Roles of land surface albedo and horizontal resolution on the Indian summer monsoon biases in a coupled ocean–atmosphere tropical-channel model

Abstract

precipitations are underestimated over India while they are overestimated over the 37 southwestern Indian Ocean, South-East Asia and the Maritime Continent. The ISM 38 onset is delayed by several weeks, an error which is also very common in current 39 CGCMs. 40We show that land surface temperature errors are a major source of the ISM 41 low-level circulation and rainfall biases in our model: a cold bias over the East (ME) region weakens the Findlater jet while a warm bias over India strengthens 43 the monsoon circulation over the southern Bay of Bengal. A surface radiative heat 44 budget analysis reveals that the cold bias is due to an overestimated albedo in this 45 desertic ME region. Two new simulations using a satellite-observed land albedo show 46 a significant and robust improvement in terms of ISM circulation and precipitation. 47Furthermore, the ISM onset is shifted back by one month and becomes in phase with 48 observations. Finally, a supplementary set of simulations at 0.25°-resolution confirms 49 the robustness of our results and shows an additional reduction of the warm and dry 50 bias over India. These findings highlight the strong sensitivity of the simulated ISM 51 rainfall and its onset timing to the surface land heating pattern and amplitude, 52 especially in the ME region. It also illustrates the key-role of land surface processes 53 and horizontal resolution for improving the ISM representation, and more generally 54 the monsoons, in current CGCMs. 55 56 57 Keywords 58Indian Summer Monsoon; land surface albedo; horizontal resolution; 59 precipitation biases; monsoon onset; CGCM 60 3 Introduction 61 62The Indian Summer Monsoon (ISM; see Table 1 for acronyms) brings 63 substantial rainfall from June to September to some of the world most populated 64 regions, whose economy relies mainly on agriculture and water resources. But despite 65 recent progress in our understanding of mechanisms driving ISM precipitation, 66Coupled General Circulation Models (CGCMs) are still not able to correctly represent 67 its main spatial and temporal characteristics (Sperber et al. 2013) and the skill of 68 seasonal ISM predictions by dynamical or statistical models remains currently very 69 low, contrary to what is observed in other tropical regions (Wang et al. 2015). 70While some improvements have been achieved with the last generation of 71CGCMs, especially in terms of intraseasonal variability (Abhik et al. 2014, Sabeerali 72 et al. 2013, some basic features of the ISM, such as the onset or 73 the rainfall spatial distribution, are still poorly captured with a persisting (wet) dry 74 bias over (ocean) land (see Fig. 2 of Sperber et al. 2013). 75The limited horizontal resolution of CGCMs is frequently listed as a major 76 caveat because current coarse atmospheric models cannot properly resolve orography 77 2013) or convection (Pattnaik et al. 2013, Ganai et al. 2015, which all significantly 80 contribute to the total ISM rainfall, especially in the monsoon trough region. 81Regional Climate Models (RCMs) al...

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