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Previous studies suggest that convectively coupled Kelvin waves (KWs) are likely maintained by two distinct processes: (a) the internal thermodynamic feedback between KW diabatic heating and temperature, and (b) the external momentum forcing from the midlatitude Rossby waves exerting on the KW zonal wind. This study quantifies the relative importance of the two processes on KW maintenance by comparing the growth rates of eddy available potential energy (EAPE) and eddy kinetic energy (EKE) within KWs using satellite and reanalysis data. Results show that the growth rate of KW EAPE is greater than that of KW EKE in most regions and seasons, while their relative magnitude varies across different regions and seasons. The observed relative importance of the two maintenance processes can serve as a reference for numerical simulations of KWs.

Using the fifth generation European Center for Medium-range Weather Forecasts (ERA5) reanalysis data, we present a detailed examination of the climatological features of convectively coupled Kelvin waves (CCKWs) over the Indian and Pacific basins. The composited horizontal structure of Indian CCKWs resembles the theoretical Kelvin waves, with a maximum wave response at the equator. In contrast, the Pacific counterpart exhibits a very different pattern, characterized by a significant northward shift of the convective center, along with enhanced meridional winds and a relatively stronger wave response. The moist static energy (MSE) budget analysis is conducted to elucidate the physical factors that control the energetics of CCKWs. Despite the marked contrast in horizontal structure between Pacific and Indian CCKWs, the energy cycle and the physical factors that maintain this cycle are rather similar. During the recharge period (days -2 and -1), the column process (including vertical MSE advection, apparent heat source and moisture sink) functions to destabilize the atmosphere by importing the MSE; while the horizontal MSE advection tends to destabilize the atmosphere on day -2 but starts to stabilize the atmosphere earlier on day -1. During the discharge and transition period (from days 0 to + 2), the column process functions to stabilize the atmosphere by exporting the MSE; while the horizontal MSE advection inclines to stabilize the atmosphere on days 0 and + 1 but again starts to destabilize the atmosphere earlier on day + 2. The leading of horizontal MSE advection to the recharge-discharge cycle clearly points out the importance of the former in driving the eastward propagation of CCKWs. Both the horizontal MSE advection and column process are vital in maintaining the energy cycle of CCKWs, as they often take turns leading the role in recharging and discharging the atmosphere.

This study reveals the remarkable differences in the synoptic‐scale tropical waves before and after the South China Sea (SCS) summer monsoon (SCSSM) onset. Before the monsoon onset, the dominant synoptic‐scale systems around the SCS are the eastward‐propagating equatorial Kelvin waves. After the SCSSM onset, while the Kelvin waves remain largely unaltered, the transitions from mixed Rossby‐gravity waves to tropical depression (TD)‐type waves become more pronounced. Such distinctive features of the tropical waves before and after SCSSM onset are attributable to the adjustments in atmospheric mean flow. An anomalous low‐level cyclone develops around the SCS after the monsoon onset. The convergence and shear of this anomalous low‐level cyclone contribute to the barotropic energy conversion from the mean flow to the synoptic‐scale systems, which is conducive to the development of TD‐type waves and genesis of tropical cyclones. Plain Language Summary Previous studies have shown that the South China Sea summer monsoon onset is important on the large‐scale: signify the establishment of summer monsoon over East Asia‐Southeast Asia‐western North Pacific, the arrival of the main rainy season in these locations, and the adjustment of the atmospheric circulation from winter‐type to summer‐type. We find that the changes in atmospheric mean flow before and after monsoon onset can have a significant modulating effect on the synoptic‐scale perturbations gestated in them. Prior to the monsoon onset, the most obvious permutations are those propagating eastward near the equator. In contrast, after the monsoon onset, the northwestward propagating tropical disturbances off‐the‐equator become more active. Specifically, tropical depressions and tropical cyclones occur more frequently and begin to affect Southeast and East Asia. These distinctive features of tropical waves can be understood in terms of the energy conversion between the mean circulation and the disturbances. Key Points Before the South China Sea (SCS) summer monsoon onset, the dominant synoptic‐scale systems are the eastward‐propagating equatorial Kelvin waves After the monsoon onset, the northwestward‐propagating tropical depression‐type waves become more pronounced The distinctive features of the tropical waves before and after monsoon onset are attributable to the adjustments in atmospheric mean flow

