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Marine heatwaves (MHWs) are extreme climatic events that can have a significant impact on marine ecosystems and their services across the world. We examine the spatiotemporal variation of summer MHWs in the north Indian Ocean (NIO) and find that the whole NIO Basin exhibits a pronounced spatial variability as well as a significant increasing trend in MHW frequency. We show that the NIO has two leading MHW modes linked to two distinct sea surface temperature (SST) patterns during summer. The first MHW mode is associated with basinwide warming, which is preconditioned by a decaying El Niño-Southern Oscillation (ENSO) and sustained throughout the summer by anomalous northeasterlies extending from the anticyclonic circulation of the western North Pacific subtropical high (WNPSH). The combined effect of thermocline warming due to downwelling oceanic planetary waves, decreased wind-induced evaporative cooling, and enhanced insolation cause basinwide summer MHWs. The second MHW mode exhibits a zonal dipole pattern, which has unfavorable cooling conditions in the previous seasons. The second MHW mode is associated with a phase change of ENSO and is greatly influenced by the formation of an interhemispheric pressure difference (IHPD) due to strengthening of the Australian high (AH) and weakening of the WNPSH. The IHPD induces cross-equatorial southerly winds across the eastern Indian Ocean. These winds favor the transformation of basinwide cooling conditions into zonal SST patterns via wind-evaporation-SST and thermocline-SST feedback, causing MHWs with a zonal dipole pattern. These MHW modes have a significant influence on the distribution and intensity of summer precipitation in the NIO. [ABSTRACT FROM AUTHOR]

In this study, the relationship between AO and winter synoptic temperature variability (STV) over the Northern Hemisphere is examined in 34 CMIP5/CMIP6 model outputs. With significant model bias around the North Pacific and North Atlantic, most models fail to capture the correct AO-STV pattern in historical simulations compared to observations. To investigate the bias of AO-STV relationship simulations, AO-related processes for the connection between AO and winter STV are examined in high pattern score (HPS) models and low pattern score (LPS) models, respectively. Furthermore, the bias of AO impact can be traced back to AO pattern simulations. On the one hand, compared to observations, HPS models can overall capture the intensity in the North Pacific and North Atlantic center of AO. On the other hand, LPS models tend to overestimate the North Pacific center and underestimate the North Atlantic center. In addition, similar to historical simulations, a robust AO-STV relationship can still be found over the Northern Hemisphere in future projections based on HPS models. Meanwhile, the uncertainty of the projected AO-STV relationship in the multimodel ensemble is confined mainly to the North Pacific, consistent with the large diversity of intensity over the North Pacific center of AO, which is related to the uncertainty of the relationship between AO and regional mode variability. [ABSTRACT FROM AUTHOR]

The impact of El Nino –Southern Oscillation (ENSO) on the winter stratospheric polar vortex intensity (PVI) is re-examined, using JRA-55 datasets and the CESM2 model. Our results show that the negative correlation between ENSO and PVI has weakened in recent decades and is no longer statistically significant after the mid-1990s. This weakening ENSO–PVI relationship after the mid-1990s is associated with wave-1 fluxes entering the stratosphere; that is, the ENSO-induced increasing wave-1 fluxes are weaker after the mid-1990s than before the mid-1990s. Except for the Pacific–North America (PNA)-like wave train, the stratospheric wave flux changes are related to ENSO-induced geopotential height changes over East Asia. The ENSO-induced tropical zonal circulation anomalies are enhanced from the central Pacific to East Asia, leading to more substantial wave flux anomalies propagating westward from the central Pacific to East Asia and a more significant high center over Northeast Asia after the mid-1990s than before the mid-1990s. This ENSO-induced high center after the mid-1990s is out of phase with the climatological center of wave 1, leading to a relatively weaker increase in wave 1. Model results show that changes in the ENSO–PVI relationship and ENSO–Asia circulation are associated with changes in ENSO-related SST combined with global mean SST. [ABSTRACT FROM AUTHOR]

