## 5 Variability of QBIE Temperature Events
### Long-Term Variability  
We first examine the long-term variability in the occurrence frequency, intensity, and duration of QBIE heat wave and cold surge events in Hong Kong. Although a number of research studies have reported that climate change leads to more frequent and stronger extreme weather events, our results suggest that this is not entirely applicable to QBIE temperature events in Hong Kong. When considering all types of QBIE events together, we observe statistically significant increasing trends only in the occurrence frequency (+0.11 events/decade) and the number of event days (+1.24 days/decade) of QBIE cold surges from 1885 to 2022. However, those of QBIE heat waves exhibit insignificant changes over the same period. In addition, no statistically significant trends are observed in the average intensity and duration of either QBIE heat waves or cold surges in the past 138 years (Table 4). These indicate that the overall statistics of QBIE temperature extremes in Hong Kong have not undergone substantial changes under the past anthropogenic warming climate. The only notable change is a modest increase in the occurrence of QBIE cold surge events. Similar results are observed across different types of QBIE temperature events, as most long-term trends from 1885 to 2022 are statistically insignificant (Table 4). Significant changes are observed only in specific categories and metrics of QBIE events. For example, for the number of event counts, significant increasing trends are found solely for CDN heat waves and cold surges. This indicates that while climate change has increased the likelihood of the strongest type of QBIE temperature events, it has not similarly affected other types. However, despite the increase in CDN heat wave and cold surge events since 1885, the long-term trends in the annual average event days for these events remain statistically insignificant. This may be due to the lack of significant changes in the average event durations of CDN heat waves and cold surges. On the other hand, significant increases in average annual event days are observed for WNT (+0.46 days/decade) and SNT (+0.74 days/decade) cold surges, indicating that Hong Kong has experienced more nights affected by QBIE \begin{table}
\begin{tabular}{c c c c c c c} \hline  & **ALL** & **CDN** & **WNT** & **SNT** & **WDT** & **SDT** \\ \hline Event count (heat wave) & 0.05 & **0.05** & \(-\)0.01 & \(-\)0.02 & 0.02 & \(-\)0.00 \\ Event count (cold surge) & **0.11*** & **0.04** & 0.01 & 0.02 & 0.01 & 0.03 \\ Event intensity (heat wave) & 0.01 & 0.01 & 0.01 & \(-\)0.02 & \(-\)0.02 & 0.01 \\ Event intensity (cold surge) & 0.00 & \(-\)0.00 & \(-\)0.02 & \(-\)0.01 & \(-\)**0.05*** & 0.03 \\ Event duration (heat wave) & \(-\)0.021 & 0.03 & \(-\)0.10 & \(-\)0.06 & 0.08 & 0.01 \\ Event duration (cold surge) & 0.044 & \(-\)0.06 & **0.24*** & \(-\)0.02 & 0.06 & 0.17 \\ Event days (heat wave) & 0.40 & \(-\)0.16 & 0.01 & \(-\)0.24 & 0.46 & 0.15 \\ Event days (cold surge) & **1.24*** & 0.44 & **0.46*** & **0.74*** & \(-\)0.04 & 0.38 \\ \hline \end{tabular}
\end{table} 
Table 4: Long-term linear trends of event count (unit: events/decade), intensity (unit: \({}^{\circ}\)C/decade), duration (unit: days/decade), and event days (unit: days/decade) of QBIE heat waves and cold surges from 1885 to 2022. Note that data from 1940 to 1947 were not included in the calculations. Asterisks indicate the confidence levels: 1 asterisk (*) = 90%, 2 asterisks (**) = 95%, and 3 asterisks (***) = 99%. cold surges. Notably, the average event duration of WNT cold surges has increased significantly by 0.24 days/decade. In terms of average event intensity, a significant decreasing trend is observed only for WDT cold surges (\(-0.05\)\({}^{\circ}\)C/decade), while the others exhibit insignificant trends (Table 4). Overall, the above results indicate that most statistics of QBIE temperature events in Hong Kong do not show significant changes from 1885 to 2022.