Explore the vast range of titles available.

In this study, the effect of multiple timescale wind fields on the westerly wind burst (WWB) was investigated during the onset of super (1982, 1997, and 2015) and regular El Niño events. The results revealed that extreme WWBs during the onset of the super El Niño group were attributed to low-frequency westerly (≥ 90 days, LFW), medium-frequency westerly (20–90 days, MFW, or intraseasonal) and high-frequency westerly (≤ 20 days, HFW) components, accounting for approximately 56, 22 and 22%, respectively. Thus, the extreme WWBs during the onset of super El Niños were primarily contributed by LFWs. By contrast, the WWBs during the onset of regular El Niños were determined primarily by MFWs (37%) and HFWs (35%), whereas the LFW contribution is relatively small (28%). Further analysis indicated that LFWs during the onset of the super El Niños were primarily a response to a positive SST anomaly in the tropical to eastern North Pacific resembling the Pacific Meridional Mode (PMM), which had persisted during the preceding 9–12 months in the extratropical eastern North Pacific. A significant lagged correlation between the tropical and extratropical North Pacific SST was identified, and their correlation has become stronger since the late 1980s. MFWs during the onset of the super El Niños were primarily associated with the Madden–Julian Oscillation.

Since Shi et al. proposed that the climate in the drylands of Northwest China experienced a significant transition from a “warming and drying” trend to a “warming and wetting” trend in the 1980s, researchers have conducted numerous studies on the variations in precipitation and humidity in the region and even in arid Central Asia. In particular, the process of the “warming and wetting” trend by using obtained measurement data received much attention. However, there remain uncertainties about whether the “warming and wetting” trend has paused and what its future variations may be. In this study, we examined the spatiotemporal variations in temperature, precipitation, the aridity index (AI), vegetation, and runoff during 1950–2019. The results showed that the climate in the drylands of Northwest China and the northern Tibetan Plateau is persistently warming and wetting since the 1980s, with an acceleration since the 1990s. The precipitation/humidity variations in North China, which are mainly influenced by summer monsoon, are generally opposite to those in the drylands of Northwest China. This reverse change is mainly controlled by an anomalous anticyclone over Mongolia, which leads to an anomalous easterly wind, reduced water vapor output, and increased precipitation in the drylands of Northwest China. While it also causes an anomalous descending motion, increased water vapor divergence, and decreased precipitation in North China. Precipitation is the primary controlling factor of humidity, which ultimately forms the spatiotemporal pattern of the “westerlies-dominated climatic regime” of antiphase precipitation/humidity variations between the drylands of Northwest China and monsoonal region of North China. The primary reasons behind the debate of the “warming and wetting” trend in Northwest China were due to the use of different time series lengths, regional ranges, and humidity indices in previous analyses. Since the EC-Earth3 has a good performance for simulating precipitation and humidity in Northwest and North China. By using its simulated results, we found a wetting trend in the drylands of Northwest China under low emission scenarios, but the climate will gradually transition to a “warming and drying” trend as emissions increase. This study suggests that moderate warming can be beneficial for improving the ecological environment in the drylands of Northwest China, while precipitation and humidity in monsoon-dominated North China will persistently increase under scenarios of increased emissions.

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

Changes in accumulated snowfall over the Antarctic Ice Sheet have an immediate and time-delayed impact on global mean sea level. The immediate impact is due to the instantaneous change in freshwater storage over the ice sheet, whereas the time-delayed impact acts in opposition through enhanced ice-dynamic flux into the ocean1. Here, we reconstruct 200 years of Antarctic-wide snow accumulation by synthesizing a newly compiled database of ice core records2 using reanalysis-derived spatial coherence patterns. The results reveal that increased snow accumulation mitigated twentieth-century sea-level rise by ~10 mm since 1901, with rates increasing from 1.1 mm decade−1 between 1901 and 2000 to 2.5 mm decade−1 after 1979. Reconstructed accumulation trends are highly variable in both sign and magnitude at the regional scale, and linked to the trend towards a positive Southern Annular Mode since 19573. Because the observed Southern Annular Mode trend is accompanied by a decrease in Antarctic Ice Sheet accumulation, changes in the strength and location of the circumpolar westerlies cannot explain the reconstructed increase, which may instead be related to stratospheric ozone depletion4. However, our results indicate that a warming atmosphere cannot be excluded as a dominant force in the underlying increase.

