Since the 2000s, there has been a marked acceleration in the decline of summer Arctic sea ice, accompanied by an increase in Arctic sea ice variability. However, the impact of Arctic sea ice decline on changes in Arctic atmospheric circulation and its implications for mid-latitude summers remain unclear. Here we propose that the emergence of a dipole-like mode of Arctic atmospheric circulation, driven by the Arctic sea ice decline, is linked to the intensification of temperature rise and increased European heat waves. Idealized model experiments support that the recent intensification of the Arctic sea ice decline is a key driver for the emergence of dipole-like atmospheric circulation in mid-latitude summer. The accelerated Arctic sea ice decline weakens the meridional temperature gradient and the zonal wind in Eastern Europe, develops an anti-cyclonic circulation, and contributes to the intensification of European heat waves. The recent acceleration of Arctic sea ice decline is linked to the emergence of a dipole-like Arctic atmospheric circulation, which amplifies temperature rise and increases European heat waves, according to idealized model experiments.

Understanding the relationship between fire activity and climate variability is a major concern for the scientific community and is essential for reducing economic losses and life-threatening fire hazards.However,the drivers of fire activity and the influence of climate variability remain uncertain.Here,we show that the Madden-Julian Oscillation(MJO)—a dominant tropical subseasonal variability-influences fire activity by modulating local fire-supporting weather through atmospheric teleconnections.Our results show that midlatitude fire emissions exhibit significant subseasonal variability,with MJO-related weather influencing the fire intensity and contributing to large fire events.MJO-related fire events account for about 10％-20％of total midlatitude fire events,suggesting that if MJO teleconnections strengthen in the future,fire emissions and associated economic losses could worsen.

Daily precipitation anomalies in the western North Pacific (WNP) and East Asia (EA) exhibit significant intraseasonal variability, peaking at 10–30-day time scales. It has been suggested that boreal summer intraseasonal oscillation (BSISO) on 30–60-day time scales is strongly modulated by El Niño–Southern Oscillation (ENSO) with stronger intensity and propagation during La Niña compared to El Niño summers, but the dependency of 10–30-day BSISO on ENSO has not been well understood. Here, we show that the intensity and northward propagation of the 10–30-day BSISO convection over the WNP-EA region are stronger and more organized during El Niño developing summers than other summers, including neutral summers. During El Niño developing summers, the BSISO-induced precipitation and low-level circulation tend to exhibit a stronger meridional tripolar pattern than those during neutral summers. We highlight that the strengthening of 10–30-day BSISO northward propagation and associated rainfall anomalies over EA in El Niño developing summers is contributed by not only the previously proposed stronger air–sea interaction with a larger meridional gradient of sea surface temperature, but also an enhanced dynamic process with stronger relative vorticity and moisture convergence.

The complex interaction between the Pacific decadal oscillation (PDO) and East Asian winter temperatures remains unclear. This study reveals that since the early 2000s, East Asia has experienced a strengthening of Aleutian low (AL) and Siberian high (SH) during negative PDO phases, leading to an intensified East Asian winter monsoon (EAWM). The increased pressure gradient between the SH and the AL, driven by warming in the western Pacific associated with the negative PDO phase, has significantly contributed to a shift toward cooling in East Asia (105 degrees-150 degrees E, 20 degrees-50 degrees N) since the early 2000s. Observations and model simulations provide evidence that the enhanced tropical convection in the western Pacific under a negative PDO phase has intensified the atmospheric circulations associated with the EAWM since the early 2000s. Understanding these dynamics is crucial for improving winter temperature forecasts in East Asia.

