Trends in extreme mean sea level pressure and their characteristics during the summer monsoon season over the Indian region

Abstract

Using daily National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) (1948–2009) and European Centre for Medium-Range Weather Forecasts (ECMWF) ReAnalyses (ERA)-40 (1958–2001) reanalysis mean sea level pressure (MSLP) data, the frequencies in the extremes of low/high MSLP days were computed with the 10th and 90th percentiles during the summer monsoon season (June–September) over 11 zones of India, the Arabian Sea, the Bay of Bengal and all-India as a whole. We observed that the trends in the frequencies of high (low) MSLP days are increasing (decreasing), indicating a increase/decrease in anticyclonic/cyclonic activities, respectively, which affect the monsoon performance over the Indian region. The decreasing (increasing) tendency of the frequency of the low (high) MSLP days suggests a consistency between the extreme MSLP and its associated rainfall activities. The frequency of the high MSLP days over India is highly correlated negatively with the Indian summer monsoon rainfall (ISMR). The linear trends in the frequencies of the extreme low/high MSLP days for different zones and all-India are found to be highly significant (at the 0.1% level and above). After 1978, the trends in the series of low/high MSLP days are found to be more towards decreasing/increasing tendencies, respectively, whereas the frequencies of low MSLP days are much higher than those of high MSLP days during the late 1970s. The same characteristics were also evidenced from the analysis based on ECMWF ERA-40 data for the period 1958–2001. The possible causes of this may be El Niño events, greenhouse gases (GHGs), unprecedented surface warming and also tropospheric cooling trends during recent decades over the northern hemisphere as well as over the Indian region and major volcanic eruptions that occurred during the period 1978–2009.