Journal of Climate

Kaminski, Kristie; Ashley, Walker S.; Haberlie, Alex M.; Gensini, Vittorio A.

2025 Journal of Climate

doi: 10.1175/jcli-d-23-0633.1

AbstractThis study uses high-resolution, convection-permitting, dynamically downscaled regional climate simulation output to assess how long-lived, convectively induced, extratropical windstorms known as derechos may change across the CONUS during the twenty-first century. Three 15-yr epochs including a historical period (1990–2005) and two separate late-twenty-first-century periods (2085–2100) employing intermediate (RCP4.5) and pessimistic (RCP8.5) greenhouse gas concentration scenarios are evaluated. A mesoscale convective system (MCS) identification and tracking tool catalogs derecho candidates across epochs using simulated radar reflectivity and maximum 10-m wind speed as a proxy for near-surface severe wind gusts. Results indicate that MCS-based windstorms, including derechos, are more frequent, widespread, and intense in both future climate scenarios examined for most regions of the central and eastern CONUS. Increases are suggested across all parts of the year, with significant changes in populations concentrated during the early spring and summer months, suggesting the potential for a longer, more extreme MCS windstorm season. This research provides insights for forecasters, emergency managers, and wind-vulnerable stakeholders on how these events may change across the twenty-first century so that they may mitigate, adapt to, and become resilient against severe convective storm perils.Significance StatementLong-lived, thunderstorm-induced, damaging wind events known as derechos may increase across most portions of the central and eastern United States in the future, with projections indicating a near doubling or tripling of annual cases in the Midwest, eastern Great Plains, and Mississippi and Ohio Valley regions by the end of the twenty-first century. Modeling projections suggest that future derechos could generally be longer-lived, more expansive, and capable of producing more severe wind gusts and damage, which will ultimately increase risk to life, infrastructure, and wind-sensitive industries affected by these extreme thunderstorm events.

Fasullo, John T.; Rosenbloom, Nan; Buchholz, Rebecca

2025 Journal of Climate

doi: 10.1175/jcli-d-24-0148.1

AbstractThe influence of biomass burning (BB) aerosols arising from wildfires and agricultural fires on the transient coupled evolution of El Niño–Southern Oscillation (ENSO) is explored in Community Earth System Model, version 2 (CESM2). For both El Niño and La Niña, two 20-member ensembles are generated from initial states that are predisposed to evolve into ENSO events. For each ENSO phase, one ensemble is forced with the observed BB emissions during satellite-era ENSO events while the other is forced with a climatological annual cycle, with the responses to anomalous BB emissions estimated from interensemble differences. It is found that the regional responses to anomalous BB emissions occur mainly during boreal fall, which is also the time of the climatological seasonal maximum in emissions. Transient responses are identified in precipitation, clouds, and radiation in both the tropics and extratropics. At the onset of El Niño, these include increased precipitation in the northern branch of the intertropical convergence zone (ITCZ) and an enhancement of cloud albedo and amount across the Maritime Continent and eastern subtropical Pacific Ocean. Additional responses are identified through the course of El Niño and successive La Niña events, the net effect of which is to strengthen sea surface temperature (SST) anomalies in the eastern Pacific Ocean during El Niño and warm the tropical Pacific Ocean during La Niña. These responses improve the simulation of ENSO power, diversity, and asymmetry in CESM2.Significance StatementBiomass burning emissions from both wildfires and agricultural fires during ENSO act as a climate feedback, as they are both driven by fire weather anomalies tied to ENSO and influence clouds, radiation, and precipitation on a global scale. While the largest biomass burning (BB) emissions emanate mainly from Indonesia, other regions also exhibit systematic responses. When simulated in CESM2, the emissions collectively impact the flows of energy across the tropical Pacific Ocean in a spatially and temporally complex fashion, with the net effect of improving CESM2’s simulation of ENSO events.

Leitmann-Niimi, Nicolas M.; Kummerow, Christian D.; Hsiao, Wei-Ting; Maloney, Eric D.

