Why a wetter Tibetan Plateau still faces 'hot-dry' extremes: The roles of ENSO and the North Atlantic

Since the 1950s, the Tibetan Plateau has experienced warming and increased rainfall. However, this overall wet trend does not mean that drought risk has decreased. Instead, the plateau now faces an increasing risk of compound hot-dry events, which are characterized by extreme heat and drought. These events can have serious impacts on ecosystems, water resources, glaciers, and the risk of secondary hazards like avalanches and landslides.

Yet, the factors that drive the year-to-year variability of these events are not well understood. A new study, led by Professor Tianjun Zhou at the Institute of Atmospheric Physics, Chinese Academy of Sciences, sheds light on this issue. The researchers found that two major climate modes, the El Niño Southern Oscillation (ENSO) and the Summer North Atlantic Oscillation (SNAO), play a crucial role in shaping the occurrence of compound hot-dry events.

ENSO conditions in the preceding winter significantly influence the number of compound hot-dry days in the following summer. During El Niño years, the number of these days increases by about 1.85 days, while during La Niña years, it drops by roughly 1.13 days. Interestingly, the impact of ENSO varies depending on whether it is an Eastern Pacific (EP) or Central Pacific (CP) type.

EP El Niño years see a more pronounced increase in hot days and drought days, attributed to changes in cloud radiative effects and clear-sky shortwave radiation. On the other hand, CP La Niña has the strongest suppressive effect on compound hot-dry events, reducing the number of hot and drought days. Local land-atmosphere interactions further amplify these effects over the southern Tibetan Plateau endorheic region, creating a self-reinforcing feedback cycle of hotter and drier conditions.

Additionally, the SNAO also contributes to the variability of these events over the eastern Tibetan Plateau. When the SNAO is in its positive phase, these compound hot-dry events become more frequent across the eastern plateau. However, the impact varies across different parts of the plateau, with the southeastern region experiencing reduced precipitation and cloud cover, while the northeastern region sees more warming due to changes in downward radiation under clear-sky conditions.

This study fills the gap between large-scale climate drivers and local surface processes, providing a comprehensive understanding of why these compound hot-dry events vary so much from year to year over the Tibetan Plateau. By linking ENSO and SNAO teleconnections with land-atmosphere feedbacks, the researchers have developed a physical framework to better understand and predict these extreme climate events, which can help improve seasonal predictions and risk assessments.