Changes in the summer seasonal cycle of lakes in the Inner Tibetan Plateau since the 21st century

In this study, 96 lakes on the Inner Tibetan Plateau (ITP) with an area larger than 40 km2 are selected and the time series of lake surface water temperature (LSWT) with a daily temporal resolution is decomposed using a seasonal-trend decomposition based on loess (STL). Annual changes in LSWT characterized by a trend component show that most of the lakes on the ITP are warming at a rate of 0.07 °C/10 yr, lower than the global average(0.34 °C/10 yr). The seasonal component is used to reclassify the lakes' summer season based on specific thresholds; indices characterizing summer include the summer onset (SO), summer end (SE), and summer duration (SD). On average, SO was essentially unchanged and SE has been delayed at a rate of 0.11 d/yr during 2000–2011. The situation changed considerably during 2012–2022, with SO and SE arriving earlier at a rapid rate of 0.34 d/yr and 0.18 d/yr, respectively. There is a strong link between these changes and climate factors in the period before the seasonal shift occurs, and the main climatic factors considered are air pressure, air temperature, downward surface solar radiation and wind speed. The asymmetric summer onset (SO)/ summer end (SE) variations (ASV) are quantified using a new index. The value of ASV characterizes both the variability of ITP lakes and the dominant factors that produce this variability. The classification of lakes in the ITP according to the value of ASV shows that most of the lakes in the ITP have prolonged summers and this prolongation was dominated by SO during 2012–2022, which is quite different from the situation in 2000–2011.