The role of internal variability in multi-decadal trends of summer rainfall over East Asia-Northwest Pacific

The impacts of internal variability on East Asia-Northwest Pacific (EA-NWP) summer rainfall trends on the multidecadal time scale are invested based on three large ensemble simulations, which have ensemble member of 30, 40 and 100. In all the three simulations, the summer rainfall trends during 1970-2005 are remarkably diverse across the individual ensemble members over the EA-NWP, and the signal-to-noise ratio is lower than 1 over the EA-NWP, suggesting a strong impact of internal variability on EA-NWP summer rainfall trends at this interval. Moreover, we found that the diversity of EA-NWP summer rainfall trends across individual members has a similar leading spatial pattern in all the three ensembles, featuring reverse trends between in Mei-yu region and in the tropical NWP. The leading pattern is likely caused by a gradient between the sea surface temperature (SST) trends in the North Indian Ocean (NIO) and in the tropical western Pacific (WP). When there is a warming trend in the NIO and a cooling trend in the tropical WP, a low-level anomalous anticyclone strengthens over the subtropical NWP, causing a dipole rainfall trend over the EA-NWP. The impact of the east-west SST gradient pattern is confirmed by numerical experiments. Our findings highlight that the internally-generated gradient of NIO-WP SST trends is an important source of the uncertainty in EA-NWP summer rainfall decadal changes in simulations.

Automated summary

The impacts of internal variability on East Asia-Northwest Pacific summer rainfall trends are significant, with distinct diversity across ensemble members and a leading spatial pattern caused by sea surface temperature gradient between the North Indian Ocean and tropical western Pacific. The internally-generated NIO-WP SST trends gradient is an important source of uncertainty in EA-NWP summer rainfall decadal changes in simulations.