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Qiyan Lin
and
Jiacan Yuan

Abstract

Humid heat extremes, taking account of both temperature and humidity, have adverse impacts on society, particularly on human health. It has been suggested that quasi-stationary waves (QSWs) with anomalously high amplitudes contribute to the occurrence of near-surface precipitation extremes and temperature extremes in the midlatitudes of the Northern Hemisphere. However, little attention has been paid to the linkages between amplified QSWs and humid heat extremes. Using the ERA5 dataset, we identify amplified QSWs of zonal wavenumbers 5–7 (Wave 5–7) in summer months from 1979 to 2020. These amplified QSWs show clear circumglobal wave patterns horizontally and nearly barotropic structure vertically. Linking amplified Wave 5–7 to wet-bulb temperature (WBT) extremes, we find that amplified QSWs preferentially induce prominently prolonged WBT extremes in specific regions: north-central North America for amplified Wave 5; western United States, south-central Asia, and eastern Asia for amplified Wave 6; and western Europe and the Caspian Sea region for amplified Wave 7. Analyses of physical processes indicate that, accompanied by the amplification of Wave 5–7, the changes in horizontal temperature advection, adiabatic heating associated with descending motion, downward solar radiation, moisture transport and moisture flux convergence, and surface latent heat fluxes largely account for the increase in persistence of WBT extremes.

Significance Statement

In the context of climate change, humid heat extremes exhibit different changes and impacts from high-temperature extremes, but the physical processes that may cause them remain unclear. This study aims to explore the atmospheric dynamic processes leading to the concurrence of humid heat extremes, which may exacerbate the risk from heat stress in today’s interconnected world. In specific regions over the Northern Hemisphere midlatitudes, durations of humid heat extremes are found to be elevated simultaneously by amplified quasi-stationary waves. We further identify the physical connections between amplified quasi-stationary waves and humid heat extremes over targeted regions. This would help in better understanding the role of changing atmospheric circulations in the humid heat extremes.

Open access
Gabriel M. P. Perez
,
Pier Luigi Vidale
,
Helen Dacre
, and
Jorge L. García-Franco

Abstract

Precipitation often happens along organized filaments or bands of moisture such as convergence zones. Recent regional studies have shown that these moisture filaments arise from synoptic-scale mixing features known as attracting Lagrangian coherent structures (LCSs). In this study, we present a climatology of synoptic-scale mixing and investigate its covariability with precipitation on temporal scales ranging from weekly to interannual. We characterize mixing with the finite-time Lyapunov exponent (FTLE), a measure of parcel deformation, in ERA5 data between 1980 and 2009. Attracting LCSs are identified as ridges of the FTLE. At the interannual time scale, we compare El Niño and La Niña events and find that composites of precipitation and mixing anomalies share similar spatial patterns. We also compare summer and winter seasons and find that composites of seasonal-mean precipitation and mixing anomalies present similar characteristics, with precipitation being particularly intense (weak) where mixing is strong (weak). In particular, these patterns closely match the typical signatures of the intertropical convergence zone (ITCZ) and monsoon systems and the migrations of extratropical cyclone tracks. At the subseasonal scale, we employ daily composites to investigate the influence of the Madden–Julian oscillation and the North Atlantic Oscillation on the mixing regimes of the Atlantic and east Pacific; our results indicate that these oscillations control the synoptic-scale horizontal mixing and the occurrence of LCSs as to suppress or enhance precipitating systems like the ITCZ and the South Atlantic convergence zone. The results presented in this first climatology of synoptic-scale mixing and LCSs indicate that these are powerful diagnostics to identify circulation mechanisms underlying precipitation variability.

Open access
Min-Gyu Seong
,
Seung-Ki Min
, and
Xuebin Zhang

Abstract

Recent studies showed that anthropogenic greenhouse gas (GHG) increase is a major driver of the observed increases in extreme temperatures at global and regional scales using an optimal fingerprint (OF) method, which is a frequentist approach based on linear regression. Here, a Bayesian decision method is employed, which finds the most probable cause of the observed changes by comparing likelihoods of different forcings in view of observations. To quantify individual forcing contributions, a new modified attribution procedure based on Bayesian decision is proposed, i.e., computing the likelihood ratio [Bayes factor (BF)] between different forcings. First, the contribution of anthropogenic forcing (ANT) is measured by BF between anthropogenic-plus-natural forcing (ALL) and natural forcing (NAT) using a threshold for “substantial” evidence (lnBF ≥ 1). Similarly, the NAT contribution is assessed by BF between ALL and ANT. Further, the GHG contribution to the detected ANT is quantified by BF between ANT and anthropogenic aerosols (AA), and the AA contribution is evaluated by BF between ANT and GHG. The devised Bayesian approach is applied to HadEX3 observations and CMIP6 multimodel simulations for extreme temperature intensities (warmest day/night and coldest day/night) for global, continental, and regional domains following previous studies. Bayesian attribution results indicate that the ANT signal is detected in many continental and subregions for all extremes indices. This is generally consistent with OF-based results but with less frequent detection, indicating that the Bayesian method is slightly stricter than the OF method. However, GHG contributions to the detected ANT are identified over more subregions in the Bayesian attribution, suggesting its potential advantage over conventional methods in case of low signal-to-noise ratio and high collinearity.

