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Utilizing spaceborne cloud radar and lidar (CloudSat/CALIPSO) observation products, we examine vertical distributions of clouds and quantify their radiative effects associated with equatorial Rossby and Kelvin waves. The most important result is that the radiative heating substantially increased the generation of the eddy available potential energy by 19% and 40%, in Rossby and Kelvin waves, respectively, adding to the convective latent heating. Composite analyses indicate a simultaneous development between deep‐convective anvil clouds and stratiform clouds of mesoscale convective systems in the Rossby waves, and a transition from low‐level clouds, anvil clouds to stratiform clouds in the Kelvin waves. These are consistent to precipitation characteristics provided by precipitation radar observation, and thus the apparent heat source can be estimated by combining convective heating and radiative heating. Plain Language Summary Radiative forcing is a key issue in coupling mechanisms between equatorial waves and convective activity in the tropical meteorology. While many previous idealized studies suggested some instability mechanisms, cloud distributions and radiative heating effects associated with convectively coupled equatorial waves are not enough investigated. We utilize spaceborne cloud radar and lidar observation to quantify cloud and radiation statistically and compare them in equatorial Rossby and in equatorial Kelvin waves. We found the stacked clouds from middle‐ to upper‐level in the Rossby waves and the cloud transition from low‐, middle‐, to upper‐level in the Kelvin waves. These are consistent to the precipitation characteristics, and thus these enable us to estimate apparent heat source more realistically. Consideration of the radiative heating is essential because it enhances the energy generation of the waves with 19% and 40%, for Rossby and Kelvin waves, respectively. Key Points Clouds associated with equatorial Rossby waves indicate simultaneous development from low‐ to upper‐level Clouds associated with equatorial Kelvin waves indicate gradual development from shallow, deep convection to mature mesoscale convective systems Radiative heating increase eddy available potential energy generation by 19% and 40% in Rossby and Kelvin waves respectively

Extreme precipitation events (EPE) are often associated with severe floods and significant damages in Central Sahel. To better understand their formation and improve their forecasts, we investigate the sub‐seasonal drivers of EPEs. A composite analysis reveals that moist, cyclonic and upper‐level divergence anomalies are found on average as a result of several tropical waves. The equatorial Rossby wave (ER) dominates at large scale providing a moist and convectively‐active anomaly over the northern Sahel together with a smaller‐scale African Easterly Wave (AEW). The Madden‐Julian Oscillation provides upper‐level divergence anomalies and a Kelvin wave increases convection during the EPE. Statistics show the prevalence of AEW and emphasize ER as a key driver of EPE. The co‐occurrences of several tropical waves, especially those involving AEW, ER, and Kelvin waves, increase the probability of EPE. Monitoring these tropical waves combinations could improve EPEs forecasts. Plain Language Summary The drivers of extreme precipitation events (EPE) over the Central Sahel are studied at subseasonal scales. A statistical approach is adopted to build an average extreme rainfall event. EPEs occur within a large‐scale moist anomaly, an upper‐level divergence, and at shorter scales an intense vortex. These features are provided by multiple tropical meteorological systems, called tropical waves, that have typical spatio‐temporal scales greater than that of a convective storm. The four tropical waves studied here contribute to this favorable environment. However two of them—an Equatorial Rossby wave, rather slow, and an African Easterly Wave (AEW) (more rapid)—explain the largest part of the favorable conditions leading to an EPE. Statistics show that a single intense AEW or the combination of multiple tropical waves increases the probability of EPE. Monitoring these tropical waves combinations could improve EPEs forecasts. Key Points Tropical waves are key to build the atmospheric environment conducive to an extreme precipitation event Equatorial Rossby waves appear as a new driver of extreme rainfall in the Central Sahel when combined with African Easterly Waves The combination of African Easterly Wave with an equatorial Rossby wave and/or Kelvin wave, increase the probability of extreme rainfall

