Journal of Climate

The Silk Road pattern (SRP) is an important summer teleconnection pattern across the midlatitude Eurasia and is featured by alternate southerly and northerly wind anomalies along the upper-tropospheric westerly jet. It affects significantly the boreal summer climate, including those around the Tibetan Plateau (TP), which has been investigated much based on the summer mean results. However, the SRP shows substantial differences between early and late summer, which are separated at early July. This study identifies that the SRP’s impacts on precipitation surrounding the TP differ much between the two periods: The SRP-related precipitation anomalies are significant over the northeastern TP (NETP) in early summer, but shift northwestwards to domains around the northwestern TP (NWTP) in late summer. These differences can be attributed to the subseasonal change of the SRP and its configurations with the thermal condition surrounding the TP. The SRP is located more westward in late summer than in early summer. Correspondingly, the SRP-related southerly anomalies around the TP prevail over the NETP in early summer but over the NWTP in late summer. Superimposed onto the climatological thermal contrast between the humid southern TP and the dry north, these southerly anomalies favor the northward moisture transport and local precipitation genesis. NETP and NWTP are two domains with greater precipitation amount surrounding the northern TP and are crucial for the Asian ecological and environmental balance. Therefore, the present study holds practical implications for enhancing the accuracy of summer precipitation forecasts surrounding the TP on a subseasonal time scale and further improves regional climate adaptation and water resource management.

Monsoon intraseasonal oscillation (MISO) is a predominant feature of the Indian summer monsoon (ISM). The meridional gradient of sea surface temperature (SST) at intraseasonal time scales plays a key role in influencing MISOs via warm SST anomalies to the north of convection and cold SST anomalies around or lagging convection. Previous studies have proposed two major meridional SST gradient feedback routes: pressure adjustment (PA) mechanism and vertical mixing (VM) mechanism. In this study, we find that the contribution of the VM mechanism to the total wind convergence reaches up to 75% in the lower troposphere, whereas the PA mechanism accounts for only 50% of the total. The moisture budget reveals that the meridional SST gradient affects moisture supply mainly through the moisture convergence in the lower troposphere. The VM mechanism accounts for 90% of the moisture convergence induced by the meridional SST gradient, corresponding to the major effect of wind convergence, whereas the PA mechanism accounts for approximately 65%. The overwhelming contribution indicates that the VM mechanism plays a dominant role in lower-tropospheric dynamics and thermodynamics via the turbulent momentum flux. Besides, the two mechanisms result in their combined effect exceeding 100% in both wind and moisture convergences, which is caused by the strong nonlinearity in the low troposphere. Conversely, the PA and VM mechanisms have a similarly important role in the vorticity propagation in the free troposphere. Either of them can cause the interruption of the meridional shear of the intraseasonal vertical velocity in the northward propagation, affecting the propagation of vortex tilting, and thus the MISO propagation. Overall, this study quantitatively demonstrates the contribution of different mechanisms in the oceanic feedback to MISOs. Among them, the VM mechanism mainly dominates the wind and moisture convergences in the lower troposphere, whereas the PA and VM mechanisms jointly affect the vorticity in the free troposphere. These quantitative results advance our detailed understanding of ocean feedbacks during MISOs and ISM rainfall, which have important implications for improving model simulations and predictions.

The longwave feedback λ characterizes how Earth’s outgoing longwave radiation changes with near-surface air temperature Tas, directly impacting climate sensitivity. Compared to λ, its spectrally resolved counterpart λν offers deeper insights into the underlying physical processes. Both λ and λν vary with Tas, but this Tas dependence has so far only been investigated using models. Here, we derive the clear-sky λν for Tas between 210 and 310 K based on observations of the AIRS instrument. We disentangle the radiative signatures of the atmospheric general circulation by simulating λν based on a single-column model with different degrees of idealization. We find that at low Tas, the observed λν is dominated by the surface response and sensitive to biases in Earth’s skin temperature. At higher Tas, changes in the vertical distributions of atmospheric temperature and relative humidity play an important role in shaping λν. These changes impact both the absorption of surface emission in the atmospheric window and the atmospheric emission in the water vapor and CO2 absorption bands. Our results demonstrate that we can fully understand the observed λν at a wide range of Tas using a simple model of Earth’s atmosphere, lending further support to estimates of the clear-sky longwave λ, Earth’s most fundamental climate feedback. They also highlight the effect of different assumptions about Earth’s atmosphere on λ. Similar approaches can be used to better constrain changes in relative humidity and temperature with warming using satellite observations, as well as for paleoclimate and exoplanet studies. Significance Statement The longwave feedback describes how much more thermal radiation Earth emits to space as it warms. It is the strongest feedback in Earth’s climate and determines the warming in response to an increase in CO2 concentration. We use satellite observations to analyze the dependence of the longwave feedback on radiation frequency at a wide range of surface temperatures which allows insights into the underlying physical processes. We reproduce these observations using a simple model of Earth’s atmosphere, demonstrating the complete physical understanding of this fundamental climate feedback. Our results can be used to constrain changes in atmospheric temperature and humidity with warming from satellite observations and to better understand the climates of Earth’s past and of other planets.