After consecutive two years with the La Niña phenomenon in 2020–2021, cold sea surface temperature (SST) anomalies in the central-eastern equatorial Pacific persisted in 2022, known as a “triple-dip” La Niña event. These conditions have had a profound impact on global weather and climate. Understanding the underlying processes and mechanisms is crucial for improving ENSO prediction and has significant socioeconomic implications. In this study, we investigate the processes responsible for the evolution of the third-year La Niña event in 2022 based on related observations and an intermediate coupled model (ICM). The results show that two factors are essential for the development of the third-year La Niña event: surface easterly wind anomalies over the central-western equatorial Pacific in early 2022 and the induced cold SST anomalies by the former. In the first half of 2022, mainly due to the off-equatorial downwelling Rossby wave reflection at the western boundary, the equatorial downwelling Kelvin waves continuously induce warming effect in the upper ocean, acting to weaken the cold SST anomalies in the central-eastern Pacific. However, the easterly wind anomalies over the central-western equatorial Pacific during February–March 2022 induce upwelling Kelvin waves, whose cooling effect reverses the warming SST tendency. Afterward, despite the pronounced warming effect induced by the reflected downwelling Kelvin waves, the wind-induced cold SST anomalies persist on the equator during the equatorial cold season, which are further intensified by the Bjerknes feedback. Sensitivity experiments based on the ICM further confirm these outcomes. When the cooling effect induced by the easterly wind anomalies is included in ocean initial conditions, the ICM can accurately capture the reappearance of the La Niña phenomenon. These results highlight the importance of anomalous easterlies in the central-western equatorial Pacific during the decaying phase of the La Niña event.

An extreme Atlantic Niño developed in the boreal summer of 2021 with peak‐season sea surface temperature anomalies exceeding 1°C in the eastern equatorial region for the first time since global satellite measurements began in the early 1970s. Here, we show that the development of this outlier event was preconditioned by a series of oceanic Rossby waves that reflected at the South American coast into downwelling equatorial Kelvin waves. In early May, an intense week‐long westerly wind burst (WWB) event, driven by the Madden‐Julian Oscillation (MJO), developed in the western and central equatorial Atlantic and greatly amplified one of the reflected Kelvin waves, directly initiating the 2021 Atlantic Niño. MJO‐driven WWBs are fundamental to the development of El Niño in the Pacific but are a previously unidentified driver for Atlantic Niño. Their importance for the 2021 event suggests that they may serve as a useful predictor/precursor for future Atlantic Niño events. Plain Language Summary Atlantic Niño is the Atlantic counterpart of El Niño in the Pacific, often referred to as El Niño's little brother. It was previously thought to have only regional influence on rainfall variability in West Africa, but a growing number of studies have shown that Atlantic Niño also plays an important role in the development of El Niño–Southern Oscillation, as well as in the formation of powerful hurricanes near the coast of West Africa. This study investigates the development of an extreme Atlantic Niño in the summer of 2021. Here, we show that the 2021 event was preconditioned by warm waters piled up near the South American coast, and then directly triggered by a westerly wind burst event that drove the warm waters eastward. The westerly wind burst event was driven by a patch of tropical thunderstorms that formed across the Indian Ocean and moved slowly eastward across the Pacific, South America, and the Atlantic, also known as the Madden‐Julian Oscillation. Westerly wind bursts driven by the Madden‐Julian Oscillation are fundamental for the development of El Niño in the Pacific, but a previously unidentified driver for Atlantic Niño, and thus may improve our ability to predict future Atlantic Niño events. Key Points The extreme 2021 Atlantic Niño was preconditioned by a series of oceanic Rossby waves reflected into downwelling equatorial Kelvin waves One of the Kelvin waves was greatly amplified by an intense week‐long westerly wind burst event, initiating the 2021 Atlantic Niño The westerly wind burst was driven by the Madden‐Julian Oscillation, which is a previously unidentified driver for Atlantic Niño

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.