2025 Journal of Climate

doi: 10.1175/jcli-d-23-0500.1

AbstractThe water budget components of an atmospheric column are precipitation, evaporation, and horizontal water vapor divergence. This study finds that when precipitation from the Global Precipitation Climatology Project (GPCP), evaporation from the SeaFlux product, and water vapor divergence from ERA5 are employed, the degree of budget closure depends strongly on the location and time period. Variations in this error are not random, and this study seeks to better understand these biases as the climate system evolves. Errors are particularly significant over ocean regions in and near the west Pacific warm pool, where there are multiyear budget residuals of roughly 10% of the magnitude of precipitation. Biases in other tropical ocean basins are more seasonal and smaller in magnitude. Time-varying budget errors are strongly linked to variations in convective organization that vary with the large-scale environment; errors correlating with deep organized rain show coefficients of 0.62 and 0.56 in the west Pacific and central Pacific, respectively. Errors are linked to a lesser extent with cloud microphysical structures in the East Indian. Both factors affect rainfall retrievals through well-known precipitation bias mechanisms (beam-filling, convective/stratiform effects). Characteristics of the large-scale environment that produce changes in convective organization and precipitable ice water content are explored.

Tomassini, Lorenzo

2025 Journal of Climate

doi: 10.1175/jcli-d-24-0068.1

AbstractOne-year-long global kilometer-scale simulations with prescribed sea surface temperatures are presented that are used to study how clouds, convection, and the tropical circulation change under two different warming patterns. The warming patterns, one of which exhibits La Niña–like characteristics and the other an El Niño–like feature, are added to the historical year 2020 and derived from past observed sea surface temperatures. They contain both signatures of natural variability and climate change. Climate sensitivities distinctly differ depending on the warming pattern, mainly due to diverse changes in low clouds over the eastern tropical Pacific. These changes are connected with alterations in larger-scale circulations. Processes related to the interaction between clouds, convection, and circulation are further examined with a focus on the tropical Pacific. Better understanding those processes helps elucidate apparent discrepancies between recent observed trends over the tropical Pacific and trends simulated by low-resolution global climate models. Evidence suggests an active role of moisture dynamics in shaping low-level moist static energy anomalies over the western tropical Pacific and a positive feedback of resulting moist convection on larger-scale zonal circulations, including over the eastern Pacific subsidence region. Complementary to the global kilometer-scale simulations, the same experiments are conducted with a lower-resolution model version in which convection is parameterized. Even though climate sensitivities and the broad responses are not strongly changed, the differences in the convection-permitting experiments might still be consequential in global coupled climate simulations.

Zhang, Ruhua; Zhou, Wen; Zhang, Yue; Xu, Xiran

2025 Journal of Climate

doi: 10.1175/jcli-d-24-0207.1

AbstractThis study analyzes the response of East Asian surface air temperature (SAT) to the quasi-biennial oscillation (QBO) in boreal winter using the JRA-55 reanalysis dataset and the CAM6 model. East Asian SAT patterns in response to the boreal winter QBO vertical profile are derived from singular value decomposition (SVD) analysis. The leading mode of SVD (SVD1) shows an evident cold center over East Asia, while warm and cold centers show an east–west dipole in the second mode of SVD (SVD2). The corresponding QBO pattern in SVD1 has two opposite centers, with an upper-stratospheric easterly phase and a lower-stratospheric westerly phase; however, there is only one middle-stratospheric westerly center in SVD2. The SVD1-like QBO tends to cause more significant East Asian SAT anomalies than the SVD2-like QBO pattern. This difference originates from the response of the stratospheric polar vortex and the North Pacific circulation to the QBO. The SVD1-like QBO forces a cyclonic anomaly over the midlatitude North Pacific and an anticyclonic anomaly over China via a subtropical path and polar path, leading to a zonal pressure gradient and meridional wind anomaly over East Asia. Temperature advection driven by this meridional wind affects cooling and warming over East Asia. By contrast, there are no significant responses to the SVD2-like QBO over the midlatitude North Pacific, and thus, the corresponding East Asian SAT response is weak.Significance StatementEast Asian surface air temperature (SAT) variation is often attributed to variations in lower-atmospheric dynamics and sea surface temperature. Few studies focus on the role of the QBO. In this study, we diagnose the influence of the stratospheric QBO on East Asian SAT during boreal winter. Both reanalysis and simulated results indicate that the response of East Asian SAT is sensitive to the selection of multilevel QBO indices. This sensitivity comes from the changes in QBO’s subtropical and polar influencing path. Our results provide a new perspective on East Asian SAT changes.

