Satellites reveal the strongest increase in duration of extreme dry periods in global monsoon regions

Drought is arguably the climate phenomenon that most strongly impacts societies worldwide, causing severe socioeconomic and ecologic damage. While models unanimously project an overall increase in aridity and drought occurrence in the future, observational evidence has so far been inconclusive. The discrepancies between the various drought definitions and drought indices has been a major factor contributing to our low confidence in observed dryness trends. In this study we investigate global trends in meteorological dry spells using a simple, unambiguous and intuitive diagnostic: the maximum annual number of consecutive dry days (CDDs). In contrast to popular drought indices, the number of CDDs is a direct measure of the duration of rainfall scarcity, easy to quantify based on rain data, free of parametrizations, and independent from other proxies.

Because the time-span of available satellite-based precipitation data records is constantly increasing, current products are becoming an alternative to in situ rain gauges for studying long-term trends. In particular, the Tropical Rainfall Measuring Mission (TRMM) has now been operational for over twenty years, thus offering a unique opportunity to analyse temporally-coherent, single-platform precipitation data. Here, we use TRMM3B42 3-hourly data for 1998–2018 to calculate and analyse changes in the maximum annual number of CDDs worldwide. The robustness of the identified relationships among observational products is tested using recently-compiled gauge and satellite precipitation data from the Frequent Rainfall Observations on GridS (FROGS) database.

The results reveal that almost 70% of the continental land monitored by TRMM has experienced an increase in duration of the longest annual dry period over the past 20 years, and in 20% of these regions trends are found to be significant (p < 0.01). Agreement among various observational products is regionally dependent. However, most of the data sets suggest that the signal largely originates, not from arid regions (which would support the dry–gets–drier paradigm), but from monsoon areas. Further analysis shows that the same areas experience clear increasing (decreasing) trends in rain seasonality (amount), suggesting a link to the monsoon circulation dynamics. A preliminary analysis confirms this connection and additionally points to the potentially important role of land feedbacks, revealing a tendency for later moisture build up and, hence, a monsoon onset after more prolonged dry seasons. Altogether, our findings emphasize the vulnerability of global monsoon regions to climate change. An increasing length of dry spells as we progress into the future might lead to devastating socioeconomic and ecologic consequences in these regions.