The January 2022 eruption of Hunga Tonga‐Hunga Ha'apai (HTHH) injected a huge amount (∼150 Tg) of water vapor (H ₂ O) into the stratosphere, along with small amount of SO ₂ . An off‐line 3‐D chemical transport model (CTM) successfully reproduces the spread of the injected H ₂ O through October 2023 as observed by the Microwave Limb Sounder. Dehydration in the 2023 Antarctic polar vortex caused the first substantial (∼20 Tg) removal of HTHH H ₂ O from the stratosphere. The CTM indicates that this process will dominate removal of HTHH H ₂ O for the coming years, giving an overall e‐folding timescale of 4 years; around 25 Tg of the injected H ₂ O is predicted to still remain in the stratosphere by 2030. Following relatively low Antarctic column ozone in midwinter 2023 due to transport effects, additional springtime depletion due to H ₂ O‐related chemistry was small and maximized at the vortex edge (10 DU in column).

The causes of ENSO diversity, although being of great interest in recent research, do not have a consistent explanation. This study provides a possible mechanism focused on the background change of the tropical Pacific as a response to global warming. The second empirical orthogonal function mode of the sea surface temperature anomalies (SSTA) in the tropical Pacific, namely the cold tongue mode (CTM), represents the background change of the tropical Pacific under global warming. Using composite analysis with surface observations and subsurface ocean assimilation data sets, we find ENSO spatial structure diversity is closely associated with the CTM. A positive CTM tends to cool the SST in the eastern equatorial Pacific and warm the SST outside, as well as widen (narrow) zonal and meridional scales for El Niño (La Niña), and vice versa. Particularly in the positive CTM phase, the air‐sea action center of El Niño moves west, resembling the spatial pattern of CP‐El Niño. This westward shift of center is related to the weakened Bjerknes feedback (BF) intensity by the CTM. By suppressing the SSTA growth of El Niño in the eastern equatorial Pacific, the CTM contributes to more frequent occurrence of CP‐El Niño under global warming.

When long time series are analyzed, two nearby periods may show significantly different trends, which is known as trend turning. Trend turning is common in climate time series and crucial when climate change is investigated. However, the available detection methods for climate trend turnings are relatively few, especially for the methods which have the ability of detecting multiple trend turnings. In this article, we propose a new methodology named as the running slope difference (RSD) t test to detect multiple trend turnings. This method employs a t-distributed statistic of slope difference to test the sub-series trend difference of the time series, thereby identifying the turning points. We compare the RSD t test method with some other existing trend turning detection methods in an idealized time series case and several climate time series cases. The results indicate that the RSD t test method is an effective tool for detecting climate trend turnings.

Time-slice experiments with the Whole Atmosphere Community Climate Model, version 4 (WACCM4), and composite analysis with satellite observations are used to demonstrate that the Indo-Pacific warm pool (IPWP) can significantly affect lower-stratospheric water vapor. It is found that a warmer IPWP significantly dries the stratospheric water vapor by causing a broad cooling of the tropopause, and vice versa for a colder IPWP. Such imprints in tropopause temperature are driven by a combination of variations in the Brewer–Dobson circulation in the stratosphere and deep convection in the troposphere. Changes in deep convection associated with El Niño–Southern Oscillation (ENSO) reportedly have a small zonal mean effect on lower-stratospheric water vapor for strong zonally asymmetric effects on tropopause temperature. In contrast, IPWP events have zonally uniform imprints on tropopause temperature. This is because equatorial planetary waves forced by latent heat release from deep convection project strongly onto ENSO but weakly onto IPWP events.

Lower Stratospheric water vapor (SWV) is one of important drivers of global climate change. Increases and decreases in lower SWV have been found to strengthen and offset global warming effects, respectively. Using several data sets, we find that sea surface temperature (SST) warming in the past 100 years has caused an increase in SWV. SST warming over the tropical Indian Ocean and the western Pacific has resulted in a drier stratosphere. However, tropical Atlantic Ocean warming has resulted in a significantly wetter stratosphere and is the main contributor to the increasing trend of SWV in the past 100 years. The responses of Rossby and Kelvin waves over the Indian Ocean and western Pacific to Atlantic warming have led to a warmer tropopause temperature, resulting in more water vapor entering the stratosphere. This study suggests that SWV trend may simply be the result of a game between warm pool SST and tropical Atlantic SST changes.