Precipitation patterns and their variations over the Tibetan Plateau (TP) are mainly dominated by the Asian summer monsoon, westerlies, and their interactions. The exact extent of the Asian summer monsoon’s influence, however, remains undetermined. Referencing the climatological northern boundary index of the East Asian summer monsoon, we demonstrate that the 300 mm precipitation isoline from May to September can be utilized as an indicator of the northern boundary of the Asian summer monsoon over the TP, allowing for an analysis of the spatial distribution characteristics of the climatological and interannual northern boundary. Our results indicate that the climatological northern boundary of the Asian summer monsoon over the TP lies along the eastern Qilian Mountains-Tanggula Mountains-Qiangtang Plateau-Gangdise Mountains-Western Himalayas during 2001–2020. This position corresponds well with the position of the convergence of westerly (westerlies) and southerly wind (monsoon) in the lower troposphere, representing the interface between dry and wet regions in the rainy season over the TP. There is a significant positive correlation between changes in the zonal/meridional water vapor budget and variations in precipitation to the north/south of the climatological northern boundary, respectively. Additionally, a close relationship exists between the interannual fluctuation range of the northern boundary and the distribution of vegetation across the TP. Compared to the northern boundary of the summer monsoon defined by meteorological criteria, which is established based on 5-day (pentad) mean precipitation (exceeding 4 mm day −1 ), our climatological northern boundary offers a more objective portrayal of the region that experiences persistent influence from the summer monsoon. These indicate that climatological northern boundary has a clear significance for natural geographical distribution such as the westerlies-monsoon circulation, ecology, and climate. Based on the interannual fluctuation range of the northern boundary, we divided the TP into domains of westerlies, monsoon, and westerlies-monsoon transition. This study could serve as a foundation for further investigation into the interactions between westerlies and monsoon, variations in precipitation patterns and hydrological-ecological systems over the TP.

The Mozambique Channel trough (MCT) is a cyclonic region prominent in austral summer in the central and southern Mozambique Channel. It first becomes evident in December with a peak in strength in February when the Mozambique Channel is warmest and the Mascarene high (MH) is located farthest southeast in the Indian Ocean basin. The strength and the timing of the mean MCT are linked to that of the cross-equatorial northeasterly monsoon in the tropical western Indian Ocean, which curves as northwesterlies toward northern Madagascar. The interannual variability in the MCT is associated with moist convection over the Mozambique Channel and is modulated by the location of the warm sea surface temperatures in the south Indian Ocean. Variability of theMCTshows a strong relationship with the equatorial westerlies north of Madagascar and the latitudinal extension of the MH. Summers with strong MCT activity are characterized by a prominent cyclonic circulation over the Mozambique Channel, extending to the midlatitudes. These are favorable for the development of tropical–extratropical cloud bands over the southwestern Indian Ocean and trigger an increase in rainfall over the ocean but a decrease over the southern Africanmainland.Most years with a weak MCT are associated with strong positive south Indian Ocean subtropical dipole events, during which the subcontinent tends to receive more rainfall whereas Madagascar and northern Mozambique are anomalously dry.

Previous studies have confirmed the diverse spatiotemporal characteristics of Atlantic Niño events. Our research further reveals the crucial preparatory role of equatorial western Atlantic barrier layers (BL) and the triggering effect of westerly wind bursts (WWB) on different varieties of Atlantic Niño. Strong easterly winds typically facilitate the formation of thick BL by deepening isothermal layer depth in the western Atlantic through horizontal transport. The existence of BL accumulates the necessary heat for the onset of Atlantic Niño. Additionally, the timing of BL occurrences, the presence of easterly wind anomalies preceding WWB, and the duration of westerly wind anomalies jointly contribute to Atlantic Niño diversity. Persistent westerly wind anomalies following strong easterly winds often lead to Atlantic Niño events lasting over 6 months, while short‐lived events occur when westerly wind anomalies cease shortly after their onset. Plain Language Summary The tropical Atlantic Ocean experiences significant year‐to‐year climate variability known as Atlantic Niño or Niña, similar to the warm and cold phases of El Niño–Southern Oscillation (ENSO) in the Pacific. Atlantic Niño has a considerable impact on local rainfall and cyclone activity. However, each instance of Atlantic Niño has unique spatiotemporal development characteristics, which can be classified into four varieties. While previous research has demonstrated that distinct preconditions give rise to different varieties of Atlantic Niño events, there hasn't been any investigation into the common factors among them. The salinity stratified isothermal layer between the base of the mixed layer and the top of the thermocline is referred to as the barrier layer (BL), which is a common feature of tropical western Pacific and Atlantic. The BL in the tropical western Pacific has been confirmed to facilitate the accumulation of heat in the upper ocean and can provide favorable thermal conditions for the onset of El Niño events. Our study reveals the key role of BL induced heat accumulation in various Atlantic Niño onsets. This suggests that anomalies of BL can be reliable indicators for predicting the onset of Atlantic Niño events. Key Points The role of barrier layers in heat buildup is confirmed during the development of the four varieties of Atlantic Niño The heat buildup caused by barrier layers, combined with zonal wind events, regulates the diversity of Atlantic Niño The sustainability of westerly wind anomalies links to the strength of Atlantic Niño events