Southerly wind in the lower troposphere is an essential feature of East Asian summer monsoon (EASM) circulation, which is reported to be enhanced under global warming scenarios and interglacial epochs. Based on an analysis of an ensemble of CMIP6 models, this study shows that the magnitude of intensification of the EASM circulation is much smaller under global warming scenarios than during interglacial epochs. Distinct changes in the western North Pacific subtropical high (WNPSH) are responsible for the different responses of the EASM circulation. The WNPSH is substantially enhanced during interglacial epochs, which acts to strengthen the southerly wind associated with the EASM on the western flank of the WNPSH. However, the change in the WNPSH is insignificant and cannot strengthen the EASM under global warming scenarios, and the weakly enhanced EASM circulation may be a direct response to intensified heating over the Tibetan Plateau. The land-ocean thermal contrast explains the different responses of the WNPSH. During interglacial epochs, the summertime surface warming over the subtropical North Pacific is much weaker than over Eurasia due to the large thermal inertia of the ocean to increased insolation, and the WNPSH is intensified as a response to the suppressed latent heating over the subtropical North Pacific. The fast response of the WNPSH to abrupt quadrupling of CO2 without sufficient ocean warming is an analog to the interglacial epochs, but it is offset by the effect of slow oceanic warming, resulting in an insignificant change of the WNPSH under global warming scenarios. [ABSTRACT FROM AUTHOR]

Summer monsoon rainfall supplies over 55% of annual precipitation to global monsoon regions. As shown by more than 70% of models, including 30 models from CMIP5 and 30 models from CMIP6 under high-emission scenarios, North American (NAM) monsoon rainfall decreases in a warmer climate, in sharp contrast to the robust increase in Asian- African monsoon rainfall. A hierarchy of model experiments is analyzed to understand the mechanism for the reduced NAM monsoon rainfall in this study. Modeling evidence shows that the reduction of NAM monsoon rainfall is related to both direct radiative forcing of increased CO2 concentration and SST warming, manifested as fast and slow responses to abrupt CO2 quadrupling in coupled GCMs. A cyclone anomaly forms over the Eurasian-African continental area due to enhanced land-sea thermal contrast under increased CO2 concentration, and this leads to a subsidence anomaly on its western flank, suppressing the NAM monsoon rainfall. The SST warming acts to further reduce the rainfall over the NAM monsoon region, and the El Niño-like SST warming pattern with enhanced SST warming over the equatorial Pacific plays a key role in suppressing NAM rainfall, whereas relative cooling over the subtropical North Atlantic has no contribution. A positive feedback between monsoon precipitation and atmospheric circulation helps to amplify the responses of monsoon rainfall. [ABSTRACT FROM AUTHOR]

In winter 2018/19, southeastern coastal China experienced extreme warm temperatures that were due to a weak East Asian winter monsoon. On the basis of observations from 10 meteorological stations and reanalysis data, the large-scale circulation patterns associated with this extreme warm winter and the possible driving mechanism of its related sea surface temperature (SST) anomalies are investigated in this study. During this winter, many places in this region reached their highest winter mean temperature record and had more extreme warm days and fewer extreme cold days relative to climatology. According to the circulation patterns during winter 2018/19, several large-scale circulation conditions associated mainly with the weak East Asian winter monsoon are identified: the eastward shift of the Siberian high and a shallower East Asian trough, which is related to the low blocking frequency over the Aleutian region, are both unfavorable for cold-air intrusion southward. Meanwhile, strong low-level southerly wind anomalies over southeastern China are related mainly to the 2018/19 El Niño event. Furthermore, the possible role of SST anomalies over the North Atlantic and tropical western Pacific Oceans is examined by using an atmospheric general circulation model, suggesting that both the ''tripole pattern'' of North Atlantic SST and tropical western Pacific SST anomalies in winter 2018/19 played a role in influencing the East Asian trough. The combined effect of all of these factors seems to be responsible for this extreme warm winter over southeastern coastal China. [ABSTRACT FROM AUTHOR]

The Silk Road pattern (SRP) is a well-known teleconnection pattern along the upper-level westerly jet over the Eurasian continent during boreal summer. The SRP has experienced an interdecadal change around the late 1970s. The present study identified a new change of the SRP around the late 1990s, which is characterized by significant weakening and zonal phase shift of the major centers of the SRP during the recent decades. The recent reshaping of the SRP is attributed to an enhanced impact of precipitation anomalies over the northeastern Indian summer monsoon (ISM), which is associated with the leading mode change of the ISM precipitation anomalies around the late 1990s. The interdecadal weakening of the upper-level westerly jet over central and East Asia also favors the southward movement of the SRP during recent periods. The differences of the features, climate impact, and causes related to the recent SRP change from those related to the SRP change around the late 1970s are also contrasted in this study. [ABSTRACT FROM AUTHOR]

