Abstract Cloud cover impacts Earth's radiation budget as both a cause and an effect of climate change. While long-term satellite observations are vital for climate studies, existing products are limited by coarse resolutions that fail to capture fine-scale cloud heterogeneity. Despite the five-decade Landsat archive, global pixel-level analysis remains constrained by computational limitations. We investigated spatiotemporal patterns of cloud occurrence probability (COP) and cloud contaminated area (CCA) from continental to global scales using 1984–2023 data from Landsat-5, Landsat-7, and Landsat-8. Results reveal pronounced variations: global average COP is 34.39%, with Europe recording the highest rate (43.95%) and Oceania the lowest (21.23%). Tropical regions exhibit higher COP (48.10%) than extra-tropical zones (39.51%). Global mean CCA covers 47.05% of Earth’s surface (7.01 × 10 8 km 2 ), with Asia contributing the largest share (36.6%). Unlike the stable tropics, CCA shows significant seasonality at mid- and high-latitudes, peaking in July and reaching a minimum in December. A key finding is the synchronized decline in global cloud cover COP and CCA during the 2014–2023 OLI era ( p < 0.05), with South America exhibiting the most pronounced regional reduction. Beyond validating the Landsat archive, our findings demonstrate the unique capacity of 30-m observations to resolve sub-grid cloud heterogeneity, offering a potential benchmark for assessing partial coverage biases in coarser global products. This high-resolution record provides an essential foundation for refining cloud-climate feedbacks in future climate modeling.
Abstract Convectively coupled Kelvin waves (CCKWs) are a prominent mode of tropical variability that organize convection and modulate rainfall, providing an important source of synoptic-to-subseasonal predictability. Most current understanding of CCKW structure and evolution is derived from oceanic environments, where surface conditions are relatively uniform and wave–convection coupling is coherent. Over the land, however, strong diurnal variability, heterogeneous surface properties, and complex terrain introduce additional processes that can substantially modify this coupling. Using reanalysis and satellite observations, we show that CCKWs over the Congo Basin depart from the canonical-ocean-based framework. The typical shallow–congestus–deep convective transition is weak, with limited shallow cloud development ahead of the active phase, particularly during daytime. Surface latent heat flux anomalies occur in phase with enhanced incoming solar radiation, indicating a thermodynamically driven response rather than the wind-stress-dominated mechanism common over oceans. During the suppressed phase, reduced cloud cover allows increased solar radiation to warm the surface, strengthening latent heat fluxes and promoting boundary layer growth and moistening. These processes progressively erode convective inhibition from below, while free-tropospheric adjustment becomes dominant only near the convective peak, implying a distinct pathway of convective preconditioning over the land. Topography and diurnally forced circulations further modulate wave evolution by locally triggering convection ahead of the large-scale active phase, partially offsetting wave-scale inhibition and advancing convective maxima. This work demonstrates that land–atmosphere interactions substantially reshape CCKW structure and convection over central Africa, highlighting the limitations of ocean-based Kelvin wave frameworks for continental environments. Significance Statement Convectively coupled Kelvin waves are large-scale weather systems that help organize tropical rainfall and affect weather patterns from daily to weekly time scales. Most of what we know about these waves comes from studies over the ocean. Here, we examine how they behave over the Congo Basin in central Africa. We find that conditions over the land, such as strong daytime heating, changes in surface moisture, and complex terrain, alter how these waves interact with clouds and rainfall. This study shows that Kelvin waves do not behave the same way over the land as they do over the ocean, emphasizing the need to better understand land–atmosphere processes to improve rainfall prediction in tropical Africa.
Abstract The uplift of major mountain ranges can exert a strong influence on ocean circulation through large-scale atmosphere–ocean interactions. Using topography-sensitive simulations with the Community Earth System Model 1.0, we isolate the role of the Andes in shaping Pacific climate and ocean circulation. Andean uplift weakens the equatorial easterlies and shifts the Hadley circulation, leading to surface warming along the equator and in the eastern North Pacific, accompanied by regional cooling in the southeastern Pacific. These atmospheric changes drive basin-scale oceanic adjustments, including the intensification of the North Pacific subtropical gyre and a strengthening of the Pacific Meridional Overturning Circulation (PMOC). The enhanced subtropical gyre increases warm-water transport into the subpolar northeastern Pacific, leading to pronounced surface warming and salinification. However, these surface buoyancy changes alone are insufficient to explain the intensified deep-water formation. Instead, they occur in concert with large-scale ocean dynamical adjustments that favor the development of North Pacific Deep Water (NPDW). The resulting PMOC intensification emerges through coupled dynamical and thermohaline adjustments on multicentennial timescales. Our results demonstrate that Andean topography alone can promote a vigorous PMOC without requiring elevated atmospheric CO 2 concentrations or changes in tropical seaways, highlighting an important role of continental-scale tectonics in regulating Pacific overturning circulation.
