Origins of the Decadal Predictability of East Asian Land Summer Monsoon Rainfall

Abstract

AbstractThe present study aims to explore the origins of decadal predictability of East Asian land summer monsoon rainfall (EA-LR) and estimate its potential decadal predictability. As a preliminary study, a domain-averaged EA-LR index (EA-LI) is targeted as it represents the leading mode of variability reasonably well. It is found that the decadal variations of EA-LI are primarily linked to a cooling over the central-eastern tropical Pacific (CEP) and a warming over the extratropical North Pacific and western tropical Pacific (NWP) during May–October. Two numerical experiments suggest that the CEP cooling may be a major driver of EA-LR, while the NWP warming, which is largely a response, cannot be treated as a forcing to EA-LR. However, this does not mean that the NWP sea surface temperature anomalies (SSTAs) play no role. To elaborate on this point, a third experiment is conducted in which the observed cooling is nudged in the CEP but the SST is nudged to climatology in the NWP (i.e., atmosphere–ocean interaction is not allowed). The result shows anomalous northerlies and decreased rainfall over East Asia. Results of the three experiments together suggest that both the forcings from the CEP and the atmosphere–ocean interaction in the NWP are important for EA-LR. Assuming that the tropical and North Pacific SSTAs can be “perfectly” forecasted, the so-called perfect prediction of EA-LI, which is achieved by a physics-based empirical model, yields a significant temporal correlation coefficient skill of 0.70 at a 7–10-yr lead time during a 40-yr independent hindcast (1968–2009), providing an estimation of the lower bound of potential decadal predictability of EA-LI.