The summer moisture circulation anomaly over East Asia and the western North Pacific (WNP) couples well with the El Niño-Southern Oscillation (ENSO) in a quasi-4-yr period. The moisture circulation is dominated by two well-separated modes. The first mode exhibits an anticyclonic (cyclonic) moisture circulation over tropical-subtropical East Asia-WNP with an easterly (westerly) transport over the tropical WNPIndian Ocean; the second mode displays an alternating pattern with an anticyclonic (cyclonic) moisture circulation over the subtropical WNP layered between two cyclonic (anticyclonic) circulations. Both modes couple well with the ENSO signal during its quasi-4-yr cycle. Within the cycle, in the summer of a developing warm episode, the positive phase of the second mode plays a key role, while in the transitional summer between a decaying warm episode and a developing cool episode, the positive phase of the first mode tends to take effect. In the summer of a developing cool episode, the negative phase of the second mode plays an important role, while the negative phase of the first mode tends to take effect in the transitional summer between a decaying cool episode and a developing warm episode. The anticyclone (cyclone) over the Philippine Sea region serves as a bridge in the quasi-four-year coupling. Its establishment and eastward extension modify moisture circulation over East Asia-WNP. Conversely, the easterly (westerly) wind to the south of the anticyclone (cyclone) is beneficial for the formation and eastward propagation of the Kelvin wave and, hence, to the development of the quasi-4-yr periodic ENSO episode. [ABSTRACT FROM AUTHOR]

This study examines the interannual variability of three groups of tropical cyclones (TCs)-super typhoons (STYs), typhoons (TYs), and tropical storms and tropical depressions (TSTDs)-and their relationship with the El Niño-Southern Oscillation (ENSO). Both wavelet analysis and correlation studies of upper-ocean heat content reveal significant differences for the three types of TCs. In particular, an increase (decrease) in the frequency of STYs is usually associated with the mature phase of El Niño (La Niña) events, while the converse is true for TSTDs. In contrast, the frequency of TYs increases (decreases) during the transition period from La Nina to El Niño (El Niño to La Nina) events. The results suggest that the timing with which ENSO impacts STYs, TYs, and TSTDs varies and that their corresponding changes in frequency closely follow the evolution of the ENSO cycle. Empirical orthogonal function analysis is also conducted to investigate the impact of different environmental factors influenced by ENSO on TCs. The vertical wind shear and moist static energy associated with ENSO are identified as the dominant factors that control the frequency of STYs. In comparison, the frequency of TYs is found to be closely related to the relative vorticity and vertical wind shear associated with both the transition phase of ENSO and with other types of climate variability. [ABSTRACT FROM AUTHOR]

The characteristics of the climatological distribution of the anticyclonic (LC1) and cyclonic (LC2) Rossby wave breaking (RWB) in the Southern Hemisphere (SH) are investigated by calculating the occurrence frequency of the LC1- and LC2-like stratospheric potential vorticity (PV) streamers in the SH during the austral summer [[December--February (DJF)]] and wintertime [[June--August (JJA)]] on several isentropic surfaces by using the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) daily dataset. The results show that 1) on the equatorward flank of the climatological midlatitude jet (MLJ), the LC1-like PV streamers are frequently found over the central oceanic regions, whereas the LC2-like PV streamers are almost absent. On the poleward flank of the climatological MLJ, both types of PV streamers are frequently observed and the LC2-like PV streamers predominate; 2) the regions where the occurrences of the PV streamers are frequent overlap the weak zonal wind regions; and 3) in austral winter, a ''double-jet'' setting is evident in two regions of the SH [[the double-jet upstream (DU) and the spilt jet region]]. In the double-jet setting regions, the LC1-like PV streamers are frequently found both in the DU and the split-jet regions, while the occurrence of the LC2-like PV streamers is frequent in the split-jet region but is rather infrequent in the DU region. [ABSTRACT FROM AUTHOR]