Abstract Weather typing enhances understanding of extreme precipitation mechanisms by identifying variations in atmospheric circulation patterns. This study investigates the typical circulation patterns and associated moisture source distinctions for extreme precipitation over the Yangtze River Basin (YRB). Based on 310 detected extreme precipitation days (EPDs), four typical circulation patterns (P1–P4) are identified. These patterns, distinguished by circulation over the East Asia–western North Pacific region, include the dipole pattern (P1), the coherent anticyclonic pattern (P2), the blocking pattern (P3), and the coherent cyclonic pattern (P4). Distinctions in the moisture source distribution and the moisture source contribution from geographical sectors are analyzed across the four patterns. The moisture sources in P1 exhibit a distribution similar to that of all EPDs, while the moisture source anomalies in P2–P4 show dipole structures: southwest-northeast in P2, east-west in P3, and north-south in P4. The spatiotemporal variations of precipitation across the four patterns are examined. A west–east oscillation is observed in the spatial distribution. In P1 and P3, precipitation is biased to the east, with significantly higher precipitation mainly in the lower and middle reaches of the YRB, respectively. In P2 and P4, precipitation is biased to the west, with higher values predominantly in the northern and central parts of the upper YRB. This study contributes to the understanding of variations in large-scale dynamics and moisture transport for extreme precipitation over the YRB, and may provide insights for forecasting, particularly regarding its spatial distribution.
Abstract An ensemble of seven integrations of the HadGEM3-GC5 coupled model is used to investigate the sensitivity of the simulated climate system to settings in the ocean component that have been shown to affect the level of numerical mixing in forced simulations. This configuration is closely related to the UK contribution to CMIP7. The ensemble is integrated for 60 years with constant year 2000 greenhouse forcing. The ocean surface temperature has a strong and consistent response to numerical mixing, with increased mixing leading consistently to warming over the global ocean by up to 0.5°C, while reducing mixing cools the surface by a similar degree. The response of the surface air temperature is very similar to that of the SST, but is seasonally amplified at high latitudes in the respective winter. Robust and strong sensitivities are also found for sea ice cover, rainfall, and the surface shortwave and latent heat fluxes, the latter two showing opposing changes in their global means of over 2 Wm −2 across the ensemble. We present large-scale ocean and atmospheric metrics and discuss mechanisms for the sensitivity of surface temperatures in these simulations to numerical mixing in the ocean, in which more mixing warms the surface and vice versa. The magnitude of this sensitivity of the surface temperature is significant, since it is comparable with the changes found in CMIP simulations with historical or future greenhouse scenario forcings, and we speculate on the implications for modelling future climates.
Abstract Wind gusts—rapid surges in wind speed occurring within minutes or even seconds—pose significant risks to aviation, infrastructure, and human safety. Yet, the changes in extreme wind gusts (EWGs) across China remain poorly understood. Here, using the fifth generation ECMWF atmospheric reanalysis (ERA5) data, we examine long-term trends and variability in summer EWGs across China from 1961 to 2024. Southern China (SC) and northwestern China (NWC) emerge as key hotspots of summer EWG variations, exhibiting significant increases in EWG days and frequency over the past six decades. In SC, EWGs are associated with low pressure anomalies and intensified surface disturbances over the northern South China Sea and adjacent waters. In NWC, EWGs are significantly correlated with widespread low pressure anomalies from the Middle East to central Asia and increased surface disturbances in NWC. Anomalous midlatitude Rossby waves play a crucial role in both regions. In SC, they induce a northward shift of the northwestern Pacific subtropical high, heightening the likelihood of convective systems and typhoon landfalls. In NWC, they generate northeastward-tilting low pressure anomalies between the lower and midtroposphere, enhancing atmospheric baroclinicity. Both processes can result in more variable surface pressure, thereby being consistent with amplified disturbances and more frequent EWGs in SC and NWC. These findings enhance our understanding of the variability and dynamics of wind extremes in China, offering valuable implications for regional climate risk assessment and disaster preparedness. Significance Statement This study addresses a critical gap in understanding wind extremes across one of the world’s most populous and economically vital regions. We identify southern and northwestern China as emerging hotspots with rapidly increasing frequencies of summer extreme wind gusts, associated with distinct atmospheric circulation patterns. Our findings reveal that anomalous Rossby waves modulate wind gusts through region-specific mechanisms—displacing the subtropical high in southern China (SC) and enhancing baroclinicity in northwestern China. These insights advance our understanding of the variability and dynamics of wind extremes, with important implications for aviation safety, infrastructure resilience, and renewable energy management.
