\section{Data, models and methods}\label{sec:data}
For observed precipitation, we use: version 6 of  Global Precipitation Climatology Centre (GPCC) monthly precipitation \citep{Rudolf_2010}, which is a gauge-based,  0.5$^{\circ}$ longitude $\times$ 0.5$^{\circ}$ latitude gridded global land surface  dataset for  the period of 1901--2010; version TS 3.21  of monthly precipitation over global land areas from the Climatic Research Unit at the University of East Anglia (CRU hereafter, \citealt{Harris_2014}) covering from 1901 to 2009; version 2.2 of the Global Precipitation Climatology Project (GPCP) monthly precipitation dataset from 1979 to 2010 \citep{Huffman_2009}, which combines gauge observations and satellite  data into 2.5$^{\circ}\times$2.5$^{\circ}$ global grids; Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) \citep{Xie_1997}, which is also monthly satellite and gauge data covering from 1979 to 2011;  version 7 of Tropical Rainfall Measuring Mission (TRMM) daily precipitation \citep{Huffman_2007} covering from 1998 to 2014; precipitation from the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-Analysis (ERA-Interim) \citep{Dee_2011}, which covers the post-1979 period and is the latest of the ECMWF reanalyses