Restricted access
Bing Pu
,
Qinjian Jin
,
Paul Ginoux
, and
Yan Yu

Abstract

California is one of the nation’s top agriculture producers and is vulnerable to extreme events such as droughts and heat waves. Concurrent extreme events may further stress water and energy resources, exerting greater adverse socioeconomic, environmental, and health impacts than individual events. Here we examine the features of compound drought, heat wave, and dust events in California during spring and summer. From 2003 to 2020, 16 compound events are found in warm seasons, with a mean duration of ∼4 days. Compound events are characterized by enhanced surface temperature up to 4.5°C over northern and western California, reduced soil moisture and vegetation density, and an increase in dust optical depth (DOD) by 0.05–0.1 over central and southern California. The enhanced DOD is largely associated with severe vegetation dieback that favors dust emissions and southeasterly wind anomalies that support northward transport of dust from source regions in southern California. Surface fine dust and PM2.5 concentrations also increase by more than 0.5 and 5 μg m−3, respectively, during compound events associated with both enhanced dust emissions and a relatively stable atmosphere that traps pollutants. The development of the compound events is related to an anomalous high over the west coast in the lower to middle troposphere, which is a pattern favoring sinking motion and dry conditions in California. The anomalous high is embedded in a wave train that develops up to 7 days before the events. In comparison with heat wave extremes alone, compound events show significantly higher DOD and lower vegetation density associated with droughts.

Open access
Olivier Arzel
,
Thierry Huck
,
Antoine Hochet
, and
Alexandre Mussa

Abstract

Identifying the primary drivers of North Atlantic interdecadal climate variability is crucial for improving climatic prediction over the coming decades. Here the effect of thermal coupling on the leading energy sources of the interdecadal variability of the ocean–atmosphere system is examined by means of a stochastically forced idealized coupled model. The effect of coupling is quantified from a comparison of the buoyancy variance budget of coupled and uncoupled model configurations. The simplicity of the model allows us to contrast the effect of coupling between a supercritical regime where the deterministic ocean dynamics drive the variability and a damped regime where noise forcing is central to its existence. The results show that changes in surface buoyancy fluxes act as a sink of temperature variance in the supercritical regime, and only become a source in the strongly damped regime. By contrast, internal ocean dynamics associated with the interaction of transient buoyancy fluxes with mean buoyancy gradients always act as a source of interdecadal variability. In addition to the reduced thermal damping effect in coupled integrations, thermal coupling with the atmosphere is shown to significantly increase the role of internal ocean dynamics in the variability, particularly in the regime where interdecadal modes are damped. Only for oceanic background states in the strongly damped regime do changes in surface buoyancy fluxes play a leading role in the upper-ocean variability. A stochastically forced coupled box model is proposed that captures the basic effect of thermal coupling on atmospheric and oceanic energy sources of variability.

Significance Statement

The purpose of this study is to better understand the impact of ocean–atmosphere thermal coupling on the leading energy sources of Atlantic interdecadal variability. Increasing our understanding of the physical mechanisms driving climate variability at interdecadal time scales is important to improve climate prediction. We show that the effect of ocean–atmosphere thermal coupling, as measured by the atmospheric feedback on sea surface temperature anomalies, is to substantially increase the role of internal ocean dynamics in the low-frequency variability of the upper-ocean heat content and sea surface temperature. Atmospheric stochastic forcing only becomes the primary driver of the oceanic temperature variability in the large dissipative limit, when internal ocean modes are strongly damped.

