A numerical diagnosis of leading signals facilitating a “north-ridge south-trough” dipole for the early-2008 south-China freezing-rain events

Previous studies emphasize the important role of a “north-ridge versus south-trough” dipole (affecting the latitudes from 20° to 75° N around the Tibetan Plateau) of anomalous geopotential height ( ) in the early-2008 abnormal cryogenic freezing-rain-and-snow events in the southern part of China. The present study intends to extract the leading signal facilitating the dipole based on the numerical outputs of a full -linear model for diagnosing the global . Using this model built on full primitive equations in spherical-isobaric coordinates, we can further split the anomaly of (representing the component not explicitly associated with the Coriolis parameter and its meridional derivative ) into 15 components. With the model-output (mainly accounting for the dipole under the geostrophic balance) and matrices as the left and right singular vectors respectively, a maximum covariance analysis (MCA) is performed to extract the significant 2-4-day leading signal carried by the MCA mode in the upstream area of the dipole. This leading signal is mainly attributed to 1) the abnormally strong westerlies centered around the exit region of the Atlantic jet-stream and 2) the corresponding anomalous 950-300 hPa anticyclone to the south of the abnormally strong center of westerlies. The energy of the positive wave center around this jet exit region favors the downstream north-ridge while the energy of the negative wave center associated with the anomalous anticyclone favors the downstream south-trough.