Spatiotemporal structures of the intraseasonal oscillations of precipitation over northern Eurasia during summer

This study identifies intraseasonal oscillations in summer precipitation over northern Eurasia using multi-channel singular spectrum analysis (MSSA) and a gridded daily precipitation data set for northern Eurasia. The analysis period is JuneAugust of 1979-2002. Empirical orthogonal function analysis was first performed on 8-day low-pass-filtered precipitation anomalies at 2.5 degrees grid resolution for northern Eurasia; MSSA was then carried out on the ten leading principal components (PCs). Three quasi-periodic oscillations with timescales of 45, 15, and 9 days were identified. The spatiotemporal structures of precipitation oscillations were defined by composite analysis based on the reconstructed time series of the spatiotemporal PCs obtained from the MSSA. The composite life cycle of each mode was classified into eight phase categories. The 45-day oscillation is characterised by a slow eastward progression of a broad east/west contrastive pattern which is associated with a replacement of elongated dry and wet zones across northern Eurasia. The 15-day oscillation shows a regular eastward phase propagation of precipitation anomalies with a tripole structure across the domain. The eastward displacement of dry and wet zones occurs in association with this oscillation. Spatiotemporal behaviours of the precipitation anomalies associated with the 9-day oscillation are similar to those of the 15-day oscillation, but the spatial scale of this oscillation is somewhat smaller. To explore the connections between the oscillations of precipitation and large-scale atmospheric circulation patterns, a similar composite method was applied to 300-hPa geopotential height anomalies over the Northern Hemisphere. The circulation patterns responsible for the 45-day oscillation are characterised by a wave train extending across the northeastern Atlantic/northern Eurasian sector. The wave train, with zonal wavenumbers of 34, indicates slow eastward moving and quasi-stationary features. The circulation patterns linked to the 15-day oscillation exhibit an eastward propagating wave train extending from northeastern Europe into the North Pacific/North American sector. In spatial scale, this wave train has zonal wavenumbers of 56. The wave trains associated with the 9- and 15-day oscillations have similar behaviour patterns over northern Eurasian. However, the 9-day wave train has a comparatively smaller spatial scale with zonal wavenumbers of 67. Therefore, the intraseasonal oscillations of precipitation are connected to the well-organised wave trains that extend from the Euro-Atlantic region to northern Eurasia, suggesting that the oscillation on each intraseasonal time scale must be produced by propagation of the corresponding wave train. Copyright (C) 2011 Royal Meteorological Society