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Teleconnection patterns associated with the Madden–Julian oscillation (MJO) and El Niño–Southern Oscillation (ENSO) impact weather and climate phenomena in the Pacific–North American region and beyond, and therefore accurately simulating these teleconnections is of importance for seasonal and subseasonal forecasts. Systematic biases in boreal midwinter ENSO and MJO teleconnections are found in eight subseasonal to seasonal (S2S) forecast models over the Pacific–North America region. All models simulate an anomalous 500-hPa geopotential height response that is too weak. This overly weak response is associated with overly weak subtropical upper-level convergence and a too-weak Rossby wave source in most models, and in several models there is also a biased subtropical Pacific jet, which affects the propagation of Rossby waves. In addition to this overly weak response, all models also simulate ENSO teleconnections that reach too far poleward toward Alaska and northeastern Russia. The net effect is that these models likely underestimate the impacts associated with the MJO and ENSO over western North America, and suffer from a reduction in skill from what could be achieved.

Using 17 CMIP5 and CMIP6 models with a spontaneously generated quasi-biennial oscillation (QBO)-like phenomenon, this study explores and evaluates three dynamical pathways for impacts of the QBO on the troposphere: 1) the Holtan–Tan (HT) effect on the stratospheric polar vortex and the northern annular mode (NAM), 2) the subtropical zonal wind downward arching over the Pacific, and 3) changes in local convection over the Maritime Continent and Indo-Pacific Ocean. More than half of the models can reproduce at least one of the three pathways, but few models can reproduce all of the three routes. First, seven models are able to simulate a weakened polar vortex during easterly QBO (EQBO) winters, in agreement with the HT effect in the reanalysis. However, the weakened polar vortex response during EQBO winters is underestimated or not present at all in other models, and hence the chain for QBO, vortex, and tropospheric NAM/AO is not simulated. For the second pathway associated with the downward arching of the QBO winds, 10 models simulate an inconsistent extratropical easterly anomaly center over 20°–40°N in the Pacific sector during EQBO, and hence the negative relative vorticity anomalies poleward of the easterly center is not present in those models, leading to no consensus on the height response over the North Pacific between those models and the reanalysis. However, the other seven models do capture this effect. The third pathway is only observed in the Indo-Pacific Ocean, where the strong climatological deep convection and the warm pool are situated. Seven models can simulate the convection anomalies associated with the QBO over the Maritime Continent, which is likely caused by the near-tropopause low buoyancy frequency anomalies. No robust relationship between the QBO and El Niño–Southern Oscillation (ENSO) events can be established using the JRA55 reanalysis, and 10 models consistently confirm little modulation of the ocean basinwide Walker circulation and ENSO events by the QBO.

Using the historical, moderate emission scenario (RCP45/SSP245), and high emission scenario (RCP85/SSP585) experiments provided by the Coupled Model Intercomparison Project Phases 5 and 6 (CMIP5/6), future changes of stratospheric final warming (SFW) events are explored in this study. Most CMIP5/6 models project a delay of SFWs in the two future scenarios, compared with historical simulation in both hemispheres (4–8 days shift in the multimodel mean). The projected delay in SFWs suggest a later seasonal transition from the climatological wintertime to summertime circulation in both hemispheres. In the Southern Hemisphere (SH), essentially all of the delay in the SFW occurs in the era with strong ozone depletion (1980–2040), and the SFW date is largely unchanged as ozone recovers through the end of the century. Both CMIP5 and CMIP6 multimodel ensemble means (MMEs) do not project any significant change in reversal of westerlies and the stratosphere-troposphere coupling strength during the Northern Hemisphere (NH) SFW. In contrast, both CMIP5 and CMIP6 MMEs project a significant decrease in the strength of SH SFW, but the lower tropospheric response to the SH SFW changes little during 1980–2040. However, the near-surface response to SH SFWs is projected to be significantly stronger during 2040–2100 than during 1980–2040, as well as in CMIP6 than in CMIP5. Biases in SFW over the historical period are generally larger in the NH than in the SH, and show little improvement from CMIP5 to CMIP6.

