Early prediction of Indian Summer Monsoon Onset
Update (May 31): The Indian Summer Monsoon (ISM) onset prediction for 2022, which we issued with data until March 11 (see below), turns out to be correct. Based on the tropospheric temperature gradient ∆TT, we predicted the ISM onset to occur on May 26 (median) with Interquartile range from May 24 to May 28, and indeed, the ∆TT-based ISM onset in 2022 occurred on May 25 (Fig. 1). Note that the regional monsoon onset over Kerala (MOK) occurred on May 29.
The Indian Summer Monsoon (ISM) sets in at the Southern tip of India, typically around the beginning of June. The prediction of the onset date is crucial for effective agricultural planning and general water-resource management in India.
The meridional gradient of the tropospheric temperature over the southern Eurasian region (∆TT) is a widely-used index for an objective definition of the ISM onset date [Xavier et al., 2007]. The onset date is defined as the day of year at which ∆TT changes from negative to positive (Fig. 1a and 1b, black). The ∆TT-based ISM onset date is strongly correlated with the Monsoon onset over Kerala (MOK) (Fig. 1b, green). The MOK is declared by the Indian Meteorological Department and forecasted at the end of April and at the middle of May. Therefore, a prediction of the ∆TT-based ISM onset date at longer lead times than two months would be helpful.
Mitsui and Boers (2021) proposed a neural-network-based prediction method for the ∆TT-based ISM onset date, with a seasonal-scale lead time (~83 days) (Fig. 2, left). So far, we have succeeded in hindcasting the onset dates during 1981－2021 with an accuracy of 4.9 days (RMSE) and correlation coefficient of 0.58 (Fig. 1b). Based on this method and the observed ∆TT by March 11th 2022, the Indian Summer Monsoon in this year 2022 is predicted to start around May 26th (median), with an interquartile range from May 24th to 28th. Note that this is the prediction for ∆TT-based onset date (not for the MOK). This neural-network prediction is partly related to a statistical correlation between the average winter ∆TT and the onset dates (Fig. 2, right). The predicted date (May 26th), which is earlier than the average (June 2nd), is overall consistent with above-average winter ∆TT, as observed by March 11th.
Figures:
Fig. 1. Prediction of the Indian Summer Monsoon onset date for 2022 (May 26th (median), with an interquartile range from May 24th to 28th) and those for the past 41 years. (a) Annual variation of the ∆TT thermal index. (b) Predicted onset (magenta circle) and actual onset dates (black diamond). MOK is the Monsoon Onset over Kerala, as declared by the Indian Meteorological Department (green cross).
Fig. 2. (Left) Schematic of our method. We construct recurrent neural networks that approximate proximity to the onset of Indian Summer Monsoon (Mitsui and Boers, 2021). (Right) Empirical anti-correlation between the average winter ∆TT (the first 70 days of year) and the accrual onset date (R=-0.53)
Reference: Takahito Mitsui and Niklas Boers 2021 Environ. Res. Lett. 16 074024
