Characteristics Of Freshwater Flux And Its Impact On The Regional Climate Change

Air-sea freshwater flux, the difference between Evaporation and Precipitation at the surface of the Atmosphere and the Ocean, is an important indicator to represent the global exchange of moisture between them. Observed and model results both indicate the water cycle is accelerating under the circumstance of global warming; the spatial distribution of water is changing. Tropical extreme precipitation is becoming more frequent and fierce, leading to the change of global freshwater utilizability and further threatens global ecosystem and human society stability. In the long run, the strengthening of water cycle could induce the change of sea surface salinity through changing the freshwater flux, and then induce the current and temperature anomalies, finally feed back to the change of climate. By using several reanalysis and observed datasets, freshwater flux results of 13 models from CMIP5 project and the coupled model CESM, we analyze the historical freshwater variations and then project the changing pattern based on different Representative Concentration Pathways. Based on which, we focus on the significant role of freshwater flux on the Ocean and the effect of the anomalous precipitation of the tropical Pacific Ocean on the climatology states of the Ocean and the Atmosphere as well as the North Atlantic Ocean and the East-Asia monsoon region.Recent observational freshwater demonstrates significant seasonal characteristics, especially where is vulnerable to the East-Asia monsoon and the sub-polar region where is susceptible to the solar radiation. ENSO modulates the interannual variations of freshwater, and reflect on the opposite spatial distribution pattern on the middle tropical Pacific and the western Pacific. In the high latitudes region of the North Hemisphere, middle eastern equatorial Pacific Ocean and the southern Indian Ocean, freshwater shows increasing trend; while in the northern hemisphere subtropical latitudes and middle to high latitudes in the southern hemisphere, it is characterized by decreasing trend. Spatial distribution of freshwater trend is consistent with the first mode of annual mean freshwater EOF analysis. By evaluating the CMIP5 models results by the freshwater climatology and zonal mean distribution, Taylor diagram analysis, seasonal to interannual EOF analysis, the results show that all models could capture the climatological space distribution and zonal mean characteristics as well as the spatial and temporal features on seasonal timescale and spatial characteristics on the interannual timescale. Its defienciency are mainly in following several aspects:the double ITCZs are widespread among the models and their magnitudes are smaller than the reanalysis results, the temporal characteristics on the interannual timescale are poorly simulated. Results of multi-model ensemble are superior to single model in all respects of evaluation. Tropical Ocean will change significant under the circumstance of global warming with a range between 15% and 20%, the spatial distribution pattern will not change markedly under different scenarios, but the amplitudes of freshwater flux and the consistency among the models will strengthen under RCP8.5 compared with the RCP4.5.The extreme positive precipitation anomalies in the tropical Pacific Ocean could induce a El Nino-alike mode, which lift the western tropical Pacific mix layer depth(MLD) and drop the eastern Pacific MLD. The north and south equatorial current weaken and even disappear; meanwhile, the equatorial undercurrent weakens significantly with a magnitude up to 50% of its climatology value. The south part of north equatorial current which closes to the equator strengthens obviously which the north part weakens slightly.Freshwater enters into the Ocean in the form of virtual salt flux, showing a pathway as follows. The freshwater flux increase (decrease) anomalies can quickly cause the decrease (increase) of the sea surface salinity by influencing the density changes, leading to the sea surface height decrease (increase); Under the effect of geostrophic balance, increase of the northern hemisphere sea surface height anomaly can induce a clockwise circulation anomalies, while in the southern hemisphere, it’s opposite. This finally leads to the redistribution of ocean surface heat, and dramatic changes will take place in SST.The existence of freshwater flux is crucial for the temperature and salinity variations on the Atlantic high-latitude Ocean, it could help to maintain the North Atlantic meridional overturning circulation and prevent the sharply cooling and freshening phenomenon.Equatorial Pacific abnormal increase of precipitation (10%) can induce an El Nino-alike mode of the sea surface temperature and the mixed layer depth in central equatorial Pacific, leading to the weakening of Walker circulation western Pacific Hadley circulation and Atlantic Hadley circulation, whereas the strengthening of eastern Pacific Hadley circulation. Similar changes happen to the equator currents which are found based on the reanalysis datasets,thermoline rises on the western Pacific Ocean while declines on the eastern part, it induces the weakening of south(north) equatorial currents and the upper part of the western equatorial undercurrent, the strengthening of the lower part of the eastern equatorial undercurrent.The El Nino-alike mode could affect the precipitation of the East Asian monsoon region and the North Hemisphere (especially the North Atlantic) high latitudes region through the equator coupled wave train, polar vortex and the arctic oscillation. The reason for the abnormal cooling and freshening of sea water in the high latitudes of the North Atlantic Ocean could be explained by the anomalous southward transport ice mass and freshwater along the sea ice boundary through the East Greenland currentin winter. The accumulated sea ice will melt in summer and finally result in the cool and fresh water. Furthermore, with the integral of freshwater on the ocean surface, the buoyancy flux increases, weakens the mix and entrainment effect within the mix layer depth, form a positive feedback mechanism which contribute to a cooler and fresher surface layer.