Effects of Runoff Changes on the Climate Simulations of the Middle Miocene Climate Optimum

Maintaining global freshwater conservation in climate models is crucial for accurately simulating Earth's hydrological cycle. This property particularly deserves specific attention in the deep-time paleoclimate simulations for the different geographies that changes the river route to the ocean. Changes in the volume of runoff directly exert significant impact on the ocean circulation. Large uncertainties in paleo-topography causes the uncertainties in runoff, but the latter receives less attention in the model simulations. To investigate the effects of the uncertainties on the model simulations, climate simulations of the Pre-Industrial (PI) and the Middle Miocene Climatic Optimum (MMCO) are compared by two sets of experiments —— freshwater conservative and non-conservative experiments that with sufficient and insufficient runoff import to the ocean model, respectively. Responses of the differences between the MMCO and the PI to the runoff changes are investigated. For the mean state, large qualitive and quantitative differences appear in the North Atlantic. Compared to the non-conservative experiments, the conservative experiments show the reduced salinity in the North Atlantic and collapsed Atlantic Meridional Overturning Circulation (AMOC), in contrast to the high salinity distribution and much strong AMOC in the non-conservative experiments. These differences lead to the discrepancies in volume transport through the oceanic seaway, as well as contribute to the temperature, sea ice and surface albedo changes in different amplitude in the North Atlantic. Although the climatic variabilities are affected by the runoff changes, enhanced Atlantic Multidecadal Oscillation (AMO) and reduced El Niño–Southern Oscillation (ENSO) are simulated in the MMCO regardless of the quality of freshwater conservation. Besides, the MMCO simulations show that the intensity of the Asian monsoon is greater in the South Asia and lower in the East Asia compared to PI. The study suggests that runoff changes have great effect on the climate change in the North Atlantic and need extra attention in the paleoclimate study.