Subseasonal-to-Seasonal Predictability of Onset Dates of South China Sea Summer Monsoon: A Perspective of Meridional Temperature Gradient

Abstract

AbstractThe onset of the South China Sea summer monsoon (SCSSM) has traditionally been ascribed to the El Niño–Southern Oscillation (ENSO) on an interannual timescale, but the two do not correspond in some years. The present study applies harmonic analysis on the meridional temperature gradient (MTG) in mid–upper troposphere over South China Sea (SCS) and decomposes the onset process to be a slow-varying seasonal cycle and transient subseasonal component. The ENSO-related air temperature anomaly in the southern SCS provides seasonal predictability of SCSSM onset by a stable and robust relationship between ENSO and MTG seasonal cycle. However, in the northern SCS, the MTG is regulated by an intraseasonal oscillation (ISO) of extratropical air temperature with a significant 10–30-day period. This ISO originates over the western TP, then propagates eastward and gets enhanced by anomalous diabatic heating due to spring rainfall anomaly over South China, as a result of subseasonal thermal forcing of TP. When the ISO arrives to the north of the SCS, it directly changes the tropospheric temperature to modulate the MTG. Meanwhile, the upper-level circulation associated with the ISO alters the meridional potential vorticity advection and pumping effect, followed by the anomalous low-level westerly wind and monsoon convection over the SCS. The SCSSM onset is evidently disrupted from its seasonal cycle when this ISO is more active. Since the independence of its intensity from ENSO, this extratropical ISO over TP and South China provides additional subseasonal predictability of the onset dates of the SCSSM.