Haibo Hu

Based on atmospheric reanalysis data from 1979 to 2024 during the cold season, this study investigates the evolutionary mechanisms and predictability of the leading EOF mode of the Eurasian subseasonal surface air temperature (SAT). The results indicate that the early‐stage anomalous subseasonal signal appears to be associated with Arctic stratospheric potential vorticity (PV) anomalies and subsequently propagates southeastward toward Eurasia. The tripolar pattern of North Atlantic sea surface Net Turbulent Heat Flux (NTHF) anomalies relates to the secondary enhancement of the geopotential height anomalies over Eurasia by transmitting eastward wave activity flux. This enables the subseasonal signal to propagate downward into the lower‐tropospheric temperature field. To quantify the contribution of each factor, a piecewise potential vorticity inversion method is employed. The analysis indicates that the Arctic stratospheric PV intrusion and the upper‐tropospheric dynamic processes associated with the North Atlantic tripolar NTHF anomalies are the dominant factors regulating geopotential height anomaly during the SAT subseasonal events. In contrast, mid‐tropospheric latent heat release makes no significant contribution, while lower‐tropospheric static stability and baroclinicity even suppress its development. Based on these findings, two predominant predictors are defined: the 8‐day leading Arctic PV index and the 4‐day leading North Atlantic tripolar NTHF index. Using a bivariate linear regression model, the Eurasian cold‐season subseasonal SAT can be preliminary predicted. Validation results show a daily correlation coefficient of 0.41 between the predicted and observed first principal component, providing theoretical support for improving the forecast accuracy of the Eurasian cold‐season subseasonal SAT variability.

This study examined the effects of air‐sea coupling and oceanic mesoscale eddies on wintertime low‐level wind events within the marine atmospheric boundary layer (MABL) over the Kuroshio front zone in the East China Sea. High‐resolution (eddy‐resolving, 5 km) and low‐resolution (50 km) simulations were conducted using prescribed sea surface temperature (SST) forcing and air‐sea coupled simulations in both resolutions. Among all simulations, the eddy‐resolved air‐sea coupled (EC) one exhibited the strongest atmosphere‐to‐SST feedback, closest to the reanalysis data. In the EC simulation, complex interactions of numerous oceanic eddies enhanced the oceanic front intensity. Compared with uncoupled and low‐resolution simulations, it better reproduced wind event statistics from reanalysis. Highly resolved eddy pairs with positive northwest‐to‐southeast SST anomalies strengthened SST gradients, altering sea surface heat fluxes and MABL stability, and thereby distinctly modulating wind event durations. Specifically, short‐lived events were suppressed while long‐lived ones persisted, consistent with reanalysis. These effects are attributed to the influence of numerous oceanic eddy pairs by enhancing oceanic front intensity, air‐sea temperature differences, sea surface heat fluxes, and lower atmospheric stability. Anomalous winds were restrained during initiation but prolonged as events evolved.

Both conventional (cold cyclonic and warm anticyclonic) and unconventional (warm cyclonic and cold anticyclonic) oceanic eddies have been frequently observed near the Kuroshio axis in the East China Sea (ECS), as identified from daily oceanic reanalysis data sets during winter months since 1993. These four types of eddies exhibit remarkably different temporal evolution characteristics and spatial distribution patterns along with contrasting 3D thermal and dynamic structures. Each eddy type demonstrates significantly varying intensities in multiple oceanic and atmospheric physical quantities, including sea surface temperature (SST) anomalies, vorticity fields, and heat flux exchanges. In particular, the air‐sea coupling relationships presented by the correlations between SST and air‐sea turbulent heat flux over the conventional and unconventional eddies exhibit opposite characteristics, which are likely to contribute to longer (shorter) lifespans of conventional (unconventional) eddies. In addition, the difference in air‐sea heat flux anomalies associated with the four eddy types determines the corresponding responses of the lower and middle atmosphere over the oceanic eddies, including the 3D air temperature, horizontal wind, vertical velocity at various pressure levels, and atmospheric boundary layer height.

This study investigates that the concurrent influence of El Niño‐Southern Oscillation (ENSO) on the Arctic Oscillation (AO) mediated through the poleward translation of extra‐tropical cyclones (ECs) over the North Atlantic is more significant than the one‐year‐lagged impact of ENSO through poleward propagating atmospheric angular momentum. Specific results show that during El Niño (La Niña) winter, the anomalous atmospheric horizontal heat advection from the Pacific to the Atlantic, which is caused by the southward (northward) displacement of the westerly jet stream, enhances (weakens) the atmospheric baroclinicity over the subtropical North Atlantic. Subsequently, the changed baroclinicity drives intensified (reduced) baroclinic energy conversion from the eddy available potential energy to the eddy kinetic energy, which shifts the genesis locations of ECs southward (northward) and suppresses (enhances) their poleward translation into the Arctic. Ultimately, through the combined thermodynamic and dynamical forcing associated with EC activity, the negative (positive) AO pattern is generated in the concurrent winter.

