Soil-Moisture and Precipitation coupling in observations and GCM over intra-seasonal timescales

Abstract:

Soil Moisture and precipitation are coupled and the strength of coupling varies from region to region. Using ground based and satellite based observational estimates of soil moisture, we notice that SM and precipitation are more strongly coupled for drought years over northern India. Power spectrum analysis for drought year showed prominent peaks in 15-20 days window for both variables. This coupling was absent during a year with higher than normal precipitation. The stronger coupling between SM and precipitation in below normal monsoon years can affect the intra seasonal variability of monsoon.

Model experiments were performed to understand the sensitivity of precipitation to SM conditions using a global general circulation model forced with observed sea surface temperature. Three runs were performed. One with variable hydrology (control), and two with non-varying soil moisture conditions (a dry condition and a wet condition). It was observed that the experiment with soil moisture close to dry simulated higher precipitation over northern India as compared to the the simulation with wet soil moisture. This was on account of higher advection of moisture toward this region that surpassed increased evaporation in the wet simulation. A bi-stable nature of the control simulation was noticed that was closer to the dry simulation in the early part of the monsoon season and closer to the wet simulation during the later part of the season, which was unlike in the observations.