A global-scale test for monsoon indices used in palaeoclimatic reconstruction

The monsoon has received broad attention because of its economic and ecological importance. Many studies based on isotopic, palaeomagnetic, sedimentological, palaeobotanical or palaeontological proxy data have addressed the Neogene evolution of the Asian monsoon. Quantitative estimations of monsoon intensity in geological history are needed to study its evolution. Good tools are also needed to discriminate between past monsoonal or non-monsoonal climates. Several indices have been proposed to describe the seasonality in precipitation or monsoon strength based mainly on precipitation parameters. However, these indices have never been tested nor validated on a global scale. Herein, we look at four indices used in recent studies. The values of these indices are calculated for the present condition on a global scale using a geographical information system. We found that all four indices reconstruct false positive scenarios, i.e., non-monsoonal regions (either high latitudes or equatorial regions) with high index values. And some indices fail to give high values for some monsoonal regions like India. These indices cannot be used to discriminate between monsoonal and non-monsoonal climates through geological time. However, they can be used when the presence of the monsoon is confirmed by other means, and in such cases they are useful for measuring monsoon intensity. The monsoonal precipitation is characterized by its amount, annual range, and seasonal distribution but these three aspects of the monsoon are not sufficiently summarized in one index value. Parameters useful for palaeomonsoon description, such as the ratio of summer precipitation and the daily rate of summer precipitation, should be directly included in methods of palaeoclimatic reconstruction.