East Asia is a region characterized by a typical monsoon climate, which is accompanied by strong precipitation with complex spatiotemporal variability during summer. Previous works have emphasized the impact of tropical signals on extreme summer precipitation over East Asia, but the roles of the mid-high latitude cyclones are still unclear. Using a reanalysis dataset, this study discloses the synergistic influences of anomalous signals from different latitudes on the extreme precipitation event in the Beijing-Tianjin-Hebei (BTH) region during the summer of 2023. The main conclusions are obtained as follows: the decreased sea ice density caused more Arctic cyclones to generate at positions further west in the Barents Sea and the west of the Kara Sea and then move southeast to East Asia in 2023. Furthermore, the synergistic influences of the outward Arctic cyclones and anomalous signals from middle and low latitudes are discussed. First, the significant northward jump of the Western Pacific Subtropical High (WPSH) provided the favorable condition of large-scale background circulation for summer precipitation in the BTH region in 2023. In addition, the southward intrusion of the Arctic cyclones and the mid-latitude zonal wave trains transported massive cold air to the BTH region. Subsequently, the cold air masses met with the warm moist air carried by the landfall typhoon ‘Doksuri’, which generated strong fronts and triggered the extreme precipitation on July 29. However, another severe typhoon, ‘Kanu’, generated and moved northward from the tropical Pacific, which caused the further northward shift of the WPSH and the termination of this persistent extreme precipitation on August 1.

In boreal winter, the Madden‐Julian Oscillation (MJO) is the dominant mode of tropical intraseasonal oscillation (ISO), which bridges weather and climate. In addition to the dominant eastward propagating MJO mode, this study identifies the existence of the westward propagating and stationary oscillating ISO modes according to the spatio‐temporal evolution of large‐scale precipitation. At the pentad timescale, each ISO mode tends to suppress the occurrence of the other two. The diagnostic of column‐integrated moist static energy (MSE) tendency anomalies reveals the dominant contribution from horizontal MSE advection to each ISO mode. Synthesis results indicate that vertical zonal wind shear anomalies exhibit opposite distributions between the propagating and stationary oscillating ISO events over the tropical Indo‐Pacific Oceans. Subsequent simulations prove that the easterly (westerly) wind shear anomalies could suppress the eastward (westward) propagating convection of the ISO, thus favoring a stationary oscillating ISO.

This study examines the cross‐seasonal influence of the Arctic Oscillation (AO) on winter El Niño, using atmospheric and oceanic reanalysis data sets. It has been revealed that the linkage between positive AO and subsequent El Niño is closest during spring, indicating the seasonal dependent and selective impact of AO on ENSO. Composition analyses illustrate that positive spring AO is often associated with a dipole circulation anomaly over the North Pacific. The corresponding cyclonic circulation anomaly in the subtropical North Pacific initiates the Pacific Meridional Mode (PMM), which persists and evolves through the wind‐evaporation‐SST (WES) feedback from, triggering the westerly anomalies in the tropical western Pacific and subsequently triggering the onset of El Niño. Further clustering analysis reveals that the positive spring AO events exhibit two distinct spatial patterns: Type1 with prominent positive SLP anomaly over the midlatitude North Pacific and Type2 with the positive SLP anomaly over the North Atlantic. Both types can trigger El Niño by stimulating the subtropical cyclone anomaly in the North Pacific but through different mechanisms. In Type1 events, high‐pressure anomalies in the midlatitude North Pacific trigger southward‐propagating atmospheric waves, generating the cyclone anomalies in the subtropical North Pacific. Although in Type2 events, the AO‐related circulation anomalies in the North Atlantic stimulate teleconnection wave trains that cross the Eurasian continent and propagate southward to the subtropical North Pacific, inducing the cyclone anomalies. However, Type1 events occur more frequently and are more closely linked to El Niño, highlighting the significant role of air‐sea coupling processes in the North Pacific in triggering El Niño.

The Arctic Oscillation (AO) is the most significant mode of sea level pressure (SLP) anomalies in the Northern Hemisphere, exhibiting significant multiple‐timescale variability from synoptic to decadal. Using NCEP Climate Forecast System Reanalysis data from 1979 to 2022 during the cold season (November–April), this study identifies the relationship between the number of extra‐tropical cyclones entering and exiting the Arctic and the AO synoptic variability. The Joint Net Cyclone Flux (JNCF) is significantly correlated with the spatio‐temporal evolution of the synoptic AO and the composites of SLP associated with the JNCF produce AO‐like patterns. Subsequent piecewise potential vorticity inversion reveals the impacts of extratropical cyclones on the synoptic‐scale AO‐like geopotential height anomalies at different altitudes. The effects of extratropical cyclones are more important than Arctic stratospheric PV intrusions. Furthermore, the upper‐level dynamic processes among all extratropical cyclone effects dominate the evolution of synoptic‐scale AO‐like geopotential height anomalies.