Zhao, Yang; Lu, Mengqian; Zhang, Lujia; Cheng, Tat Fan

2025 Journal of Climate

doi: 10.1175/jcli-d-23-0706.1

AbstractThis study examines the remote influence of the Tibetan Plateau (TP) diabatic heating on atmospheric river (AR) activity in the North Pacific. First, we identify a sensitive heating region on the southern TP establishing a positive correlation between its heating and AR frequency. This correlation is attributed to latent heat release supported by a substantial moisture supply. Further analysis reveals that the dynamic effect of the eastward-propagating Rossby waves, originating from the Atlantic Ocean and modulated by the TP, facilitates upward moisture lifting. Using the Water Accounting Model-2Layers, we demonstrate that this anomalous heating is primarily due to increased moisture from the Indian Ocean, the Arabian Sea, and the Bay of Bengal, amplified by the westward extension of the western North Pacific subtropical high (WNPSH). Additional moisture contributions are observed from Eurasia. Moreover, Rossby wave activity over the TP propagates to the east of Japan, enhancing westerlies with upper-level divergence field which develops a cyclonic and anticyclonic vortex structure. This structure attracts abundant moisture to the North Pacific, thereby increasing AR activity. This study also highlights a positive feedback mechanism involving the southern TP heating and the eastward-propagating upper-level anticyclone, which enhance the western extension of the WNPSH. These findings underscore the global climatic impacts of the TP, emphasizing its role as a critical factor in AR dynamics across the North Pacific.Significance StatementThis study explores the remote impact of the Tibetan Plateau (TP) heating on the atmospheric river (AR) in the North Pacific. Our findings indicate that southern TP heating resulting from remote moisture contribution, quantitatively assessed using Water Accounting Model-2Layers, coupled with the western extension of the western North Pacific subtropical high (WNPSH) and sea surface temperature warming anomaly as well as the eastward-propagating Rossby waves derived from the Atlantic Ocean, plays a crucial role in AR activity in the North Pacific. TP heating affects Rossby waves propagating east of Japan enhancing the westerlies and the easterlies. Moreover, the coupled divergence and cyclonic vortex structures attract more moisture toward the North Pacific fostering AR activity. Additionally, the interplay between southern TP heating and WNPSH builds a positive feedback mechanism.

Choi, Nakbin; Stan, Cristiana

2025 Journal of Climate

doi: 10.1175/jcli-d-24-0078.1

AbstractThis study aims to understand the source of surface air temperature bias over the contiguous United States (CONUS) during boreal summer (June–September) in the Unified Forecast System (UFS) coupled model prototype 8 (P8), developed by the National Centers for Environmental Prediction (NCEP) and the National Oceanic and Atmospheric Administration (NOAA). The focus is on the subseasonal variability defined as a weekly average in weeks 2–5 of forecast leads (total 224 cases; 4 weeks × 2 initialization dates × 4 months × 7 years). The large-scale surface air temperature bias pattern is extracted using the empirical orthogonal function (EOF) analysis. The associated principal component describes the variability of bias for each reforecasting week throughout the 2011–17 reforecasting period. The leading EOF of surface air temperature bias exhibits an east–west dipole pattern over the CONUS, explaining 31.6% of the total variability of weekly temperature bias. This bias pattern is strongly related to the upper-level Rossby wave induced by a bias in convection over the central tropical Pacific. Furthermore, the mean bias of background flow in the extratropics degrades the representation of teleconnections from the tropics to the midlatitudes. UFS P8 has weaker zonal wind over the North Pacific with stronger vertical wind shear than the ERA5 reanalysis. The weak zonal wind hampers the Rossby wave’s propagation, while strong vertical shear reduces its amplitude.

Xu, Ke; Lu, Riyu

2025 Journal of Climate

doi: 10.1175/jcli-d-23-0776.1

AbstractThis study identifies 86 active events and 66 break events of the western North Pacific summer monsoon (WNPSM) from 1979 to 2020. These active and break events exhibit sharp contrast in large-scale convection and circulation fields. During the active period, deep convection almost covers the entire WNP domain (0°–25°N, 120°–170°E), accompanied by the strengthening of a local monsoon trough and monsoon westerlies extending from the Arabian Sea to the WNP. In contrast, during the break period, along with weakened monsoon westerlies, the subtropical high replaces the monsoon trough to dominate the WNP domain, leading to the disappearance of deep convection. Results about the convection evolution at multiple time scales indicate that active/break events are primarily contributed to by the wet/dry phases of intraseasonal oscillations (ISOs). The phase transitions of ISOs are nearly symmetric between active and break events. However, these two types of events present notable asymmetric features in convection anomalies relative to climatology, which can be attributable to the modulation of the seasonal mean component. Specifically, both active and break events correspond to anomalously strong seasonal mean convection, which amplifies (weakens) the convection enhancement (suppression) during the active (break) period and results in normal (enhanced) convection during adjacent periods. The preferred occurrence of active and break events in strong summer monsoons is because related mean backgrounds, including increased low-level positive vorticity, specific humidity, and easterly vertical shear, facilitate the intensification of ISOs. These mean backgrounds are likely induced by SST cooling in the north Indian Ocean and warming in the equatorial central Pacific during the simultaneous summer.Significance StatementThe subseasonal variation in monsoon rainfall typically manifests as fluctuations between active events with abundant rainfall and break events with deficit rainfall, which can induce converse extremes such as floods and droughts, significantly affecting water management, agriculture planning, and economic activities. This study aims to reveal the characteristics, formation, and occurrence regularity of active and break events during the western North Pacific summer monsoon (WNPSM). We note that the occurrence of these active and break events is determined not only by intraseasonal oscillations but also by seasonal mean backgrounds, which result in certain asymmetrical features between the two types of events. These results will enhance our understanding of the monsoon’s intraseasonal variability and the physical association between multiple time scales over the WNP domain (0°–25°N, 120°–170°E).

