Nonlinear Meridional Moisture Advection and the ENSO-Southern China Rainfall Teleconnection

In the boreal cooler months of 2015, southern China (SC) experienced the largest rainfall since 1950, exceeding 4 times the standard deviation of SC rainfall. Although an El Nino typically induces a positive SC rainfall anomaly during these months, the unprecedented rainfall increase cannot be explained by the strong El Nino of 2015/2016, and the dynamics is unclear. Here we show that a nonlinear meridional moisture advection contributes substantially to the unprecedented rainfall increase. During cooler months of 2015, the meridional flow anomaly over the South China Sea region, which acts on an El Nino-induced anomalous meridional moisture gradient, is particularly large and is supported by an anomalous zonal sea surface temperature gradient over the northwestern Pacific, which recorded its largest value in 2015 since 1950. Our study highlights, for the first time, the importance of the nonlinear process associated with the combined impact of a regional sea surface temperature gradient and large-scale El Nino anomalies in forcing El Nino rainfall teleconnection. Plain Language Summary El Nino affects regional climate extremes, but the teleconnection is not robust and a linear relationship often underestimates the impact, as in 2015 over Southern China. We found that nonlinear meridional moisture advection, involving the combined impact of a regional SST gradient over the northwest Pacific and large-scale El Nino-induced moisture anomalies, significantly modulates the teleconnection. The nonlinear process is important and must be taken into account in evaluating and predicting El Nino's impact. This result is of interest to El Nino modelers, seasonal forecasters, and broader communities in general