Every year, the Indian Meteorological Department officially predicts the onset date of the Indian summer monsoon (ISM), which is characterized by heavy rainfall throughout the season. However, this prediction is based on a regional definition of ISM onset that has little bearing on the overall seasonal variation of the monsoon and prevents precise evaluation of model simulations developed to predict its timing. In a new paper, Noska and Misra propose a new, objective definition for ISM onset that is based on measurements of average rainfall across India.
The researchers used rain gauge data collected between 1902 and 2005 to develop calculations for an objective definition of both the start and end dates of the ISM in a given year. Their new definition is consistent with other important ISM characteristics—including atmospheric temperature gradient reversal, changes in wind direction, and ocean heat transport. It also aligns with the variability associated with El Niño and La Niña events.
To determine the beginning and end of the ISM, the researchers consider the daily rainfall anomaly—the difference between a given day’s rainfall and the annual mean—averaged over all of India. The onset of ISM is declared when the all-India average daily rainfall begins to exceed the all-India averagedannual mean in a sustained manner. The demise of the ISM is declared when the all-India average daily rainfall begins to recede from the all-India average annual mean.
The authors argue that the strength of the new definition lies in its simplicity: It is based on the sole variable of rainfall. This simplicity could enable people outside of the scientific community to more easily track and understand ISM variability. The new definition could also improve prediction of anomalous rainfall patterns for a given summer season. (Geophysical Research Letters, doi:10.1002/2016GL068409, 2016)
—Sarah Stanley, Freelance Writer