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1. Slow Modes of the Equatorial Waveguide

   https://doi.org/10.1175/jas-d-19-0281.1  

   Emanuel, Kerry (May 2020, Journal of the Atmospheric Sciences)

   Abstract A recently developed linear model of eastward-propagating disturbances has two separate unstable modes: convectively coupled Kelvin waves destabilized by the wind dependence of the surface enthalpy flux, and slow, MJO-like modes destabilized by cloud–radiation interaction and driven eastward by surface enthalpy fluxes. This latter mode survives the weak temperature gradient (WTG) approximation and has a time scale dictated by the time it takes for surface fluxes to moisten tropospheric columns. Here we extend that model to include higher-order modes and show that planetary-scale low-frequency waves with more complex structures can also be amplified by cloud–radiation interactions. While most of these waves survive the WTG approximation, their frequencies and growth rates are seriously compromised by that approximation. Applying instead the assumption of zonal geostrophy results in a better approximation to the full spectrum of modes. For small cloud–radiation and surface flux feedbacks, Kelvin waves and equatorial Rossby waves are destabilized, but when these feedbacks are strong enough, the frequencies do not lie close to classical equatorial dispersion curves except in the case of higher-frequency Kelvin and Yanai waves. An eastward-propagating n = 1 mode, in particular, has a structure resembling the observed structure of the MJO. 

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2. Strong remote control of future equatorial warming by off-equatorial forcing

   https://doi.org/10.1038/s41558-019-0667-6  

   Stuecker, Malte F.; Timmermann, Axel; Jin, Fei-Fei; Proistosescu, Cristian; Kang, Sarah M.; Kim, Doyeon; Yun, Kyung-Sook; Chung, Eui-Seok; Chu, Jung-Eun; Bitz, Cecilia M.; et al (February 2020, Nature Climate Change)
3. Disentangling the mechanisms of equatorial Pacific climate change

   https://doi.org/10.1126/sciadv.adf5059  

   Kang, Sarah M.; Shin, Yechul; Kim, Hanjun; Xie, Shang-Ping; Hu, Shineng (May 2023, Science Advances)

   Most state-of-art models project a reduced equatorial Pacific east-west temperature gradient and a weakened Walker circulation under global warming. However, the causes of this robust projection remain elusive. Here, we devise a series of slab ocean model experiments to diagnostically decompose the global warming response into the contributions from the direct carbon dioxide (CO₂) forcing, sea ice changes, and regional ocean heat uptake. The CO₂forcing dominates the Walker circulation slowdown through enhancing the tropical tropospheric stability. Antarctic sea ice changes and local ocean heat release are the dominant drivers for reduced zonal temperature gradient over the equatorial Pacific, while the Southern Ocean heat uptake opposes this change. Corroborating our model experiments, multimodel analysis shows that the models with greater Southern Ocean heat uptake exhibit less reduction in the temperature gradient and less weakening of the Walker circulation. Therefore, constraining the tropical Pacific projection requires a better insight into Southern Ocean processes. 

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4. Seasonality of the QBO Impact on Equatorial Clouds

   https://doi.org/10.1029/2022JD037737  

   Sweeney, Aodhan; Fu, Qiang; Pahlavan, Hamid A; Haynes, Peter (April 2023, Journal of Geophysical Research: Atmospheres)

   Abstract The Quasi‐Biennial Oscillation (QBO) dominates the interannual variability in the tropical lower stratosphere and is characterized by the descent of alternating easterly and westerly zonal winds. The QBO impact on tropical clouds and convection has received great attention in recent years due to its implications for weather and climate. In this study, a 15‐year record of high vertical resolution cloud observations from CALIPSO and a 50 hPa zonal wind QBO index from ERA5 are used to document the QBO impact on equatorial (10°S–10°N) clouds. Observations from radio occultations, the CERES instrument, and the ERA5 reanalysis are also used to document the QBO impact on temperature, cloud radiative effect (CRE), and zonal wind, respectively. It is shown that the QBO impact on zonal mean equatorial cloud fraction has a strong seasonality. The strongest cloud fraction response to the QBO occurs in boreal spring and early summer, which extends from above the mean tropopause to ∼12.5 km and results in a significant longwave CRE anomaly of 1 W/m². The seasonality of the QBO impact on cloud fraction is synchronized with the QBO impacts on temperature and zonal wind in the tropical upper troposphere. 

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5. Seasonality in the relationship between equatorial-mean heat content and interannual eastern equatorial Atlantic sea surface temperature variability

   https://doi.org/10.1007/s00382-021-06116-w  

   Turner, Kyle J.; Burls, Natalie J.; von Brandis, Anna; Lübbecke, Joke; Claus, Martin (January 2022, Climate Dynamics)

   Abstract Interannual sea surface temperature (SST) variations in the tropical Atlantic Ocean lead to anomalous atmospheric circulation and precipitation patterns with important ecological and socioeconomic consequences for the semiarid regions of sub-Saharan Africa and northeast Brazil. This interannual SST variability is characterized by three modes: an Atlantic meridional mode featuring an anomalous cross-equatorial SST gradient that peaks in boreal spring; an Atlantic zonal mode (Atlantic Niño mode) with SST anomalies in the eastern equatorial Atlantic cold tongue region that peaks in boreal summer; and a second zonal mode of variability with eastern equatorial SST anomalies peaking in boreal winter. Here we investigate the extent to which there is any seasonality in the relationship between equatorial warm water recharge and the development of eastern equatorial Atlantic SST anomalies. Seasonally stratified cross-correlation analysis between eastern equatorial Atlantic SST anomalies and equatorial heat content anomalies (evaluated using warm water volume and sea surface height) indicate that while equatorial heat content changes do occasionally play a role in the development of boreal summer Atlantic zonal mode events, they contribute more consistently to Atlantic Niño II, boreal winter events. Event and composite analysis of ocean adjustment with a shallow water model suggest that the warm water volume anomalies originate mainly from the off-equatorial northwestern Atlantic, in agreement with previous studies linking them to anomalous wind stress curl associated with the Atlantic meridional mode. 

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