While decadal changes in Madden–Julian oscillation (MJO) have received considerable attention, the corresponding changes in Boreal Summer Intraseasonal Oscillation (BSISO) have yet to be well understood. In this study, we show the enhanced northward propagation of BSISO in the Western North Pacific (WNP) during the 2000s compared to the 1980s–1990s. Observational analyses and model experiments suggest this enhancement is partially attributed to the tropical Indian Ocean (TIO) warming. The TIO warming tends to increase the air-sea interaction, enhancing moisture anomalies in the free atmosphere. Consequently, this increase in moisture anomalies strengthens BSISO-scale convection through increased convective anomalies at the north of the BSISO center, thereby enhancing the northward propagation of BSISO. Additionally, vorticity changes resulting from mean-state zonal vertical shear also contribute to the BSISO decadal change. Our findings underscore the importance of considering the interaction between BSISO and changes in the ocean mean state in future assessments.

Understanding heavy fire activities tied up to climate and its future change is of emerging scientific concern and essential for reducing huge economic costs and life losses from substantial heavy fire hazards under greenhouse warming. Madden-Julian Oscillation (MJO) is a planetary-scale tropical convective system and moves from Asia to the US, generating extreme weather in the midlatitudes. Here, we show that when MJO convective anomalies occur in the western Pacific, the fire emissions in the midlatitudes are two times more likely than when MJO convection is in the Indian ocean. The changes in MJO related to increased fire emissions in the tropics and midlatitudes. Precise MJO predictions may contribute to enhanced subseasonal forecast of the fire activities, aggravating vast social and economic losses (124 words)

Wildfires, which occur sporadically and irregularly worldwide, are distinct natural disturbances in combustible vegetation areas, important parts of the global carbon cycle, and natural disasters that cause severe public emergencies. While many previous studies have investigated the variability and changes in wildfires globally based on fire emissions, burned areas, and fire weather indices, studies on East Asia are still limited. Here, we explore the characteristics of variability and changes in wildfire danger over East Asia by analyzing the fire weather index for the 40 years-1981-2020. The first empirical orthogonal function (EOF) mode of fire weather index variability represents an increasing trend in wildfire danger over most parts of East Asia over the last 40 years, accounting for 29% of the total variance. The major contributor is an increase in the surface temperature in East Asia associated with global warming and multidecadal ocean variations. The effect of temperature was slightly offset by the increase in soil moisture. The second EOF mode exhibits considerable interannual variability associated with the El Nino-Southem Oscillation, accounting for 17% of the total variance. The increase (decrease) in precipitation in East Asia during El Nino (La Nina) increases (decreases) soil moisture, which in turn reduces (increases) wildfire danger. This dominant soil moisture effect was slightly offset by the temperature increase (decrease) during El Nino (La Nina). Improving the understanding of variability and changes in wildfire danger will have important implications for reducing social, economic, and ecological losses associated with wildfire occurrences.

Boreal summer intraseasonal oscillation (BSISO) strongly interacts with background mean fields and tends to be stronger and longer in its northward propagation during La Niña than El Niño summers. However, BSISO dependence on El Niño‐Southern Oscillation phase has not been well understood. Here, we show that BSISO‐related convections are stronger and more organized with northward propagation on 30–60‐day timescales during El Niño developing (E‐DV) than decaying (E‐DC) summers over the western Pacific. The easterly vertical shear, responsible for the next vorticity generation north of the BSISO convection, in the E‐DV (E‐DC) remarkably increases (decreases) due to the enhanced (weakened) upper‐level easterly anomaly. The air‐sea interaction in E‐DV gets stronger due to the larger meridional gradient of sea surface temperature, amplifying northward propagation than in E‐DC.

Earth and Space Science Open Archive posterOpen AccessYou are viewing the latest version by default [v1]Variability of the Southern Annular Mode and Southern Ocean Surface Westerly Winds in E3SMAuthors Doo Young Lee iD Mark Petersen Wuyin Lin See all authors Doo Young LeeiDCorresponding AuthorLos Alamos National LaboratoryiDhttps://orcid.org/0000-0002-6129-2241view email addressThe email was not providedcopy email addressMark PetersenLos Alamos National Laboratoryview email addressThe email was not providedcopy email addressWuyin LinBrookhaven National Laboratoryview email addressThe email was not providedcopy email address