Restricted access
Zhun Guo
,
Kalli Furtado
,
Tianjun Zhou
,
Vincent E. Larson
, and
Ling Zhang

Abstract

During the winter and subsequent spring of an El Niño year, the East Asian marginal sea (EAMS) exhibits positive sea surface temperature anomalies (SSTAs) and fewer low clouds, while the western North Pacific experiences negative SSTAs. In this study, we suggest that the positive SSTAs in EAMS are maintained by a positive low cloud–SST feedback. In neutral winters and springs, the EAMS is covered by low clouds, which have a cooling effect on surface temperatures. During an El Niño year, a western North Pacific anomalous anticyclone is established, and along its northwestern flank, there are favorable conditions for convergence of moisture and weaker surface latent heat flux over the EAMS. Once a positive SSTA has been established, a further reduction of turbulent mixing results in less low cloud and enhanced solar heating of the ocean mixed layer; this reinforces and maintains both the positive SSTA and the lack of low cloud via a positive feedback mechanism. The concurrent increase of low cloud–SST feedback and anticyclone circulation strengths is evident in the coupled-model simulations from phase 6 of the Coupled Model Intercomparison Project. Furthermore, sensitivity experiments, performed with the atmospheric components of Community Earth System Model (CESM2), reveal that a positive SSTA helps to maintain the western North Pacific anomalous anticyclone. Four pacemaker-coupled experiments by CESM2, with sea surface temperature in the equatorial Pacific restored to the observational anomalies plus the model climatology and altered low cloud feedback over EAMS, suggest that the low cloud–SST feedback results in more than the maintenance of a positive SSTA over the EAMS: the positive feedback is also a previously overlooked mechanism for the maintenance of the western North Pacific anomalous anticyclone.

Significance Statement

The East Asian marginal sea (EAMS) and western North Pacific are important areas that bridge El Niño and the climate of East Asia. Unlike the cold sea surface temperature anomaly (SSTA) over the western North Pacific during El Niño, the positive SSTA over EAMS, which is covered by winter low cloud, has received less attention. We suggest that a “low cloud–SST” feedback—namely, one in which decreasing low-level clouds allows more sunlight to strike the ocean surface and favors higher SST—maintains the positive SSTA over EAMS. We also configure a widely used atmospheric model with a set of preset SSTA patterns that mimic different climate patterns. Our experiments with different climate patterns and CMIP6 historical runs show that the low cloud–SST feedback (through the positive SSTA) is a possible supplementary mechanism for reinforcing the WNP anomalous anticyclone.

Restricted access
Daehyun Kang
,
Daehyun Kim
,
Stephanie Rushley
, and
Eric Maloney

Abstract

This study investigates why the major convective envelope of the Madden–Julian oscillation (MJO) detours to the south of the Maritime Continent (MC) only during boreal winter [December–March (DJFM)]. To examine processes affecting this MJO detour, the MJO-related variance of precipitation and column-integrated moisture anomalies in DJFM are compared with those in the seasons before [October–November (ON)] and after [April–May (AM)]. While MJO precipitation variance is much higher in the southern MC (SMC) during DJFM than in other seasons, the MJO moisture variance is comparable among the seasons, implying that the seasonal locking of the MJO’s southward detour cannot be explained by the magnitude of moisture anomalies alone. The higher precipitation variance in the SMC region is partly explained by the much higher moisture sensitivity of precipitation in DJFM than in other seasons, resulting in a more efficient conversion of anomalous moisture to anomalous precipitation. DJFM is also distinguishable from the other seasons by stronger positive wind–evaporation feedback onto MJO precipitation anomalies due to the background westerly wind in the lower troposphere. It is found that the seasonal cycle of moisture–precipitation coupling and wind–evaporation feedback in the SMC region closely follows that of the Australian monsoon, which is active exclusively in DJFM. Our results suggest that the MJO’s southward detour in the MC is seasonally locked because it occurs preferentially when the Australian monsoon system produces a background state that is favorable for MJO development in the SMC.

Open access
Zhangqi Dai
,
Bin Wang
,
Ling Zhu
,
Jian Liu
,
Weiyi Sun
,
Longhui Li
,
Guonian Lü
,
Liang Ning
,
Mi Yan
, and
Kefan Chen

Abstract

Atlantic multidecadal variability (AMV) is a cornerstone for decadal prediction and profoundly influences regional and global climate variability, yet its fundamental drivers remain an issue for debate. Studies suggest that external forcing may have affected AMV during the Little Ice Age (AD 1400–1860). However, the detailed mechanism remains elusive, and the AMV’s centennial to millennial variations over the past 2000 years have not yet been explored. We first show that proxy-data reconstructions and paleo-data assimilations suggest a significant 60-yr AMV during AD 1250–1860 but not during AD 1–1249. We then conducted a suite of experiments with the Community Earth System Model (CESM) to unravel the causes of the changing AMV property. The simulation results under all external forcings match the reconstructions reasonably well. We find that the significant 60-yr AMV during 1250–1860 arises predominantly from the volcano forcing variability. During the period 1–1249, the average volcanic eruption intensity is about half of the 1250–1860 intensity, and a 20–40-yr internal variability dominates the AMV. The volcanic radiative forcing during 1250–1860 amplifies AMV and shifts the internal variability peak from 20–40 years to 60 years. The volcano forcing prolongs AMV periodicity by sustaining Arctic cooling, delaying subpolar sea ice melting and atmospheric feedback to reduce surface evaporation. These slow-response processes over the subpolar North Atlantic results in a persisting reduction of sea surface salinity, weakening the Atlantic overturning circulation, and warm water transport from the subtropical North Atlantic. The results reveal the cause of the nonstationary AMV over the past two millennia and shed light on the AMV’s response to external forcing.

