Assessing the role of local air–sea interaction over the South Asia region in simulating the Indian Summer Monsoon (ISM) using the new earth system model RegCM-ES

The understanding and prediction of the monsoon variability over South Asia region is one of the biggest challenges for climatologist and meteorologist today. The Indian Summer Monsoon (ISM) has different temporal and spatial scales of variability and it is mainly driven by strong air sea interactions. In this thesis we evaluate the performance of the new regional Earth System Model (ESM) RegCM-ES in reproducing the main characteristics of the ISM rainfall (ISMR). We performed two sets of simulations, one with the new RegCM-ES and another with the stand-alone version of the atmospheric component i.e. the regional climate model (RCM) RegCM4. RegCM-ES is composed mainly by three components, the RegCM4, the ocean model MITgcm and the hydrological model HD. Another experiment, performed using RegCM-ES with a more high-resolution hydrological model implemented ad hoc for this study, has been added to this two set of experiments. The climatological mean state of the monsoon is well represented by mostly all the experiments although not with the same skills. The most interesting results are observed in simulating the variability of the monsoon and here we highlight some of them. The intraseasonal northward(eastward) propagation of the convection has been analysed using lag/lead map of the regressed anomalies as a function of latitudes(longitudes). The two propagations are better reproduced by the set of ESM simulations thanks to the role of the air-sea coupling. For what concerns the interannual variability (IAV) of the ISMR the air-sea coupling plays an important role. The time series of simulated anomalies by RCM exhibit no correlation with the observed anomalies obtained from the dataset of the Indian Meteorological Department (IMD). On the other hand, the corresponding ESM simulations exhibits good skills in reproducing the IAV of ISMR and good correlation coefficients are observed with IMD. One new finding of this study is a new source of predictability with one-year lag for the ISMR. It is well known that El Niño Southern Oscillation (ENSO) plays a quite important role in modulating the precipitation over most of the intertropical belt and over the South Asia region. As seen in previous findings, the response of the ISMR to ENSO can be delayed of two seasons through the contribution of different air-sea coupled mechanisms as the decrease of Western Arabian Sea Upwellingthe Indo-western Pacific ocean capacitor and the Indo-Tropical northwest Pacific ocean-atmosphere interaction. Our findings extend and confirm the possibility that this response may have a longer feedback time, a year or so. The coupled simulations are used to explain the mechanism and investigate the models response. They appear quite similar to those proposed in the previous studies but further investigations are needed to understand more in deep the phenomena involved. The role of the new hydrological model (CHyM) implemented inside RegCM-ES on the ISMR is also investigated. Due to the main role that the freshwater discharge plays on the formation of a shallow mixed layer depth on the Bay of Bengal that influences the air-sea coupling and the formation of deep convection over this area, the correct estimation of the freshwater discharge is quite important. Although the new hydrological model produces a more realistic annual cycle of the discharge (verified only for a limited set of data due to the lack of observations over this region), this doesn't seem to have a relevant effect on both the mean climatological state of the monsoon and in its variability. A possible explanation for this comes from a missing representation of the barrier layer (BL) in the Bay of Bengal (BoB) due to many different reasons. Two of them are a not enough resoluted ocean model as well as a too strong wind stress forcing that doesn’t allow the formation of the BL.