White, Kayla; Liu, Dunyu; Persad, Geeta

2025 Journal of Climate

doi: 10.1175/jcli-d-23-0755.1

AbstractThe aerosol indirect effect (AIE) dominates uncertainty in total anthropogenic aerosol forcing in phase 6 of the Coupled Model Intercomparison Project (CMIP6) models. AIE strength depends on meteorological conditions that have been shown to change between preindustrial (PI) and present-day (PD) climates, such as cloud cover and atmospheric moisture. Hence, AIE strength may depend on background climate state, impacting the dependence of model-based AIE estimates on experiment design or the evolution of AIE strength with intensifying climate change, which has not previously been explicitly evaluated. Using atmosphere-only simulations with prescribed observed sea surface temperatures (SSTs) and sea ice in the National Center for Atmospheric Research (NCAR) Community Earth System Model 2, version 2.1.3 (CESM2), Community Atmosphere Model, version 6.0 (CAM6), model, we impose a PD (2000) aerosol perturbation onto a PI (1850), PD, and PD with a uniform 4 K increase in the SST (PD + 4 K) background climate to assess the dependence of the total aerosol effective radiative forcing (ERF) and AIE on background climate. We find statistically insignificant increases in aerosol ERF when estimated in the different background climates, almost entirely from increases in direct ERF but with some regionally significant compensating signals in PD + 4 K. The absence of an AIE dependence on background climate in our PD simulation may be tied to documented differences in cloud responses to the observed SSTs used in our simulations versus SSTs produced by the fully coupled models from which most cloud feedback studies are derived, known as the “pattern effect.” Our findings indicate that AIE and aerosol forcing overall may not have a strong dependence on the background climate state in the near future but could regionally under extreme climate change.Significance StatementDiverse model representations of aerosol–cloud interactions strongly contribute to uncertainty in historical anthropogenic aerosol forcing and are associated with uncertainty in climate sensitivity. This study aims to highlight the dependence of aerosol indirect effects on the background climate state in Community Earth System Model 2, version 2.1.3 (CESM2), Community Atmosphere Model, version 6.0 (CAM6), by identifying microphysical and meteorological changes between aerosol-driven atmospheric responses in present-day and preindustrial climate states to understand anthropogenic aerosol-driven forcing more thoroughly.

Miyamoto, Ayumu; Xie, Shang-Ping

2025 Journal of Climate

doi: 10.1175/jcli-d-24-0015.1

AbstractOver the subtropical Northeast Pacific (NEP), highly reflective low clouds interact with underlying sea surface temperature (SST) to constitute a local positive feedback. Recent modeling studies showed that, together with wind–evaporation–SST (WES) feedback, the summertime low cloud–SST feedback promotes nonlocal trade wind variations, modulating subsequent evolution of El Niño–Southern Oscillation (ENSO). This study aims to identify drivers of summertime low-cloud variations, using satellite observations and global atmosphere model simulations forced with observed SST. A transbasin teleconnection is identified, where the north tropical Atlantic (NTA) warming induced by the North Atlantic Oscillation (NAO) increases precipitation, exciting warm Rossby waves that extend into the NEP. The resultant enhancement of static stability promotes summertime low cloud–SST variability. By regressing out the effects of the preceding ENSO and NTA SST, atmospheric internal variability over the extratropical North Pacific, including the North Pacific Oscillation (NPO), is found to drive the NEP cooling by latent heat loss and subsequent summer low cloud–SST variability. With the help of the background trade winds and WES feedback, the SST anomalies extend southwestward from the low-cloud region, accompanied by ENSO in the following winter. This suggests the nonlocal effects of low clouds identified by recent studies. Analysis of a 500-yr climate model simulation corroborates the NTA and NPO forcing of NEP low cloud–SST variability and subsequent ENSO.