Over recent decades, the Southern Ocean (SO) has experienced multi‐decadal surface cooling despite global warming. Earlier studies have proposed that recent SO cooling has been caused by the strengthening of surface westerlies associated with a positive trend of the Southern Annular Mode (SAM) forced by ozone depletion. Here we revisit this hypothesis by examining the relationships between the SAM, zonal winds and SO sea‐surface temperature (SST). Applying a low‐frequency component analysis to observations, we show that while positive SAM anomalies can induce SST cooling as previously found, this seasonal‐to‐interannual modulation makes only a small contribution to the observed long‐term SO cooling. Global climate models well capture the observed interannual SAM‐SST relationship, and yet generally fail to simulate the observed multi‐decadal SO cooling. The forced SAM trend in recent decades is thus unlikely the main cause of the observed SO cooling, pointing to a limited role of the Antarctic ozone hole. Plain Language Summary Despite increasing greenhouse gases, the Southern Ocean sea‐surface temperatures have cooled over the recent several decades. The cause of Southern Ocean cooling remains a puzzling feature of recent climate change. Earlier studies have proposed that this multi‐decadal cooling in the Southern Ocean has arisen in part from the strengthening of surface winds associated with a positive trend in a mode of climate variability known as the Southern Annular Mode (SAM). Here we employ a new statistical method to examine this proposed relationship in both observations and climate models. We found that SAM variability only changes Southern Ocean surface temperature on short‐term timescales and makes little contribution to observed long‐term trends. Our results thus suggest the SAM trend, via the strengthening of circumpolar westerlies, is unlikely to be the main cause of the observed long‐term Southern Ocean cooling. Key Points Austral summer Southern Annular Mode (SAM) anomalies affect Southern Ocean (SO) sea‐surface temperature (SST) only on seasonal to interannual timescales Multi‐decadal observed SAM trends make little contribution to observed Southern Ocean SST trends Global climate models (GCMs) capture the observed seasonal SAM‐SST relationship and yet fail to simulate the observed long‐term SO cooling

We evaluate and compare the simulation of the main features (low-level westerlies (LLWs) and the Congo basin (CB) cell) of low-level circulation in Central Equatorial Africa (CEA) with eight climate models from Phase 6 of the Coupled Model Intercomparison Project (CMIP6) and the corresponding eight previous models from CMIP5. Results reveal that, although the main characteristics of the two features are reasonably well depicted by the models, they bear some biases. The strength of LLWs is generally overestimated in CMIP5 models. The overestimation is attributed to both divergent and rotational components of the total wind with the rotational component contributing the most in the overestimation. In CMIP6 models, thanks to a better performance in the simulation of both divergent and rotational circulation, LLWs are slightly less strong compared to the CMIP5 models. The improvement in the simulated divergent component is associated with a better representation of the near-surface pressure and/or temperature difference between the Central Africa landmass and the coastal Atlantic Ocean. Regarding the rotational circulation, and especially for HadGEM3-GC31-LL and BCC-CSM2-MR, a simulated higher 850 hPa pressure is associated with less pronounced negative vorticity and a better representation of the rotational circulation. Most CMIP5 models also overestimate the CB cell intensity and width in association with the simulated strength of LLWs. However, in CMIP6 models, the strength of key cell characteristics (intensity and width) are reduced compared to CMIP5 models. This depicts an improvement in the representation of the cell in CMIP6 models and this is associated with the improvement in the simulated LLWs.

The present study investigates the impact of various central Pacific (CP) and eastern Pacific (EP) warming on tropical cyclones (TCs) over the western North Pacific (WNP) for the period 1948–2015 based on observational and reanalysis data. Four distinctly different forms of tropical Pacific warming are identified to examine different impacts of locations and intensity of tropical Pacific warming on the WNP TCs. It is shown that WNP TC activity related to ENSO shows stronger sensitivity to the intensity of CP SST warming. The locations of TC genesis in an extreme EP El Niño featuring concurrent strong CP and EP warming (CEPW) display a notable southeastward shift that is generally similar to the CP El Niño featuring CP warming alone (CPW). These influences are clearly different from the effects of moderate EP El Niño associated with EP warming alone (EPW). The above influences of Pacific warming on TCs possibly occur via atmospheric circulation variability. Anomalous convection associated with CP SST warming drives anomalous low-level westerlies away from the equator as a result of a Gill-type Rossby wave response, leading to an enhanced broad-zone, eastward-extending monsoon trough (MT). An anomalous Walker circulation in response to EP SST warming drives an increase in anomalous equatorial westerlies over the WNP, leading to a narrow-zone, slightly equatorward shift of the eastward-extending MT. These changes in the MT coincide with a shift in large-scale environments and synoptic-scale perturbations, which favor TC genesis and development. In addition, during weaker EP SST warming (WEPW) with similar intensity to CPW, local SST forcing exhibits primary control on WNP TCs and atmospheric circulation.