We propose the lag‐1 autocorrelation of daily precipitation as a simple diagnostic of tropical precipitation variability in climate models. This metric generally has a relatively uniform distribution of positive values across the tropics. However, selected land regions are characterized by exceptionally low autocorrelation values. Low values correspond to the dominance of high frequency variance in precipitation, and specifically of high frequency convectively coupled equatorial waves. Consistent with previous work, we show that CMIP6 climate models overestimate the autocorrelation. Global kilometer‐scale models capture the observed autocorrelation when deep convection is explicitly simulated. When a deep convection parameterization is used, though, the autocorrelation increases over land and ocean, suggesting that land surface‐atmosphere interactions are not responsible for the changes in autocorrelation. Furthermore, the metric also tracks the accuracy of the representation of the relative importance of high frequency and low frequency convectively coupled equatorial waves in the models. Plain Language Summary Rainfall in the tropics is influenced by many atmospheric processes that depend on geographic location. We use the lag‐1 autocorrelation as a metric for the day‐to‐day persistence of rainfall. We find that rainfall is very persistent in most parts of the tropics with a few exceptions over land, for example, the Sahel, where high frequency rainfall events dominate. Our results show that models with a horizontal resolution of a few kilometers reproduce the autocorrelation, in contrast to coarser climate models. We also analyze atmospheric waves and find that they are important for the autocorrelation pattern in the observations and the simulations. Key Points The lag‐1 autocorrelation pattern of daily precipitation in the tropics is robust across different observation‐based data sets The lag‐1 autocorrelation reflects the relative variance of high frequency and low frequency convectively coupled equatorial waves Kilometer‐scale models capture the observed autocorrelation, but models with parameterized deep convection overestimate it

Mesoscale convective systems (MCSs) produce over 50% of tropical precipitation and account for the majority of extreme rainfall and flooding events. MCSs are considered the building blocks of larger‐scale convectively coupled equatorial waves (CCEWs). While CCEWs can provide favorable environments for convection, how CCEWs can systematically impact organized convection and thereby MCS characteristics is less clear. We examine this question by analyzing a global MCS tracking data set. During the active phase of CCEWs, MCS frequency increases and MCSs rain harder, produce more lifetime total rain, and grow larger in size. The probability of extreme MCSs also elevates. These changes are most pronounced when MCSs are associated with Kelvin waves and tropical depression‐type waves while less so with the Madden‐Julian Oscillation. These results can be benchmarks to improve model representation of MCS interactions with large‐scale circulations and can be leveraged for operational forecasts of high‐impact MCSs at extended lead times. Plain Language Summary Satellite observations show that the population of tropical clouds tends to cluster in a variety of sizes. A type of cluster with a size of around 100 km, known as mesoscale convective systems (MCSs), accounts for over 50% of tropical precipitation and often causes extreme rainfall and flooding events because MCSs can produce heavy rainfall for a long duration. Other larger clusters spanning from 1,000 to 10,000 km, such as convectively coupled equatorial waves (CCEWs), can favor the formation of convection within them. However, how CCEWs can systematically change MCS development and characteristics is not well understood. We investigate this by analyzing an MCS tracking data set. When MCSs occur within CCEWs, their frequency increases and they rain harder, produce more lifetime total rain, and grow larger in size. The probability of extreme MCSs is also elevated. These changes are most pronounced when MCSs occur within two types of CCEWs, Kelvin waves and tropical depression‐type waves while less so with another, the Madden‐Julian Oscillation. These results can be benchmarks to improve computer simulations of MCS interactions with large‐scale circulations. Because CCEWs are better predicted than MCSs beyond 1 week, these results can also be leveraged to extend weather forecasts of high‐impact MCSs. Key Points Mesoscale convective system (MCS) frequency increases during the active phase of convectively coupled equatorial waves (CCEWs) MCSs tend to rain harder, produce more lifetime total rain, and grow larger in size when they occur during the active phase of CCEWs The probability of extreme MCSs rises during active CCEWs. This provides an opportunity for extended forecasts of extreme rainfall events

The simulation of the Madden–Julian Oscillation (MJO) and convectively coupled equatorial waves (CCEWs) is considered in 13 state-of-the-art models from phase 6 of the Coupled Model Intercomparison Project (CMIP6). We use frequency–wavenumber power spectra of the models and observations for Outgoing Longwave Radiation (OLR) and zonal winds at 250 hPa (U250), and consider the historical simulations and end of twenty-first century projections for the SSP245 and SSP585 scenarios. The models simulate a spectrum quantitatively resembling that observed, though systematic biases exist. MJO and Kelvin waves (KW) are mostly underestimated, while equatorial Rossby waves (ER) are overestimated. Most models project a future increase in power spectra for the MJO, while nearly all project a robust increase for KW and weaker power values for most other wavenumber–frequency combinations, including higher wavenumber ER. In addition to strengthening, KW also shift toward higher phase speeds (or equivalent depths). Models with a more realistic MJO in their control climate tend to simulate a stronger future intensification.