An idealized ice-ocean model is used to study the time-dependent Atlantic Meridional Overturning Circulation (AMOC) responses to a sudden uniform surface warming and/or an amplified evaporation minus precipitation (E-P) forcing. At transient timescales, the AMOC initially weakens in response to both types of forcing as a result of buoyancy gain in the North Atlantic, but the amplified E-P response is an order of magnitude smaller when its amplitude is chosen based on Clausius-Clapeyron scaling, consistent with its weaker initial buoyancy flux anomaly. At equilibrium, the AMOC here weakens under warming, contrasting with previous idealized modelling studies. The difference is attributed to a larger role of North Atlantic warming (acting to weaken the AMOC), and a weaker role of reduced brine rejection around Antarctica (acting to deepen and strengthen the AMOC). When E-P forcing is amplified, the AMOC strengthens, qualitatively consistent with a previously proposed passive response that predicts an enhancement of the existing salinity pattern in equilibrium, although the amplification of the salinity contrast is significantly damped by a negative salt advection feedback. For a small-amplitude change of both temperature and E-P, the AMOC response can be approximated by the linear combination of the individual responses. However, large-amplitude warming and amplified E-P forcing can lead to a positive salt advection feedback that collapses the AMOC in our simulations. To understand why the sign of the salt advection feedback varies across different simulations, its multifaceted roles are further investigated using box-model theories, and their relevance to comprehensive models is discussed.

Atmospheric jet streams belong to the most fundamental elements of the global general circulation system and are susceptible to climate change. Jet stream variability in our present climate is usually studied from modern reanalysis products, although uncertainties arise due to insufficiently strong constraints of the underlying global wind field by the available observational records, especially concerning their long-term trends. This motivates the use of observation-based metrics to track dynamical variability and historical trends. Here, we investigate how the zonal-mean ozone structure can be used to indirectly infer changes in the strength and latitudinal position of the subtropical jet streams (STJs). We mainly consider the winter-mean ozone distribution and analyze different diagnostics that track anomalies of the sharp ozone gradients near the subtropical tropopause, based on either vertically resolved or total-column ozone (TCO) fields. Using ERA5 reanalysis output, we find significant correspondence of these sharp ozone gradients with the STJ’s strength and location, with the jet acting as a tracer transport barrier and, hence, governing wave-induced horizontal ozone transport across the jet core. The ozone gradient metrics obtained from vertically resolved ozone observations agree well with ERA5 in more recent years when densely sampled satellite measurements were included. We furthermore obtain mostly consistent historical trend signals from both conventional STJ metrics from reanalyses and more independent TCO records. Chemistry–Climate Model Initiative phase 1 (CCMI-1) and CMIP6 climate simulations suggest a strong correspondence between changes in subtropical ozone gradient maxima and projected STJ trends under different climate forcing scenarios.

Climate models exhibit a weakening of the Indian Ocean Walker Circulation (IWC) under global warming. However, the model projections are highly uncertain. As the ocean and atmosphere are fully coupled in the tropical Indian Ocean, it is difficult to attribute this circulation uncertainty to the sea surface temperature warming pattern through local dynamics. This study investigates the projected IWC uncertainty from the perspective of interbasin and extratropical processes. The sources of intermodel uncertainty vary across seasons. In boreal winter, tropical processes are the primary factors contributing to the IWC uncertainty, as the convection anchored over the Maritime Continent significantly modulates changes in the tropical atmospheric circulations. Particularly, the Pacific teleconnection plays a key role because the IWC and Pacific Walker Circulation co-vary and are both regulated by warm pool convections. In boreal summer, warm pool convection is insufficient to dominate the IWC uncertainty as the precipitation maximum has migrated northward to the monsoon region. Extratropical factors such as changes in the Atlantic Meridional Overturning Circulation and sea ice, which potentially contribute to the interhemispheric warming contrast, are suggested to dominate the IWC uncertainty. Our findings reveal the distinct sources of intermodel uncertainty in the IWC change in different seasons and advance the understanding of the relative importance of tropical and extratropical processes in shaping the Indian Ocean climate change.

