Zhiqi Zhang

Eurasian teleconnection pattern (EU) and its two variants (EU1 and EU2) are the representative wintertime atmospheric teleconnections over Eurasian continent. They are mainly indicative of the local features and closely related to other teleconnection patterns. What are the major interannual climate modes over mid‐high latitude Eurasia in boreal winter is still an open question. With the ERA5 reanalysis data sets after removing the linear impact of El Niño‐Southern Oscillation (ENSO), three wintertime climate modes over mid‐high latitude Eurasia are identified by the first three empirical orthogonal function (EOF) modes of the anomalous relative tendency (ART) of 500 hPa geopotential height. They approximately explain 75% of the interannual variance in total. The three climate modes have combined features of EU‐like patterns with Arctic Oscillation (AO), North Atlantic Oscillation (NAO) and West Atlantic (WA) teleconnections, respectively, and they are named EU‐AO, EU‐NAO and EU‐WA climate modes accordingly. All the three climate modes originate mainly from the North Atlantic and demonstrate clear Rossby wave trains downstream to East Asia along the great circle route, and they can be primarily stimulated and maintained by positive air‐sea feedback over North Atlantic regarding to the obvious North Atlantic tripole‐like sea surface temperature patterns. Interannual climate variations over the most Eurasian continent are strongly linked to and well reproduced by the three ENSO‐independent climate modes, which can be applied as the important signals for monitoring and predicting winter interannual climate variabilities over mid‐high latitude Eurasia.

A three‐dimensional lake‐ice coupled model is used to investigate the space‐time variations of ice and underlying mechanisms in Lake Nam Co (LNC), the third largest lake over Tibetan Plateau (TP), during 2007–2017. The model reasonably reproduces the in situ measured ice thickness and water temperature profile, and satellite retrieved ice coverage and lake surface temperature. Seasonally, the lake ice first forms in the eastern basin during early January, expands from east to west during January and February, covers nearly the entire LNC in March, starts melting from west to east in April, and eventually disappears in May. The eastward drift of thin ice throughout the ice‐covered phase and the eastward water heat transport during the ice melting phase are key factors to determine the spatial variation of ice and freeze‐thaw processes. A multiple linear regression analysis confirms that the eastward drift of thin ice can be mostly attributed to the prevailing westerly. During 2007–2017, ice volume, duration, ice‐on and ice‐off dates show significant interannual variations, and they are highly correlated with the surface air temperature (T_2m) averaged over January‐March, from the preceding December to May, in December and over March–May, respectively, suggesting the “cumulative effects” of T_2m. Seasonal and interannual variations of ice drift are attributed to the combined effects of wind and ice volume variations. Sensitivity analysis further points out the important impacts of ice on the lake temperature and circulation structure in winter and spring, hence the necessity of hydrodynamic‐ice coupled models in large TP lakes.

The Meiyu season is a typical rainy season in East Asia that is controlled by summer monsoon. Despite extensive research on its impact, it is unclear how urbanization modifies precipitation during the Meiyu season in the background of the monsoon influence. To address this gap, this study investigated the effects of urbanization and monsoon on the modification of precipitation during the Meiyu season (PDM) in the megacity of Shanghai, China. Through homogenization analysis of the original observational data, we assessed the temporal and spatial variation in PDM in Shanghai during two stages of urbanization. Our findings revealed that both total precipitation and extreme daily precipitation during the Meiyu season in Shanghai have significantly increased since 1961. The spatial heterogeneity of PDM has also enhanced during the rapid urban process that has occurred since 1986. The long-term trend of increasing precipitation in Shanghai showed a synchronous variation with the East Asian subtropical summer monsoon (EASM) in 1961–2021. Over the interannual time scale, the significant positive correlation between PDM and EASM during the slow urbanization period (Stage 1: 1961–1985) changed to a non-significant correlation during the rapid urbanization period (Stage 2: 1986–2021), which was associated with the enhanced convective precipitation in Shanghai during the Meiyu season. Urbanization induced more convective precipitation and further weakened the association between PDM and EASM over the central city and nearby areas during Stage 2. The rapid urbanization process also resulted in increased differences in near-surface wind between urban and non-urban areas, which facilitated more PDM over the central city due to the urban friction effect and wind shear in Stage 2. Furthermore, our analysis suggests that the increase in precipitation may be associated with the enhanced coupling of cold air intrusion with the warmer climate background due to the urban heat effect occurring in Stage 2. These findings contribute to a better understanding of how urbanization and monsoons affect PDM in East Asian megacities and serve as a unique reference for climate prediction in this region.

Previous studies showed that random atmospheric forcing can stimulate oceanic low frequency variability in midlatitudes, since ocean has huge heat capacity that can redden the atmospheric high frequency variability. However, whether the oceanic stochastic forcing can in turn influence atmospheric variability is still unclear. To answer this question, two parallel 63‐year numerical experiments are conducted with an atmospheric general circulation model. In the control experiment, the atmosphere is forced by observed global sea surface temperature (SST). After adding stochastic forcings that are constrained by observed standard deviations of mesoscale SST anomalies (SSTAs) over the North Pacific in a sensitivity experiment, large‐scale atmospheric variability is greatly increased over the midlatitude northwest Pacific, especially on decadal timescale. Moreover, the anomalous atmospheric circulations associated with the Pacific Decadal Oscillation (PDO), like the enhanced westerly wind in the Kuroshio‐Oyashio and its extension (KOE) region, are more consistent with that resolved by a reanalysis model after the stochastic SSTAs are added. Dynamic diagnostics show that under the background of PDO warm phase, stochastic SSTAs can generate larger diabatic heating over the north of KOE region by inducing more moisture condensation in the mid‐lower troposphere, which is balanced by anomalous cold low‐level air temperature advection. Thus, the atmospheric baroclinicity is increased by the cold temperature advection anomaly, and the transient eddy activity and westerly jet are both enhanced above the KOE region accordingly. Our results suggest that to accurately represent the oceanic effect on large‐scale atmospheric variability and anomalies, oceanic stochastic SSTAs should also be considered.

East Asian summer monsoon precipitation (EASMP) features complicated interdecadal variability with multiple time periods and spatial patterns. Using century-long datasets of HadISST, CRU precipitation, and the ECMWF twentieth-century reanalysis (ERA-20C), this study examines the joint influence of three oceanic interdecadal signals [i.e., Pacific decadal oscillation (PDO), Atlantic multidecadal oscillation (AMO), and Indian Ocean Basin mode (IOBM)] on the EASMP, which, however, is found not to be simply a linear combination of their individual effects. When PDO and AMO are out of phase, the same-sign SST anomalies occur in the North Pacific and North Atlantic, and a zonally orientated teleconnection wave train appears across the Eurasian mid-to-high latitudes, propagating from the North Atlantic to northern East Asia along the Asian westerly jet waveguide. Correspondingly, the interdecadal precipitation anomalies are characterized by a meridional tripole mode over eastern China. When PDO and AMO are in phase, with opposite sign SST anomalies in the North Pacific and North Atlantic, the sandwich pattern of anomalous stationary Rossby wavenumber tends to reduce the effect of the waveguide in the eastern Mediterranean region, and the teleconnection wave train from the North Atlantic travels only to western central Asia along a great circle route, causing Indian summer monsoon precipitation (ISMP) anomalies. The ISMP anomalies, in turn, interact with the teleconnection wave train induced by the PDO and AMO, leading to a meridional dipole mode of interdecadal precipitation anomalies over eastern China. Through the impact on the ISMP, the IOBM exerts significantly linear modulation on the combined impacts of PDO and AMO, especially over northern East Asia.