| Summary: | In order to study the impact of global runoff, sensitivity experiment (NoRiv) was carried out by shutting down the entire runoff into the ocean for a period of 200 years. The changes in NoRiv, compared to the reference simulation (CR), shows the impact of global runoff (NoRiv minus CR). The evolution of mean ocean SSS in the major ocean basins show that the upper– ocean reached a quasi-stabilized stage in the first 100 years in the NoRiv. Globally, when there is no runoff, the surface ocean turns saltier with the largest increase occurring off the mouths of major river systems. High salinity patches of more than 2 psu are found near the mouths of Amazon, Congo, Ganga–Brahmaputra, Yangtze, etc. The upper–ocean currents distribute the positive salinity anomalies generated near the river mouth of NoRiv to far away regions and open ocean. In accordance with our results using CCSM3.0 model, CESM1.0 also shows that the impact of runoff into the ocean on SSS is significantly higher in the extratropical ocean than in the tropics, though the tropical runoff is considerably higher (Vinayachandran, Jahfer, and Nanjundiah, 2015). Arctic Ocean recorded the highest rise in salinity as the ratio of runoff to the surface area of the basin is highest. The open ocean regions of Pacific exhibited the least change in salinity due to the vastness of the basin. We further analyzed the significance of this rise in SSS on the upper–ocean temperature, air–sea interaction, and rainfall. A saltier ocean in the NoRiv deepens the mixed layer of the upper–ocean owing to the increase in surface density. Though the mean MLD in the NoRiv is found to be deeper than the CR, the SST response to this change in ML is complex. In the simulation without runoff, the northern hemisphere oceans recorded a comparatively higher rise in surface temperature, mostly in the Atlantic Ocean. The change in SST in the NoRiv is primarily due to the weakening of upper–ocean stratification or due to changes in air–sea interaction in the coupled system. The equatorial ...
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