Impact of large-scale circulation on the water vapour balance of the Tibetan Plateau in summer

Using the monthly Interim Reanalysis (ERA-Interim) provided by the European Centre for Medium-Range Weather Forecasts (ECMWF), water vapour transported by the large-scale circulation over the Tibetan Plateau (TP hereafter) for the period of 1979-2014 has been investigated. The TP is divided into five boundaries by a reference to the 3000 m contour, and the flux of horizontally and vertically integrated atmospheric water vapour over the TP with its surrounding has been calculated. The time-averaged vertically integrated water vapour fluxes enter the TP through the southern, southwestern, western and northern boundaries with the southern boundary contributing 414.8 × 106 kg s-1. And water vapour transport is not simply input or output through one boundary. The time-averaged fluxes of water vapour on each layer show that water vapour transport to the outside/inside TP comes from different level (above/below 600 Pa) of the eastern boundary and the main exchange of water vapour occurred below the level of 500 Pa, which cannot reach the higher layer of the southern and northern boundary. There is an outflow region from 76°E to 83°E on the vertical section of northern boundary. It may be related to the topography blocking effect of northwestern TP. Then, we chose Regional Westerly and Webster and Yang monsoon indices to describe the mid-latitude westerly and the South Asian Monsoon, respectively. Through the correlation coefficient between the vertically integrated water vapour transport flux and two circulation indices, it is found that in the same area, the influence of mid-latitude westerly shows the opposite effect of the South Asian Monsoon. In years of strong/weak mid-latitude westerly or weak/strong South Asian Monsoon, the water vapour transport from northern boundary becomes stronger/weaker and from southern boundary becomes weaker/stronger. Here, the water vapour transport processes are revealed to the influence of two large-scale circulation systems. © 2016 Royal Meteorological Society