In P2, the influence of the EAWM is weaker, and the southerly anomaly over the coastal southeastern China is mainly caused by the anticyclone over Philippines, which is related to the ENSO.