Abstract This paper compares three methods for quantifying stochastic ocean forcing to low-frequency sea surface temperature (SST) variability from the surface heat budget in the framework of a simple stochastic climate model, especially in the case of red noise ocean forcing. The three methods are PT21 (Patrizio and Thompson), PT22 (Patrizio and Thompson), and LGD23 (Liu et al.). PT21 estimates the ratio of ocean over atmosphere forcing as the ratio of the covariance of SST tendency with ocean heat transport over the covariance with surface heat flux, while PT22 and LGD23 first derive the time series of oceanic and atmospheric forcing before estimating their ratio. The three methods are first applied to synthetic data of the stochastic climate model and then to the midlatitude North Atlantic in observations. It is found that the LGD23 method provides an unbiased estimation of oceanic forcing with a modest sampling error at low frequency, if the persistence time of the sea surface salinity can be treated as a good approximation of that of SST associated with ocean heat transport. The PT22 method has the smallest sampling error but tends to underestimate the ocean forcing modestly when the ocean forcing is a red noise process. The PT21 method gives the correct ratio of oceanic over atmospheric forcing in spectral density in theory but suffers from a very large sampling error for practical application to a dataset of a finite length of decades. We recommend the use of both LGD23 and PT22 as two complimentary methods for the estimation of ocean forcing, with the PT22 providing likely a lower bound for red noise ocean forcing. Significance Statement This paper compares three methods for estimating ocean forcing from the surface heat budget systematically and, therefore, provides a guideline for estimating ocean forcing in observations and climate models.
Abstract A pronounced interdecadal increase in summer (June–August) precipitation over southern China has been evident around 1993. Yet, the relative contributions of extreme precipitation (EP) and non-EP to this increase and the physical drivers remain poorly understood, despite their critical implications for water resource management and disaster preparedness. Our analysis shows that both EP and non-EP increased from 1979 to 1992 and from 1993 to 2006. Although EP accounts for only 26% of climatological rainfall, it explains a disproportionate 43% of the total increase, indicating an increased fractional contribution of EP to total precipitation amount. Notably, these increases are driven primarily by more frequent rainfall events rather than greater event intensity. The enhanced frequency of monsoon and tropical cyclone precipitation events is mainly caused by strengthened monsoon circulation and increased tropical cyclone passages over southern China, respectively, both tied to more occurrences of westward extensions of the western Pacific subtropical high. These findings highlight that circulation-driven increases in event occurrence frequency, rather than rainfall intensification, have governed the interdecadal precipitation increases in southern China. The results underscore the importance of considering precipitation occurrence frequency and intensity and their links to large-scale circulation for improving the simulation, projection, and interpretation of regional hydrological changes. Significance Statement Southern China has experienced a prominent interdecadal increase in summer rainfall around 1993, yet the processes driving this shift have remained unclear. This study shows that the rise was dominated by more frequent rainfall events rather than stronger ones. These changes are primarily driven by enhanced monsoon circulation and increased tropical cyclone passages associated with westward extensions of the western Pacific subtropical high. The results highlight the dominant role of circulation-driven frequency changes in shaping regional hydroclimate. Recognizing this mechanism is crucial for enhancing the reliability of regional climate projection and improving water resource management.