Using state-of-the-art models from the Coupled Model Intercomparison Project Phases 5 and 6 (CMIP5/6), future changes of sudden stratospheric warming (SSW) events under a moderate emission scenario (RCP45/SSP245) and a strong emissions scenario (RCP85/SSP585) are evaluated with respect to the historical simulations. Changes in four characteristics of SSWs are examined in 54 models: the SSW frequency, the seasonal distribution, stratosphere-troposphere coupling, and the persistency of the distorted or displaced polar vortex. The composite results show that none of these four aspects will change robustly. An insignificant (though positive) change in the SSW frequency from historical simulations to RCP45/SSP245 and then to RCP85/SSP585 is consistently projected by CMIP5 and CMIP6 multimodel ensembles in most wintertime months (December-March). This increase in the SSW frequency is most pronounced in mid- (late-) winter in CMIP6 (CMIP5). No shift in the seasonality of SSWs is simulated especially in the CMIP6 future scenarios. Both the reanalysis and CMIP5/6 historical simulations exhibit strong stratosphere-troposphere coupling during SSWs, and the coupling strength is nearly unchanged in the future scenario simulations. The near surface responds immediately after the onset of SSWs in both historical and future scenarios experiments, denoted by the deep downward propagation of zonal-mean easterly anomalies from the stratosphere to the troposphere. On average, the composite circumpolar easterly winds persist for 8 d in the reanalysis and CMIP5/6 historical experiments, which are projected to remain unchanged in both the moderate and strong emissions scenarios, implying the lifecycle of SSWs will not change.

An intermediate complexity general circulation model is used to isolate the effect of vertical resolution and gravity wave parameterization on the simulated monthly quasi‐biennial oscillation (QBO)‐tropical precipitation linkage. For low vertical resolution, the model is able to simulate QBO in the lowermost stratosphere, and its impact on the tropical upper troposphere‐lower stratosphere (UTLS) and precipitation, only after optimizing the gravity wave parameterization. For increased vertical resolution, the impact of the QBO on UTLS static stability is stronger. However, the tropical precipitation response is qualitatively different from that at low resolution. The precipitation response contains a substantial zonal and meridional structure that differs qualitatively between low and high vertical resolution. Two factors appear to explain this difference: the meridional width of the QBO, and the presence of a warmpool in the West Pacific. These results have implications for the ability of comprehensive models to simulate a tropical response to the QBO.

The distribution of near-surface and tropospheric temperature variability in midlatitudes is distinguishable from a Gaussian in meteorological reanalysis data; consistent with this, warm extremes occur preferentially poleward of the location of cold extremes. To understand the factors that drive this non-Gaussianity, a dry general circulation model and a simple model of Lagrangian temperature advection are used to investigate the connections between dynamical processes and the occurrence of extreme temperature events near the surface. The non-Gaussianity evident in reanalysis data is evident in the dry model experiments, and the location of extremes is influenced by the location of the jet stream and storm track. The cause of this in the model can be traced back to the synoptic evolution within the storm track leading up to cold and warm extreme events: negative temperature extremes occur when an equatorward propagating high–low couplet (high to the west) strongly advects isotherms equatorward over a large meridional fetch over more than two days. Positive temperature anomalies occur when a poleward propagating low–high couplet (low to the west) advects isotherms poleward over a large meridional fetch over more than two days. The magnitude of the extremes is enhanced by the meridional movement of the systems. Overall, horizontal temperature advection by storm track systems can account for the warm/cold asymmetry in the latitudinal distribution of the temperature extremes.

Stratospheric sudden warmings (SSWs) predominantly occur in the Northern Hemisphere (NH) with only 1 major event recorded in the Southern Hemisphere in the satellite era. Investigating factors that contribute to this asymmetry can help to reveal the cause of SSWs and lead to improved forecasts. Here we use climate model simulations to investigate the impact of boundary conditions (topography and ocean circulation) on the hemispheric asymmetry. Flattening topography eliminates NH SSWs, while removing the ocean meridional overturning circulation reduces their frequency by half. The SSW response to boundary conditions is controlled by the hemispheric asymmetry of eddy heat flux. The reduction is driven by a decrease in amplitude of both eddy meridional wind and eddy temperature, as well as an increase in the cosine of the difference between their phases. The results suggest boundary conditions play an important role in shaping SSWs, especially topographic forcing, but that the boundary condition interactions are nonlinear. Plain Language Summary Stratospheric sudden warmings (SSWs) are powerful events that affect surface weather and climate. They mostly happen in the Northern Hemisphere, with very few occurring in the Southern Hemisphere. Understanding why this happens is important. Using climate model simulations, we quantify how boundary conditions, such as topography and ocean circulation, affect SSWs. The results suggest topography is the primary factor influencing the difference between hemispheres in SSWs. Topography is shown to control how much heat is transferred poleward by deviations from the zonal mean, which are known to drive SSWs. More specifically, flattening topography leads to changes in the wave phase and amplitude of meridional wind and temperature, which in turn causes a decrease in poleward eddy heat flux. Key Points Climate model simulations are used to quantify the impact of topography and ocean circulation on stratospheric sudden warmings (SSWs) Topography is found to play a dominant role in shaping the hemispheric asymmetry of SSWs through its control on eddy heat flux Topography amplifies eddy heat flux by increasing the amplitude of eddy meridional wind and temperature while decreasing their phase difference

