Decadal change of spring extreme consecutive dry days in the Yangtze River Valley around the early 2000s: Synergistic effect of mega-El Niño/Southern Oscillation, Atlantic Multidecadal Oscillation, and Arctic sea ice

The Yangtze River Valley (YRV) is the main rice-growing region in China and agriculture production in YRV plays a vital role in ensuring food security. Spring is the key season of plowing preparation and sowing, and drought during this period could cause serious threats to agricultural activity in YRV. As a basic feature of drought, consecutive dry days (CDDs), especially the extreme-CDDs with long duration, could directly reflect the drying degree and serve as a good indicator of drought. Therefore, knowledge of variations and mechanisms of spring extreme-CDDs has significant implications for a comprehensive view of spring drought in YRV. Based on daily station precipitation data, the variability of spring extreme-CDDs in YRV is investigated. It is found that the extreme-CDDs in YRV experienced a significant decadal increase around the early 2000s. Associated with this decadal change, the Mongolian high (MH) and western North Pacific anticyclone (WNPA) are significantly intensified and weakened, respectively. The intensified MH and weakened WNPA lead to anomalous northerlies and water vapor divergence over YRV, providing favorable atmospheric conditions for more extreme-CDDs over the region. Further mechanism analyses suggest that the transition of mega-El Niño/Southern Oscillation (mega-ENSO) from the negative-phase to positive-phase contributes to the decadal weakening of WNPA. And the phase transition of Atlantic Multidecadal Oscillation (AMO) and decadal decrease of sea ice over the Barents Sea lead to intensified MH through exciting atmospheric wave train. Multiple linear regression shows that there could be a synergistic role of mega-ENSO, AMO, and sea ice over the Barents Sea in the decadal change in YRV extreme-CDDs around the early 2000s. Analysis on the simulation of 14 models in the Atmospheric Model Intercomparison Project (AMIP) experiment from phase 6 of the Coupled Model Intercomparison Project (CMIP6) shows that the models can reproduce the observed decadal intensification of MH and weakening of WNPA around the early 2000s, indicating the contribution of mega-ENSO, AMO, and sea ice over the Barents Sea to the decadal changes in MH, WNPA and extreme-CDDs in YRV.