Using a long-term high-resolution ocean reanalysis dataset, interannual-to-decadal variability of ocean heat content (OHC) in the Japan Sea is investigated with focus on the role of the Tsushima Warm Current (TWC) and its relation to the Kuroshio Extension (KE) variability. A lagged-covariance between detrended OHC and each termin the OHC heat budget suggests that the TWC heat transport is the main factor for the interannual OHC variability and the surface heat flux responds passively to OHC changes. It is further suggested that the TWC plays an important role in determining the time scale of the decadal OHC variability. We also find a close relationship between the sea-level variability at the Japanese coast and the TWC volume transport, which is explained by propagation of Kuroshio-induced coastal trapped waves (CTWs) shown by previous studies and resulting changes in sea-level difference across the Tsushima Strait. In addition, considering the relationship between the Kuroshio-induced CTWs and the wind-forced KE variability indicated by previous studies, we propose a mechanism that the interannual-to-decadal OHC variability in the Japan Sea is forced by the KE variability through adjustments of the sea-level at the Japanese coast and the TWC transport.

Most climate models from the Coupled Model Intercomparison Project (CMIP) underestimate the strengthening of the equatorial Pacific Ocean’s zonal surface temperature gradient observed since the mid-1970s, raising concerns about biases in their mean states that could adversely affect their warming projections. We investigate whether higher-resolution models, with ocean and atmospheric horizontal resolutions of up to 10 km and 25 km respectively, mitigate mean state temperature biases compared to their lower-resolution counterparts. While some models better simulate the mean sea surface temperatures (SSTs) in the central-to-eastern equatorial Pacific, their simulated warming from 1979 to 2014 remains inconsistent with observations. An analysis of causal relationships among observed SSTs, surface zonal winds, and subsurface temperature changes confirms the role of the transient ocean thermostat mechanism in regulating SSTs in this region. However, models, regardless of resolution, exhibit a weak ocean thermostat effect due to asynchronous trends among these variables. Our findings suggest that resolving tropical Pacific warming patterns in models requires not only the inclusion of relevant sub-mesoscale processes but also improvements in the representation of long-term ocean-atmosphere coupling and external forcings.

Arctic precipitation increases in a warmer world as more moisture is available to condense. These increases in precipitation and moisture result from both local surface evaporation and remote transport. Despite ongoing efforts, the relative contribution of each moisture source to future Arctic precipitation change remains unknown. Here, new atmospheric model experiments are used to isolate the contributions of these two moisture sources to Arctic precipitation. During the pre-industrial era, remote transport controls Arctic precipitation in all months. By the late 21 st century, local surface evaporation becomes increasingly important due to local surface ocean warming and sea ice loss. In fact, late 21 st century fall and winter precipitation increases are driven by these local surface ocean changes. In contrast, late 21 st century moisture transport driven by non-local surface ocean warming entirely explains late 21 st century summer precipitation increases. Furthermore, these new experiments show that late 21 st century Arctic precipitation increases are directly driven by surface ocean warming and sea ice loss, indicating that surface ocean evaporation directly drives these increases rather than land-sourced moisture. Additionally, our experimental design enables an understanding and quantification of the co-influence between local surface evaporation and remote moisture transport. This co-influence acts to reduce late 21 st century Arctic precipitation during the fall and early winter. Overall, these results show when and where surface ocean warming and sea ice loss affect future Arctic precipitation increases.

Extratropical cyclones are responsible for most precipitation falling north of 40°N, especially in winter. Greater moisture availability in a warmer world is expected to boost the intensity of cyclone-associated precipitation (CAP), but how changes in cyclone frequency and intensity impact this trend is uncertain. Here we use two atmospheric reanalyses and eighteen climate models participating in version 6 of the Coupled-Model Intercomparison Project (CMIP6) to update projections of future CAP. Models project that nearly the entire Northern Hemisphere exhibits increasing winter CAP with continued warming (by at least 5% per 1°C global warming throughout, and over 30% in the Arctic and eastern Asia). Summer CAP increases over the Pacific Ocean (2-10%) and Arctic (up to 20%) but decreases over mid-latitude continents and the Atlantic Ocean (exceeding 20% in places). These outcomes result from the relative balance between two overarching and often opposing trends: Extratropical cyclones (and therefore CAP events) become less frequent (except in the Arctic), but the average event produces more precipitation in the future (especially by more intense precipitation rates). Historically, CAP intensity trends are driven more by moisture availability than cyclone intensity (i.e., stronger winds); projections indicate future CAP intensity enhancement will be driven almost entirely by moisture availability. The strongest CAP trends historically are increases on the west side of the mid-latitude oceanic storm tracks, but projections indicate the Arctic Ocean will exhibit the strongest positive future trends because of exceptional increases in moisture availability combined with little change to storm frequency or intensity.

