Intraseasonal oscillations of the Silk Road pattern lead to predictability in East Asian precipitation patterns and the Mei Yu front

Abstract

The Silk Road pattern (SRP) is analysed on intraseasonal timescales over summer using empirical orthogonal functions (EOFs) of the meridional wind at 200 hPa. The first two principal components explain almost equal amounts of variance, hence both are required to represent the intraseasonal SRP. The associated spatial loadings are 90° out of phase with each other, providing evidence that propagating oscillations are a natural mode of variability of the intraseasonal SRP. This is supported by Hovmöller diagrams of the meridional wind at 200 hPa and by phase diagrams of the first two EOFs, which both show a predominantly eastward-propagating oscillation. The oscillations are identified as plausibly being Rossby waves by means of waveguide theory. The subtropical westerly jet and East Asian rainfall patterns are found to be dependent on the phase of the oscillation: wet anomalies occur to the east of troughs in the jet, which are also regions where local jet entrances cause upper-level divergence via an ageostrophic circulation. Dry anomalies occur to the west of troughs, which are regions of upper-level convergence. The time-delayed location of the summer Mei Yu front relative to its climatology is dependent on the phase of the oscillation: when there is an upper-level trough located over the Korean Peninsula, the Mei Yu front is likely to be located further north than normal 3–8 days later, before returning to its climatological position. This suggests that the phase of the intraseasonal SRP acts as a potential source of predictability of the location of the Mei Yu front, which might allow for better prediction of the associated rains.