Haipeng Yu

In June 2024, the Eastern Mediterranean experienced its hottest June since 1960. However, the relative contributions of anthropogenic forcing and natural variability to this event, and the roles of atmospheric circulation and soil moisture, remain unclear. Using ERA5 reanalysis and HadGEM3‐A‐N216 attribution simulations, we estimate that anthropogenic forcing contributes roughly half of the temperature anomalies, through greenhouse gas‐induced warming and through soil drying that amplifies the heatwave via soil moisture‐temperature feedback. The remaining half attributable to natural variability is dominated (>90%) by an anomalous anticyclone driven by an upstream wave train and associated with positive North Atlantic SST anomalies. Additionally, wetter soil within the natural variability component, caused by above‐normal preceding precipitation, produces a slight cooling that partially offsets the circulation‐induced temperature anomalies. These findings highlight contrasting soil moisture roles during this event, as anomalously wet soil within the natural variability component prevented an even more intense heatwave.

Spring precipitation over western Central Asia (SCAP) experienced an abrupt and unprecedented wetting during 1988–2007, yet the relative contributions of external forcing and internal variability to this change and the underlying drivers remain unclear. Using observation data sets and large ensemble simulations, our study demonstrates that this wetting is dominated by internal climate variability. Specifically, mid‐high‐latitude North Atlantic sea‐surface temperature (NAS) variability drives interdecadal SCAP changes by exciting a Eurasian Rossby wave train, which enhances ascent motion over western Central Asia and promotes moisture transport into the region. A negative‐to‐positive NAS phase transition explains 44.82% of the 1988–2007 wetting, far exceeding the externally forced contribution (21.72%). Under mid‐intensity emission scenario, removing NAS‐related rainfall variability reduces near‐term SCAP projection uncertainty by 28.39%, highlighting NAS as a key source of predictability for hydroclimate risk on western Central Asia.

Storm tracks play a vital role in modulating midlatitude weather and global heat, momentum, and moisture transport. Under global warming, their position and intensity are projected to undergo substantial changes. Using Coupled Model Intercomparison Project Phase 6 simulations, this study examines future storm track responses and the underlying dynamics. Storm tracks are diagnosed using a classical Eulerian metric—synoptic-band (2.5- to 6-d) band-pass-filtered geopotential-height variance—while the mechanisms are quantified through a full-form local eddy kinetic energy budget that partitions baroclinic conversion, barotropic conversion, energy redistribution, and geopotential (pressure) flux convergence. Results show a weakening tendency of Northern Hemisphere storm tracks in June–July–August, consistent with recent evidence of boreal-summer storm track weakening, but a more regionally structured intensification accompanied by a poleward displacement in December–January–February. In the Southern Hemisphere, storm tracks intensify in June–July–August and shift poleward in December–January–February. These changes are linked to enhanced background baroclinicity measured by the Eady growth rate, arising mainly from strengthened meridional temperature gradients with a secondary contribution from changes in static stability. Interpreted through the eddy kinetic energy budget, strengthened high-latitude baroclinic conversion, together with weakened barotropic damping and energy redistribution by flux convergence, supports an upper-tropospheric increase and poleward displacement of transient eddy activity, highlighting the central role of eddy energy conversion and redistribution processes in shaping storm tracks in a warming climate.

Northwest China (NWC) has experienced “warming and wetting” in recent decades. Reliable projections of temperature and precipitation are crucial for adaptation planning, yet CMIP6 models exhibit substantial spread. Using 26 CMIP6 models, we decompose projection uncertainty into internal variability, model differences, and scenario choice. During 1960-2020, the multi-model mean overestimates warming and underestimates wetting in NWC. For temperature, internal variability dominates near term projections (~54%), whereas scenario uncertainty prevails by the end of the century (~65%), with model uncertainty remaining substantial throughout (~27–49%). For precipitation, internal variability is overwhelmingly dominant (>80%) from near- to mid-term projections. Selecting high-skill models reduces projected anomalies and associated uncertainty for both temperature and precipitation. For the near-term future, temperature and precipitation anomalies decrease from 2.08 ± 0.92 °C and 6.58 ± 19.41 mm to 1.69 ± 0.77 °C and 5.87 ± 18.15 mm, respectively, after model selection. Across future periods, high-skill model selection reduces projected temperature uncertainty by ~16.3–20.9% (mainly from model uncertainty), whereas precipitation uncertainty is reduced only by ~3.6–6.5% (arising primarily from internal variability and model uncertainty). Overall, model selection cannot effectively reduce the substantial uncertainty associated with internal variability, particularly for near-term precipitation changes. Internal variability remains the primary source of near-term uncertainty, while scenario uncertainty dominates mid- to long-term temperature projections. These results indicate that isolating and representing key sources of internal variability will be critical over the next 10–30 years to improve NWC climate projections, thereby informing model development and adaptation decisions.

The Tibetan Plateau (TP) is well known for its unique summer monsoon structure driven by thermal air pumping. This structure is characterized by low-level convergence and upper-level divergence. Based on JRA-3Q reanalysis data, this study proposes an Index named Pumping Plateau Monsoon Index (PPMI). This index is defined as the divergence difference between 200 hPa and 600 hPa and is used to characterize the intensity of the TP thermal pumping effect and its associated three-dimensional circulation structure. The results show that the PPMI is significantly correlated with the TP heat source, the east-west displacement of the South Asian High (SAH), and circulation changes over the East Asian Summer Monsoon (EASM) region. During weak Tibetan Plateau Summer Monsoon (TPSM) years, an anomalous anticyclonic circulation is observed over the Iranian Plateau, which is associated with a westward displacement of the SAH toward the Iranian High mode. In contrast, strong TPSM years correspond to a cyclone–anticyclone–cyclone–anticyclone wave-train-like pattern extending from the Iranian Plateau and the TP to eastern China and the Korean Peninsula (KP). The associated upper-tropospheric circulation anomalies over the IP favor changes in the meridional potential vorticity gradient and are accompanied by bifurcation of the Rossby wave train, weakened eastward propagation, and reduced downstream wave activity energy transport. When the TPSM intensifies, anomalous descending-ascending-descending-ascending motion develops from the IP and the TP to Eastern China and the KP. These vertical motion anomalies further modulate the East Asian Trough and moisture transport over the EASM region. Further analyses of the intraseasonal evolution of TPSM and EASM precipitation indicate that the TPSM related teleconnection exhibits nonlinear characteristics. Specifically, the positive phase in June is characterized by decreased precipitation over Eastern China and increased precipitation over the KP, whereas the opposite pattern appears in July-August. This study reveals an important dynamical linkage through which TPSM thermal forcing influences EASM circulation anomalies. The results provide a new perspective for understanding the relationship between TPSM and East Asian summer climate and sub-seasonal variability.