Intraseasonal oscillation of land surface moisture and its role in the maintenance of CTCZ during the active phases of the Indian summer monsoon (Mar, 10.1007/s00382-023-07070-5, 2024)

What is the role of soil moisture in maintaining the continental tropical convergence zone (CTCZ) during the active phase of the monsoon? This question has been addressed in this study using soil moisture datasets from ERA5 reanalysis and is validated with other datasets. The analysis shows that, like rainfall, soil moisture also shows prominent intraseasonal oscillation. Furthermore, the sub-seasonal and seasonal features of soil moisture are different from each other. During the summer monsoon season, the seasonal mean maximum soil moisture is found over western coastal regions, central parts of India, and the northeastern Indian subcontinent. However, during the sub-seasonal active phases of the monsoon, the maximum positive soil moisture anomaly was found in northwestern parts of India. Our analysis also suggests soil moisture plays a pre-conditioning role during active phases of the monsoon over the monsoon core zone of India. It is found that the pre-conditioning depends on that region's soil type (texture) and climate patterns. Analysis of the moist static energy (MSE) budget during the active phases shows that soil moisture feedback affects the boundary layer MSE and rainfall. We also evaluate the pre-conditioning feedback in the Climate Forecast System (CFS) model-free run to understand the well-known CFS model rainfall bias. CFS runs do not show the realistic pre-conditioning role of soil moisture and its feedback on the rainfall as in observations. Thus, the rainfall dry bias in CFS operational forecasts may be partly due to this error in soil moisture-rainfall feedback. Another important conclusion from this study is that the soil texture has an important role in changing the soil–moisture and rainfall intraseasonal pattern over the region. Using a WRF regional model experiment, we find that the pre-conditioning of soil moisture over sandy soil texture leads to increased precipitation during the active phase of the monsoon. © Springer-Verlag GmbH Germany, part of Springer Nature 2024.