Understanding the variability and mechanisms of monsoon onset is extremely prominent for water management and rain-fed agriculture. Previous studies have shown a significant interdecadal advance in Asian summer monsoon (ASM) onset after the late-1990s and attributed it to the sea surface temperature anomalies (SSTA) in the tropical Pacific. However, the westward-propagating mechanisms revealed by previous studies (Walker circulation, equatorial Rossby wave response) are gradually decaying westward, which cannot explain the observational facts of stronger low-level winds over the Arabian Sea than the South China Sea. Based on longer datasets and multiple methods, this study reveals the influences of Pacific SST on the interdecadal changes of ASM onset through two eastward-propagating mechanisms: the equatorial Kelvin wave response to the SSTA in the equatorial central Pacific, and the extratropical Rossby wave train associated with SSTA in the subtropical North Pacific. These two eastward-propagating mechanisms mainly modulate the ASM onset via altering the meridional temperature gradient, which is more evident over the Arabian Sea and is more consistent with the observations. Special attention has been paid to the generation and maintenance of the extratropical Rossby wave train, which is less understood compared to the other mechanisms. This Rossby wave train can be excited by the upper-level divergence associated with the warm SSTA in the subtropical North Pacific. In addition, it can effectively gain available potential energy and kinetic energy from the basic flow, and exhibits strong positive interactions with the synoptic-scale eddies. This Rossby wave train is a newly recognized mechanism by which the extratropical Pacific SSTA influences the tropical ASM.

The tropical cyclone (TC)-centric approach developed in previous studies tends to overemphasize the direct impact of tropical waves and underestimates their large-scale modulation of western North Pacific (WNP) tropical cyclogenesis (TCG). To overcome these limitations, this study proposes a new approach based on empirical orthogonal function analyses to re-examine associations of multiple waves, including the Madden–Julian oscillation (MJO), quasi-biweekly oscillation (QBWO), convective equatorial Rossby (ER) waves, and synoptic scale waves (SSWs), with WNP TCG events. There is a close association between WNP TCG events (total of 273 TCs in this study) and SSWs (~ 64%), MJO (~ 68%), QBWO (~ 64%) and ER (~ 65%) waves. These suggest that SSW is critical for many TCG events, and all of these intra-seasonal waves significantly modulate TCG events. A majority of TCs (~ 79%) were found to be related to more than one wave type, indicating an important role for a combination of dynamic and thermodynamic conditions associated with multiple waves in TCG events. Further analyses show that SSW activity is strongly modulated by the MJO and ER waves, while the QBWO has no significant impact on SSWs. During convectively active MJO and ER phases, stronger SSW trains with more distinct southeast-northwestward aligned structures can be found compared to convectively inactive phases. Similar intra-seasonal modulation of the transition from the mixed Rossby-gravity waves from the equatorial central Pacific to the SSW trains over the WNP by the three waves is noted, with such transitions largely favored during convectively active MJO and ER phases.

A better understanding and simulation of the tropical intraseasonal oscillation (ISO) is the cornerstone of subseasonal-to-seasonal predictions. Here, we have revealed crucial roles of interference effects between Madden–Julian Oscillation (MJO) and low-frequency equatorial Rossby (ER) waves on the intensity, structure and initiation of the ISO. Over where ER waves are sufficiently strong, the ISO convection usually manifests localized strengthening or weakening due to constructive or destructive MJO-ER interferences. For the ISO interannual variability, though the strength is determined by the MJO, the area is largely controlled by ER waves. The Maritime Continent MJO (ER) varies synchronously (asynchronously) with El Niño-Southern Oscillation likely controlled by the meridional mean moisture gradient. Additionally, separating MJO and ER components helps explain the northwest-southeast tilted structure of boreal summer ISOs, which dramatically arises from decreased phase speeds of the MJO away from the equator. Moreover, the considerable damping of ISO deep convection over the Maritime Continent partly arises from the further weakened MJO aloft due to the dry intrusion of ER waves triggered from the central-eastern Pacific. Finally, a primary ISO event with jumping-like propagation behaviors is likely initiated over the Indian Ocean with the strengthening of preceding ER waves, implying a novel “initiation-propagation” linkage of the ISO. These ER signals may be detected approximately 20 days ahead from the western Pacific and are thus potentially useful for monitoring and predicting ISO initiation early. Taking together, the findings here highlight the importance of ER waves in understanding the dynamics and predictability of the ISO/MJO.