Abstract Barrier layers in the upper ocean suppress the upward entrainment of cold thermocline water, trapping heat and momentum near the surface and thereby influencing tropical air-sea interaction. However, the mechanisms governing their spatial and temporal variability are not fully understood. This study highlights the importance of salinity-induced vertical stratification in shaping the climatology and variability of the barrier layer in the Pacific Ocean on subannual and interannual timescales. Compared to observational and reanalysis data, coupled ocean-atmosphere models simulate a less eastward-extending warm pool, along with a thin barrier layer bias. This bias is linked to a saltier upper western Pacific and can be attributed to weaker precipitation and stronger easterly winds along the equator. Consistently, models with a more eastward-extending warm pool tend to exhibit a thicker barrier layer and lower salinity over the western Pacific. On interannual timescales, models agree with observational and reanalysis data that anomalous westerly winds and increased precipitation develop 10–13 months before the peak eastward shift of the warm pool eastern edge (WPEE), accompanied by an upper-ocean freshening. These anomalies peak with the WPEE shift and persist for another 9–10 months. Subannual variations exhibit more complex temporal patterns. Anomalous westerly winds and increased precipitation emerge 3–4 months prior to the WPEE peak extension and reverse rapidly one month after the peak. While models capture the timing and magnitude of wind and precipitation changes, they fail to reproduce subannual salinity variations. Improving salinity climatology and subannual variability in models remains essential for simulating barrier layers.
Abstract Mediterranean hurricanes, or “Medicanes,” are rare, tropical-like cyclones producing intense rainfall and posing significant hazards to coastal populations. This study estimates medicane precipitation hazards using a statistical-deterministic approach that generates synthetic tracks from reanalysis data and global climate models. Coupling these tracks with the Tropical Cyclone Rainfall (TCR) algorithm, the precipitation field for each synthetic event is calculated, enabling spatially and temporally resolved hazard assessments. Validation against ERA5, satellite products, and surface observations confirms the model’s performance, particularly in coastal regions, despite a localized rainfall underestimation in the Central Mediterranean Sea. Sensitivity analyses (via track sub-sampling and TCR-ERA5 coupling) reveal that this discrepancy is not driven by sample size or the rainfall algorithm, but likely by synthetic track-generation or input data limitations. Under the RCP8.5 scenario, projected future changes indicate a marked increase in rainfall extremes: 100-year return period rainfall is expected to increase by up to 140 mm along the Adriatic coast, 250-year events by up to 160 mm, and 500-year events may exceed 800 mm total rainfall in some areas. In particular, regions such as southern Italy, northern Algeria, Sardinia, and Corsica are expected to see an increase of 100–160 mm in extreme Medicane-induced rainfall. Conversely, no substantial change is projected over southern Greece. This methodology offers a robust framework for quantifying hydrological impacts of low-frequency, high-impact storms in the Mediterranean. Our findings underscore the potential for more destructive rainfall events under climate change, highlighting the urgent need for enhanced monitoring, improved local observational networks, and targeted adaptation strategies.
Abstract The 2025 Indian summer monsoon exhibited an early onset on 24 May, a week before the climatological onset (1 June), accompanied by high rainfall activity (∼30 mm day −1 ) compared to early onset composites (∼10 mm day −1 ). This study investigates the synoptic- and large-scale air–sea processes that triggered the 2025 early onset and compared with those of early and normal-onset years. An enhanced land–sea thermal contrast (>4°C) and strong meridional tropospheric temperature gradient (∼6°C) created a favorable thermodynamic background. The upper-tropospheric heating extended equatorward to ∼15°N, unlike its climatological confinement over 25°–30°N during other early and normal-onset years. The eastward displacement and intensification of the South Asian high, together with enhanced Rossby wave activity, helped the early establishment of monsoon semipermanent circulations. A stronger low-level jet (∼14 m s −1 ), compared to early onset composites (∼8 m s −1 ), enhanced moisture transport from the Arabian Sea and Bay of Bengal branches, leading to intensified moisture convergence (∼1.3–1.9 × 10 −7 kg kg −1 s −1 ) over northwest and central India. Meanwhile, the Madden–Julian oscillation transitioned from phase 7 (10 days before onset) to phase 4 (at onset), further supporting the monsoon advancement. Though the teleconnections remained largely neutral, a weak transition toward La Niña–like conditions provided