The Impact Of Hydrological Cycle Changes On The Ocean Circulation And Climate

Both observations and climate models indicate that the hydrological cycle(evaporation minus precipitation, EmP) has been intensified as a result of globalwarming. Hydrological cycle change can affect oceanic freshwater flux on one hand,which leads to changes in ocean salinity, currents, and temperature field. Anomalies inocean can further feedback to atmosphere, inducing an adjustment of global climate.On the other hand, hydrological cycle change can also affect atmospheric water vaporcontent and the associated diabatic heating. Water vapor acts as a dominantgreenhouse gas and provides the largest known feedback to amplify global warming.The phase transition of water vapor is usually accompanied with latent heat release,which in turn generates fluctuations in poleward heat transport.In this thesis, we focus on the significant role of EmP induced freshwater fluxand water vapor changes in climate system based on the coupled model FOAM1.5(Fast Ocean-Atmosphere Model, version1.5), observation, reanalysis data and modelresults from intergovernmental panel on climate change (IPCC).Recent observational analysis further demonstrates a complicated spatial patternof trend in freshwater changes. One distinct feature is a significant freshwater lossover the global western boundary currents and their extensions, particularly in theKuroshio-Oyashio Extension (KOE), gulf stream and its extension. That means thereis a tendency to loss freshwater over the western boundary currents and theirextensions. Firstly, we focus on the coupled ocean-atmosphere responses to regionalfreshwater change over KOE, Gulf stream and its extension where freshwater exertsthe maximum change over the global ocean based on the coupled model FOAM. The model explicitly demonstrates that the positive EmP forcing in the KOE region can setup a cyclonic gyre straddling the subtropical and the subpolar gyre, which induces ananomalous southward cold advection in the west and a northward warm advection inthe interior. This leads to a formation of temperature dipole in the midlatitudes with acooling in the west and a warming in the east. With the positive EmP forcing in theKOE, the response of the extratropical atmospheric circulation in the North Pacificsector is characterized by an equivalent barotropic low originating primarily from thewestern tropical Pacific changes and countered by the extratropical SST forcing. Thepositive EmP forcing also strengthens the tropical zonal SST gradient and thus ENSOthrough several competing processes including surface coupledwind-evaporative-SST (WES) mechanism, subduction of extratropical warmanomalies, and spin-up of the density-driven meridional overturning circulation.Freshwater loss over the Gulf Stream and its extension region also forces ananomalous cyclonic gyre and triggers a SST dipole, but with a northeast-southwest tiltrather than a east-west line. The freshwater loss also forces a significant response inthe atmosphere with a negative NAO-like response in early winter and a basin-scaleridge resembling the Eastern Atlantic Mode (EAM) in late winter. The salinificationalso strengthens the Atlantic meridional overturning circulation and thus the polewardheat transport, leading to tropical cooling. The freshwater loss over the Gulf Streamand its extension also leads to an El Nino-like warming in the tropical Pacific andcooling in the North Pacific, similar to the responses in previous water-hosingexperiments with an input of freshwater in the subpolar North Atlantic. The tropicalPacific responses subsequently strengthen the northern hemispheric atmosphericanomalies in early winter, but reverse them in late winter through an emanation ofRossby wave trains. Overall, the tropical Pacific air-sea coupling plays a damping role,while local air-sea coupling tends to enhance the ocean and atmospheric responsesover the North Atlantic.After exploring coupled ocean-atmosphere adjustments to the regionalfreshwater loss over the KOE, Gulf stream and its extension, we then focus on therole of global freshwater change induced by the hydrological cycle change in climate system. We assess the role of freshwater change in global warming based on theFOAM model in which CO2concentration is doubled. The model simulationsdemonstrate that the warm climate leads to an acceleration of global hydrologicalcycle which causes freshening in the high latitudes and salinification in the subtropicsand midlatitudes. It is found that the freshwater flux changes tend to amplify ratherthan suppress the global warming. Over the global scale, this amplification is largelyassociated with high latitude freshening in a warm climate, which leads to ashallowing of the mixed layer depth, reduced vertical mixing and thus a trapping ofCO2-induced warming in the surface ocean. The latitudinal distribution of SSTchanges due to the effects of freshwater flux changes in a warm climate iscomplicated. In the Northern Hemisphere, the freshening leads to not only a trappingCO2-induced warming in the surface layer, but also a slowdown of the meridionaloverturning circulation and thus warming of the deep ocean. However, the latitudinaldifference in the surface salinity also leads to anomalous convergent flows in themidlatitudes, which amplify the warming in the midlatitudes but reduce the warmingin the high latitudes. In the Southern Hemisphere, the warming in the high latitudesdue to the freshening triggers easterly anomalies through local coupledocean-atmosphere feedback, induces poleward anomalous Ekman warm advection,which significantly offsets the salinity-driven equatorward cold advection as seen inthe Northern high latitudes, and thus sustains the warming in the southern highlatitudes. In addition, atmospheric feedbacks associated with global warming alsoamplify the SST warming.After doing some researches about the role of freshwater flux in climate system,we finally turn our eyes to the atmospheric water vapor based on the reanalysis data20CRv2. It is found that annually water vapor varies on three major time scales: asecular trend, an interannual variation and a multidecadal variation. The long termincreasing trend is suggested to be associated with global warming as a result ofanthropogenic forcing, whereas the latter two are largely due to natural variability inclimate system, pointing to the Atlantic multidecadal Oscillation (AMO) and ElNio-Southern Oscillation (ENSO), respectively. The synchronization of water vapor and SST on global scale can be explained by the simple thermal Clausius-Clapeyrontheory under the conditions of constant relative humidity. However, on regional scale,water vapor spatial pattern associated with the global warming, AMO and ENSO inthe tropics is largely attributed to the mean circulation dynamics, particularly theplanetary divergent circulation change induced by the SST change. In the middle andhigh latitudes, transient eddy fluxes and the thermodynamics also play a significantrole.