This study presents the first attempt to simulate a full cycle of the quasi‐biennial oscillation (QBO) in a global storm‐resolving model (GSRM) that explicitly simulates deep convection and gravity waves instead of parameterizing them. Using the Icosahedral Nonhydrostatic (ICON) model with horizontal and vertical resolutions of about 5km$5\\,\\mathrm{k}\\mathrm{m}$and 400m$400\\,\\mathrm{m}$ , respectively, we show that an untuned state‐of‐the‐art GSRM is already on the verge of simulating a QBO‐like oscillation of the zonal wind in the tropical stratosphere for the right reasons. ICON shows overall good fidelity in simulating the QBO momentum budget and the downward propagation of the QBO jets in the upper QBO domain (25–35 km). In the lowermost stratosphere, however, ICON does not simulate the downward propagation of the QBO jets to the tropopause. This is the result of a pronounced lack of QBO wave forcing, mainly on planetary scales. The lack of planetary‐scale wave forcing in the lowermost stratosphere is caused by an underestimation of planetary‐scale wave momentum fluxes entering the stratosphere. We attribute this lack of planetary‐scale wave momentum fluxes to a substantial lack of convectively coupled equatorial waves (CCEWs) in the tropical troposphere. Therefore, we conclude that in ICON, simulating a realistic spatio‐temporal variability of tropical deep convection, in particular CCEWs, is currently the main roadblock toward simulating a reasonable QBO. To overcome this intermediate situation, we propose to aim at an improved explicit simulation of tropical deep convection by retuning the remaining parameterizations of cloud microphysics and vertical diffusion, and by increasing the horizontal resolution. Plain Language Summary The quasi‐biennial oscillation (QBO) is a wind system located in the equatorial stratosphere between ∼17${\\sim} 17$and ∼35km${\\sim} 35\\hspace*{.5em}\\mathrm{k}\\mathrm{m}$and consists of westerly and easterly wind jets that alternately propagate downward with time. The QBO has been shown to influence surface weather, so it is important to simulate the QBO realistically in the computer models typically used for climate research. However, these models often struggle to simulate a realistic QBO because they represent the processes leading up to the QBO, that is, tropical rain showers and short atmospheric waves excited by these rain showers, only empirically through so‐called parameterizations. In this study, we attempt for the first time to simulate the QBO in a model that directly represents these processes through an ultra‐fine grid. We find that our model maintains QBO‐like stratospheric winds throughout the simulation, and in the central stratosphere, the model simulates the characteristics of the QBO reasonably well for the right reasons. However, in the lowermost stratosphere, the simulated QBO is not realistic and does not move downward with time as observed due to a misrepresentation of long waves in the tropical atmosphere. These results will guide future model development to improve the model's representation of the QBO. Key Points The ability of the general circulation model ICON, which explicitly simulates deep convection and gravity waves, to simulate a quasi‐biennial oscillation (QBO) is tested ICON simulates a reasonable downward propagation and momentum balance of QBO‐like jets in the upper QBO domain in the first simulation year Simulated QBO‐like jets stall in lower QBO domain due to a lack of planetary wave forcing due to weak convectively coupled equatorial waves

Convectively coupled equatorial waves are a significant source of atmospheric variability in the tropics. Current numerical models continue to struggle in simulating the coupled diabatic heating fields that are responsible for the development and maintenance of these waves. This study investigates how the diabatic fields associated with Mixed Rossby–Gravity waves (MRGs) are represented in four reanalysis products by using a unique observational dataset from the TRMM‐KWAJEX (Tropical Rainfall Measuring Mission‐Kwajalein Experiment) field campaign. These reanalyses include ERA5, Japanese 55‐year Reanalysis (JRA‐55), Climate Forecast System Reanalysis (CFSR), and Modern‐Era Retrospective Analysis for Research and Applications (MERRA). We found that all four reanalyses captured the MRG structures in winds and temperature, and to a lesser degree in the humidity field except in the boundary layer. However, only the ERA5 and MERRA reanalyses captured the gradual rise and succession of the diabatic heating from boundary layer turbulence, shallow convection, cumulus congestus, and deep convection within the waves. ERA5 is the only product that also captured the gradual rise of the subgrid‐scale vertical transport of moist static energy. All reanalysis products underestimated the diabatic heating from cumulus congestus. Results provide observational basis on what aspects of MRG can be trusted and what cannot in the reanalysis products. While ERA5 and MERRA are found to capture the gradual rise of the wind divergence field, and hence the titling structure in the diabatic heating and moisture sink, JRA‐55 and CFSR failed to capture the coherent rising structure in the wind divergence field. This failure is then reflected in their upright structures of the diabatic heating and moisture sink fields. All reanalysis products underestimated the diabatic heating from cumulus congestus.