Change Of Synoptic-scale Wave Train Activity Over Western North Pacific In A Warming Climate

The change of summertime synoptic-scale wave train(SWT)activity over western North Pacific(WNP)since 1950 was investigated using reanalysis data from National Center for Environmental Prediction(NCEP).It is found that the intensity of the SWT has a rising trend,while its structure and phase propagation characteristics remain unchanged.Environmental factors responsible for the rising trend are investigated.By separating the whole period into three warming phases(P1: 1950-1958;P2: 1978-1986;P3: 2006-2014),we found that background low-level vorticity over the monsoon trough region increases from P1 to P2 but decreases from P2 to P3.So is low-level barotropic energy conversion.Thus the environmental dynamic factor alone could not explain the continuous rising SWT trend.The thermodynamic factors such as sea surface temperature,moisture and atmospheric instability,on the other hand,shows a clear step-by-step increasing trend.A non-dimensional synoptic activity index(SAI)that combines the dynamic and thermodynamic factors is then proposed.This index well captures the observed long-term trend of the SWT intensity.Furthermore,the data analysis results of CMIP5 show that the existing model data cannot simulate the changes of synoptic wave train in the northwest Pacific Ocean well,and only a few models can simulate the structure and propagation characteristics of such synoptic signal to a certain extent,but most of these models have defects in intensity simulation.