additional background support. The synergistic interaction of thermodynamic forcing and upper-level dynamics primarily supported the early establishment of monsoon circulations, while the MJO-driven intraseasonal convection and teleconnections supplemented the early onset of 2025. These findings improve the understanding of multiscale mechanisms for monsoon onset and help improve its subseasonal prediction and numerical model performance. Significance Statement The 2025 Indian summer monsoon began unusually early on 24 May, nearly a week ahead of its normal-onset date, making it the earliest onset since 2009. This study investigates the key atmospheric and oceanic factors that contributed to this early arrival. Prior to the onset, abnormal warm sea surface temperatures in both the Arabian Sea and the Bay of Bengal, combined with a strong low-level jet, enhanced moisture transport toward the Indian subcontinent. At the same time, warming in the upper atmosphere over the land strengthened the conditions necessary for monsoon development. Rapid changes in the Madden–Julian oscillation also supported the early onset. By the onset date, the temperature contrast between the land and ocean had increased significantly, along with notable warming in the upper troposphere. Upper-level warming is usually confined to 25°–30°N during normal-onset and early onset cases, whereas this event showed a stronger and more southward extension, reaching as far as 15°N. In addition, large-scale circulation features such as the South Asian high and Rossby wave activity played an important role in triggering the early monsoon. Although major climate modes over the Pacific and Indian Oceans remained largely neutral, brief cooling signals were observed before and during the onset period. Overall, the findings highlight that enhanced upper-atmospheric heating, stronger land–sea temperature contrast, and changes in large-scale circulation systems were key drivers of the unusually early monsoon onset in 2025. These findings improve our understanding of monsoon onset and can help enhance the accuracy of monsoon forecasts and climate models.
Abstract The lack of warming in the central-to-eastern equatorial Pacific under anthropogenic forcing is a well-documented feature of the observed sea surface temperature (SST) trend. Less recognized is its strong seasonal dependence, characterized by pronounced cooling during boreal winter and mild warming from spring to summer across the central-to-eastern equatorial Pacific, which amplifies the local seasonal cycle and delays the seasonal maximum by one month. In the far eastern Pacific, the SST warming trend likewise peaks in spring–summer and is less in winter–early spring, leading to a reduced amplitude of the seasonal cycle with little change in phase. Consequently, the zonal SST gradient across the equatorial Pacific strengthens most in winter and less in other seasons. This seasonal structure of the long-term trend is consistently identified across observational SST datasets. We further show that the variation in SST is largely associated with wind-driven seasonally varying upper-ocean dynamics, particularly the weakening of geostrophic zonal currents in all seasons except winter and strong thermocline shoaling during winter, while the influence of surface heat fluxes is not well constrained. The lack of long-term warming in the central-to-eastern equatorial Pacific is a net effect of these opposing seasonal dynamical processes rather than a uniform cooling trend.
Abstract Wave breaking is the dominant physical process responsible for energy dissipation of ocean surface waves and is critical for exchanges of momentum, energy and mass across the air-sea interface. Over the past two decades, extensive research efforts have been devoted to investigating the climatology and trends of wind speed and wave height in global oceans, revealing their upward trends over the past several decades. Nonetheless, research on global-scale wave breaking statistics remains relatively limited. It is not immediately clear whether wave steepness and wave breaking have intensified because the trends of wavelength are unknown. In this paper, we first present a simplified theoretical analysis based on the empirical wind-sea fetch laws and Toba’s 3/2 power law. A climatological relation between wind-sea wave steepness and wind speed is obtained, which suggests that as a consequence of accelerating winds on a climatological timescale, wave height grows faster than wavelength does and thus results in amplified steepness and intensified wave breaking. We then conduct a thorough analysis of global climatology and trends of the dominant wave breaking probability b T , an important proxy of wave breaking estimated empirically from a recently produced global wave hindcast over the period from 1992 to 2019. It is confirmed that in the Southern Ocean (global oceans), b T has increased by approximately 4% (2%) over the past 28 years, and these relative trends of b T are comparable to those for wind speed and wave height.