Significance Statement

AMV plays an important role in the regional and global climate variability. The purpose of this study is to better understand the secular change of AMV during the past 2000 years and its response to the external forcing. Proxy data and model simulation consistently show a significant 60-yr AMV during AD 1250–1860 that is absent during AD 1–1249. Active volcanic eruptions during 1250–1860 amplify the AMV and shift its intrinsic 20–40-yr to a prominent 60-yr variance peak. Volcanoes prolong AMV periodicity by sustaining Arctic cooling, delaying subpolar sea ice melting, reducing evaporation, and increasing surface salinity. These results help us better understand nonstationary AMV and highlight the role of external forcing over the past two millennia.

Open access
Nicole K. Neumann
and
Nicholas J. Lutsko

Abstract

The factors controlling the present-day pattern of temperature variance are poorly understood. In particular, it is unclear why the variance of wintertime near-surface temperatures on daily and synoptic time scales is roughly twice as high over North America as over Eurasia. In this study, continental geometry’s role in shaping regional wintertime temperature variance is investigated using idealized climate model simulations run with midlatitude continents of different shapes. An isolated, rectangular midlatitude continent suggests that in the absence of other geographic features, the highest temperature variance will be located in the northwest of the continent, roughly collocated with the region of largest meridional temperature gradients, and just north of the maximum near-surface wind speeds. Simulations with other geometries, mimicking key features of North America and Eurasia, investigate the impacts of continental length and width, sloping coastlines, and inland bodies of water on regional temperature variance. The largest effect comes from tapering the northwest corner of the continent, similar to Eurasia, which substantially reduces the maximum temperature variance. Narrower continents have smaller temperature variance in isolation, implying that the high variances over North America must be due to the nonlocal influence of stationary waves. Support for this hypothesis is provided by simulations with two midlatitude continents, which show how continental geometry and stationary waves can combine to shape regional temperature variance.

Significance Statement

Wintertime temperature variance over North America is roughly twice as high as over Eurasia, but the reasons for this are unknown. Here we use idealized climate model simulations to investigate how continental geometry shapes regional temperature variance. We find that the smaller variance over Eurasia is largely due to the tapering of its northwest coast, which weakens temperature gradients in the continental interior. Our simulations also suggest that in isolation a narrow continent, like North America, should have weak temperature variance, implying that stationary waves are responsible for the high variance over North America. Understanding the controls on regional temperature variance is important for interpreting present-day winter climates and how these will change in the future.

Open access
Chao He
and
Tianjun Zhou

Abstract

The subtropical North Pacific and North Atlantic are controlled by basin-scale anticyclones in boreal summer. Based on a novel metric regarding the strengths of the rotational and the divergent circulation of anticyclones, we investigated the possible future responses in the intensity of these two subtropical anticyclones to global warming. While the North Atlantic subtropical anticyclone (NASA) is projected to strengthen, the North Pacific subtropical anticyclone (NPSA) is projected to weaken, in terms of both the rotational and the divergent circulation. The distinct responses of the NPSA and NASA are corroborated by the models participating in the fifth and sixth phases of the Coupled Model Intercomparison Project (CMIP), under both intermediate and high emission scenarios. We further investigated the possible mechanism for their distinct responses by decomposing the effect of greenhouse gas forcing into the direct effect of increased CO2 concentration and the indirect effect through sea surface temperature (SST). The intensified NASA results from the CO2 direct forcing while the weakened NPSA is dominated by the SST warming. The CO2 direct forcing enhances the NASA by increasing land–ocean thermal contrast anchored by the largest subtropical continental area, the Eurasian–African continent. Both the uniform SST warming and the change in SST pattern act to weaken the NPSA by increasing the latent heating over the subtropical North Pacific basin, as suggested by atmospheric component model simulations. The distinct responses of the NPSA and the NASA may lead to zonal asymmetry of the subtropical climate change.

Restricted access