The drivers of low-frequency (i.e., interannual-to-multidecadal) variability and change in ocean heat content (OHC) on the U.S. Northeast Shelf (USNES) are investigated through heat budget analysis and Regional Ocean Modeling System experiments. Surface heat flux on the USNES has been responsible for warming since 1977, and it dominates the interannual-to-decadal OHC variability in the shallow shelf near the coast. In contrast, remote forcing from the open Atlantic has a weak impact on the warming trend due to contrasting effects from the northern and eastern parts of the Atlantic. Still, it plays a more significant role in interannual-to-decadal OHC variability in deep shelf near the continental break. Both regional and remote forcings are important for the interannual-to-decadal variability of OHC integrated over the entire USNES. Regionally and remotely forced sea surface temperature (SST) anomalies alter surface heat flux over the USNES, inducing OHC variability. The remotely forced OHC anomalies result primarily from the advection of remotely forced temperature anomalies from the Scotian Shelf and along the shelf break by the currents driven by both regional and remote forcings. Furthermore, the interannual variability of Shelfbreak Jet significantly contributes to OHC advection through the northern boundary of the USNES. In contrast, the influence of the Gulf Stream on OHC advection across the USNES boundaries is relatively weak.

Overly smooth topography in general circulation models (GCMs) underestimates the blocking effect of the steep mountain ranges flanking the eastern Pacific. We explore the impact of this bias on common biases in Pacific climate simulation [i.e., the unrealistic cross-equatorial symmetry of near-surface winds, sea surface temperatures (SSTs), and precipitation] through sensitivity experiments with modified Central and/or South American topography in an atmosphere–ocean coupled GCM. Quantifying orographic blocking potential via the Froude number, we determine that an envelope topographic interpolation scheme best captures observed blocking patterns. Implementing envelope topography only in Central America reduced model biases as greater blocking of the trade winds warmed SST and enhanced convergence in the northeastern Pacific. Doing so additionally over the Andes improved the simulation of South Pacific circulation and the South Pacific convergence zone as stronger deflection of the westerlies intensified the South Pacific anticyclone. This mitigated convection biases in the southeast Pacific by increasing subsidence and cooling SST. However, remote impacts of the Andes exacerbated the dry bias in the northeast tropical Pacific, resulting in negligible improvement in the East Pacific double-ITCZ. We find that, due to the significant role of large-scale convergence in driving precipitation patterns, other model biases, such as cloud-radiative biases, may modulate the impact of altering topography. Our results highlight the importance of considering alternate methods for calculating model topographic boundary conditions, though the optimal interpolation scheme will vary with model resolution and the impact of topography on GCM biases can be sensitive to choices made in formulating parameterizations. Significance Statement In this study, we explore how the mountain ranges spanning Central and South America shape the climate of the Pacific by blocking large-scale midlatitude and tropical winds. We show that the height of these mountains is typically too low in climate models and that elevating them can improve patterns of rainfall, surface ocean temperatures, and near-surface winds in the Pacific. This is important because model biases in the Pacific climate limit their utility for understanding current and future climate variability. Improving the representation of blocking by mountains can thus be a simple method for reducing uncertainties in future climate projections.

Achieving multidecadal seamless continuity in the surface radiation budget derived from satellite observations and ancillary input datasets is challenging owing to changes in the input data stream with time. We present a revision of the Clouds and the Earth’s Radiant Energy System (CERES) Energy Balanced and Filled surface irradiance product that overcomes this challenge by limiting the input cloud property information used in deriving surface irradiances to imagers on sun-synchronous orbits. We show that cloud properties derived from Terra and Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) are sufficient to produce accurate and seamless regional monthly mean surface shortwave and longwave irradiances with time. Although a single sun-synchronous orbit introduces significant bias in surface irradiances for regions with strong cloud diurnal cycles, monthly regional surface anomalies align with those from two sun-synchronous orbits. In transitioning the CERES record to and from the combined Terra and Aqua period to one sun-synchronous satellite (e.g., Terra-only period from March 2000 through July 2002 and NOAA-20-only period from April 2022 onward), therefore, regional surface irradiance climatological means derived from one sun-synchronous orbit are adjusted to match the corresponding climatological means derived from Terra + Aqua. The climatological adjustment places surface irradiances of the Terra-only and NOAA-20-only periods on the same radiometric scale as surface irradiances of the Terra + Aqua period. While the uncertainty in regional mean irradiances is the same as the uncertainty in the previous version (edition 4.1), the regional surface irradiance anomaly time series is significantly improved, especially for longwave downward irradiances. Significance Statement The diurnal cycle of clouds influences the top-of-atmosphere and surface radiative energy budget. The study shows that cloud observations taken from two sun-synchronous orbits 3 h apart in mean local equator crossing time around local solar noon are sufficient to capture cloud diurnal cycles to compute the regional (1° × 1°) surface radiative energy budget. The bias of regional surface irradiance computed with one sun-synchronous observation is nearly constant with time. Observations from one sun-synchronous orbit are, therefore, sufficient to capture the variability of the regional surface radiation budget. These key results are used to extend the time period of the Clouds and the Earth’s Radiant Energy System surface radiation budget climate data over the time period consisting of observations taken from multiple satellites.