Abstract Cross-equatorial flow (CEF) represents a fundamental atmospheric phenomenon in the tropical troposphere. While low-level CEF (LCEF) has been extensively studied, the understanding of high-level CEF (HCEF) remains insufficient. Based on three reanalysis datasets (ERA-5, JRA-55, and MERRA-2) and CMIP6 models, this study conducts a comprehensive analysis of HCEF, including its vertical structure, multiple time-scales variability, and relationship with the El Niño-Southern Oscillation (ENSO). The results indicate that HCEF has four primary channels located over the eastern Pacific, the Atlantic, West Africa, and the Indian Ocean-Maritime Continent, respectively. While interannual variability of HCEF is relatively consistent across the datasets, substantial discrepancies exist in its decadal variability and trend. The linkage between HCEF and the underlying LCEF exhibits dataset dependence and channel difference at interannual scales. The eastern Pacific HCEF (EPHCEF) is directly modulated by two types of ENSO (eastern Pacific ENSO and central Pacific ENSO) due to sea surface temperature anomalies that excite anomalous convection, which in turn modifies patterns of upper tropospheric convergence and divergence, thereby driving HCEF anomalies. During eastern Pacific ENSO events, Atlantic HCEF (ALHCEF) and West Africa HCEF (WAHCEF) also exhibit significant interannual variability through a physical process similar to that affecting EPHCEF. Statistical analysis suggests that the Atlantic Niño is more likely to be the primary driver of their variability compared with ENSO. The simulation results of CMIP6 for EP ENSO are similar to those of the reanalysis datasets, but show systematic biases for CP ENSO. These findings significantly advance our understanding of the tropical upper tropospheric circulation.
Abstract Salinity plays a crucial role in the global hydrological cycle and climate system by regulating upper ocean stratification and sea surface temperature (SST). Past studies have revealed the dynamics of salinity variation in the world’s major ocean basins and its linkage to global climate on interannual to decadal time scales. However, understanding of upper ocean salinity variability in the Indian Ocean, especially on decadal time scale and its global implications, remains limited. This study identifies a prominent Horseshoe Salinity Pattern (HSP), present in the Indian Ocean, extending from surface to 150m depth. It exhibits pronounced decadal variability and is closely associated with SST variability in the southeastern Indian Ocean (SEIO) with a lag of ∼18 months. Composite maps and a sea surface salinity budget analysis reveal that SST anomalies in the SEIO region modulate the local Walker and Hadley circulation anomaly causing the variation in the precipitation pattern. These changes drive upper ocean freshening or salinification, producing the observed HSP. Advection contributes regionally to salinity variability, particularly in the southern Indian Ocean, but does not play a dominant role in shaping the coherent HSP structure. Further, this pattern modulates upper-ocean stratification, leading to the development of SST anomalies in the western tropical Indian Ocean. These findings offer novel insight into the Indian Ocean salinity dynamics, highlighting the role of SST-salinity coupling and have the potential for improving predictions of decadal climate variability and its global implications.
Abstract Subseasonal to seasonal (S2S) prediction skill of wintertime surface air temperature (SAT) over the contiguous United States (CONUS) remains limited by systematic model biases and poorly understood sources of predictability. Using deterministic reforecasts from the NOAA Unified Forecast System prototype 8 (UFS P8) for December–March 2011–18, we examine dominant SAT bias patterns and their physical origins associated with Arctic variability, tropical forcing, and land–atmosphere interactions. Empirical orthogonal function analysis applied to UFS P8 identifies two leading modes of winter SAT bias. The first mode is linked to Arctic surface temperature variability over the East Siberian–Chukchi Seas, where forecast skill decays beyond week 2. The second mode features a dipole pattern associated with an exaggerated upper-level circulation response and local surface processes. In boreal winter, UFS P8 exhibits an unrealistic upper-level circulation modulated by an erroneous, summer-like Rossby wave. Over the eastern CONUS, surface warming is primarily driven by excessive downward longwave heating. Despite a wet soil moisture bias, increased soil moisture does not enhance latent heat flux, implying inefficient surface energy partitioning and reduced evaporative cooling that amplify the warm bias. These combined deficiencies contribute to systematic warm biases in winter SAT over the eastern CONUS. They indicate that surface warming is primarily driven by cloud-induced downward longwave radiation, but its magnitude and variability are modulated by land–atmosphere interactions via surface energy partitioning. Thus, these coupled processes represent an unexploited source of predictability for S2S forecasts and highlight land surface processes as a barrier to advancing extended-range forecast skill.
