Relationship between summer rainfall anomalies and sub-seasonal oscillation intensity in the ChangJiang Valley in China

- There are high positive correlations between the summer precipitation anomaly and the intensity of the QBW (10-20 days) and ISO (20-60 days) modes in ChangJiang Valley.
- The QBW mode is characterized by a northwest-southeast oriented wave train pattern, moving southeastward. The perturbations associated with the ISO mode propagate northwestward in strong ISO years but southeastward in weak ISO years.
- A marked feature is the phase leading of low-level moisture to convection in both the QBW and ISO mode.
- The significant positive correlation between the summer precipitation and the strength of sub-seasonal variability arises from the large-scale control of the summer mean flow to perturbations.

Sub-seasonal variability of summer (May-October) rainfall over the ChangJiang Valley exhibits two dominant timescales, one with a quasi-biweekly (QBW) period (10-20 days) and the other with an intraseasonal oscillation (ISO) period (20-60 days). A significant positive correlation (at a 99% confidence level) was found between the summer precipitation anomaly and the intensity of the QBW and ISO modes in the region. By examining the composite structure and evolution characteristics, we note that the QBW mode is characterized by a northwest-southeast oriented wave train pattern, moving southeastward. The perturbations associated with the ISO mode propagate northwestward in strong ISO years but southeastward in weak ISO years. A marked feature is the phase leading of low-level moisture to convection in both the QBW and ISO mode. When the summer rainfall is strong in the ChangJiang Valley, large-scale atmospheric conditions in the strong QBW/ISO activity region are characterized by deeper moist layer, convectively more unstable stratification and greater ascending motion. Such mean conditions favor the growth of the QBW and ISO perturbations. Thus, a significant positive correlation between the summer precipitation and the strength of sub-seasonal variability arises from the large-scale control of the summer mean flow to perturbations.