Climatological Characteristics of Flow around and Flow over the Tibetan Plateau

In this study, surface wind data from NCEP/NCAR for 1951-2008 are decomposed into flow around and flow over the Tibetan Plateau (TP) according to equations that control the surface flow near the orography, and the climatological characteristics of the flows around and over the TP are examined by calculating their zonal and meridional components. The results indicate that the annual mean flow around the TP bifurcates on the southwest part of the TP at nearly (32°N, 75°E), presenting an approximately anti-cyclonic pattern in the north and a cyclonic pattern in the south downstream; in contrast, the annual mean flow over the TP diverges along the Himalayas, with southerly upslope winds in the main body and northerly upslope winds in the northeastern region. In summer, the flow around the TP manifests a cyclonic pattern, with its center located at nearly (35°N, 90°E) on the central TP; in autumn, it presents mainly an anti-cyclonic pattern along the edges. The flow over the TP converges approximately at the north-south midline of the TP in summer, but it diverges along the Himalayas in other seasons. The flows around and over the TP were also found to have different seasonal evolution characteristics in the main body of the TP and key areas nearby. On the basis of the decomposition equations, the relationship between the different surface flow components and the terrain height is also investigated. The ratio of the zonal and meridional components of flow around and flow over the TP were found to remain constant, suggesting that they rely only on the topography gradient of the locations. The annual mean intensities of flow around and over the TP all increase with increasing altitude, and the flow around prevails in terms of the regional distribution, in that there are more regions in which flow around the TP is stronger than flow over the TP. The vertical motions forced mechanically by the TP correspond strongly with those in the observational data by position, but a discrepancy exists in the magnitude in both summer and winter.