Using the Model of an Idealized Moist Atmosphere (MiMA) capable of spontaneously generating a quasibiennial oscillation (QBO), the gradual establishment of the extratropical response to the QBO is explored. The period and magnitude of the QBO and the magnitude of the polar Holton–Tan (HT) relationship is simulated in a free-running configuration of MiMA, comparable to that in state-of-the-art climate models. To isolate mechanisms whereby the QBO influences variability outside the tropical atmosphere, a series of branch experiments are performed with nudged QBO winds. When easterly QBO winds maximized around 30 hPa are relaxed, an Eliassen–Palm (E-P) flux divergence dipole quickly forms in the extratropical middle stratosphere as a direct response to the tropical meridional circulation, in contrast to the HT mechanism, which is associated with wave propagation near the zero wind line. This meridional circulation response to the relaxed QBO winds develops within the first 10 days in seasonally varying and fixed-seasonal experiments. No detectable changes in upward propagation of waves in the midlatitude lowermost stratosphere are evident for at least 20 days after branching, with the first changes only evident after 20 days in perpetual midwinter and season-varying runs, but after 40 days in perpetual November runs. The polar vortex begins to respond around the 20th day, and subsequently a near-surface response in the Atlantic Ocean sector forms in mid-to-late winter runs. These results suggest that the maximum near-surface response observed in mid-to-late winter is not simply due to a random seasonal synchronization of the QBO phase, but is also due to the long lag of the surface response to a QBO relaxation in early winter and the short lag of the surface response to a QBO relaxation in mid-to-late winter.

Stratospheric water vapor is a powerful greenhouse gas, and it can directly affect the radiative balance and temperature structure of the stratosphere. Although previous studies have investigated the water vapor variability associated with the quasi-biennial oscillation (QBO), the seasonal differences in the water vapor QBO are still not well understood. Using the ERA5 reanalysis and SWOOSH observations, this study compares the stratospheric water vapor distribution in northern winter and summer under different QBO phases. The QBO (represented by the 30 hPa QBO index) exerts the greatest influence on 100 hPa water vapor at a lag of six months. During northern summer, the peak amplitude of 100 hPa water vapor under different QBO phases in tropical regions reaches ±0.12 ppm at a six-month lag, while in winter it reaches ±0.2 ppm. The dehydration effect by cold temperature in the lower stratosphere is also more effective in boreal winter than in summer. The intensity of the QBO-related secondary circulation is stronger in the boreal winter than in summer, which not only influences the cold point tropopause temperature in tropical regions but also drives the transport of stratospheric water vapor. The mean vertical transport term via the QBO-related residual circulation is the leading factor controlling the water vapor distribution in the tropical lower stratosphere. Although the CMIP6 models simulated the lagged effect of the 30 hPa QBO on lower stratospheric water vapor, they tend to underestimate the water vapor amplitude, and the seasonal contrast is underrepresented in most models.

Southwest China experienced a severe drought during winter 2022–spring 2023. This drought mainly struck Yunnan Province and surrounding regions (21°–30° N, 97°–106° E), with precipitation deficit lasting for about 8 months from Oct 2022 to May 2023. The area-mean precipitation and surface soil moisture in the study region during the drought were both the lowest recorded for the same period since 1950. The Standardized Precipitation Evapotranspiration Index (SPEI) also reached its lowest level since 1950 at −2.76. Quantitative analysis shows that precipitation deficit and potential evapotranspiration (PET) increase contributed 71.36%, and 28.64% to the SPEI, respectively. Of the raw contribution of PET, 7.05% can in turn be attributed to the changes in precipitation. Using data from the CMIP6 Detection and Attribution Model Intercomparison Project (DAMIP), we found that anthropogenic forcing increased the likelihood of a PET anomaly such as the one during the drought by about 133 times, with a fraction of attributable risk (FAR) of 0.99 [0.98, 1.00]. For the precipitation anomaly, we obtained a FAR of 0.26 [−1.12, 0.70], suggesting that anthropogenic forcings may have little impact. The extreme drought also increased the risk of fires, with the Fire Weather Index reaching its second-highest value since 1950 and abnormally high burned areas observed by satellites.