The characteristics of the wintertime Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) and their impacts on climate variability over the Northern Hemisphere are important metrics for evaluating a climate system model. Observational analyses reveal that the horizontal and vertical structures in the AO and NAO exhibit a meridional dipole and a large-scale barotropic pattern between the Arctic and mid-latitudes. Historical model simulations from the Energy Exascale Earth System Model (E3SM-HIST) are used to identify how well it captures these major climate modes. It is found that the simulated AO and NAO modes have spatial structures similar to the observed features. In addition, the observed frequency bands in the AO and NAO-related time variability are captured well in the E3SM-HIST simulation. Associated with the positive phase in wintertime AO and NAO, zonal flow and warm advection in mid-latitude continents are enhanced, along with stronger cold flow from enhanced northerly winds over high latitudes. These features are linked to the atmospheric circulation pattern reflected by lower SLP anomalies over the Arctic and higher SLP anomalies over the mid‐latitudes. In E3SM-HIST, these spatial associations and main structural features are analogous to those in observations. In the time-height evolution related to winter AO and NAO modes, it can also be seen that the simulations reproduce the downward propagating patterns in observations. Nevertheless, the vertical structures associated with AO and NAO in E3SM-HIST exhibit substantial biases in the lower stratosphere. The cause of these stratospheric biases is investigated using the strength of climatological stratospheric polar vortex (SPV) and wave activity fluxes. The results herein suggest that E3SM-HIST has a reasonable skill in reproducing the observed characteristics related to the winter AO and NAO, although there exist systematic biases in the associated climate variability.

Climate variability and change in the Southern Hemisphere (SH) are influenced by the Southern Annular Mode (SAM) and are closely related to changes in the kinematic properties of the SH surface zonal winds. The SAM and SH surface zonal winds have strong effects on the atmospheric and oceanic circulation system. In this study we investigate the variability and trend in the SAM and position and strength of the surface zonal wind stress (TAUX), using two ensembles of simulations covering the historical record from the Energy Exascale Earth System Model (E3SM‐HIST and Atmospheric Model Intercomparison Project) for 1979–2014. In addition, performance of two CO 2 forcing simulations from the E3SM (E3SM‐1pctCO2 and 4xCO2) is assessed to examine the sensitivity of the variability and changes in the SAM and SH surface TAUX to climate forcing. In general, all E3SM simulations tend to capture the dominant feature of the SAM pattern reasonably well. The annual SAM index in the E3SM‐HIST simulation shows a significant increasing trend. These features are similar to the trends in the strength (along with poleward shift in the position) of the annual surface TAUX. For the climatological surface TAUX position and strength, the two CO 2 forcing simulations show slightly poleward movement and stronger intensity, while the E3SM‐HIST is equatorward and weaker than observations. In the relationship between the SAM and surface TAUX, we show that the SAM index exhibits a positive (negative) relationship with the strength (position) of the surface TAUX in the variability for all seasons and annual mean.

An assessment of the forecast quality of 10 m wind speed by deterministic and probabilistic verification measures has been carried out using the original raw and two statistical bias-adjusted forecasts in global coupled seasonal climate prediction systems (ECMWF-S4, METFR-S3, METFR-S4 and METFR-S5) for boreal winter (December–February) season over a 22-year period 1991–2012. We follow the standard leave-one-out cross-validation method throughout the work while evaluating the hindcast skills. To minimize the systematic error and obtain more reliable and accurate predictions, the simple bias correction (SBC) which adjusts the systematic errors of model and calibration (Cal), known as the variance inflation technique, methods as the statistical post-processing techniques have been applied. We have also built a multi-model ensemble (MME) forecast assigning equal weights to datasets of each prediction system to further enhance the predictability of the seasonal forecasts. Two MME have been created, the MME4 with all the four prediction systems and MME2 with two better performing systems. Generally, the ECMWF-S4 shows better performance than other individual prediction systems and the MME predictions indicate consistently higher temporal correlation coefficient (TCC) and fair ranked probability skill score (FRPSS) than the individual models. The spatial distribution of significant skill in MME2 prediction is almost similar to that in MME4 prediction. In the aspect of reliability, it is found that the Cal method has more effective improvement than the SBC method. The MME4_Cal predictions are placed in close proximity to the perfect reliability line for both above and below normal categorical events over globe, as compared to the MME2_Cal predictions, due to the increase in ensemble size. To further compare the forecast performance for seasonal variation of wind speed, we have evaluated the skill of the only raw MME2 predictions for all seasons. As a result, we also find that winter season shows better performance than other seasons.

