Impact of the March Arctic Oscillation on the South China Sea summer monsoon onset

This study reveals that the South China Sea summer monsoon (SCSSM) onset is closely linked to the preceding March Arctic oscillation (AO), the dominant mode of atmospheric circulation over extratropical Northern Hemisphere. The physical processes for the impact of the March AO on the SCSSM onset are further examined. March AO leads to an anomalous low‐level cyclone over subtropical western North Pacific (WNP) via eddy–mean flow interaction. The southwesterly wind anomalies to the southeastern side of the anomalous cyclone weaken the climatological trade winds, which contribute to warm sea surface temperature (SST) anomalies over tropical western‐central North Pacific via reduction of surface evaporation and latent heat flux. These warm SST anomalies excite an atmospheric Rossby wave response and reinforce the anomalous cyclone over the subtropical WNP. Via the above positive atmosphere–ocean interaction, the anomalous cyclone induced by the March AO could persist through the whole spring and shift equatorward. This anomalous cyclone creates favourable environment for the SCSSM onset via weakening the WNP subtropical high and is conducive to the transition of the low‐level zonal wind. Further analysis indicates that the March AO‐SCSSM onset relationship is independent of El Niño‐Southern Oscillation (ENSO). This result suggests that, besides the dominant tropical system (i.e., ENSO), interannual variation of the SCSSM onset also has a close relation with the dominant extratropical atmospheric system (i.e., AO), which may provide additional source for the seasonal prediction of the monsoon onset. Figure presents (a) the time series of original SCSSM onset date (grey bar) and the March AO index (black curve). And (b) is the same as (a), but for the 10‐year highpass filtered data. Their correlation (partial correlation after removing the preceding ENSO) is −0.371 (−0.375) and is significant at 95% confidence level.