The Monsoon as a Hydrological Capacitor: Memory Effects and Interannual Variability

The monsoon plays a very important role in controlling water availability over land, especially in regions like South Asia. While monsoon rainfall is often studied as a seasonal event, it is also influenced by what happens before and after the monsoon season. In this work, we study the idea that the monsoon system behaves like a **hydrological capacitor**, where land water storage accumulates during the monsoon and slowly releases afterward, affecting future conditions.

In this framework, soil moisture and subsurface water storage act as a memory of past rainfall. During the monsoon, rainfall adds water to the land surface, similar to charging a capacitor. During the dry season, this stored water is lost through evaporation, transpiration, and runoff, which is like discharging the capacitor. Because this discharge happens slowly, the land retains memory of past monsoon conditions over several months or even years.

We develop a simple mathematical model to describe how water storage changes from year to year under monsoon rainfall forcing. The model shows that the amount of storage before the monsoon can strongly influence surface dryness and land–atmosphere interactions in the following season. Even small changes in monsoon duration or intensity can lead to large differences in pre-monsoon dryness, especially when the storage decay timescale is long.

Using idealized stochastic rainfall forcing, we derive expressions for the variability and persistence of land water storage. The results show that interannual variability in monsoon rainfall naturally produces correlations across years because of this storage memory. The model also suggests that a shift in monsoon onset or withdrawal by about 10–20 days can significantly change the amount of water stored in the land system.

As part of the ongoing work, observational data from reanalysis and gridded precipitation products will be used to estimate realistic storage timescales and to test whether the predicted relationships are seen in real monsoon regions. The model will also be extended to study how large-scale climate variability influences the monsoon through changes in rainfall statistics.

Overall, this study shows that viewing the monsoon as a capacitor-like system provides a simple and useful way to understand monsoon memory, interannual variability, and the persistence of dry and wet conditions.