Cold-season precipitation statistics in simulations from the storm-resolving WRF Model at 6-km and 1-h resolution over western North America are analyzed. Pseudo–global warming future simulations for the 2041–80 period, constrained by GCMs under the RCP8.5 scenario, are compared to the 1981–2020 historical simulation. The analysis focuses on the dynamical properties of precipitation time series at subdaily scales and on the morphology of storms. The statistical distribution of precipitation intensities in each pixel of the simulation domain is characterized through nonparametric statistical indicators: frequency of wet hours, mean wet-hour precipitation intensity, and Gini coefficient as a measure of the temporal concentration of the precipitation volume. Additionally, the temporal and spatial Fourier power spectra of precipitation time series and precipitation fields are analyzed. The half-power period (HPP) and half-power wavelength (HPW) are defined as spectral measures of the characteristic scales of precipitation’s temporal and spatial patterns. The results show statistically significant increases in the mean wet-hour precipitation intensity and in the Gini coefficient in 99% of the pixels, indicating that the seasonal precipitation volume becomes more concentrated within a smaller number of hours with higher precipitation intensity. The statistics of change in the frequency of wet hours are more contrasted across the simulation domain. The changes are also reflected in the power spectra, which show the spatial and temporal variability increasing proportionally more with finer spatial and temporal scales and the HPW and HPP decreasing. These projected changes are expected to have consequences, not only in terms of hydrologic impacts but also in terms of the predictability of precipitation patterns. Significance Statement The precipitation characteristics of winter storms over the western United States and southwestern Canada are analyzed in future climate simulations for the 2041–80 period. As compared to present-day climate, the most intense parts of the storms are projected to produce a higher rainfall volume, with increased concentration over smaller areas and shorter time intervals. The propensity of rainfall intensity to vary rapidly over time will be enhanced in the future according to the simulations. These model predictions imply an increased risk of rapid flooding in small basins. They also suggest that predicting several hours ahead the time and location at which a storm will produce maximum rainfall may become more challenging in the future.

The North Atlantic warming hole is an area of relative cooling in the North Atlantic subpolar gyre. Observations and models have suggested numerous causes of the warming hole, including a role for wind-driven ocean circulation changes. We investigate the role of wind-driven ocean circulation changes on the development and projected future of the North Atlantic warming hole by comparing two ensembles within the Community Earth System Model, version 2 (CESM2). One ensemble includes wind-driven ocean circulation changes, while the other does not. The difference between the ensemble means isolates the role of wind-driven ocean circulation changes on the externally forced North Atlantic warming hole. We find that wind-driven ocean circulation changes do not alter the timing of the formation of an externally forced warming hole. However, anthropogenic changes to the near-surface winds lead to enhanced upwelling near Greenland, and wind stress changes enable a positive feedback loop that relies on changes to mechanical stirring. These mechanisms amplify the cooling in the high latitude North Atlantic and lead to increased sea level pressure and reduced precipitation near the southern tip of Greenland. Thus, changes to wind-driven ocean circulation are a crucial component of future changes in North Atlantic climate. Improved understanding of ocean–atmosphere coupling in this region will improve projections of sea surface temperatures and associated atmospheric impacts. Significance Statement The purpose of this study is to quantify the role that changes to the wind-driven component of ocean circulation have on future sea surface temperatures in the North Atlantic subpolar gyre region. This region has warmed less than the global average, often referred to as a “warming hole.” We use a targeted climate model experiment to demonstrate that wind-driven ocean circulation changes do not cause the modeled North Atlantic warming hole. However, wind-driven ocean circulation changes alter the warming hole beginning in 2040. This demonstrates that monitoring and understanding changes to the surface winds and ocean currents in the North Atlantic is important for understanding future climate changes in the region.

Signal separation and spatiotemporal reconstruction of multi-timescale climate fields are conducted based on the Multi-Taper Method–Singular Value Decomposition (MTM–SVD) in this study, with a focus on interannual precipitation responses over eastern China to sea surface temperatures (SST) forcing of the Indo-Pacific at a specific cycle and the related mechanisms illustrated. Results show that the precipitation and SST exhibit two synergistic interannual cycles: the quasi-biennial (QB, 2.4-year) and quasi-quinquennial (QQ, 4.7-year) cycles. These modes are influenced by multiple atmospheric responses associated with the central Pacific and eastern Pacific types of El Niño, including the Pacific-Japan/East Asia-Pacific teleconnection, western North Pacific subtropical high, and western North Pacific anomalous anticyclone. On the QB cycle, precipitation over South China correlates positively with the QB component of El Niño. On the QQ cycle, the meridional tri-pole precipitation pattern is dominated during the mature stage of the QQ component associated with El Niño. The combined effects of these two modes are demonstrated for the 1997/1998 precipitation events, highlighting regional differences in the relative contributions of QB and QQ variabilities. During the winter of 1997, the increased precipitation over South China is explained by the collective effects of the QB and QQ variabilities. During the summer of 1998, the decreased precipitation south of the Yangtze River is attributed to the QB variability, and this effect is mitigated by the QQ variability; the increased precipitation north of the Yangtze River is attributed to the combined effects of the QB and QQ variabilities. This research aims to separate the impacts and mechanisms in associated with the Indo-Pacific SST forcing on precipitation over China at interannual scales, providing scientific guidance for corresponding climate predictions.