Abstract To better understand the effects of anthropogenic forcings on tropical cyclone (TC) activity, we investigate the individual impacts of anthropogenic aerosols (AAER), greenhouse gases (GHG), and biomass burning aerosols (BMB) on TC frequency using CESM2 large ensemble simulations. Changes in TC genesis are estimated using a dynamic genesis potential index (DGPI), which reveals distinct impacts from AAER and GHG. Increasing GHG emissions result in a decrease and southward shift in DGPI over the North Atlantic and eastern North Pacific since 1940, whereas the time-dependent AAER emissions lead to a decrease and southward shift in DGPI before 1980s and an increase and northward shift thereafter. In addition, BMB emission increases lead to a slight increase in TC frequency in the North Atlantic. Various dynamic and thermodynamic parameters act cooperatively to change TC frequency associated with GHG and AAER over the North Atlantic, while the increase in DGPI associated with BMB is mainly due to the changes in dynamic variables. AAER effects largely dominate DGPI changes in earlier decades, but GHG effects become increasingly dominant in recent decades. Near-future projections, characterized by continued increases in GHG emissions and steady or declined AAER emissions, suggest decreased DGPI over the North Atlantic and eastern North Pacific driven primarily by GHG forcing. A comparison between DGPI and an alterative, thermodynamic GPI shows similar increases or decreases in TC genesis potential with quantitative differences. However, comparisons between CESM2, CESM1, and six CMIP6 models show considerable inter-model spread in DGPI response, highlighting model uncertainties and the need for caution when applying and interpreting multi-model ensemble means.
Abstract The El Niño–Southern Oscillation (ENSO) is important to the interannual variability of tropical cyclone (TC) genesis in the Western North Pacific (WNP). However, the mechanisms for the TC seasonal changes in ENSO developing phase remain unclear. This study addresses the critical influences of the background sea surface temperature (SST) annual cycle and ENSO related sea surface temperature anomalies (SSTA) in regulating TC genesis seasonal changes. The results show that during eastern Pacific (EP) El Niño years, TC genesis increases in the southeastern WNP and decreases in the northwest in summer, while in autumn, TC genesis strengthens in the east and weakens in the west. For central Pacific (CP) El Niño events, basin-wide enhancement occurs in summer, with a southeast–northwest contrast in fall, whereas La Niña developing years exhibit nearly opposite patterns. These variations arise from anomalous convection generated by the combination of warm background SST annual cycle and ENSO SSTA, which modify large-scale conditions for TC genesis. Background SST can also strengthen ENSO evolution via the Bjerknes feedback, amplifying circulation impacts. This work provides a new perspective on the critical role of background SST annual cycle in shaping the ENSO–TC relationship, which is potentially helpful for improving seasonal TC prediction over the WNP.
Abstract The ocean and land surface both influence rainfall in southern Africa, but recent work has highlighted the often complex, inconsistent, and heterogenous connections between these processes and rainfall in the region. Here, we use a moist static energy (MSE) framework to more deeply investigate the connections between southern Africa rainfall and the ocean and land surface during the wet season (October–March). South of 13°S, rainfall is negatively correlated with saturated MSE in the free troposphere at 650 hPa ( h * 650 ), especially over the western (Angola, Namibia) and southern (South Africa) regions of our domain and during the core months (December–February) of the wet season. This correlation is much weaker over the wettest region in the east (Malawi, Mozambique, Zambia, Zimbabwe), though all areas have strongly positive and significant correlations between rainfall and surface MSE ( h sur f ) and the vertical MSE gradient ( h sur f – h * 650 ; Δ h vert ). High levels of h * 650 , indicating strong stability constraints, are associated with warmer conditions in the tropical Indian and Pacific Ocean basins, patterns typically associated with El Ni˜ño, a classic expression of the ocean forced “remote” pathway. However, in the wetter eastern part of our domain (where h * 650 constraints on rainfall are weaker), h sur f , and by extension Δ h vert , are strongly correlated with sea surface temperatures in the southwest Indian Ocean, a pattern similar to positive phases of the Subtropical Indian Ocean Dipole. Soil moisture (surface and root zone) and leaf area index (LAI) are strongly and positively correlated with h sur f in the first half of the wet season (October–December), but these correlations are much weaker in January–March, suggestive of a shift from a supply-limited to an energy-limited evaporative regime. Our analyses highlight how interactions between MSE and rainfall vary seasonally and spatially across southern Africa, including the shifting connections with local land surface and remote ocean processes.