Multi-model ensembles (MMEs) are powerful tools in dynamical climate prediction as they account for the overconfidence and the uncertainties related to single-model ensembles. Previous works suggested that the potential benefit that can be expected by using a MME amplifies with the increase of the independence of the contributing Seasonal Prediction Systems. In this work we combine the two MME Seasonal Prediction Systems (SPSs) independently developed by the European (ENSEMBLES) and by the Asian-Pacific (APCC/CliPAS) communities. To this aim, all the possible multi-model combinations obtained by putting together the 5 models from ENSEMBLES and the 11 models from APCC/CliPAS have been evaluated. The grand ENSEMBLES-APCC/CliPAS MME enhances significantly the skill in predicting 2m temperature and precipitation compared to previous estimates from the contributing MMEs. Our results show that, in general, the better combinations of SPSs are obtained by mixing ENSEMBLES and APCC/CliPAS models and that only a limited number of SPSs is required to obtain the maximum performance. The number and selection of models that perform better is usually different depending on the region/phenomenon under consideration so that all models are useful in some cases. It is shown that the incremental performance contribution tends to be higher when adding one model from ENSEMBLES to APCC/CliPAS MMEs and vice versa, confirming that the benefit of using MMEs amplifies with the increase of the independence the contributing models. To verify the above results for a real world application, the Grand ENSEMBLES-APCC/CliPAS MME is used to predict retrospective energy demand over Italy as provided by TERNA (Italian Transmission System Operator) for the period 1990–2007. The results demonstrate the useful application of MME seasonal predictions for energy demand forecasting over Italy. It is shown a significant enhancement of the potential economic value of forecasting energy demand when using the better combinations from the Grand MME by comparison to the maximum value obtained from the better combinations of each of the two contributing MMEs. The above results demonstrate for the first time the potential of the Grand MME to significantly contribute in obtaining useful predictions at the seasonal time-scale.

ABSTRACTThe current status and future changes in the frequency and intensity of climatological blocking activity over the North Pacific region are investigated using historical and two Representative Concentration Pathway (RCP4.5 and 8.5) simulations in the coupled climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) for boreal winters (December–February) over a 30‐year period. The future change in the Pacific blocking frequency and intensity are examined in terms of the projected meridional thickness gradient, Hadley circulation changes, and changes in the probability distribution of categorized blocking strength. The five CMIP5 models that show better performance in reproducing climatological blocking events in the historical simulations for the Pacific region are selected for the analyses of the projected blocking activities. The climatological winter Pacific blocking frequencies of most of the individual models and model mean values show a tendency to decrease under global warming conditions. The trend is closely linked with the strong upper level westerly wind, resulting in less meandering air flow, consistent with the enhanced meridional temperature gradients at mid‐latitude in the future climate. The decreased frequency in climatological atmospheric blocking over the Pacific under warming may also be influenced by the strengthening of the north–south temperature gradients due to the poleward extension of Hadley circulation in the subtropics. The climatology of the Pacific blocking intensity in boreal winter also tends to decrease slightly due to a future reduction in the number of strong blocking events.

The research leading to these results has received funding by the Seventh Framework Programme (FP7) of the European Commission (GA30837, GA308291), the Ministerio de Economia y Competitividad (MINECO) under the project CGL2013-41055R and the project PCIN-2014-012-C07-07.