Cut-off lows (COLs), defined as isolated mid-tropospheric low pressure systems, are responsible for a large fraction of the annual mean and extreme precipitation over the Mediterranean Sea region. In this study we quantify the impacts from Arctic sea-ice loss on the frequency and distribution of COLs. We use model outputs from the Polar Amplification Model Intercomparison Project (PAMIP) forced only by the projected reduction in the amount of Arctic sea ice in a globe 2°C warmer than in the preindustrial period. We find that sea-ice loss can, through alterations to the upper-level jet already documented in previous studies, significantly affect the frequency of COLs over southern Europe: in particular, a sharp reduction is simulated over the northeast Atlantic and the Iberian Peninsula as a consequence of more anticyclonic conditions prevailing over that region. This reduction in the number of COLs is accompanied by significantly less precipitation over the western Mediterranean, which could potentially lead to water availability affectation there.

This study investigates how tropical cyclones (TCs) in the western North Pacific (WNP) respond to the global warming trend using TC-track clustering analysis and data from a modified High Resolution Atmospheric Model (HiRAM). The dataset includes four future projections driven by different CMIP5-based warming patterns of sea surface temperatures (SSTs), incorporating interannual SST variability aligned with the present-day simulation. Under the representative concentration pathway 8.5 (RCP8.5) scenario, WNP TCs are projected to undergo the following changes across all the six categorized clusters and four projections in the late twenty-first century: fewer TCs, the distribution of TC lifetime maximum intensity (LMI) extending toward higher intensity, and enhanced mean intensification rates. Intercluster and interensemble variations exist in projected changes in other TC parameters. For instance, two clusters demonstrate a substantial and statistically meaningful increase in the mean LMI, resulting from enhanced mean intensification rates and nearly unchanged mean intensifying durations. One of these two clusters comprises stronger TCs affecting a wide range of coastal regions, with characteristics well replicated in the HiRAM present-day simulation. Our calculations of the seasonal-mean ventilation index suggest either less-supportive or mostly unchanged environmental favorability for WNP TC development under the warming scenario. This contrasts with the projected enhancement of TC intensification rates across all clusters and does not comprehensively explain the dramatic reduction in HiRAM TCs. The main text also delves into changes in the geographic distribution of TC occurrence, genesis and LMI, and the interplay between each TC parameter and environmental favorability for TC development under the imposed global warming trend.

We present a novel approach to studying the regime behavior of the Euro-Atlantic summer circulation by analyzing its spatiotemporal preferred trajectories. Our approach is inspired by the dynamical system concept of unstable periodic orbits (UPOs) and motivated by the potential practical utility in modeling the circulation as a sequence of short-lived but well-defined trajectories. Here, we identify the dominant spatiotemporal preferred trajectories in a large ensemble (2000-yr) of simulated present-day climate data with a multistage clustering algorithm. Unlike conventional regime definitions based solely on spatial patterns, our method also explicitly takes into account the temporal trajectory through phase space. We find that 13 spatiotemporal clusters together capture over 80% of the circulation dynamics over the Euro-Atlantic domain. We distinguish between clusters with a quasistationary tendency and clusters with a transient tendency. Markov transition probabilities between the clusters reveal that the circulation tends to alternate between quasistationary and transient episodes, instead of transitioning between quasistationary clusters directly. We show that traversing the phase space between quasistationary blocking and southerly shifted zonal jet states takes at least 10–15 days and that trajectories tend to stay close to these states in phase space for prolonged periods. Taken together, our results demonstrate that the spatiotemporal clusters capture a diverse range of well-known circulation regimes while also revealing more nuanced behavioral characteristics of the Euro-Atlantic summer circulation.

A classic example of a marine heatwave (MHW) was the 2014–16 warm event that spread across the northeastern Pacific (NEP) Ocean. We use an adjoint sensitivity approach to shed new light on potential causes for such reoccurring NEP MHW events. The study is based on the Massachusetts Institute of Technology General Circulation Model (MITgcm) and its adjoint, for which the mean top 100-m potential temperature during different target years was set as the objective function, separately for the two target regions (145°–160°W, 48°–56°N) and (130°–145°W, 40°–48°N). Resulting adjoint sensitivities show that during MHW years, local turbulent surface heat flux is the dominant atmospheric driver, with air temperature, specific humidity, and longwave radiation leading to up to 80% of the temperature anomaly of the NEP; during normal years, this is only about 60%. In contrast, increased wind typically does not lead to an MHW occurrence as it is associated with a deepening of the mixed layer. We find the horizontal temperature advection, i.e., the impact of the basinwide ocean circulation, to be less important during an MHW year, but it could act as a preconditioning of MHW through its role in climate oscillations. Response analysis shows that atmospheric forcing anomalies occurring within 3 months (from October to December) prior to an MHW year play a critical role in driving the MHW. The reconstruction using various sensitivity periods suggests that the leading 6-month atmospheric conditions should have potential predictive skills for the next year. Reconstruction that includes leading 36-month atmospheric conditions performs better than persistence.