We employed zonal wind data from Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics Doppler Interferometer, equatorial electrojet (EEJ) measurements from Jicamarca (12°S, 77°W), and global ionospheric total electron content (TEC) maps to investigate the effects of ultra‐fast Kelvin waves (UFKW) with zonal wavenumbers 2 and 3 propagating eastward (E2, E3) in the equatorial mesosphere on both ionospheric TEC and EEJ signatures, as well as their differences in ionospheric response characteristics compared to E1 waves. Periodic components in zonal wind, EEJ, and TEC are quantified through the least squares spectral fitting. Our findings reveal three distinct categories of UFKW events: Type 1 exhibits both TEC and EEJ responses, Type 2 shows TEC response without EEJ signature, and Type 3 displays neither TEC nor EEJ response. The finding reveals distinct seasonal dependencies in TEC responses: E2 and E3 waves exhibit significant seasonality, whereas E1 waves show negligible seasonal variation. Furthermore, E1 waves demonstrate higher ionospheric response occurrence rates compared to E2 and E3 waves. For E1 waves, shorter periods, larger amplitudes, and longer vertical wavelengths correlate strongly with enhanced ionospheric responsiveness. Conversely, amplitude exerts minimal influence on ionospheric responses for E2 and E3 waves. For E2 and E3 waves temporally coincident with E1 waves, E2 and E3 waves may elevate Type 2 event occurrence among concurrent E1 waves, while E1 waves tend to suppress ionospheric response capability in coincident E2 and E3 waves, increasing Type 3 event prevalence.

The rare triple-dip 2020–2023 La Niña event has resulted in a series of extreme climate events across the globe. Here, we reveal the role of tropical Indo-Pacific oceanic interactions in driving the first triple-dip La Niña of the twenty-first century. Specifically, we found that the eastern Indian Ocean subsurface warming anomalies were associated with the re-intensification of the subsequent La Niña event. The subsurface warming anomaly signals were propagated eastward by equatorial and coastal subsurface Kelvin waves from the eastern Indian Ocean to the western Pacific Ocean through the Indo-Pacific oceanic pathway, which contributes to the accumulation of heat content and deepens the thermocline in the western tropical Pacific. The westward Indonesian Throughflow (ITF) transported more heat during multi-year La Niña events from the western Pacific Ocean to the eastern Indian Ocean than during single-year events, resulting in the injection of more warm water into the eastern Indian Ocean. The combination of subsurface Kelvin wave propagation and increased ITF volume transport in the Indo-Pacific region acted to prolong the heat content in the western Pacific during the decay phase of La Niña, ultimately leading to the rare triple-dip 2020–2023 La Niña event.

Previous studies disagree on whether tropical cyclogenesis is significantly modulated by convectively coupled Kelvin waves (KWs), partially due to limited observations over the satellite era. This study investigates interactions between tropical cyclogenesis and KWs using 10 simulations over 2001 to 2010 produced by the deep‐learning model Ai2 Climate Emulator version 2 (ACE2) trained on reanalysis data. The 100 years of simulations show a robust modulation of tropical cyclogenesis by KWs in all basins. Tropical cyclogenesis preferentially occurs after the peak KW precipitation by 1 day in the Indian Ocean and Western Pacific, mainly due to the enhanced column relative humidity and horizontal vorticity by the KWs. This study demonstrates that large‐ensemble data generated from deep‐learning climate emulators can help strengthen relationships difficult to detect in observations alone. While we focus on tropical cyclogenesis, this approach may also benefit other types of weather or climate variability.

Marine Heat Waves (MHWs) can cause significant distress to marine environment and modulate air-sea interaction, which in turn can have economic and societal impacts. This study aims to identify surface and subsurface MHWs in the Bay of Bengal using available sea-surface temperature data and buoy observations spanning four decades. The results show significant increase in the number, frequency, duration, and intensity of surface MHWs in recent years. To better understand the relationship between MHW occurrences and different phases of two climate phenomena, namely, the Indian Ocean Dipole (IOD) and the El Niño-Southern Oscillation, multiple extreme MHW event years between 2008 and 2018 are analyzed. The findings show that the surface MHWs become more prominent during El Niño/positive IOD events due to weakened winds, increased net heat flux input towards the ocean, increased stratification and warming tendency through vertical processes in the presence of inversion. This, in turn, affects the surface biological productivity in the region. Additionally, surface MHWs were also found to be driven by surface currents and eddies. On the other hand, subsurface MHWs develop during negative IOD/La Niña conditions due to the deepening of the thermocline, which is forced by coastally trapped downwelling Kelvin waves and reflected downwelling Rossby waves, and is followed by substantial freshening of deeper layers. This study provides a deeper understanding of the causes of surface and subsurface MHWs in the Bay of Bengal, which is a crucial basin in influencing monsoon and cyclonic events affecting its surrounding landmasses.