This article investigates the year-to-year variability of the onset of the South China Sea summer monsoon (SCSSM) and the possible influences exerted by the surface temperature anomalies over land and sea. Early and late monsoon onsets are related to the temperature anomalies in different regions. It is found that an early onset follows negative sea surface temperature (SST) anomalies in the central tropical Pacific (CP) Ocean during the preceding winter and spring, corresponding to a CP La Niña. In contrast, a late onset is preceded by the negative surface air temperature anomalies over land in the central Asian continent. Negative SST anomalies in the central-eastern equatorial Pacific Ocean and the associated warming in the western Pacific induce an anomalously enhanced Walker circulation. This anomalous Walker cell leads to an increase in convection, causing more latent heat release and a subsequent decrease of surface pressure. The anomalous Walker cell and the enhanced latent heat release weaken the Western North Pacific subtropical high and the Philippine Sea anticyclone, favouring a westerly flow from the Indian Ocean, resulting in an early SCSSM onset. On the other hand, negative land surface temperature anomalies cool the atmosphere over land, and locally modify the Hadley circulation, accompanied by the anomalous divergence in the low-level atmosphere over the western equatorial Pacific. This divergence anomaly reduces the latent heat release and strengthens the anticyclone in the Philippine Sea, thus preventing the westward extension of the westerlies from the Indian Ocean and causing a late SCSSM onset.

A composite analysis of Northern Hemisphere's midwinter tropospheric anomalies under the conditions of strong and weak stratospheric polar vortex was performed on NCEP/NCAR reanalysis data from 1948 to 2013 considering, as additional grouping criteria, the coincidental states of major seasonally relevant climate phenomena, such as ENSO, QBO and strong volcanic eruptions. The analysis reveals that samples of strong polar vortex nearly exclusively occur during cold ENSO states, while a weak polar vortex is observed for both cold and warm ENSO. The strongest tropospheric and near-surface anomalies are found for warm ENSO and weak polar vortex conditions, suggesting that internal tropospheric circulation anomalies related to warm ENSO constructively superpose on dynamical effects from the stratosphere. Additionally, substantial differences are found between the continental winter warming patterns under strong polar vortex conditions in volcanicallydisturbed and volcanically-undisturbed winters. However, the small-size samples obtained from the multi-compositing prevent conclusive statements about typical patterns, dominating effects and mechanisms of stratosphere-troposphere interaction on the seasonal time scale based on observational/reanalysis data alone. Hence, our analysis demonstrates that patterns derived from observational/reanalysis time series need to be taken with caution as they not always provide sufficiently robust constraints to the inferred mechanisms implicated with stratospheric polar vortex variability and its tropospheric and near-surface signature. Notwithstanding this argument, we propose a limited set of mechanisms that together may explain a relevant part of observed climate variability. These may serve to define future numerical model experiments minimizing the sample biases and, thus, improving process understanding.

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