This study compares the characteristics of global mesoscale convective systems (MCSs) simulated in a global storm-resolving model (GSRM) and a high-resolution (∼25-km) general circulation model (GCM), both developed at the Geophysical Fluid Dynamics Laboratory. By comparing with two satellite datasets, we examine the spatial distribution, seasonal/diurnal cycles, and event-based features such as duration, size, intensity, and propagation of MCSs across six global hotspots. MCS-related precipitation features and their contribution to the total precipitation are also analyzed. Our results show that both models effectively capture the observed spatial patterns and seasonal cycles of MCSs, although notable differences exist in absolute values, particularly in the GCM. Both models not only simulate event-based statistics but also show large geographical variations with an overall tendency to produce longer-lasting and larger MCSs. The GSRM performs better in simulating MCS diurnal cycle and MCS intensity. While both models replicate spatial patterns of MCS-related precipitation, they struggle with accurately capturing intensity, and their contributions to total precipitation vary. This comparison highlights strengths and limitations of these two types of models, calling for further process-level investigation of model deficiencies and a detailed evaluation of observations due to dataset discrepancies. Significance Statement Mesoscale convective systems (MCSs) are organized deep convective systems that play a significant role in total precipitation, particularly in tropical and midlatitude regions. Due to their larger spatial coverage and longer lifespan compared to individual thunderstorms, MCSs can cause extreme weather events like flooding, gusty winds, hail, and tornadoes. Accurately simulating MCSs is essential for predicting mean climate patterns and extreme events. In this study, we compared MCS features simulated by a 10-yr high-resolution global climate model (25 km) and a 2-yr global storm-resolving model (3.25 km) developed at the Geophysical Fluid Dynamics Laboratory (GFDL), highlighting the strengths and limitations of each model in capturing MCS features and their interaction with large-scale circulation and climate change.

Aerosol effects on precipitation are crucial factors in climate change, yet they remain poorly understood, representing a large source of uncertainty in climate models. In the Geophysical Fluid Dynamics Laboratory (GFDL) Earth system model, version 4 (ESM4), simulated historical century-scale trends of global land precipitation demonstrate significant drying biases compared to observations, even when imposing observed historical variations of sea surface temperature and sea ice concentrations (LongAMIP simulations). These biases manifest as overestimated decreasing trends in precipitation over tropical–subtropical land and underestimated increases in higher latitudes. In this study, we investigate the “fast response” of land precipitation to historical anthropogenic aerosol emissions and its contributions to the model trend biases, by conducting idealized ESM4 LongAMIP experiments with emissions of either black carbon (BC) or sulfate (SO4) aerosol precursors set to near-preindustrial levels (1850). Local aerosol effects, occurring through alteration of atmospheric energy balance and circulation, emerge as critical drivers of excessive precipitation declines in the LongAMIP runs in some regions: 1) over East Asia, a negative SO4 effect and a positive BC effect contribute to the simulated negative trend bias in LongAMIP; 2) for regions of Africa, the negative fast response to BC and SO4 partially contributes to the overestimated precipitation decline; and 3) over west-central North America, the negative fast response to BC in the model contributes toward underestimating a modest observed increasing precipitation trend. However, over South Asia, eastern North America, and northwest Eurasia, the fast responses of precipitation to aerosols cannot account for the LongAMIP model bias (in the opposite direction), indicating the dominant influence of other factors. Significance Statement Aerosol effects are crucial for understanding and predicting precipitation change. In this paper, we conduct numerical experiments to investigate the fast responses of land precipitation to historical emissions of black carbon and sulfate aerosols, utilizing the GFDL ESM4 model. The results demonstrate the critical roles of aerosol direct radiative effects in driving precipitation changes and contributing to model biases across East Asia, West Africa, South Africa, and western North America. Over eastern North America and northwest Eurasia, the fast responses to aerosols cannot explain the precipitation changes and model biases, indicating the dominant influences of other factors. This study will advance the comprehension of precipitation biases in the GFDL model and also help improve other climate models.

