A New Initialization Strategy for IITM Decadal Prediction System and Its Impact on the Representation of Meridional Heat and Salt Transport in the Indian Ocean

A new initialization strategy is proposed in this study which provides more accurate representation of the initial state of ocean and atmosphere for seasonal to decadal prediction. The present study demonstrates the role of four dimensional subsurface nudging of ocean state in improving the representation of heat and salt transport associated with Shallow Meridional Overturning Circulation (SMOC) in the Indian Institute of Tropical Meteorology Decadal Prediction System (IITM DPS). For this, the monthly temperature and salinity profiles in the model are nudged towards that obtained from the ensemble of Ocean Reanalysis System version 5 (ORAS5), using Newtonian damping method for the period 1959-present while giving the realistic surface forcing for both the ocean and atmospheric components. Nudging is applied only outside the 3°S-3°N band to avoid spurious near equatorial upwelling. The preliminary validation of physical parameters from model simulation such as temperature, salinity, currents display good agreement with available observations and reanalysis datasets. It is observed that over the Tropical Indian Ocean, the mean bias in upper 100m averaged temperature and salinity with respect to a free run is reduced significantly. A detailed analysis suggests that the new initialization strategy reduces more than 60% mean bias in heat transport relative to forced simulation. The season wise correlation analysis of SMOC and its associated heat and salt transport variations is carried out. Power spectrum analysis of the heat and salt transport associated with SMOC is carried out to understand its dominant modes of variability. Thus the study provides insights on how the temperature and salinity nudging impact the representation of heat and salt transport associated with SMOC with respect to recent ocean reanalysis.