Reports from western U.S. firefighters that nighttime fire activity has been increasing during the spans of many of their careers have recently been confirmed by satellite measurements over the 2003–20 period. The hypothesis that increasing nighttime fire activity has been caused by increased nighttime vapor pressure deficit (VPD) is consistent with recent documentation of positive, 40-yr trends in nighttime VPD over the western United States. However, other meteorological conditions such as near-surface wind speed and planetary boundary layer depth also impact fire behavior and exhibit strong diurnal changes that should be expected to help quell nighttime fire activity. This study investigates the extent to which each of these factors has been changing over recent decades and, thereby, may have contributed to the perceived changes in nighttime fire activity. Results quantify the extent to which the summer nighttime distributions of equilibrium dead woody fuel moisture content, planetary boundary layer height, and near-surface wind speed have changed over the western United States based on hourly ERA5 data, considering changes between the most recent decade and the 1980s and 1990s, when many present firefighters began their careers. Changes in the likelihood of experiencing nighttime meteorological conditions in the recent period that would have registered as unusually conducive to fire previously are evaluated considering each variable on its own and in conjunction (simultaneously) with one another. The main objective of this work is to inform further study of the reasons for the observed increases in nighttime fire activity. Significance Statement Western U.S. firefighters have reported a problematic rise in nocturnal wildfire activity. Verifying their hypothesis that meteorological variability is responsible is a first step toward better understanding the predictability of the underlying processes. This study expands upon our previous investigation of multidecadal change in seasonally averaged nocturnal vapor pressure deficit by looking at changes in the frequency of dry-fuel nights over the western United States and their coincidence with other fire-conducive nocturnal meteorological conditions. Dry-fuel nights have become >10× more frequent in the 2010s compared to the 1980s and 1990s in some regions. Over 81% of the study area, increasing dry-fuel night frequency has been compounded by the double, or triple, threats of simultaneously windier and deeper planetary boundary layers.

Extreme strong winds (ESWs) can pose a threat to human safety, influence air quality, and affect wind power generation, thus necessitating an urgent exploration of the long-term variations in ESWs under global warming. However, compared with studies focused on the mean surface wind speed, studies focused on ESW variations in China remain limited. Here, our results revealed that from 1970 to 2017, ESWs across China exhibited a significant decreasing trend during both the warm and cold seasons, with values of −0.96% and −1.05% per decade, respectively. Furthermore, via the use of the weather regime (WR) classification method, the contributions of dynamic (atmospheric circulation) and thermodynamic factors (factors other than atmospheric circulation, such as radiation changes resulting from human activity) were quantified. Atmospheric circulation contributed negligibly to the long-term declining trend in ESWs. Instead, thermodynamic factors were the dominant drivers, accounting for 93.9% and 94.6% of the declines observed during the warm and cold seasons, respectively. These findings indicate that the anthropogenic effects on ESWs should be explored further.

Persistent extreme precipitation events (PEPEs) have dramatic socioeconomic impacts in the Yangtze–Huaihe River basin (YHRB). However, the possible role of the Northeast China cold vortex (NEC-CV) in modulating the PEPEs over the YHRB remains unresolved. In this study, the contribution of NEC-CVs to summer precipitation is first examined over central-eastern China, which is characterized by a local and long-distance effect, along with distinct geographic variability. Limited influence is found for the areas outside a threshold of 4× radius of NEC-CV. The YHRB is one of the regions significantly affected by the NEC-CVs, which accounts for about 35%–40% of the total extreme precipitation. During 1961–2022, about 27.7% of the total PEPEs are found to be closely related to the NEC-CVs. In addition, two types of PEPEs (type W/type E) are identified based on the position of corresponding NEC-CV tracks. Significant impacts are found for the opportune configurations of NEC-CVs. The PEPEs are found to be located more westward/eastward for type W/type E, with the anomalous moisture mainly coming from the western North Pacific/South China Sea. The two PEPEs exhibit the anomalous eastward/westward extension of the South Asian high/western North Pacific subtropical high and anomalous southward shift of the upper-level jet with respect to the climatology. Meanwhile, the lower troposphere is dominated by a large-scale low pressure, strong wind shear, and intense moisture transport in the YHRB. The concurrent combinations of the upstream Ural blocking and the downstream Okhotsk blocking are favorable for the development and southward intrusion of NEC-CVs to the YHRB in type W. However, the counterparts in type E are closely associated with the upstream Baikal blocking. The precursor signals of the NEC-CVs can be detected 12/8 days prior to the peak PEPE occurrence at 500 hPa for type W/type E. Significance Statement Persistent extreme precipitation events (PEPEs) can cause catastrophic flooding. This study demonstrates the role of the Northeast China cold vortex (NEC-CV) in influencing such high-impact weather events in the Yangtze–Huaihe River basin (YHRB). Using the latest reanalysis datasets and neural network technology, we quantitatively conclude that the NEC-CVs have contributed to about 35%–40% of the total extreme precipitation and 27.7% of the total PEPEs in the YHRB over the past 60 years. The relevant PEPEs are dominated by the NEC-CVs, with the opportune configuration of the upper and lower circulation systems. These key findings present a new perspective on the meteorology of the PEPEs with implications for the medium-range weather forecasts.